The more important difference is that the photoresponse under sub-bandgap excitation exhibits clear environment dependence. A similar behavior has also been observed by Tamang et al. . The i p in the vacuum is roughly
three times higher than that in air. This observation is consistent with the OS mechanism in metal oxide semiconductors. Although the mechanism is usually described by the spatial separation of the electron–hole pair under above-bandgap excitation, CHIR-99021 molecular weight the sub-bandgap light that excites electrons from the surface trap state to conduction band could result in a similar effect [46, 47]. The schematic PC processes of hole trapping in the bulk and surface state excitations is shown in Figure 5. Although electron transition from the valence band to surface states may also generate a free hole which is able to recombine with oxygen ions and release trapped electrons leading to similar OS effect, the surface states are mostly occupied and negatively charged (i.e., the surface-adsorbed oxygen molecules are mostly ionized). STI571 solubility dmso The result indicates that the transition selleck chemical probability is rather low, which allows us to neglect the minor contribution. As light absorption only takes place at the surface, this could explain the very high power that is required
to produce an observable photoresponse using the 808-nm excitation source. Figure 5 The schematic PC processes for V 2 O 5 NW. Hole trapping effect in the bulk region by inter-bandgap excitation and oxygen sensitization effect in the surface by sub-bandgap excitation are illustrated respectively. Step (1a) electron–hole pair is generation by band-to-band
excitation (λ = 325 nm) in the bulk; step (1b) hole is captured by the trap state leaving the unpaired electron with long lifetime. Step (2a) free electron is solely generated from the negatively charged surface state (or oxygen ion) by sub-bandgap excitation (λ = 808 nm); step (2b) electron attracted to the core with less recombination probability also exhibits prolonged lifetime. The recombination will only take place while foreign oxygen molecule recaptures electron on surface. To compare the PC efficiencies between the above- Anidulafungin (LY303366) and below-bandgap excitations and between the V2O5 NWs grown by PVD and hydrothermal approaches, a new photoconductor parameter named normalized gain (Γn) is adopted and discussed [45, 48]. As the frequently used Γ is physically defined as the ratio of τ to transit time (τ t ) between two electrodes of a device, i.e., where v is the carrier drift velocity which is equal to the product of mobility (μ) and applied electric field (F), i.e., v = μF, where , Γ can be rewritten as . Accordingly, Γ depends on l and V. In terms of engineering application, photodetectors can be operated with high Γ by shortening l and increasing V.