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Register a new userSelf-assembled Behaviors of Desulphurized $MoS_2$ Monolayer Sheets
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Self-assembled topological structures of post-processed two-dimensional materials exhibit novel physical properties distinct from those of their parent materials. Herein, the critical role of desulphurization on self-assembled topological morphologies of molybdenum disulfide ($MoS_2$) monolayer sheets is explored using molecular dynamics (MD) simulations. MD results show that there are differences in atomic energetics of $MoS_2$ monolayer sheets with different desulphurization contents. Both free-standing and substrate-hosted $MoS_2$ monolayer sheets show diversity in topological structures such as flat surface, wrinkles, folds and scrolls, depending on the desulphurization contents, planar dimensions and ratios of length-to-width of $MoS_2$ monolayer sheets. Particularly, at the critical desulphurization contents, they roll up into nanotube morphology, consistent with previous experimental observations. Moreover, the observed differences in the molecular morphological diagrams between free-standing and substrate-hosted $MoS_2$ monolayer sheets can be attributed to unique interatomic interactions and van der Waals interactions in them. The study provides important insights into functionalizing structural morphological properties of two-dimensional materials, e.g., $MoS_2$, via defect engineering.
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The doping and strain effects on the electron transport of monolayer MoS_2 are systematically investigated using the first-principles calculations with Boltzmann transport theory. We estimate the mobility has a maximum 275 cm^2/(Vs) in the low doping level under the strain-free condition. The applying a small strain (3%) can improve the maximum mobility to 1150 cm^2/(Vs) and the strain effect is more significant in the high doping level. We demonstrate that the electric resistance mainly due to the electron transition between K and Q valleys scattered by the M momentum phonons. However, the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the K and Q valleys. This sensitivity of mobility to the external strain may direct the improving electron transport of MoS_2.
We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare them to those of the InAs/GaAs QDs. We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structure differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizeable hole $p$, $d$ intra-shell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small $p$, $d$ level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV ($sim$ 1.55 $mu$m), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iii) The widths of the exciton $P$ shell and $D$ shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (iv) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1$bar{1}$0] directions.
Epitaxial self-assembled quantum dots (SAQDs) are of interest for nanostructured optoelectronic and electronic devices such as lasers, photodetectors and nanoscale logic. Spatial order and size order of SAQDs are important to the development of usabl
e devices. It is likely that these two types of order are strongly linked; thus, a study of spatial order will also have strong implications for size order. Here a study of spatial order is undertaken using a linear analysis of a commonly used model of SAQD formation based on surface diffusion. Analytic formulas for film-height correlation functions are found that characterize quantum dot spatial order and corresponding correlation lengths that quantify order. Initial atomic-scale random fluctuations result in relatively small correlation lengths (about two dots) when the effect of a wetting potential is negligible; however, the correlation lengths diverge when SAQDs are allowed to form at a near-critical film height. The present work reinforces previous findings about anisotropy and SAQD order and presents as explicit and transparent mechanism for ordering with corresponding analytic equations. In addition, SAQD formation is by its nature a stochastic process, and various mathematical aspects regarding statistical analysis of SAQD formation and order are presented.
Epitaxial self-assembled quantum dots (SAQDs) represent an important step in the advancement of semiconductor fabrication at the nanoscale that will allow breakthroughs in electronics and optoelectronics. In these applications, order is a key factor. Here, the role of crystal anisotropy in promoting order during early stages of SAQD formation is studied through a linear analysis of a commonly used surface evolution model. Elastic anisotropy is used a specific example. It is found that there are two relevant and predictable correlation lengths. One of them is related to crystal anisotropy and is crucial for determining SAQD order. Furthermore, if a wetting potential is included in the model, it is found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical surface height for three-dimensional film growth.
Molybdenum disulfide (MoS2) is layered transition-metal dichalcogenide (TMDC), which in its monolayer form, has the direct bandgap of 1.8 eV. We investigated the effect of width and strain on quantum transport for MoS2 armchair nanoribbons. That indicates MoS2 armchair nanoribbons are a good candidate for transistors even with strain.
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