# Direct observation of the band gap transition in atomically thin ReS$_2$

## TL;DR

This study uses advanced microscopy and calculations to explore the electronic band structure of ReS$_2$, revealing layer-dependent properties and identifying bilayer ReS$_2$ as having a direct band gap, which is promising for 2D device applications.

## Contribution

It provides the first momentum-resolved measurements of ReS$_2$'s electronic structure across different layers, highlighting the direct band gap in bilayer ReS$_2$ and the delocalization of valence electrons.

## Key findings

- Valence electrons are delocalized across the van der Waals gap in bulk ReS$_2$
- Valence band dispersion varies with the number of layers
- Only bilayer ReS$_2$ exhibits a direct band gap

## Abstract

ReS$_2$ is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of the van der Waals compound ReS$_2$ leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS$_2$ using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS$_2$ are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing a significantly increased effective electron mass in single-layer crystals. We also find that only bilayer ReS$_2$ has a direct band gap. Our results establish bilayer ReS$_2$ as a advantageous building block for two-dimensional devices and van der Waals heterostructures.