Ambient-Induced Selenium Segregation and Nanoparticle Formation in 2H-HfSe2: An Experimental and Theoretical Study

TL;DR

This study combines experimental and theoretical methods to understand how ambient air causes selenium segregation and nanoparticle formation in HfSe2, revealing degradation mechanisms and emphasizing the need for encapsulation.

Contribution

It provides new insights into the air-induced degradation process of HfSe2 through combined experimental observations and ab initio simulations.

Findings

Se-rich nanoparticles form upon air exposure

Se atoms migrate to flake edges during degradation

Oxidation leads to Se expulsion and structural reorganization

Abstract

We investigate the air-induced degradation of few-layer hafnium diselenide (HfSe$_2$) through combined experimental and theoretical approaches. AFM and SEM reveal the formation of selenium-rich spherical features upon ambient exposure, while EDS confirms Se segregation. \textit{Ab initio} molecular dynamics simulations show that Se atoms migrate to flake edges and that O/O$_2$ exposure leads to selective Hf oxidation, breaking Se--Hf bonds and expelling Se atoms. No stable Se--O bonds are observed, indicating structural reorganization rather than oxidation. These findings emphasize the material's instability in air and the importance of encapsulation for preserving HfSe$_2$ in practical applications. Scanning tunneling spectroscopy confirms the semiconducting character of the nanoparticles, with an electronic bandgap compatible with that of elemental Se. These results highlight the critical role of lattice defects and oxidation dynamics in the degradation process and underscore the need for encapsulation strategies to preserve the integrity of HfSe$_2$-based devices.