Two dimensional crystals and van der Waals heterostructures for nanoelectronics

Resume : 2-Dimensional semiconductors, have recently received considerable attention, as they are compatibile with current microelectronics production processes. Most of the work done so far has been on metal surfaces: 2D lattices of Ge have been observed on various metallic substrates such as Pt(111), Au(111) and Al(111). Metallic surfaces are poorly compatible with silicon technology and efforts are devoted for obtaining 2D layers on more appropriate substrates. 2D graphite-like hexagonal AlN (h-AlN), a metastable phase favored at small thickness against the most known bulk wurtzite-AlN phase, is a promising insulating substrate for germanene growth, since it has been theoretically predicted that the silicene counterpart remains stable when encapsulated between two 2D h-AlN layers. In this work, we present evidence of germanene formation 2D h-AlN insulating layers on Ag(111). By combining a number of experimental techniques such as RHEED, STM and UPS we first report the successful epitaxial growth of sp2-hybridized h-AlN layers on Ag(111) with a flat geometry by plasma assisted MBE. Subsequently, Ge layers were also deposited by MBE on an h-AlN buffer layer and were structurally characterized by RHEED, XAS and DFT calculations. RHEED data indicate a faint 4x4 superstructure with respect to (1x1) Ag(111), or a (3x3) reconstruction with respect to (1x1) germanene. XAS provides evidence of 2D Ge layer formation with an interatomic distance of dGe-Ge=2.38 Å, characteristic of free-standing germanene, also compatible with first-principles DFT calculations performed on the Ge/h-AlN/Ag(111) system with a buckling of 0.7 Å. The perspective of extending this method to other 2-D systems will be discussed.

Resume : Black phosphorus (BP) has a layered structure thus it can be exfoliated down to the monolayer. In our labs, good quality phosphorene flakes were prepared by sonicating BP in dimethylsulfoxide. Afterwards transition metals nanoparticles of nickel, ruthenium, palladium and gold have been immobilized on it. The resulting nanocomposites were characterized by TEM, AFM and XPS. As preliminary application, Ni-2D BP composite was tested in the hydrogenation of phenylacetylene and exhibited high catalytic activity and selectivity towards styrene. The applications of Ru-2D BP, Pd-2D BP and Au-2D BP in several catalytic reactions is in progress and their electronic properties will be studied as well and compared with that of pristine phosphorene. Acknowledgments: The authors thank the European Research Council for funding the project PHOSFUN “Phosphorene functionalization: a new platform for advanced multifunctional materials” (Grant Agreement No. 670173) through an ERC Advanced Grant to MP.

Resume : Graphene and graphene-based materials possess a rather exclusive set of physicochemical properties holding great potential for biomedical applications, in particular neural prostheses. In this presentation, I will provide an overview on fundamentals and applications of several graphene-based technologies and devices aiming at developing an efficient bidirectional communication with electrogenic cells and nerve tissue. To this end, I will discuss several device technologies based on graphene that are used to investigate the electrical activity in cell cultures and in acute experiments (nerve tissue slices); finally, we will disclose recent in-vivo experiments in which flexible graphene devices are used to record brain activity. The results presented in this talk highlight the great potential of graphene technologies in neuroprosthetics.

Resume : Strain engineering of graphene has attracted great interest as it allows tuning of the electronic and optical properties of graphene.1-3 Raman spectroscopy is an ideal tool for characterization of strained graphene.4-5 Many studies have concentrated on biaxial strain as this allows more reliable calculation of the Gruneisen parameters than uniaxial strain.4 However, the application of biaxial strain is rather difficult to achieve experimentally, so all previous works reported on graphene subjected to relatively small biaxial strain (0.1-1%), in contrast to uniaxial strain, where strain above 10% was reported.6 Here we show a simple fabrication technique to produce pressurized and stable graphene membranes which can hold up to 14 bar differential pressure and reversible strain up to ~2%. The Grüneisen parameters have been observed not to change up to 2% strain, in agreement with calculations.7 However, for strain close to 2%, a characteristic broadening of both the G and 2D peak was observed for the first time for biaxial strain. This has been attributed to nanoscale variations of strain in the membrane within the laser spot size.8 1. Guinea, F., Katsnelson, M.I. & Geim, A.K. Nature Physics 6, 30-33 (2009). 2. Pereira, V.M., Neto, A.C. & Peres, N. Physical Review B 80, 045401 (2009). 3. Pereira, V.M. & Neto, A.C. Physical Review Letters 103, 046801 (2009). 4. Zabel, J. et al. Nano Lett 12, 617-21 (2012). 5. Mohiuddin, T. et al. Physical Review B 79, 205433 (2009). 6. H. Hugo Pérez Garza, Nano Lett., 2014, 14 (7), pp 4107–4113 7. Cheng, Y., Zhu, Z., Huang, G. & Schwingenschlögl, U. Physical Review B 83, 115449 (2011). 8. Neumann, C et al, Nature Comms, doi:10.1038/ncomms9429

Resume : Carrier transport in monolayer molybdenum disulfide (MoS2) field-effect transistors on layered hexagonal boron nitride (h-BN) nano sheet were systematically investigated in here to have a clear insight of the charge carrier scattering mechanism at low temperature down to 25K. The presence of h-BN layers enables to prevent an inevitable hole doping mainly occurring from Coulomb impurities located in silicon dioxide substrates, giving rise to negative shift of flat band voltage and considerably reduction of Schottky barrier height compared to MoS2 device on silicon dioxide. Besides, the surface trap density of hexagonal boron nitride dielectric estimated by low frequency noise analysis falls it off two orders of magnitude compared to that obtained from silicon dioxide, bringing about an enhancement of charge carrier mobility and an early observance of a metal-insulator transition at lower carrier density. These results will shed more light on the way to further performance enhancements in two-dimensional electronic systems.

Resume : Two-dimensional crystals have emerged as a class of materials that may influence future electronic technologies. In particular, few-layer black phosphorus (BP) presents anisotropic in-plane lattice structure with consequent highly anisotropic electronic and optoelectronic properties. Moreover, the energy bandgap in few-layer BP increases monotonously with reducing number of layers, eventually reaching 2 eV for monolayer. Thus, BP has demonstrated intriguing properties and promising potential for applications in infrared optoelectronics and photonics which may enable the realization of conceptually new devices. However, the current method for fabricating few-layer BP is of rather low yield since it is based on mechanical exfoliation from bulk BP crystals. There is an urge need to develop cost-effective large-area synthesis and processing methods to produce wafer-scale few-layer black phosphorus. Here we demonstrate the patterning of few-layer BP into arrays of micro- and nanostripes on large area substrate by means of wet deposition in confinement [1]. The flake-based stripes are fabricated across the channel of a field-effect transistor (FET) in top-contact/bottom-gate configuration by a stamp-assisted deposition of solution of few-layer BP flakes which is obtained by liquid exfoliation of BP under sonication in DMSO [2]. The few-layer BF flakes self-organize into ordered stripes as the critical mass concentration is reached in the solution menisci under the stamp protrusions. Our final goal is the realization of multifunctional FETs based on few-layer BF such as light-emitting FETs [3] by means of innovative cost-effective large-area wet technique capable to control material arrangement at multiple length scales. Acknowledgments: The authors thank the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (Grant Agreement No. 670173) for funding the project PHOSFUN “Phosphorene functionalization: a new platform for advanced multifunctional materials” through an ERC Advanced Grant. [1] D. Gentili, F. Valle, C. Albonetti, F. Liscio, M. Cavallini Acc. Chem. Res. 47 (2014) 2692-2699. [2] M. Serrano-Ruiz, M. Caporali, A. Ienco, V. Piazza, S. Heun, M. Peruzzini Adv. Mater. Interfaces (2015) 15000441. [3] M. Muccini, W. Koopman, S. Toffanin Laser Photonic Rev. 6 (2012) 258–275.

Resume : We investigate the band gap structure of exfoliated MoTe2, from bulk-like crystals to monolayers, and find an anomalous evolution with decreasing thickness, differing from that more studied semiconducting transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2. Spectroscopic measurements based on ionic-liquid gated ambipolar transistors, scanning tunneling microscopy and optical transmission measurements show that in thick flakes the indirect gap is only ~ 100 meV smaller than the direct one. Since the indirect band gap of most TMDs is known to increase significantly upon reducing thickness, this small difference suggests that in MoTe2 the transition from indirect to direct gap may happen already in rather thick multilayers (before reaching monolayer level). We confirm that this is the case by investigating the thickness evolution of the integrated PL intensity (both exciton and trion transitions) and of microreflectance (MR). Indeed, contrary to other TMDs, in MoTe2 the integrated PL intensity is identical for mono and bilayer, decreases slightly for trilayer, and becomes significantly lower only in tetralayers. These observations, consistently with the MR data, indicate that mono and bilayer MoTe2 are direct band gap semiconductors, tetralayer MoTe2 has an indirect gap and in trilayers the direct and indirect gaps nearly coincide.

Resume : Black phosphorus (BP) is one of the phosphorus allotropes, which can be stabilized in high-quality monolayer (phosphorene) flakes [1], that is currently gaining an outstanding scientific and technological interest because of its tunable energy bandgap (as a function of its inherent thickness) and its relatively high carrier mobility, which makes it suitable for optoelectronic devices [2]. We present a thorough characterization of few BP layers down to the single layer by means of direct atomic resolution by Z-contrast scanning transmission electron microscopy (STEM), and for the first time we concomitantly report on an extensive experimental study of energy loss near edges structure (ELNES) associated with the corresponding density of states (DOS) for both K and L atomic edges via electron energy loss spectroscopy (EELS). We also prove how the combined use of modern transmission electron microscopy, entered into the era of sub-Å resolution, and EELS spectroscopy can be exploited to simultaneously estimate the crystal structure, band gap, and electronic density of the single crystal, or even a single layer, of the studied material at atomic level. Acknowledgments: Thanks are expressed to MIUR under project Beyond-Nano (PON a3_00363) and ERC for funding the project PHOSFUN (Grant Agreement No. 670173). [1] M. Serrano-Ruiz, M. Caporali, A. Ienco, V. Piazza, S. Heun and M. Peruzzini, The Role of Water in the Preparation and Stabilization of High-Quality Phosphorene Flakes, Advanced Materials Interfaces (2015) doi:10.1002/admi.201500441. [2] L. Viti, J. Hu, D. Coquillat, W. Knap, A. Tredicucci, A. Politano and M. S. Vitiello, Black Phosphorus Terahertz Photodetectors, Adv. Mater. 27 (2015) 5567.

Resume : Recently, the integration of silicene [1] in a field-effect transistor (FET) [2] attracted huge interest thus representing an intriguing option to overcome the scaling issues in the nanoelectronics field and being, at the same time, fully compatible with the current ubiquitous semiconductor technology. Therefore, the study of Si thin films at the two-dimensional (2D) limit is highly demanded in order to understand either structural or electronic properties. In particular, multilayer silicene on Ag(111) [3] might represent an advance with respect to the monolayer because of its weaker interaction with the Ag substrate at the bottom and a higher stability towards air exposure on top. Hence, we report here on the comparison of material microstructures and electronic properties between multilayer silicene films grown at different substrate temperature regimes. By means of Raman spectroscopy, a clear trend of the E2g mode is found out pointing towards bulk-Si condition as the substrate temperature is increased. Moreover, the realization of multilayer silicene FETs allowed for discriminating an ambipolar behavior from a trivially one as a function of the Si growth temperature. These outcomes show that the structural and electronic properties of Si at the 2D limit can be successfully manipulated through carefully tuning the growth conditions, paving the way to an advanced control of Si properties at the nanoscale. [1] Grazianetti et al., 2D Mater. 3, 012001 (2016). [2] Tao et al., Nature Nanotech. 10, 227 (2015). [3] De Padova et al., 2D Mater. 1, 021003 (2014).

Resume : Alternative group-IV 2D materials to graphene, i.e. silicene, germanene and stanene (also called Xenes), are gaining a lot of interest, due to their unique properties and potential compatibility with the existing Si nanotechnology platform. We will present recent theoretical predictions regarding the structural, electronic and transport properties of these novel 2D materials. Particular attention will be given to their natural tendency to form buckled structures, and the impact of buckling on their electronic properties. The stronger spin-orbit coupling, going down in the periodic table (from Si to Sn), and the occurrence of a topological insulating state in Xenes will also be discussed; the possibility to induce a topological to trivial insulator phase transition in these materials, either by an out-of-plane electric field or by quantum confinements, will be highlighted. Finally, the interaction of Xenes with different substrates will be discussed, distinguishing between weak van der Waals and stronger covalent Xenes/substrates interactions.

Resume : Two-dimensional (2D) materials attract major interest in both basic and applied research due to their unique structural and electronic properties. On special focus are those 2D materials which do not exist in nature[1]. Here, we will show the structural and electronic properties, using synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction techniques, from a single layer to few layer germanene, these layers we create artificially by dry epitaxy on a gold template[2]. In fact, until now germanene have been experimentally successfully synthesized recently on several metal substrates; Pt(111)[3], Au(111)[2] and Al(111)[4]. For germanene are expected many behaviors, such as the tunable band-gap by electric-field and much stronger spin–orbit interaction, displaying great potential in electronic and spintronic applications[5]. However, germanene shares a problem with silicene: one cannot utilize germanene’s ultimate properties unless suitable substrates are found for germanene. Currently, ML germanene as mentioned can only be synthesized on metal surfaces, until now only in the case of Au(111) we show that few layer germanene posseses Dirac cones thanks to a reduced interaction, but electronic devices such as FETs require substrates with a large band-gap for this reason we will compare the results using different substrates and hopefully this will end up with the fabrication of the first germanene field-effect transistor (FET) in the near future as already exist on silicene[6]. [1]- J. Wang et al. National Science Review, 2, 22-39 (2015). [2]- M.E. Dávila et al. New Journal of Physics 16 (9): 095002 (2014) and M.E. Dávila and G. Le Lay, accepted Nature Scientific Reports. [3]-L. Li et al. Advanced Materials 26 (28): 4820–4824 (2014). [4]-M. Derivaz et al. Nano Lett. 15, 2510−2516 (2015). [5]- A. Acun et al. J. Phys. Condens. Matter. 27, 443002 (2015). [6]- L. Tao et al., Nat. Nanotechnol.,10, 227-231 (2015).

Resume : Although dozens of two- dimensional (2D) materials have been found in the last few years, single-element systems, such as graphene and black phosphors, are still very rare. Boron, the fifth element in the periodic table, has short covalent radius and flexibility to adopt sp2 hybridization as carbon, which favors the formation of 2D allotropes. Here we show that 2D boron sheets can be epitaxially grown on a Ag(111) substrate. Two types of boron sheet, corresponding to a triangular lattice with different arrangements of periodic holes, have been observed. Moreover, the boron sheets were found to be quite inert to oxidization, and interact only weekly with the substrate. The realization of such long expected boron sheets paves the way toward possible 2D boron electronics in the future.

