Science and technology of two-dimensional materials

Resume : Graphene for large-scale industrial production can be obtained starting from graphene oxide (GO). Indeed, GO can be treated to reduce the number of oxygen functional groups and consequently the energy gap and other physical/chemical properties, becoming similar to graphene but with higher residual defects density. In this work, GO was prepared by a modified Hummers method and reduced by pulsed laser irradiation using visible wavelength (532 nm). This process is suitable to finely tune the degree of reduction permitting to evaluate how the amount and the kind of oxygen functional groups affect several properties: the removal ability of contaminants from water, the antibacterial activity and toxicological level of these material in water. Although several models have been proposed, the composition and the structure of GO and reduced GO has not yet been clarified. Transmission electron microscopy analyses were performed and dual electron energy-loss (EEL) spectra were acquired in different regions of GO and reduced GO flakes. Experimental results show a series of characteristic peaks related to C and O K-edge shells. Simulations of the high-loss EEL spectra at atomic level allow associatin the observed experimental peaks to the presence of different oxygen functional groups on the graphene surface and the corresponding atomic configurations (C-OH, C-O-C).

Resume : The doping of graphene provides control over its electrical and chemical properties, but it can also introduce defects that reduce its excellent intrinsic electrical conductivity. Here, we show that nitrogen-doped graphene can be produced without inducing additional structural defects during the growth of GaN micro and nanorods on single-layer graphene/sapphire substrates by metal-organic vapor phase epitaxy. Mapping Raman Spectroscopy reveals that the CVD graphene remains virtually unchanged with respect to the defect density after the high temperature epitaxy up to 1200oC implying negligible increase in the D peak intensity and decrease in the 2D/G peak intensity ratio. However, the G and 2D peak positions upshift demonstrating the n-doping of graphene. It should be noted that the contribution to these shifts due to the strain related to the sapphire substrate was subtracted based on samples undertaking the same heat treatment but without doping. X-ray Photoelectron Spectroscopy confirms the existence of 86% pyridinic N-C bonds in our doped graphene films. Nanoprobe electrical measurements in a SEM demonstrate the good electrical conductivity of graphene itself as well as between individual GaN nanorods and the underlying graphene. This work establishes graphene as a conductive electrode in the case of GaN LEDs on insulating substrates. Our approach can be extended to other hybrid (opto)electronic systems based on technologically relevant semiconductor nanostructures.

Resume : Membranes obtained by stacking graphene oxide (GO) nanosheets have demonstrated great potential in gas separation and liquid filtration. GO membranes in a hollow fibre shape are of particular interest because of their high-efficiency and easy-assembly at module level which can enable industrial scale applications. Up until now, GO membranes have been made primarily on planar supports which are not suitable for large-scale production and in addition, planar modules are not as efficient as currently utilized technologies. We have fabricated GO membranes with controllable thickness on ceramic hollow fibre substrates using vacuum filtration method. However, we have found that the membranes are highly unstable, mainly due to shrinkage occurring during post-deposition drying processes. We show that we can overcome this fundamental limitation by chemical processing of GO membranes. In aqueous environment, the membranes exhibit high rejection rates of different alcohols and dyes, with molecular weight cut off less than 300 Dalton. The rejection rates are comparable with commercially available membranes and stability has been confirmed over several weeks. This open to significant opportunities in wet separation processes such as nanofiltration and pervaporation.

Resume : Supramolecularly engineered hybrid materials based on graphene are key as multifunctional systems for applications in (opto)electronics and energy. However, their practical use requires the optimization of the processing, self-assembly of multimodular architectures at surfaces using non-conventional methods, their controlled manipulation and responsiveness to external stimuli, and the quantitative study of various physico-chemical properties at distinct length- and time-scales. My lecture will review our recent findings on: (i) The harnessing of the yield of exfoliation of graphene in liquid media by mastering the supramolecular approach via the combination with ad-hoc functional molecules possessing high affinity for the graphene surface, leading ultimately to the bottom-up formation of optically responsive graphene based nanocomposites for electronics. [1] (ii) The tuning of the graphene properties by combining them with organic semiconductors as a strategy both to promote hole mobility in an otherwise electron transporting material and to exploit the tunable ionization energy of liquid phase exfoliated graphene to modulate the transport regime as well as to fabricate new memory devices.[2] (iii) The bottom-up formation of graphene based 3D covalent frameworks with tunable intersheet distance, exhibiting large specific surface areas which determine an ability to adsorb CO2 which is the highest reported among carbon-based materials and extremely high performance in supercapacitors.[3] Our approaches provide a glimpse on the chemist?s toolbox to generate nanocomposites with ad-hoc properties for science and technology of 2D materials. [1] (a) A. Ciesielski, S. Haar, M. El Gemayel, H. Yang, J. Clough, G. Melinte, M. Gobbi, E. Orgiu, M.V. Nardi, G. Ligorio, V. Palermo, N. Koch, O. Ersen, C. Casiraghi, P. Samor?, Angew. Chem. Int. Ed. 2014, 53, 10355?10361. (b) S. Haar, A. Ciesielski, J. Clough, H. Yang, R. Mazzaro, F. Richard, S. Conti, N. Merstorf, M. Cecchini, V. Morandi, C. Casiraghi, P. Samor?, Small 2015, in press. (c) S. Haar, A. Ciesielski, M. Bruna, J.X. Lian, J. Moran, Y. Olivier, D. Beljonne, A.C. Ferrari, P. Samor?, 2015 in preparation. (d) M. D?bbelin, S. Haar, M. Bruna, S. Osella, F. Richard, A. Minoia, R. Mazzaro, E. Adi Prasetyanto, L. De Cola, V. Morandi, R. Lazzaroni, A.C. Ferrari, D. Beljonne, A. Ciesielski, P. Samor?, 2015 in preparation. [2] (a) M. El Gemayel, S. Haar, F. Liscio, A. Schlierf, G. Melinte, S. Milita, O. Ersen, A. Ciesielski, V. Palermo, P. Samor?, Adv. Mater. 2014, 26, 4814-4819. (b) T. Mosciatti, S. Haar, F. Liscio, A. Cieselski, E. Orgiu, P. Samor?, 2015 submitted. [3] X. Zhang, A. Ciesielski, F. Richard, P. Chen, E. Adi Prasetyanto, L. De Cola, P. Samor?, 2015 submitted

Resume : Transition metal dichalcogenides (TMDs) have been received much attention due to novel physical and chemical phenomena. In particular, TMDs families such as MoS2 and WS2 etc. have been widely explored in recently, because of their potential possibility in high-performance devices application, including photodetector, non-volatile memory, and biosensor. Single-layer MoS2 field-effect transistor (FET) with universal back-gated configuration (MoS2/SiO2) reveal poor carrier mobility (0.1~10 cm2/Vs). Alternatively, the devices under top-gated geometry (HfO2/MoS2) with high-κ material suppression results as high as 200 cm2/Vs of mobility. This suggests that underlying substrate and its associated interface trap states play a crucial role in MoS2 based FETs. In this study, we demonstrated the observation of localized electronic states by minimizing the effect of underlying SiO2/Si substrate via establishing suspended few-layer MoS2 channel of FETs. The distribution of interface trap states can be obtained by measuring variable-temperature I-V measurement. We find that suspended MoS2 provided U shape of trap state distribution in the band gap and as low as 10^10 states/eVcm2 was evaluated at the middle gap which is two orders of magnitude lower than density of interface states of 10^12 states/eVcm2 in conventional devices.

