Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensionalMoS2

Abstract

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly investigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the electronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low-energy valence electrons in the freestanding MoS 2 layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4 ± 0.2 eV in monolayer MoS 2 . This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS 2 systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement effect.

Nonvolatile Resistive Switching in Nanocrystalline Molybdenum Disulfide with Ion‐Based Plasticity

“…These ions are expected to originate from water molecules that have been split by the catalytic effect of MoS 2 to water 43–45. This is in line with other reports where the layered crystal structure of MoS 2 and the associated anisotropic electronic properties facilitate ion transport along the layers 28,46,47…”

Optical absorption and transmission in a molybdenum disulfide monolayer

“…2(a) and 3 of Ref. [39] which demonstrate that EELS spectra decrease as in-plane component of momentum transfer (q y ) increases (which is analogous with increasing incident angle).…”

Momentum-Resolved Dielectric Response of Free-Standing Mono-, Bi-, and Trilayer Black Phosphorus

“…This is consistent with the selection rules related to optical processes for non-zero values of the momentum and with the exciton dispersion predicted for BP and other 2D materials. 32,33 Now, we focus on the dispersion of the plasmonic contributions, which is characterized by a maximum intensity at 19.3 eV (for q = 0 Å -1 ), using the TEM in-column energy filter to record the scattering pattern at fixed energies (EFSP). Figure 3b shows the intensity repartition of the dielectric function as a function of momentum and direction for fixed values of energy.…”