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van der Waals heterostructure (vdWH) is an attractive technique to enhance the behavior of two-dimensional materials for designing viable products for electronic, spintronics and clean energy source application. Inspired by the recent synthesized of CrSSe monolayer, we have calculated the structural, electronic, Rashba parameters and photocatalytic properties of MoS 2 -CrXY (X≠Y S, Se, Te) vdWH by using density functional theory calculation. Two different model of MoS 2 -CrXY (X≠Y S, Se, Te) vdWH are presented due to the alternative order of chalcogen atom attached to MoS 2 layer. Phonon band structure, thermal stability and binding energies confirms, the most favorable stacking configuration is dynamically, thermal and energetically feasible. Indirect type-I (MoS 2 -CrXY (X≠Y S, Se)) and type-II (MoS 2 –CrSeTe) band alignment are observed in understudy vdWH. Large Rashba spin splitting is observed in model 2. Electrostatic potential and Bader charge analysis show charges are transferred from MoS 2 layer to CrXY layer, while the potential drop separate charge carrier (holes and electrons) at the interface. Work function and charge density difference are also calculated and presented. A red shift is observed in the position of excitonic peaks from model 1 to model 2. In last we investigated the performance of these vdWH for photocatalytic water splitting at pH = 0. Our results shows that MoS 2 -CrXY (X≠Y S, Se, Te) vdWH may be promising for future nanoelectronics, spintronics and photocatalytic water splitting. • JTMDCs having strong intrinsic dipole and efficient for spintronics applications. • We construct vdWH by vdW stacking of MoS 2 and CrXY monolayer. • Bader charge analysis confirm the charge transfer in these vdWH. • Imaginary part of dielectric function is calculated for understanding the optical properties. • Photocatalytic response confirms that, MoS 2 -CrXY vdWH can split water into O 2 / H 2 O and H + / H 2. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) monolayer pristine MoS 2 transition metal dichalcogenide (TMDC) is the most investigated material due to its potential applications as a non-precious electrocatalyst for the hydrogen evolution reaction (HER). However, the active edge sites created by sulfur non-stoichiometry and a defect-engineering by selective doping play a vital role in activating inert basal planes and enhancing the HER activity. Herein we report the synthesis of highly disordered Mn-doped 2D-MoS 2 nanospheres by a simple hydrothermal route under a non-equilibrium growth condition. Excess sulfur content in the precursor and electron-rich atmosphere created by Mn-doping results in an increased domain-based defects in the lattice that enhances the number of exposed edge sites, and also exposing more catalytically active high-index planes. Thereby improving its physicochemical properties. The developed electrode shows a low overpotential of 186 mV at a current density of 10 mA/cm2 and a low Tafel slope of 79.9mV/dec in acidic electrolytes. In addition, Density Functional Theory based calculation shows that Mn-doping in non-stoichiometric MoS 2 enhances the electronic properties and its role in facilitating hydrogen adsorption and desorption processes with free energy close to that of benchmark Pt is established. • Mn-doped MoS 2 nanosheets grown under a non-equilibrium hydrothermal process. • Nanosheets of MoS 2 were highly defect rich due to excess sulfur. • Mn-doping causes two order increase in the electrical conductivity of the catalyst. • The catalyst shows remarkably high hydrogen turnover rate of 1.45 s−1 at 0.2V. • DFT calculation shows a 10% substitutional doping by Mn would create thermodynamic condition similar to Pt. [ABSTRACT FROM AUTHOR]

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS2 transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS2 devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe2/MoS2 devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance. [ABSTRACT FROM AUTHOR]

