Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe$_{2}$

TL;DR

This study uses angle-resolved photoemission spectroscopy and theoretical calculations to analyze the electronic band structure of monolayer ReSe₂, revealing its anisotropic properties, orbital contributions, and effects of doping on its band gap.

Contribution

It provides the first detailed polarization-dependent electronic structure analysis of ReSe₂, highlighting its unique orbital contributions and doping effects compared to other 2D semiconductors.

Findings

ReSe₂'s valence band is dominated by in-plane orbitals, unlike 2H materials.

The band gap is approximately 1.7 eV in pristine ReSe₂.

Potassium doping reduces the band gap to at least 1.4 eV.

Abstract

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials, the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications on the gap following surface-doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe$_{2}$. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals.