Parts per million water in gaseous vapor streams dramatically accelerates porous silicon oxidation

Substantial research has focused on exploiting and understanding porous silicon (pSi) photoluminescence (PL) for applications in areas ranging from chemical sensing to solid-state lighting. At ambient temperature, pure H ₂O is well-known to slowly (over a time scale of hours to days) and irreversibly oxidize as-prepared pSi (ap-pSi) to form oxidized pSi (ox-pSi). In this paper, we report that the apparent ap-pSi to ox-pSi oxidation rates can be orders of magnitude faster in the presence of nonaqueous vapor streams that contain just ppm H ₂O levels. When H ₂O is removed from the nonaqueous vapor stream, ap-pSi oxidation ceases. The nonaqueous analyte vapors serve as a vehicle to transport H ₂O directly into the hydrophobic, ap-pSi matrix where the H ₂O then oxidizes the ap-pSi leading to ox-pSi, permanently changing the pSi PL and surface chemistry. The ap-pSi oxidation rate is much faster in the presence of nonaqueous vapors because H ₂O transport into the pSi matrix is no longer limited by H ₂O slowly percolating-oxidizing-percolating through the ap-pSi matrix.