2D semiconductors: applications and perspectives

Resume : Made from stacks of two-dimensional materials, van der Waals heterostructures exhibit unique light-matter interactions and are promising for novel optoelectronic devices. The performance of such devices is governed by near-field coupling through, e.g., interlayer charge and/or energy transfer. New concepts and experimental methodologies are needed to properly describe 2D heterointerfaces. Here, we report an original study of interlayer charge and energy transfer in atomically thin metal-semiconductor [i.e., graphene-transition metal dichalcogenide (TMD)] heterostructures using a combination of micro-photoluminescence (PL) and Raman scattering spectroscopies. By measuring light emission from the TMD, we demonstrate that interlayer coupling to graphene drastically reduces the PL yield and exciton lifetime and leads to significant modifications of the temperature dependent PL spectra. Additionally, using Raman spectroscopy, we finely probe the frequency and linewidth of the optical phonon modes in the monolayers. These parameters are highly sensitive to the charge carrier density and show a net photo-induced charge transfer from TMD to graphene that is sensitive to the sample environment and temperature. Remarkably, exciton dynamics in TMD-graphene heterostructures is largely independent of the existence of this net charge transfer. This key result suggests that fast interlayer energy transfer from TMD to graphene dominates the photoresponse of these heterostructures.

Resume : Molybdenum ditelluride (α-MoTe2) is a layered semiconductor that belongs to the family of transition metal dichalcogenides (TMDCs). MoTe2 is unique because it exhibits the smallest direct (indirect) band gap, around 1.15 eV (0.85 eV) amongst all group VI TMDCs semiconductors.[1] Thus, allowing to extend the optoelectronic applications of TMDCs into the near-infrared range. Also, the existence of spin-polarized bands in some bulk TMDCs[2] allows to explore an exciting range of potential applications in spintronics and valleytronics for multilayered TMDCs. In order to design novel devices it is crucial to accurately determine the electronic band spin-splitting. In this regard, MoTe2 is an excellent candidate to be investigated since it exhibits the largest spin-orbit coupling amongst all Mo-containing TMDCs. We investigated high-quality MoTe2 crystals[3] by means of photoreflectance (PR) under high pressure. We measured the pressure dependence of six direct excitonic transitions and assigned them with the aid of first-principles calculations based on density functional theory. We discussed the presence of excited-states, interlayer states and dark excitons. The obtained pressure coefficients of the A and B transitions are 2.40(3) and −3.42(18) meV/kbar, respectively. The difference in sign between both transitions is attributed to a strong splitting of the conduction band with increasing pressure and the presence of spin-valley locked bands. 1 S.J. Zelewski and R. Kudrawiec, Scientific Reports 7, 15365 (2017). 2 X. Xu, W. Yao, D. Xiao, and T.F. Heinz, Nature Physics 10, 343 (2014). 3 [online] [access: 10 January 2018] http://www.hqgraphene.com/

Resume : High-pressure experiments are a controllable and comprehensive approach to tune materials’ band structure. The results offer useful guidelines for the design of semiconductor devices. In this case, the pressure can be generated by strain induced from the device substrate, or isovalent chemical substitution. I will present experimental results on semiconducting transition metal dichalcogenides (TMDs) of different crystal symmetry (2H-WSe2, 1T-ZrS2, 1T-ZrSe2), showing the evolution of the optical absorption in the pressure range 0-20 GPa. For the case of 2H-WSe2, we observed a non monotonous evolution of the optical absorption onset with pressure. Thanks to ab-initio calculations, we explain the experimental observation as manifestation of a competition between band-gap closing and exciton binding energy increasing under pressure. Completely different behavior is observed for the 1T semiconducting TMDs (1T-ZrS2, 1T-ZrSe2), for which high pressure induces an in-plane symmetry breaking of the crystal symmetry. The structural transformation is responsible for a semiconductor to metal transition.

Resume : Two dimensional (2D) materials have received much attention of the scientific community owing to their unique properties and as the materials becoming an intriguing and promising candidates for photonic and optoelectronic devices with high performance and unique functions [1-4]. Layered transition metal dichalcogenides (TMDs) are more attractive for specific applications in electronics and optoelectronics than their mature sibling, graphene, due to presence of a finite band gap [1-2]. TMDCs exhibit a significant change in their electronic structure with the layer thickness [1]. To realize TMD based next generation ultra-thin, flexible photonic and electronic devices enormous efforts have been devoted to enhance and tune their electronic and optical properties[5,6]. Recently, a considerable attention has been paid to tune the emission using hybrid systems composed of a TMD and metal nanoparticles (NPs) since NPs have the ability to enhance and localize the incident electromagnetic field [5, 6]. Furthermore, these hybrid systems show great interest from the standpoint of fundamental science as it constitutes an excellent platform to investigate plasmonic-exciton interactions and charge transfer. Here we realized WS2–Au hybrids structures by growing Au NPs at the edge of the mechanically exfoliated bilayer WS2, by simple chemical treatment. We study the interactions between WS2 and Au NPs through Raman and Photoluminescence (PL) spectroscopy. A significant enhancement of the PL intensity in the WS2-Au composite with respect to bare bilayer WS2 has been observed, and it increases as the number and size of the Au-NPs on WS2 is increased [7]. Spatial and spectral overlapping between the localized surface plasmon polariton waves and that from WS2 emission modulates the relative intensity between trion and exciton emissions[7]. We probe the mechanism of the intensity change through polarization dependent PL measurements and simulation [7]. Our study sheds light on developing the TMDs based photonic devices. References [1] K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010) [2] A. Splendiani et al., Nano Letters 10, 1271 (2010) [3] O. Lopez-Sanchez et al., Nature Nanotechnology 8, 497 (2013) [4] D. Ovchinnikov et al., ACS Nano 8, 8174 (2014) [5] J. Kern et al., ACS Photonics 2, 1260 (2015) [6] K. C. J. Lee et al., Scientific Reports 5, 16374 (2015) [7] T. S. Bhattacharya et al., Manuscript under preparation.

Resume : Transition metal dichalcogenides monolayers are attracting extraordinary attention for its intriguing optical, electronic and mechanical properties. One of the advantages of this class of 2D semiconductors, with respect to graphene, is the indirect-to-direct bandgap transition as a function of the thickness. In this work we develop Quantitative Nanoscale Absortion Mapping, a novel non-destructive and non-contact technique for the analysis of the absortion properties of 2D materials at the nanoscale. The technique is based on the cathodoluminescence of the underlying substrate and the concurrent optical absoprtion of the 2D materials. The electron probe allows to overcome the light diffraction limit and analyze 2D layers with nanometer spatial resolution. The QNAM technique can employ standard growth substrate of transition metal dichalcogenides, as oxidized silicon and sapphire. In particular, the defect related emissions of the different oxide provide different spectral range of analysis from UV to visible. The QNAM map allow to study the optical properties of the 2D materials such as adlayer pyramidal defects, grain boundaries and bilayer terraces. This technique is also suitable for studying the absorption properties of Van der Waals heterostructures based on MoS2 and MoSe2. This imaging technique opens the way to in-situ studies at the nanoscale on working devices based on a wide range of 2D materials in different spectral range.

Resume : The exceptional semiconducting properties of exfoliated transition metal dichalcogenides (TMDs) nanosheets have inspired their use in applications such as transistors, photodetectors, biosensors and photoelectrodes. Nano-TMDs not only provide a new range of tunable optoelectronic properties, but also create a direct path towards ultrathin, flexible devices. However, a major challenge in commercializing TMD-based devices is their large-scale production.1 Our group recently addressed this issue by developing methods for solution-based fabrication of nanoflake TMD thin films for large area solar energy conversion applications.2 In this presentation we advance these methods toward a continuous roll-to-roll deposition of TMD-based thin films from nanoflake dispersions using a liquid-liquid self-assembly technique. We demonstrate the printing of 100mm wide TMD flake films on plastic substrates at a rate of 60mm/min. In addition, we address the effects of processing conditions on TMD flake quality and optoelectronic device performance. We find that exfoliation method and solvent choice impact the defect density within the flakes, which effects the harvesting of photogenerated charge carriers. We observe this for MoS2 and WSe2-based devices, suggesting that this is a fundamental limitation of liquid exfoliated TMDs and further we present defect mitigation strategies to overcome this.3 1. ACS Energy Lett. 1, 315–322 (2016). 2. Nat. Commun. 6, 7596 (2015). 3. Nano Lett. 18, 215–222 (2018).

Resume : Transition metal dichalcogenide (TMD) materials have been widely investigated because of unique electronic and optical properties. Among the monolayer TMD, a number of studies of tungsten disulfide (WS2) have been conducted because monolayer WS2 has a relatively high photoluminescence quantum yield. According to these properties, the high-quality WS2 are required to fabricate the device and an evaluation of sample quality is necessary by using the non-destructive technique like Raman spectroscopy. However, the conventional Raman spectroscopy has a limit to analyze nanoscale defects such as vacancies, substitutions, and grain boundaries which affect the electronic and optical properties of WS2. Therefore, the research using tip-enhanced Raman spectroscopy (TERS) called a nano-Raman technique is absolutely necessary to investigate the defects on nanometer scale for monolayer WS2. In this study, we perform TERS experiments for the monolayer WS2. The high-resolution images of the TERS and scanning tunneling microscopy (STM) show TERS spectra depending on STM topography. We also demonstrate that the red-shifted A1g mode accompanied with the D and D′ modes can be attributed to the defects in monolayer WS2. Furthermore, we identify that the emergence of new Raman modes can be induced by sulfur vacancies through the density functional theory calculations.

Resume : Surface functionalization of phosphorene (2D BP) is a key factor for the development of this 2D semiconductor as an effective platform to be implemented in real-setting applications. Indeed, several chemical strategies can be adopted in order to modulate phosphorene band gap [1], to realize active hybrid heterostructures and to protect 2D BP from oxidation [2]. Herein, we perform a throughout photo- and chemical-physical study in order to get insights on the nature of the chemical interactions between 2D BP and organic luminescent conjugated compounds bearing different functionalities. We selected two pyrene derivatives with boron-functionalities such as the pyren boronic acid (PBA) and a pyren boronic cyclic ether (PBE). 31P deuterated magic-angle spinning NMR spectrum (DE-MAS) and DFT calculations highlighted a non-covalent nature of the interaction, while time-resolved fluorescence spectroscopy revealed that both PBA and PBE are energetically stabilized with respect to unsubstituted pyrene. The results showed up (i) the intimate nature of the interaction between 2D BP and active molecules and (ii) that the energy-stabilized hetero-system, constituted by 2D BP and suitably-functionalized pyrene molecules, is prone to be used as active layer in optoelectronic devices. Acknowledgements This work was supported by an ERC Advanced Grant PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials” (Grant Agreement No. 670173) to M. P. [1] T. Hu et al, ACS Appl. Mat. Int. 2015, 7, 23489−23495 [2] V. Korolkov et al, Nat. Comm. 2017, 8, 1385

Resume : Among the family of bidimensional materials, exfoliated black phosphorus, 2D bP, came out as a hot and intriguing topic over the last few years.[1, 2] The sp3 hybridization and the formal presence of a lone pair on each phosphorus atom offers the potential to get involved in strong interactions with metal fragments. We report here the preparation of a palladium/black phosphorus nanohybrid (Pd/bP) by in situ growth of Pd nanoparticles on bP nanosheets. Extensive characterization of the material highlighted the nature of the Pd-P interaction. An X-ray Absorption Spectroscopy (XAS) study carried out at the Pd-K edge reveals the presence of both Pd-Pd and Pd-P bonds. Considered the high fraction of Pd atoms on the nanoparticles surface this finding evidences the interaction between Pd and bP nanosheets. As a survey of its potential application, the material has been tested as catalyst in the selective hydrogenation of chloronitrobenzene to chloroaniline, which results in a much higher selectivity in comparison to other Pd-based catalysts. Acknowledgements This work was supported by an ERC Advanced Grant PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials” (Grant Agreement No. 670173) to M. P., and project Beyond-Nano (PON a3_00363) CNR-IMM. ___________ [1] Pumera M., Sofer Z., Gusmão R., Angew.Chem. Int. Ed. 2017, 56, 8052–8072. [2] Peruzzini M. et al., Eur. J. Inorg. Chem., DOI: 10.1002/ejic.201801219.

Resume : It is broadly accepted that the inherently low photoluminescence quantum yields (PLQY) in transition metal dichalcogenide (TMD) monolayers are due to chalcogen vacancies (Refaely-Abramson, 2018). The most prolific attempt to passivate these defects thus improving PLQY has been demonstrated by treating mechanically exfoliated monolayers with non-oxidizing organic ?super-acid? bis-trifluoro sulfonic acid (TFSI) yielding spatially and spectrally uniform near-unity PLQY molybdenum disulphide (MoS2) and tungsten disulphide (WS2) monolayers (Amani,2015-2016). The chemical and photophysical reasons for the said improvements remain under debate. This study demonstrates an alternative treatment on WS2 with oleic acid (OA) ligands. Using PL mapping, enhancement statistics are generated to compare OA and TFSI treatments. On average, we find that PL enhancement at low fluence excitation for both treatments are comparable. In both cases, the enhancement is neither spatially or spectrally uniform with notable blue shifts in emission closer to the expected energy of the neutral exciton. A Steady state PL intensity series on OA treated sample reveals the evolution of an additional red-shifting peak, which is attributed to trion formation due to photoionization at high excitation intensity. Time Correlated Single Photon Counting (TCSPC) reveals similar recombination lifetimes between pristine and OA treated WS2, perhaps serving as evidence of defect passivation by ligands. In the TFSI case, excitons are longer lived, indicating the existence of a trap state as identified in other TCSPC studies (Goodman, 2017).

Resume : The discovery of 2D materials based on transition metal dichalcogenides (TMDs), has opened up new interesting possibilities in optoelectronic devices, as monolayer TMDs possess direct bandgaps with absorption in the visible to near-infrared (NIR) spectral region. However, monolayer TMDs often exhibit poor photoluminescence quantum yields (PLQEs) and mobilities, which are signs of a poor-quality semiconductor material. While surface passivation by chemical treatment of TMDs has been explored by a number of groups, thus far only a few methods have been shown to improve a few, but not all of the semiconducting properties. For instance, the use of the ‘super-acid treatment’ with trifluoromethanesulfonimide (TFSI) improves PLQE greatly but gives rise to considerable exciton and charge trapping. At the basis, the chemical mechanisms behind such passivation schemes is unclear, allowing little room for their optimisation and the generation of high-quality materials. Here, I will present a new chemical functionalization approach to greatly enhance the PL intensity of mechanically exfoliated monolayer molybdenum disulphide (MoS2), while simultaneously enhancing the charge and exciton transport properties. In addition, through a range of characterization tools such as transient absorption measurements, X-ray photoelectron spectroscopy, and surface enhanced Raman spectroscopy, an unprecedented detailed understanding of the passivation mechanisms is obtained.

Resume : Nowadays, the graphene research is focused on the possibility to modify the band structure of graphene without a deterioration of the charge carrier mobility and the destruction of its basic electronic properties. Pristine graphene, at the k point of the Brillouin zone, is a gapless semiconductor with almost no density of states at the Fermi level. Due to the vanishing density of state of graphene in this condition, various approaches are being tested to modify the charge carrier concentration such as the direct doping of graphene through the chemical modification, either by the introduction of defects or through molecular adsorption. At the same time, an electronic bandgap in the graphene is mandatory to develop the graphene-based electronic devices. Being simple and scalable, chemical modification is becoming a promising approach to modify the graphene electronic structure. The method of graphene chemical modification includes the covalent attachment of aryl based molecules onto the graphene surface. In our experiments, the graphene layer has been grafted by 3,4,5-trimethoxybenzenediazonium (TMeOD) molecules. In particular we studied the effect of different concentration of grafted molecules on the core level and on the valence band of graphene with photoemission spectroscopy. These results are interpreted on the basis of density functional theory (DFT) calculations carried out on TMeOD alone, as well as on TMeOD covalently bonded to graphene. The photoemission results were also supported with the information given by the Raman spectroscopy and Scanning Tunneling Microscopy Measurements (STM). The talk will be focused on the modification of the electronic properties of graphene induced by the covalent attachment of the diazonium based molecules. The element of novelty is the study of the properties of modified graphene at the k point of the Brillouin zone, never discussed in the literature for this system.

Resume : Although two-dimensional (2D) molybdenum disulfide (MoS2) has gained a great attention due to its unique physical properties, the limited electrical contact to 2D semiconductor still impedes to realize high-performance of 2D MoS2-based electronic devices. In this regard, many studies have been conducted to improve the carrier injection properties by inserting thin tunneling layers, such as hexagonal boron nitride or graphene, between semiconductor channel and electrodes.[1,2] However, the reported strategies require relatively low-yield and time-consuming transfer processes on MoS2 flakes. Here, we suggest a simple contact modification method which introduces chemically adsorbed thiol-molecules as thin tunneling barriers between the metal electrodes and MoS2 channels. The directly deposited thiol-molecules via the vapor-deposition process introduce additional tunneling paths at the contact regions, improving the carrier injection properties with lower activation energies in MoS2 field-effect transistors. Additionally, by inserting thiol-molecules at the only one contact region, asymmetric carrier-injection was feasible depending on the temperature and gate bias.[3] References [1] J. Wang et al., Adv. Mater. 28, 8302 (2016). [2] X. Cui et al., Nano Lett. 17, 4781 (2017). [3] K. Cho et al., Adv. Mater. 30, 1705540 (2018).

Resume : Layered transition metal dichalcogenides are well known for their active catalytic behavior toward hydrogen evolution reaction (HER). Taking molybdenum disulfide (MoS2) as an example, its trigonal prismatic 2H-phase edge sites, octahedral 1T-phase high conductivity, and distorted 1T’-phase active basal planes are all beneficial for HER electrocatalysis [1-3]. A variety of structural engineering approaches has been developed to further optimize the catalytic performance of layered TMD materials. One simple method successfully applied to MoS2 is to reduce the dimension of 3D bulk to 2D nanosheets and down to 0D nanodots, which increases the surface-to-volume ratio and hence exposes more active sites for the catalysis [4]. Even though many research works have proven the promising potential of MoS2 nanomaterials toward HER, one critical issue of these materials is the low stability of the active 1T and 1T’ phases, hindering their practical application in water splitting. In our work, we prepare ReSSe nanodots as an alternative solution for the metastable MoS2 nanomaterials [5]. Our ReSSe nanodots not only stand out with its top HER catalytic activity but also show long-term stability after 20,000 continuous cycles. These results are attributed to the ultrasmall size, the alloying effect and the high density of sulfur vacancies and edge sites of the nanodots. By employing the stable 1T’- phase ReS2 and ReSe2 as well as the ball milling and lithium intercalation techniques to reduce the size of ReSSe bulk crystal to uniform 0D nanodots, we have also realized the influence of distorted anisotropic crystal phase on HER performance. There are two types of anionic vacancies on ReSSe nanodots based on the positions of chalcogen atoms with respect to Re atoms. Low-site sulfur vacancies on ReSSe alloyed nanodots have higher HER activity than all other active sites, which also implies the best catalytic performance of ReSSe nanodots in ReS2xSe2(1-x) family. References 1. Nano Lett., 2013, 13 (12), 6222–6227. 2. Science, 2007, 317 (5834), 100-102. 3. J. Mater. Chem. A, 2014, 2 (48), 20545-20551. 4. Adv. Mater., 2018, 30 (9), 1705509. 5. J. Am. Chem. Soc., 2018, 140 (27), 8563–8568.

Resume : Cancer phototherapy (PT) has attracted extensive interest in recent years due to the advantages over the traditional cancer therapeutic approaches such as surgery, radiotherapy, and chemotherapy, in terms of side-effects and damages to healthy tissues. PT, including photodynamic therapy (PDT) and photothermal therapy (PTT), is a non-invasive and region-specific therapy activated solely by light irradiation, thus offering targeted treatment of cancer with minimized destruction to normal organs. We have been developing an efficient and scalable method to produce 2D nanosheets from the bulk materials by use of exfoliant through wet bath-sonication or dry ball-milling [1,2]. In this context, we found a very efficient method for fabricating graphene-chlorine e6 (G-Ce6) composite with high Ce6 loading ratio through liquid phase exfoliation [3]. The G-Ce6 composite exhibited much higher (7-75 times higher) phototherapeutic efficacy to kill cancer cells than that of other nanomaterial-Ce6 composites. However, the composite has problem in its stability in cell culture medium, though it was stable in phosphate-buffered saline (PBS). In this study, we prepare Ce6 loaded-MoS2 (MoS2-Ce6) in a similar process to that of G-Ce6 [3]. After bath-sonication and centrifugation of the mixture of bulk MoS2 and Ce6 in water, the nanocomposite from the supernatant, composed of a few layer MoS2 and Ce6, exhibited good dispersibility and stability in cell culture medium. The loading capacity of Ce6 in MoS2-Ce6 is 121 wt%, which is lower than G-Ce6 (160 wt%) mainly because of the large difference in their atomic weights of C, Mo and S. Then, we carried out cell experiments with or without light irradiation at 660 nm to evaluate cytotoxicity of MoS2-Ce6. The nanocomposite did not show any cytotoxicity up to 0.20 g mL-1 of Ce6 concentration without light irradiation. Under irradiation of light, however, more than 95% of the cancer cells were killed at Ce6 concentration larger than 0.050 μg mL-1. This efficiency is higher than that of G-Ce6 and other nanocarriers loading photosnesitizer. [1] G. Liu, N. Komatsu* ChemNanoMat, 2 (6), 500 - 503 (2016) [highlighted at the front cover]. [2] G. Liu, N. Komatsu* ChemPhysChem, 17 (11), 1557–1567 (2016) [highlighted at the front cover]. [3] G. Liu, H. Qin, T. Amano, T. Murakami, N. Komatsu,* ACS Appl. Mater. Interfaces, 7 (42), 23402-23406 (2015).

Resume : Heterojunctions based devices utilizing both 2D and 3D materials opens the way to explore new devices. The heterojunction of 2D semiconductors with wide bandgap 3D semiconductors could increase the functionality of wide bandgap semiconductors which are limited due to low mobility. The important consequence of 2D/3D heterojunctions is the enhancement in the photoresponse. GaN is an ideal candidate for UV photodetection whereas MoS2 shows response in visible to near infrared region. Thus, Integration of MoS2 and GaN can lead to broadband photodetection ranging from NIR to UV. Here, we report about the fabrication and electrical characterization of MoS2/GaN heterojunction devices. The MoS2/GaN heterojunction device was fabricated using electron beam lithography in a cleanroom environment. The device exhibited current rectifying behavior. The diode like behavior was attributed to unique type II band alignment of the heterojunction as confirmed by Kelvin Probe Force Microscopy. Further, the photoresponse of the device was investigated with laser illumination and photoresponsivity was calculated to be about 105 A/W. The heterojunction also shows very high detectivity of the order of 1014 Jones. Thus, photoexcitation in the heterojunction of MoS2 with III-nitrides could lead to new optoelectronic devices.

