Ultrafast electron hole plasma dynamics in chemically pristine and Ag-doped ZnO nanorods

The ultrafast dynamics of femotosecond laser excited carriers in the assembly of chemically pristine and Ag-doped ZnO nanorods is studied using the two-color pump-probe spectroscopy technique with the central photon energy of the pump pulse equal to the bandgap of the nanorods ensemble (∼3.14 eV) and the central photon energy of the probe pulse kept at 1.57 eV. On photo-excitation at 3.14 eV with an excited carrier density of ∼1020cm−3, about an order higher than the Mott density of bulk ZnO, the time delayed probe transmission displays tri-exponential decay exhibiting three decay mechanisms: (i) ultrafast electron-phonon thermalization of the order of ∼0.40 ps, (ii) trap mediated decay with a time constant of ∼9 ps, and (iii) e-h recombination with a time constant of ∼650 ps. It is observed that as the carrier density increases, the first two decay processes get longer on contrary to the recombination time which gets shorter. A detailed analysis of the faster decay process through the Two Temperature Model of ultrafast carrier relaxation supports our observation. Doping of Ag in these nanorods is found to introduce new defect states which alter the carrier dynamics significantly.