Resume : In this work are examined the crystal structure and optical properties of composites obtained by intercalation of Cd and Zn atoms in the single crystalline plates of AIIIBVI semiconductors (GaS, GaSe, GaTe and InSe). The intercalation has been carried out by thermal treatment at temperatures in the range of 620-1070K, during from 10 minutes to 60 hours, of AIIIBVI semiconductor plates in Zn and Cd vapor with the pressure of 10-2-5 mm Hg. Were obtained composite materials containing micro-lamellar AIIIBVI crystallites and AIIBVI micro-crystallites (CdS, CdSe, CdTe, ZnS, ZnSe and ZnTe). Absorption spectra at 300K and 80K show that the absorption band edge is composed of two slopes characteristic for absorption of AIIIBVI and AIIBVI semiconductor crystallites. The crystallite sizes were determined from the analysis of XRD patterns and from the anti-Stokes shift of photoluminescence (PL) bands. Intercalation process contributes to the formation of structural defects which are intensively screening the excitons in the crystallites of the composite. From the analysis of the PL spectra, temperature dependence of PL intensity, recombination processes and thermal stimulation were determined the recombination and capture energy levels in both the initial crystals and the composite. From the analysis of structural and optical properties was established the technological regime for obtaining nano-lamellar composites containing AIIIBVI and AIIBVI semiconductors, respectively.

Resume : A material composed of GaTe and CdTe crystallites was obtained by heat treatment of GaTe single crystalline plates in Cd vapor, at pressure of 10-1-5.0 mm Hg. The holes concentration and mobility in the primary GaTe single crystals at 300K were 1.8•1015 cm-3 and 18 cm2/(V•s) respectively. The crystalline structure and composition of the obtained material were determined by XRD measurements. The size of CdTe crystallites was determined from the analysis of the XRD line contour registered from the (1 0 1) planes. The CdTe-GaTe composite obtained by treatment of the GaTe plate for 60 minutes in Cd vapor with pressure of ~0.3 mm Hg exhibits photoconductivity in the spectral range of 450-950 nm, with a broad peak in the region of 700-750 nm. The photocurrent relaxation curves at chamber temperature contain two slopes with time of τ1 ≤ 10-7 s and τ2 ≈ 10-2 s. By heat treatment in Cd vapor at 920K for 20 minutes, of GaTe single crystals doped with Sn, were obtained n-GaTe/p-CdTe planar structures photosensitive in the 500-950 nm spectral region. It was determined the mean free path of electrons in CdTe layer which is ~ 1.0 μm. The recombination and capture levels energies in GaTe crystallites were determined from the photoluminescence (PL) and thermoluminescence spectra analysis at temperatures from 80K up to 300K. Varying the spectral composition of the PL excitation beam was determined the diagram of localized states in the band gap of CdTe crystallites from composite.

Resume : Atomically thin transition metal dichalcogenides (TMDs) are emerging two-dimensional materials with interesting electronic and optical properties. Out of all TMDs explored till now, semiconducting MoS2 is the most intensively studied one. Recent advances in chemical vapor deposition (CVD) have shown great promise in scaling up the synthesis of TMD materials including MoS2 with excellent electronic quality [1]. Single layer MoS2 has been synthesized in a wide range of temperatures ranging from 700 °C to 1000 °C using CVD technique. However, a systematic comparison of samples grown at different temperatures is scarce. Here, we study the effect of deposition temperature on the growth of MoS2 using CVD technique and present a comparison of their morphological and optical qualities. Detailed optical and SEM studies reveal the formation of different structures on the substrates and the same is explained on the basis of MoO3-x vapor flux arriving at a certain location. Raman and PL studies indicate that single layered MoS2 grown at 650 °C has different optical quality than films grown at 700 °C, even though their morphology is same. Our work suggests that growth temperature has a significant effect on the quality of CVD grown 2D TMDs using solid precursors, which in turn can affect the performance of devices based on these materials. [1] Y. Shi, H. Li, L.-J. Li, “Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniques,” Chem. Soc. Rev., vol. 44, no. 9, pp. 2744–2756, Apr. 2015.

Resume : The advent of new facile fabrication techniques of producing the single- and the multi-layered graphene, has led to large amount of research over other two-dimensional materials such as MoS2, WS2, MoSe2, WSe2, and MoTe2. These materials in principle can be utilized for semiconductor devices such as transistors, emitters, and detectors as well as exhibit interesting two-dimensional physics. Especially, WS2 bilayer shows mobility above 300 cm2V-1S-1 in low temperature that implies possible low losses through the Joule effect. In this study, we made WS2 samples by exfoliation method. We deposited WS2 flakes on Si substrates with a 300 nm SiO2 layer, and the typical size of thin layer of WS2 flakes were about few micrometers. For measuring basic characteristics such as lattice properties and information regarding the electronic band structures of the samples, we performed Raman scattering spectroscopy using four different excitation energies of 457 nm (2.71 eV), 488 nm (2.54 eV), 514nm (2.41 eV), 532 nm (2.33 eV), and 632.8 nm (1.96 eV) in different temperatures. Also, we measured Raman spectra including the low energy breathing and shear modes. We report anomalous phonon behavior that depends on the number of layers and the resonant effect reflecting the underlying electronic band structure.

Resume : Top-down exfoliation from bulk transition metal dichalcogenides like MoS2 usually leads to micron-sized flakes. We are using an alternative growth method of two-dimensional transition metal diselenides (TMDS) from multilayers down to a single layer based on the Van der Waals (VdW) epitaxy. In the VdW epitaxy, the TMDS is grown either on a passivated surface with a very low density of dangling bonds (it is then called quasi-VdW epitaxy) or on a layered VdW substrate. The basic concept of this growth method relies on the very weak interaction between the epilayer and the substrate in order to largely release the constraint of lattice matching. Therefore it leads to the formation of fully relaxed TMDS layers. Moreover, this technique allows for the growth of uniform layers over centimeter scale surfaces making it compatible with the development of a large scale 2D electronics based on these materials. In this poster, we present our recent results on three different topics relying on the VdW epitaxy: (i) the epitaxy of MoSe2 on polycrystalline graphene grown by CVD on Pt, (ii) the fabrication and characterization of MoSe2 and WSe2 transistors on SiO2/Si substrates to study magnetotransport under illumination with circularly polarized light and (iii) the growth and characterization of single crystalline MoSe2 and WSe2 monolayers and multilayers on single crystalline insulating substrates AlN/Si(111) and sapphire to study spin transport in model 2D systems.

Resume : Silicene and Germanene, two honeycomb crystal structures composed of a mono-layer of Si and Ge respectively, combine the advantages of the 2D ultimate-scaled electronics, with their compatibility with industrial processes presently based on Si and Ge. We envisage pseudomorphic lateral heterostructures based on ribbons of Silicene and Germanene, which are the 2D analogous of conventional 3D Si/Ge super-lattices and quantum wells. In spite of the considerable lattice mismatch (~4%) between free-standing Silicene and Germanene, our ab initio simulations predict that, considering striped 2D lateral hetero-structures made by alternating Silicene and Germanene ribbons of constant width, the Silicene/Germanene junction remains pseudomorphic -- i.e. it maintains lattice matched edges -- up to a critical ribbon widths that can reach some decades of nanometers. Such critical widths are one order of magnitude larger than the critical thickness measured in 3D pseudomorphic Si/Ge heterostructures, and than the resolution of lithography, thus enabling this technique to pattern Silicene/Germanene junctions. We computed how the strain produced by the pseudomorphic growth modify the crystal structure and electronic bands of the ribbons, providing a mechanism to engineer the population of high mobility carriers within the Dirac cone. Our results pave the way to lithography patterned lateral heterostructures that can constitute the building blocks of the novel 2D-electronics.

Resume : In this study we developed a cost-economical, time-saving and efficient ultrasound-assisted solvent method to exfoliate graphene and MoS2 simultaneously in a proper solvents mixture. The Hansen Solubility Parameter strategy basing on the theory of surface energy equilibration was applied to predict and optimize the composition of solvents mixture. A ternary-solvent system was used to minimize the difference of surface energy between solvents mixture and solutes. We demonstrated that the addition of solvent species provided an extra dimension to tune the surface energy of solvents mixture, and enabled us to brew the best solvents mixture for exfoliation process. Impressively, graphene-MoS2 hybrid structure was observed after heat treating co-exfoliated graphene-MoS2 solution, which was due to the restacking effect. The graphene-MoS2 nanocomposite was drop-casted on the sensor substrates and tested with various volatile organic compounds (VOCs) gases, including ethanol, methanol, benzene, and toluene, at room temperature with visible light irradiation. The experimental results revealed that the nanocomposite sensor exhibited n-type sensing response towards reducing VOCs gases. The sensing performance of nanocomposite sensor surpassed both graphene-based and MoS2-based sensor owing to the synergetic effect. Remarkably, the sensing property of nanocomposite sensor underwent degradation when the visible light irradiation was removed.

Resume : Organic materials containing aromatic structure were reported to promote the synthesis of Molybdenum disulfide (MoS2). However, the role of the organic materials in the growth of MoS2 has not been understood. Here in, we systematically study the effect of the organic materials as a nucleation promoter on the growth of MoS2 by chemical vapor deposition (CVD). For these study, we grew high-quality MoS2 flakes with Ammonium Molybdate Tetrahydrate (AMT) as a Mo precursor and sulfur. We also employed perylene-3,4,5,1-tetracarboxylic acid tetrapotassium salt (PTAS) which contain potassium and sodium chloride (NaCl) as seeding promoters. First, we investigate the effect of the size of the promoter clusters on growth of the MoS2 flakes by using varying concentration of the promoters. Secondly, we examined the role of elements in the promoters in growth of the MoS2 flakes by comparing the flakes grown with PTAS and NaCl promoters. The size and the thickness of the flakes were similar with the cluster size of both promoters. Thus, we can conclude that the amount of alkali ions in seeding promoter clusters is the key factor to determine the thickness and the size of MoS2 flakes. Finally, we suggest the growth mechanisms of MoS2 flake with a seeding promoter. Our studies not only open the way for finding the growth conditions of scalable and high-quality crystalline MoS2 but also accelerate the commercialization of MoS2 in the future applications.

Resume : Layered rare-earth hydroxides (LRHs) are ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules [1]. The metal cations occupy the centers of the edge sharing octahedral, whose vertex contains hydroxide ions that connect to form infinite 2D sheets compounds-based on lanthanide elements. Those materials are very attractive because they can be used in several technological applications due to their optical, magnetic, electronic and catalytic properties [2]. Here layered Eu hydroxides have been produced by homogeneous precipitation of Europium chloride hexahydrate with hexamethylenetetramine (HMT) [3]: this gives rise to relatively high concentration of flakes of 700 nm thickness and 3 µm lateral size. Micro-mechanical exfoliation and liquid-phase exfoliation have been then used to exfoliate the as-produced material to single and few-layers. X-ray diffraction, Raman spectroscopy, UV-Vis spectroscopy, Fluorescence spectroscopy, Scanning Electron Microscope and Atomic Force Microscope have been used to characterize the exfoliated material. 1. Qiang Wang, Dermot O’Hare. Recent advances in the synthesis and application of Layered Double Hydroxide (LDH) nanosheets. Chemical Reviews, 112, 07 2012. 2. Qi Zhu et al. Tens of micron-sized unilamellar nanosheets of Y/Eu layered rare-earth hydroxide: efficient exfoliation via fast anion exchange and their self-assembly into oriented oxide film with enhanced photoluminescence. Science and Technology of Advanced Materials, 15, 02 2014. 3. F. Geng et al. New Layered Rare-Earth Hydroxides with Anion-Exchange Properties. Chemistry - A European Journal, 14, 2008.

Resume : Graphene nanoribbons (GNRs), defined as nanometer-wide strips of graphene, are attracting increasing attention as highly promising candidates for electronics since they combine the outstanding properties of graphene with the presence of a finite bandgap [1]. However, the electronic properties of GNRs critically depend on the ribbon width and its edge morphology [1]. Recently it has been shown that bottom-up chemical synthetic approaches based on solution-mediated [2] cyclodehydrogenation are able to produce ultra-narrow (<< 10nm width) and structurally well-defined GNRs. Since GNRs properties are strongly sensitive to their precise atomic structure, it is essential to develop a fast and non-destructive characterization tool. Raman spectroscopy is a well-established technique for the investigation of carbon nanostructures [3-5]. Here we report a detailed experimental study of the Raman spectrum of ultra-narrow GNRs with different edge morphology, functional groups, length and width. We show that the Raman spectrum is strongly sensitive to the specific modification at the edges, in particular in the acoustic region, where a characteristic radial-like breathing mode [6] is observed. 1. O.V. Yazyev, A guide to the design of electronic properties of graphene nanoribbons. Acc. Chem. Res. 46, 2319 (2013). 2. A. Narita et al, Nature Chem., 6, 126 (2013). 3. A. Jorio, R. Saito, G. Dresselhaus, M.S. Dresselhaus. Raman Spectroscopy in Graphene Related Systems. Wiley-VCH Verlag GmbH, 2011. 4. C. Casiraghi. Raman spectroscopy of graphene. In Spectroscopic Properties of Inorganic and Organometallic Compounds: Techniques, Materials and Applications, Volume 43, pages 29{56}. The Royal Society of Chemistry, 2012. 5. A. C. Ferrari, D. M. Basko. Nature Nanotech., 8, 235 (2013) 6. J. Zhou, J. Dong, Appl. Phys. Lett. 91, 173108 (2007).

Resume : Van der Waals (vdw) heteroepitaxial stacking growth of dissimilar layered materials can form a new class of superlattices, and thus hold promises for new two-dimensional (2D) electronic and optical phenomena in solid state. Conceptually the 2D vdw heteroepitaxial growth does not strictly require the close lattice-match between the constituent 2D crystals, due to the weak vdw interlayer interactions. However, it is yet to be experimentally verified. Here we report the two different growth mechanisms of vdw heteroepitaxial Bi2Te3/Sb2Te3 few-layers stacking by choosing different substrates. We show that the growth mode becomes the layer-by-layer growth (Volmer-Weber growth) on h-BN substrates, and the 3D island growth (Frank-van der Merwe growth) on SiO2/Si. We find that the compressive strain in the substrates imposed by the lattice-mismatch plays a crucial role to determine different growth modes in these 2D nucleation kinetics models. Our work suggests general implications for large-area 2D stacking growth of various layered materials.