Resume : Two dimensional materials have demonstrated attractive properties for physical and technological applications since the discovery of graphene. In contrast to graphene as zero gap semiconductor, 2D single layer crystalline phase of group V elements with a buckled honeycomb structure, such as arsenene, bismuthene and antimonene, were predicted to exhibit a broad range of band gap and high mobilities in optoelectronic applications. Here we derive the electronic structure of the heterostructure comprised of Bi and Sb bilayer (BL) on the top of Bi2Te3 and Sb2Te3 topological insulators. With scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we confirmed that well-order Bi and Sb BL thin films were successfully prepared on Bi2Te3 and Sb2Te3 substrate respectively. In Bi bilayer/Bi2Te3 system, a large Rashba splitting and charge transfer was observed by band mapping result, but not proposed in previous studies. In Sb bilayer/Sb2Te3 system, the strong hybridization between Sb bilayer and Sb2Te3 cause a newly emerged Dirac cone and is in agreement with the first-principle calculation. Our finding provides a potential system to fabricate future spintronic devices.

Resume : Two-dimensional (2D) layered materials based on transition metal dichalcogenides (TMDCs); MX2 (M = Mo, W; X = S, Se) recently have received much attention because of highly important potential applications in flexible nanoelectronics, sensors, and photodetectors as a substitute for silicon or organic semiconductors. A few works reported the dielectric function (ε) of monolayer MoS2 at low and room temperatures by using spectroscopic ellipsometry (SE) [1,2]. To apply properly for device applications, the dielectric function of 2D-MoS2 had better be well known at arbitrary temperature. We report the dielectric function parametric model of ε of MoS2 from 1.4 to 6.0 eV and for temperatures from 35 to 350 K measured by SE. The existence of ten critical point (A-, A0, B, E0, C, and EI-V) structures could be observed clearly as shown in figure 1. The parameters were extracted by fitting the spectra with the reconstruction from ten dispersive oscillators at each measured data. We observed the fundamental absorption peak E0, which occurs at the K-point of the Brillouin zone. The dependence of temperature is achieved from the model parameters that are fitted by polynomial and then the dielectric functions of MoS2 for arbitrary temperature is determined. These results are expected to be useful in design and understanding in applied device technologies based on MoS2.

Resume : New generation of high performance technologies for energy and environment requires the design and development of new materials with improved properties. Materials with nano-hybrid structure show promising properties by synergically combining the characteristics of each single component into a new system. In this work all-inorganic perovskite, with formula CsPbBr3 and black phosphorus (BP) nanosheets were combined in an innovative composite tested for photocatalytic applications. The CsPbBr3 nanocrystals were prepared by precipitation method of Cs-oleate and Pb-oleate with the injection of a Br-precursor obtained from tetrabutylammonium bromide. The perovskites were analyzed morphologically with SEM and XRD, while the UV-Vis absorbance analysis helped to study the photocatalytic activity. The synthetic procedure optimized for CsPbBr3 alone was repeated adding BP nanosheets into the reaction environment. The new composite was characterized with SEM, TEM, XRD and UV-Vis. Photocatalytic degradation tests of two different organic pollutant dyes, rhodamine B and methyl orange, were performed for perovskites and the hybrid nanocomposite. The latter shows a better photocatalytic activity in the degradation thanks to the synergic effect due to the catalytic properties of CsPbBr3 and the light absorption capability of the perovskite and BP. To the author knowledge, this is the first work that reports the use of the CsPbBr3-BP composite for photocatalytic applications. Acknowledgements This work was supported by an ERC Advanced Grant PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials” (Grant Agreement No. 670173) to M. P.

Resume : Phosphorene, a single layer of black phosphorus, is attracting interest in the scientific community for its semiconducting properties. However, phosphorene also possesses a great potential for tribological applications due to its layered structure and for the capability of P-based lubricant additives to reduce friction and adhesion in steel-steel contacts. In spite of these promising characteristics, the investigations of the lubricating properties of phosphorene are still at their infancies. Here we present a comprehensive analysis of the tribological properties of phosphorene based on first principles calculations. Firstly, we estimate the interlayer work of separation and shear strength for the bilayer system. A structural superlubricity is identified for a perpendicular orientation of the layers. Then, we evaluate the lubricating effects of phosphorene intercalated at the iron-iron interface, comparing the results with graphene and MoS2, which are widely used as lubricants. Our results indicate that a consistent reduction of the metal adhesive and shear strengths can be obtained with just one phosphorene layer embedded at the interface. The atomic planes, constituting the layer, separate and fully passivate the mated metal surfaces. This intriguing behaviour produces lubricant effects attainable only with bilayer films of the other 2D materials.[1,2] [1] G. Losi, P. Restuccia, and M. C. Righi, to be published (2019) [2] P. Restuccia and M. C. Righi, Carbon 106, 118 (2016)

Resume : Transition metal dichalcogenides (TMDs) is a promising material as a channel of thin film transistor (TFT) due to its applicability for transparent and flexible electronics. In order to maximize the advantage of being able to control the thickness and to make hetero-structures of various stacks, we introduce a simple circuit transfer method to transfer multiple TMDs field-effect transistors (FETs) onto flexible and transparent substrates such as PET. We compared the characteristics of the MoS2 and WSe2 FETs transferred and the characteristics of the same structures of devices transferred after channel passivation with Al2O3 layer, on a flexible substrate. The Al2O3 passivation showed that the process of transferring the entire device with the electrodes was further stabilized. Based on this result, the Complementary metal-oxide-semiconductor (CMOS) inverter circuit implemented with MoS2 and WSe2 materials could be successfully transferred to the PET substrate after channel passivation. This implies that the TMDs circuit can be easily implemented on various polymer substrates that are vulnerable to semiconductor processing, and also shows the possibility of various derivative applications such as NO2 gas voltage inverter sensor.

Resume : Van der Waals (VDW) heterostructures such as MoS2-graphene, are emergent and promising materials for nanoelectronics and nanophotonics. One of the main parameters for heterostructures is the valence band offset (VBO), which in case is defined as the difference between the graphene Fermi level and the MoS2 valence band maxima. The processing induced VBO evolution is of great importance for the proper VDW devices operation. In this work the method of X-ray photoelectrom spectroscopy (XPS) was used to evaluate effect of thermal annealing on the MoS2-graphene VBO. MoS2 films were CVD grown and transferred by PMMA onto the graphen/SiO2/Si substrates. The thicknesses of graphene and MoS2 were investigated by Raman spectroscopy and were found out to be a single and a few-layers, respectively. The obtained MoS2-graphene heterostructures were annealed in a nitrogen/ ultra-high vacuum at 200-300°C. The annealing temperature increase led to a decrease of the VBO from 1.6 eV to 1.3 eV. This was accompanied by a shift of the XPS Mo3d5/2 and S2p3/2 peaks on 0.3 eV, too. Moreover, accoding to Raman spectroscopy (RS), the G-peak of graphene shifted from 1600 cm-1 to 1595 cm-1, and the 2D peak from 2652 cm-1 to 2664 cm-1. The observed change in XPS and RS data with the annealing temperature increase is evaluated both in terms of the increased VDW interaction between the graphene and MoS2 and in the charge transfer between the layers induced by a removal of the adsorbed contaminant molecules from the MoS2 surface and from its interface with graphene.

Resume : In this study, we have integrated a molybdenum ditelluride (MoTe2) field effect transistors (FETs) wih a perovskite photovoltaic solar cell for self-powered photosensing circuit. Among many transition metal dichalcogenide (TMD) semiconductors, MoTe2 is selected because of stable electrical properties compared to the other representative 2D TMD semiconductors: MoS2, MoSe2, WSe2, and etc. The few nm-thin n-type MoTe2 FET operating at a low voltage is fabricated on a glass substrate as back-to-back bonded to the other glass which has perovskite PV cell. As the PV cell has large open circuit voltage (VOC) over 0.9 V under artificial sun (AM 1.5) or visible photon illumination, we simply have used a red light emitting diode (LED) to generate ~1 V of photovoltage. Surprisingly, two operation modes in circuit are possibly observed depending on the light intensity of red LED: self-power current/voltage source and photosensor mode. Interestingly, despite of small output current and voltages in the circuit, 940 nm near infrared (NIR) is also sensitively detected. As a result, we highly regard that our self-power circuit approaches combining PSC diode and 2D thin TMD FET would be worth much attention toward future ubiquitous electronics and energy harvest.

Resume : Nanoelectromechanical (NEM) systems have attracted a lot of attention since electrical and mechanical degrees of freedom in NEM devices provide great potential for utilizing the devices as sensors and actuators in electronic, mechanical, and photonic applications. Graphene is one of the best candidate materials for the NEM device thanks to its superior mechanical properties as well as high electrical conductivity. In this presentation, graphene NEM resonator based ultra-sensitive mass detection will be demonstrated. The amount of mass loaded on the graphene NEM resonator can be measured by resonant frequency shifts. The sensitivity of frequency change detection by mass loading could be improved by improving the quality of device structure and effective mass of graphene. After Joule heating process by applying high current through suspended graphene, the residual materials on the graphene surface could be evaporated so that a very clean graphene ribbon was obtained. Our studies on the investigation of mechanical properties of suspended graphene based nanomechanical resonators gave us an inspiration to apply graphene based nanomechanical device to ultra-sensitive mass detection based sensors. A possibility to detect a specific gas or single bio molecule using our device is being investigated.

Resume : Monolayer transition metal dichalcogenides (TMDs) have attracted great interest due to their physical properties including large spin−orbit coupling, strong quantum confinement, and the indirect-to-direct bandgap transition for monolayer. Understanding the stability of monolayer transition metal dichalcogenides at high temperatures has important consequences for their handling, life-span, and utilization in high temperature applications. We demonstrate the high temperature-induced instability of WS2 monolayer, grown by chemical vapor deposition, by means of optical and morphological analyses. Raman and photoluminescence measurements assess that long-term annealing at 300°C in vacum causes a degradation of the light emission properties, while retaining the crystalline quality, indicating an important generation of non-radiative centers, probably due to the formation of point defects. Annealing of WS2 monolayers at 600°C, even for short times, is instead observed to cause massive degradation of the crystalline quality, as demonstrated by Raman analyses. In conclusion, this study demonstrates that monolayer TMDs are very sensitive to high temperatures and therefore not suitable for any application in such enviroments.

Resume : Molybdenum disulfide (MoS2) has received extraordinary interest, due to its optical, electronic and mechanical properties. In this work we report on the synthesis of hybrid, organic–inorganic light-emitting nanofibers doped with MoS2 nanoparticle obtained through a sonication process in N-methyl-2-pyrrolidone. The gentle fragmentation method, used to produce the MoS2 nanoparticles leads to low defectiveness and preserves the stoichiometry of the two-dimensional material, demonstrated by Raman analysis. A careful morphological study reveals that the nanoparticle size ranges from 20 to 80 nm. The size-related quantum confinement is also confirmed by photoluminescent analysis, evidencing an intense emission at 550 nm. In addition, they show significant waveguiding of self-emitted photons along their longitudinal axes, thus working as miniaturized optical fibers. These findings suggest light-emitting, MoS2-doped fibers obtained by gentle fragmentation as highly promising optical materials for sensing and photonics. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 306357 (ERC Starting Grant “NANO-JETS”).

Resume : In recent years researches have devoted much attention to study of intravalley double electron-phonon processes in single- and bilayer graphemes, which determined main strips of Raman spectra. We experimentally investigated dispersion (dependence of phonon energy from quantum energies of exciting radiation) of 2D' band in micro-Raman spectra of light scattering in single- and bilayer graphemes. D' band, as a D band are defect-activated. Its frequencies depend on wavelengthes of exciting radiation. But unlike the D band, for which intravalley electron scattering mechanism is implemented, D' band associated with intravalley electron scattering on defects. The 2D' strip is it’s overtone. Established non-monotonic, but the same for both structures dispersion nature of 2D' bands, which allowed by selection rules, and defined their half-widths are equal 10cm-1. It is shown that “sharpness” of the intravalley double electron-phonon resonance processes in graphemes can be used to experimental determination of localization state density maxima (or phonon energy maxima) on i-LO phonon branches in single- and bilayer graphemes.

Resume : We report a systematic data analysis and modelling of experimental data of the resistance of Reduced Graphene Oxide single sheet and thin films of different thicknesses, sp2 fraction [1]. Since the nature of charge transport in single sheet is established [2], the thin film are still object of debate: Efros-Shklovskii or 2D Mott variable-range hopping (ES-VRH / 2D-VRH). Such films are described as multi-layered networks where hopping events affect the in-plane charge transport whereas out-of-plane transport is quasi-metallic. Based on analysis of the reduced activation energy W(T)[4], we show that the charge transport mechanisms is the Efros-Shklovskii Variable Range Hopping, in 2-300K temperature range [5]. The probability that charges can circumvent the hopping barriers increases with film thickness, with a corresponding increase in the effective delocalization of the electronic states up to the µm-scale [6]. [1] Joung, PRB, 86, 235423 (2012) [2] Vianelli, Carbon, 89, 188-196 (2015) [3] Kovtun, Carbon, 143, 268-275 (2019) [4] Zabrodskii, Zh. Eksp. Teor. Fiz, 742, 425-33 (1984) [5] Kovtun, submitted paper [6] Kovtun, PhD thesis, università di Modena e Reggio Emilia, Italy, 2017, https://morethesis.unimore.it/theses/available/etd-02032017-103522/

Resume : Changes in electrical properties of graphene devices induced by energetic ion irradiation are very important for their application in harsh radiation environment. We investigate the modulating behavior of electrical properties of graphene-based device irradiated by swift heavy ions (SHIs). Graphene field effect transistors (GFETs) were irradiated by 1.79 GeV Ta ions and it was found that at lower fluence (10^9 ions/cm^2~10^10 ions/cm^2), the SHIs irradiation can effectively optimize the performance of GFET while at higher fluence, the electrical properties of the devices were significantly deteriorated after the irradiation process. The length and width ratio of the graphene ribbon, electronic energy loss (dE/dx)e of the SHIs and the irradiation fluence are the three factors that determine the improvement in performance of the GFET. In this work, Raman spectroscopy was employed to figure out the correlation between the changes in electrical performance of GFET and initial defect density in graphene. The competition among various factors such as the doping, local annealing and defect creation dominates the GFET performance. Moreover, not only the defects and doping in graphene make significant contribution to the deterioration of electrical properties of GFET but also the defects in substrate and doping at SiO2/Si interface can contribute substantially to these changes. The resistance, carrier mobility and position of Dirac point can be modified by the SHIs irradiation, which paves a way for the optimization of GFETs. This work provides an important reference for the graphene based irradiated devices in aerospace electronics as well.

Resume : As a rapidly emerging group of materials, transition metal dichalcogenides (TMDs) are formed by stacked monolayers with van der Waals (vdW) interaction and have indirect bandgap. However, the properties such as bandgap of ultrathin 2D-TMDs change significantly, leading them to have wider applications including optical electronics. Among 36 combinations of TMDs, MoS2 gets the most concern due to relatively high stability in low dimension. Although MoS2 has been explored for transistors and energy storage devices with excellent performance, there are limited studies on its electrical transfer characteristics which likely have a better understanding of MoS2. Moreover, in situ transmission electron microscopy (TEM)-scanning tunneling microscopy (STM) techniques benefit to examine structural dynamics of various nanomaterials, the measurements of electrical properties of nanowires/nanotubes, the manipulation, and micro-control of the nanotubes or nanowires. In this work, Nanofactory in situ TEM-STM holder was applied to stimulate the 2D-MoS2 by mechanical force or bias during TEM observation to study the relationship between structure and properties, including stress- and thickness-dependent conductivity, as well as the conductivity difference between edge contact and face contact. Taking advantage of higher accuracy in probe-specimen contact, a high-resolution controller was also used for the investigations. This is a powerful method to monitor the changes in the nano-scale specimens. It is found that edge contact and thinner specimen have a better conductivity than face contact and relative thicker specimen, respectively. The results may help to develop next generation instruments based on better understanding of MoS2.

Resume : We show that a stacking defect or a shift has a striking effect on carrier transport of bilayer graphene. Charge transport through a ballistic n–neutral–n junction of shifted bilayer graphene may result in minimal conductivity and shot noise anomalies which are found to be sensitive to the shift defect. Minimum conductivity and shot noise in shifted bilayer graphene exhibit an anisotropic beahavior as a function of the orientation of the electrodes. The minimum conductivity could be suppressed for somme specific value of twist defect while the shot noise takes the unit value. Our results provide a way to control transport properties in an undoped graphene bilayer structure by adjusting the layer stacking.

Resume : Since discovery of direct band gap and broad absorption range from UV to IR regions at monolayer, transition metal dichalcogenides (TMDCs) such as MoS2, WS2 and WSe2 are regarded as promising materials in nano-scale optoelectronic devices. However, 1L-MoS2 is difficult to applicate for practical optoelectronic devices such as photodetector, solar cell and light emitting diode due to an extremely low optical quantum yield (QY). To enhance the optical QY of 1L-MoS2, the prospective method with the surface plasmon (SP) effect have been used by metal nanostructure. In this investigation, a high-gain 1L-MoS2 photodetector was successfully realized, based on the SP effect of the polyvinylpyrrolidone (PVP) encapsulated Ag NW. The role of PVP is to prevent the charge transfer effect. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS2 and the Ag NW network, it was determined that a strong SP effect influenced a greatly enhanced optical QY. The PL emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS2. Consequently, the overall photocurrent of the hybrid 1L-MoS2 photodetector was observed to be 250 times better than that of the pristine 1L-MoS2 photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ~ 1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs.

Resume : Recently, the two dimensional (2D) transition metal dichalcogenides (TMDs) materials attract a lager research interests. TMDs materials are developed and used for different devices such as tunnel field effect transistor, photodetector and solar cell owing to their favourable electronic and optoelectronic properties. Additionally, TMDs materials are also used as saturable absorbers in the laser cavity for laser pulses generation. The saturable absorber (SA) is a critical component used within the passively Q-switched or mode locked laser systems due to their strong optical saturable absorption property. The Q-switching and mode locking laser pulses have a high peak power and short pulse duration for a wide range of practical applications including laser marking, optical communication, sensing, medical treatment and scientific research. It has been demonstrated that the group 6 TMDs materials e.g. WS2, WSe2, MoS2 and MoSe2 were used for laser cavity for generating stable Q-switched laser or mode-locked laser pulses [1,2]. In this project, the nonlinear optical properties of platinum dichalcogenides (PtX2) has been studied (X can be S, Se, or Te). PtX2 is a member of metallic noble transition metal dichalcogenides (MNTMDs) layered materials, which provide a broadly tunable energy bandgap, nonlinear optical properties and excellent stability at the room temperature and atmospheric pressure. Previously, the saturable absorption property of platinum disulfide (PtS2) was investigated by our research group [3, 4]. It was used as saturable absorber within a Q-switched or mode-locked Er doped fiber laser cavity. The tunable repetition rate and pulse duration of 18.1 kHz ~ 24.6 kHz and 9.6 µs ~ 4.2 µs are achieved within the Q-switched laser system [3]. And the ultrashort laser pulse duration achieved from the mode locking laser system is about 2.1 ps [4]. Moreover, we have successfully fabricated the platinum diselenide (PtSe2) saturable absorber by selenization method with magnetron sputtered platinum film [5]. Then, these fabricated PtSe2-SA are inserted within a Nd:LuVO4 solid state laser for producing ultrafast laser. We have also first demonstrate successful Q-switching pulses achieved by using Platinum ditelluride (PtTe2), a new member of the PtX2 family recently. The PtTe2 offers some new features compared with other Pt based 2D materials due to its smaller bandgap and larger tunable range in the infrared range. This works will open up new opportunity for the group 10 TMDs 2D materials for the pulse laser applications. Acknowledgement: The project is supported by The Research Grants Council of Hong Kong, China (GRF 152109/16E PolyU B-Q52T) and The Hong Kong Polytechnic University (G-YBVG). Reference list: 1. B. Chen, X. Zhang, K. Wu, H. Wang, J. Wang, and J. Chen, “Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2,” Opt. Express 23(20), 26723-26737 (2015). 2. H. Xia, H. P. Li, C. Y. Lan, X. X. Zhang, S. J. Zhang, and Y. Liu, “Ultrafast erbium-doped fibre laser mode-locked by a CVD-grown molybdenum disulfide (MoS2) saturable absorber,” Opt. Express 22, 17341–17348 (2014). 3. X. Wang, P. K. Cheng, C. Y. Tang, H. Long, H. Yuan, L. Zeng, S. Ma, W. Qarony, and Y. H. Tsang, “Laser Q-switching with PtS2 microflakes saturable absorber,” Opt. Express 26(10), 13055-13060 (2018). 4. H. Long, C. Y. Tang, P. K. Cheng, X. Wang, W. Qarony, and Y. H. Tsang, “Ultrafast laser pulses generation by using 2D layered PtS2 as a saturable absorber,” IEEE J. of Light. Tech. DOI: 10.1109/JLT.2018.2889289 (2018). 5. L. Tao, X. Huang, J. He, Y. Lou, L. Zeng, Y. Li, H. Long, J. Li, L. Zhang, and Y. H. Tsang, “Vertically standing PtSe2 film: a saturable absorber for a passively mode-locked Nd:LuVO4 laser,” Photon. Res. 6(7), 750-755 (2018).

Resume : Transition metal dichalcogenides, such as MoS2, have recently gathered a lot of interest as promising candidates for post-Si CMOS devices. However, several challenges need to be faced to exploit their potentialities in real applications. Although many progresses have been done in the vapor phase growth of MoS2, uniformity on large area still needs to be improved. Furthermore, several processing and materials integration issues (contacts, dielectrics, mobility engineering) need to be solved to achieve optimal MoS2 transistors performances. In particular, current injection mechanisms from metals to MoS2 are still object of intense debate, due to their implications in the final device performances. In this work, we report a conductive atomic force microscopy (CAFM) investigation of the current injection to MoS2 films grown by CVD onto a SiO2/Si substrates. MoO3 and S powders were used as Mo and chalcogen precursors. An organic promoter (PTCDA) was used to enhance the MoS2 flakes nucleation.[1] After preliminary characterization by optical microscopy, Raman, photoluminescence, and atomic force microscopy, we performed nanoscale current mapping and local I-V measurements by CAFM on selected monolayer and few layer MoS2 triangular flakes. The distribution of the Schottky barrier height values in MoS2 regions with different thickness, and in different areas of the flakes (center/border) was evaluated. These results will be compared with those recently obtained by CAFM on exfoliated MoS2 samples [1,2], to elucidate the peculiar aspects of current injection mechanisms in the case of CVD grown MoS2. [1] Y. Okuno et al, Nanoscale 10, 14055-14059 (2018) [2] F. Giannazzo et al, Phys. Rev. B 92, 081307(R) (2015). [3] F. Giannazzo et al, ACS Appl. Mat. & Int. 9, 7761-7771 (2017).

Resume : Gallium selenide (GaSe) from a class of 2D layered semiconductors is a promising material for nonlinear optical applications in the mid-infrared wavelength region in the quality of detectors and sources of THz waves; the basis of various optoelectronic devices for visible wavelength region; a matrix for hydrogen storage; substrates in planar nanotechnologies, etc. In this report we demonstrate that GaSe interacts with water under day light illumination. Interaction is initiated on atomically smooth van der Waals (0001) surface of GaSe formed by close-packed chemically saturated Se atoms and accompanied by the formation of highly toxic strong oxidants - selenic acids H2Se, H2SeO3 and H2SeO4 in its turn resulting in revealing of non-basal dislocations etch pits, and than to etching of samples’ side edges obtained by mechanical destruction of each layer and their expansion prior to their full dissolution. Here we discuss reasons of light-induced interaction of GaSe with H2O by way of model based on peculiarities of crystallographic structure, and changes of GaSe properties under illumination (essential increasing of static dielectric constant, i.e., photo dielectric effect). Because cancer cells and tissues mainly contain excess water in comparison to normal ones, the obtained results allow to predict the perceptivity of GaSe nanoparticles usage for cancer therapy. The closest analogue of such application is photodynamic therapy of cancer by porphyrins or dyes.