The transition-metal dichalcogenide (TMDC) in the family of MX 2 ( M = Mo , W ; X = S , Se ) and the graphene (Gr) monolayer are an atomically thin semiconductor and a semimetal, respectively. The monolayer MX 2 has been discovered as a new class of semiconductors for electronics and optoelectronics applications. Because of the hexagonal lattice structure of both materials, MX 2 and Gr are often combined with each other to generate van der Waals heterostructures. Here, the MX 2 / Gr heterostructures are investigated theoretically based on density functional theory (DFT). The electronic structure and the optical properties of four different MX 2 / Gr heterostructures are computed. We systematically compare these MX 2 / Gr heterostructures for their complex permittivity, absorption coefficient, reflectivity and refractive index. [ABSTRACT FROM AUTHOR]

A Density Functional Theory based simulation study on cubic crystal orientation dependent transition metal contact with monolayer MoS2/WS2 in reference to exchange energy, density of states and thermodynamic entropy measurements are studied. Exchange energy states the stability of the interfaces between MoS2/WS2 and transition metals. The density of states calculation reveals the transport dominance with respect to charge polarity (electron or hole) and magnitude of charge flow through the interface. Entropy gives an alternative view to analyse carrier-phonon scattering at the interface. Our calculations suggest that (100) and (111) orientated transition metals from 3d group like Sc, Ti, Cr, Co, and Ni, (110) orientated transition metals from 4d group like Tc, Pd, Zr, and Ag and from 5d group (100), (110) orientated elements like Ta, W, Os, Ir, Pt are better suited for elemental contact applications in MoS2 and WS2 channel-based CMOS logic FETs. [ABSTRACT FROM AUTHOR]

Vertically stacked van der Waals heterostructures are a lucrative platform for exploring the rich electronic and optoelectronic phenomena in two-dimensional materials. Their performance will be strongly affected by impurities and defects at the interfaces. Here we present the first systematic study of interfaces in van der Waals heterostructure using cross-sectional scanning transmission electron microscope (STEM) imaging. By measuring interlayer separations and comparing these to density functional theory (DFT) calculations we find that pristine interfaces exist between hBN and MoS2 or WS2 for stacks prepared by mechanical exfoliation in air. However, for two technologically important transition metal dichalcogenide (TMDC) systems, MoSe2 and WSe2, our measurement of interlayer separations provide the first evidence for impurity species being trapped at buried interfaces with hBN interfaces that are flat at the nanometer length scale. While decreasing the thickness of encapsulated WSe2 from bulk to monolayer we see a systematic increase in the interlayer separation. We attribute these differences to the thinnest TMDC flakes being flexible and hence able to deform mechanically around a sparse population of protruding interfacial impurities. We show that the air sensitive two-dimensional (2D) crystal NbSe2 can be fabricated into heterostructures with pristine interfaces by processing in an inert-gas environment. Finally we find that adopting glovebox transfer significantly improves the quality of interfaces for WSe2 compared to processing in air. [ABSTRACT FROM AUTHOR]

[Display omitted] • The BC-stacking Zr 2 CO 2 / M Se 2 bilayers are energetically favourable. • The twisted Zr 2 CO 2 /WSe 2 -46.8 bilayer has flat–like bands near Fermi level. • The Zr 2 CO 2 –WSe 2 – BC bilayer is a type-II heterojunction. • The Zr 2 CO 2 –WSe 2 – BC bilayer is a promising candidate for photocatalysis. The heterojunctions based on two–dimensional materials have aroused much research interest as they exhibit excellent properties and can be used in various fields such as catalysis, nanoelectronics, and superconductivity. In this study, the typical MXene monolayer, Zr 2 CO 2 , was integrated with the M Se 2 (M = Mo and W) monolayers to build van der Waals Zr 2 CO 2 / M Se 2 bilayers with and without twisted angles. The density functional theory (DFT) calculations were conducted at different levels to study the stability, electronic structure, optical absorption spectra, and carrier mobility of the designed bilayers. The BC –stacking Zr 2 CO 2 / M Se 2 bilayers are energetically favorable and thermodynamically stable. The rotation angle between the Zr 2 CO 2 and M Se 2 layers can effectively modulate the band structures by shifting the band edges. The Zr 2 CO 2 /WSe 2 bilayer with the twisted angle of 46.8° has the flat–like bands in the vicinity of the Fermi level with the bandwidths less than 0.1 eV. The BC –stacking Zr 2 CO 2 /WSe 2 bilayer (Zr 2 CO 2 –WSe 2 – BC) is a type–II heterojunction with an indirect band gap of 1.44 eV. The valence band maximum and conduction band minimum of Zr 2 CO 2 –WSe 2 – BC are dominated by the electronic states of WSe 2 and Zr 2 CO 2 layers, respectively. Zr 2 CO 2 –WSe 2 – BC can be considered a favorable candidate photocatalyst for overall water splitting due to its appropriate band structure, spatial separation of photogenerated carriers, improved response to light, and fast carrier mobility. [ABSTRACT FROM AUTHOR]