Resume : Two-dimensional materials have been proposed as the building-blocks for future (opto-) electronic and catalytic systems, exploiting their properties that strongly deviate from bulk materials. The monolayer crystalline phase of silicon oxide [1], a honeycomb structure composed of [SiO4] tetrahedrons, is potentially an ultimately thin support for other two-dimensional materials,. that could be fabricated over large areas, and in this respect an alternative to h-BN [2]. Also, the structure of monolayer silicon oxide mimics the surface of zeolites, and for this reason is considered a model support in catalytic systems [3] suited for extensive scrutiny via surface probing techniques, which has been impossible in bulk systems so far. In this contribution, we report on reflection high energy diffraction (RHEED) and scanning tunneling microscopy (STM) measurement, together with density functional theory (DFT) calculations applied to monolayer silicon oxide [4]. We precisely unravel the atomic structure, beyond the resolution achieved so far [3]. The formation of domains of monolayer silicon oxide on Ru(0001) and their lateral stitching, resulting in intrinsic defects in terms of antiphase domain boundaries (APDBs) is also rationalized. [1] Huang, P. et al. Nano Lett. 12, 1081 (2012) [2] Britnell, L. et al. Science 335, 947 (2012) [3] Shaikhudtinov, S. et al. Adv. Mater. 24, 49 (2013) [4] Mathur, S., et al. Phys. Rev. B 94, 161410(R) (2015)

Resume : Atomically resolved STM and PES were applied to study a surface of Zr(0001)-1x1 single crystal. The surface was cleaned with ion bombardment and annealing at 750°C. LEED shows a simple 1x1 patterns. Different degree of surface contamination with C and O was detected with PES. The amount of both species was less than one monolayer and the C1s and O1s spectra did not show the presence of carbide or zirconia forms. On the other side, relatively high background of LEED patterns signalises a certain disorder at the surface. STM images give a more complex view of the “clean” surface. We found a high density of steps that separate terraces no bigger than 1 µm in diameter. Most of the step edges are decorated. In the interior of the terraces, three different regions are found: i) Small ones with hexagonal ordering, typical for a metallic surface. We identified it as the Zr(0001)-1x1 clean parts. ii) Some terraces or their parts are covered with a layer showing a well-known feature – Moire patterns. Atomic resolution at such layer shows hexagons in the form of benzene ring i.e. strong covalent bonding. On the base of these data and narrow C1s peak, we prescribe it to graphene formation at 750°C. iii) The third region is covered with chain like structure formed either from the single atoms (first stage) or massive rope like agglomerates. This feature is time dependent and increases at the same rate like O1s signal. We believe that it is an initial stage of the Zr oxidation.

Resume : Monolayer MoS2, one of two-dimensional (2D) transition metal dichalcogenide materials, offers unique physical and chemical properties that are not attainable in bulk form. These 2D structures are grown laterally in various shapes from triangles to hexagons or in different sizes and at an angle to the substrate by chemical vapor deposition (CVD) on a single run depending on the substrate position [1-2]. Laterally-grown 2D MoS2 crystals can also be merged to form polycrystalline monolayer films. Different forms of 2D crystals can potentially be employed in different device applications; e.g., large triangles can be used in field effect transistors (FETs), large-area films in solar cells and vertically aligned structures in catalytic reactions [2]. In this work, we report on the CVD growth of monolayer MoS2 crystals with controlled flake geometry and size also describing the transition mechanism from separate flakes into monolayer thin films of merged flakes. We evaluate properties of different formations by analyzing photoluminescence (PL) spectra, Raman scattering spectra and scanning electron microscopy (SEM) images. PL peaks for small hexagonal crystals (1-2 µm) and large (60–90 µm) triangles were observed at 660 to 680 nm, respectively, due to changes in the film morphology. In this study, we also compare the performance of FETs that we fabricated on different shaped monolayer MOS2 formations. References [1] Wang et al. Chemistry of Materials 2014 26 (22), 6371-6379 [2] Perkgoz et al. Nano-Micro Lett. (2016) 8(1):70–79

Resume : Layer Transition Metal Dichalcogenides (LTMDs) have been investigated in the past decades as two dimensional materials for their potential applications spanning from catalysis to optoelectronic devices. This is due to the transition of their band gap from indirect to direct when thickness reduced to monolayers. While graphene hydrogels and aerogels formed from graphene oxide suspensions have been extensively demonstrated, challenges still remain in the case of LTMDs, due to the smaller dimensions of the layers and the absence of functional groups generating hydrogen bonding, which can link the flakes together. We demonstrate the formation of free-standing networks of exfoliated MoS2 flakes via hydrothermal method. Highly concentrated inks of predominantly single layered MoS2 flakes, exfoliated in water via lithium intercalation method, have been treated at 185 °C in presence of cross-linking agents. The final shape of the network is conformal to the gelation vessel and the resulting structure present random macroporosity, good mechanical stability and it is electrically conductive. The 3D structures have been then employed as photoanode in photoelectrochemical cells.

Resume : We report on p-type doping effect of oxygen and ozone molecules on mono- and few-layer WSe2 and MoSe2 field effect transistors. We find that the adsorption of oxygen and ozone molecules is reversible and leads to substantial p-type doping and corresponding increase in hole density via non-covalent charge transfer process. Our control experiments demonstrate that water adlayer on the surface plays a crucial role in solubilizing oxygen and ozone and in forming a redox couple with a chemical potential higher than that of WSe2 and MoSe2. Due to the higher oxidation potential and water solubility, ozone is significantly faster and more efficient in doping compared to oxygen. DOI: 10.1039/C5CP07194A

Resume : Two-dimensional (2D) transition metal dichalcogenides (TMDC) have been initially studied mainly in form of exfoliated flakes1,2. Recently an increasing interest is reported for 2D layers grown by CVD-related techniques. The reason is that the mass production of 2D TMDC-based devices demands a method for the synthesis of large-scale, layer-controlled flakes. In this work we report on the first Cathodoluminescence (CL) study of the optical emissions of mono- and bi-layer MoS2 flakes grown by Chemical Vapor Transport on large area SiO2/Si substrates. Thanks to the peculiar excitation mechanism of CL, the radiative recombinations of the SiO2 layer typically used as gate insulator in FETs are also studied. This is particularly appealing since it permits to correlate the device performance with SiO2 intra gap states. Raman spectroscopy and imaging, Scanning Transmission Electron Microscopy (STEM) and atomic Force Microscopy (AFM) investigations are also used to correlate the structural and optical properties of the CVT grown flakes. AFM imaging reveals the presence of bi- and multi-layers of MoS2 film. The bi-layer corresponds to triangular grains with sides 1-2-micrometers long. Then triangular adlayers with sides around 100 nm are found to grow on the top of the bi-layers. A rotation of 180° appeared after each monolayer increase as expected by the plane stacking of the 2H phase of MoS2 as confirmed by STEM images. Further, an overgrowth at the larger grain edges is imaged. Three main CL bands peaked around 1.9 eV, 2.3 eV and 2.7 eV are found at room temperature and ascribed to the excitonic direct band gap transition at K point of MoS2 single layers, to the SiODCII in SiOx and to self-trapped exciton in SiOx respectively. The number of monolayers is correlated to the integrated intensity of the 1.9 eV CL emission. Contrary to what has recently been reported in the literature3, the nanometric triangular adlayers show the highest CL emission intensity. This result is discussed in terms of excitation mechanisms. Monochromatic CL imaging also indicates a homogeneous optical emission inside mono- and bi-layer MoS2 flakes. The Raman spectra show a typical separation of 20 cm-1 between E2g and A1g for 1-2 monolayer samples with E2g more intense than A1g, while the E2g - A1g separation for single-layers is of 18 cm-1. The coverage uniformity of the single-layer MoS2 film is found to be partially discontinuous, as shown by Raman and CL maps. Further, in agreement with AFM studies, an overgrowth at the large grain edges is revealed by CL mapping and confirmed by the E2g/A1g Raman intensity maps. References 1 H. Wang et al., Nano Letters 12, 4674 (2012). 2 O. Lopez-Sanchez, et al., ACS Nano 8, 3042 (2014). 3 J. Jeon et al.; Nanoscale, 2015, 7, 1688–1695

Resume : Among the 2D transition metal dichalcogenides (TMDs), MoS2 is the one that has attracted the most attention due to its structure dependent electronic properties. Monolayer MoS2 is known to exist in three polymorphs: 2H, 1T and 1T’. The former is thermodynamically stable and semiconducting. The metallic 1T structure, on the contrary, was found to be metastable and undergoes a Peierls transition to the 1T’ structure. In the process a small energy gap is opened. In recent years much effort has been made to stabilize T phases over 2H with the aim of gaining enhanced electrical performance or better catalytic activity. Although, several techniques were proposed so far a proper control of the transition is yet to be achieved. In this theoretical work we propose a new route for the MoS2 T phases stabilization: alloying with an MX2 material, SnS2 in particular, for which the T phase is the thermodynamically stable one. Results, carried out by means of Density Functional Theory and Cluster Expansion simulations, show that, even for small amounts of Sn in MoS2, the T and T’ phases formation energy is reduced making the switch between H and T easier. For low doping concentration, it is also reported that the MoS2 and SnS2 semiconductive and most stable phases turn into metals, opening up to the creation of metal-semiconductor junctions for PV cells.

Resume : By displaying a measurable direct band-gap, semiconducting transition metal dichalcogenides (TMDs) such as tungsten disulfide (WS2) offer exciting prospects for the development of novel electronic and optoelectronic devices. Indeed, opportunely designed 2D-heterostacks hold the potential to outperform conventional materials for a wide variety of applications including flexible and transparent electronics with low power consumption. In this work, we report a robust CVD recipe to grow WS2 on both traditional bulk materials and other 2D materials of interest towards optoelectronic applications in 2D-only devices. All the growths were performed using a vapor-phase reaction from solid powders in aquartz tube, steadily keeping to the same growth recipe, allowing for reproducibility of our experiments. As a commonly used reference substrate SiO2 was chosen, while CVD Graphene transferred on Quartz, epitaxial graphene from Silicon Carbide (SiC) substrate and exfoliated h-BN on Quartzserved as substrate for 2D material stacking. The quality, crystallinity, thickness and optical properties of the synthesized films and of the resulting heterostacks were investigated via Scanning Electron Microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), time-resolved Photoluminescence (PL) and Trasmission Electron Microscopy (TEM) will be presented in detail.

Resume : KCa2Na2Nb5O16 ceramics, Dion-Jacobson layered perovskite materials, have been widely investigated because they can be easily exfoliated to the nano-sized sheets which can be used to the future multilayer capacitors with a small size and high performance. A dense KCa2Na2Nb5O16 phase was formed at 1250oC. A proton exchange process was carried out to produce the intermediate phase of HCa2Na2Nb5O16. Stable Ca2Na2Nb5O16- nanosheet colloids were formed using the mixture of the HCa2Na2Nb5O16 powders, tetrabutylammonium hydroxide (TBAOH) solution and water having the same TBA+/H+ ratio. The exfoliated Ca2Na2Nb5O16- nanosheets were dispersed into the acetone medium, which was used to form the thin film by the electrophoretic method under 100 V electric source. The electrophoretic thin films were then annealed at various temperatures for 30 min under ambient condition and all the films have the Ca2Na2Nb5O16 phase. The structural and dielectrical properties of these films will be presented in this work.

Resume : A KSr2NaNb4O13 compound, Dion-Jacobson layered perovskite materials, was synthesized at the very first because it can be used to be the future multilayer ceramic capacitors with a small size and high performance by exfoliating the precursor. The KSr2NaNb4O13 phase was formed at 1200oC with the lateral grain size of several hundred nanometers. Moreover, stable [Sr2NaNb4O13]- nanosheet colloids were finally formed by reacting intermediate phase powders with tetrabutylammonium hydroxide solution. The inorganic [Sr2NaNb4O13]- nanosheets were dispersed into the acetone medium, which was used to deposit the thin film on Pt/Ti/SiO2/Si substrate by the electrophoretic method. The electrophoretic thin films were then annealed at various temperatures under air condition due to organic defects. When the annealing temperature increased until 600oC, all the films have the stable Sr2NaNb4O13 phase with removing organic compounds. In particular, the Sr2NaNb4O13 film annealed at 500oC showed an εr of 73 and a dielectric loss of 1.8 % at 100 kHz. This film also exhibited a high breakdown electrical field of 0.73 MV/cm and a low leakage current density of 5 x 10-8 A/cm2 at 0.7 MV/cm.

Resume : Although hexagonal boron nitride (h-BN) has been known to be a good candidate for gate insulating materials by minimizing interaction from substrate, further applications to electronic devices with available two-dimensional semiconductors are still limited by the flake size. While monolayer h-BN has been synthesized on Pt or Cu foil using chemical vapor deposition (CVD), multi-layer h-BN is still absent. Here, we use Fe foil and synthesize large-area multi-layer h-BN film by CVD with a borazine precursor. These films revealed strong cathodoluminescence and high mechanical strength (Young’s modulus: 1.16 ± 0.1 TPa), reminiscent of formation of high-quality h-BN. The CVD-grown graphene on multi-layer h-BN film yielded a high carrier mobility of ~24,000 cm2V-1s-1 at room temperature, higher than that (~13,000 cm2V-1s-1) with exfoliated h-BN. By placing additional h-BN on SiO2/Si substrate for MoS2 (WSe2) field effect transistor, the doping effect from gate oxide was minimized and furthermore the mobility was improved by 4 (150) times.

Resume : Liquid phase exfoliation is a versatile technique for the production of large-scale, stable dispersion of nanosheets of layered two dimensional materials, but is mostly based on the use of organic solvents as stabilizing reagents, N-Methyl-2-pyrrolidone (NMP) and N-Cyclohexyl-2-pyrrolidone (CHP), which are toxic and health hazardous. In the present study, we report on the production of few layered nanosheets of MoS2 and WS2 in presence of bio-compatible and cost-effective solvent, i.e., water and ethanol, assisted by fungal hydrophobin Vmh2 . This protein is endowed with peculiar physicochemical properties and allows to obtain a stable dispersion of the exfoliated material. Moreover, few steps of cascaded centrifugation enable the selection of bio-functionalized nanosheets of MoS2 and WS2, with large monolayer content, as assessed by Raman and photoluminescence spectroscopy and atomic force microscopy. The analysis of surface charge density, performed with zeta-potential technique, shows that the electrostatic repulsion between the sheets is responsible for the stabilization. The sign of the charge of the nanosheets is negative in the bare solvent and becomes positive when biofunctionalized with hydrophobin.