Resume : Recently, graphene is the subject of considerable attention in technological applications become increasingly attractive in electrochemical and biomedical fields because of their remarkable electronic and catalytic properties, such as its good detection capability and excellent mechanical, thermal and electrical properties. In addition, graphene is a large area, high conductivity, residual oxygen functionality and accessible sites for catalysis, biocompatibility and good stability. Therefore, graphene-based nanomaterials have recently gained great popularity in biological sensing applications. In our work, the graphene-based biological biosensor is processed for the precise detection of biomolecules, in particular glucose, by the presence of efficient immobilization of glucose oxidase enzymes. Graphene-based biosensors have performed remarkably well with high sensitivities, large wide linear detection areas, low detection limits, and long-term stability.

Resume : Interest in bringing p- and n-type monolayer semiconducting transition metal dichalcogenides (TMD) into contact to form rectifying pn diode has thrived since it is crucial to control the electrical properties in two-dimensional (2D) electronic and optoelectronic devices. Usually this involves vertically stacking different TMDs with pn heterojunction or, laterally manipulating carrier density by gate biasing. Here, by utilizing a locally reversed ferroelectric polarization, we laterally manipulate the carrier density and created a WSe2 pn homojunction on the supporting ferroelectric BiFeO3 substrate. This non-volatile WSe2 pn homojunction is demonstrated with optical and scanning probe methods and scanning photoelectron micro-spectroscopy. A homo-interface is a direct manifestation of our WSe2 pn diode, which can be quantitatively understood as a clear rectifying behavior. The non-volatile confinement of carriers and associated gate-free pn homojunction can be an addition to the 2D electron–photon toolbox and pave the way to develop laterally 2D electronics and photonics.

Resume : Monolayer transition metal dichalcogenides have emerged as a new class of direct-bandgap semiconductors, but consensus over the bandgap nature of tungsten diselenide (WSe2) has not been reached yet. Here we identify indirect-bandgap excitons in large-area monolayer WSe2 both in the absorption spectrum and in the emission with µs-long lifetime. We provide direct experimental evidence that direct and indirect excitons coexist and give rise to distinct emission peaks. Near absolute zero temperature, the indirect excitonic emission peak is almost two orders of magnitude higher than the direct one, thus proving that monolayer WSe2 is an indirect-bandgap semiconductor. At room temperature, fast non-radiative channels outpace the emission from indirect-bandgap excitons, and only direct-bandgap transitions are detectable. We conclude that it is not correct to infer the bandgap nature from the emission spectra at room temperature without time decays. The richness of excitonic states in WSe2, featuring fast and slow-lived states as well as spin dependence, will enable novel photon-exciton interaction schemes with possible applications in quantum information processing, telecommunication and energy.

Resume : Monolayer transition metal dichalcogenides have wide application areas such as electronics, optoelectronics, biosensing and catalysis. Among them, monolayer MoSe2 based optoelectronic gain an increasing interest because of its room temperature high carrier mobility, high current on/off ratio and adjustable carrier type achieved by nonmetal doping. In this study, the transport properties of as-grown, monolayer MoSe2 that has P-type carrier doping and its heterosturctures with other TMDs have been investigated and optimized to enhance the electronic performance of the fabricated devices in terms of carrier mobility, current on/off ratio and hysteresis range, by optimization of the fabrication process and device structure. Monolayer TMDs have been grown by chemical vapor deposition. The transistors have been fabricated by using electron beam lithography. Devices were annealed in the N2 atmosphere for 2 hours. The transfer characteristics of the MoSe2 based devices after annealing exhibit a p-type FET characteristic with a maximum drain source current of 320 nA , 10^5 current on/off ratio, 2.62 cm2V-1s-1 carrier mobility and 50 V hysteresis range without encapsulation and doping. The measured results prove the tunability of the electronic structure of MoSe2 with device performance being comparable with previous studies in the literature. We have shown that the post annealing process has crucial importance on device performance for the future electronic applications of MoSe2 in.

Resume : Molybdenum disulfide (MoS2), a two-dimensional (2D) transition metal dichalcogenide, has attracted remarkable attention as a promising semiconductor of next-generation nanoelectronics. To utilize MoS2 in large-area integrated applications, it is essential to grow uniform large-area MoS2 using techniques such as the chemical vapor deposition (CVD) synthesis method. However, the electrical performance of MoS2 films is limited by structural defects created during synthesis process, such as grain boundaries. In this regard, the effect of grain boundaries on the electrical performance and noise characteristics in CVD-grown MoS2 require a thorough understanding to increase their potential for future applications. In this presentation, we will report the electrical and noise characteristics of MoS2 field effect transistors (FETs) fabricated with CVD-grown MoS2. The electrical transport in CVD-grown MoS2 FETs were hampered by the grain boundary, which can be considered as the dominant noise source. The dependence of the Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

Resume : Two-dimensional (2D) nanostructures in the honeycomb lattice are currently materials of interest due to their unique electronic properties. Their large surface-to-volume ratio makes them ideal for chemical detectors. In the present work, the structural and electronic properties of mono- and bilayers of germanene are investigated. Research on the adsorption of some gas molecules (O_2, N_2, CO, NO, NO_2, SO_2) [1] on the surface of pristine germanene and transition metal (TM)-decorated (Cu, Ag and Au) germanene is presented. Geometries and partial density of states are calculated through density-functional-theory (DFT) calculations. The most stable adsorption geometries, the adsorption energies and the charge transfers of the gas molecules on a germanene 4 × 4 supercell are presented. The most stable site for TM atoms is on the sites over the center of the hexagons of the honeycomb lattice. The germanene without TM atom presented adsorption energies below 1 eV, however our calculations found that embedded TM elements in general can significantly enhance the interactions between gas molecules and germanene. Silver (Ag) may be the best option among all the metallic elements used. A detailed knowledge of the geometry and electronic properties of these systems are required for the design and further optimization for future applications of germanene-based gas sensing, catalysis and microelectronics. [1] F. de Santiago, et.al., Applied Surface Science 475 (2019) 278-284.

Resume : Cadmium telluride is the leading technology for thin-film solar cells. As deposited, CdTe films contain a very high concentration of stacking faults and exhibit poor photovoltaic efficiency. An extra annealing treatment with CdCl2 is required to produce high efficiency cells. Although the precise mechanism for this performance increase is unknown, the concentration of stacking faults is greatly reduced in treated cells. This work uses high accuracy density functional theory calculations to investigate the role of stacking faults in CdTe. All experimentally observed faults are found to be very low energy, correlating with their high concentrations. No fault clustering mechanism is found, suggesting a cause for the large observed quantities of small lone faults despite larger twin structures being more stable. While polytype faults are shown to significantly reduce CdTe’s bandgap they are found to have high defect formation energies and are not observed experimentally, restructuring to bulk-like CdTe when multiple such faults are simulated. However, increased lone tetrahedral fault density may produce shallow carrier trap states, thereby reducing cell performance. The removal of these defects may therefore be a leading cause of CdTe's photovoltaic efficiency improvement through CdCl2 treatment.

Resume : The isolation of graphene has led to a new field of research on two-dimensional (2D) materials which exhibit interesting electrical, optical and opto-electronic properties. In this study, 2D materials fabricated by chemical vapor deposition have been investigated. Raman spectroscopy has been employed to obtain the magnitude of difference between E2g (~385 & 358 cm-1) and A1g (~404 & 419 cm-1) peaks of MoS2 and WS2 which has been used as a signature of the number of layers. In order to characterize the dependence of the electrical properties of these materials on the number of layers, two types of electrode patterns were fabricated. In the first case, interdigitated Ti/Au (4 nm / 110 nm) electrodes were deposited onto a single MoS2 flake. In the second case, MoS2 flakes were deposited onto a pre-deposited Au interdigitated electrodes and electrical and opto-electronic properties were studied. Drain and transfer characteristics were measured as a function of applied drain-source, and gate-source bias. For the MoS2 device, a current of about 7.8 mA at a gate voltage of 1 V was measured. The dependence of the optoelectronic properties, photoresponse measurements (I-t characteristics) as a function of varying gate voltages demonstrated that the photo-response and mobility of MoS2 based devices in the visible and IR part of the spectrum can be varied by controlling the number of MoS2 layers.

Resume : Exfoliated black phosphorus (bP) has found a prominent place in the family of van der Waals materials, thanks to its direct band-gap, tunable with layer number, and to its strong in-plane anisotropy. Another peculiarity of bP is its high reactivity, especially with oxygen and moisture, which results in water-soluble phosphorus oxiacids. This represents a major drawback for surface science studies of this material. Here, we investigate surface modifications on exfoliated bP flakes upon subsequent annealing steps, focusing on the temperature range around 375 °C, when sublimation starts and a controlled desorption from the surface occurs with the formation of well-aligned craters. There is an open debate in the literature about the crystallographic orientation of these craters, i.e. whether they align along the zigzag1 or armchair2 direction. Thanks to the atomic resolution provided by STM, we can directly identify the crystallographic orientation of the craters with respect to the crystal, solving the controversy3: the long axis of the craters is aligned along the zigzag direction. The authors thank the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 670173) for funding the project PHOSFUN by an ERC Advanced Grant to MP. References: [1] M. Fortin-Deschenes et al., J. Phys. Chem. Lett., 7, 1667 (2016) [2] X. Liu et al., J. Phys. Chem. Lett., 6, 773 (2015) [3] A. Kumar et al., 2D Materials 6, 015005, (2019)

Resume : Two-dimensional materials with tunable band gap in the range of 0.3–1.5 eV are highly desirable for electronic and optoelectronic applications. Palladium diselenide (PdSe2), a less explored group-10 transition metal dichalcogenide, is one such material of particular interest that demonstrates a gradual transition from a semiconductor (monolayer) to semimetal (bulk) [1, 2]. In this work, we report the transport and optoelectronic properties of p-type PdSe2 field effect transistors (FETs) with laterally spaced graphene electrodes. The fabricated devices can be understood as a pair of back-to-back Schottky diodes created at the graphene/PdSe2 interface and a series resistor presented by the PdSe2 channel. The devices demonstrate hole dominated transport with gate tunable Schottky barrier height. Next, the current induced PdSe2 channel decomposition has been studied under various bias conditions. Similar material transitions have been further observed under 532 nm laser irradiation, where the structural transitions have been simultaneously controlled with Raman spectroscopy. References: [1] L. Zeng et al. Adv. Funct. Mater. 1806878, 1–9 (2018). [2] A. D. Oyedele J. Am. Chem. Soc. 139, 14090–14097 (2017)

Resume : Two-dimensional (2D) layered materials include transition metal dichalcogenides. The structure of these materials is similar to that of graphite and characterized by strong in-plane chemical bonds and weak coupling between layers. When the thickness of layered material is reduced to single or only few atomic layers, their electronic properties are strongly affected. Our previous MD-EXAFS studies of different materials such as Cu3N, ZnO, UO2 and ScF3 suggest that the use of molecular dynamics simulations provides reliable and detailed information on the thermal disorder as far as up to 6-7 Å from the absorber. In the framework of this project, we plan to apply this methodology to layered materials for the first time. In this study, we performed ab initio molecular dynamics (AIMD) simulations of WS2 compound in the temperature range from 300 K to 900 K in order to elucidate the details of thermal disorder and structural anisotropy on the lattice dynamics of layered WS2 structures. In addition, we validated the obtained AIMD results by a direct comparison with the W L3-edge absorption spectra using the MD-EXAFS approach developed in [1] and based on the analysis of EXAFS data using molecular dynamics simulations. Financial support provided by project No. 1.1.1.2/VIAA/l/16/147 (1.1.1.2/16/I/001) under the activity ”Post-doctoral research aid” realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. [1] A. Kuzmin and R. A. Evarestov, Quantum mechanics-molecular dynamics approach to the interpretation of X-ray absorption spectra, J. Phys.: Condens. Matter 21 (2009) 055401.

Resume : Signatures of conductance quantization have been observed in suspended graphene, however with limited control on constriction width [1]. In graphene nanoconstrictions sandwiched by hexagonal boron nitride evenly spaced modulations have been found and interpreted as size quantization signatures, away from the Dirac point, superimposed on the linear conductance [2]. However, mainly due to the charging of localized edge states from the rough edges the precise observation of charge quantization in graphene at the zero external magnetic field is still lacking. Instead of reactive ion etching, using atomic force microscope-based technique to pattern narrow constrictions can lead to unique insight of charge quantization. Here we report on the observation of quantum confinement through graphene constrictions prepared by AFM-based lithography. Constrictions with smooth edges have been formed by mechanical scratchings through specific directions aligned to the atomic orientations of the graphene. The conductance through the constrictions have been measured as a function of back gate voltage applied on the Si substrate to tune the carrier density in the graphene layer at temperatures near 1.5 K. At zero external magnetic field even-spaced plateau-like features have been identified, roughly spaced by 2 times e^2/h. References [1] N. Tombros et al., Nat. Phy. 7, 697-700 (2011). [2] B. Terrés et al., Nat. Comm. 7, 1-7 (2016).

Resume : Chlorine-doped tungsten disulfide monolayer with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating a reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated. This indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. This electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance with theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in the air. Scanning Auger Microscopy and X-ray photoelectron spectroscopy are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. That photo-assisted chemical doping provides a useful and fast tool for bidirectional control of the Fermi level in 1L-WS2 and can be extended to other TMD monolayers.

Resume : Various quasiparticles like excitons, biexcitons are able to co-exist in the exfoliated two-dimensional (2D) stratum of transition metal dichalcogenides (TMDs), which have gained massive attention due to their layer dependent band gap modulations and diverse electro-optical properties. The knowledge about the properties and dynamics of these metastable carriers turn out to be essential for nanofabrication of various optoelectronic components. Therefore, an advanced time-resolved pump-probe spectroscopy has been utilized to investigate these ultrafast processes, as it is beyond the limit of conventional optical studies. In our case, we have explained the transient absorption spectra (TAS) of chemically exfoliated mono-to-few layers WS2 dispersion at 300 K temperature. This shows six major features including three saturation absorptions (SA) peaks and three excited state absorption (ESA) peaks for a pump of 405 nm (3 mW). The density functional theory (DFT) calculations predict that the nature of the band structure remains unaffected from mono to few-layers transformation, which has also been independently verified by our TAS analysis. We are able to identify the existence of both the biexcitons in the multi-layered stratum of WS2 through the cooling process of hot exciton population as well as calculate their corresponding binding energies (AA ~ 69 meV and BB ~ 66 meV) via a new and more robust method. Furthermore, using many-body physics, we demonstrate that the excitons behave like Wannier-Mott excitons and explain their origins via first-principles calculations. Indeed, our results unravel the complex optical response of biexcitons in layered WS2, which should lead to numerous technological applications for developing biexciton-based valleytronic devices and ultrafast biexciton lasers at room temperature.

Resume : Graphene has been regarded as a prospective transparent electrode for light-emitting diodes or solar cells, but fundamental knowledge about the intrinsic properties of the graphene/material surface contact is still lacking. In this presentation, we will report optical and electronic properties of graphene exfoliated on ZnO surface. The graphene optical visibility was simulated based on Fresnel?s law [1] and confirmed with an optical microscope and micro-Raman spectra. The interfacial chemical and electronic properties were studied with a scanning photoelectron microscope [2]. The single layer graphene (SLG) showed a p-type doping property while stacked on ZnO surface and forming a pn junction. The energy band alignment of SLG/ZnO reveals a flat band condition, which may lead to an Ohmic contact between the graphene/ZnO interface. References: [1] P. Blake, E. W. Hill, A. H. Castro Neto, K. S. Novoselov, D. Jiang, R. Yang, T. J. Booth, and A. K. Geim, Appl. Phys. Lett. 91, 063124 (2007) [2] H. W. Shiu, L. Y. Chang, K.-H. Lee, H.-Y. Chen, S. Gwo, and C.-H. Chen, App. Phys. Lett. 103, 081604 (2013)

Resume : Transition metal dichalcogenides (TMDs) are attractive materials as they display transparency, flexibility, and good electrical properties- similar to graphene. However, unlike graphene, many 2D TMDs are semiconducting, giving application in transistors. Furthermore, as the demand for smaller devices is increasing downward scaling of 2D TMD transistors is very attractive [1]. 2D TMDs also posses a tunable bandgap, which occurs as the bulk material is reduced to monolayers due to quantum confinement and surface effects [2], giving rise to desirable optical properties such as strong photoluminescence and large exciton binding energy, making TMDs ideal for optoelectronic devices. Despite the remarkable properties of these relatively new materials, there are still many unanswered questions regarding their fundamental properties. One question we aim to answer is what makes up the electronic structure of bulk MoS2, MoSe2 and MoTe2, a property that has been surprisingly neglected in the literature. This is due to the complex van der Waals bonding between the interlayers. Here, we use a combination of x-ray photoemission spectroscopy with hybrid density functional theory calculations to probe the bulk electronic structure of the Mo-dichalcogenide series. Hard and soft x-rays are used to provide insight into the orbital contributions to the valence band. [1]R.Cheng, et. al., Nat. Comms., vol. 5, no. 5143, 2014. [2]Y.Zhang, et. al., Nat. Nanotech, vol. 9, no. 111, 2013.

Resume : The critical behavior of single-crystalline layered ferromagnet Fe1/4TaS2 were studied by bulk magnetization around the paramagnetic to ferromagnetic phase transition. Critical exponents β = 0.459(6) and γ = 1.205(11) are extracted from the Kouvel-Fisher plot, whereas δ = 3.69(1) is obtained by the critical isotherm analysis at Tc = 100.7 K. These critical exponents obey the Widom scaling relation δ=1+γ/β. Moreover, the self-consistency and reliability of the results are further verified by scaling equations. The determined exponents match well with those calculated from the results of renormalization group approach, and our analysis suggests that Fe1/4TaS2 possesses three-dimensional long-range magnetic interaction with the exchange distance decaying as J(r) ≈ r^−4.8.

Resume : Black phosphorus (BP), the most stable allotrope of phosphorus, is a material stacking individual atomic layers together through van der Walls interactions. The band gap of BP is tunable from 0.3eV for bulk BP to 2.0eV for phosphorene (monolayer BP) depending on the number of stacked layers. two-dimensional black phosphorus (phosphorene) dispersed in a solution is obtained by the solvent exfoliation. Among various solvents, N-methylpyrrolidone (NMP) is found to provide stable, highly concentrated BP dispersions. However, its instability under ambient conditions material deposition options for device fabrication. black phosphorous thin films were deposited on the substrates using ink-jet printing method. Physical properties of the films were systematically characterized by atomic force microscope (AFM), scanning electron microscopy (SEM), photoluminescence(PL), transmission electron microscope (TEM) and Raman spectroscopy. In this study, the stable, highly concentrated, electronic-grade phosphorous thin films were successfully deposited by combining the solvent exfoliation with the ink-jet printing deposition method. Considering our result obtained in this study, it is believed that the black phosphorene prepared in this study could be applied to large-area, high-performance phosphorene devices.

Resume : Electronic and optical properties of monolayer molybdenum dichalcogenide MoS2 were ab initio investigated upon substitution of S atoms by Te ones. It was found that such substitution leads to the reduction of the band gap from 1.84 in the case of monolayer MoS2 to 1.17 eV in the case of MoTe2. While bulk MoS2 and MoTe2 compounds are indirect-gap semiconductors, in monolayer form they are direct-gap materials, as well as MoS2-xTex compound with x < 0.5 or x > 1.5. The dependence of in-plane lattice constant on the concentration of Te atoms is almost linear for all stoichiometries considered. The band gap demonstrates linear behavior when the concentration of tellurium atoms is less than of sulfur ones. The first direct transition in direct-gap compounds is allowed in dipole approximation that suggests them to be suitable for effective light emission. The averaged absorption coefficients (> 105 cm-1 in the visible spectrum) are comparable to GaAs that shows the compounds considered to be promising for photovoltaic applications. The model of MOS transistor structure with a channel made of a two-dimensional crystal is proposed and charge transport properties of such structure are investigated.

Resume : Two-dimensional materials like graphene and transition metal dichalcogenides (TMDCs) show interesting optical, electrical, and mechanical properties differing from their bulk equivalents. However, TMDCs offer the possibility of combining more than one type of metal cation to form mixed solid solutions, which can confer tunability in various properties of the single-component constituents. In this work, we have synthesised monolayer alloy of molybdenum tungsten diselenide (MoWSe2) via chemical vapour deposition. The samples were thoroughly characterised via Raman/ PL/ XPS spectroscopy and AFM/STEM microscopy. We used Raman point and mapping spectra to analyze the behaviour of the 2D film under biaxial strain. The nature of crack propagation near Mo- and W-rich nanoscopic regions were observed to be different leading to an irreversible structural transformation due to stress buildup at tip of propagating crack. This behaviour was observed in STEM under electron-beam irradiation-induced cracking. Molecular dynamics simulations confirmed the nature of the crack spreading as well as predicting improved mechanical properties in alloys over single-component TMDCs.

Resume : The photoluminescence spectra of mono- and bilayer WS2, gated by the ionic liquid, were systematically studied at 77 K. Interesting phenomena, such as a redshift of the exciton peaks and a change in the spectral weight of the exciton, trion, and biexciton peaks, were observed at intermediate doping levels. By increasing the doping level, all the exciton, trion, and biexciton peaks vanished, which is attributed to the phase-space filling effect and the Coulomb screening effect. The variation in the band structure, which was induced by the quantum-confined Stark effect in both the mono- and bilayer WS2, was also studied using first-principle calculations.

Resume : Scattering-type scanning near-field optical microscopy (s-SNOM) has emerged as a key technology to gain chemical information, study structural properties and observe charge carrier distributions on the 10 nm length-scale. s-SNOM employs a metallic AFM tip which is illuminated to create a 10 nm small optical hot-spot at the tip apex independent from the wavelength of incident light. The concentrated light in the optical hotspot interacts with the sample surface below the tip and is modified by the local dielectric properties (absorption, reflection) of the sample. Detection of the elastically scattered light simultaneous to AFM imaging yields near-field images and broadband near-field spectra (nano-FTIR) with <20 nm spatial resolution. Using infrared s-SNOM imaging, the free carrier distribution in Bi2Se3 plates could be determined and it could be revealed that polyol-synthesized plates feature a non-uniform distribution of Se vacancies which cannot be cured by post-annealing. CVD-grown Bi2Se3 plates on the contrary don’t show such inhomogeneities. [1] Furthermore, s-SNOM images, recorded using mid-IR light, reveals the highly symmetric optical pattern of solvothermally grown Sb2Te3 hexagonal nanoplatelets. The superordinate optical patterns of the spiral growth patterns are shown to represent domains of distinct electronic properties, resulting from so-called growth twins with different densities of antisite defects. [2] [1] X. Lu et al., Adv. Electron. Mater., 2018, 4, 1700377 [2] B. Hauer et al., Nano Lett., 2015, 15 (5), 2787

Resume : Today’s conventional von-Neumann computer architecture, memory and processor are physically separated by a data channel, and data travels to and from the processor and memory, but it can’t process and save at the same time. In contrast, in human’s brain, data is stored and processed in the same place at the same time, so it enables a quick process of a massive amount of information. Therefore, many researchers believe that brain-inspired computing, which mimics the characteristics of human’s brain can break the problem of von-Neumann computer architecture. Here, we demonstrate a synaptic device on vdW heterostructures, and perform a pattern recognition simulation based on MLP. We firstly implement a plasma treatment on h-BN surface to make interfacial traps which can have synaptic functionalities. And we deposit WSe2 and MoS2 on plasma treated h-BN surface respectively. And we combine these two electronic devices to represent a unit synapse. Since the conductance of each electronic device can only be positive, it is necessary to have a pair of electronic device in order to implement the positive and negative weights. By utilizing this synapse, we figure out a way to enhance symmetry and linearity of LTP/LTD curve which can affect the recognition rate of artificial neural network. In this study, we suggest one of candidates for synaptic device structure and hope that we can inspire the other researchers in this field.

