Abstract
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.