Resume : Transition metal dichalchogenides (TMDs) can be regarded as two-dimensional (2D) building blocks to design multistacked van der Waals (vdW) heterostructures. The vdW character of the interlayer coupling can be exploited in epitaxial growth of artificial engineered 2D layers upon them, as recently reported for graphene-like silicon on nonmetallic substrates [1] and 2D highly buckled Si nanosheets (NSs) on MoS2 substrates with local hexagonal arrangement [2]. The distortion of the Si NS lattice may influence the electronic coupling at the Si/MoS2 heterosheet (HS) interface, with possible impact on the electronic and optic applications, depending on MoS2 thickness from single layer to bulk regime. To this purpose MoS2 in the form of mechanically exfoliated flakes and of nanosheet grown via chemical vapour deposition are used as substrates for the Si atom deposition. With the aim to unravel the electronic band line-up at the Si/MoS2 HS interface, we used angle resolved photoemission spectroscopy to inspect the valence band structure at the Si/MoS2 HS interface. The Si NS is seen to bend the electronic bands of the bulk MoS2 topmost layers enough to cause an electron accumulation at the Si/MoS2 HS interface [3]. [1] E. Scalise et al., 2D Materials 1 011010 (2014) [2] D. Chiappe, et al., Adv. Mater. 26 2096 (2014) [3] A. Molle et al., Adv, Mater. Interf. (submitted)

Resume : Two dimensional (2D) transition metal dichalcogenides (TMD) are emerging materials for optoelectronic applications. The 2D features and varying band gap depending on the number of layers make them attractive in optoelectronics. However, their room temperature photoluminescence (PL) is rather low due to the intrinsic defect density. Here in, we report the simple technique of few hours dipping of TMD materials in methanol for uniform enhancing of PL. We found the remarkable enhancement in PL from naturally n-type monolayer (1L) MoS2 and WS2, and p-type 1L WSe2. We observed more PL enhancement from further annealing at 80 0C for few minutes. From the study of various time series optical (PL and Raman) mapping data of 1L TMD dipping in methanol, very interestingly, we observed that methanol is inducing duel phenomemon of n-type doping effects and fixing of intrinsic defects for enhancing the PL. We also checked this technique in many layered MoS2, the phenomenon was clearly observed until 5 layers as the indication of effectiveness of the technique in thick TMD materials too. We more confirmed the ongoing phenomenon from electrical transfer characteristics of all the mentioned 1L TMD materials for various time series data of dipping in methanol . Details will be presented.

Resume : Controllable CVD Growth of Monolayer MoS2(1-X)-Se2x Alloys: Size and Bandgap Tuning Mehmet Bay, Ayberk Özden, Aydan Yeltik, Feridun Ay, Cem Sevik, and Nihan Kosku Perkgöz Abstract Two-dimensional (2D) monolayer molybdenum disulfide-selenium alloys (MoS2(1-X)-Se2x) have attracted significant attention thanks to their tunable electronic structure and controllable optical properties, allowing for different device applications including photodetectors, solar cells, and supercapacitors [1]. Using chemical vapor deposition (CVD), it is possible to tune the bandgap of MoS2(1-X)-Se2x by simply changing precursor ratios of S and Se [2]. However, it is still challenging to obtain large, controllable and uniform 2D MoS2(1-X)-Se2x films [3]. In this study, monolayer CVD-grown MoS2(1-x)-Se2x flakes and films were realized by using the process parameters of pressure, reaction time and temperature. Here we show that a systematic optimization of these parameters enables to the growth of MoS2(1-x)-Se2x monolayers as large as ~60 µm on one side. It was found that the mass ratio of MoO3 precursor to Se S precursors is a critical parameter to grow the monolayer MoS2(1-x)-Se2x. In these alloys, we observed the Raman peaks at 403 cm-1 assigned to A1g mode and at 382 cm-1 assigned to E2g mode for MoS2 and those peaks at 240 cm-1 assigned to A1g mode and 270 cm-1 assigned to E2g mode for MoSe2. Intense photoluminescence peaks indicate that the obtained formations are single layer, as also supported by scanning electron microscopy analyses. References [1] A. L. Harris., et al. Inorganic Chemistry, 48 (8), 3882-3839 (2009). [2] Q. Feng, et al. Advanced Materials, 26(1), 2648-2653 (2014). [3] W. Zhang, et al. Nanoscale 7(1), 13554-13560 (2015). Keywords: MoS2 (1-x)-Se2x, CVD, Two Dimensional Alloys.

Resume : In 2009, the estimation of intrinsic and Rashba spin-orbit coupling interaction in clean graphene were found to lie in the ?eV range, which together with the absence of a hyperfine interaction, yield to calculation of spin lifetimes for supported graphene in the unbelievable micro or even milliseconds. In all other materials, metals and semiconductors, the best measured values at room temperature in ?ballistic systems? have been limited to the nanosecond scale. Therefore such theoretical prediction has sparked a lot of expectations heralding graphene as an exceptional (unrivalled) material for the development of lateral spintronic. However, despite monumental efforts and progress in improving graphene quality, resolving contact issues, and reducing substrate effects, the experimentally estimated spin lifetime remains too many orders of magnitude shorter (from 3 to 6 !), even for best mobility samples, with the mysterious incapacity to surpass the nanosecond scale after almost one decade of intense works. This could misleadingly be seen as a critical issue for the advent of practical graphene spintronic, as well as beyond concerning spin transport in all Dirac matter (topological insulators, 3D Dirac semimetal?). In this talk, I will discuss spin transport in graphene based materials, accounting for the effect of substrate, impurities, and ad-atoms and using a fully quantum description of spin dynamics which goes beyond the usual semi-classical theory and conventional approximations made in the literature. The role of spin-pseudospin entanglement in driving spin dephasing and relaxation will be discussed in the ultraclean limit for which electron-hole puddles and micron eV spin-orbit interaction are actually enough to capture the essentials of spin lifetimes observed experimentally. I will further present how chemical functionalization can be further harvested to generate multiple quantum phases and how surface distribution of ad-atoms drives a crossover from the Quantum Spin Hall state to the Spin Hall effect in graphene; a cornerstone spin manipulation phenomenon for the design of innovative spin-based device applications such as zero-power spin processing technologies. Bibliography [1] L. E. F. Foa Torres, S. Roche, and J. C. Charlier, Introduction to Graphene-Based Nanomaterials: From Electronic Structure to Quantum Transport (Cambridge University Press, Cambridge, 2014). [2] Dinh Van Tuan, F. Ortmann, D. Soriano, S. O. Valenzuela, S. Roche, Nature Physics, 10, 857?863 (2014); D. Van Tuan et al., Scientific Reports (submitted) [4] A. Cresti, D. Van Tuan, D. Soriano, A. W. Cummings, S. Roche, Phys. Rev. Lett 113, 246603 (2014)

Resume : We investigate low-temperature transport in ionic-liquid gated transistors based on bulk WS2 and few-layers thick MoS2, exploiting the large gate capacitance to induce very high electron density (~1014 cm-2). In WS2 we observe for the first time a superconducting transition with a fully developed zero-resistance state [1]. The analysis of our results provide evidence for a 2D Berezinskii-Kosterlitz-Thouless mechanism, as expected considering the thickness of the accumulation layer at this carrier density. In MoS2 we observe a SC transition for layers of all thicknesses, down to individual monolayers, which represents the first observation of gate-induced SC in atomically thin van der Waals materials [2]. We compare the superconducting transitions in layers of different thicknesses (from 1 ML to 6 ML) and observe a drastic weakening of the SC in monolayers as compared to all other thicker multilayers crystals, which manifests itself in a significantly lower critical field and temperature. We discuss possible physical mechanisms that could account for our observations.

Resume : Graphene [1] is one of the most studied materials due to its unique properties such as hardness, flexibility and high electric and thermal conductivity. However, probably the best quality of graphene is that it has opened the field to many other 2D crystals [2] including superconductors and topological insulators. In this work, the synthesis and characterization of metal chalcogenides from bulk to thin layers are discussed. As an example, transport measurements in thin layers of 2H-TaS2 are presented: it is observed a superconducting critical temperature enhancement by decreasing the number of atomic layers (from 0.8 K in the bulk sample to ca. 2K in a ~3 nm layer). This behaviour is the opposite of the one reported in other superconductor 2D crystals [3]. This result brings superconductivity into the flatland and may open the door for their future use in magnetic sensors or low energy applications. [1] K. S. Novoselov et al., Science 306 (2004) 666. [2] L. Britnell et al., Science 340 (2013) 1311. [3] M. M. Ugeda, M., Nature Physics 12 (2015) 12, 92–97; M. S. El-Bana et al., Superconductor Science and Technology 26 (2013) 1; A. W. Tsen et al., Nature Physics (2015), online, doi:10.1038/nphys3579.

Resume : The moire pattern, created by the deliberate alignment of crystallographic directions in graphene and a hexagonal boron-nitride underlay, offers a new opportunity to create lateral superlattices and to tune graphene's electronic properties [1,2]. To date, experiments have elucidated the novel miniband structure of this material, where miniband edges and van Hove singularities in the electronic dispersion can be reached by electrostatic gating. Here, we investigate the dynamics of electrons in the moire minibands by Transverse Electron Focusing: the measurement (and theory) of ballistic transport between adjacent local contacts in a magnetic field [3]. At low temperatures, we observe the caustics of skipping orbits extending over hundreds of superlattice periods, the reversals of the cyclotron revolution for successive minibands, and the breakdown of cyclotron motion near van Hove singularities. This information is also used to extract the parameters of the moire superlattice potential. At high temperatures, we study the suppression of electron focusing effects due to the inelastic scattering of electrons. [1] M. Yankowitz, et al., Nature Physics 8, 382 (2012). [2] J. Wallbank, et al., Physical Review B 87, 245408 (2013) [3] V. S. Tsoi, JETP Lett. 19, 70 (1974).

Resume : Either in bulk form, or when exfoliated into atomically thin crystals, layered transition metal dichalcogenides are continuously leading to the discovery of new phenomena. The latest example is provided by 1T’-WTe2, a semimetal recently found to exhibit the largest known magnetoresistance in bulk crystals, and predicted to become a two-dimensional topological insulator in strained monolayers. Here, we show that reducing the thickness through facile exfoliation provides an effective experimental knob to tune the electronic properties of WTe2, which allows us to identify the microscopic mechanisms responsible for the observed classical and quantum magnetotransport down to the ultimate atomic scale. We find that the longitudinal resistance and the very unconventional B-dependence of the Hall resistance are reproduced quantitatively in terms of a classical two-band model for crystals as thin as six monolayers, and that for thinner crystals a crossover to an insulating, Anderson-localized state occurs. Besides establishing the origin of the very large magnetoresistance of bulk WTe2, our results represent the first, complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals, from which we conclude that searching for a topological insulating state by straining monolayers is a challenging, but feasible experiment.

Resume : Graphene nanoribbons (GNRs) are promising materials for electronics, optoelectronics and photovoltaics since they inherit many electronic properties of graphene but have a bandgap as a result of quantum confinement in the lateral dimension. It has been shown recently that GNRs can be chemically synthesized using a bottom-up approach [1]. The ribbon width (and hence the electronic bandgap), edge morphology and position of functional groups, such as alkyl chains, can be controlled during the chemical synthesis. The conductive properties of GNRs might be sensitive to the abovementioned ribbon features. Here we present an experimental study of transient THz photoconductivity [2,3,4,5] in two kinds of GNRs dispersed in organic solvent (1,2,4–trichlorobenzene, TCB), and optically excited above the bandgap. Our GNRs have the same structure of a backbone while the functional groups (C12H15 alkyl chains) are attached at the different peripheral positions. In our experiments, we find that both types of nanoribbons, in spite of different positions of functional groups, feature similar photoconductive response dominated by a mixture of free and excitonically bound carriers. 1. A. Narita et al, Nature Chem., 6, 126 (2013). 2. S. Jensen et al, Nanolett., 13(12), 5925 (2013). 3. A. Narita et al, ACS Nano, 8(11), 11622 (2014). 4. R. Ulbricht et al, Rev. Mod. Phys., 83, 543 (2011). 5. J. Lloyd-Hughes and T-I Jeon, J. Infrared Millim. THz Waves 33, 871 (2012)

Resume : Recently 2D materials (2Ds), particularly the 2Ds beyond graphene, have attracted considerable attention in the transistor community. The demonstration of the first single-layer MoS2 MOSFET in 2010 has led to an explosion of research activities on MOSFETs using semiconducting 2Ds beyond graphene for the channel. Meanwhile a variety of different 2Ds are used as MOSFET channels and many studies on the simulation of such 2D MOSFETs have been reported. On the other hand, there is still a controversial debate on the real prospects of 2D MOSFETs for future electronics. In the present paper we discuss the potential of 2D transistors for ultimately scaled CMOS logic. After introducing scaling-relevant transistor FOMs (figure of merit) and reviewing the state of the art of 2D transistors, we examine 2D MOSFETs in terms of scaling. We show that the subthreshold operation of scaled 2D MOSFETs is governed almost solely by the surrounding of the channel, i.e., gate oxide above and backside oxide underneath the channel, and that for both oxides a low dielectric constant is beneficial for improving the scaling-related FOMs. For the on-state performance, on the other hand, in addition the channel properties related to carrier transport are relevant. Finally, we examine direct source-drain tunneling in ultimately scaled MOSFETs and show that 2Ds with heavy carrier effective mass are promising to suppress tunneling.

Resume : Molybdenum disulfide (MoS2) has recently emerged as a new channel material in field effect transistors. To maximize its potential, it must be associated with an efficient gate dielectric. The combination of 2D materials and new dielectrics could also benefit to large-area/printable electronics, sensors and display technologies, especially if these dielectrics present additional advantages in terms of mechanical flexibility, low temperature processes, conformability to structured substrates, cost and simplicity of equipment and processes, etc. In this context, we developed new dielectrics based on electrografted organic thin films on metallic electrodes. These dielectrics are produced at room temperature and under mild conditions. The process yields uniform films of nanometer thickness (4-8 nm range). We demonstrated the first transistors combining MoS2 as channel material and an electrografted organic ultrathin film as gate dielectric. The transistors exhibit high ION/IOFF ratio together with steep subthreshold slope as low as 110 mV/decade. Besides, their transfer characteristics are totally hysteresis-free due to the hydrophobic and trap-free nature of this dielectric. The first transistors were fabricated on rigid substrates and using mechanically exfoliated MoS2. Their potential in large scale (based on CVD MoS2) and flexible electronics will be discussed on the basis of our latest results.