Resume : We report on the growth of graphene on Ru ultra-thin films (2-10 nm) in a combined CVD-PVD system. The growth temperature of Ru films and the substrate (fused silica and sapphire) are relevant parameters that influence the morphology of the film, which in turn determines the quality of the obtained graphene. The optimum temperature for the graphene single layer (SLG) formation is 1000 ºC while lower temperatures produce multilayer graphene (MLG). The 2 and 5 nm Ru films obtained at room temperature are extremely flat, no plasmon is detected and the resistance is low while those grown at 650 ºC on fused silica are granular, insulating and present a Ru plasmon resonance at 4.91 eV. The Ru films morphology after graphene deposition depends on the Ru growth temperature as observed by AFM and SEM. The resonance is shifted depending on Ru thickness to a range from 4.81 to 4.51 eV very close to the SLG interband transition at 4.6 eV indicating that graphene electronic structure is probably interacting with Ru free electrons. Interestingly, when MLG is present the Ru plasmon resonance is very much enhanced and shifted to higher energies, 5.3 eV. These characteristics may be of interest for plasmonic applications. Raman spectra of graphene on thick Ru films grown on sapphire are not detectable while on Ru ultra-thin films the observed spectra coincide with MLG or SLG. In all cases the defects peak D is present and its intensity related to the size of the Ru granular morphology.

Resume : A large variety of applications are based on materials derived from graphene oxide (GO) monolayers, either forming thin films or hybrid materials combined with organic or inorganic compounds, where adequate elastic properties are essential. The elastic properties of GO monolayers have been studied by AFM indentation and, for thick GO paper, conventional mechanical techniques were used resulting in a wide range of reported Young’s modulus values (5 - 260 GPa). The peculiarities of the GO paper and the difficulty to perform conventional mechanical experiments on more controlled samples explain the disparity between reported values and the absence of experimental values for the shear elastic constants of graphene oxide. In the present work we propose a methodology to obtain information on the intrinsic elastic constants (related to the in-plane and interlayer bonding) of GO through the surface acoustic waves detected by Brillouin spectroscopy of few-layer films of GO monolayers deposited on glass. The temperature dependent elastic behavior is studied in-situ up to 200 ºC. The effect of the elimination of the embedded water molecules and of part of the functional groups, thus increasing the elastic constants, is analyzed and discussed. Structural parameters as film thickness and density have been deduced from the dependence of the interlayer distances in graphene oxide few-layer films with temperature obtained from synchrotron diffraction experiments.

Resume : We have study the conditions of synthesis of single layer nitrogen-doped graphene from a liquid precursor (acetonitrile) using a LPCVD (Low pressure CVD) apparatus with base pressure of 10-4 Pa. 25 µm thick ultra pure Cu foils were used as substrate. Cu oxide reduction was performed in hydrogen atmosphere before to expose the samples to the precursor vapor. Samples were prepared at 800oC, 900oC and 1000oC, at different pressures and for different exposition times to the precursor vapor. The samples were transferred to a Si oxide substrate for their characterization. Pristine sample were prepared by using the same apparatus but with methane as precursor. Raman and XPS techniques were used for sample characterization. Raman mapping results show that samples prepared at exposition time shorter than 25 minutes are inhomogeneous, while the samples prepared at 800oC are essentially amorphous. For longer processing times samples prepared at both 900 and 1000OC are defective graphene and the Raman spectra reveal the presence of the D’ band. However, the ratio between the D and D’ band does not fit any model proposed by A. Eckmann et al. (Nanoletters 12 (2012) 3925). XPS spectra of the N1s core-level indicated the presence of N atoms in two environment, one corresponding to N in substitutional sites (peak at 400.6 eV) and the second corresponding to N in pyridinic configurations (peak at 399 ev), as attributed by R.J. Koch et al. (Phys. Rev. B 86 (2012) 075401)

Resume : Graphene, a one-atom-thick sp2-hybridized carbon sheet, has attracted a great deal of scientific interest since its discovery by Geim and co-workers graphene holds enormous potential in a variety of applications, including electronic devices, transparent electrodes, super capacitors, sensors, and composites. On the other hand, semiconductor-based photocatalysts have attracted much attention over the world for environmental remediation, of which ZnO is considered as the most promising material due to its relatively high activity, nontoxicity, , and low cost, have emerged as important components for building various optoelectronic and electronic devices especially photovoltaic ones. Graphene film was formed on the surface of ZnO nano wires arrays through in situ electrochemical reduction of a graphene oxide dispersion by cyclic voltammetry. The residual oxygen-containing groups and other structural defects such as sp3-hybridized carbons in the electrodeposited graphene were further removed by photo assisted reduction of the underlying ZnO nanowires, thus achieving the maximum restoration of π-conjugation in the graphene planes. Spectroscopic, electrochemical, and photo electrochemical techniques were used to characterize the graphene films, and the use of the resulting graphene?ZnO nanowires material in photocatalysis and photovoltaic was investigated. keywords: graphene,

Resume : In this work we are using First Principles calculations within the Density Functional Theory (DFT) framework we to study the deposit of a graphene bilayer over different GaN surface structures with the aim to find defect free epitaxial growth which could serve as a basis to introduce defects in order to fine tuning a band gap. In previous work we have found that a defects free graphene monolayer over GaN that maintains its hexagonal honeycomb structure with the C C bonds intact possible to obtain1. Our preliminary results in bilayer show that it is possible to growth and the determination of the best GaN surface structure to do so is underway. Acknowledgments: DGAPA project IN102714. Calculations were performed in the DGCTIC-UNAM supercomputing center. The authors are grateful to A. Rodriguez for his technical assistance. Calculations were performed at the DGCTIC-UNAM under project SC14-1-I-48. References 1. Miguel Espitia-Rico, Jairo Arbey Rodriguez-Martinez, Maria Guadalupe Moreno-Armenta, and Noboru Takeuchi, Applied Surface Science 326 (2015) 7–11

