Ultrasensitive Transient Absorption Study on Charge Trapping Dynamics in Rutile TiO ₂ Single Crystals

Charge recombination and trapping processes in rutile TiO2 single crystals were studied through ultrahigh-sensitivity transient absorption spectroscopy. I previously reported that secondary charge recombination of a polaron was the dominant response in rutile TiO2 single crystals within the subnanosecond time range (Chem. Phys. Lett. 2008, 461, 238–241). This conclusion did not necessarily indicate the absence of trapping processes because charge recombination occurs preferentially over trapping processes if the excitation light intensity is high. Therefore, transient absorption measurements have to be examined carefully at low excitation light intensities to explore charge-trapping processes. Transient absorption measurements in rutile TiO2 single crystals under very weak excitation conditions (<10 μJ cm–2) were thus examined using an ultrahigh-sensitivity transient absorption spectrometer developed recently in my laboratory. I succeeded in observing trapping processes and estimated the trapping time constant of a polaron to be 40 ns. Time-resolved microwave conductivity measurements were also examined, and I confirmed that the polaron underwent an immobile state.