Direct Measurement and Identification of Nonradiative Processes in ZnO Nanocrystallines by Combining Photoacoustic Spectroscopy with Surface Photovoltage Spectroscopy

Nonradiative transitions (NRTs) are relaxation processes competing with radiative process. In light emitting diodes and room temperature laser devices, NRTs lower the emission efficiency. In order to prohibit these processes, better understanding is practically needed. But nowadays knowledge of NRTs comes from the analyses of the steady-state and time-resolved photoluminescence spectra, which are indirect evidences because of their radiative nature. Here we report a direct detection of nonradiative processes of ZnO nanocrystallines by combination of photoacoustic spectroscopy (PAS) with field-induced photovoltaic spectroscopy (FISPS) methods. In photoacoustic spectrum of ZnO nanocrystals, a main feature centered at 374 nm and a shoulder feature at 441 nm have been recorded. The surface photovoltage spectrum (SPS) displays a main peak at about 364 nm, which is assigned to band-gap transition. And the FISPS spectrum shows a main feature at 380 nm and a pronounced shoulder at 450 nm. The relative energy of the PAS main feature locates at about 0.1 eV lower than that of band-gap, and the relative energy of the PAS main and shoulder features locates at 60 meV higher than that of FISPS main and shoulder features. These energy spaces are in good consistent with the exciton binding energy reported for ZnO nanostructures. Thus we tentatively ascribe the NRTs to the trapping of the photogenerated excitons to the surface states of ZnO nanocrystallines.