Resume : The combination of outstanding chemical, electrical, optical, mechanical and thermal properties of graphene and other 2D materials make them interesting for a multitude of applications in flexibleelectronics and sensor systems. Sensor and flexible electronics applications of graphene and other 2D materialsare discussed, and Nokia’s key results in applying 2D materials are described by examples in sensors, memory and battery technologies [1,2,3,4]. Finally, commercial and technical challenges in productising graphene technologies are discussed. References: (1) S.Borini, R.White, Di Wei, M. Astley, S. Haque, E. Spigone, N. Harris, J. Kivioja, and T. Ryhänen, Ultrafast Graphene Oxide Humidity Sensors, ACS Nano, 2013, 7 (12), pp 11166–11173. (2) Di Wei, S. Haque, P. Andrew, J. Kivioja, T. Ryhänen, A.Pesquera, A. Centeno, B. Alonso, A. Chuvilin and A. Zurutuza, Ultrathin rechargeable all-solid-state batteries based on monolayer graphene, J. Mater. Chem. A 1, 3177 (2013). (3) Di Wei, M.R.Astley, N.Harris, R.White, T.Ryhänen, and J. Kivioja, Graphene nanoarchitecture in batteries, Nanoscale 6, 9536-9540 (2014). (4) A.A. Bessonov, M.N. Kirikova, D.I. Petukhov, M. Allen, T. Ryhänen, and M.J.A. Bailey, Layered memristive and memcapacitive switches for printable electronics, Nature Materials 14, 199-204 (2015).

Resume : Graphene and other two-dimensional materials (2DMs) consist of atomically thin covalently bound layers linked together via much weaker van-der-Waals interactions, causing extreme anisotropy in their electrical, mechanical and thermal properties. Using combination of scanning probe microscopy, ultrasonic vibrations and electrostatic interactions, we explore applications of 2D materials in the nanomechanical and nanoelectromechanical (NEMS) systems where the atomically thin layers are subjected to the flexural and normal stresses. We show that a new behaviour of 2DMs – a 2D buckling transition - that can significantly increase the sensitivity of NEMS sensors to applied stimuli, is directly linked to the local in-plane stresses in 2DM and its adhesion to the substrate. Our analysis of stress propagation through a few layer 2DM, a transversely isotropic material - indicate that it is directly governed by the ratio of the out-of-plane Young modulus and the in-plane shear modulus, explaining unusual recent experimental observation of “ultrasonic transparency” of few FLG and MoS2 that allows to observe defects and structures deep under immediate surface of such materials. Finally, we demonstrate that highly anisotropic properties of 2DMs allow exploration of local electrostatic interactions between the material and the substrate via nanomechanical actuation, thus revealing and mapping with nanoscale resolution the charges hidden under the layers of such materials.

Resume : A 2D porous layer is an ideal membrane for filtration because its infinitesimal thickness promises ultimate permeation. Unique properties of graphene such as great mechanical strength, chemical stability and inherent impermeability altogether lend this 2D material adaptability for this membrane platform, if perforated precisely, for both filtration applications and basic transport study. This talk will present synthesis and physical perforation of graphene, followed by mass transport characterizations leading to ultimate permeation across 2D porous membranes. Chemical vapor deposition of graphene with an ethylene precursor obeyed a Gompertzian kinetics. Focused-ion-beam drilling of nanometer-sized holes on this graphene enabled observation of unique mass transport phenomena. The measured transport rates of gases and liquid show a good agreement with theoretical predictions yet are far in excess of those of current membrane materials of finite thicknesses, highlighting the potential this 2D membrane material poses.

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 and ultrafast carrier dynamics, graphene is a promising material for 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 thecreation of novel and versatile optoelectronic devicesthat 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 materialheterostructures, which probes the transit of photoexcited charges across the photoactive TMD layer – and thus current generation – directlyin the time domain. In addition,nano-structured sandwiches of graphene with boron nitride have resulted in high quality plasmonic systems for infrared light. We discuss new configurations of these electrically tunable metamaterials and plasmonic circuits with in-situ tunable control of light at length scales more than a factor 150 below the free-space wavelength. We report strong improvements of the graphene plasmon lifetime and propagation lengths and we assess the intrinsic loss mechanisms [2,3]. Finally, we show how scanning near-field photocurrent microscopy is a promising new technique to gain insight into different device properties of standard graphene devices with a nanometre-scale resolution. References: [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] Highly confined low-loss plasmons in graphene–boron nitride heterostructures A. Woessner, M. B. Lundeberg, Y. Gao, A. Principi, P. Alonso-González, M. Carrega, K. Watanabe, T. Taniguchi, G. Vignale, M. Polini, J. Hone, R. Hillenbrand, F. H. L. Koppens Nature Materials [online DOI: 10.1038/NMAT4169] (2014) [3] Plasmon losses due to electron-phonon scattering: the case of graphene encapsulated in hexagonal Boron Nitride Alessandro Principi, Matteo Carrega, Mark Lundeberg, AchimWoessner, Frank H.L. Koppens, Giovanni Vignale, Marco Polini Phys. Rev. B 90, 165408 (2014) [4] Electrical Control of Optical Emitter Relaxation Pathways enabled by Graphene K.J. Tielrooij, L. Orona, A. Ferrier, M. Badioli, G. Navickaite, S. Coop, S. Nanot, B. Kalinic, T. Cesca, L. Gaudreau, Q. Ma, A. Centeno, A. Pesquera, A. Zurutuza, H. de Riedmatten, P. Goldner, F.J. García de Abajo, P. Jarillo-Herrero, F.H.L. Koppens Nature Physics [online DOI: 10.1038/nphys3204] (2015) [5] Ultrafast electronic read-out of diamond NV centers coupled to graphene Andreas Brenneis, Louis Gaudreau, Max Seifert, Helmut Karl, Martin S. Brandt, Hans Huebl, Jose A. Garrido , Frank H.L. Koppens, Alexander W. Holleitner Nature Nanotechnoly [online DOI: 10.1038/nnano.2014.276] (2014)

Resume : In this talk, I will summarize the recent research on synthesis, characterization and applications of two-dimensional nanomaterials in my group. I will introduce the synthesis and characterization of novel low-dimensional nanomaterials, such as graphene-based composites including the first-time synthesized hexagonal-close packed (hcp) Au nanostructures on graphene oxide, and the epitaxial growth of Pd, Pt and Ag nanostructures on solution-processable MoS2 nanoshees at ambient conditions, single- or few-layer metal dichalcogenide nanosheets and hybrid nanomaterials, and large-amount, uniform, ultrathin metal sulfide and selenide nanocrystals. Then I will demonstrate the applications of these novel nanomaterials in chemical and bio-sensors, solar cells, water splitting, hydrogen evolution reaction, electric devices, memory devices, conductive electrodes, other clean energy, etc.

