Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional MoS$_2$

TL;DR

This study investigates how the anisotropic electronic transitions in freestanding MoS$_2$ layers depend on layer thickness, revealing layer-dependent optical gaps and the three-dimensional nature of certain electronic states.

Contribution

It provides the first experimental analysis of layer-dependent anisotropic electronic transitions in freestanding MoS$_2$, highlighting the differences between monolayer and multilayer systems.

Findings

Direct gap at 1.8 eV is excited by in-plane polarization.

Out-of-plane optical gap is 2.4±0.2 eV in monolayer MoS$_2$.

Indirect transition shows a three-dimensional-like character.

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 in- vestigated 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 elec- tronic 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$\pm$0.2 eV in monolayer MoS$_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.