Characterization of Carrier Dynamics in Semiconductor Nanomaterials Using Near-Field Heterodyne Transient Grating Method

Abstract

The authors developed a near-field heterodyne transient grating (NF-HD-TG) technique using femtosecond pulsed lasers to study ultrafast photoexcited carrier dynamics in semiconductor nanomaterials including TiO2, deposited CdSe and CdS QDs on nanostructured electrodes, and nanocrystalline silicon. Chemical deposition method was used to adsorb CdSe QDs and CdS QDs onto nanostructured TiO2 and SnO2 electrodes. Ultrafast photoexcited carrier dynamics was also studied in the QDs in air and in electrolyte of Na2S. They found that photoexcited hole and electron dynamics can be measured at the same time for CdSe and CdS QDs using the NF-HD-TG technique. The hole relaxation was attributed to surface trapping and/or relaxation into intrinsic nanocrystal states. Electron transfer rate ket was estimated from the QDs to TiO2 or SnO2 by comparing the electron relaxation of the QDs adsorbed on glass substrate in the NF-HD-TG responses. For nanocrystalline silicon fundamental processes related to light emission were determined including trapping to surface states and two-body recombination of excited carriers, with trapping being the dominant source of light emission.