Resume : We present a high (A/W range) responsivity Si-graphene Schottky avalanche photodetector (PD) for visible (642nm) and telecom (1550nm) wavelengths. The device is fabricated by contacting chemical vapor deposited (CVD) graphene with a p-type Si substrate forming a Schottky junction 1,2. Upon device illumination with photon energy above the Si bandgap (1.12eV), photodetection happens due to direct (band-to-band) photo-generation of electron-hole pairs in Si 3. At 1550nm the photon energy (0.8eV) is below the Si bandgap, and the PD operation relies on internal photoemission, where photoexcited free carriers are injected from the graphene electrode to Si above a Schottky barrier 3. To achieve a photogain, the PD is designed to perform under avalanche multiplication of the photoexcited carriers in the Si depletion region for elevated (higher than 4V) reverse biases. As a result, the device has an external responsivity up to ~1A/W for 642nm and ~0.5A/W for 1550nm. This is the highest achieved so far amongst Si-PDs for telecom wavelengths 4, and comparable with state-of-the-art Si-Ge devices currently employed in Si photonics 5,6. Our device paves the way towards graphene-Si optoelectronic integration. 1 X. An et al., Nano Lett. 13, 909 (2013). 2 S. Tongay et al., Phys. Rev. X 2 (2012). 3 S. Sze et al, Wiley, New York (2006). 4 M. Casalino, Int. J. Opt. Applications 2, 1 (2012). 5 Y. Kang et al., Nat. Photonics 3 (2009). 6 J. Michel et al., Nat. Photonics 4 (201

Resume : This study reports a novel approach of chlorine doping of graphene by plasma, which results in the significant reduction (~60%) of sheet resistance, while keeping optical transparency very high. To achieve it, a parallel grounded mesh has been inserted between ICP source and substrate in order to reduce the ion bombardment energies, which is a potential cause of induced the damage onto graphene network. Analysis reveals that this approach avoids the D-band (defects) formation, while maintaining the π-bonding of the graphene, which affects conductivity. By the pre-doping technique on graphene/copper foil, we obtained the pristine sheet resistance and optical transmittance of the chlorine doped-single layer graphene 240 Ω/sq and 98.5% at 550 nm wavelength, respectively. X-ray photoelectron spectroscopy revealed that an extremely high Cl coverage of 47.3% of monolayer graphene surface was achieved as the highest surface-coverage graphene doping material ever reported. The C:Cl ratio can effectively tuned by controlling Cl concentration through the plasma treatment time.

Resume : Two-dimensional materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. As is common for new materials, much of the early work has focused on measuring and optimizing intrinsic properties on small samples (e.g., micromechanically exfoliated flakes) under idealized conditions (e.g., vacuum and/or cryogenic temperature environments). However, real-world devices and systems inevitably require large-area samples that are integrated with dielectrics, contacts, and other semiconductors at standard temperature and pressure conditions. These requirements are particularly challenging to realize for two-dimensional materials since their properties are highly sensitive to surface chemistry, defects, and the surrounding environment. This talk will thus explore methods for improving the uniformity of solution-processed two-dimensional materials with an eye toward realizing scalable processing of large-area thin-films. For example, density gradient ultracentrifugation allows the solution-based isolation of transition metal dichalcogenides (e.g., MoS2, WS2, MoSe2, and WSe2) and boron nitride with homogeneous thickness down to the single-layer level.Similarly, two-dimensional black phosphorus is isolated in solution with the resulting flakes showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes. In addition to solution processing, this talk will also report on the integration of two-dimensional materials with dielectrics and other semiconductors. In particular, atomic layer deposition of dielectrics on two-dimensional black phosphorus suppresses ambient degradation, thereby preserving electronic properties in field-effect transistors at atmospheric pressure conditions. Finally, p-type semiconducting carbon nanotube thin films are combined with n-type single-layer MoS2 to form p-n heterojunction diodes. The atomically thin nature of single-layer MoS2 implies that an applied gate bias can electrostatically modulate the doping on both sides of the p-n heterojunction concurrently, thereby providing five orders of magnitude gate-tunability over the diode rectification ratio in addition to unprecedented anti-ambipolar behavior when operated as a three-terminal device.

Resume : Graphene and graphene-based materials show good potential to replace indium tin oxide (ITO) as a transparent conductor for electronic applications because of its exceptional electrical and optical properties. Among several well-known techniques to produce graphene and its derivatives, oxidising graphite to graphite oxide followed by dispersing in aqueous media to obtain graphene oxide (GO) flake suspensions, is one of the most efficient techniques for the large scale production of graphene-based materials. The GO dispersions can be processed as inks for use in manufacturing processes, e.g. inkjet printing. In this work, we present a study of high conductivity reduced graphene oxide films produced by inkjet-printing. We have investigated the influence of GO sheet size on the electrical conductivity of reduced GO films. We demonstrated how the mean sheet size of GO dispersions can be controlled by using different sonication and centrifugation steps. The mean sheet size of small GO dispersions was 0.7±0.3 µm, while the mean sheet size of large GO dispersions was 35.9±23.2 µm. The conductivity of printed GO films after reducing increased from 1.6 ± 0.3×10+4 S/m to 2.4 ± 0.3×10+4 S/m with increasing GO sheet size. We believe that this solution process production of graphene-like materials combined with inkjet printing technology is of great significance for low cost and large-scale applications such as flexible electronics.

Resume : 3d printing is a rapidly growing field. It consists in thedeterministic layer-by-layer printing of 3d objects of almost any shape,from a Computer Aided Design source file. Its versatility[1], the limited number of process steps[1]and reproducibility[2], make 3d printing one of the most promising techniques for rapid design and prototyping. Fused Modeling Deposition (FMD) (i.e. layer-by-layer deposition of melted and extruded polymer through a nozzle[3]) is the most popular 3d printing technique thus far, because of its simplicity and the use of common thermoplastic polymers, such as Poly Lactic Acid (PLA)[4], Acrylonitrile Butadiene Styrene (ABS)[4], Polycarbonate (PC)[4], etc. However, polymer composites with Young’s Modulus higher than few GPa[5] or electrical conductivity higher than few S/m[6], or with a combination of both properties are still not available. Graphene’s electrical, thermal, optical and mechanical properties can be used to create composites with higher electrical and mechanical properties than the starting materials[7].Here, we demonstrate 3d printing of a graphene-polymer filament by FMD. Graphene powder produced from methane plasma cracking is blended with PLA using two different routes: wet mixing in chloroform and dry mixing at 250°C. The composite is then extruded at 160°C into a solid filament and 3d printed by FMD. The electrical and mechanical properties of the 3d printed structures are investigated by conductivity and tensile strength measurements. Ink-jet printing of graphene is now an ever growing field, with increasing industrial interest and now including a variety of other 2d materials. We believe our demonstration of 3d printing of graphene will also stimulate a new field, soon to include a variety of other layered materials. References [1] E M. Sachs, et al. Three-dimensional printing techniques, US5204055 [2] D. Dimitrov, et al. CIRP Annals - Manufacturing Technology, 52, 189 (2003) [3] S.S. Crump, Apparatus and method for creating three-dimensional objects, US5121329 [4] L. Novakova-Marcincinova et al. World Academy of Science, Engineering and Technology 70, 396 (2012) [5] M. Nikzad,et al. Materials & Design 32, 3448 (2011) [6] S.J. Leigh et al. PLoS ONE 7, e49365 (2012) [7] A.C. Ferrari et al. Nanoscale, 7, 4598 (2015)

