Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

Interface contact and modulated electronic properties by external vertical strains and electric fields in graphene/MoS2 heterostructure

Based to the first-principles calculations, we study the electronic properties of graphene/MoS₂ heterostructure by modulating the vertical strains and applying external electric field. Graphene/MoS₂ heterostructure is a van der Waals heterostructure (vdWH) with the interlayer spacing is 3.2 Å for the equilibrium state, and the contact property of the interface is n-type Schottky contact. The Schottky barrier height (SBH) changes with vertical strains which induces a change of charge transfer between graphene and MoS₂ layer. In addition, with strain or without strain, the applied positive electric field can effectively promote the charge transfer from graphene to MoS₂, while the negative electric field has the opposite effect. These findings support for the design of field effect transistors based on graphene vdWHs.     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Dependence of electronic and optical properties of multilayer SiC and GeC on stacking sequence and external electric field

The electronic and optical properties of different stacked multilayer SiC and GeC are investigated with and without external electric field (EEF). The band gaps of multilayer SiC and GeC are found smaller than that of monolayer SiC and GeC due to the interlayer coupling effect. When EEF is applied, the direct band gaps (ΔK–M) of multilayer SiC and direct band gaps (ΔK–K) of multilayer GeC all turn to indirect band gaps (ΔK–G) as the band at the G point drops dramatically toward zero. The imaginary part ε₂(ω)s of multilayer SiC and GeC show that new absorption peaks between 2–5 eV appear when the polarized direction is perpendicular to the layer plane, and new absorption peaks in infrared region appear as the EEF is higher than a certain point when the polarized direction is parallel to the layer plane. Our calculations reveal that different stacking sequences and EEF can provide a wide tunable band structures and optical properties for multilayer SiC and GeC.     

Band gap modulation of transition-metal dichalcogenide MX2 nanosheets by in-plane strain

The electronic properties of quasi-two-dimensional honeycomb structures of MX₂ nanosheets (M=Mo, W and X=S, Se) subjected to in-plane biaxial strain have been investigated using first-principles calculations. We demonstrate that the band gap of MX₂ nanosheets can be widely tuned by applying tensile or compressive strain, and these ultrathin materials undergo a universal reversible semiconductor-metal transition at a critical strain. Compared to WX₂, MoX₂ need a smaller critical tensile strain for the band gap close, and MSe₂ need a smaller critical compressive strain than MS₂. Taking bilayer MoS₂ as an example, the variation of the band structures was studied and the semiconductor-metal transition involves a slightly different physical mechanism between tensile and compressive strain. The ability to tune the band gap of MX₂ nanosheets in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of application.     

A theoretical study on tuning band gaps of monolayer and bilayer SnS2 and SnSe2 under external stimuli

Based on density functional theory, we systematically study the mechanical and electronic properties of monolayer and bilayer SnS₂ and SnSe₂. The electronic properties of these layers can be significantly tuned by applying in-plane strains and electric fields perpendicular to the sheets. The band gaps of monolayer SnS₂ and SnSe₂ slightly increase with the in-plane tensile strains, and they start to decrease after critical strains (5% for monolayer SnS₂ and 7% for monolayer SnSe₂). The band gaps of bilayer SnS₂ and SnSe₂ have a similar tendency to the monolayers with smaller critical strains (1% for bilayer SnS₂ and 2% for bilayer SnSe₂), which enables a semiconductor-to-metal transition at 10% strain for bilayer SnSe₂. We also find that an external electric field perpendicular to bilayer SnS₂ and SnSe₂ modulates their electronic band gaps. Semiconductor-to-metal transitions are achieved at the electric fields of 0.27 V/Å for bilayer SnS₂ and 0.13 V/Å for bilayer SnSe₂.     