Resume : A simple fabrication of highly active and robust hexagonal iridium and ruthenium oxide nanosheets for the electrocatalytic oxygen evolution reaction (OER) will be presented. Iridate and Ruthenate nanosheets were synthesized from the layered bulk materials by tetrabutylammonium hydroxide assisted exfoliation. Electron diffraction indicates the retention of the hexagonal arrangement of the edge shared octahedra observed in the bulk precursors. Upon deposition on etched titanium plates the iridate nanosheets reach a current density of 10 mA cm-2 at overpotential of 344 mV, thus outperforming rutile IrO2 (415 mV) and bulk IrOOH (433 mV). Under the same conditions, the ruthenate nanosheets require only 255 mV, rendering them the most active OER electrocatalysts among all solution processed acidic medium catalysts reported in acidic media. Theoretical calculations reveal that the edge sites are likely the origin of the high OER activity of the nanosheets.Moreover, post OER analyses indicate a sustained stability of the nanosheets and the oxidation states of iridium or ruthenium during the electrocatalytic process. Therefore, the present investigation suggests that both iridate and ruthenate nanosheets are promising and highly material efficient acid medium OER catalysts with an application potential in photoelectrochemical cells, proton exchange membrane (PEM) electrolyzers and beyond. 1. Chem. Mater. 2017, 29, 8338–8345. 2. J. Mater. Chem. A 2018, 6, 2158-21566. 3. submitted.

Resume : Non-volatile memory based on two dimensional materials represents the ultimate scaling limit of this technology. The metallic T (or semimetallic T?) and semiconducting H phases in transition metal dichalcogenides have great potential for phase change resistive memory. Recent experiments, previously predicted by first principle calculations, have demonstrated that monolayer MoTe2 can switch phase under low external perturbation including strain and electrostatic gating. Moreover, the energy input to trigger the phase transition was evaluated to be 100-10,000 times lower than in state-of-the-art Ge2Se2Te2. However, slow kinetics (seconds) and high voltages (? 2 V) are still above nowadays memory standards. In this work we used density functional theory to predict low voltage, low energy ultrafast switching materials based on doped MoTe2. We explored all transition metals as potentials dopants to substitute Mo and, we rationalized their effect to the phase stabilization of MoTe2 via ligand field and charge density wave mechanisms. Based on this we predicted the electrostatic voltage and strain required to trigger the phase transition in each doped materials and evaluated the corresponding energy input of some of the best candidates. We found that a small amount of dopant (few %) dramatically affects the relative energy of H and T? phases. Moreover, the kinetics corresponding to the phase transition in the proposed doped materials are several order of magnitude faster than in pristine MoTe2. For example, we predict 6.3 % Mn doped MoTe2 to switch phase under 1.19 V gate voltage in less than 1 ?s with an input energy of 0.048 aJ/nm3, approximately 40 % lower than in the pristine materials. Interestingly, the inclusion of transition metal to monolayer MoTe2 often introduces phase dependent magnetic moment, due to their localized d-electrons. This leads to multifunctional phase transition as the change in the electronic conductivity is accompanied with a change of magnetic moment.

Resume : Many two-dimensional transition-metal dichalcogenides (TMDs) such as MoSe2, are direct bandgap semiconductors at monolayer thickness. Controllable doping in TMD ultrathin layers to tune their electronic properties are of great fundamental interests as well as device applications. Here we report controllable hole doping in epitaxial MoSe2 by phosphorus during growth by molecular-beam epitaxy. Examinations by annular dark field scanning transmission electron microscopy reveal substitutional doping of P, where P atoms replace Se in MoSe2 monolayer. X-ray photoelectron spectroscopy and Auger spectroscopy measurements confirm the formation of Mo-P bonds. Scanning tunneling spectroscopy and angle resolved photoemission spectroscopy experiments reveal in-gap defect states close to the valence band edge and apparent Fermi-level shifts. These are consistent with our density functional theory calculations, affirming the p-type shallow acceptor doping in MoSe2 by P-substitution. The hole doping effect can be tuned by changing the P/Se flux ratio and substrate temperature during growth. Also a tensile strain is shown to gradually build in the doped epi-film according to in situ reflection high-energy electron diffraction measurements, which is believed to result from the relatively longer Mo-P bonds than that of Mo-Se. This work is financially supported by a General Research Fund (No. 17327316) from the Research Grant Council, Hong Kong Special Administrative Region, and a NSFC/RGC joint research grant (No. N_HKU732/17 and 51761165024).

Resume : Monolayer (1L) tungsten diselenide (WSe2) is of interest for next generation ultrathin flexible electronic devices. However, typical WSe2 field effect transistors (FETs) show ambipolar characteristics that are not desirable for complementary field-effect-transistors and circuits. Here, significant suppression of the ambipolar characteristics of a 1L-WSe2 FET is demonstrated by using an electron withdrawing functional group of graphene oxide (GO). The pristine 1L-WSe2 FET shows n-type dominant ambipolar characteristics, whereas the GO coated 1L-WSe2 FET shows unipolar p-type behavior with a huge decrease (1/106) of current level of the n-channel. Also, the current level of the p-channel increases up to ten times that of the pristine 1L-WSe2 FET. These results are applicable for the realization of flexible nanoscale digital logic devices by using transition metal dichalcogenides.

Resume : Resistivity measurements of a few-layer black phosphorus (bP) crystal in parallel magnetic fields up to 45 T are reported as a function of the angle between the in-plane field and the source-drain (S-D) axis of the device. The crystallographic directions of the bP crystal were determined by Raman spectroscopy, with the zigzag axis found within 5° of the S-D axis, and the armchair axis in the orthogonal planar direction. A transverse magneto-resistance (TMR) as well as a classically forbidden longitudinal magneto-resistance (LMR) are observed. Both are found to be strongly anisotropic and non-monotonic with increasing in-plane field. Surprisingly, the relative magnitude (in %) of the positive LMR is larger than the TMR above ~32 T. Considering the known anisotropy of bP whose zigzag and armchair effective masses differ by a factor of approximately seven, our experiment strongly suggests this LMR to be a consequence of the anisotropic Fermi surface of bP. The authors thank the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 670173) for funding the project PHOSFUN by an ERC Advanced Grant.

Resume : Lattice engineering and control over the electronic band structure of the hexagonal transition metal dichalcogenide (TMDs) material such as molybdenum disulfide (MoS2) is challenging task among other telluride and selenide TMDs. Herein, we report on long range electron doping induced 2H to 1T? phase conversion in a few-layer MoS2 along with the giant band gap re-normalization (~200 meV) of the bulk MoS2 as observed in the optical reflection and photoluminescence (PL) spectra by realizing a hetero-structure with electride material such as dicalcium nitride [Ca2N)]+.e-, where the large work function difference between them (>2 eV) creates electron density of ~1014 cm-2 on the MoS2. Commonly observed Raman modes of the bulk 2H-MoS2 were hugely softened by the ??=10 cm-1 at the hetero-structure indicating the beginning of the lattice engineering caused by long range electron doping. As the MoS2 thickness was decreased, well defined Raman peaks of 2H crystal were gradually disappearing with the emergence of other lower frequency Raman modes (< 400 cm-1) describing the 1T' phase configuration. Strikingly, a few-layer MoS2/[Ca2N)]+.e- hetero-structure remains in a mixture of the 2H and 1T' phase, demonstrating the anisotropic Raman intensities, the presence of the hugely shrunk (250 meV) optical gap and enhanced PL intensity than a pristine 1L. This is attributed to the long-range electron doping induced structural change across the K-? line, hence provided an opportunity to discover various polymorphs of the 2D-MoS2 using a single 2D-dopant suitable for the future optoelectronic application.

Resume : Exploration of oxygen evolution reaction catalysts based on precious-metal-free catalysis is of great importance in renewable energy conversion and storage technologies. We report the correlation between the oxygen evolution reaction (OER) activities and the electronic properties of pure and first row transition metal doped monolayer AlN/GaN. Ni doped systems are found more active for OER compared with the pristine AlN/GaN and other first row transition metal doped AlN/GaN due to lower overpotential of about 0.3 V. Based on electronic structure, magnetic moment and crystal field theory analysis, the d electrons of the dopants are assigned to the degenerate energy levels for the different adsorption states during the reaction mechanisms. It is found that the potential determining step is related to the unpaired electrons of the dopants. In specific, the potential determining step is the step from a smaller number of unpaired electrons to a larger number of unpaired electrons. This is the first time to put forward the relationship between potential determining step and unpaired electrons of transition metal dopants.

Resume : Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in applications such as quantum optoelectronics. However, electrically-driven light emission from such excitons species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe2 with graphene as electrode. We observe that the relative intensity of the different exciton complexes strongly depends on the pulse parameters. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV.

Resume : Graphene nanoribbons (GNRs) are one dimensional materials with precisely defined electronic band structures. GNRs are synthesized by evaporation of organic precursors onto metallic substrates and subsequent polymerization and cyclodehydrogenation. A magnitude of GNRs with different widths, edge shapes and functionalizations can be produced in this way. Usually the polymerization and cyclodehydrogenation steps are performed by modest annealing to ~200C and ~400C, respectively. The downside of this approach is, that the synthesized GNR film completely covers the surface and any nanostructuring needs to be performed post-growth. To address this point, we hereby introduce a new, facile way to prepare N=7 GNRs in a highly precise fashion which overcomes the downsides of the bulk synthesis approach. By using a 532 nm laser, we were able to polymerize a typical organic precursor, yielding N=7 GNRs inside the focus of the laser. This enables us to ''print'' conducting paths of GNRs and therefore providing a first step towards printable graphene nanodevices.

Resume : The past decade has seen incredible progress in the ability to isolate and manipulate two-dimensional crystals. Due to their unique structure and dimensionality, it is possible to confine charge carriers in two dimensions, resulting in peculiar physical, chemical and electronic properties. Such novel properties can be further controlled and tuned through defect engineering at the atomic level, where a combination of low voltage scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and ab-initio calculations provides not only the most powerful means of characterization, but also a unique tool for manipulating the single atom structures and engineer their electronic interaction with the host matrix. This approach will be demonstrated on a number of 2D systems, for which the low energy ion implantation of single atom dopants such as N and B (for graphene [1]) or Se (for MoS2 [2]) can be successfully implemented to tailor their electronic structure at the atomic scale. STEM-EELS analysis provides an unambiguous fingerprint of these modifications but also offers the prospect to controllably drive the diffusion of substitutional dopants through the host 2D matrix, one atomic jump at a time, making STEM a promising alternative method for the direct assembly of nanostructures [3]. [1] F.S. Hage et al., ACS Nano 12, 1837-1848 (2018) [2] U. Bangert et al., Ultramicroscopy 176, 31-36 (2017) [3] T. Susi et al., 2D Materials 4, 042004 (2017)

Resume : The decoration of a single Co atom on a MoS2 basal plane (Co-MoS2) shows high-stability, -catalytic reactivity and -efficiency for the hydrodeoxygenation (HDO) reaction.1 In this work, the roles of the single Co atom and S vacancy defect on Co-MoS2 for the deoxygenation of 4-methyphenol (4MP) toward toluene were investigated using density functional theory (DFT) calculation. The insights into the reaction mechanism of HDO and H2 activation were systematically determined on this catalyst. The DFT results show that the strong interaction of Co substituted sulfur vacancy on MoS2 basal plane is responsible for the durability of the catalyst. The strongest 4MP and H2 adsorption sites can be observed at the Co site. This Co site also plays an important role for the H2 activation process before one dissociated H is trapped at the nearby S vacancy site nearby the Co atom. The most favorable pathway reveals that 4MP initially adsorbs on the Co atom with its carbon backbone followed by the C-OH bond dissociation. The dissociated hydroxyl moiety is bound to the pre-adsorbed H atom to form H2O molecule, while toluene is formed and stabilized on Co atom. Our results demonstrate that cooperation of S vacancy and single Co sites is essential key for enhancing the catalytic performance of monolayer MoS2 catalyst for the HDO reaction.

Resume : We have studied the (0001) surface of 2H-MoS2 by means of helium atom scattering (HAS). The electron-phonon coupling constant of this system has been determined by measuring the thermal attenuation of the specular peak at surface temperatures between 100 and 500 K. HAS diffraction also reveals a 3% planar dilation of the surface layer, while step interference indicates a slight contraction of the surface layer thickness. By employing a recently developed quantum-theoretical approach applied to the case of layered degenerate semiconductors, we find lambda=0.40 [1]. The present HAS data provide evidence for a surface enhancement of electron-phonon interaction and a possible relevant role of surface relaxation. The pronounced out-of-plane features observed confirm the high corrugation of the 2D surface unit cell. The observation of diffraction features along the two main high symmetry directions allows determining the in-plane surface lattice constant with high accuracy. The measured lattice constant is a = (3.25 ± 0.05) Å. Within experimental error, the MoS2 lattice parameter was found to remain constant in the temperature range between 90 and 522 K, in good agreement with previously reported DFT based calculations [2]. [1] G. Anemone, A. Al Taleb, G. Benedek, A. Castellanos-Gomez, D. Farías. J. Phys. Chem. (in press). [2] G. Anemone, A. Al Taleb, A. Castellanos-Gomez, D. Farías. 2D Materials 5, 035015 (2018).

Resume : Nano-flowers of MoS2 have been prepared by one step hydrothermal process at lower temperature (≈ 180°C). The morphologies and compositions of the as-prepared nanoflowers were characterized by XRD, field-emission scanning electron microscopy, high resolution transmission electron microscopy, Raman spectroscopy and KPFM analysis. From XRD and HRTEM confirms the interplanar spacing along 0.62 nm along the orientation of 002 planes without any impurity phase. From Raman analysis we concludes signature of E2g & A1g for a nanofloral structure. KPFM analysis clearly shows the sheets of MoS2 having surface potential around 430 mV. In case of bulk MoS2 band gap is 1.3 eV but as these grown nanoflowers shows a direct band gap of 1.9eV which has great impact on formation of solar cell devices. These grown nanoflowers can be used for different application such as hydrogen evolution, solar cells & li-ion battery preparation.

Resume : Two-dimensional transition metal dichalcogenides possess large surface-to-volume ratios that make them ideal candidates for sensing applications such as detecting the surface adsorption of specific gas molecules. The resulting changes of the electrical and optical properties allow for detection and analysis of interaction mechanisms at the sensing interface. Specifically, we investigate the influence of O2 adsorption on monolayer MoS2 and the role of the Fermi level energy in this process. We record the response in photoluminescence and transport properties of monolayer MoS2 upon O2 adsorption and the impact of external electric gating. We find an increase of the photoluminescence intensity and a reduction of the conductivity upon O2 adsorption, and show that the adsorption can be enhanced by an increase of the Fermi level energy. These results demonstrate that ionosorption of O2 on MoS2 by charge transfer only occurs if free carriers are available in the conduction band of MoS2. This free-carrier-supported adsorption-mechanism is corroborated by density functional calculations. Furthermore, the resulting reduction in screening of the Coulomb interaction between photo-excited electron-hole pairs amplifies the effect of the electron transfer on the excitonic recombination, causing a strong change of the photoluminescence intensity and rendering photoluminescence recording advantageous for sensing applications.

Resume : Two-dimensional (2D) layered semiconductors, such as black phosphorus and 2H transition-metal dichalcogenides, have emerged as a class of materials that may impact our future electronics technologies. The widely tunable band gap of 2D semiconductors is important not only to explore a new class of Dirac and Weyl fermions, but also to systematically investigate topological phase diagrams. In this talk, I am going to introduce our recent angle-resolved photoemission spectroscopy (ARPES) studies on black phosphorus. Surface doping to black phosphorus modulates the band gap from +0.3 eV to -0.6 eV by the surface Stark effect [1,2]. This energy range is large enough to achieve a topological phase transition from a trivial insulator or a semiconductor to a Dirac semimetal with a pair of Dirac points [3]. The Dirac point of black phosphorus is different from that of graphene in that it is protected by the crystal symmetry (spacetime inversion symmetry), offering an unprecedented opportunity to explore a 2D analogue of Weyl fermions. References 1. J. Kim et al., Science 349, 723 (2015). 2. M. Kang et al., Nano Lett 17, 1610 (2017). 3. J. Kim et al., Phys. Rev. Lett. 119, 226801 (2017).

Resume : Atomic scale defects greatly limit the electronic transport properties of Van der Waals layered black phosphorus (BP). While capping layers can protect devices from ambient degradation, a good understanding of in-grown defects remains essential for the reliability and performance of phosphorus-based devices. To support the production of high quality nanoscale applications with controllable properties, we present a systematic investigation into the purity of BP samples and the nature of these defects, combining Scanning Tunnelling Microscopy/Spectroscopy (STM/STS), chemical analysis, and multi-scale modelling. We show the characteristic double lobed defects are part of a greater set of hydrogenic orbitals arising from the Coulomb potential around a charged defect, and hence are not specific to any one defect. We find the defects produce localised states in the bandgap and can transition through p- and s-like states in positive and negative bias. Through STM, we resolve different electronic rearrangements at the core of these defects, further suggesting multiple origins are responsible for the defects. Using XPS and SIMS chemical analysis, we show the presence of Sn impurities in BP samples created through both common synthesis methods. We find substitutional Sn defects to be a good fit to both our multi-scale DFT and tight-binding calculations and STM/STS data, and emphasise Sn should be reconsidered in the ongoing investigation of defects in BP.

Resume : The covalent functionalization of phosphorene (Pn), obtained upon exfoliation of layered black phosphorus, has scarce experimental support especially when involved transition metals, TM.(1) For this, we explored in silico models via solid state DFT calculations with the program CRYSTAL.(2) The high density of the facial P atoms, with outpointing but not fully independent lone pairs, offers potential Pn reactivity with mono-, bi- and three-functional acidic units. In the first group, we examined adducts with BH3, I2 and the ClAu(I) fragment and established stereochemical and electronic relations with those of a single phosphine or white phosphorus (P4). Other unsaturated TM fragments of the L2M and L3M type were chosen on the basis of the isolobal analogy concept (3) for combining neighbor Pn atoms with a single metal that carries multiple vacant lobes. Selected examples of new species will be highlighted.(4) Finally, the study is being extended to the O or S atom transfer from a corresponding donor (e.g., DMSO or R3SbS) to a Pn atom, the electronic mechanisms being focused on. This work was supported by European Research Council through the ERC Advanced project PHOSFUN (Grant Agreement No. 670173). [1] V. V. Kulish et al. Phys. Chem. Chem. Phys. 2015, 17, 992. [2] R. Dovesi et al. CRYSTAL17. [3] R. Hoffmann Nobel Lecture Angew. Chem. Int. Ed. Engl. 1982, 21, 711 [4] A. Ienco, G. Manca, M. Peruzzini, C. Mealli Dalton Trans. 2018, 47, 17243.

Resume : Although the “library” of 2D materials is very large listing more than one thousand different 2D materials there is little information on 2D nitride semiconducting materials. However, there are model simulations, which predict their existence and properties. In the literature 2D GaN is one exception that has been synthetized and characterized. The reported physical properties include the bandgap, which is much larger than for thick GaN layers. This encouraged our team to carry out MOCVD growth for a range of nitrides using intercalation between graphene/SiC templates. The first successful case we report here is the growth 2D InN as intercalated between graphene/SiC. These thin, embedded layers emerge as challenging for characterization both by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The highest quality equipment (aberration corrected electron microscopy) was employed to verify the 2D layers at atomic resolution by application of atomic mass contrast. Our results are confirmed by spectroscopic techniques like energy dispersive X-ray Spectroscopy (EDS) and electron energy loss spectroscopy (EELS) as well. We report on cases, when two layers of In2O3 semiconductor was formed instead of the expected InN. Current injection across the graphene/In2O3/SiC heterostructure was investigated by nanoscale resolution conductive atomic force microscopy. By changing the growth parameters we were able to grow InN with the thickness of 3-4 atomic layers. In such a layer much higher bandgap is expected than the 0.7 eV of thick InN. Support for this work through FLAG-ERA JTC project GRIFONE is acknowledged.

Resume : The band structure and electronic properties of graphene are influenced by number of layer, and oxidized level. In this presentation, soft X-ray is employed to study the band structure of graphene under two aspects: number of layer and oxidized level. The dependence of photoelectron spectra (PES) of mechanical exfoliated graphene on number of layer by the scanning photoelectron microscopy (SPEM). The work function of graphene with various number of layer was measured by electrostatic force microscopy (EFM). According the results of SPEM and EFM, we found that the C1s core-level energy (E1s, core-level to vacuum level) of graphene decreases with the increase of NL. The E1s of single layer and multilayer graphene is ~289.42 eV and ~289.13 eV, respectively. In addition, the influence of oxidized level to the electronic band structure of graphene was investigated by soft X-ray PES. We demonstrated that X-ray irradiation is able to induce the reduction of graphene oxide (GO) by the characterization of X-ray PES. The number of C-O bonds of GO exhibits an exponential decay with exposure time. The X-ray reduction rate of GO is positively correlated with the intensity of low-energy secondary electrons excited from substrates by soft X-ray exposure. The reduction of GO is due to the secondary electrons induce the dissociation of the C-O bonds, not the direct interaction between X-ray and GO. According the results of XPS and valence band maximum measurements, we obtained the variation in band structure from GO to reduced GO.

Resume : We present the first experimental report on the transport properties of graphene devices on ultrathin-Si3N4. Our study provides a quantitative understanding of the physics of ultrathin Si3N4 as a gate dielectric for graphene-based devices. The Si3N4 ultrathin film was grown on Si(111) under UHV conditions and investigated by scanning tunneling microscopy (STM). Subsequently, an exfoliated graphene flake was deposited on top of it by a PMMA-based transfer technique. A Hall bar device was fabricated from the graphene flake. STM was again employed under UHV conditions to study the graphene flake after device processing, showing that its surface quality is preserved. Back gate modulation of the carrier type and density in the graphene channel, weak localization, and tunneling across the dielectric are observed in magneto-transport measurements at 4.2 K. Experimental data of tunnel current due to back gate modulation as a function of temperature is discussed in terms of the Poole-Frenkel and Fowler-Nordheim mechanisms.

Resume : Graphene (Gr), thanks to its extraordinary properties and a peculiar low-dimensional morphology, is a promising 2D nanomaterial for future scaling of microelectronics devices. Gr features a strong sensitivity to the environment through charge-transfer processes driven by interactions with the substrate or adsorbed molecules. This feature can be used to finely tune the doping of Gr, a key purpose in order to develop Gr-based nano-scale devices. Herein, we report the study of the p-doping process due to the adsorption of O2 molecules intercalated between Gr and SiO2 substrate by thermal treatment. In particular, the role of the substrate chemistry is investigated, by comparing the different native configuration of Gr transferred on three different oxides — SiO2, Al2O3, and HfO2 —, as well as the different doping outcomes. We used Atomic Force Microscopy and Contact Angle measurements to characterize the morphological and chemical properties of the substrates. In addition, the structural and electronic properties of both native and O2 treated Gr were investigated, through the evaluation of its strain and doping, respectively, by means of Raman Spectroscopy. The strong substrate influence on the temperature distribution of reaction sites, as well on the reaction kinetics is revealed. The experimental results suggest that the highest doping level is related to the substrate water affinity, whereas the reaction kinetics to is related diffusional process.