Rashba spin-orbit coupling (SOC) plays a vital role in transporting carriers under external electric field or strain in two-dimensional transition metal di-chalcogenides in the absence of inversion symmetry. Here, the effect of SOC on the electronic and thermoelectric properties of two-dimensional monolayer HfS 2 , HfSe 2 and their Janus monolayer HfSSe has been investigated using density functional theory (DFT) and Boltzmann transport equation (BTE). A considerable enhancement in thermoelectric power factor (PF) for n-type carriers has been observed due to the SOC in all the monolayers. This enhanced power factor for n-type carriers is mainly because of increased valley degeneracy at the conduction band minima (CBM), as there is no enhancement in electron relaxation time (τ) with SOC. The lattice thermal conductivity (κ ph) at room temperature is very low for these monolayers compared to extensively used TMDCs such as MoS 2 and WS 2. This is due to the strong coupling between acoustic and optical modes, which results in low group velocity and a short lifetime of phonons in HfS 2 , HfSe 2 and HfSSe Janus monolayers. The ultralow value of κ ph results a very high thermoelectric figure of merit (ZT) at room temperature for n-type carriers and which increases significantly with SOC. The highest ZT values of 0.90 (HfSe 2), 0.84 (HfS 2) and 0.81 (HfSSe) for n-type at 600 K has been achieved with SOC. Our theoretical investigation predicts that there is a significant effect of SOC on the thermoelectric performance of HfX 2 (X = S, Se) and its Janus monolayer HfSSe and they can be a revolutionary candidate for the fabrication of a highly efficient thermoelectric power generator. • SOC induced enhancement of the thermoelectric properties of HfS 2 , HfSe 2 , and Janus monolayer of HfSSe was observed. • The enhanced power factor for n-type carriers is mainly because of increased valley degeneracy at the CBM with SOC. • Highest ZT value of 0.90 for n-type carriers , has been obtained at 600 K in HfSe 2 monolayer with SOC. [ABSTRACT FROM AUTHOR]

Due to their atomic thicknesses and semiconducting properties, two-dimensional transition metal dichalcogenides (TMDCs) are gaining increasing research interest. Among them, Hf- and Zr-based TMDCs demonstrate the unique advantage that their oxides (HfO2 and ZrO2) are excellent dielectric materials. One possible method to precisely tune the material properties of two-dimensional atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. In the present work, the molecular adsorption of NO2 and NH3 on the two-dimensional trigonal prismatic (1H) and octahedral (1T) phases of Hf and Zr dichalcogenides (S, Se, Te) is studied using dispersion-corrected periodic density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are investigated. In addition, the effects of the molecular dopants (NH3 and NO2) on the electronic structure of the materials are studied. It was observed that the adsorbed NH3 donates electrons to the conduction band of the Hf (Zr) dichalcogenides, while NO2 receives electrons from the valance band. Furthermore, the NO2 dopant affects than NH3 significantly. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This study explores, not only the properties of the two-dimensional 1H and 1T phases of Hf and Zr dichalcogenides (S, Se, Te), but also tunes their electronic properties by adsorbing non-bonded dopants. [ABSTRACT FROM AUTHOR]