Resume : The electronic quality of graphene is of great importance to ensure the high performance in various device applicationssuch as transparent conductors,ultrahigh-sensitive sensors/actuators, ultrafast transistors, photovoltaics, etc. However, even the best quality graphene can be easily degraded during the transfer process mainly because of defects and cracks induced by mechanical deformation. Recently, we report an ultraclean, cost-effective, and easily scalable method of transferring and patterning large-area graphene using pressure sensitive adhesive films (PSAF) at room temperature.1 This simple transfer is enabled by the difference in wettability and adhesion energy of graphene with respect to PSAF and a target substrate. The PSAF is found to be free from residues and also recyclable because it doesn't need to be dissolved for removal. As a result, the graphene transferred by PSAF shows the excellent charge carrier mobility as high as 17,700 cm2/V•sec with less doping effect compared to conventionally transferred graphene.In addition, the recyclability of PSAF films and the uniformity of the transferred large-area graphene were confirmed, which is important for more cost-effective production of graphene films for practical applications. Finally, various examples showing the importance of graphene transfer for high-performance quantum electronic, electromagnetic devices,2advanced chemical analysis,3 and biological applications4 will be discussed in this presentation. References: 1. S. J. Kim et al. Ultra-Clean Patterned-Transfer of Single-Layer Graphene by Recyclable Pressure Sensitive Adhesive Films. (submitted) 2. Y. Kimet al.Giant orbital diamagnetism in graphene at room temperature. (submitted) 3. Dongha Shin et al. Growth Dynamics and Gas Transport Mechanism of Nanobubbles in Graphene Liquid Cells. Nature Commun. 6, 6068 (2015). 4. Jong Bo Park et al. Non-destructive electron microscopy imaging and analysis of biological samples with graphene coating. arXiv:1407.2070 (2014).

Resume : Recently, two dimensional (2D) transition metal dischalcogenides (TMDCs) nanosheetshave attracted great attention, since theyhave exotic electronic and opticalproperties:Indirect-to-direct bandgaptransition depending on thenumber of layers, highcarrier mobility and strong spin-orbit coupling due to theirbroken inversion symmetry.However, the synthesis of wafer scale 2D TMDCsnanosheetswith layer controllability is absolutely necessary to utilize these remarkable properties in practical applications. In contrast to conventional chemical vapor deposition (CVD), atomic layer deposition (ALD) is inherently based on self-limited surface reaction of chemicals, which makes it a promising synthesis method for 2D TMDCs. In this presentation, I will present the synthesis ofvarious 2D TMDCs nanosheetsincludingMoS2, WS2, WSe2and Mo(1-x)WxS2alloybased on ALD.The synthesis of 2D TMDCs with layer controllability down to monolayer with wafer scale uniformity was realized by ALD. Especially, chemical reaction mechanism which is unique to ALD of 2D TMDCs was identified, which we call self-limiting layer synthesis.Furthermore,various applicationsof synthesized 2D TMDCs nanosheets, includingphotodetector, gas sensor, and catalyst for hydrogen evolution reaction, will be presented.

Resume : MoS2 and transition metal dichalcogenides have opened numerous research directions and potential applications for this diverse family of nanomaterials. The combination of these 2D materials in heterostructures can result in a huge number of potentially interesting new materials. Most of the attention in this field is focused on heterostructures composed of different 2D materials. In my talk, I will present some of our recent efforts in this direction, oriented towards realizing combinations of 2D and 3D materials into van der Waals heterostructures. I will report on high-performance photodetectors based on 2D/3D heterostructures that can operate with internal gain and high sensitivity. Our devices also show very low noise, due to the unique architecture of the 2D/3D heterojunction. Next, I will give an update on our efforts to realize high-performance electrical circuits based on TMD materials.

Resume : In the last years, novel device concepts exploiting the vertical current transport in graphene (Gr) heterostructures with ultra thin insulators or semiconductors have been demonstrated, showing on/off current ratios >1e4 not reachable by conventional Gr FETs. Among these devices, the Gr Base Hot Electron Transistor (GBHET) is very promising for high frequency electronics (fT>1THZ) due to the ballistic transport in the Gr base. The GBHET demonstrators reported so far (based on metal/high-k/Gr/SiO2/Si vertical stacks) suffer from low collector current density (Jc ~1e-7 A/cm^2) and current gain (1e-4 - 1e-2), due to the use of Si as the emitter and of SiO2 as the tunneling barrier. In this work, we discuss the design, implementation and expected performances of a metal/high-k/Gr/AlGaN/GaN vertical transistor, where the 2DEG at AlGaN/GaN interface is the emitter (E), Gr is the base (B) and AlGaN is the E-B barrier. The key element of this device, i.e. the Gr/AlGaN/GaN junction, was obtained by transfer of CVD Gr on an Al0.25Ga0.75N/GaN heterostructure grown on Si. The characterization of the vertical current transport at Gr/AlGaN junction reveled a low Schottky barrier height of ~0.4 eV and a high n-type doping of Gr (~1e13 cm^-2). This low barrier height is expected to allow an efficient current injection in the Gr base of the GBHET and, as consequence, an increase in the collector current densities Jc>1A/cm^2 and current gain >0.1 with respect to the state of the art Si GBHETs.

Resume : Silicene and germanene are attracting considerable interest, due to their unique electronic properties and their potential good compatibility with the existing Si nanotechnology. Using first-principles calculations, we studied the impact of various point defects on the ballistic transport properties of armchair silicene and germanene nanoribbons (NRs). First, we examined the effect of a Stone-Wales defect, an edge dangling bond (DB), and an edge/interior vacancy. We found that the ballistic current is increased with the presence of an edge vacancy and a DB. The increase arises from a defect level, produced near the valence band maximum. This energy level pins the Fermi level at the contact/channel interface and leads to the reduction of the “transport gap” and subsequently, to a lower onset voltage for the ballistic current. Next, we examined various configurations accounting for double vacancies. We found that divacancy defects cause significant changes to the electronic structure of the NRs, and can either improve or degrade the electron transport. In general, our simulations reveal that some types of defects are favorable for ballistic transport as compared to the defect-free case. These findings are promising for future Si/Ge NR-based devices.

