Raman shift and electrical properties of MoS2 bilayer on boron nitride substrate

Adsorption of water on C sites vacancy defected graphene/h-BN: First-principles study

Heterostructures (HS), vacancy defects in HS, and molecular adsorption on defected HS of 2D materials are fervently inspected for a profusion of applications because of their aptness to form stacked layers that confer approach to an amalgamation of favorable electronic and magnetic properties. In this context, graphene (Gr), hexagonal boron nitride (h-BN), HS of graphene/h-BN (Gr/h-BN), and molecular adsorption on Gr/h-BN offer promising prospects for electronic, spintonic, and optoelectronic devices. In this study, we investigated the structural, electronic, and magnetic properties of C sites vacancy defects in Gr/h-BN HS and adsorption of water molecule on defected Gr/h-BN HS materials by using first-principles calculations based on spin-polarized density functional theory method within van der Waals (vdW) corrections DFT-D2 approach. We found that these considered materials are stable 2D vdW HS. Based on band structure calculations, they are semimetallic, and on density of states and partial density of states analysis, they are magnetic materials. The magnetic moment developed in these defected systems is due to the unpaired up-spin and down-spin states in the orbitals of atoms present in the materials created by the vacancy defects.

Unusual properties and potential applications of strain BN-MS2 (M = Mo, W) heterostructures

Heterostructures receive intensive attentions due to their excellent intrinsic properties and wide applications. Here, we investigate the natural physical properties and performances of strain BN-MS₂ (M = Mo, W) heterostructure by density functional theory. Different to compressive monolayer MS₂, corresponding BN-MS₂ heterostructures keep direct band-gap characters because effects of charge transfer on anti-bonding dz² orbitals are stronger than those of Poisson effect. Mexican-hat-like bands without magnetic moments are observed at strain BN-MS₂ heterostructures when the compression is enough. Consequently, electron mobilities of strain BN-MS₂ heterostructures are slightly reduced at first and then enlarged with increasing compressive strain. Note that, strain BN-MS₂ heterostructures reduce the band edges of MS₂ layers and extend their application in photocatalytic water splitting. But just the n-type and p-type Schottky barriers of devices with strain BN-MS₂ heterostructures are reduced and even vanished with the increasing tensile and compressive, respectively. Besides, electron mobilities of strain BN-MoS₂ and BN-WS₂ heterostructures can be enhanced to 1290 and 1926 cm²  V ⁻¹ s⁻¹, respectively, with increasing tensile strain. Interestingly, the exciton binding energies of strain BN-MS₂ heterostructures exhibit oscillation variations, different to those of strain monolayer MS₂.

Tunable electrical properties of multilayer HfSe2 field effect transistors by oxygen plasma treatment

HfSe₂ field effect transistors are systematically studied in order to selectively tune their electrical properties by optimizing layer thickness and oxygen plasma treatment. The optimized plasma-treated HfSe₂ field effect transistors showed a high on/off ratio improvement of four orders of magnitude, from 27 to 10⁵, a field effect mobility increase from 2.16 to 3.04 cm² V^-1 s^-1, a subthreshold swing improvement from 30.6 to 4.8 V dec^-1, and a positive threshold voltage shift between depletion mode and enhancement mode, from -7.02 to 11.5 V. The plasma-treated HfSe₂ photodetector also demonstrates a reasonable photoresponsivity from the visible to the near-infrared region of light.

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS2/h-BN Heterostructure

Layered hexagonal boron nitride (h-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS₂ can induce ∼6.5 × 10¹¹ cm^-2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS₂ on h-BN was found to be ∼5 × 10¹³ cm^-2 at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS₂/h-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal-insulator transition at a much lower charge density of ∼1.0 × 10¹² cm^-2 (T = 25 K). The reduced effective Schottky barrier height in MoS₂/h-BN is attributed to the decreased effective work function of MoS₂ arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO₂.

Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures

Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.