Exploring the air stability of PdSe₂ via electrical transport measurements and defect calculations

In this work we investigate the effects of ambient exposure on CVD grown PdSe₂ and correlate density functional theory calculations of various physisorption and chemisorption binding energies and band structures to the observed changes in the electrical transport. Pristine PdSe₂ is n-type due to intrinsic selenium vacancies, but shows increased p-type conduction and decreased n-type conduction as a function of ambient aging during which various aging mechanisms appear to be operative. Short term aging (<160 h) is ascribed to an activated chemisorption of molecular O₂ at selenium vacancies; first-principles calculations suggest a ~0.85 eV activation energy and adsorption geometries with binding energies varying between 1.3–1.6 eV, in agreement with experimental results. Importantly, this chemisorption is reversible with a low temperature anneal. At long term aging (>430 h), there is a total suppression of n-type conduction, which is attributed to a dissociative adsorption/reaction of the O₂ molecules to atomic O and subsequent PdO₂ formation. XPS confirms the presence of PdO₂ in long term aged flakes. At these extended aging times, the low temperature anneal restores low n-type conduction and suppresses p-type conduction due to the low thermal stability of PdO₂ which, in agreement with XPS measurements, sublimates during the anneal. Thus PdSe₂ devices can be processed into device architectures in standard laboratory environments if atmospheric exposure times are limited to on the order of 1 week.