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1.
ACS Nano ; 2020 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-32496750

RESUMO

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment (θ = 0°), associated with flat bands in the Brillouin zone of the moiré pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS2/WSe2 system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moiré unit cell of a θ = 0° MoS2/WSe2 heterobilayer and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data. We further explore the role of a twist angle by showing experimental data for a twisted MoS2/WSe2 heterobilayer structure with a twist angle of θ = 15°, which exhibits a moiré pattern but no confinement.

2.
Nano Lett ; 18(3): 1849-1855, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29415536

RESUMO

A two-dimensional (2D) heterobilayer system consisting of MoS2 on WSe2, deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS2 and WSe2. Significant energy shifts are observed in tunneling spectra observed at the maxima of the moiré corrugation, as compared with spectra obtained at corrugation minima, consistent with prior work. Furthermore, at the minima of the moiré corrugation, sharp peaks in the spectra at energies near the band edges are observed for spectra acquired at 5 K. The peaks correspond to discrete states that are confined within the moiré unit cells. Conductance mapping is employed to reveal the detailed structure of the wave functions of the states. For measurements at 80 K, the sharp peaks in the spectra are absent, and conductance maps of the band edges reveal little structure.

3.
Nanotechnology ; 2018 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-29384500

RESUMO

ReS<sub>2</sub>, a layered two-dimensional material popular for its in-plane anisotropic properties is emerging as one of the potential candidates for flexible electronics and ultrafast optical applications. It is an n-type semiconducting material having a layer independent bandgap of 1.55 eV. In this paper we have characterized the intrinsic electronic noise level of fewlayer ReS<sub>2</sub> for the first time. Fewlayer ReS<sub>2</sub> FET devices show 1/f nature of noise for frequency ranging over three orders of magnitude. We have also observed that not only the electrical response of the material is anisotropic; the noise level is also direction dependent. In fact the noise is found to be more sensitive towards the anisotropy. This fact has been explained by evoking the theory where the Hooge parameter is not a constant quantity, but has a distinct power law dependence on mobility along the two axes direction. The anisotropy in 1/f noise measurement will pave the way to quantify the anisotropic nature of two-dimensional (2D) materials, which will be helpful for the design of low noise transistor in future.

4.
Nanotechnology ; 29(14): 145706, 2018 04 06.
Artigo em Inglês | MEDLINE | ID: mdl-29457965

RESUMO

ReS2, a layered two-dimensional material popular for its in-plane anisotropic properties, is emerging as one of the potential candidates for flexible electronics and ultrafast optical applications. It is an n-type semiconducting material having a layer independent bandgap of 1.55 eV. In this paper we have characterized the intrinsic electronic noise level of few-layer ReS2 for the first time. Few-layer ReS2 field effect transistor devices show a 1/f nature of noise for frequency ranging over three orders of magnitude. We have also observed that not only the electrical response of the material is anisotropic; the noise level is also dependent on direction. In fact the noise is found to be more sensitive towards the anisotropy. This fact has been explained by evoking the theory where the Hooge parameter is not a constant quantity, but has a distinct power law dependence on mobility along the two-axes direction. The anisotropy in 1/f noise measurement will pave the way to quantify the anisotropic nature of two-dimensional (2D) materials, which will be helpful for the design of low-noise transistors in future.

5.
ACS Nano ; 12(2): 965-975, 2018 02 27.
Artigo em Inglês | MEDLINE | ID: mdl-29360349

RESUMO

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 µA·µm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

6.
Nano Lett ; 18(2): 1049-1056, 2018 02 14.
Artigo em Inglês | MEDLINE | ID: mdl-29342357

RESUMO

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe2 domains on c-plane sapphire for the epitaxial growth of large area monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in H2Se to promote surface diffusion of tungsten-containing species to form oriented WSe2 islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe2 monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe2 domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication.

7.
Nanoscale ; 10(1): 336-341, 2017 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-29215125

RESUMO

Two and three-dimensional (2D/3D) hybrid materials have the potential to advance communication and sensing technologies by enabling new or improved device functionality. To date, most 2D/3D hybrid devices utilize mechanical exfoliation or post-synthesis transfer, which can be fundamentally different from directly synthesized layers that are compatible with large scale industrial needs. Therefore, understanding the process/property relationship of synthetic heterostructures is priority for industrially relevant material architectures. Here we demonstrate the scalable synthesis of molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) via metal organic chemical vapor deposition (MOCVD) on gallium nitride (GaN), and elucidate the structure, chemistry, and vertical transport properties of the 2D/3D hybrid. We find that the 2D layer thickness and transition metal dichalcogenide (TMD) choice plays an important role in the transport properties of the hybrid structure, where monolayer TMDs exhibit direct tunneling through the layer, while transport in few layer TMDs on GaN is dominated by p-n diode behavior and varies with the 2D/3D hybrid structure. Kelvin probe force microscopy (KPFM), low energy electron microscopy (LEEM) and X-ray photoelectron spectroscopy (XPS) reveal a strong intrinsic dipole and charge transfer between n-MoS2 and p-GaN, leading to a degraded interface and high p-type leakage current. Finally, we demonstrate integration of heterogeneous 2D layer stacks of MoS2/WSe2 on GaN with atomically sharp interface. Monolayer MoS2/WSe2/n-GaN stacks lead to near Ohmic transport due to the tunneling and non-degenerated doping, while few layer stacking is Schottky barrier dominated.

9.
Nano Lett ; 17(5): 3202-3207, 2017 05 10.
Artigo em Inglês | MEDLINE | ID: mdl-28414459

RESUMO

Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe2 regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

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