Time-resolved photoluminescence of the ZnO nanorods: size and temperature dependences

The dynamics of the bound and free excitons and exiton polaritons of the ZnO nanorods have been investigated by time resolved photoluminescence in the temperature range from 10 K to 300 K. The samples have been fabricated by catalyst-free metal organic chemical vapor deposition (MOCVD), and have a diameter 35 nm and lengths in the range of 150 nm to 1.1 μm. In the temperature range of 10 K to 50 K, the photoluminescence lifetime of the bound exciton increases as the temperature increases. Photoluminescence lifetime of the free excitons, however, decreases with the temperature. The low temperature (10 K) time resolved photoluminescence spectra reconstructed from the time profiles measured at different frequencies clearly show that the bound exciton decay faster than the free A exciton. This result may be due to the transition from the bound exciton to free exciton because of the local temperature increase. Free B exciton is dominant above 50 K, and forms exciton polariton at high temperatures. At low temperature, photoluminescence lifetimes of the free A and B excitons do not show a clear correlation with the length of the nanorods. At room temperature, however, the photoluminescence lifetime increases monotonically as the length of the nanorods increase in the range of 150 nm to 600 nm. Decrease of the radiative decay rate of the exciton polariton has been invoked to account for the results.