Spectral Dependence of Nanocrystal Photoionization Probability: The Role of Hot-Carrier Transfer

We conduct: measurements of photocharging of PbSe and PbS nanocrystal quantum ;dots (Ms) as a function of excitation energy ((h) over bar omega). We observe a rapid growth of the degree of photocharging with increasing (h) over bar omega, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (E-g) and scale as similar to 1.54 and similar to 3E(g), indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability Increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (gamma 10(-6) to 10(-3) for 1.5 eV and similar to 10(-4) to 10(-1) for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.