Time-resolved X-ray excited optical luminescence (TRXEOL) from ZnO quantum dots: tracking hot-electron thermalization using luminescence dynamics

Time-resolved X-ray excited optical luminescence from uniform, perfectly crystalline ZnO quantum dots (QDs) (∼10 nm in diameter) excited using a 35 ps synchrotron light pulse from the Canadian Light Source with 570 ns repetition is reported. The experiment was performed with excitation at the O K-edge and recorded using an optical streak camera, which monitors simultaneously the signal in both the energy and time domain. It is found that the ZnO QD only emits at its optical band gap of ∼380 nm and the decay within the first 20 ns can be described by two exponentials. The decay lifetime is excitation energy dependent—the optical decay is faster with excitation energy just above the O K-edge than just below and becomes progressively faster with increasing excitation energy above the O K-edge in the XANES region. This behaviour is attributed to the redistribution of excitation sites near the surface of the QD and a change in the thermalization dynamics resulting from the introduction of O KLL Auger electrons turned on by O 1s excitation.