Resume : Thermoelectric effects allow the generation of electrical power from waste heat and the electrical control of cooling and heating, leading to applications that are compact, fast and robust. These effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can therefore be exploited as probes of distortions in the electronic structure at the nanoscale. In this work we consider two-dimensional graphene as an excellent nanoscale carbon material for exploring the interaction between electronic and thermal transport phenomena, by presenting a direct and quantitative measurement of electronic cooling and heating via the Peltier effect. Thanks to an architecture including nanoscale thermometers we detected temperature modulation of up to 15 mK for currents of 20 uA at room temperature, and observed a full reversal between cooling and heating for electron and hole regimes. This effect, a fundamental thermodynamic property, is also a potential technological tool to achieve refrigeration in mesoscopic systems.

Resume : The optoelectronic response of two-dimensional (2D) crystals, such as graphene and transition metal dichalcogenides (TMDs), is currently subject to intensive investigations. Owing to its broadband absorption, gapless character and ultrafast carrier dynamics, graphene is a promising material for nano-photonics and high-speed photodetectors [1], whereas TMDs have emerged as potential candidates for sensitive photodetection thanks to their enhanced photon absorption. Vertically assembling these crystals in so-called van der Waal heterostructures allows the creation of novel and versatile optoelectronic devices that combine the complementary properties of their constituent materials. Here we present a various new device capabilities, varying from nano-photonic devices to ultra-fast and broadband electrical detectors [1-5]. We applied femtosecond time-resolved photocurrent measurements on 2d material heterostructures, which probes the transit of photoexcited charges across the photoactive TMD layer ? and thus current generation ? directly in the time domain. Remerkably fast photoresponse of 5 ps is observed. In adition, we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic (PTI) effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath, leading to a thermalized ?hot? carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the PTI effect enables detection of sub-bandgap photons, while being size- scalable, electrically tunable, broadband and ultrafast. [1] Photodetectors based on graphene, other two-dimensional materials and hybrid systems F. H. L. Koppens, T. Mueller, Ph. Avouris, A. C. Ferrari, M. S. Vitiello, M. Polini Nature Nanotechnol. 9, 780-793 (2014) [2] High-Responsivity Graphene?Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit. Ren-Jye Shiue, Yuanda Gao, Yifei Wang, Cheng Peng, Alexander D Robertson, Dmitri K Efetov, Solomon Assefa, Frank HL Koppens, James Hone, Dirk Englund Nano Letters, 15 (11), 2015. [3] Picosecond photoresponse in van der Waals heterostructures M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, K. J. Tielrooij and F. H. L. Koppens Nature Nanotechnology 11.1 (2016) [4] Hot-carrier photocurrent effects at graphene?metal interfaces K. J. Tielrooij, M. Massicotte, L. Piatkowski, A. Woessner, Q. Ma, P. Jarillo-Herrero, N. F. van Hulst, F. H. L. Koppens J. Phys.: Condens. Matter 27, 164207 (2015) [5] Generation of photovoltage in graphene on a femtosecond timescale through efficient carrier heating K. J. Tielrooij, L. Piatkowski, M. Massicotte, A. Woessner, Q. Ma, Y. Lee, K. S. Myhro, C. N. Lau, P. Jarillo-Herrero, N. F. van Hulst, and F. H. L. Koppens Nature Nanotechnology 10, 437-443 (2015)

Resume : Heterostructures produced from various 2D materials have led to the development of vertical tunneling transistors(1), photovoltaic devices(2) and light emission devices(3). In this presentation I will present the light emission characteristics of quantum well light emitting devices constructed by stacking graphene, hexagonal boron nitride (hBN) and various transition metal dichalcogenides (TMDC's). In such devices, graphene serves as a transparent charge injection electrode, hBN as an ultra clean tunnel barrier and various single layer direct band gap TMDC's as the active light emitting layer. Interestingly, MoSe2 and WSe2 have opposite signs of the spin orbit splitting of the conduction band, this leads to the lowest energy excitonic states being dark in WSe2 while bright in MoSe2. The result is that WSe2 based light emitting quantum wells become more efficient as the temperature is increased while MoSe2 based light emitting quantum wells become dimmer with increasing temperature(4). (1) T. Georgiou et al., Nature Nanotechnology, 8, 100-103 (2013) (2) L. Britnell et al., Science 340, 1311 (2013) (3) F. Withers et al., Nature Materials, 14, 301-306 (2015) (4) F. Withers et al., Nano Letters, 15(12), 8223-8228 (2015)

Resume : Planar electroluminescence from two-dimensional (2D) semiconductors such as MoS2 is often a low-yield process due to short resident time of carriers that inhibit efficient formation of excitons. While multiple quantum well structures based on alternating layers of 2D semiconductors and insulators has proved useful for achieving longer carrier resident time, higher emission quantum yield is achieved at a cost of increase in device resistance and complex device structure. Here we report bright planar electroluminescence from a simple metal-insulator-semiconductor (MIS) diode built entirely from 2D materials. We show, in our graphene-hBN-WS2 devices, holes in the graphene electrode tunnel into WS2 through the insulating hBN layer and radiatively recombine with electrons that are present in the WS2 layer. Since the hBN layer acts as an electron blocking layer, a large fraction of electrons recombine with holes. This allows observation of electroluminescence at a current density as low as 1 nA/um2. We verify the operation mechanism by studying emission intensity as a function of gate-modulated electron density in WS2.

Resume : The discovery of graphene marked the start of research in 2D electronic materials which was expanded in new directions with MoS2 and other layered semiconducting materials. They have a wide range of promising potential applications, including those in digital electronics, optoelectronics and flexible devices. Combining 2D materials in heterostructures can increase their reach even further. In my talk, I will review the status of our research in 2D transition metal dichalcogenides (TMDCs) and present our current level of understanding on the influence of contacts, material quality and the environmental effects on 2D materials, all critical for achieving high performance levels in devices based on 2D semiconductors. I will also update on our efforts to achieve high operation frequencies in scaled TMDC devices. Next, I will present our recent work on electromechanical response of MoS2 and graphene.

Resume : We present experimental results on operating temperatures and heating coefficients measured in a multilayer black phosphorus device as a function of injected electrical power. By combining micro-Raman spectroscopy and electrical transport measurements, we have observed a linear temperature increase up to 600 K at a power dissipation rate of 0.896 K μm3/mW. By further increasing the bias voltage, we determined the threshold power and temperature for electrical breakdown and analyzed device failure by means of scanning electron microscopy and atomic force microscopy. Further, we present recent low temperature and time resolved measurements on other van der Waals heterostructures.

Resume : Because suspended monolayer transition metal-dichalcogenides (TMD) are ultra-thin membranes with exceptional optical properties, they are ideal materials for opto-mechanical applications. Here we developed an ultrasensitive optical readout of monolayer TMD resonators that allows us to reveal their mechanical properties at cryogenic temperatures. We find that the quality factor of monolayer WSe2 resonators greatly increases below room temperature, reaching values as high as 1.6E4 at nitrogen temperature and 4.7E4 at helium temperature. This surpasses the quality factor of monolayer graphene resonators with similar surface areas. Upon cooling the resonator, the variation of the resonant frequency unveils the thermal expansion coefficient of WSe2 monolayers, which reaches 1e-5 K-1 at room temperature in agreement with recent theoretical calculations. The high quality-factor found in this work opens new possibilities for coupling mechanical vibrational states to two-dimensional excitons, valley pseudospins, and single quantum emitters.

Resume : Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic and quantum computing devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics, but several challenges remain unsolved: how can molecular spins be assembled into hybrid structures? What is the influence of the graphene environment on the spin? Can molecules be used to control coherent currents in graphene devices? To answer these questions we created van der Waals molecular heterostructures for graphene-based spintronics. A complete, detailed picture of the assembly process showcases the relevance of dynamic scaling theory in the creation of such heterostructures, and offers the first insight into the dynamics of molecular magnet assembly on surfaces. We then show the relevance of residual molecular mobility in creating self-assembled networks, and explore the importance of graphene surface defects, by using different graphene surfaces. We eventually show how such tuning of the heterostructures produces dramatically different magnetic behaviours, and is thus pivotal for molecular spintronic devices.

Resume : The two-dimensional nature and low mass of graphene, combined with its high Young?s modulus and stretchability, makes it a promising candidate for nanoelectromechanical systems (NEMS) [1]. Experimental data on graphene-membrane based pressure sensors will be discussed that rely on the strain-induced piezoresistive effect. Even though the piezoresistive gauge factor is quite low, the ultimate thinness of the material leads to an extremely high sensitivity compared to commercial and other sensors based on nanotechnology. In addition to large area CVD graphene, the talk will cover graphene ink-based devices for strain measurements. Nevertheless, a number of parasitic effects can influence and falsify the response of a graphene sensor [2]. This talk aims to carefully balance the discussion about the merits and disadvantages of graphene and related 2D-Material NEMS based on experimental data. [1] A. D. Smith, F. Niklaus, A. Paussa, S. Vaziri, A. C. Fischer, M. Sterner, F. Forsberg, A. Delin, D. Esseni, P. Palestri, M. Östling, M. C. Lemme, ?Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes,? Nano Lett., 13(7), 2013. [2] A. D. Smith, K. Elgammal, F. Niklaus, A. Delin, A. C. Fischer, S. Vaziri, F. Forsberg, M. Råsander, H. Hugosson, L. Bergqvist, S. Schröder, S. Kataria, M. Östling, M. C. Lemme, ?Resistive graphene humidity sensors with rapid and direct electrical readout,? Nanoscale, 7(45), 2015.

Resume : The elimination of extrinsic sources of spin relaxation is the key to the realization of the exceptional intrinsic spin transport performance of graphene. Towards this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture that allows us to make a comparative study for separately investigating the roles of substrate and polymer residues on spin relaxation. The comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. On the other hand, the observation of a 5-fold enhancement in spin relaxation time in the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin relaxation length of ~10 μm in the encapsulated bilayer with a charge mobility of 24,000 cm 2 /Vs. The carrier density dependence of spin relaxation time has two distinct regimes; n<4x10 12 cm -2 , where spin relaxation time decreases monotonically as carrier concentration increases and n>4x10 12 cm -2 , where spin relaxation time exhibits a sudden increase. The sudden increase in the spin relaxation time with no corresponding signature in the charge transport suggests the observation of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of spin signal was observed while a transport gap is induced, which is interpreted as a novel manifestation of impedance mismatch within the spin channel.

Resume : High frequency devices and circuits, the core of modern information and communication systems, have been recognized from the very beginning as a promising field of applications for 2D materials, having the potential to significant outperform established devices based on Silicon or III/V semiconductors in terms of speed, functionality or flexibility. This large expectation has been fuelled mainly by graphene?s outstanding charge carrier mobility, the large saturation velocity and the 2D nature giving prospects for ultimately scaled devices. But also the specific properties of other 2D materials like MoS2, phosphorene or WSe2 have attracted significant interests especially for printed and/or flexible RF electronic applications and heterostructures. In this talk I will review the current state-of-the-art for RF devices and circuits based on 2D materials and point out specific challenges and opportunities. Significant efforts have been directed towards the realization of field effect transistors as one key RF component. Also other devices like diodes, varactors, vertical transistor and etc. based on different 2D material provide a promising or already outstanding performance. However the realization of more complex circuits containing several devices would be the next essential step towards the success of 2D materials in applications. This requires first a certain level of process control to reproducible fabricated these devices with predefined performance parameters and also significant efforts on the circuit and system level to fully exploit the specific parameters of devices based on 2D materials.

Resume : An important class of 2D nanomaterials beyond graphene are transition metal dichalcogenides such as MoS2. The coexistence of metallic (1T) and semiconducting (2H) phases in MoS2 represents a distinct polymorphism in terms of structural and electronic properties, an important deviation from graphene [1]. Our investigations focus on a metallic 1T-MoS2 strip embedded in semiconducting 2H-MoS2. The whole material is referred to as a ‘hybrid MoS2’ monolayer. We report on four aspects of such a hybrid system: (i) the metallic/semiconductor interface, (ii) the variation of the electronic properties with respect to the width of the embedded metallic strip, (iii) the point defect formation in hybrid MoS2, and (iv) the electronic transport properties of the hybrid MoS2 sheet. Our work is based on quantum-mechanical simulations within the density functional theory approach. The quantum transport calculations are carried out using the non-equilibrium Green's functions formalism. We discuss the relevance of our results with respect to potential nanotechnological applications such as nanoscale biosensing devices [2]. [1]Song et al, RSC Adv. 5, 7495 (2015) [2]Feng et al, Nano Lett. 15, 3431 (2015)

Resume : A single material logical junction is designed by exploiting strong quantum confinement effects in a layered noble transition metal dichalcogenide (TMCs). The proposed device is based on 1T PdS2 with metallic bilayers acting as electrodes and a semiconducting monolayer as the scattering channel, what results in strongly reduced contact resistance. The effective electron mass is similar to that of the well-known 2D MoS2 crystal. The channel length of only 2.45 nm shows negligible tunnel loss and thus marks the limit of integration density for a conventional transistor. Similar phenomena might be observed in other noble-metal TMCs.

Resume : In the last couple of years, a novel field has emerged which deals with structures and devices assembled layer-by-layer from various atomically-thin crystals. These new multilayer structures have proved to be extremely versatile, showing exceptional electronic and optical properties, new physics and new functionality. This is mostly due to the fact that each atomic layer can be chosen among many different materials including metals, semiconductors, superconductors or even topological insulators. For example, graphene?s ?sister? material hexagonal boron nitride has similar mechanical properties but on the contrary is a wide gap insulator. Another example is that by using transition metal dichalcogenides we can now overcome the band gap problem by producing tunnelling transistors that consist of two layers of graphene separated by a few atoms thick layer of molybdenum disulphide. In this talk I will review recent progress in this field and present important milestones in its development. Specific attention will be paid to fabrication of such structures and their chemical stability as well as charge transport and optical experiments demonstrating various proof-of-concept functionalities. I will also discuss new additions to the 2D material family such as black phosphorus and niobium diselenide and present results on their properties down to one atomic layer thickness.

Resume : Tungsten diselenide (WSe2) and Molybdenum diselenide (MoSe2) monolayers are interesting two dimensional transition metal dichalcogenides (TMDs) materials possessing direct band gap and tunable transport properties which make them suitable for variety of optoelectronics applications.(1,2) Nowadays, most of the researches are focused on the study of hybrid structures of TMDs and their properties. In this work, we demonstrated optical tailoring of CVD grown p-type WSe2 monolayer by decorating MoSe2 quantum dots on it to make their hybrid structure. We synthesized the MoSe2 quantum dots by the combination of sonication and solvothermal process from its bulk powder.(3) We observed the reduction of photoluminescence (PL) spectra of these quantum dots decorated on monolayer WSe2 as well as the quenching and blue shifting of A exciton PL peak of WSe2 monolayers, which suggests the charge transfer occurring from MoSe2 quantum dots to WSe2 monolayer. Details of the hybrid structure preparation and nanoscale optical characterization results will be presented. 1. Xingli et al. ACS Nano. 2014, 8, 5125-5131. 2. Bilu et al. ACS Nano. DOI: 10.1021/acsnano.5b01301 3. Shengjie et al. Adv. Funct. Mater. 2015, 25, 1127-1136.