Resume : Van der Waals semiconductors display some distinctive properties with huge potential for technological exploitation in nanoelectronics, catalysis, gas sensors, nanocomposites, and seawater desalination, arising both from the reduced thickness and from the modified electronic structure. In particular, plasmonic modes in each class of van der Waals semiconductors have their own peculiarities. Plasmons of transition-metal dichalcogenides share features typical of graphene, due to their honeycomb structure, but with damping processes dominated by intraband rather than interband transitions, unlike graphene. Spin-orbit coupling strongly affects the plasmonic spectrum of buckled honeycomb lattices (silicene and germanene), while the anisotropic lattice of phosphorene determines different propagation of plasmons along the armchair and zigzag direction. The recent advancement in the growth/synthesis techniques and in the control over surface phenomena enable novel applications based on van der Waals semiconductors. As an example, plasmonic excitations in van der Waals semiconductors can be used to improve the efficiency and the selectivity of membrane distillation to produce drinking water from the sea only by using sunlight, with also recovery of minerals from the sea. Another possible application is related to plasma-wave photodetection of Terahertz light. Finally, van der Waals semiconductors can be combined to obtain van der Waals heterostructures for innovative low-loss plasmonic devices.

Resume : Two dimensional van der Waals (vdW) PN junctions have been studied for heterojunction diodes which basically utilize out-of-plane current across the junction interface. In fact, the same vdW PN junction structure can be utilized for another important device application, junction field effect transistors (JFETs), where in–plane current is possible along with 2D-2D heterojunction interface. Also, the 2D TMD-based JFET can use both p- and n-channel for low voltage operation, which might be its unique feature. Here, we report vdW JFETs as an in-plane current device with heterojunction between semiconducting p- and n-TMDs. Since this vdW JFET would have low density traps at the vdW interface unlike 2D TMD-based metal insulator semiconductor field effect transistors (MISFETs), little hysteresis of 0.05~0.1 V and best subthreshold swing of ~100 mV/dec were achieved. Easy saturation was observed either from n-channel or p-channel JFET as another advantage over 2D MISFETs, exhibiting early pinch-off at ~1 V. Operational gate voltage for threshold was near zero volt and our highest mobility reaches to ~more than 500 cm2/V s for n-channel JFET with MoS2 channel. For 1 volt JFET operation, our best ON/OFF current ratio was observed to be ~104

Resume : Transition metal dichalcogenides (TMDs) have brought a great attraction in nanoelectronics by the virtue of feasibility of band structure design. In this study, p-type tungsten diselenide (WSe2) and n-type molybdenum disulfide (MoS2) were chosen for the fabrication of large-area heterojunction device arrays. By utilizing selenization and thermal decomposition process, few-layer WSe2 and MoS2 films can be obtained in large scale, respectively. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were carried out to investigate the structure and compositions of the samples, and atomic force microscopy (AFM) was conducted to analyze the thickness and the uniformity of films. Heterojunction devices composed of WSe2 and MoS2 were fabricated via PMMA-mediated transfer method on either rigid substrates or flexible polyimide (PI) substrate afterwards. Moreover, both of WSe2 and MoS2 devices exhibit the excellent electrical characteristics. In conclusion, based on highly transparent and flexible essence for TMDs, the combinations of these materials show the great potential for the fabrication of large-area, transparent and flexible heterojunction devices.

Resume : Mono-Layer and few-layer Transition Metal Dichalcogenides (TMDCs) have attracted great interest since the discovery of graphene due to their outstanding optoelectronic properties. Recent studies have shown the capability of fabricating P-N heterojunctions using different approaches like material stacking or chemical doping. The burden of these methods appears when involving fabrication steps like resist deposition, chemical manipulation and aligning/lithography procedures that increase the cost and the damage probability of the device's properties. In our group, we developed a direct fabrication method, Pulsed Focused Electron Beam Induced Etching (PFEBIE), which allows the scissoring of TMDCs regions on the devices once their fabrication steps have been completed, saving intermediate fabrication processes. This method enables the possibility to fine-tune the device's optoelectronic properties. When the etching is done a p-doping mechanism occurs only in the etched zone. Taking the advantage of this doping phenomenon, we can fabricate a lateral P-N homo-junction from an intrinsically N field effect transistor. Field effect devices were fabricated from mechanically exfoliated MoS2 flakes via optical beam lithography followed by a metal evaporation and a lift-off process to define the gate-contact structures showing a transistor behavior. The devices were characterized employing Raman and Photoluminescence mapping spectroscopy, transport measurements and AFM/SEM microscopy. Afterwards, PFEBIE was utilized to fabricate a lateral P-N homo-junction that was characterized with the same methods as before. Electric measurements after PFEBIE show a N-P diode behavior with excellent white light photoresponse.

Resume : Homojunction PN and PIN diodes are reported, using two dimensional transition metal dichalcogenide MoTe2. Recently, type II-based heterojunction diodes have been mainly seen in report, but homojunction PN diodes using 2D materials are still rare. Recently, hydrogen-doped n-type MoTe2 was reported using atomic layer deposition (ALD) on top of p-type MoTe2 surface. As a result, lateral homojunction PN diode could be realized by H doping in selective area. In fact, ultra-thin MoTe2 based device could be applied for a near-infrared detection in range of ~1300 nm, which is the wavelength that Si-based devices might not effectively cope with. Here, seamless MoTe2 homojunction PIN diode has been first time demonstrated, detecting visible-to-1300 nm SWIR photons. Our 2D-like thin MoTe2 initially forms PN junction, but it becomes PIN diode using two split gates. Compared to PN diode mode, PIN mode much enhances the photo-response in visible-to-1300 nm IR measurement range due to enhanced built-in electric field. In particular, the response to 1300 nm is not good enough in initial PN mode but remarkably increases in PIN mode as controlled with two split gates. Because I region must be under high electric-field during PIN device operation, Franz-Keldysh effect is presumed to work for effective absorption of 1300 nm in MoTe2 with energy band gap of ~0.95 eV. Superior responsivity of PIN photodiode to that of PN photodiode is well known, but PIN structure in 2D TMD semiconductor is very rare in report and we anticipate that it may support Si photodetector as integrated on Si devices.

Resume : Black phosphorus (BP) is a two-dimensional layered material, which possesses ultrathin body thickness, widely tunable band gap (ranging from 0.35 eV for bulk to 2.0 eV for monolayer), and high hole mobility (102-103 cm2V-1s-1) at room temperature. It has been predicted that BP has higher carrier mobility than MoS2 for both electrons and holes, which facilitates its application for complementary logic circuits. However, the extensively reported BP transistors exhibit p-type or ambipolar transport property due to the Fermi level pinning at the contacts and the suppressed electron transport caused by oxygen and moisture exposure. In recent studies, n-type BP transistors are achieved by utilizing contact metals with low work function such as aluminum (Al) and scandium (Sc). The n-type characteristic is mainly attributed to the well-match between the conduction band of few-layer BP and the work function of contact metal electrode. In this work, we demonstrate unipolar BP n-type transistors with copper (Cu) contact. The contact metal Cu experiences penetration to BP flakes during metal evaporation, resulting in BP with interstitial Cu at the contact region. The interstitial Cu n-dopes the BP and narrows the bandgap of BP without changing the crystal structure of BP, which benefits the excellent Cu-doped BP edge contact with channel pure BP. The BP transistors with Cu contact exhibit strong n-type transport property with high electron mobility of ~ 60 cm2V-1s-1 and on/off ratio of ~ 103 at room temperature. The n-type current can reach 58 μA/μm with 3 μm channel length. The Cu-doped edge contact changes the energy level match between BP and Cu, promoting the electron transport in BP.

Resume : Achieving a good quality Ohmic contact on van der Waals materials is a challenge, since at the interface between metal and van der Waals material, different interface conditions could apply, ranging from the presence of a large energy barrier to the metallization of the layered material below the contacts. In black phosphorus (bP) a further challenge is its high reactivity to oxygen and moisture, which makes comparison among different experiments difficult, since the presence of uncontrolled oxidation can substantially change the behavior of the contacts. In this study, we tested the influence of the metal used for the contacts against flakes and sample variability, using three of the most used metals as contacts: chromium, titanium, and nickel. From an analysis of nine different devices, using the transfer length method at both room and at low temperature, Ni appears to be the best candidate for Ohmic contacts to bP, providing the lowest contact resistance and a very low scattering between different devices. Moreover, we investigated the gate dependence of the current-voltage characteristics of these devices. In the accumulation regime, we observed good linearity for all metals investigated. The authors thank the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 670173) for funding the project PHOSFUN by an ERC Advanced Grant to MP.

Resume : 2D materials are exciting for two reasons: i) layer number dependent properties and ii) the broad palette of accessible layered crystals potentially giving access to any desired function. Ten years ago, it was demonstrated that 2D nanosheets can be obtained from layered crystals via liquid phase exfoliation (LPE) resulting in colloidal dispersions. However, sample polydispersity was a problem until recently. Now, we have arrived at a point where size selection (e.g. liquid cascade centrifugation, LCC) and size measurement protocols are in place, which can be readily applied to the whole nanosheet zoo. By comparing various materials, we have now developed a model to understand the exfoliation. With our realisation that both size and thickness result in changes in optical extinction and absorbance spectra due to edge and confinement effects, it became possible to quantitatively determine the nanosheet dimensions optically. Such metrics have now been developed for ~15 materials. The understanding of the optical spectra is useful to monitor degradation kinetics in various liquids as function of time/temperature. Activation energies can be determined and passivation of defects, e.g. by functionalisation, subsequently studied. Functionalisation in general has the potential to ultimately enable the fabrication of hybrids, vertical and horizontal heterostacks etc. for new functional materials.

Resume : Two-dimensional (2D) nanomaterials have recently attracted tremendous research attention. Their unusual properties, originating from the quantum size effects, have led to many promising applications in nanoelectronics, catalysis and energy storage. Theoretical studies demonstrated that black phosphorus (BP) forms strong bonds with metal adatoms while still preserving its structural integrity, unlike graphene whose atomic lattice is strongly distorted. This unique combination of high reactivity with good structural stability is very promising for potential applications of BP. Structural and spectroscopic study by STEM-EELS have been already reported in literature and unveiled for the first time the atomic structure and electronic properties of pristine BP[1-3], During the present research activity, hybridization of exfoliated black phosphorus with different metals was carried out, either as metal nanoparticles in the case of nickel and palladium or as molecular fragment in the case of rurthenium. Thus, Ni/BP, Pd/BP and Ru/BP have been synthesized and their structural and electronic properties studied by means of Scanning Transmission Electron Microscopy (STEM), Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray Spectroscopy (EDX) at atomic level. In particular, EELS spectra were numerically simulated by means of Density Functional Theory (DFT) calculations in order to assess the effect of the different metals adatoms on the phosphorene electronic Density of States (DOS), a key parameter to understand the electronic properties of the system. Acknowledgements This work was performed at Beyond- Nano CNR-IMM, which is supported by the Italian Ministry of Education and Research (MIUR) under project Beyond-Nano (PON a3_00363). The authors thanks EC for financing an ERC Advanced Grant PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials” (Grant Agreement No. 670173) to M. P. __________ [1] Nicotra G., Mio A.M.et al.,, Microsc. Microanal 2015 427, No. 0214 suppl 3 [2] Nicotra G. Politano A. et al., Phys. Status Solidi B 2016 253, No. 12, 2509–2514 [3] Nicotra G, van Veen E. et al, Nanoscale 2018, 21918-21927, vol 10, 46

Resume : Recently, orthorhombic group IV metal monochalcogenides (MXs; M=Ge/Sn and X=S/Se) with a uniquely distorted layered structure has emerged as a novel electronic material in two-dimensional (2D) family. [1,2] Among them earth abundant tin (II) monosulfide (SnS) has sparked as a rising star material due to its intriguing optical, electrical, and mechanical properties. [3,4] However, the fabrication of chemically stable, electronic grade, few- to monolayers of SnS has been significantly challenged due to its strong interlayer forces originates from active lone pair electrons. [5] Here, we utilized a simple approach to prepare large area, ultrathin (as thin as two of monolayers) SnS sheets through a sonication assisted liquid phase exfoliation process. The isolated SnS nanosheets were verified with various microscopic and spectroscopic techniques, which are ultrathin (bi-, quadri-, hexa-, and octa- layer) with high crystallinity. Further, the thin layer of SnS nanosheets were employed as an interface- /inter- layer of a functional perovskite solar cells to enhance selective charge collection and environmental device stability. The charge separation at the perovskite/SnS interface probed by the ultrafast transient absorption spectroscopy, which directly impact on the increment of overall device performance. A significant environmental device stability was achieved due to the earth abundant SnS protective layer on perovskite solar cells. References [1] T. Rangel, B. M. Fregoso, B. S. Mendoza, T. Morimoto, J. E. Moore, and J. B. Neaton, Phys. Rev. Lett., 2017, 119, 067402. [2] Lei Xu, Ming Yang, Shi Jie Wang, and Yuan Ping Feng, Phys. Rev. B., 2017, 95, 235434. [3] Z. Tian, C. Guo, R. Li and J. Xi, ACS Nano, 2017, 11, 2219-2226. [4] S. Lin, A. Carvalho, S. Yan, R. Li, S. Kim, A. Rodin, L. Carvalho, E. M. Chan, X. Wang, A. H. C. Neto & J. Yao, Nat. Commun., 2018, 9, 1455. [5] N. Higashitarumizu, H. Kawamoto, M. Nakamura, K. Shimamura, N. Ohashi, K. Ueno and K. Nagashio., Nanoscale, 2018, 10, 22474.

Resume : 2D semiconductor nanocrystals are attracting increasing attention due to their unique properties. Ultrathin colloidal semiconductor nanosheets (NSs) with thickness in the strong quantum confinement regime are of particular interest, since they combine the extraordinary properties of 2D nanomaterials with versatility in terms of composition, size, shape, and surface control, and the prospects of solution processability. However, bottom-up synthesis procedures for colloidal NSs are still underdeveloped. Compound Cu-chalcogenides are an interesting class of materials due to their low toxicity, low costs, and wide range of compositions. This latter point makes them extremely versatile. In this contribution, we discuss recent work by our group on ultrathin (1.5-2 nm thick) colloidal NSs of both binary (Cu2-xA; A= S, Se) and ternary (CuInA2) compound Cu-chalcogenides, with lateral dimensions in the 100 nm to 1 µm range. The formation mechanisms of these 2D NSs were investigated in detail, and involve 2D-constrained growth in soft templates, oriented attachment of building blocks, and self-limited cation exchange, depending on the NS composition. Moreover, the NS composition can be post-synthetically tailored by exploiting topotactic cation exchange reactions. The bottom-up methods developed in our work thus hold great promise as a route to solution-processable compositionally diverse colloidal 2D semiconductors.

Resume : Pt-based materials have attracted considerable attention due to the superior performance of Pt in applications such as catalysis, fuel cells, and sensors. Recent efforts to improve the utilization efficiency have been directed toward tuning specific structural features of Pt nanostructures to produce catalysts with high surface area, and consequently, to achieve superior catalytic performance. The synthesis of Pt-based nanomaterials with a controlled structure and morphology or surface area is expected to lead to their use in new applications. We have been studied 2D metallic material. However, metallic atoms tend to exhibit 3D close-packed structures, and ultra-thin metal structures with excessive unsaturated atoms are difficult to stabilize. Several studies have shown that developing 2D is a task that has proven difficult. Regarding methods of synthesizing 2D monolayered materials, exfoliating layered materials such as graphite is an effective method. Therefore, we have prepared the nanosheets using Pt-based layered materials. However, Pt-based layered materials were scarcely maintainable no investigation. Here we report the preparation of Pt-based layered materials and new nanosheets with homogeneous thickness by the exfoliation of the layered materials. We have synthesized two kind of Pt-based layer materials. At first, the ｌayered alkaline platinum oxide was obtained via the facile and direct calcination of Pt black and alkaline carbonate at high temperature. Next, we had layered platinum hydroxide by soft, solution method, tried exfoliation the materials and analyzed their materials by XRD, TEM, AFM, and so on. These Pt-based nanosheets will prove essential for new nanodevices and will further the investigation of the characteristics of electrochemical reactions.

Resume : Most metals feature an atomically-thin oxide layer at the metal air interface. This also applies to liquid metals including molten tin, indium, gallium and their alloys.1 In many cases this oxide layer grows in a self-limiting reaction providing a pathway towards atomically-thin, two-dimensional materials.1-4 This talk will discuss different liquid metal-based synthesis strategies for 2D materials and will highlight how large area ultrathin sheets can be isolated form the liquid metal interface. The isolated oxide sheets may then be used as a precursor for compound semiconductors. We report the synthesis of centimeter sized ultrathin GaN and InN.4 The synthesis relies on the ammonolysis of liquid metal derived two-dimensional (2D) oxide sheets that were squeeze-transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical bandgap of 3.5 eV and a carrier mobility of 21.5 cm2V-1s-1, while the InN featured a thickness of 2.0 nm. The deposited nanosheets were highly crystalline, grew along the (001) direction and featured a thickness of only three unit cells. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices. References 1. Zavabeti, A.; Ou, J. Z.; Carey, B. J.; Syed, N.; Orrell-Trigg, R.; Mayes, E. L. H.; Xu, C.; Kavehei, O.; O’Mullane, A. P.; Kaner, R. B.; Kalantar-zadeh, K.; Daeneke, T., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science 2017, 358 (6361), 332-335. 2. Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesman, A. S. R.; Russo, S. P.; Lau, D. W. M.; Xu, Z.-Q.; Bao, Q.; Kavehei, O.; Gibson, B. C.; Dickey, M. D.; Kaner, R. B.; Daeneke, T.; Kalantar-Zadeh, K., Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals. Nature Communications 2017, 8, 14482. 3. Daeneke, T.; Atkin, P.; Orrell-Trigg, R.; Zavabeti, A.; Ahmed, T.; Walia, S.; Liu, M.; Tachibana, Y.; Javaid, M.; Greentree, A. D.; Russo, S. P.; Kaner, R. B.; Kalantar-Zadeh, K., Wafer-Scale Synthesis of Semiconducting SnO Monolayers from Interfacial Oxide Layers of Metallic Liquid Tin. ACS Nano 2017, 11 (11), 10974-10983. 4. Syed, N.; Zavabeti, A.; Messalea, K. A.; Della Gaspera, E.; Elbourne, A.; Jannat, A.; Mohiuddin, M.; Zhang, B. Y.; Zheng, G.; Wang, L.; Russo, S. P.; Dorna, E.; McConville, C. F.; Kalantar-Zadeh, K.; Daeneke, T., Wafer-Sized Ultrathin Gallium and Indium Nitride Nanosheets through the Ammonolysis of Liquid Metal Derived Oxides. Journal of the American Chemical Society 2018.

Resume : Two-dimensional (2D) polymers (2DPs) that are laterally infinite, one atom- or monomer-unit thin, free-standing, covalent networks with long-range order along two orthogonal directions have attracted intense attention in recent years due to their wide applications in electronics, membrane, and sensing[1][2]. At present, one of the key challenges in two-dimensional (2D) materials is to go beyond graphene, a prototype 2D polymer (2DP), and to synthesize its organic analogues with structural control at the atomic- or molecular level. Here, we present the bottom-up synthesis of 2DPs towards large-area, free-standing feature, high crystallinity, tunable pore structures, and controllable thickness (from single-layer to multilayers) by reliable interfacial synthesis strategies (such as air/water and liquid/liquid interfaces). The monolayer 2DP has a thickness of∼0.7 nm with a lateral size of 4-inch wafer, and it has Young’s modulus of 267±30 GPa[3]. Notably, the monolayer 2DP functions as an active semiconducting layer in a thin film transistor. Aberration-corrected high-resolution transmission electron microscopy is utilized to visualize time-dependent crystal growth of multilayer 2DP. The grain boundaries and the crystalline defects of such 2DP were further investigated at the molecular level, which is critical for understanding its crystal growth mechanism as well as the corresponding physical and chemical properties. Moreover, proof‐of‐concept applications of such 2DPs suggest that they are promising materials in energy storage and conversion, catalysis and electronic application. This work opens the door for the interfacial synthesis of functional 2DPs using reversible polycondensation reaction, which may pave the way for the rational synthesis of 2D organic soft materials as promising candidates for next-generation electronics and energy-related applications. . Reference: [1] J. Sakamoto, J. van Heijst, O. Lukin, A. D. Schlüter, Angew. Chem. Int. Ed. 2009, 48, 1030–1069. [2] X. Zhuang, Y. Mai, D. Wu, F. Zhang, X. Feng, Adv. Mater. 2015, 27, 403–427. [3] H. Sahabudeen, H. Qi, B. A. Glatz, D. Tranca, R. Dong, Y. Hou, T. Zhang, C. Kuttner, T. Lehnert, G. Seifert, et al., Nat. Commun. 2016, 7, ncomms13461.

Resume : MoTe2 belongs to layered 2D materials (2DM) with interesting electronic properties. As many other 2DMs, MoTe2 occurs in several polytypes, e.g., semiconducting hexagonal 2H phase, or metallic 1T’ monoclinic one. We have grown MoTe2 layers by molecular beam epitaxy (MBE). Since 2DMs grow in the Van der Waals mode, the moderate influence of the substrate on the MBE growth character and quality of epitaxial MoTe2 films has been noticed - layers grown on GaAs, Si and sapphire exhibit similar properties. However, the MBE growth conditions have essential influence on the crystallographic structure and quality of MoTe2 layers. At lower growth temperatures (about 300 °C) semiconducting hexagonal 2H MoTe2 phase is obtained, whereas at about 100 °C higher one, films in 1T’ monoclinic Weyl semimetal phase are crystallized. Moreover, by an appropriate choice of the MBE growth conditions, one can switch from the 2D layer-by-layer MoTe2 growth to 3D growth of Mo6Te6 nanowires. Structural properties of the layers and nanowires have been thoroughly studied by high resolution transmission electron microscopy. Optical and transport properties of MoTe2 layers have also been investigated by Raman spectroscopy and magnetotransport measurements. The latter technique reveals the signature of Weyl-semimetal properties of the 1T’ MoTe2 phase. This work has been supported by the National Science Centre (Poland) through project No. 2017/27/B/ST5/02284

Resume : 2D mono-elemental crystals, namely X-enes, promise to introduce novel opportunities in several technological fields due to their fascinating physical properties.[1] As such, developments in the synthesis by technologically relevant processes are key to their integration into functional devices. [2,3] Among X-enes, antimonene, the allotrope β-phase of antimony, is formed by atoms arranged in buckled hexagonal rings and offers several advantages including high environmental stability. Here, we investigated the growth of antimonene multi-layers on Ge (100) and (111) single crystal substrates by a process based on MOCVD. The catalytic effect of Au nanoparticles (NPs) was also investigated. The growth with and without Au NPs were analyzed by SEM, XPS, XRD and HR-TEM, revealing that antimony forms extended domains of layered β-phase films with an epitaxial relationship with the substrate. In particular, the Sb grows with (012) planes parallel to the Ge (111) planes. The study of the antimonene layers was further refined by means of Raman spectroscopy, following the evolution of the Raman peaks as a function of the growth parameters and comparing the frequency positions of the phonon modes with theoretical data [2,4] obtained by first principles simulations. 1 A. Molle et al., Nat. Mater., 2017, 16, 163–169. 2 J. Ji et al., Nat. Commun., 2016, 7, 13352. 3 D. Singh et al., J. Mater. Chem. C, 2016, 4, 6386–6390. 4 C. Gibaja et al., Angew. Chemie Int. Ed., 2016, 55, 14345–14349.