Resume : Achieving Ohmic contacts to layered transition metal dichalcogenides (MoS2, WS2, WSe2 and MoSe2) has been a challenge to researchers owing to the Schottky barrier between metal and semiconductor. This has resulted in low on-currents, mobilities and poor sub-threshold slopes in the devices made with these materials. Here we report a chemical approach to reversibly transform semiconducting phase (2H) and metallic phase (1T), which is universally applicable to all semiconducting Transition metal dichalcogenides. Taking advantage of the metallic phase, we fabricated hybrid transistors which have 1T phase of the material at the contacts and 2H phase of the material as the channel. The 1T phase significantly reduces the Schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues and allowing for probing the true intrinsic optoelectronic properties of transition metal dichalcogenides. Material synthesis, compositional, optical and electrical characterization results will be discussed in detail. This strategy has implications for several applications in optoelectronics, catalysis, supercapacitors and batteries.

Resume : One of the major challenges in the implementation of 2D MoS2 and other isoelectronic compounds into high performance electronic, spintronic and valleytronic devices is optimizing their electronic quality, which is often limited by various intrinsic and extrinsic factors. For instance, electron transport in these materials is strongly dominated by band edge disorder and mid-gap localized states. Here, we investigate the intrinsic magneto-transport properties of mono- and bilayer MoS2 sheets in the high carrier densities regime where conduction occurs via extended states. We achieve high doping by dual gating the layers with ion gel and SiO2 back gate. We found that carrier mobility saturates with carrier density and with decreasing temperature, indicating that short-range scattering is responsible in limiting carrier mobility. Our magneto-transport further studies reveal gradual transition from strong to weak localization and to weak anti-localization regime with increasing carrier density beyond 1013 cm-2 at low temperatures. The phase coherence length of both monolayer and bilayer devices was found to be around 10 nm at 2K and tunable by gate voltages. We will discuss the dominant carrier scattering mechanisms based on theoretical fits.

Resume : The interest on graphene (Gr) electronic properties has stimulated the development of advanced methods for large-area material growth on different substrates. The Gr can be grown by Chemical Vapor Deposition (CVD) on a metal substrate (such as Cu, Ni) and successively transferred onto a different substrate. In situ doping during CVD growth or post-growth doping treatments are necessary to tailor the properties of this material (carrier density, conductivity) for specific electronics or optoelectronics applications. In this work, we report the effects of thermal treatments in vacuum and in oxygen controlled atmosphere of a single layer of CVD grown Gr transferred on a 300 nm thick SiO2 layer on Si. Atomic force microscopy (AFM) and micro-Raman spectroscopy characterization of Gr before and after the O2 treatments, enabled to monitor the changes in the morphology, defects density, strain and doping. Electrical measurements on backgated Gr field effect transistors provided information on the changes in the sheet resistance and carrier mobility. Heat treatments at low temperature (from 0 to 400 °C) by varying the pressure of O2 from below ambient pressure up to 100 bar enabled to highlight stable and permanent p-type doping of Gr and to determine the onset temperature and the kinetics of the doping process also by treatments at different temperatures and time.

Resume : We present a computational study on the impact of intrinsic rippling and defects on the quantum transport properties of 2 D phosphorene flakes. As phosphorene is a major candidate for p-type MOS devices, we study both electron and hole transport in such freestanding phosphorene flakes in detail. We employ the meta-genaralized gradient approximation (MGGA) density functional theory (DFT) for evaluating the material properties of perfect and various rippled and defective sheets of 2-D phosphorene. The energy resolved transmission spectra is computed with the non-equilibrium green’s function (NEGF) approach. We also study the transmission pathways and the phase of the transmission eigenstates for evaluating the breakdown of coherent transport in the face of elastic scattering from defects and undulations in the 2-D sheet. Our studies show a significant suppression of conductance due to the presence of defects and rippling in phosphorene sheets.

Resume : The incorporation of heteroatoms in graphene have attracted much attention due to their potential applications in batteries, catalyst supports, super capacitor and fuel cells. We have study the conditions of synthesis of single layer boron-doped graphene from a liquid precursor (triisoporpyl borate) using a LPCVD (Low pressure CVD) apparatus with base pressure of 10-4 Pa. 25 µm thick ultra pure Cu foils were used as substrate. Samples were prepared at 800oC, 900oC and 1000oC, at different pressures and for different exposition times to the precursor vapor. Pristine sample were prepared by using the same apparatus but with methane as precursor. SEM, Raman and XPS techniques were used for sample characterization. We correlated XPS measurements with Raman spectra. B1s spectra reveals the boron incorporation in substitutional sites. Theses results together with the observed shift to higher frequencies of the G and the 2D bands suggested the p-doping of graphene layer for a sample prepared at 1000oC, 25 mtorr and 50 minutes of exposition to the precursor vapor. Raman mapping indicate that this sample is homogenous with nearly constant ratios of the D and G bands, as well as the ratio between 2D and G bands.

Resume : Two-dimensional phases of silica [1] on metal support consists of mutually shifted crystalline domains [2] and vitreous regions [3]. The existence of these domains has never been yet explained in the literature. In this paper, we present a systematic study of the nucleation and the growth of domains in monolayer silica on Ru (0001) [4]. By means of scanning tunneling microscopy and in operando reflection high energy electron diffraction, we found that the anti-phase domain boundaries are associated with the in-plane shifts which are inherited from the successive oxygen reconstructions occurring onto Ru (0001) surface. The latter, thus acts as seed for the silica domains. Based on this observation, we propose a route to control the domain size, which is desirable in view of applications exploiting defects or, contrarily, requiring high quality silica. References: [1] Huang, P. et al. Nano Lett. 12, 1081(2012). [2] Yang, B. et al. Phys. Chem. Chem. Phys. 14, 11344 (2012). [3] Lichtenstein, L. et al. Angew. Chemie. Int. Ed. 51, 404 (2012). [4] Mathur, S. et al., submitted.

Resume : In the present work we used confocal micro-Raman spectroscopy as sensitive tool to study the nature of defects in single-layer graphene induced by laser irradiation at varied laser power densities and dozes. The minimal power threshold of the exciting radiation of the structural defects generation is found. Appearance and drastic intensity increase of zone-edge D-like modes caused by introduction of structural defects in the graphene layer were observed in the Raman spectra at high powers of excitation. Time-dependent evolution of Raman spectra is studied. From the analysis of intensities of defective D and D’ bands relative to G-band, the structural defects generated under laser irradiation of graphene with power density higher than threshold are shown to be mainly vacancy-type defects [1]. The surface density of structural defects is estimated from the intensity ratio of D and G bands. Stokes and anti-Stokes components of the Raman spectra are analyzed to estimate the lattice temperature of graphene on the power density of exciting radiation. 1. Axel Eckmann et al., Probing the nature of defects in graphene by Raman spectroscopy. Nano Lett. 12, 3925-3930 (2012).