Resume : Atomically thin semiconductors are technologically promising materials that can complement graphene in applications where its lack of band gap hinders its use. Inherent to their reduced out-of-plane dimension, atomically thin semiconductors present a marked thickness-dependent band structure due to the quantum confinement of the charge carriers. This thickness-dependent band gap can be very advantageous for applications as photodetectors because one can select the sensing photon energy window by simply selecting the right material thickness for the photodetector device. Up to now, however, there is a broad spectral window from 2 eV to 3 eV uncovered by 2D materials that could be very relevant for applications requiring near-UV light detection. I In this work we demonstrate a very strong quantum confinement effect in the optical properties of atomically thin α-In2Se3 crystals. We observe a marked thickness-dependent shift in the optical absorption spectra of mechanically exfoliated In2Se3 flakes. While for the thicker flakes we extract band gap values of 1.45 eV (in good agreement with that of bulk α-In2Se3), thin flakes show a remarkable increase of their optical band gap, reaching 2.8 eV for 3.1 nm thick flakes. Such increase of the optical band gap is among the largest reported to date in atomically thin semiconductors.

Resume : Excitonic systems separated by a thin barrier layer strongly interact via near-field coupling. A vertical stack of semiconducting two-dimensional (2D) crystals is effectively a coupled quantum well that is expected to allow efficient exciton energy transfer across a sub-nanometer van der Waals gap via near-field interaction. In previous studies on hetero-bilayers of group 6 transition metal dichalcogenides (TMDs), the interlayer charge transfer processes was assumed to dominate exciton decay dynamics over energy transfer due to type-II band alignment. In this talk, I will discuss our experimental observation of fast interlayer energy transfer in MoSe2/WS2 hetero-bilayer using photoluminescence excitation (PLE) spectroscopy. We find that the energy transfer is Förster-type involving excitons in the WS2 layer resonantly exciting higher order excitons in the MoSe2 layer. Our results indicate that energy transfer competes with interlayer charge transfer with efficiencies exceeding 80% despite the type-II band alignment [1]. [1] Kozawa et al. ?Efficient interlayer energy transfer via 2D dipole coupling in MoSe2/WS2 heterostructures? arXiv:1509.01875

Resume : Two-dimensional (2D) ternary semiconductor single crystals, an emerging class of new materials, have attracted significant interest recently owing to the great potential for academic interest and practical applications. In addition to other types of metal dichalcogenides, 2D tin dichalcogenide semiconductors are also important layered compounds with similar capabilities. Yet, multi-elemental single crystals enable to assist multiple degrees of freedom for tuning physical properties via ratio variation. Pursuing such novel 2D materials is a high priority in the discovery of suitable and promising candidates for high performance-based devices. The discovery of graphene has ignited tremendous development in materials science because of its unique, novel, and exciting electric and mechanical properties. However, the absence of a band gap and weak light absorption in graphene limit its advantageous optoelectronic applications, therefore paving the way for other 2D layered semiconducting materials with versatile electronic and optical employments. To date, a few layered materials, such as transition metal dichalcogenides (e.g., MoS2, MoSe2, WS2, and MoTe2) and other 2D crystals (e.g., GaS, GaSe, and InSe) have been investigated. In particular, the layered chalcogenides of group IV-VI (GeS, GeSe, SnS, SnSe, etc.) have attracted wide research interest because of their suitable band gaps, high absorption coefficients, anisotropic optical/electrical properties, high thermal stability, low toxicity, and environmental sustainability. These encouraging results motivated us to investigate the photoresponsive properties of Se doped SnS2 or [Sn(SxSe1-x)2] alloys, and demonstrate their great candidate potential for photo-transistors with impressive performance, which have not been reported thus far. Current research on 2D materials has been almost exclusively focused on single elemental and binary systems. The investigation of the ternary Sn(SxSe1-x)2 alloys system may also bring a new dimension to 2D material research. Herein, we report the successful synthesis and unambiguously demonstrate the isolation of ultra-thin few-layered (~ 6 nm thick) Sn(SxSe1-x)2, as a 2D atomically layered ternary compound. A comprehensive study of high performance few-layered SnSSe photo-transistors fabricated on both rigid SiO2/Si and flexible polyethylene terephthalate (PET) substrate has been performed. The few-layered Sn(SxSe1-x)2 channel exhibits the n-type characteristics of a field-effect transistor (FET) with a carrier mobility of ∼4.6 V−1 cm2 s−1. To the best of our knowledge, this is the first report of ternary semiconductor of few-layered SnSSe to serve as photo-transistors, capable of conducting photodetection with high photoresponsivities of up to 6000 AW−1 (on SiO2/Si) and 1.2 AW−1 (on PET) at 633 nm, which are superior to those of other reported 2D crystal-based (SnS2, SnSe2, MoS2, and GaTe) photodetectors. It produces ultra-high gain (η) over ∼8.8×105 and the measured specific detectivity (D*) ∼8.2×1012 J of few-layered SnSSe photo-transistor is comparable to that of commercial silicon photodiodes. Taking a step further, the photoresponsivities of the SnSSe device on the flexible PET film were examined with and without bending, of which the measured data are inimitable to those on a rigid SiO2/Si substrate. These figures-of-merit demonstrate that few-layered SnSSe nanosheets hold great potential for device applications, especially for fabrication of photo-transistors with ultrahigh photoresponse and flexible optoelectronics.

Resume : Two-dimensional (2D) Transition Metal Dichalcogenides (TMDs) semiconductor materials are becoming increasingly attractive as possible channel alternatives for future field effect transistors (FETs), due to their attractive electronic properties [1]. In particular, TMDs inks are now available [2-3], opening the possibility to make functional transistors on flexible and transparent substrates by using simple techniques such as ink-jet printing. Dielectric and semiconductor materials that can be processed from solution are indeed promising candidates for printed electronics [4]. However, achieving high performance devices using solution-processed materials is very challenging. In this work we investigated the use of printable inks of exfoliated WS2 nanoflakes as semiconducting layer in field-effect transistors. The electrical characteristic of the FETs is studied for different solvents (NMP, water, etc) and for different inks composition (WS2/Graphene inks with variable ratio). [1] B. Radisavljevic et al, Nature Nanotechnology 6, 147–150 (2011) [2] V. Nicolosi et al, Science, 340 no. 6139 (2013) [3] H. Yang et al, 2D Materials, 1 (1), 011012 (2014) [4] Sirringhaus, H. et al. Science 290, 2123-2126 (2000)

Resume : By means of density functional theory calculations, we predict that several two-dimensional AB binary monolayers, where A and B atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-splitting phenomena. We show that valley-related properties originate mainly from the binary nature of AB monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.

Resume : In this work, electrical conductivity as a function of doping and temperature in two-dimensional (2D) metal oxides has been investigated for use as transparent conducting electrodes. The monolayer nanosheets with a thickness of ~ 1 nm were chemically exfoliated from a single-crystalline bulk metal oxide using an electrostatic-assembly method. In order to investigate the effect of doping on electrical conductivity in 2D metal oxides, the nanosheets were doped with metal ions. For accurate electron transport measurements, a four-point probe measurement system was used to measure electrical conductivity in the temperature range 7 – 300 K under a high vacuum. Electrical conductivity in bare nanosheets was obtained to be ~ 1.35ⅹ10^5 S/m. On the other hand, we found that metal ions doping and reduction processes are very effective to improve electrical conductivity of 2D metal oxides. Electrical conductivity in the doped and reduced nanosheets were measured to be 2.48ⅹ10^5 S/m and 6.97ⅹ10^5 S/m, increased by 84% and 417%, respectively, as compared to that in the bare nanosheets. We also found electrical conductivity in 2D metal oxides decreases with decreasing temperature. Our results indicate that the 2D metal oxides are semiconducting due to the change of the band structure in nanoscale, in contrast to metallic bulk metal oxides [1]. The atomic structure and band structure in the 2D metal oxides are also discussed in detail. [1] W.D. Ryden et al, Phys. Lett., 1968, 209-210.

Resume : Heterogeneous integration of 2D materials in vertically stacked monolayers enables the engineering of novel material architectures with highly tuneable electronic and optical properties. Interfacing transition-metal dichalcogenides with graphene is considered a promising approach for creating field effect transistors, photo-responsive memory devices, and ultrafast, high-gain photodetectors. However, the underlying characteristics of the charge transfer across the interface, as well as the effect that inter-layer electronic coupling has on the excitonic properties are yet to be fully elucidated. We present a comprehensive study that correlates the electronic and optical properties of semiconductor-semimetal heterostructures built from tungsten disulphide (WS2) directly grown on epitaxial graphene on silicon carbide. We employ scanning Kelvin probe microscopy to study the local electronic properties of monolayer islands of WS2, with lateral sizes typically larger than 10 μm. Novel excitonic effects are revealed by room temperature photoluminescence spectroscopy experiments, which show that the optical properties of WS2 can be effectively tuned by the number of supporting graphene layers. These effects are rationalised in terms of efficient charge transfer between WS2 and graphene layers, together with the inter-play between the low energy exciton and trion photoluminescence. The results are supported by ab-initio calculations.

Resume : Interest in the superconducting properties of layered transition metal dichalcogenides (TMDs) has been recently renewed by several key discoveries such as electric-field-induced superconductivity and the role of spin-valley locking in MoS2. Superconductivity in intercalation compounds of TMDs has been studied since the 60’s [1] but its connection to the recently observed electric-field-induced superconductivity remains elusive [2]. In this contribution, we report superconducting transition in mechanically exfoliated flakes of LixMoS2 grown by chemical vapor transport (CVT). We observe multiple transition temperatures ranging from 3 to 7 K. Interestingly, depending on the samples, resistivity was found to show insulator-like temperature dependence above the transition temperature in contrast to typical superconducting materials, including electrostatically doped MoS2. This change in the sign of temperature coefficient near superconducting transition and its magnetic-field dependence are consistent with a superconductor-to-insulator transition driven by induced localization of Cooper pairs. We attribute our observation to the multi-phase structure of LixMoS2 in which superconducting and insulating phases form a disordered network. [1] Intercalated layered materials, ed. F. Levy (D. Reidel Pub. Co, Holland, 1979). [2] J. T. Ye et al., Science 338, 1193 (2012); Y. Saito et al., Nat. Phys., DOI:10.1038/nphys3580 (2015).

Resume : Recent theoretical studies predict that multilayer transition metal dichalcogenides are strongly sensitive to vertical electric fields and exhibit electro-optic effects such as field-tunable dielectric constant and giant Stark effect [1,2]. However, experimental investigation of these effects is largely unexplored. Here we report a systematic study of electronic and optical properties of dual-gated monolayer and bilayer WSe2 layers. The dual gate configuration allows us to control the doping level and vertical electric field independently and correlate these parameters to the optical and transport gap of these materials. We studied photoluminescence and transfer characteristics of these materials under electric fields up to 2 V/nm. We show that optical and transport gap of these materials exhibit weak dependence on electric field contrary to the theoretical predictions. We attribute our observation to screening of external electric fields by trapped charges at the interface of the semiconductor and the dielectrics. [1] A. Ramasubramaniam, et al., Phys. Rev. B 84, 205325 (2011). [2] E. J.G Santos and K. Efthimios, ACS Nano 7, 10741 (2013).

Resume : The great versatility of carbon materials arises from the strong dependence of their physical properties on the ratio of sp1 (carbyne-like), sp2 (graphite-like) to sp3 (diamond-like) bonds. An amorphous carbon can have any mixture of sp3, sp2 and even sp1 sites, with the possible presence of hydrogen and nitrogen [1]. One of the recent developments in the field is the ion-assisted condensation of carbyne-like sp1 chains, in-plane ordered in a hexagonal structure with nearly 5 angstroms interchain distance. Such structures have been studied by a plenty of experimental and numerical techniques, including electron and atomic force microscopy, IR and Raman spectroscopy. Comprehensive ab initio calculations were performed to predict the structure of carbyne crystals [2]. However the set of Raman bands obtained still lacks well-grounded interpretation. The primary aim of this work is to combine the obtained experimental Raman spectra and calculated phonon modes of the simple carbyne crystal structural model. As a sample system we used the 2000 – 4000 angstrom chains on glass substrate synthesized by ion-assisted condensation in a high vacuum (10-7 Torr) where the streams of carbon and inert gas ions (Ar+) impinge on the substrate. Density functional theory calculations allowed to establish the dependence between chain length, IR- and Raman-active frequencies, giving interpretation to high-energy and low-energy vibrations. Laser annealing results confirm the proposed explanation.

Resume : Transition metal dichalcogenides (TMDCs) are promising materials because of high performance in electronic and optical properties. One of interesting studies in TMDCs is the application of field-effect transistors (FETs) based on a few layers MoS2. In order to enhance the performance of TMDCs-based FETs, polarization characteristics must be improved. A few layers of molybdenum diselenide (MoSe2) on PbTiO3 (PTO) epitaxial thin-films were used for this study. The MoSe2-PTO heterostructures were characterized by scanning probe microscopy. Nanoscale electrical transport and surface potential were observed by using conductive-atomic force microscopy and Kelvin force microscopy, respectively. Tuning of polarization on PTO with MoSe2 thin layers were performed. Remnant polarization values at zero voltage were enhanced and difference of polarization at the up- and the down-states is increased. As a result, we can control the performance of the MoSe2 by manipulating the non-volatile ferroelectric characteristics.