Resume : Since thin layer 2D materials have been attracting enormous interest, various processes have been investigated so far to obtain these materials efficiently. In view of their practical applications, the most desirable source is the pristine bulk material with stacked layers such as pristine graphite. On their exfoliation, we have many options in terms of conditions such as wet or dry, with or without additive, and kind of solvent. In this context, we have found versatile exfoliant, 2,3,6,7,10,11-hexahydroxytriphenylane, which works efficiently for exfoliation of typical 2D materials, graphene, MoS2, and h-BN, in both wet and dry processes using sonication and ball-milling, respectively, in aqueous and organic solvents [1,2]. As for graphene, stable dispersions with relatively high concentration (up to 0.28 mg/mL) in water and tetrahydrofuran were obtained from graphite in presence of hexahydroxytriphenylene by wet process using bath sonication and via dry process using ball-milling. Especially, most of graphite was exfoliated and dispersed as thin layer graphene in both aqueous and organic solvents through ball-milling even at large scale (47 - 86% yield). In addition, the exfoliant can be easily removed from the precipitated composite by heat treatment without disturbing the graphene structure. Bulk MoS2 and h-BN were also exfoliated in both wet and dry processes. As in graphene, MoS2 and h-BN dispersions of high concentrations in water and DMF were produced in high yields through ball-milling. [1] G. Liu, N. Komatsu* ChemNanoMat, 2 (6), 500 - 503 (2016) [highlighted at the front cover]. [2] G. Liu, N. Komatsu* ChemPhysChem, 17 (11), 1557?1567 (2016) [highlighted at the front cover].

Resume : Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have stimulated appreciable attentions due to their unique properties and enormous potential in various applications. Therefore, governable synthesis and its parameters’ interpretation are required to achieve desired high quality product. Among synthesis methods, chemical vapor deposition (CVD) is a powerful method which may meet the above requirements in spite of a few unresolved challenges, arising from lack of understanding the growth mechanisms and precisely controlling the growth parameters. All the proposed mechanisms and their relation to the growth conditions are inferred from characterizing intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin WSe2 and MoSe2 crystals in salt assisted TMDC synthesis. With the support of our real time observations, both the vapour-solid-solid (VSS) and vapour-liquid-solid (VLS) growth routes are identified by investigating the reaction between the salt and the metallic precursor as well as their intermediate product. We show that by directing the liquid intermediate compound through pre-patterned channels, it is viable to guide the crystal formation. Furthermore, we demonstrate the role of H2 which dominates the growth routes based on the growth temperature. Finally, we observe the synthesis of the MoSe2/WSe2 heterostructures optically, and elucidate the conditions required for both lateral and vertical heterostructure synthesis.

Resume : The magnetic ordering associated with the degree of freedom of spin in two-dimensional (2D) materials is subjected to intense investigation because of its potential application in 2D spintronics and valley-related magnetic phenomena. We report here a bottom-up strategy using molecular beam epitaxy to grow and dope few-layer MoSe2 on mica with magnetic dopant Mn. High-quality Mn-doped MoSe2 layers was found for the Mn content less than 5% (atomic) while we observe a clear transition from layer-by-layer growth to cluster phase with increasing the Mn content. Magnetic measurements, which were performed with care involving an approved transfer process of the doped layers on 100-micron-thick silicon substrate, show plausible proof of high-temperature ferromagnetism in the doped layers. Although we could not point to a correlation between magnetic and electrical properties, we also demonstrate that the transfer process used in this study permits to achieve conventional electrical measurements on the doped layers on any substrates we want. Therefore, this study provides promising alternative route to synthetize and characterize stable and ferromagnetic 2D layers, which is broadening the current start-of-the-art of 2D materials-based applications.

Resume : Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe2 mono- and multi-layers on SiO2/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO2/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe2 monolayers over 1 cm2. To grow MoSe2 multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe2 monolayer which serves as a van der Waals growth tem- plate. By annealing, we obtained continuous MoSe2 multilayers over 1 cm2. Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10 % of Mn in MoSe2 bilayers.

Resume : In the last years, the field of two-dimensional materials has been the target of intense study due to the unique properties that single-atom or few-atoms thick materials exhibit compared to their bulk counterparts. The most extensively studied 2D material is graphene, however a class of two-dimensional materials beyond graphene is rapidly emerging, comprising a wide spectrum of organic, inorganic and hybrid materials1,2. Pyridinylboronic acids have recently drawn attention in crystal engineering due to their self-complementary nature. Directed by the strongly selective and directional N-B bond, such compounds displayed a multitude of peculiar packing motifs, ranging from polymers to macrocages3,4. Herein we report the case of 4-pyridinylboronic acid. This small molecule spontaneously assembles in the solid state in a low-density structure, characterized by the stacking of 2D sheets formed by a rare cooperation of N-B and H-bonds. Recrystallization was obtained only from mixtures of water and DMSO, while the compound is insoluble in both pure solvents. The so obtained crystals were characterized by means of XRD techniques and electron microscopy, while Hirshfeld surface analysis and Intermolecular Interaction Energies (IIEs) calculations were performed to probe the properties of the compound as a potential two-dimensional material. Exfoliation was explored through liquid-phase sonication performed in different solvents. The nanosheets obtained by exfoliation were characterized by TEM imaging showing that the solvent has an influence on the morphology of the flakes. References [1] Bhimanapati, G. R., Lin, Z.; Meunier, V., Jung, Y.; Cha, J., Das, S., Xiao, D., Son, Y., Strano, M. S., Cooper, V. R., Liang, L., Louie, S. G., Ringe, E., Zhou, W., Kim, S. S., Naik, R. R., Sumpter, B. G., Terrones, H., Xia, F., Wang, Y., Zhu, J., Akinwande, D., Alem, N., Schuller, J. A., Schaak, R. E., Terrones, M. & Robinson, J. A. (2015). ACS Nano. 9, 11509?11539. [2] Boott, C. E., Nazemi & A., Manners, I. Angew. Chemie - Int. Ed. (2015). 54 (47), 13876?13894. [3] Salazar-Mendoza, D., Cruz-Huerta, J., Höpfl, H., Hernández-Ahuactzi, I. F. & Sanchez, M. (2013). Cryst. Growth Des. 13(6), 2441?2454. [4] Salazar-Mendoza, D., Guerrero-Alvarez, J. & Höpfl, H. (2008). Chem. Commun. 44(48), 6543.

Resume : 2D TMDs(transition metal dichalcogenides) are able to enable high performance electronic devices because of its excellent scalability and relatively high mobility. However, in order to realize practical applications based on 2D TMDs, a technique capable of uniformly growing a large-scale and high quality 2D TMDs on wafer-sized substrate is indispensable. The challenging issue has been considered as a serious bottle neck up to now even if there have been many CVD methods for single crystalline 2D TMDs. RF magnetron sputtering of TMDs can be one of physical vapor deposition method of feasibly and quickly depositing large-area two-dimensional layered materials. However, it is highly hard to accurately control chalcogen atoms in 2D TMDs and successfully achieve crystallinity, which occasionally cause a large amount of chalcogen deficiency. Thus, the previous researches involving sputtering-based 2D TMDs synthesis require additional sulfurization process under chalcogen-based precursor condition. However, the relatively complex two step processes should be simplified into simple one step process. Here, we developed facile and simple one step synthesis process where 2D TMD could be achieved by co-sputtering process of WSe2 and Se targets, followed by in-situ annealing within sputtering chamber. Meanwhile, a single sputtering process using only WSe2 target often causes considerable amount of Se vacancies and WO3 compound unnecessary. Our co-sputtering process could control effectively the stoichiometry of 2D WSe2, which was confirmed by XPS. Characterization analysis results of Raman, XRD, and XPS proved high quality 2D WSe2. We investigated reasonably excellent electrical properties of an electronic field effect transistor based on co-sputtered WSe2. Furthermore, simple annealing in ambient air would be able to form WOX-WSe2 heterostructure, leading to higher switching characteristics. Ultimately, the facile co-sputtering method will be widely applied to diverse 2D TMDs layered nanomaterials.

Resume : In the last few years the family of two-dimensional (2D) transition metal dichalcogenides (TMDs) has attracted increasing attention due to their unique optical, electronic and catalytic properties. In particular, WSe2 presents direct bandgap close to near infra-red range (1.65 eV), high charge carrier mobility and valley-spin degree of freedom that make it a promising candidate for advanced optoelectronic devices and spintronic applications. However, the fabrication of high quality monolayer films remains a major challenge. To date, monolayered isolated flakes and continuous polycrystalline films of WSe2 have been achieved employing either H2 in support of Se powder or H2Se as Se source in order to provide selenium in a reduced oxidation state for the formation of WSe2. However, both gases are sources of potential hazard for full-scale production. Here, we report the synthesis of high quality monolayer WSe2 starting from ZnSe powders as Se source. We have demonstrated the synthesis of a highly dense distribution of monolayered flakes without the use of H2 or H2Se. The grown materials present high optical properties characterized by sharp room temperature PL peak and low temperature PL features comparable to mechanical exfoliated flakes. The critical role played by ZnSe will be discussed in this presentation.

Resume : Tungsten disulfide (WS2) is an emerging 2D material with unique electrical and optical properties. Although chemical vapor deposition and mechanical exfoliation could lead to large area layers, the sonication solvent-based exfoliation is a convenient method to realize mono and few layer flakes. Various solutions as NMP and DMF are being used for the exfoliation of WS2 flakes. In this study we have investigated a mixture of Dimethyl-Sulfoxide (DMSO) and water as a safe, facile and user-friendly exfoliation solvent for synthesis of large-scale WS2 sheets. While water molecules weaken the Van der Waals force between the stacked layers, DMSO molecules facilitate the exfoliation process. To improve the exfoliation and achieve large area sheets, an oxygen plasma pre-treatment has been exploited on bulk WS2 crystals just prior to the exfoliation process. This strategy not only moderates the boiling-point of the solution, it improves the stability and yield of monolayer nanosheets which can be applied for various purposes. The effect of oxygen in the water has also been examined. SEM, TEM, AFM, Raman spectroscopy and DLS analyses have been employed to understand the mechanism of the exfoliation and to study the effects of various parameters as water temperature, duration and plasma power. Sheets as large as 1um have been obtained. Field effect transistors have been made using these sheets and relatively high carrier mobility of around 4-5 cm2/V.s has been extracted.

Resume : Many kinds of research have been dedicated to exploiting the unique functionalities of 2-D metal chalcogenides which can hardly be expected from bulk materials. A challenging task for implementation of 2-D metal chalcogenides in emerging devices is to synthesize the well-crystallized layer on large area substrates at low temperatures. SnS, a p-type layered semiconductor with high hole mobility, is a promising candidate for the realization of the thin film growth on a large-area at low temperatures because of its low melting point (882 ºC). Several techniques such as spray pyrolysis, chemical vapor transport, sulfurization, and e-beam evaporation have been introduced to synthesize 2-D SnS thin films. However, There are difficulties in synthesizing phase-pure SnS thin films because tin sulfides exist in various phases such as SnS, Sn3S4, Sn2S3, and SnS2. Here, we demonstrate a successful synthesis of single phase p-type SnS thin films using an atomic layer deposition (ALD) technique at low temperatures (< 240 ºC). The use of bis(1-dimethylamino-2-methyl-2propoxy)tin(II) as the Sn-precursor synthesizes single phase SnS(II) thin films at temperatures ranging from 90 ºC to 240 ºC, which is an exceptionally wide temperature window for an ALD process. The SnS grain size increases with increasing the growth temperature. It is also found out that the van der Waals interlayers of SnS are well aligned in parallel to the substrate at 240 ºC. Impurities such as carbon, oxygen, and nitrogen are negligibly detected in the SnS(II) films, and other phases such as Sn2S3 and SnS2 are not incorporated. Furthermore, we investigate the feasibility of the SnS(II) thin films as a functional material in emerging devices such as thin film transistors and gas sensors.

Resume : We designed and synthesized 2D and 3D thiadiazoloquinoxaline containing long N-Nanorribbons with a size approaching 11 nm. Crystal structure analysis demonstrated in-plane extension through close contacts of thiadiazoles and layered packing enabling in-plane and interlayer electron transport. Organic field-effect transistor devices provided electron mobilities, which supply a potential way to enhance the charge transport in long N-heteroacenes.

Resume : MoS2 ultrathin films have been deposited on Si and sapphire substrates with different deposition parameters by dc-magnetron sputtering. The influence of discharge power, argon pressure and substrate surface type on grain size, surface roughness, bandgap energy of MoS2 ultrathin films is discussed. The biggest grain size of MoS2 films structure was achieved at the lower discharge power (10W) and at the lower argon pressure (5×10^‒1 Pa). Spectrophotometer was used for detecting reflectance spectra and then for calculating thin films optical bandgap by using Tauc plot method. The MoS2 ultrathin films samples, deposited on Si substrates at the lower discharge power (10W) and the lower argon pressure (5×10^‒1 Pa) had the highest value of bandgap energy of about 1.5 eV.

Resume : Poly-Si thin films have received considerable attention due to their great potential for advanced electronic and photovoltaic applications. In particular, they are now drawing attention as a promising alternative for solar cells that exhibit a high conversion efficiency; consequently considerable research efforts have been devoted to the fabrication of high-quality poly-Si films with electrical properties that are suitable for low-cost photovoltaic applications. Aluminum (Al)-induced crystallization (AIC) of amorphous silicon (a-Si), wherein the growth of a poly-Si film is achieved simply via thermal annealing of a-Si/Al bilayers at temperatures below the eutectic temperature of the Al–Si alloy system, is known as a method for fabricating a poly-Si film at low cost. Regarding the formation of poly-Si film by AIC, when SiOx layer is used as a source material for poly-Si formation instead of a-Si layer, it is possible to reduce the production cost. Here, we report the synthesis of poly-Si films via the crystallization of SiOx films with different values of x using a thin Al layer as a catalyst. The fabrication of poly-Si films was achieved by thermal annealing of a SiOx/Al/glass layered structure. The resulting poly-Si layer was found to exhibit a highly preferential (111) orientation. In particular, poly-Si has been found to be more effectively formed as x value increases. The growth of the poly-Si film is also attributable to the layer exchange mechanism.

Resume : Among the proposed routes to synthesize MoS2 nanosheets, chemical vapor deposition (CVD) based approach is gaining consideration because of the good balance between relatively low installation and running costs, and the rich flexibility in terms of process parameters tuning and sample surface sizing and conditioning. We grew MoS2 nanosheets by two CVD methods: (i) the Solid Precursor Film (SPF) approach [1,2] where metal oxide thin film is sulfurized in a single thermal zone furnace and (ii) the Vapour Phase Reaction (VPR) approach [3] where solid powder precursors react in a multiple zones furnace. SiO2/Si substrates are used as is or pre-treated with organic molecules, namely perylenes [4]. The so-grown mono or few-layered MoS2 sheets are characterized by several techniques such as Raman spectroscopy, SEM, XPS, AFM, Kelvin probe microscopy. By combining the results from the analyses and the growth conditions, we could rationale peculiar MoS2 features, such as the number of MoS2 sheets, the shape and size of the domains or the degree of granularity, the size/amount of substrate area covered by MoS2. Selected films were patterned by lithography methods for electrical testing to extract transport characteristics, such as mobility and carrier concentration. [1] S. Vangelista et al., Nanotechnology 2016, 27, 175703 [2] C. Martella et al., Adv. Electron. Mater. 2016, 2, 1600330 [3] C. Martella et al., Adv. Mater. 2018, 30, 1705615 [4] K.K.H. Smithe et al., 2D Mater. 2017, 4, 011009

Resume : A growth technique to directly prepare 2D materials onto conventional semiconductor substrates, enabling low-temperature and large-area capability, is strongly demanded to realize highly functional 2D-3D heterojunction devices.1 Herein, we demonstrated plasma-sputtering technique which could enable effectively deposition of MoS2 multilayers directly on 4-inch Si substrates at relatively low temperatures of <400 °C. The as-fabricated MoS2/Si heterojunctions exhibited large and fast photocurrent responses under illumination condition with visible range wavelength. The measured photocurrent was linearly proportional to the laser power, indicating that trapping and detrapping of the photogenerated carriers at defect states could not significantly suppress the collection of photocarriers. All the results demonstrated that our sputtering method could produce high-quality TMD/Si 2D–3D heterojunctions for optoelectronic applications.

Resume : We report the formation of nickel-incorporated silicon sheets using hydrogen plasma during thermal treatment of nickel sheets. Originally, a 4-8 nm nickel sheet is deposited on (100) or (111) silicon substrate using e-beam evaporation at 150oC. Later on, a hydrogen plasma treatment at 400oC to 600oC is carried out in a sequential manner. During this critical step, the sample is exposed to trace values of CH4 as a source for carbon incorporation and grain formation. As a result, anomalous in-depth or lateral diffusion of nickel through silicon substrate has been observed. Depending on the type of the silicon substrates, whether (100) or (111), in-depth or planar sheets are evolved with applications for nano-channels as well as light emitting diodes. In-depth nano-channels with a depth of 500 nm and width of 50-100 nm have been realized while the length of channel is more than 100 µm. Apart from channel formation, we have observed silicon-based quantum dots of nickel within the silicon sheets using TEM analysis. The physical and morphological properties of the films have been investigated using AFM, SEM, Raman spectroscopy and XPS analysis. Photoluminescence spectroscopy shows the evolution of direct band gap material suitable for light emitting diode formation. The presence of a hydrocarbon gas during the plasma treatment is found critical for the in-depth diffusion of nickel in the substrate and for the formation of nickel quantum dots.

Resume : Molybdenum disulphide (MoS2) is a naturally occurring mineral known in engineering for its high-temperature lubrication properties. However, since the emergence of 2D-graphene material, the interest in the MoS2 and other transition metal dichalcogenide (TMD) materials have grown for exploring optoelectronic and photonic properties, as these layered materials on sub-nanometre scale offer opportunities to explore quantum interaction [1]. The electronic structure and stoichiometry of TMDs make them distinguishable from the metallic graphene, as the TMDs depict a clear bandgap, as in compound semiconductors [2], which is quite attractive for device engineering and applications in photovoltaic, energy storage, and bandgap engineered light-sources [3]. Recently, the 2D layered materials grown using pulsed laser deposition technique have become attractive in a wide range of device applications [4]. Here, we present an ultrashort pulsed laser deposition of undoped and rare-earth Yb3+-ion doped MoS2 films. For fabricating nanometer-scale PLD films of MoS2. The source laser was a Ti-sapphire laser mode-locked at 800 nm wavelength, with a pulse duration of 80 fs and a repetition rate of 1 KHz. The deposition parameters for doped and undoped thin films, grown on a silica substrate, was investigated by controlling and optimizing the process parameter, namely the laser fluence 3 J/cm2, argon (Ar) gas pressure inside the chamber at 10 mTorr and substrate-to-target distance at 50 mm. The characterization of undoped and doped MoS2 films were carried out using the Raman, FTIR, UV-Vis spectroscopy, transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques, which enabled us to analyse the materials structure, electronic states and phonon vibration spectrum, essential for optoelectronic and photonic device engineering. The electronic structure of molybdenum disulfide (MoS2) have been investigated through first-principles calculations using density functional theory (DFT). We have also investigated the photoluminescence and non-linear optical properties. The later was characterized using the open aperture Z-scan technique. For Z-scan measurements, a 532 nm laser source (frequency doubled YAG) with 7 ns pulse duration and 10 Hz repetition rate was used to excite the MoS2 films, the enhancement in saturable absorption (SA) at room temperature was observed. Our results on fs-pulsed laser deposition technique offers an opportunity for materials fabrication and device engineering using novel multifunction rare-earth doped 2D materials. References: 1. Mak Fai Kin, et al., Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides, Nature Photonics 10, 216-226 (2016). 2. Chhowalla. M, et al., Two-dimensional transition metal dichalcogenides (TMD) nanosheets, Chem Soc. Rev. 44, 9, 2584 (2015). 3. S. Manzeli, et al., 2D transition metal dichalcogenides, Nat. Rev. Mat. 2, 17033 (2017). 4. Yang. Z, et al., Progress in pulsed laser deposited two-dimensional layered materials for device applications, J. Mat. Chem. C. 4, 38 (2016).

Resume : Toward any practical application of 2D transition metal dichalcogenide (TMD) transistors, still many issues exist. Among them, first, instability issue generated from the interface between TMD/gate dielectric: gate voltage-hysteresis is unavoidable due to the defects and charge traps at the metal/channel or dielectric/channel interface. This is certainly undesirable in practical circuit applications (i.e. operating voltage issue). Second, source/drain metal contact is not usually ohmic due to Fermi-level pinning. Consequently, this non-ohmic characteristic between TMD semiconductor and contact metal leads to large contact resistance, and degraded performance (low field effect mobilities). In this talk, I’ll present two potential methods to circumvent these issues. Regarding the first issue, ultrathin polystyrene‐brush layer, which is robust to conventional photo-lithography process, is applied between TMD channel and 50 nm thin Al2O3 dielectric. p‐channel MoTe2 field-effect transistors (FETs) exhibit a minimum voltage hysteresis of less than 1 V. For second issue, facile oxygen plasma process is introduced onto the p-type semiconducting MoTe2 channel. Consequently, high-workfunction MoOx is induced, and serves as efficient hole-injection material for the p-type MoTe2 FETs. The on-state drain current of O2 plasma‐processed MoTe2 FET is higher than that of the pristine MoTe2 FET, and its linear mobility appears 2.5 times higher. In particular, ultrathin Pt/indium‐tin‐oxide (ITO) contact are applied onto the oxygen plasma induced MoOx to demonstrate world-first fully transparent p-type TMD transistor.

Resume : Transition metal dichalcogenides (TMDs) are typical two-dimensional (2D) atomic layered compounds which can be obtained by the exfoliation from their bulk three dimensional materials or by direct synthesis using chemical vapour deposition (CVD) technique. Despite of the progresses made during the last years, the CVD synthesis of large area 2D-samples is still a challenging to be mandatorily faced in view of their use in optoelectronic devices. With this regard micro-Raman spectroscopy turns out to be a versatile, non-destructive and fast tool for studying 2D materials and for investigating the distribution of defects within these layered materials. Here we report on the atmospheric pressure chemical vapour deposition (APCVD) synthesis of crystalline MoS2 layers grown on the top of SiO2/Si (100) and their spectroscopic characterization. The synthesis was carried in a two stage furnace in Argon flux of 100 sccm at 750 oC, using MoO3 powder and S powder as precursors. Optical and AFM microscopies revealed that the average size of the crystals was 30 micron. Micro-Raman measurements carried out under different polarization settings confirm the high quality of these crystals, either in form of single-layer or few-layers, showing an excellent selection of the phonon modes with different symmetry. Moreover, the analysis of their low-wavenumber Raman scattering allows for detection of diverse interlayer modes observed from regions with different thickness and, also, for their unambiguous assignment in symmetry.