Resume : The properties of silicon and the MOS device technology start to put limitations on the further development of microelectronic devices. The dusk of silicon in advanced integrated circuits stimulated a renewed extensive search of new materials and device principles able to substitute the dominating CMOS paradigm. The alternative device principles are not anymore based on the classical field-effect paradigm used up to now in most of the electronic devices. One of the alternative device concepts is based on T- or Y-shaped junctions made of nanowires or nano ribbons fabricated in top down or bottom-up approaches. These junctions can operate as ballistic rectifiers or logic gates. The T- or Y-shaped junctions. In this report we demonstrate the fabrication of all carbon graphene nano ribbon three terminal junctions and their electric properties. The graphene was grown on semi insulating silicon carbide. It will be demonstrated that the device properties initially predicted only for ballistic electrons or holes can be achieved if pure diffusive transport in the devices appears. Additionally, current amplification was observed in the three terminal junctions when they were operated in transistor configuration. It will be shown that the current amplification can be tuned by changing the device design.

Resume : Synthesis of single graphene layers can be obtained over large areas on Si/SiO2/Cu wafers by Chemical Vapor Deposition (CVD). This simple and cost-efficient large scale method of graphene facilitates the breakthrough approach of producing high quality graphene films for potential electronics applications. This study highlights current advances in the development of graphene on large wafer scale and in the reliable transfer method that can be performed to graphene layers onto Glass/ΙΤΟ substrates for use as transparent anode electrodes in polymer BHJ photovoltaic cells. The stacking order of graphene onto ITO was with layer number up to three. The device performance of these Bulk Heterojuction polymer solar cells depends on the morphology of the active derivative system P3HT: ICBA and the interface modification layer, PEDOT: PEG that inserts between graphene and PEDOT: PSS. By inserting this polymeric interface between graphene surface and the copolymer PEDOT: PSS, it has been shown that hydrophobic nature of graphene converted to be hydrophilic. This attainment results to better wettability and coverage uniformity of PEDOT: PSS on graphene surface by leading to improved OPVs efficiency. Study of structural, optical and morphological properties of graphene layers onto Glass/ΙΤΟ substrates was conducted by Raman Spectroscopy, Spectroscopic Ellipsometry and Atomic Force Microscopy respectively.

Resume : Raman spectroscopy is a fast, non-destructive tool to characterise graphene1-3. Doping, defects and contaminants can influence the device performance strongly. We first investigate contaminants trapped between graphene/boron-nitride heterostructures. BN is considered an ideal substrate to optimise graphene's mobility, given a clean interface4. Raman spectroscopy identifies contaminants as tape residuals. After successive steps of annealing the contaminants Raman signal progressively decreases along with the height of trapped contaminants measured by AFM. A systematic Raman and AFM study reveals that the contamination peaks only occur when acetone is used as additional cleaning step. Avoiding this allows us to produce atomically flat interfaces. We then consider the effect of doping and defects in graphene. To investigate the doping dependence of D and D’ for a fixed amount of defects5 we combine polymer-electrolyte gating2 and in-situ Hall effect measurements. I(D)/I(G) and I(D)/I(G) both decrease significantly for increasing doping due to the additional effect of electron-electron scattering. We present a general formula to estimate the defect density in samples with non-negligible doping. 1 A.C.Ferrari and D.M.Basko,Nature Nanotech.8,325(2013) 2 A.Das et al.,Nature Nanotech.3,210(2008) 3 L.G.Cancado et al.,Nano Lett.11,3190(2011) 4 C.R.Dean et al.,Nature Nanotech.5,722(2010) 5 M.Bruna et al., ACS Nano8,7432(2014)

Resume : Significant progress has been made in graphene spintronics since the first demonstration of a graphene-based spin valve [1]. Due to low spin-orbit coupling [2] and hyperfine interaction [2], spin diffusion lengths have been measured in the range from 1.5 m [3] up to 285 m [4]. Here we present spin valves formed by combining La2/3Sr1/3MnO3 (LSMO) electrodes and few layer graphene channels. LSMO exhibits interfacial spin-polarization close to 100% at low temperature [5], making it a promising material for spin valves with highly spin-polarized electrodes [6]. We report spin transport on a device fabricated combining a 5 layer graphene and LSMO. The resistance difference between the antiparallel and parallel configurations is ΔR=1.0 MΩ, corresponding to a magnetoresistance of 5.5% at 10 K, and a spin diffusion length~100 μm. Importantly, X-ray magnetic circular dichroism and magneto-optic Kerr effectanalysis exclude the contribution from tunnelling anisotropic magnetoresistance (TAMR), and allows us to attribute the recorded magnetoresistance entirely to spin transport. [1] E.W. Hill et al., IEEE Trans. Magn. 42 (2006) 2694. [2] D. Huertas-Hernando et al., Pys. Rev. B 74 (2006) 155426. [3] N. Tombros et al., Nature.448 (2007) 571. [4] B. Dlubak et al., Nat. Phys. 8 (2012) 557. [5] M.Bowen et al., Appl. Phys. Lett. 82 (2003) 233. [6] L. Hueso et al., Nature 445 (2007) 410.

Resume : Noble metal nanoparticle / graphene hybrid systems exhibit promising functionality for implementation in electronic, optoelectronic and SERS devices. Here, a detailed analysis of the structural, optical, and transport properties of Ag/graphene system after sequential Ag nanoparticle (Ag NP, D~5 nm) deposition on graphene by sputtering is reported. Insight into the evolution of the physical properties of these graphene-based materials will provide valuable tools to understand and tailor the functionality of these hybrid materials. Results reveal two different regimes in their optical and electric performance, depending on graphene coverage with Ag NPs. Optical transmittance spectra reveal that full coverage (at a deposition time ~60s) sets a significant increase of the plasmon intensity with increasing Ag NPs deposition time. This concomitantly yields an abrupt enhancement of the graphene Raman signal, determining the SERS performance of Ag NPs/graphene-based devices. On the other hand, the evolution of the transport properties reveals an increase of the sheet resistance of graphene with Ag NP incorporation below the full coverage regime accompanied by a progressive n-doping. These results are consistent with the charged-impurity scattering model at this regime. However, above the full coverage point, no significant variation of resistivity or density of carriers is observed, suggesting that subsequent Ag NPs deposition does not modify the electric performance of graphene.

