Mahdi Ghorbani-Asl

Electromechanics in MoS2 and WS2: nanotubes vs. monolayers

The transition-metal dichalcogenides (TMD) MoS2 and WS2 show remarkable electromechanical properties. Strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition. Providing strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16%. We show that Raman signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain, showing that Raman spectroscopy is an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments in situ. TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter.

Defect-induced conductivity anisotropy in MoS2monolayers

Various types of defects in MoS2 monolayers and their influence on the electronic structure and transport properties have been studied using the Density-Functional based Tight-Binding method in conjunction with the Greens Function approach. Intrinsic defects in MoS2 monolayers significantly affect their electronic properties. Even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS2, strong anisotropy is observed in the presence of defects. Localized mid-gap states are observed in semiconducting MoS2 that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models.

Electromechanics in MoS 2 and WS 2 : nanotubes vs. monolayers

The transition-metal dichalcogenides (TMD) MoS2 and WS2 show remarkable electromechanical properties. Strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition. Providing strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16%. We show that Raman signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain, showing that Raman spectroscopy is an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments in situ. TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter.

Two Dimensional Materials Beyond MoS2: Noble‐Transition‐Metal Dichalcogenides

The structure and electronic structure of layered noble-transition-metal dichalcogenides MX2 (M=Pt and Pd, and chalcogenides X=S, Se, and Te) have been investigated by periodic density functional theory (DFT) calculations. The MS2 monolayers are indirect band-gap semiconductors whereas the MSe2 and MTe2 analogues show significantly smaller band gap and can even become semimetallic or metallic materials. Under mechanical strain these MX2 materials become quasi-direct band-gap semiconductors. The mechanical-deformation and electron-transport properties of these materials indicate their potential application in flexible nanoelectronics.

Spontaneous Ripple Formation in MoS2 Monolayers: Electronic Structure and Transport Effects

The spontaneous formation of ripples in molybdenum disulfide (MoS2 ) monolayers is investigated via density functional theory based tight-binding Born-Oppenheimer molecular dynamics. Monolayers with different lengths show spontaneous rippling during the simulations. The density of states reveals a decrease in the bandgap induced by the stretching of the MoS2 units due to ripple formation. Significant quenching in electron conductance was also observed. The ripples in the MoS2 monolayers have an effect on the properties of the material and could impact its application in nanoelectronics.

Nearly free electron states in MXenes

Using a set of first-principles calculations, we studied the electronic structures of two-dimensional transition metal carbides and nitrides, so called MXenes, functionalized with F, O, and OH. Our projected band structures and electron localization function analyses reveal the existence of nearly free electron (NFE) states in a variety of MXenes. The NFE states are spatially located just outside the atomic structure of MXenes and are extended parallel to the surfaces. Moreover, we found that the OH-terminated MXenes offer the NFE states energetically close to the Fermi level. In particular, the NFE states in some of the OH-terminated MXenes, such as

A Single‐Material Logical Junction Based on 2D Crystal PdS2

\mathrm{T}{\mathrm{i}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{Z}{\mathrm{r}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{Z}{\mathrm{r}}_{2}\mathrm{N}{(\mathrm{OH})}_{2},\mathrm{H}{\mathrm{f}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{H}{\mathrm{f}}_{2}\mathrm{N}{(\mathrm{OH})}_{2},\mathrm{N}{\mathrm{b}}_{2}\mathrm{C}{(\mathrm{OH})}_{2}

Effect of compression on the electronic, optical and transport properties of MoS2/graphene-based junctions

, and

Is MoS2 a robust material for 2D electronics

\mathrm{T}{\mathrm{a}}_{2}\mathrm{C}{(\mathrm{OH})}_{2}

Efficient Quantum Simulations of Metals within the Γ-Point Approximation: Application to Carbon and Inorganic 1D and 2D Materials.

, are partially occupied. This is in remarkable contrast to graphene, graphane, and