Resume : Recently, two dimensional materials with noncentrosymmetric structure have received significant interest due to their potential usage in piezoelectric applications. It has been reported by first principles calculations that relaxed-ion piezoelectric strain (d11) and stress (e11) coefficients of some transition metal dichalcogenide monolayers (TMDCs) are comparable or even better than that of conventional bulk piezoelectric materials. Furthermore, piezoelectric coefficient of MoS2 has been measured as 2.9 × 10−10 C/m, which agrees well with mentioned theoretical calculations[1]. In order to deeply investigate this potential, we perform first-principles calculations and systematically investigate the piezoelectric properties various single layer structures[2]: 18 TMDCs, 7 TM oxides, and 21 II-VI compounds. We predict that not only the Mo- and W-based TMDCs but also the other materials with Cr, Ti, Zr and Sn exhibit highly promising piezoelectric properties. Moreover, we surprisingly observe that the calculated d11 coefficient of some II-VI compounds are quite larger than that of TMDCs and the bulk materials, α-quartz, w-GaN, and w-AlN which are widely used in current applications. Our calculations clearly reveal that monolayer semiconductors are strong candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices. [1] H. Shu et al. Nat. Nano. 10, 151, 2014 [2] M. M. Alyoruk et al. J. Phys. Chem. C 119, 23231, 2015

Resume : Two dimensional materials and their multilayers are emerging candidates for post CMOS nanoelectronic devices and integrated circuits. However, the fabrication of low ohmic contacts is still a challenge in device fabrication using two dimensional materials. This is caused by the pristine nature of the MoS2 surface causing a tunnel current dominated ohmic contact. A further factor hindering low contact resistance is the lack of techniques allowing the doping of the two dimensional materials below the contacts. This causes to a voltage dependence of the contact resistance obscuring the extraction of the channel resistance and the true mobility values in the fabricated transistor devices. To overcome this challenging task we applied the Y-method developed for the parameter extraction of MOSFET transistors. This method combines drain current and transfer characteristics allows to isolate the influence of the contact resistance from the channel resistance. It gives the ability to determine the low filed mobility and the mobility attenuation factor. We demonstrate the applicability of this method to determine the contact resistance, channel resistance, low field mobility, the mobility attenuation factor and the saturation drift mobility for MoS2 field effect transistors. Furthermore, low contact resistance for Ti based ohmic contacts will be demonstrated.

Resume : We report on the effects of low-energy (20 eV) hydrogen irradiation on the electronic properties of mechanically exfoliated MoSe2 flakes studied by micro-photoluminescence (μ-PL) spectroscopy. Depending on the number of layers, quite diverse and surprising H-induced effects on the emission properties of MoSe2 are observed. In the single-layer samples, we record an intensity reduction and a broadening of the PL signal and the appearance of a broad defect band below the free exciton (FE) energy. Instead, in the bilayer flakes a 10-fold enhancement in the radiative recombination efficiency is found and sharp recombination lines are observed both at higher and lower energies with respect to the band gap FE of the untreated bilayer. Finally, in the hydrogenated bulk-limit samples, we observe PL emission up to room temperature at energy 20 meV higher than that of the FE in single-layer flakes. Atomic force and scanning electron microscopy measurements unveil the formation of circular craters (diameter~1-4 μm) along with filamentary features (diameter~100-500 nm and length~1-4 μm), likely induced by chemically reactions involving H and Se atoms. Notably, 2-dimensional μ-PL maps show that radiative recombination originate from circular regions, whose size matches that of the micro-craters observed by microscopic measurements. These findings open the way to a flexible patterning of the emission properties of single- and multi-layer TMDC.

Resume : Germanium monochalcogenides (GeS and GeSe) crystallize in layered structures with a weak bonding between the layers. It is expected that single-layer germanium monochalcogenides exhibit physical properties different than their bulk counterparts. This is due to strong changes in the electronic band structure which are caused by the reduction of crystal size (from a bulk regime through a few layers to a monolayer). However, optical properties of GeS and GeSe flakes, which are composed of a few layers, have not been intensively investigated up to now. In this work we applied photoreflectance (PR) and photoacoustic (PA) spectroscopy to study the electronic band structure of GeS and GeSe bulk crystals and flakes. PR spectroscopy, due its differential and absorption-like character, is an excellent technique to study direct optical transitions in semiconductor materials, while PA spectroscopy is a good tool to study the indirect gap. In the case of bulk GeS and GeSe crystals the indirect gap has been determined by PA spectroscopy. The direct optical transitions have been clearly observed in PR spectra. Afterwards, the flakes composed of a few monolayers have been exfoliated from the bulk crystals and studied by micro-PR spectroscopy. Changes in PR spectra of GeS and GeSe flakes composed of various number of layers will be discussed in this work.

Resume : Recently, transition metal dichalcogenides (TMDs) have been received great concerns as a strong candidate for future electronics. Modulation of electrical properties is an attractive issue for high-performance complementary logic circuit. In this work, we demonstrate that ambipolar behavior in MoS2 field effect transistors (FETs) by heating MoS2 flakes under air atmosphere in the presence of cobalt oxide catalyst (MoS2 + O2 → MoOx + SOx). The catalytic oxidation of MoS2 flakes between source-drain electrodes resulted in lots of MoOx nanoparticles (NPs) on MoS2 flakes. N-type behavior of MoS2 FETs was converted to ambipolar transport characteristics by MoOx NPs which inject hole carriers to MoS2 flakes. To simulate the partly hole carriers injected MoS2 FETs, we set the equivalent circuit model with the hexagonal networks of n-type transistors. SPICE calculations were performed for each randomly exchange n-type to p-type transistors from the networks to investigate that how affect the electrical properties in MoS2 channel is affected from partial hole carrier injection.

Resume : After the isolation of graphene [1], two dimensional (2D) materials have taken the spotlight as future active components in the design and fabrication of nanoscale devices, due to the extraordinary variety of their electronic properties and to the possibility of stacking different layers in a seamless way to create the so called van der Waals heterostructures [2]. One of their most exciting applications is the design and optimization of 2D MOSFETs for their further use as A-RAM memory cells [3, 4]. In such cells, the binary value is given by the current flowing through the bridge connecting source and drain, which is controlled by the holes accumulated between the gate and the bridge under a negative gate bias. In this work, we focus on a heterostructure formed by a metallic graphene layer acting as the bridge, a semiconducting MoS2 channel and a hexagonal boron nitride (h-BN) layer acting as the gate insulator. We carried out ab initio calculations using the SIESTA method [5] in order to evaluate the electronic properties of the system and the interaction between the current flowing through the graphene layer and the charge accumulation (or lack of) on the MoS2 layer. [1] K. S. Novoselov et al, Science 306, 666-669 (2004). [2] A. K. Geim and I. V. Grigorieva, Nature 499 , 419 (2013). [3] N. Rodriguez et al, IEEE Electron Dev. Lett. 31 972 (2010). [4] N. Rodriguez et al, IEEE Trans. Electron Dev. 58 2371 (2011). [5] J. M. Soler et al, J. Phys.: Condens. Matter 14, 2745 (2002).

Resume : We have investigated the doping effects of triethanolamine(TEOA) on multilayer molybdenum disulfide (MoS2) field effect transistors (FETs). From the transfer characteristics and the Raman spectra, it is found that TEOA acts as a n-type surface dopant, where the TEOA is absorbed on the surface of MoS2. As a result of the TEOA doping process, the electrical performances of multilayer MoS2 FETs were dramatically enhanced at room temperature. Extracted field effect mobility was estimated to be ~30 cm2/Vs after the surface doping process, which is 10-times higher than that of the pristine device. Subthreshold swing and contact resistance were also improved after the TEOA doping process. These improvements can be explained by the Schottky barrier lowering and the equivalent circuit modeling of the layered structure.

Resume : Nowadays, few-layer black-phosphorus has sparked the interest in the scientific community due to its potential in nanoelectronic devices, optoelectronic applications[1] and some theoretical works, predict also great potential in thermoelectric applications[2]. According to these works, few layer black-phosphorus can reach Seebeck coefficients (S) up to +2 mV/K. Nevertheless, the transport properties of bulk[3] and few layer black-phosphorus[4] are little experimentally known. In this communication, the optical and transport properties of bulk black-phosphorus are exposed. Kubelka-Munk function, obtained from FTIR measurements at RT, has been used to determine the direct band gap energy (0.35 ± 0.05 eV), which is in good agreement with the previously reported value[5] and that estimated from the thermoelectric measurements (0.32eV) [3]. Direct measurements of electrical resistance (R) and S, in a temperature range near to RT (300-400K), yields an increasing value of S from +335 ± 10 μV/K at RT up to +400 ± 10 μV/K at 350 K. At T > 350K, S seems to stabilize around 410 ± 10 μV/K. On the other hand, R continuously decreases as temperature is increased. In consequence, the power factor of bulk black-phosphorus reaches values  3 times higher than at room temperature. This quality is highly convenient for thermal waste energy harvesting applications. [1] H. Liu, et al. Chem Soc Rev 2014, 10, 1039. [2] J.-W. Jiang, Nanotechnology 2015, 26, 055701. [3] E. Flores, et al. Appl. Phys. Lett. 2015, 106, 022102. [4] T. Hong, et al. Nanoscale 6, 2014 , 8978. [5] H. Asahina et al. J. Phys. C: Solid State Phys. 1984, 17, 1839.

Resume : Recently, layered transition metal dichalcogenide (TMdC) materials with the chemical formula MX2 (M = Mo, W, Ti, V,.. and X = S, Se, Te,..) have attracted much interest for fundamental sciences and technology applications. There have been several attempts to modify optical properties of 1L-MoS2 including enhancing or quenching the PL intensity by engineering the structural defects. Photoluminescence (PL) from monolayer MoS2 (1L-MoS2) has been modulated using plasma treatment or thermal annealing. Yet, a systematic way of engineering optical properties has not been demonstrated, let alone understanding the underlying mechanism. Here, we have investigated modification of optical properties of 1L-MoS2 by laser irradiation under ambient conditions with different exposure time. Three distinct regions exist in response to laser irradiation time; slow photo-oxidation involving physisorption, fast photo-oxidation involving chemisorption, and photo-quenching involving structural degradation, respectively. Raman spectroscopy combined with atomic force microscopy confirms morphological modification related to oxygen-associated structural defects.

Resume : One of the main technological issues to be solved in graphene technology is the realization of graphene/metal contacts with very low contact resistance. Unfortunately, the values reported in literature are not completely consistent and the mechanisms at play at the interfaces are not fully understood. The principal aim of our work is to understand the nature of the mechanisms involved in the metal-graphene interface, through an in-depth multiscale theoretical study at the fundamental level, exploiting ab-initio simulations. We have considered two different categories of metals, based on the binding energy and the metal-graphene distance, i.e., physisorbed and chemisorbed metals. We have performed electronic calculations and detailed analysis of the electrostatic potential at the interface region of the system and computed transport through the interface, so to accurately calculate the contact resistance. Our approach is able to explain experimental results available in the literature, and it can be used in order to engineer contact resistance (i.e., giving hints so to obtain optimized interfaces), and to eventually obtain high performance devices based on graphene.

Resume : The valley index in 2D semiconductor crystals based on transition metal dichalcogenide (TMDC) monolayers (MoS2, WS2...) [1] could constitute a novel degree of freedom to carry and process information. Electrical generation and control of valley-polarized carriers, constitutes a key point for the use of this valley degree of freedom in nanoelectronics, and yet remains a formidable challenge [2]. Due to the unique correlation between spin and valley indices of electric carriers in these materials [3], this crucial step is directly linked to the ability to electrically inject spin polarized (and energy selected) carriers in TMDC. Ferromagnetic metals constitute good potential candidates for such an injection. We used first principles methods based on the density functional theory to investigate the atomic and electronic structure at the interface between hcp cobalt and a single MoS2 layer. Covalent bonding between Co and S atoms strongly modifies the electronic structure of the MoS2 layer which behaves like a metal at the interface with Co. We give details on the magnetic moments of Co, S and Mo atoms at the interface, and on the nature and spin-polarization of the electron states at the Fermi level. This interface could be used to promote electrical spin-injection from a cobalt electrode to a single MoS2 layer. [1] K.F. Mak et al.,Phys. Rev. Lett. 105, 136805 (2010) [2] A. Allain et al., Nat. Materials 14, 1195 (2015) [3] D. Xiao et al.,Phys. Rev. Lett 108, 196802 (2012)

Resume : 3D graphene materials synthesised by Chemical Vapour Deposition (CVD), such as graphene foams, have attracted a lot of interest due to their ability to transfer many of the unique properties of graphene to a larger scale, with high surface area, high electrical conductivity, and good structural integrity. We present the promising preliminary results of two different applications that we are exploring for our graphene foams. In the first, they are electrochemically functionalised with iron (III) oxide and used as electrodes for Li-ion batteries. In the second, they are combined with a polyelastomer (PDMS) to create a piesoresistive sensor that can be used for health monitoring. In addition, we propose that the functional potential of graphene foams can be further improved by reducing the pore size from the 200-400 μm range typical of commercially available metal foam templates. We demonstrate a new technique to reach a pore size range of 1-10 μm using networks of nanoparticles as the templates for graphene growth, which significantly increases the graphene surface area available in a given volume.

Resume : We evaluated the change in chemical state at the interface between high-k dielectric grown by the atomic layer deposition (ALD) process and exfoliated black phosphorus (BP) as a function of air exposure time. The change in chemical state and structure on oxidized phosphorus species before and after ALD process were investigated using x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), Atomic force microscopy (AFM), transmission electron microscopy (TEM), and first-principles DFT calculations. We observed that oxidized phosphorus species (POx) were significantly reduced during the ALD-Al2O3 process. In particular, field effect characteristic of few-layer black phosphorus showed that the enhancing electrical properties (mobilties of ~ 200 cm2V-1s-1 and on-off ratios of ~10_3) at the 1 day of exposure time in BP sample. These results provide an in-depth understanding on the reduction of oxidized phosphorus species by the ALD process and enhancing electrical characteristics of the BP FET, even using an oxidized BP by exposure to air circumstance.

Resume : As a crucial step towards the identification of novel and promising 2D materials, we provide here a large scale first-principles exploration and characterization of such compounds. From a combination of 480,000 non-unique structures harvested from the ICSD [1] and COD [2] databases, three-dimensional crystals are screened systematically by checking the absence of chemical bonds between adjacent layers, identifying close to 6,000 layered systems. Then DFT calculations of the van der Waals interlayer bonding are performed with automatic workflows, while systematically assessing the metallic, insulating or magnetic character of the materials obtained. Following full atomic and cell relaxations, phonon dispersions are computed as a first step towards the assessment of thermodynamic properties. Thanks to the AiiDA materials' informatics platform [3], and in particular its automatic workflow engine, database structure, sharing capabilities, and pipelines to/from crystallographic repositories, the systematic and reproducible calculation of these properties becomes straightforward, together with seamless accessibility and sharing. [1] http://www.fizkarlsruhe.com/icsd.htm [2] S. Grazulis et al, Nucleic Acids Research, 40, D420 (2012). [3] G. Pizzi, A. Cepellotti, R. Sabatini, N. Marzari and B. Kozinsky, Comp. Mat. Sci. 111, 218 (2016).

