Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin-orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7-3.1 and 1.9-3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M and X) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo < W) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S < Se). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression

On the Suitability of Raman Spectroscopy to Monitor the Degree of Graphene Functionalization by Diazonium Salts

Raman spectroscopy is undoubtedly the most frequently used technique for universal characterization of graphene and related materials. Quantification of parameters like disorder or strain is possible through analysis of particular Raman bands. However, under certain conditions, such evaluation can be jeopardized by - sometimes hidden - convolution of more overlapping effects. In this work, graphene functionalization by the common nitrobenzenediazonium salt under simultaneous biaxial tensile deformation induced by substrate swelling was investigated by Raman spectroscopy. As expected, the disorder-related D band appeared in the spectra documenting the covalent attack on the graphene lattice. However, the strain-induced shift of the graphene bands exposed additional peaks, masked at exactly the same positions as the unstrained graphene bands. The new bands were assigned to vibrations of the diazonium molecule and its decomposition products adsorbed on top of the functionalized graphene. The external strain thus provided means for more correct quantification of the lattice disorder

Raman Spectra of Double-Wall Carbon Nanotubes under Extreme Uniaxial Stress

We investigated the pressure dependence of the Raman frequencies and intensities of the D and G bands of double-wall carbon nanotubes under strong uniaxial conditions. Using moissanite anvils, we observed for the first time the evolution of the D band under extreme stress/pressure conditions. We find that the difference between D and G frequencies remains constant over the whole stress range. In addition, we observe that double-wall carbon nanotubes behave elastically up to the maximum uniaxial stress reached in our experiments, which is estimated to be about 12 GPa

Morphological changes in carbon nanohorns under stress: a combined Raman spectroscopy and TEM study

In this work, we present the first study of highly compressed carbon nanohorns (CNHs).</p

Crossover from ballistic to diffusive thermal transport in suspended graphene membranes

We report heat transport measurements on suspended single-layer graphene disks with radius of 150-1600 nm using a high-vacuum scanning thermal microscope. The results of this study revealed a radius-dependent thermal contact resistance between tip and graphene, with values between 1.15 and 1.52 × 10 KW. The observed scaling of thermal resistance with radius is interpreted in terms of ballistic phonon transport in suspended graphene discs with radius smaller than 775 nm. In larger suspended graphene discs (radius >775 nm), the thermal resistance increases with radius, which is attributed to in-plane heat transport being limited by phonon-phonon resistive scattering processes, which resulted in a transition from ballistic to diffusive thermal transport. In addition, by simultaneously mapping topography and steady-state heat flux signals between a self-heated scanning probe sensor and graphene with 17 nm thermal spatial resolution, we demonstrated that the surface quality of the suspended graphene and its connectivity with the Si/SiO substrate play a determining role in thermal transport. Our approach allows the investigation of heat transport in suspended graphene at sub-micrometre length scales and overcomes major limitations of conventional experimental methods usually caused by extrinsic thermal contact resistances, assumptions on the value of the graphene's optical absorbance and limited thermal spatial resolution

Novel graphene electrode for retinal implants : an in vivo biocompatibility study

Altres ajuts: this work has made use of the Spanish ICTS Network MICRONANOFABS partially supported by MICINN and the ICTS 'NANBIOSIS'.Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically

Carbon incorporation in MOCVD of MoS2 thin films grown from an organosulfide precursor

Altres ajuts: CERCA Programme/Generalitat de CatalunyaWith the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices

Novel Graphene Electrode for Retinal Implants: An in vivo Biocompatibility Study

Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 785219 (GrapheneCore2) and No. 881603 (GrapheneCore3). DN has received funding from the doctoral school of Cerveau, cognition, comportement (3C) of Sorbonne Université. SP was also supported by the French state funds managed by the Agence Nationale de la Recherche within the Programme Investissements d’Avenir, LABEX LIFESENSES (ANR-10-LABX-65) and IHU FOReSIGHT (ANR-18-IAHU-0001). This work has made use of the Spanish ICTS Network MICRONANOFABS partially supported by MICINN and the ICTS ‘NANBIOSIS,’ more specifically by the Micro-NanoTechnology Unit of the CIBER in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN) at the IMB-CNM