ReS2 is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of the van der Waals compound ReS2 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 ReS2 using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS2 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 ReS2 has a direct band gap. Our results establish bilayer ReS2 as a advantageous building block for two-dimensional devices and van der Waals heterostructures.
@article{arxiv.1702.04176,
title = {Direct observation of the band gap transition in atomically thin ReS$_2$},
author = {Mathias Gehlmann and Irene Aguilera and Gustav Bihlmayer and Slavomír Nemšák and Philipp Nagler and Pika Gospodarič and Giovanni Zamborlini and Markus Eschbach and Vitaliy Feyer and Florian Kronast and Ewa Młyńczak and Tobias Korn and Lukasz Plucinski and Christian Schüller and Stefan Blügel and Claus M. Schneider},
journal= {arXiv preprint arXiv:1702.04176},
year = {2017}
}