Observation of the Polaron Excited State in a Single-Crystal ZnO

Elucidating the property of the polaronic state at ultrafast temporal resolution is of great significance, especially for photocatalytic and solar-energy materials. In this work, femtosecond transient absorption (TA) spectroscopy with state selectivity is utilized to monitor the dynamics of the polaron generated by exciting the mid-gap states from the oxygen vacancy into the polaronic state in a single-crystal ZnO. The photoexcited polaronic state is verified by the observed coherent acoustic phonon with an oscillation on the kinetics at the initial several picoseconds at a frequency of about 50 GHz. We found that the excited state of polaron is located inside the conduction band (CB), which injects an electron into the CB within about 1 ps observed via a broad-band TA spectrum ranging from 400 to 4000 nm. Compared with that of free carriers generated by direct band gap excitation, the lifetime of the carriers resulted from the electron injection from the polaronic states is significantly prolonged, an important feature beneficial to photocatalysts.