Tunable electronic and optical properties of GaS/GaSe van der Waals heterostructure

We have used first-principles calculations to investigate the electronic and optical properties of GaS/GaSe van der Waals heterostructures formed by stacking two-dimensional GaSe and GaSe monolayers. Our findings confirm that the GaS/GaSe heterostructures transform from an indirect to a direct band gap material for the two stackings considered in this study. In addition, we found that the direct band gaps are 1.780 eV and 1.736 eV for AA and AB stacking, respectively. It is observed that the behavior of the optical properties of AA stacking is similar to AB stacking with some differences in details and both heterostructures located in UV range. The refractive index values are 2.21 (AA pattern) and 2.18 (AB pattern) at zero photon energy limit and increase to 2.937 for AA and 2.18 AB patterns and both located in the visible region. More importantly, the GaS/GaSe heterostructures have a variety of extraordinary electronic and optical properties. Accordingly, these heterostructures can be useful for the solar cell, nanoelectronics, and optoelectronic applications.     

Single atom doping in 2D layered MoS2 from a periodic table perspective

Molybdenum Disulfide (MoS₂) is a well-known transition metal dichalcogenide with a hexagonal structure arrangement analogous to graphene. Two dimensional (2D) MoS₂ has attracted wide attention in various applications such as energy storage, catalysis, sensing, energy conversion and optoelectronics due to its unique properties including tunable bandgap, substantial carrier mobility, outstanding mechanical strength and dangling-bond free basal surface. Moreover, MoS₂ has shown an excellent capability to be a host for foreign atoms which tune its physicochemical properties. Herein, currently known structural changes in the MoS₂ crystals introduced by various single atom dopants coming from all over the chemical table of elements are reviewed. Accompanying electrical, optical and magnetic properties of such structures are discussed in detail. Potential applications of the doped MoS₂ are introduced briefly as well. The review concentrates on the recent state-of-the-art results obtained mostly by the high resolution scanning transmission electron microscopy (STEM), such as high angle annular dark field (HAADF) imaging as well as scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM). These techniques have been used to decipher dopant positions and other sub-atomic structural changes introduced to the MoS₂ structure by isolated dopants.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Controllable growth and characterizations of hybrid spiral-like atomically thin molybdenum disulfide

Monolayer MoS₂ is an emerging two-dimensional semiconductor with wide-ranging potential applications in novel electronic and optoelectronic devices. Here, we reported controlled vapor phase growth of hybrid spiral-like MoS₂ crystals investigated by multiple means of X-Ray photoemission spectroscopy, scanning electron microscopy, atomic force microscopy, kelvin probe force microscopy, Raman and Photoluminescence techniques. Morphological characterizations reveal an intriguing hybrid spiral-like MoS₂ feature whose lower planes are AB Bernal stacking and upper structure is spiral. We ascribe the hybrid spiral-like structure to a screw dislocation drive growth mechanism owing to lower supersaturation and layer-by-layer growth mode. In addition, the electrostatic properties of MoS₂ microflakes with hybrid spiral structures are obvious inhomogeneous and dependent on morphology manifested by kelvin probe force microscopy. Our work deepens the understanding of growth mechanisms of CVD-grown MoS₂, which is also adoptable to other TMDC materials.     

Theoretical studies on mechanical and electronic properties of s-triazine sheet

Abstract

Mechanical and electronic properties of s-triazine sheet are studied using first-principles calculations based on density functional theory. The in-plane stiffness and bulk modulus for s-triazine sheet are found to be less than that of heptazine. The reduction can be related to the nature of the covalent bonds connecting the adjacent sheets and the number of atoms per unit cell. The Poisson’s ratio of s-triazine sheet is half the value to that of graphene. Additionally, the calculated values of the two critical strains (elastic and yielding points) of s-triazine sheet are in the same order of magnitude to that for heptazine which was calculated using MD simulations in the literature. It is also demonstrated that s-triazine sheet can withstand larger tension in the plastic region. These results established a stable mechanical property for s-triazine sheet. We found a linear relationship of bandgap as a function of bi-axial tensile strain within the harmonic elastic region. The reduced steric repulsion of the lone pairs (p_x-, p_y-) causes the p_z-like orbital to shift to high energy, and consequently an increase in the bandgap. We find no electronic properties modulation of the s-triazine sheet under electric field up to a peak value of 10 V/nm. Such noble properties may be useful in future nanomaterial applications.     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension

We present first principles theory calculations on the mechanical and electronic properties of silicene and silicane structure under uniaxial tensile strain along different directions. Chirality effect is more significant in the mechanical properties of silicene than those of silicane. Different failure mechanisms are identified. A small band gap (up to 0.8 eV) is developed from zero with silicene structure under uniaxial tension and vanishes before the structure reaches its in-plane ultimate strength. However, a pre-existing band gap (2.39 eV) exists with silicane structure and decreases to zero with the increasing tensile strain without chirality effects.     

