| Optoelectronic properties of nanoscale materials are significantly affected by their surfaces because of the presence of intrinsic and extrinsic electronic states in the bandgap. Thermally or optically driven charge exchange between the bulk and these states directly affects the surface band bending of these materials. Physisorption, chemisorption or desorption of environmental species at the surface also bring in additional modifications to the surface electronic structure, thereby, changing the surface potential barrier. The focus of this work was to investigate the influence of the environmental conditions on the surface band bending in nanoporous silicon (NPS), as well as some other nanostructured materials, using transient surface photovoltage---a non-destructive and highly surface-sensitive experimental probe.;The potential barrier in NPS was monitored in a wide range of pressures, going down to high vacuum, and in a variety of gases. Due to the different rates of physisorption and chemisorption processes, we observed a coexistence of multiple components in the SPV transients occurring on different time scales and under different ambient conditions. We suggested a model elucidating the roles of intrinsic or extrinsic states in the surface transport properties of NPS. The importance of results is discussed in relation to potential applications of the studied materials in optoelectronic gas sensors.;In order to distinguish between different surface charge transport processes affected by adsorption/desorption, we employed monochromatic illumination using several different wavelengths for selective excitation of these processes and compared these results with the ones observed for a polychromatic illumination containing both super- and sub-bandgap wavelengths. The scaling of transient SPV onset slopes in NPS vs. the excitation intensity was found to deviate from the linear dependence commonly observed in conventional bulk semiconductors. Considering drift- and diffusion-mediated surface charge redistribution, we discuss possible reasons for such discrepancy in nanoscale materials. We also performed transient SPV measurements for NPS filled with magnetic metal nanoparticles, in vacuum and non-vacuum environments, and found that the embedded filler does not affect the photoresponse of the material, a finding beneficial to potential dual spintronic/sensor applications.;Finally, using similar approaches, we compared results obtained for some other nanomaterials with those described above, and demonstrated that the behavior observed for NPS is not generic, but rather unique and germane to NPS. |
