Ultrafast Charge Transfer Dynamics in Polycrystalline CdSe/TiO2 Nanorods Prepared by Oblique Angle Codeposition

Abstract

Ultrafast exciton dynamics of aligned polycrystalline nanorod arrays composed of CdSe or CdSe/TiO2 grown on conductive glass substrates using oblique angle deposition/codeposition have been studied using femtosecond transient absorption (TA) spectroscopy. Scanning electron microscopy images show that the morphology of the two samples are comparable in height, width, and tilt angle. X-ray diffraction and Raman spectroscopy indicate that the as-deposited CdSe nanorod arrays are in the hexagonal phase, while the TiO2 is amorphous. In the TA studies, a pump wavelength of 580 nm was used to determine the exciton lifetimes of CdSe in the two samples. Transient bleach dynamics probed at 695 nm can be fit with triple exponential functions with lifetimes of 7 ps, 84 ps, and ∼1.0 ns for CdSe nanorods versus 0.5 ps, 3 ps, and 24 ps for the CdSe/TiO2 composite-nanorods. These lifetimes are independent of the pump power, indicating that nonlinear processes are not involved. For CdSe nanorods, the two fast decays are mainly due to nonradiative electron–hole recombination or exciton relaxation mediated by trap states. The overall much faster decay in CdSe/TiO2 nanorods is due to electron transfer from the conduction band of CdSe to the conduction band of TiO2. The electron injection rate from CdSe into TiO2 was calculated to be 1.7 × 1011 s–1 based on the average lifetime measured for CdSe with and without TiO2. This very high rate of electron injection is attributed to the large interfacial area and strong coupling between the two materials in CdSe/TiO2 composite-nanorods. Such strongly coupled semiconductor–metal oxide heterostructures are desired for applications in solar energy conversion.