Resume : Graphene shows great potential for future nanoelectronics and related applications due to its extraordinary electronic properties. Recently various methods of graphene synthesis have been developed and in particular chemical vapor deposition (CVD) and related technologies have given insights to the possibility of large scale application. However, functional devices are still far from realization because of the lack of precise control on the process. Control and understanding of the atomic structure of synthesized graphene are crucial, because their intrinsic properties are strongly dominated by their atomic structure. Although Raman spectroscopy is commonly used as a powerful technique to statistically analyze graphene structures such as grain size and presence of defects, we also need some other complementary techniques to fully understand the detailed atomic structures. Following the invention of aberration corrector (AC), atomic resolution transmission electron microscopy (TEM) imaging has become possible on one atom thick layer of carbon [1]. In this work, a peculiar recrystallization in graphene is studied using an AC-TEM imaging. The evolution of crystallinity occurring during the CVD growth process performed with a specific configuration has been first probed by Raman spectroscopy. Atomic structures of synthesized graphene are precisely studied using the AC-TEM in order to understand this re-crystallization process. Figure 1 shows dark-field (DF) images of the graphene samples after 90 min and 5 hours of growth. The grain size becomes lager with increasing the growth time. Nanometer size grains formed at early stage are further re-oriented and merged together and finally results in large single domains up to 1 µm after long time process. The AC-TEM imaging revealed that two neighboring grains are typically aligned zig-zag to armchair at the boundary (Fig. 2a), which might be a low-energy crystallographic configuration. A lot of small domains are also observed, at the middle stage, enclosed with the same zig-zag to armchair misorientation within larger domains (Fig. 2b). Generally in CVD processes, the orientation and achievable size of grains are determined at the early nucleation stage. In our case, the small grains already form a continuous film at the early stage and further transform to low defective large crystals. We infer that such large scale re-crystallization of graphene is enhanced thanks to the platinum substrate. Finally the TEM characterization provided plenty of additional information on the detailed atomic structures and which allowed density functional theory (DFT) calculation supporting the study on this re-crystallization. References [1] Z. Lee et al., Ultramicroscopy, 112, (2012) pp39 [2] O. Lehtinen et al., Nature Communications, DOI : 10.1038 /ncomms (2013) pp3098

Resume : The set of elastic constants of a material describes its response to applied external forces, and a knowledge of them is essential to gain insight on the nature of crystal structure and bonding forces[1]. In crystals with uniaxial hexagonal layered structure, C44 is the shear modulus of the layer-layer interface, accounting for displacement of planes with respect to each other [1], whereas C33 is the Young’s modulus in the normal direction, describing the out-of-plane compression or expansion of layers[1]. Raman spectroscopy is the prime characterization tool for graphene and related layered materials (LMs)[2]. The shear (C)[3] and layer breathing modes (LBMs)[4] are due to relative motions of planes, either perpendicular or parallel to their normal, hence these modes can be associated to their respective elastic constants. We consider three LMs, i. e. NLG, NL-MoS2, NL-hBN (N being the number of layers), and we find that the positions of their C and LBMs depend strongly on N. A general linear-chain model can account for the observed trends, allowing us to evaluate C44 and C33, with applicability to any layered materials. For NLG we find C44 ~4.3 GPa and C33 ~37GPa. The C44 and C33 of NL- MoS2 are found to be ~18.9 GPa and ~59.6 GPa, respectively, whereas the C44 of NL-hBN is ~6.5 GPa. [1] G. Grimvall, North-Holland (1986) [2] A. C. Ferrari et al., Nat. Nanotechnol., 8, 235 (2013) [3] P. H. Tan et al., Nat. Mater. 11, 294 (2012) [4] X. Zhang et al., Phys. Rev. B, 87, 115413 (2013)

Resume : In ultrafast photonics, light interaction with matter is regarded on timescales less than a picosecond. Access to such short timescales is typically achieved with ultrafast lasers or mode-locked lasers [1]. Ultrafast pulse generation, in lasers, is enabled by a nonlinear optical device, called saturable absorber (SA) [1]. Two-dimensional materials offer unique proprieties to be explored for ultrafast photonics and optoelectronics [2]. In particular, graphene features ultrafast [3], wavelength independent saturable absorption response [4,5]. This opens exciting new possibilities for low cost applications in science, information, medicine, materials processing, entertainment and security [1,2]. Here, I present recent advances in implementing graphene as SA for ultrafast [6,7], tunable [8,9], wide spectral coverage [10,11] and high repetition rate [12,13] pulse generation. ~28 fs pulse duration [6], the shortest to date for graphene-based fiber lasers, is presented in a dispersion-managed fiber laser mode-locked by graphene [6,7]. Tunability is presented in a vertical-external-cavity surface-emitting laser (VECSEL), mode-locked by a graphene-integrated mirror [8]. Mode-locking at 1 [10], 1.5 [10] and 2 m [11] is also presented for graphene fiber lasers. 7.8 GHz repetition rate is demonstrated in a graphene mode-locked waveguide laser [13]. Synchronism of dual-wavelength pulse-trains through the inclusion of a common graphene-SA in a coupled cavity configuration is presented. This allowed to get sum frequency generation, with a strong coherent emission detected at ~800 nm through degenerate four wave mixing [10], extending the wavelength operation range. These results showcase the potential of graphene as SA for simple, reliable and low-cost ultrafast lasers. Introduction to other two-dimensional materials for ultrafast photonics is also discussed. [1] M. E. Fermann, Ultrafast lasers: technology and applications (CRC Press, 2003). [2] A. Ferrari et al, Nanoscale, 7, 4598 (2015). [3] D. Brida et al, Nature Communications 4, 1987 (2013). [4] F. Bonaccorso et al, Nature Photonics 4, 611 (2010). [5] Z. Sun et al, ACS Nano 4, 803 (2010). [6] D. Purdie et al, CLEO Stu1N.8 (2014). [7] D. Popa et al, Appl. Phys. Lett. 97, 203106 (2010). [8] C. A. Zaugg et al, Opt. Express 21, 31548 (2013). [9] D. Popa et al, Appl. Phys. Lett. 98, 073106 (2011). [10] M. Zhang et al, CLEO Stu1I.2 (2014). [11] M. Zhang et al, Opt. Express 20, 25077 (2012). [12] R. Mary et al, Opt. Express 21, 7943 (2013). [13] Y. Ren et al, IEEE J. Sel. Top. Quantum Electron. 21, 1 (2015).

Resume : Understanding thermal properties at the nanoscale is of fundamental importance for the development of modern technology, but the direct measurement of these presents a number of unique challenges. Here we report the exploration of the thermal properties of graphene and MoS2 from monolayer to bulk using experimental nanoscale scanning thermal microscopy (SThM) correlated with finite elements (FE) simulations. SThM is a modification of the more well-known Atomic Force Microscope employing a self-heated probe which is used to scan the sample; during probe-sample contact the corresponding drop in probe temperature can be electronically monitored and directly related to changes in the thermal properties. FE simulations were performed to correlate the measured properties of our systems with variations of graphene’s isotropy and anisoptropy as well as substrate interactions. We have investigated how these properties change as a function of sample thickness on substrates of both high and low thermal conductivities. We observe well defined values of thermal conductance for monolayer and near monolayer thicknesses, however some materials show increased conductance at increasing multilayers whilst others show a decrease – in most cases the conductance does not scale simply with thickness. We discuss and compare experimental considerations and simulation outputs in order to construct a thermal conductance models to explain these interesting results.