Resume : Large area homogeneous growth of transition metal dichalcogenide monolayers is still a challenging issue. Here, we present the growth of monolayer MoS2 via a facile vapor phase-assisted reaction sequence [1]. Starting with ultrathin MoO3-x precursor layers deposited on Si, SiO2, and Sapphire substrates, respectively, we applied varying temperature profiles with delayed H2S injection. Competing reaction pathways, including solid-gas- and vapor-gas-reactions, are controlled using an optimized combination of process temperature, MoO3-x precursor thickness, and H2S injection delay. XPS analyses of the samples at various stages of the process corroborate the occurrence of vapor-phase reactions at high process temperatures. The influence of the processing parameters on the surface coverage ratio, layer thickness, and homogeneity of MoS2 are discussed. Raman and photoluminescence (PL) spectroscopy confirm the formation of ultrathin MoS2 films with a narrow thickness distribution in the monolayer range on length scales of a few millimeters. The obtained PL yield is comparable to reference measurements on mechanically exfoliated monolayer flakes, highlighting the optical quality of the grown layers. [1] D. Pareek et al. RSC Adv. 9, 107 (2019).

Resume : Although polycrystalline hexagonal boron nitride (PC-hBN) has been realized, defects and grain boundaries still cause charge scatterings and trap sites, impeding high-performance electronics. Here, we report a method of synthesizing wafer-scale single-crystalline hBN (SC-hBN) monolayer films by chemical vapor deposition. The limited solubility of boron (B) and nitrogen (N) atoms in liquid gold promotes high diffusion of adatoms on the surface of liquid at high temperature to provoke the circular hBN grains. These further evolve into closely packed unimodal grains by means of self-collimation of B and N edges inherited by electrostatic interaction between grains, eventually forming an SC-hBN film on a wafer scale. This SC-hBN film also allows for the synthesis of wafer-scale graphene/hBN heterostructure and single-crystalline tungsten disulfide.

Resume : Progressive miniaturization in electronics is approaching the limits of bulk semiconductor materials. Therefore, there is a strong trend in research and industry towards the integration of novel materials for further device and system level improvements. In this connection, various two-dimensional (2D), as well as one-dimensional materials have been investigated. One of them, Silicon nanosheets (SiNSs) - a 2D semiconducting material - may have interesting optical and electronic properties. In this work, SiNSs are synthesized via chemical exfoliation from CaSi2, where the Ca layers are dissolved in concentrated HCl. After HF etching, functionalization of the H-terminated SiNSs is achieved which enhances the solubility in solvents. But SiNSs degrade within a few minutes if exposed to oxygen. For applications, it is a key requirement to protect SINSs efficiently from degradation. We present solution-based approaches such as drop- and spin-coating to encapsulate the SiNSs by using Polydimethylsiloxan (PDMS), Polymethylmethacrylat (PMMA), ZnO, and another 2D material, h-BN. A further route to protect SiNSs from oxidation is a blend fabrication with for example PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) or P3HT (Poly(3-hexylthiophene)). The aforementioned encapsulation approaches are compared and evaluated with respect to the implementation into electronic devices such as field effect transistors.

Resume : Effect of gamma irradiation on chemical vapor deposition (CVD) synthesized monolayer (1L) molybdenum disulfide (MoS2) flakes on SiO2/Si and sapphire substrates were studied at various doses, viz., 1-1000 kGy. Irradiated MoS2 flakes were characterized by optical microscopy (OM) and Raman spectroscopy. It was observed that up to irradiation dose of 100 kGy, no significant changes were observed. But above the dose value of 100 kGy, damage in flakes was observed by OM. Raman spectroscopy revealed that high irradiation dose affected the position of first-order Raman modes of MoS2, viz., E12g and A1g. Up to 500 kGy, E12g and A1g modes show blue shift while above a red shift. Intensity (counts) of Raman spectra decreases continuously as irradiation dose increases. This shows the degradation of flakes. Above 100 kGy, Raman peak at 230 cm-1 was observed and its intensity increased as the irradiation dose increased. This peak corresponds to MoOS2 and MoO2 which indicates the oxidation of MoS2 at high gamma dose values. Our findings show how Raman spectroscopy can be used as a non-destructive technique to measure the radiation hardness of MoS2 material and devices fabricated over it.

Resume : 2D nanomaterial graphene is at the top of the significant interest due to the giant electron and hole mobility, charge carrier multiplication, flexibility, optical transparency, chemical inertness. One of the hindrances stopping the wider application of the graphene in semiconductor device technology is complex procedure used for fabrication of the graphene layers by using graphene transfer. Graphene usually is synthesized by chemical vapor deposition on Cu or Ni catalytic foils. Long process of the graphene transfer onto the targeted semiconductor or dielectric substrates follows. During that process, graphene can be contaminated by different adsorbents. Wrinkles or ripples may appear on graphene. In such a case control of the graphene layer or graphene-semiconductor contact properties is complicated. Recently there were shown that tranfer-free and catalyst-free synthesis of the graphene on semiconducting or dielectric substrates is possible. However, such a research is at the very beginning. In present study graphene layer were directly synthesized by microwave plasma enhanced chemical vapor deposition on monocrystalline Si substrates and SiO2 films. Nitrogen doping of the directly synthesized graphene layers was considered. Structure, composition and morphology of the films was investigated by Raman scattering spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy. Deposition temperature, plasma power, synthesis time and pressure effects on the structure of the directly synthesized graphene layers were studied. Composition and structure of the nitrogen doped graphene were investigated. Effects of the synthesis conditions on orientation of the graphene flakes (vertical or horizontal) were studied.

Resume : Here, we proposed controlled growth of vertical MoS2 flakes and their utilization in developing fast and highly recover H2 and NO2 gas sensor. Controlled growth of vertical MoS2 was synthesized by modified atmospheric chemical vapor deposition (CVD) technique on SiO2/Si substrate. The detailed characterization analysis reveals that in-plane MoS2 was found to work as seed layer for initial growth of vertical MoS2 and leads to growth of an interconnected vertical MoS2 flakes with increased gas flow rate. Hydrogen gas sensor was developed on highly uniform vertical MoS2 flakes for 1 % concentration of H2 gas in temperature range of 28-150 °C. The lowest response time (14 s) and fast recovery (108 s) for bare MoS2 flakes was obtained with sensitivity increases from 1% to 11% as temperature increases. The role of MoS2 edges verified by depositing thin ZnO layer (2-3 nm) on vertical MoS2. We found a decrease in relative response of MoS2-ZnO hybrid structures. This study provides a strong experimental evidence for role of MoS2 edge-sites in developing RT, fast and low power (0.3 mW) hydrogen sensor. The photoactivated NO2 sensor developed by mixed in-plane and vertical p-MoS2 flakes (mixed MoS2). The sensor showed fast response with sensitivity of ~10.36 % for 10 ppm of NO2 at RT without complete recovery. The UV assisted NO2 sensing showed an improved performance in term of fast response and complete recovery kinetics with enhanced sensitivity to 10 ppm NO2 concentration.

Resume : Transition metal dichalcogenides like WS2 have attracted considerable attention because of their interesting physical properties and potential applications in various opto-electronic devices. In this study, WS2 monolayers were grown by chemical vapor deposition (CVD) method on a Si/SiO2 substrate using WO3 and S precursors and various temperature regimes of the two-zone furnace. In addition, the relative content of the N2 and H2 in the carrier gas mixture and flow rate were modified. Largest WS2 monolayer domains showing the highest photoluminescence emission intensity were obtained by using temperatures 200 oC and 850 oC in the sulfur and WO3 zone of the furnace, and 9% of H2 in the mixed gas. The resulted WS2 monolayers were mostly of triangular shape. WS2 monolayers grown under various conditions were characterized using Raman scattering, room-temperature photoluminescence (PL) and reflectance contrast (RC) analysis. The uniformity of single WS2 monolayer domains was studied using PL images. The intensity of the A band in the PL spectra was used to evaluate the quality monolayers. The A band peak maximum was found to vary for different WS2 monolayer domains, indicating different tensile strain values in CVD grown WS2 monolayers. The influence of the growth conditions on the exciton emission in WS2 monolayers is discussed.

Resume : Two-dimensional (2-D) metal chalcogenides have recently received great attention because of their unique characteristics. The industrially compatible development of these emerging materials is indispensable to facilitate the transition of 2-D metal dichalcogenides from the research stage to the practical industrial application stage. A critical challenge in employing 2-D metal chalcogenides in emerging devices is how to synthesize a high-quality material over large areas at temperatures compatible with current fabrication processes. Although chemical vapor deposition (CVD) has achieved great advances in the synthesis of high-quality 2-D materials, the high process temperature and the poor process tolerance in the CVD process remain critical obstacles in industrial applications. Atomic layer deposition could be a promising growth technique because of the wide tolerance for thermodynamic parameters such as dose of precursors. Here, we study low-temperature synthesis of SnSx films via the ALD technique. Tin sulfides are interesting layered materials. SnS2 and SnS, the representative materials, are n- and p-type semiconductors, respectively, and both materials have a layered structure. Additionally, the low melting point (< 900 oC) of the SnSx enables low-temperature growth. We demonstrate the phase selection of the SnSx at low temperatures (< 300 oC) The growth technologies show excellent uniformity over large areas for both materials. Based on our ability, we fabricate NMOS and PMOS transistors using SnS2 and SnS. We believe that this work would be an important step toward the incorporation of 2D metal chalcogenides in industry.

Resume : Transition Metal Dichalcogenides (TMDs) have been one of the most studied class of two-dimensional materials. Their potential applications cover a wide range of possibilities, such as sensors, opto- and nano-electronics. Nonetheless, a large-area growth technique capable of integrating them with the modern technology processes is still missing. In this work we characterise uniform thin films of PtSe2 grown by Thermal Assisted Conversion over a large area. For this process the metal initially deposited on the substrate is converted at 400oC, making it compatible with back-end-of-line processing. Several studies showed how a Forming Gas Anneal (FGA) can improve TMD-based devices, because of a better interface between the metal and the oxide. Usually, due to the lack of a proper growth technique systematic experiments are difficult due to high variability. Here the samples were grown over a large area and the variation is minimised. Then, we applied different FGA steps in 5/95% H2/N2 at 150oC, 250oC and 350oC. The samples are electrically characterised by circular Transfer Length Method structures and contact resistance and sheet resistance are analysed. The results are supported by a combination of X-Ray Photoelectron Spectroscopy, Raman spectroscopy, Energy-Dispersive X-Ray Spectroscopy and Cross-sectional Transmission Electron Microscopy. Considering similar samples the same annealing conditions will be repeated in an inert environment to consider the impact of the forming gas.

Resume : Possessing outstanding anisotropy in companion with direct optical band gap in all thicknesses turned phosphorene to a reliable material in electronics and optoelectronics. We report a novel technique to realize two-dimensional phosphorene layers directly on silicon substrates using plasma treatment and Ultraviolet (UV) irradiation. The formation of such layers is a through phase transition from amorphous red phosphorus (RP) to the layered crystal black phosphorus (BP). The energy imparted from the Hydrogen plasma or UV source results in the arrangement of phosphorene sheets with no need for any high temperature, high pressure or day-long procedures. Characterization analyses including SEM, TEM, Raman spectroscopy, AFM, EDS and electron diffraction pattern have been used to examine these layers. These flakes have been employed to fabricate a photodetector. The electrical and optical response of the device is promising, further indicating the evolution of well-crystalline phosphorene layers. In addition, a laser source has been employed to improve the crystalline quality of the films and to achieve large area sheets. This would be a great step for direct device-fabrication that completely bypasses exfoliation and transfer steps. Controlling various parameters as the source power, temperature and the environment gas pressure would have direct effect on the size and quality of the sheets. Fabrication of the field effect transistor is underway.

Resume : Transition-metal dichalcogenides (TMDCs), which can form stable three-atom-thick monolayers, are emergent semiconducting materials with a direct band gap, high charge carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance optoelectronic and nanoelectronic devices on a technologically relevant scale. However, existing growth methods such as CVD can provide the semiconductor quality of 2D TMDCs layers with lateral size up to ~100-1000 µm only. Taking into account that ALD is a well-established technique for large-scale uniform deposition of metal oxides with atomic level accuracy here introduce two-stage MoS2 growth process on a large-scale substrates (~45 cm2): the first one is an ALD growth of ultrathin and uniform MoO3 at low (~165C) temperatures using Mo(CO)6/O* process. In situ XPS studies allowed to define MoO3 purity and minimal thickness of continues film achievement. The applicability of H* active species for MoO3 to MoO2 reduction was evaluated using in situ XPS too. The second stage is MoO3/MoO2 nanolayers sulfurization under 800-1000C using sulfur or H2S in Ar atmosphere. As a result 2, 3 and 4 monolayer wafer-scale MoS2 films with high stoichiometry (XPS), crystallinity (Raman spectroscopy), smoothness (AFM) and high carrier mobility were obtained.

Resume : In this paper, we report simple, scalable and high-yield production of MoS2 and WS2 nanosheets through solid phase exfoliation using ball milling in the presence of sodium cholate (SC) [1]. The exfoliated MoS2 and WS2 nanosheets are stored as ?stock solid? and readily dispersed in water simply by shaking with hand prior to use. While solid phase exfoliation using ball milling has been applied to the production of graphene nanosheets, this methodology has not been demonstrated in the exfoliation of TMDs such as MoS2 and WS2. Although we have reported scalable method using hexahydroxytriphenylene as an exfoliant through ball milling, bath sonication for half an hour has been required to obtain stable dispersion [2]. As compared with wet-grinding and liquid phase ball milling, the dry process reported here is considered to be more preferable to keep the solid as it is for longer time, because the presence of liquid may facilitate aggregation as mentioned above. In addition, simple dispersion with controlled concentration can be prepared from the dry ball milled solid, while the liquid used for exfoliation under wet conditions may make solvent system complicate, and concentration less precise and less controllable. First, the effect of surfactant amount in the ball milling process was studied; the amount of MoS2 was fixed at 0.20 g and the amount of SC was varied from 0.010 to 0.40 g. The powder obtained after ball milling was dispersed in deionized water (100 mL) and the resulting dark-greenish suspension was centrifuged at 3000 rpm (1025g) for 60 min. The top 75% of the supernatant was subjected to UV/vis spectroscopic analysis. The yield (%), the mean number of layers, and lengths of the napnosheets can bewere calculated estimated using the extinction and the reported coefficient at 345 nmreporte, the wavelength of the exciton peak (A), and the ratio of the extinction at 605 nm and 345 nm (Ext605/Ext345), respectivelyof MoS2 in the dispersion was calculated by the extinction at 345 nm and the extinction coefficient reported. The yield of MoS2 increased according to the increase of the weight ratio of SC/MoS2. The yield of 9% obtained at the SC/MoS2 weight ratio of 2 is much larger than that obtained by liquid phase sonication. The yield of MoS2 saturated around 0.40 g of SC. In addition, the mean number of layers and lengths of nanosheets can be estimated using the wavelength of the exciton peak at 658 (A) and Ext605/Ext345 ratio, respectively. The morphology of MoS2 nanosheets was determined to be two layers and 60 - 80 nm in size, from the A of MoS2 observed at 658 nm and the value of Ext605/Ext345. [1] G. Liu, N. Komatsu* ChemNanoMat, 2 (6), 500 - 503 (2016) [highlighted at the front cover]. [2] G. Liu, N. Komatsu* ChemPhysChem, 17 (11), 1557?1567 (2016) [highlighted at the front cover].

Resume : A major challenge for the production of 2D materials is the fabrication of large areas of film without grain boundaries and with defined layer thickness. To improve the growth of 2D materials it is vital to have a tool that allows for large area characterization of these films. Often, characterization methods are either very indirect or have extremely small fields of view. We demonstrate a modified scanning electron microscope with a homemade transmission diffraction stage that allows mapping the structure of 2D films over large areas by automatically collecting thousands of electron diffraction patterns at beam energies of down to 5 keV. Damage-free observations of large areas of 2D films (80x80µm2 demonstrated, mm2 should be possible) can be facilitated to determine e.g. local layer thickness, orientation and strain from each of the acquired electron diffraction patterns and thus leads to large maps that depict regions of mono layers, grain boundaries or strain distribution. Mapping the structure of 2D materials through a support film is possible with this setup, because the slow electrons lead to strong diffraction of even a mono-layer of material. This can be utilized to map strain distribution in a 2D film that is partly free-standing and partly supported. We demonstrate that a typical transfer procedure can lead to inhomogeneous strain in a 2D film, which is difficult to measure with any other technique.

Resume : Ultrathin graphitic carbon nitride (g-C3N4) is prepared by exfoliation of bulk g-C3N4 (derived from urea) using isopropyl alcohol and used as an adsorbent for aqueous La3 , Ce3 and Gd3 . Adsorption capacity of ultrathin g-C3N4 is found to be influenced by initial concentration, solution pH, contact time and temperatures (283K to 323K). Adsorbed ions are separated from ultrathin g-C3N4 by ultracentrifuge. Adsorption kinetics is studied by using first and second order kinetic model (Langmuir and Freundlich isotherm model). Initial and La3 , Ce3 and Gd3 ions accumulated adsorbent are characterized by X-ray diffraction technique (XRD, UV-visible, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy, scanning electron microscopy (SEM) along with energy dispersive X-ray spectroscopy (EDS) for elemental mapping. Valence band of C1s, N1s, La3d, Ce3d and Gd3d are observed through X-ray photoelectron spectroscopy (XPS). Specific surface area and pore characteristics of adsorbent are measured by BET analysis. The thickness of initial and rare earth ions adsorbed two dimensional nanosheets are measured by atomic force microscopy (AFM). AFM surface profile shows that the ions are adsorbed physically on ultrathin g-C3N4 surface and increased the average thickness of nanosheets. The photoluminescence spectra are recorded for the both adsorbent and ions accumulated adsorbent. HNO3 (0.1M), NaOH (0.1M) and de-ionized water are used for desorption and around 98% quantitative recovery of rare earth ions are observed.

Resume : Atomically thin, two-dimensional silicane is a promising new material that is being investigated for future applications, such as field effect transistors or photonic sensors. Structurally, it can be thought of as the silicon counterpart to graphene. Unlike graphene, however, the silicon atoms are sp3 hybridized and hydrogenated, thus offering a reactive moiety. This can be seen as both a positive and negative, as the reactivity of the Si–H bonds leads to oxidation, but also offers possibilities for surface functionalization, which improves properties such as dispersibility and stability, could allow band-gap engineering and enables tailoring the surface for specific applications. Recently, we were able to show that diaryliodonium salts can be used to functionalize the silicane surface by initiating the hydrosilylation reaction with a variety of alkenes or alkynes. The silicane/diaryliodonium salt system is also able to initiate cationic and radical polymerizations. The silicon nanomaterial acts as a co-initiator, inducing the decomposition of the diaryliodonium salt. The decomposition products and reactive surface groups, in turn, are able to initiate both cationic and radical polymerizations thereby enabling a mild and straightforward reaction procedure to obtain a variety of functionalized silicane/polymer composites. These composites improve the ambient stability of the silicon nanosheets, enable simpler processing and facilitate subsequent device fabrication.

Resume : Hexagonal boron nitride (h-BN) is a unique 2D III nitride material system. It can find interesting application in the field of electronics and opto-electronics because of its properties such as graphite-like structure, high thermal conductivity and a wide bandgap (~ 6eV) [1-3]. However, recently the electron-hole pair recombination in bandgap of h-BN is found to be of indirect nature by both theoretical calculations and experiments, [4,5] which motivates to explore new class of alloy in the nitride group. On the other hand, aluminium nitride (AlN) is a well-known III-nitride direct bandgap semiconductor (~ 6.1eV). Forming, Al diluted BN alloys system may induce direct band transition and may result in better conductivity control. Boron rich BAlN ternary alloy system has not yet been explored since growth of single crystalline alloy may be challenging because both materials crystallize in different crystal system. In this study, we report, MOCVD growth of single-phase boron rich BAlN 2D layers with wrinkles. SEM images showed 2D wrinkles on the surface of BAlN compared to those obtained on standard 2D h-BN. The composition of Al was measured by SIMS analysis and Al incorporation was up to 8%. HRXRD and SEM were used to study the structural characteristics of the samples. The 002 peak of the h-BN shifted to lower 2 theta-omega indicating that there is change in strain state due to Al incorporation. Further, interesting optical characteristics of these samples in comparison with the pristine h-BN will be presented. References: [1] Shim et al., Science, 362, 665 (2018) [2] Li et al., Crystal Growth & Design, 16, 3409 (2016) [3] Ayari et al., APL, 108, 171106 (2016) [4] Arnaud et al., PRL, 96, 026402 (2006). [5] Cassabois et al., Nature Photonics, 10, 262 (2016).

Resume : Graphene grown using chemical vapor deposition (CVD) on metal substrates, especially on metals with low C solubility such as Cu, is a viable method to mass produce large-area graphene with relatively high quality. However, its growth mechanisms are not yet fully understood. An important growth mode, that is, the screw-dislocation driven growth, which occurs naturally in crystals, including graphite, is still surprisingly lacking in the literatures. Here, we demonstrate that the growth of few-layer graphene (FLG) with spiral structure can be obtained using a simple Cu-catalyzed ambient pressure CVD. By performing micro-Raman characterization, we discuss and compare the two different growth modes of FLG that coexist under low supersaturation conditions: (i) screw-dislocation driven growth of FLG spiral and (ii) concentrically-stacked FLG formed by nucleation at the graphene/Cu interface. The FLGs with different structures poses significantly differing properties. The unique interlayer coupling of FLG spirals which enable superior conductivity along the normal of the two-dimensional crystal with helical trajectories are expected to have new and interesting nanoscale applications.

Resume : It was recently discovered that chemical vapor deposition (CVD) of CH4 on Ge(110) can directly yield wafer-scale single-crystal monolayer graphene with high quality. However, graphene film has no bandgap, make it difficult to apply in switching devices which need a high on/off ratio. Narrow graphene nanoribbons are excellent substitutes which can induce a bandgap without degrading carrier mobility of graphene. This paper mainly talks about the process of synthesis of graphene nanoribbon arrays and some characteristics methods. Firstly, single-crystal graphene film was synthesized on germanium via CVD method. And then, Ni nanoparticles, which act as catalyst for the etching reaction, was deposited onto the graphene film by electron beam evaporation. The graphene with deposited Ni nanoparticles need be sent to an annealing procedure in two steps with Ar/H2 gas flow: annealing at 500 °C for 30 min, which resulted Ni nanoparticles on graphene surface, and then etching at 900-930 °C for 60 min. The effect of evaporation conditions on the size of Ni nanoparticles was investigated. Etching temperature is also an unnegligible factor in formation of graphene nanoribbon arrays. The morphology, aspect ratio, edge type and bandgap width of graphene nanoribbons were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and scanning tunnelling microscope (STM). Quality of graphene nanoribbons were measured by Raman spectroscopy compared to graphene film. For the carrier mobility and on/off ratio measurements, filed-effect transistors (FETs) were fabricated with graphene nanoribbon arrays transferred onto a SiO2/Si substrate. The results show that, induce of a bandgap is heavily dependent on a narrow width (<10 nm) and a high aspect ratio of graphene nanoribbon, which were affected by the size of Ni nanoparticles. And synthesis of high density of graphene nanoribbon arrays in our method is convenient and serve as a powerful technology to promote the development of graphene nanocircuits.

