Excitation energy dependent efficiency of charge carrier relaxation and photoluminescence in colloidal InP quantum dots

Femtosecond transient absorption spectroscopy has been used together with time-integrated photoluminescence (PL) measurements to study charge carrier relaxation in colloidal InP quantum dots (QDs). Measurements of band-edge photoinduced bleaching as well as PL intensity measurements indicate that the fraction of charge carriers relaxing to the band-edge states depends on excitation wavelength, with a markedly reduced relaxation efficiency observed for excitation well above the absorption edge. The results concur with previous research on CdSe and InP QDs, and suggest that with increasing excitation energy there arise relaxation pathways involving surface or external energy states exhibiting reduced radiative efficiency. Such highly excited carriers are either inhibited or deviated from reaching the band-edge states. Excitation intensity dependent measurements indicate that those charge carriers which contribute to the band-edge absorption bleaching also contribute proportionally to the time-integrated PL spectrum.