Resume : The performances of the organic semiconductor devices are determined by the charge carriers generation mechanism and their transport and collection by electrodes. Nano-patterning of the metallic electrode offers a way to improve both optical and electrical effects. The nanostructures at the metal-organic interface can increase the efficiency of the light absorption, create an intense electric field determining an increased mobility of the carriers and enhance the area of the metal-organic contact favoring carriers injection. This paper presents a comparative study between the properties of the heterostructures realized with single or multilayer organic prepared on Si substrate and on Si covered by a grating of nanostructures developed in a metallic layer. We have used the UV-Nanoimprint Lithography for the realization of some 2D periodic structures characterized by a periodicity between 250 and 400 nm and cylindrical shape with different structural parameters (diameter of few hundreds of nm, depth between 250 and 350 nm) in correlation with the processing conditions related to spin coating soft stamp and contact mode parameters. The cathode electrode has been deposited on this nanostructured surface by vacuum evaporation or sputtering. The effect of the nano-patterning on the morphology of the subsequently deposited by vacuum evaporation organic films has been evidenced by SEM and AFM measurements. The UV-Vis reflection measurements and I-V characteristics have revealed the effect of the cathode nano-patterning on the properties of the heterostructures.

Resume : Scanned probe techniques play a key role in the investigation of 2D dimensional systems due to their unique ability to provide direct information regarding the atomic structure and fundamental properties. In particular, scanning tunneling spectroscopy (STS) is a powerful tool to explore the local electronic structure of novel 2D materials at the atomic scale. In this talk I will illustrate this fact by presenting our results regarding the spatially resolved electronic characterization of MBE-grown 2D transition metal dichalcogenides (TMDs) by means of low temperature (5K) STM/STS and nc-AFM. I will first discuss the direct experimental observation of extraordinarily high exciton binding energy and band structure renormalization in a single layer of semiconducting TMD [1] due to reduced screening by using a combination of STS and photoluminescence spectroscopy. We have also studied the role of interlayer coupling and layer dependent carrier screening on the electronic structure of few layer MoSe2 [2]. We find that the electronic quasiparticle bandgap decreases by nearly 1 eV when going from one layer to three. Finally, I will describe the fate of the collective electronic phases, i.e. charge density wave order and superconductivity, of NbSe2 in the single layer limit [3]. We demonstrate that 3 x 3 CDW order remains intact in 2D. Superconductivity also still remains but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of ? = 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi level in the 2D limit. [1]Nature Materials 13, 1091 (2014). [2]Nano Letters 15, 2594 (2015). [3]Nature Physics 12, 92 (2016).

Resume : The inherent presence of atomic defects plays an important role in determining the properties of 2D materials. Monolayer semiconducting transition metal dichalcogenides (TMDs) are of particular interesting since they exhibit direct bandgaps in the visible range, high charge-carrier mobility, extraordinarily enhanced light-matter interactions and potential applications in novel optoelectronic devices. Although the presence of structural defects is expected to play a critical role on their properties, a detailed understanding of defect structure also opens new possibilities in the functionalization of the properties in 2D-TMDs. In our work, we provide direct evidence for the existence of isolated, 1D charge density waves at mirror twin boundaries (MTB) in single-layer MoSe2. Our LT-STM/STS measurements reveal a substantial bandgap of 100 meV opening at the Fermi level in the otherwise 1D metallic structure. We find an energy-dependent periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order. In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the 1D MTB by means of nc-AFM. DFT calculations reproduce both the gap opening and the modulations of the density of states.

Resume : The energy-related environmental challenges induce an intense research in the field of photovoltaic (PV) technologies. The new generation of PV devices includes organic and hybrid cells, the latter benefiting from the good stability of inorganic materials, combined with the low cost, large surface and flexibility of the organic ones. In this work, n-type fullerene-like (IF) rhenium-doped MoS2 nanoparticles (NP) are blended with the reference donor polymer (poly(3-hexylthiophene)) (P3HT). MoS2 is a semiconductor that is easily structured by forming two-dimensional crystals that self-assemble via van der Waals forces. Its electron affinity and optical band gap are suitable for the role of an electron acceptor in hybrid PV blends. Conductive-Atomic Force Microscopy (C-AFM) is used to determine the nanoscale morphology of the blends and to correlate it to the current mapping, in order to identify the different components as well as their specific electrical response (conductivity, transport). The IF-MoS2 NPs are also showed to act as nucleation centers for P3HT self-organization, enhancing both the donor/acceptor interface density and the electrical transport properties of P3HT. The photo-generation of charge under in situ illumination of the blends is investigated by Photo-C-AFM, confirming the role of IF-MoS2 as an electron acceptor in those hybrid PV blends. A positive photo-current is measured at the P3HT/IF-MoS2 interface, indicating a hole-collection at the tip.

Resume : In the rising field of two-dimensional (2D) materials, recent attention has been focused on 2D oxides, being of potential interest e.g. as high-k 2D dielectrics or as key components in flexible and transparent 2D photodetectors or electrodes. Zinc oxide has been recently observed to form a 2D graphene-like structure which can in principle be stacked in van der Waals heterostructures of different 2D materials for optoelectronic applications. Focus of our work is the synthesis and nanoscale investigation of 2D ZnO nanostructures, obtained by physical vapor deposition methods and characterized in situ by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. Two different synthesis strategies have been explored to produce 2D ZnO structures on Au(111). The first one is based on e-beam Zn evaporation under oxidizing conditions, the second on pulsed laser deposition, representing a more versatile and scalable approach to 2D oxide synthesis. The effect of several deposition and post-annealing parameters on the growth mechanisms and morphology have been investigated. STM images of the obtained structures reveal a moiré pattern with ~24 Å periodicity characterizing the ZnO surface, associated to the formation of graphene-like ZnO. Our experiments lay the basis for more detailed studies of the properties of 2D ZnO and its integration in lateral or vertical heterostructures for application purposes.

Resume : Experimental methods allowing for a large-area growth of few layers Transition Metal Dichalcogenides are technologically demanding. Chemical Vapour Deposition (CVD) is one of the most promising option in this respect [1]. Here we have grown uniform (~cm2) MoS2 sheets by CVD employing ultra-thin MoOx films as precursors. X-ray Photoelectron Spectroscopy shows several Mo oxidation states in precursor deposited by e-beam with respect to the more stoichiometric MoO3 obtained by atomic layer deposition (ALD). MoS2 was characterized in morphology by atomic force microscopy (AFM) and in structure by Raman spectroscopy. In MoS2 thicker than four layers, Raman modes exhibit an asymmetric broadening for e-beam deposited precursors. This feature is not observed when using the alternative film precursors. This asymmetry was recently reported to stem from disorder due to relaxation of the Raman selection rules [2]. Reducing the thickness of the precursor film, MoS2 bilayer is obtained for e-beam deposited MoOx, whereas multi-layer grains result from ALD-grown MoO3 (based on AFM topographies and “bulk-like” Raman modes). The results suggest that de-wetting and coalescence mechanisms concerned with the process temperature and the low thickness must be taken into account. The deposition technique of the precursor and a proper selection of the substrate can play a role in contrasting these mechanisms. [1] Y. Zhan et al., Small 8, 966 (2012) [2] S. Mignuzzi et al., Phys. Rev. B 91, 195411(2015)

Resume : The chemical vapor deposition (CVD) of layered transition metal dichalcogenides (TMDs) holds promise for the synthesis of high quality monolayered material over large areas for integration in optoelectronic devices. The CVD synthesis of TMDs, and especially of tungsten-based TMDs, is still challenging since film continuity and thickness uniformity are often limited to tens of micron-sized areas. The most widely reported CVD method for the synthesis of atomically thin WS2 layers consists of the simultaneous evaporation of tungsten trioxide (WO3) and sulphur (S) powders. Normally this methods requires temperature >900°C and lead to formation of WS2 domains with lateral size that comprises between 5-20 microns. We demonstrate the synthesis of high-quality monolayered WS2 extended over 300 microns sized areas at lower temperature (~800°C) by using monohydrated tungsten oxide as metal precursor, and halide compounds as transport agent. While using only halides as transport agents, increase in the flakes size is limited to a few tens of microns. We therefore conclude that formation of large WS2 domains is due the thermodynamically favorable chemical transport of W precursors proceeding via gaseous hydroxide and oxide halide species which are preferentially formed in presence of H2O. Photoluminescence characterization shows the high crystal quality of the grown material.

Resume : Atomic Layer Deposition (ALD) is a promising technique to grow few-layered two-dimensional (2D) transition metal dichalcogenides (MX2) for ultra-scaled nano-electronic devices at low deposition temperature with single layer growth control. However, the nucleation and growth mechanisms of MX2 ALD need to be understood, as they determine the composition and crystallinity of the layers. We report on a low temperature Plasma-Enhanced ALD process for 2D WS2 from WF6 and H2S precursors and hydrogen (H2) plasma. The composition of the layers (S/W ratio) is determined by the H2 plasma and H2S reactions, which is understood by considering the redox chemistry of the process. A low H2 plasma power is needed to confine the reduction reactions of the W atoms to the growing surface, which results in layers with a S/W ratio of 1.8-2 (Rutherford Backscattering Spectroscopy), uniform over the 300mm substrates. Nano-crystalline WS2 with the characteristic 2D structure is grown at 300°C on amorphous Al2O3 substrates. Raman spectroscopy and Photoluminescence indicate that the WS2 thickness is scalable down to the monolayer content. The crystal domain size is determined by the nucleation mechanisms. The first layers of WS2 nucleate with high nucleation density. The domain size is increased by deposition on a sapphire substrate as determined by plan-view Transmission Electron Microscopy, which also provides evidence for local epitaxial seeding of the WS2 nanocrystals.

Resume : Two-dimensional (2D) metal chalcogenide nanomaterials are receiving considerable attention in recent years, due to their extradinary properties derived from their 2D structural characteristics and great potential in a wide spectrum of applications.1,2,3,4 In this presentation, first, I will present the high yield exfoliation of two layered ternary bulk crystals in solution into ultrathin 2D ternary metal chalcogenide nanosheets, i.e. Ta2NiS5 and Ta2NiSe5, via the electrochemical Li-intercalation and exfoliation method.5 The single-layer Ta2NiS5 was used as a novel sensing platform to construct a fluorescent biosensor for DNA detection, which exhibited good selectivity and high sensitivity (with a detection limit of 50 pM). Second, I will talk about the preparation of 2D metal chalcogenide epitaxial hetero-nanostructures in solution phase by epitaxial growth of metal sulfide nanoplates, including CuS, ZnS and Ni3S2, on ultrathin 2D TiS2 nanosheet by using the bulk TiS2 crystal and metal foils as precursors via an electrochemical method.6 Last, I will introduce a facile and general assembly strategy for the high-yield and scalable preparation of chiral nanofibers based on the self-assembly of various ultrathin 2D nanomaterials, including single-layer graphene oxide, MoS2, TaS2, TiS2, few-layer TaSe2, WSe2 and hybrid nanosheets of Pt nanoparticle-decorated MoS2 (Pt-MoS2) and reduced graphene oxide (Pt-rGO), in vigorously stirred polymeric solutions.7 These chiral nanofibers can be further assembled into same-handed helical nanorings. The chiral MoS2 nanofiber with P123 was used as the active layer for a flexible nonvolatile resistive memory device, which exhibited a nonvolatile rewritable memory effect with excellent reproducibility and good stability. References [1] C. L. Tan, H. Zhang, Chem. Soc. Rev., 2015, 44, 2713-2731. [2] C. L. Tan, H. Zhang, Nat. Commun., 2015, 6, 7873. [3] C. L. Tan, H. Zhang, J. Am. Chem. Soc., 2015, 137, 12162-12174. [4] C. L. Tan, H. Zhang, et al. Chem. Soc. Rev., 2015, 44, 2615-2628. [5] C. L. Tan, H. Zhang, et al. J. Am. Chem. Soc., 2015, 137, 10430-10436. [6] C. L. Tan, H. Zhang, et al. Angew. Chem. Int. Ed., 2015, 54, 1841-1845. [7] C. L. Tan, H. Zhang, et al. J. Am. Chem. Soc., 2015, 137, 1565-1571.

Resume : In recent years, the research on 2D materials have attracted great attention due to their potential applications in many fields (electronics[1], optics, etc). In particular, 2D materials open novel possibilities to find suitable compounds to dissociate H2O molecule in a photoelectrochemical cell(PEC)[2]. However, this property has been so far little explored[3]. To this aim, we have synthesized layered trisulfides (TiS3, ZrS3, HfS3) belonging to a large class of low-dimensional compounds (MX3, M=Ti, Zr, Hf, Nb, Ta, W and X= S, Se, Te). These layered compounds exhibit properties that fit the required parameters to be used in energy conversion through photoelectrochemical processes, i.e. a huge specific area, an adequate band gap value, optimal transport properties, etc. In this work, a comparison of the ability of these materials to photogenerate H2 is investigated. We have determined their flatband potentials, and we have measured the open circuit photovoltages and the photogenerated hydrogen flows (quantified by Quadrupole Mass Spectrometry) of the different samples under 200mW/cm2 visible light illumination in a PEC, at different bias potentials. [1] J. O. Island, M. Buscema, M. Barawi, J. M. Clamagirand, J. R. Ares, C. Sánchez, I. J. Ferrer, G. A. Steele, H. S. J. van der Zant, A. Castellanos-Gomez, Adv. Opt. Mater. 2014, 2, 641. [2] A. Fujishima, K. Honda, Nature 1972, 238, 37. [3] M. Barawi, E. Flores, I. J. Ferrer, J. R. Ares, C. Sánchez, J Mater Chem A 2015, 3, 7959.

Resume : Chemical vapour deposition of graphene on copper foils is generally scalable to silicon wafer size [1]. Device fabrication, however, typically requires transfer of the graphene films onto suitable substrates. Wet transfer methods are most commonly used, where the target substrate has to withstand certain liquids. In addition, a liquid film is trapped between the graphene and the substrate, which may influence the graphene properties. Here, we report an improved PDMS assisted graphene transfer method, which avoids these issues and results in a clean graphene transfer with negligible or minimal damage. In this method, PMMA/graphene stacks are lifted with PDMS stamps and transferred lightly onto the target substrate. Adhesion is then promoted using a vacuum chamber for at least one hour. This is in contrast to typical PDMS assisted transfer processes, where mechanical pressure is applied, which often results in broken and folded graphene layers due to pressure non-uniformities. In the proposed process, the PDMS stamp is removed from the PMMA/graphene stack on a pre-heated hot plate. This results in smooth detachment of the PDMS and leaves undamaged PMMA/graphene stacks on the desired substrates. We demonstrate that the improved PMDS stamp method can also be used to transfer other CVD grown 2D materials onto arbitrary rigid and flexible substrates. [1] S. Kataria et al. , “Chemical vapor deposited graphene: From synthesis to applications,” Phys. Status Solidi A, 211(11), 2014.