Enhanced photoluminescence of MoS2–Au nanostructures: Nanotriangle and nanohole arrays

We investigated the photoluminescence (PL) characteristics of MoS₂–Au hybrid nanostructures, fabricated by nanosphere lithography and wet-transfer techniques. Two kinds of Au nanostructures - such as nanotriangles (NTs) and nanoholes (NHs) - were fabricated for comparison. MoS₂ monolayers on both NT and NH arrays exhibited enhanced PL intensity, compared with those on SiO₂/Si substrates and flat Au thin films. Numerical simulations revealed clear distinction in the electric field intensity distributions in the NT and NH arrays at the PL excitation wavelength. Such difference could be attributed to the excitation of localized and propagating surface plasmon in the NT and NH arrays. This work helps us to understand how the plasmonic NT and NH arrays affect the physical properties of the MoS₂ monolayers on them.     

Laser-assisted activation of dielectrics for electroless metal plating

2 O₃, SiC, diamond, ZrO₂, etc. are presented. The activation of the dielectric surface can be achieved in a wide range of laser wavelengths and is stable in time. This activation allows selective deposition of various metals (Cu, Ni, Pt, Pd, etc.) with lateral dimensions of several μm. The model of the activation process is discussed. This deals with the modification of the band gap of the dielectric, which involves the appearance of a non-zero density of electronic states in the vicinity of the potential of electroless metal reduction. These electronic states can arise either from the formation of point defects in the ablated surface (for example, F centers in Al₂O₃, CeO₂, or ZrO₂) or from the band bending of the dielectric caused by residual mechanical stresses left in the material after laser ablation (SiC or diamond). The data on the activation of dielectrics by mechanical indentation are qualitatively consistent with the model. Received: 5 January 1998/Accepted: 7 January 1998     

Optically probing the interaction between monolayer MoS<Subscript>2</Subscript> and single-wall carbon nanotube

Owing to outstandingly tunable optoelectronic properties, hybrid materials consisting of atomic scale thickness of two dimensional (2D) transition metal dichalcogenides (TMDs) and one dimensional (1D) nanowires have been attracting steady interests over the last several years. In this research for the first time we report optically probing the interaction between monolayer MoS₂ and single-wall carbon nanotube (SWCNT). By using Raman and photoluminescence measurements, we found the charge transfer between MoS₂ and SWCNT is sensitive to the intensity of light field. We also demonstrate that SWCNT acts as p-type dopants at physical contact with monolayer MoS₂. Our study gives new insight into the interaction between monolayer MoS₂ and SWCNT, which may allow new phenomena and ideas for novel low dimensional hybrid materials.     

Electronic structure,mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

The electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si_1-xGe_xC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243?eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si _0.75 Ge _0.25 C, Si _0.50 Ge _0.50 C, Si _0.25 Ge _0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657?eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si_1-x Ge_xC (X?=?0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.     

External strain effect on the electronic and mechanical properties of the superconductor Nb2InC

We use first-principles method to investigate the effects of external strain ε on the structural, mechanical and electronic properties for the superconductor Nb₂InC. The results show that the tensile strain induces an isostructural phase transition in Nb₂InC. The elastic constants C_ij, bulk modulus B, shear modulus G, Young's moduli E, and Poisson ratio v_ij of Nb₂InC were also investigated in the range from ε=−10% to ε=10%. It indicates that Nb₂InC is mechanically stable under external strain, and its brittle–ductile transition occurs at ε=3.5%. Moreover, Nb₂InC gets a negative Poisson ratio at ε=4%. The calculated electronic structures indicate that the Nb–C bonding is stronger than Nb–In bonding in Nb₂InC. The energy band structures and densities of states of strained Nb₂InC were also calculated and discussed in detail. From these calculations, it is clear that the related properties of Nb₂InC can be easily tuned by strain. Therefore, our findings are very useful to tailor the physical properties of Nb₂InC by using strain engineering.