Luminescence probing of surface adsorption processes using InGaN/GaN nanowire heterostructure arrays

InGaN/GaN nanowire arrays (NWAs) exhibit efficient photoluminescence (PL) which persists to temperatures of 200 °C and beyond. The PL characteristics exhibit chemical sensitivity when the NWAs are exposed to gases or liquids. Gas sensing tests on such arrays reveal both quenching and enhancing PL responses, depending on the kinds of gases chosen and NWA operation conditions employed. A characteristic observed for all kinds of analytes is that the concentration dependence of the PL response consistently follows Langmuir isotherms which are easy to interpret regarding adsorbate-specific adsorption energies. Another characteristic feature is that best-fitting adsorption energies do not reveal as species-dependent constants but rather follow linear dependencies on temperature with species-dependent slopes. We show that this behavior can be explained by a competition of test gases and background gases for common adsorption sites. A particularly interesting gas species is water vapor, which can form competing adsorbates with quenching and enhancing characteristics. Unlike oxidizing gas species (O2, NO2, and O3) and water vapor, reducing gas species do not exhibit an intrinsic ability to modify the PL yield. Reducing gas species, rather, are detected in an indirect manner by consuming quenching oxygen adsorbates and by forming enhancing H2O ones as these interact with oxygen species coadsorbed in reactive backgrounds of ambient or synthetic air.