Excitation relaxation in copper selenide nanowires

Ultrafast nonequilibrium charge carrier relaxation in highly ordered Cu_2-xSe nanowires of 8 nm, 13 nm and 25 nm diameter was investigated by means of femtosecond pump-probe absorption spectroscopy. Transient absorption bleaching was observed in the region of the near infrared absorption band, whereas an induced absorption dominated at higher energies. The transient absorption kinetics is almost independent of the excitation and probe wavelength and shows a biexpoenetial charge carrier recombination with the excitation intensity dependent decay rates. The initial ultrafast relaxation, which gets slower at higher excitation intensities, is followed by the slower decay component emerging at high intensities. These relaxation peculiarities are discussed in terms of recombination enhancement by intragap states, and relaxation kinetics is described by a theoretical model of two concurrent relaxation channels involving deep and shallow impurity levels.