# Electronic and optical excitations of two-dimensional ZrS$_2$ and HfS$_2$ and their heterostructure

## TL;DR

This study uses first-principles calculations to explore the electronic and optical properties of ZrS₂, HfS₂ monolayers, and their heterostructure, revealing their band structure, excitonic features, and hybridization effects.

## Contribution

It provides a detailed first-principles analysis of the electronic and optical excitations in ZrS₂, HfS₂, and their heterostructure, highlighting their unique band alignment and excitonic properties.

## Key findings

- Both materials have indirect quasi-particle band gaps (~2.8 eV for ZrS₂, 2.6 eV for HfS₂.
- Excitonic peaks with binding energies of 0.6-0.8 eV dominate optical spectra.
- Heterostructure shows type-I level alignment with hybridization in the valence band.

## Abstract

In a first-principles study based on density-functional theory and many-body perturbation theory we investigate the electronic properties and the optical excitations of ZrS$_2$ and HfS$_2$ monolayers and their van der Waals heterostructure. Both materials have an indirect quasi-particle band gap, which amounts to about 2.8 eV in ZrS$_2$ and to 2.6 eV in HfS$_2$. In both systems the valence-band maximum is at $\Gamma$ and the conduction-band minimum at M. Spin-orbit coupling induces a splitting of about 100 meV at the $\Gamma$ point in the valence band, while it does not affect the conduction band. The optical absorption spectra are dominated by excitonic peaks, with binding energies between 0.6 eV and 0.8 eV. The ZrS$_2$/HfS$_2$ heterobilayer exhibits a peculiar type-I level alignment with a large degree of hybridization between the two monolayers in the valence band, while the conduction bands retain either ZrS$_2$ or HfS$_2$ character, respectively. As a consequence, both the electron and the hole components of the first exciton are localized in the ZrS$_2$ monolayer with non-vanishing probability of finding the hole also in the HfS$_2$ sheet.