Search

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in mono-layered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements., 15 pages, 6 figures + SI 3 pages 3 figures. Pre-print published with the authorization of ACS Publications

International audience; Atomically thin MoS$_2$/graphene vertical heterostructures are promising candidates for nanoelectronic and optoelectronic technologies.In this work, we studied the optical and electronic properties of n doped single layer MoS$_2$ on p doped bilayer graphene vdW heterostructures. We demonstrate a non-uniform strain between two different orientation angles of MoS$_2$ monolayer on top of epitaxial bilayer gra-phene. A significant downshift of the E$^1$$_{2_g}$ mode, a slight downshift of the A$_{1_g}$ mode, and photo-luminescence shift and quenching are observed between two MoS$_2$ monolayers differently oriented with respect to graphene; This could be mostly attributed to the strain-induced transition from direct to indirect bandgap in monolayer MoS$_2$. Moreover, our theoretical calculations about differently-strained MoS$_2$ monolayers are in a perfect accordance with the experimentally observed behavior of differently-oriented MoS$_2$ flakes on epitaxial bilayer graphene. Hence, our results show that strain-induced bandgap engineering of single layered MoS$_2$ is dependent on the orientation angle between stacked layers. These findings could be an interesting novel way to take advantage of the possibilities of MoS$_2$ and deeply exploit the capabilities of MoS$_2$/graphene van der Waals heterostructures.

Two-dimensional monochalcogenides (MX) have been identified as a unique and promising class of layered materials in recent years. The valence band of single-layer MX, as predicted by theory, is inverted into a bow-shaped (often referred to as an inverted sombrero) and relatively flat dispersion, which is expected to give rise to strongly correlated effects. The inversion leads to an indirect band gap, which is consistent with photoluminescence (PL) experiments, but PL provides no direct evidence of the band inversion in the valence band. Here we demonstrate for a hexagonal MX crystal, gallium selenide (GaSe), using a combination of angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), that the valence band of monolayer (ML) GaSe exhibits a robust inversion of the valence dispersion at the \ensuremath{\Gamma} point forming a bow-shaped dispersion with a depth of 120 \ifmmode\pm\else\textpm\fi{} 10 meV between the double valence band maximum along the \ensuremath{\Gamma}$K$ direction. We also demonstrate that the deeper-lying bands detected in the ARPES spectrum are consistent with DFT calculations only if spin-orbit coupling is considered. The presented ARPES evidence that spin-orbit coupling leads to the splitting of two fourfold-degenerate states into four Kramers doublets is of significance for PL measurements, as the change in energy of the second highest valence state at the \ensuremath{\Gamma} point has a measurable effect on the PL energies in high-energy luminescence. We predict the optical absorption coefficients for the principal transitions in ML GaSe using a four-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model parametrized from first-principles theory with spin-orbit effects considered.

Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs) with graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2 on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements (ARPES) and Density Functional Theory (DFT) calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2 valence band at K points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single layer WS2. . Our results provide an important reference for future studies of electronic properties of WS2 and its applications in valleytronic devices., Comment: 11 pages, 3 figures + SI 7 pages 8 figures

Semiconducting two-dimensional (2D) materials, such as transition-metal dichalcogenides (TMDs), are emerging in nanomechanics, optoelectronics, and thermal transport. In each of these fields, perfect control over 2D material properties including strain, doping, and heating is necessary, especially on the nanoscale. Here, we study clean devices consisting of membranes of single-layer MoS

Semiconducting 2D materials, such as transition metal dichalcogenides (TMDs), are emerging in nanomechanics, optoelectronics, and thermal transport. In each of these fields, perfect control over 2D material properties including strain, doping, and heating is necessary, especially on the nanoscale. Here, we study clean devices consisting of membranes of single-layer MoS2 suspended on pillar arrays. Using Raman and photoluminescence spectroscopy, we have been able to extract, separate and simulate the different contributions on the nanoscale and to correlate these to the pillar array design. This control has been used to design a periodic MoS2 mechanical membrane with a high reproducibility and to perform optomechanical measurements on arrays of similar resonators with a high-quality factor of 600 at ambient temperature, hence opening the way to multi-resonator applications with 2D materials. At the same time, this study constitutes a reference for the future development of well-controlled optical emissions within 2D materials on periodic arrays with reproducible behavior. We measured a strong reduction of the MoS2 band-gap induced by the strain generated from the pillars. A transition from direct to indirect band gap was observed in isolated tent structures made of MoS2 and pinched by a pillar. In fully suspended devices, simulations were performed allowing both the extraction of the thermal conductance and doping of the layer. Using the correlation between the influences of strain and doping on the MoS2 Raman spectrum, we have developed a simple, elegant method to extract the local strain in suspended and non-suspended parts of a membrane. This opens the way to experimenting with tunable coupling between light emission and vibration.

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and −0.51eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV.

Two-dimensional (2D) materials have recently been the focus of extensive research. By following a similar trend as graphene, other 2D materials including transition metal dichalcogenides (MX2) and metal mono-chalcogenides (MX) show great potential for ultrathin nanoelectronic and optoelectronic devices. Despite the weak nature of interlayer forces in semiconducting MX materials, their electronic properties are highly dependent on the number of layers. Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate the tunability of the quasiparticle energy gap of few layered gallium selenide (GaSe) directly grown on a bilayer graphene substrate by molecular beam epitaxy (MBE). Our results show that the band gap is about 3.50 +/-0.05 eV for single-tetralayer (1TL), 3.00 +/-0.05 eV for bi-tetralayer (2TL) and 2.30 +/-0.05 eV for tri-tetralayer (3TL). This band gap evolution of GaSe, in particularly the shift of the valence band with respect to the Fermi level, was confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and our theoretical calculations. Moreover, we observed a charge transfer in GaSe/graphene van der Waals (vdW) heterostructure using ARPES. These findings demonstrate the high impact on the GaSe electronic band structure and electronic properties that can be obtained by the control of 2D materials layer thickness and the graphene induced doping.

Two dimensional materials like graphene, transition metal dichalcogenides, and hexagonal boron nitride (h-BN) have attracted a keen interest over the past few years due to their possible integration in the next generation of nano-components. Here, we used high resolution X-ray photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) to perform a complete study of stacking configuration and identify sp3 crystal deformations of a single domain h-BN crystal. The AA′ stacking was found to best reproduce features in the experimental B and N K-edges. The NEXAFS also shows that the splitting of the 1s to π* peak in the B K-edge, recently predicted by density functional theory, may be accounted for by the presence of AB′ stacking faults. The presence of this stacking fault has, as a result, the introduction of point defects in the crystal such as boron atoms in a pyramidal or sp3 configuration. Interstitial nitrogen defects are also present in the crystal forming a N-N pair as expected for a p-type h-BN crystal.

International audience; Suspended graphene membrane presents a particular structure with fundamental interests and applications in nanomechanics, thermal transport and optoelectronics. Till now, the commonly used geometries are still quite simple and limited to the microscale. We propose here to overcome this problem by making nanostructures in suspended epitaxial bilayer graphene on a large scale and with a large variety of geometries. We also demonstrate a new hybrid thin film of SiC-graphene with an impressive robustness. Since the mechanics and thermal dissipation of a suspended graphene membrane are strongly related to its own geometry, we have in addition focused on thermal transport and strain engineering experiments. Micro-Raman spectroscopy mapping was successfully performed for various geometries with intrinsic properties measurements at the nanoscale. Our engineering of graphene geometry has permitted to reduce the thermal transport, release and modulate the strain in our structures. © 2017 Elsevier Ltd

International audience; Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a binding energy of about −0.73 eV relative to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about m*/m0 = −1.1 was determined. By coupling the ARPES data with high-resolution X-ray photoemission spectroscopy measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow a deeper understanding of the interlayer interactions and the electronic structure of the GaSe/graphene vdW heterostructure.

International audience; In this work, we show that the sensing performance of graphene based humidity sensors can be largely improved through polymer functionalization. Chemical vapor deposited graphene is functionalized with amine rich polymer, leading to electron transfer from amine groups in the polymer to graphene. The functionalized graphene humidity sensor has demonstrated good sensitivity, recovery, and repeatability. Charge transfer between the functionalized graphene and water molecules and the sensing mechanism are studied systemically using field effect transistor geometry and scanning Kelvin probe microscopy.

In this paper, graphene/mica and graphene/SiO2 based ammonia gas sensors are compared. It is found that adsorbed NH3 molecules result in up-shifting of the Fermi level in graphene, leading to a significant increase in graphene resistance. In comparison with SiO2 supporting substrate, the mica supporting substrate is found to induce more p-doping in graphene, in favour of NH3 molecule adsorption, yielding a high sensitivity. These findings suggest that the substrate plays an important role in mediating the interaction between graphene and NH3 molecules and that mica can be an excellent underlying substrate for graphene for ammonia gas detection. Published version