Resume : Considering plenty of extraordinary properties superior to commercially available materials, graphene has great potential in many application fields. The facile and scalable synthesis of patterned graphene is still a challenge for massive applications in flexible and optical electronics. However, conventional fabrication of graphene pattern inevitably involves lithography process and gel residues result in a poor performance of graphene-based devices. Here we introduce the direct growth of high-quality patterned graphene on Germanium substrate where selective areas are passivated by aluminum oxide. We choose Germanium (110) as growth substrate because it has been demonstrated that graphene growth on Ge (110) surface is uniform single-crystal due to unidirectional grain orientation. A barrier layer of aluminum oxide is grown on Ge (110) samples by ALD and designed patterns are printed using standard lithography. Solution etching is adopted to remove Al2O3 areas not covered by gel and Ge (110) underneath appears with good crystal lattice. High quality monolayer graphene pattern is synthesized on such a Ge surface during CVD growth while graphene cannot be achieved on aluminum oxide barrier layer. Several tests are carried out to measure the properties of patterned graphene. Raman spectroscopy shows no obvious D peak and the intensity ratio I2D/IG is in the range of 1.5-2. AFM height results prove that patterned graphene is monolayer. Transmission electron microscopy images reveal that well-defined monolayer graphene is formed and no structural defects are observed in every graphene pattern. The selected area electron diffraction images acquired from different patterns all display a single crystalline lattice structure of graphene. Compared with the conventional graphene pattern process, which means that graphene is transferred to target substrate and shaped by lithography, graphene pattern fabricated by our method has less defects and better electronic properties. The proposed method for directly patterned graphene growth is expected to have great vision in novel electronic applications.

Resume : Chemical vapor deposition (CVD) of two-dimensional (2D) transition metal dichalcogenides (TMDs) always involves the conversion of vapor precursors to solid products in a vapor-solid-solid (VSS) growth mode (e.g., WO3 + S + H2 → WS2 + SO2 + H2O). This often requires very high temperature to sublimate the metal oxides (e.g., WO3, Nb2O5,). Since our pioneering work on salt-assisted CVD of 2D WS2 and WSe2 monolayers, [1] tens of papers had been published on this topic and proved that salts (MX, M = Li, Na, K; X = Cl, Br, I) can facilitate the growth of ~50 TMDs at lower temperatures by forming volatile metal oxyhalide (e.g., WO3 + NaCl → WO2Cl2). However, the functionality of alkali metal (e.g., Li, Na, K) in the salt-assisted CVD has long been ignored. Our new results show that Na leads to a distinct vapor-liquid-solid (VLS) growth mode by forming non-volatile Na-Mo-O liquid droplets (e.g., MoO3 + NaCl → Na2Mo2O7) at the growth temperatures of 700-750 oC. [2] The VLS growth shows great advantageous in wafer-scale growth of monolayer 2D TMD films and site-controlled growth of 2D TMD flakes. And thus, the VLS growth opens new research direction for the 2D community. References: [1] S. Li, S. Wang, D.-M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg, G. Eda,* Appl. Mater. Today 1, 60-66 (2015) [2] S. Li,* Y.-C. Lin, W. Zhao, J. Wu, Z. Wang, Z. Hu, Y. Shen, D.-M. Tang, J. Wang, Q. Zhang, H. Zhu, L. Chu, W. Zhao, C. Liu, Z. Sun, T. Taniguchi, M. Osada, W. Chen, Q. Xu, A. T. S. Wee, K. Suenaga, F. Ding, G. Eda,* Nat. Mater. 17, 535-542 (2018)

Resume : In order to realize competitive 2D TMDs/3D semiconductor heterojunction device, a growth technique is required that directly synthesizes 2D material on conventional semiconductor substrates, enabling low temperature, high throughput and large area performance. In this study, we successfully developed atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) process that could directly grow MoS2 and WS2 multilayers on flexible PET substrates as well as 4-inch Si substrates at temperatures below 200 °C. The fabricated MoS2/Si and WS2/Si heterojunctions showed large and fast photocurrent responses under green light irradiation. The measured photocurrent was linearly proportional to the light power and signifies that trapping and de-trapping of the photogenerated carriers in defect states could not significantly suppress collection of the photocarrier. All the results demonstrated that our AP-PECVD process could successfully produce high-quality 3D-TMDs/3D-Si heterojunctions for optoelectronic applications.[1] Reference Yonghun Kim, Soyeong Kwon, Eun-Joo Seo, Jae Hyeon Nam, Hye Yeon Jang, Se-Hun Kwon, Jung-Dae Kwon, Dong-Wook Kim, and Byungjin Cho, “Facile Fabrication of a Two-Dimensional TMD/Si Heterojunction Photodiode by Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition” ACS Appl. Mater. Interfaces 10, 36136 (2018).

Resume : Over the last few years, graphene quantum dots have shown immense potential especially in the field of optoelectronic and biological applications primarily due to tunable photoluminescence properties. However, recent trends also suggest that there is a lot to be desired when it comes to tunability of the material when functionalized appropriately with the right element or the functional group to exhibit a lot of other interesting properties for magnetic and energy transfer related application in conjunction in other 2D materials such as few layer MoS2 sheets to be used as optical sensors. In this work, we highlight a couple of such possibilities in chemically prepared amino-functionalized graphene quantum dots realised experimentally as well as using ab-initio simulations.

Resume : Monolayer transition metal dichalcogenides (TMDs) have demonstrated great potential in next-generation electronics due to their unique optical and electronic properties. However, it remains challenging to produce uniform high-quality TMDs over a large scale. The direct sulfurization method holds great promise in achieving large-scale synthesis, but the obtained materials suffer from small grain size and multilayer regions. Herein, low-cost glass substrate is used to achieve facile growth of large-area uniform and large-size monolayer MoS2 crystals via a modified direct sulfurization method. We find that the ability of glass to incorporate predeposited precursors into its molten state is the key to the production of high-quality monolayer MoS2 crystals. The monolayer MoS2 crystals possess the largest average crystal size (?100 ?m) for MoS2 grown by the direct sulfurization method, with large-area uniformity, which is only limited by the size of the substrate. A combination of low-cost, uniformity, scalability, simplicity, and high quality is achieved in our method which showed great promise for the development of wafer scale electronic devices based on 2D materials. Our work also provides a new look at the role substrates could play in the synthesis and the possibilities of using other glass or liquid substrates for the growth of high-quality 2D materials.

Resume : Transition metal dichalcogenides have attracted interests because of possibilities for various electronic device applications with atomically thin nature. In particular, TMDs can have a polymorphism and MoTe2 shows multiple crystal phase. Phase transition can be obtained relatively easily in MoTe2 because energy difference between the 2H and 1T’ phase is very small. Therefore, we can control conducting states of MoTe2 by manipulating crystal phases and electrostatic doping has been proposed to induce structural phase transition [1-3]. Here, electrostatic doping is achieved by using ferroelectric polarizations and the electrostatic doping effect can be controlled by applying poling process in ferroelectric thin films. As a result, structural deformation is obtained in MoTe2 layers and phase transition is observed by using Raman scattering spectroscopy. Structural deformation is related to change of electromechanical response and it is investigated by using piezoresponse force microscopy. In addition, change of conducting states with the doping effect was obtained by using conductive-atomic force microscopy and memristive switching behavior was achieved.

Resume : Atomically thin membranes are ideal building blocks of NEMS because of their unique mechanical properties and low mass. We make suspended membranes by transferring atomically thin layers from materials such as graphene or MoS2 on top of silicon oxide substrates pre-patterned with (circular) holes, thereby forming thin drums with (sub)micron diameters. The suspended parts form the membranes that are characterized by atomic force microscopy to determine their static mechanical properties (Young's modulus, pre-stress); complimentary to this scanning force microscopy technique, a laser interferometer set-up gives access to the dynamics in the frequency- and time-domain. The interferometer setup has also been equipped with a moveable x-y stage so that the membrane motion can be visualized with a lateral resolution of 140 nm and a displacement resolution of 11 fm/√Hz [1]; additionally, the nonlinear response of the motion can be used to extract the mechanical parameters such as the Young’s modulus [2,3]. As an application of these thin membranes, we have built a different types of pressure sensors consisting of a few-layer graphene membrane [4]. Their operation principle will be presented and their performance benchmarked with that of existing state-of-the-art devices. Work done in collaboration with Peter Steeneken and his group. Financial support is obtained from the Dutch Technology Foundation, the European Union’s Horizon 2020 research and innovation programme under grant agreement No 785219 (Graphene Flagship) and NWO/OCW as part of the Frontiers of Nanoscience program. 1. D. Davidovikj, S.J. Cartamil-Bueno, H.S.J. van der Zant, P.G. Steeneken and W. Venstra, Visualizing the Motion of Graphene Nanodrums, Nano Letters 16 (2016) 2768-2773. 2. D. Davidovikj, F. Alijani, S.J. Cartamil-Bueno, H.S.J. van der Zant, M. Amabili and P.G. Steeneken, Non-linear dynamics for mechanical characterisation of 2D materials, Nature Communications 8 (2017) 1253. 3. R.J. Dolleman, D. Davidovikj, H.S.J. van der Zant and P.G. Steeneken, Amplitude calibration of 2D mechanical resonators by nonlinear optical transduction, Appl. Phys. Lett. 111 (2017) 253104. 4. R.J. Dolleman, D. Davidovikj, S.J. Cartamil-Bueno, H.S.J. van der Zant and P.G. Steeneken, Graphene squeeze-film pressure sensors, Nano Letters 16 (2016) 568-571.

Resume : Robocasting or Direct Ink Writing, is a scalable additive manufacturing technique that brings the possibility of making electrodes and electrochemical energy storage devices in any three-dimensional (3D) geometry and dimensions. Miniaturization over three-dimensional is very attractive for future on-chip technologies where efficiencies need to be optimized over small footprints. This is a new challenge, as device miniaturization is mainly developed to achieve planar-geometries. Here, we demonstrate 3D printed supercapacitors from highly concentrated, water-based 2D atomically thin material inks. The printed architectures, from woodpile and serpentine structures to interdigitated electrodes, are extended over a few mm in three-dimensions and present structural integrity with as low as 100 μm-sized struts. The mechanical robustness allows their employment as miniaturized supercapacitor devices. The particular microstructure in the struts - which is imparted by the use of 2D nanosheets, leads to exceptionally high capacitance as well as energy and power density.

Resume : Resistive random-access memory (ReRAM) devices based halide perovskites are emerging as revolutionary data storage devices due to their switching materials (halide perovskites) received considerable attention in recent years. Among the electrical characteristics of halide perovskites, its current–voltage (I-V) hysteresis, which may occur because of defect migration, makes ReRAM employ halide perovskites as switching materials. In general, the halide perovskite means a 3-dimensional (3D) crystal structure with the general formula ABX3, where a monovalent A+ cation, a divalent B2+ cation, and the 1− charge of the X halide anion. In the resistive switching process, conductive filaments in the 3D halide perovskite show repeatable formation and rupture. However, as a switching material, the morphology of the 3D halide perovskite film is not uniform, and the ON/OFF ratio of the memory device is low. To overcome these challenges, 2-dimensional (2D) halide perovskites can be applied to the resistive memory devices. The 2D halide perovskite structure formed by inserting into large molecule at A sites in a 3D perovskite structure, which means the 3D structures are broken, and become 2D layered structures. Herein, we apply 2D layered halide perovskite in resistive switching memory devices by inserting phenylethyl ammonium (PEA) into 3D halide perovskite structure, and the device structure is Ag/2D layered halide perovskite/Pt/Ti/SiO2/Si stack. The morphology of the film is enhanced, and the resistive switching memory devices exhibit the higher ON/OFF ratio, compared with those of the 3D perovskite based devices. Also, the air stability is improved. This research will contribute to the improvement of switching behavior of memory devices and the better understanding on the resistive switching mechanisms based on the 2D layered halide perovskites.

Resume : Two-dimensional materials (2DM), including semi-metallic graphene (Gr) and semiconductor transition metals dichalcogenides (TMDs), have been widely investigated as channel materials of field effect transistors for high frequency applications. Despite its high carrier mobility allowing to achieve very high cut-off frequencies (fT>300 GHz), Gr suffers from the lack of a bandgap, that ultimately results into a poor power gain and maximum oscillation frequency (fMAX). On the other hand, the relatively low mobility of semiconducting TMDs precludes reaching very high fT and fMAX values. Vertical hot electrons transistors (HETs), relying on the ballistic transit of hot carriers through an ultrathin base layer, have been also recently considered as alternative devices potentially able to operate at THz frequencies [1]. Due to their ultimate atomic thickness, both Gr and MoS2 have been proposed as base materials for HETs. However, proper choice of the emitter and base-collector barrier materials is necessary to achieve the theoretical performances. In this work, we discuss the fabrication, characterization and simulation of a HET with a monolayer Gr base, a high quality Al(Ga)N/GaN emitter, and the base-collector barrier consisting of 10 nm Al2O3 grown on Gr by atomic layer deposition [2]. This transistor showed good modulation of the collector current by the emitter-base bias, with ION/IOFF ratio >10^6 and ON-state collector current density of 100 mA/cm^2. The implications on the device performances of replacing the Gr base with a monolayer MoS2 will be also discussed. This work has been supported by the FlagERA GraNitE project (MIUR grant no. 0001411). [1] F. Giannazzo, et al., Crystals 8 (2), 70 (2018). [2] G. Fisichella, et al., ACS Appl. Mat. & Interf. 9, 7761 (2017).

Resume : The primary gas sensing mechanisms of 2D materials are analyte-induced surface charge transfer, and Schottky barrier (SB) modulation at the channel-electrode interface. Out of the two, it is known that dramatic changes of gas sensing performance originate from SB modulation, which switches the charge transfer barrier and transport of charge carrier depending on the type of analytes. Until now, SB modulation has been mainly utilized only at the interface between the semiconducting channel material and the metal electrode in a gas sensor. Here, we designed a gas sensor with a high density of intrinsic SB interfaces within the sensing channel by the heterojunction of metallic Ti3C2Tx MXene and semiconducting TiO2. MXene, an emerging family of 2D carbides/nitrides, are known to be metallic unlike most conventional sensing channel materials, which can be transformed into metal oxides by controlled oxidation. We synthesized a TiO2/Ti3C2Tx colloidal solution by heating diluted Ti3C2Tx solutions at 80 °C. Synthesized TiO2/Ti3C2Tx was filtrated into a film by vacuum filtration to automatically form inter-flake SBs, which was then transferred to electrodes for gas sensing measurements. Gas responses of TiO2/Ti3C2Tx toward NO2 gases increased by 19 times compared to pristine Ti3C2Tx while the responses of reducing gases were almost unchanged, demonstrating a very high selectivity

Resume : Transport in systems with many particles experiencing frequent mutual collisions (such as gases or liquids) has been studied for more than two centuries and is accurately described by the theory of hydrodynamics. It has been argued theoretically for a long time that the collective behaviour of charge carriers in solids can also be treated by the hydrodynamic approach. However, despite numerous attempts, very little evidence of hydrodynamic electron transport has been found. Graphene encapsulated between hexagonal boron nitride (hBN) offers an ideal platform to study electron hydrodynamics as it hosts an ultra-clean electronic system with electron-electron collisions being the dominant scattering source above liquid nitrogen temperatures. In the first part of my talk, we will discuss why electron hydrodynamics has not been observed before and how it manifests itself in graphene. It will be shown that electrons in graphene can behave as a very viscous fluid forming vortices of applied electron current [1,2]. In the second part, we will discuss methods which can be applied to measure electron viscosity and talk about superballistic flow of viscous electron fluids through graphene point contacts [3]. Then we will talk about the behaviour of electron fluids in the presence of a magnetic field where I will report the experimental measurements of the odd (Hall) viscosity in two dimensions [4]. This dissipationless transport coefficient has been widely discussed in theoretical literature on fluid mechanics, plasma physics and condensed matter physics, yet, until now, any experimental evidence has been lacking, making the phenomenon truly a unicorn. Last but not least, we will discuss how electron hydrodynamics can help in the development of resonant terahertz detectors where I will report some recent progress in this direction [5]. [1] Negative Local Resistance Caused by Viscous Electron Backflow in Graphene, D. A. Bandurin, A. Principi, G.H. Auton, E. Khestanova, K.S. Novoselov, I. V Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, Science 351, 1055 (2016). [2] Fluidity Onset in Graphene, D. A. Bandurin, A. Shytov, L. S. Levitov, R. Krishna Kumar, A. I. Berdyugin, M. Ben Shalom, I. V. Grigorieva. A. K. Geim and G. Falkovich, Nat. Comm. 9, 4533 (2018). [3] Superballistic Flow of Viscous Electron Fluid through Graphene Constrictions, R. Krishna Kumar, D.A. Bandurin, F.M.D. Pellegrino, Y. Cao, A. Principi, H. Guo, G.H. Auton, M. Ben Shalom, L.A. Ponomarenko, G. Falkovich, I. V. Grigorieva, L.S. Levitov, M. Polini, and A.K. Geim, Nat. Phys. 13, 1182 (2017). [4] Measuring Hall viscosity of Graphene’s Electron Fluid, I. Berdyugin, S. G. Xu, F. M. D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, A. K. Geim and D. A. Bandurin, arXiv:1806.01606 [5] Resonant Terahertz Detection Using Graphene Plasmons, D. A. Bandurin, D. Svintsov, I. Gayduchenko, S. G. Xu, A. Principi, M. Moskotin, I. Tretyakov, D. Yagodkin, S. Zhukov, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, G. Goltsman, A. K. Geim and G. Fedorov, Nat. Comm. 9, 5392 (2018).

Resume : One of the most interesting 2D semiconductors are the transition-metal dichalcogenides (TMDC). Similarly to graphene, their electronic properties are governed by relativistic, albeit gapped or massive, Dirac fermions situated at the corners of the first Brillouin zone. Similarly to their high-energy counterparts, their relativistic behavior is at the origin of distinct electronic phenomena in TMDC. The analogue of the spin-orbit coupling can be elegantly described in terms of the Berry curvature. While the latter is often difficult to unveil in materials, we show here that it plays an eminent role in the spectrum of neutral [1] and charged excitons [2] formed from massive Dirac fermions. Most saliently, Berry-curvature corrections to the hydrogenic exciton model yield a short-range repulsion and thus lower the binding energy of the s-states. This, in addition to the more conventionally investigated non-local screening, is at the origin of measured deviations from the hydrogenic spectrum. [1] M. Trushin et al., Phys. Rev. Lett. 120, 187401 (2018). [2] A. Hichri et al., arXiv:1807.10838.

Resume : Hybrid superconductor/semiconductor devices constitute a powerful platform where intriguing topological properties can be investigated. Here we present Josephson junction devices formed by a high-mobility InAs quantum-well bridging two Nb superconducting contacts. We demonstrate supercurrent flow with transport measurements, high critical temperature of 8.1 K, and critical fields of the order of 3T. Modulation of supercurrent amplitude can be achieved by acting on two side gates lithographed close to the two-dimensional electron gas. Low-temperature measurements reveal well-developed quantum Hall plateaus, showing clean quantization of Hall conductance and demonstrating the potential of hybrid devices to investigate the coexistence of superconductivity and Quantum Hall effect. Moreover, we present a different, fully tunable, hybrid semiconductor/superconductor device, in which the width, area, and supercurrent of the two arms of a SQUID-like geometry can be independently controlled with high precision. We show that one can tune the device from one extreme case to another: from a SQUID with narrow arms to a Fraunhofer pattern in an extended single--armJosephson junction. Transition between these limits is investigated in a continuous manner, via electrostatic gating, without the need of additional in-plane magnetic field. Comparison between experimental results and a simple theoretical model is discussed.

Resume : Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunnelling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.

Resume : Two-dimensional materials (2DMs) can be stacked to form atomically designed heterostructures with novel properties, presenting fascinating opportunities for nanoelectronics. Although the process is in principle simple, with the new degree of freedom of the twist angle between layers, the interfaces are almost always incommensurate and so present significant challenges for theoretical studies which are typically restricted to effective (simple) models. Notably, in the popular semiconducting transition metal dichalcogenide (TMDC) heterostructures, although they are heavily studied experimentally, a complete theoretical understanding of band-alignment and hybridisation effects upon stacking has not been forthcoming. We have employed high-accuracy linear-scaling DFT calculations, as implemented in the ONETEP code, utilizing non-local van-der-Waals functionals to explore large-scale models of TMDC heterostructures. We will present results for the full set of heterostructures comprising pairs of TMDCs involving [Mo,W][S,Se]_2. Band-alignments and modifications of the electronic structure upon stacking and rotation of different monolayers are obtained by unfolding the supercell spectral function into the primitive cells. We will present a comparison of the changes in spectral weight and band-structures between the heterostructured materials and the respective monolayers. These predictions will be directly compared to experimental data on the same systems.

Resume : The tremendous interest in transition metal dichalcogenide (TMDC) semiconductors, such as 2H-MoS2, stems from their rich 2D polymorphism and extraordinary properties for various applications. For electronic devices utilizing atomically thin TMDC semiconductors, the electrical contact issue should be essentially addressed. However, the conventional top-contact geometry used to date encounters a number of obstacles, including the Schottky barrier, interface van der Waals gap and inefficient carrier injection, which have profound impacts on the device performance. Here we report a high-performance edge-contact solution based on a new stable metallic nanophase (~ 1 nm wide 1T-MoS2-xOx), transformed from semiconducting 2H-MoS2 edges by reactive oxygen-plasma. This nanophase perfectly bridges 2H-MoS2 to metal leads, leading to zero Schottky barrier, ultralow contact resistance (90 Ω·μm) and record-high field-effect mobility (ranging from 556 cm2·V-1·s-1 at room temperature to 23,700 cm2·V-1·s-1 at 2 K) in trilayer MoS2 field-effect transistors. Moreover, this nanoscale transition provides a flexibility to electrically connect full layers or only the topmost monolayer in few-layer MoS2 to switch quantum transports between Q- and K-valleys. Since this promising nanophase is produced locally along the MoS2 edges patterned by standard lithographic techniques, our edge-contact strategy is scalable for making high-density nanoelectrodes in the future MoS2 circuit integration.

Resume : Transition metal dichalcogenide (TMD) materials comprise of neutral, single-atom-thick layers of atoms that are covalently or ionically bonded with their neighbours within each layer, whereas the distinct layers are held together via van der Waals bonding along the third (thickness) axis. To date, MoS2 has been the testing ground for many TMD-based studies. Controllable doping of TMD materials is one of the main research challenges associated with the practical realization of 2D semiconductors in hetero- and homo-junctions. Doping semiconductors relies on the intentional introduction of impurity atoms into the semiconductor crystal lattice in order to modify properties of the material. In this study we provide insight in the understanding of the electronic properties of native defects in the form of vacancy and impurities in MoS2 structure by means of density functional simulations. In particular, we have focused on Re- and Nb-substitutional impurities in MoS2 films creating n-type and p-type dopants, respectively. We have also shown that single sulfur vacancy behaves as acceptor-type dopant shifting the Fermi level to lower energies; i.e., making non-intentionally doped MoS2 film, p-type. Moreover, supercell eigenstates are unfolded to band structure of a reference primitive cell to allow direct comparison of the bands with the band structure of a primitive cell or the band structure obtained from angle-resolved photoemission spectroscopy (ARPES) measurements (from literature). This analysis also reveals the localized nature of the vacancies. * Science Foundation Ireland is acknowledged for funding this project through grants 15/IA/3131 and 12/RC/2278.