Femtosecond study of photo-induced electron dynamics in AgI and core/shell structured AgI/Ag2S and AgBr/Ag2S colloidal nanoparticles

Abstract

Direct measurements of the dynamics of photoinduced electrons in AgI, AgI/Ag2S, and AgBr/Ag2S semiconductor colloidal nanoparticles have been performed using femtosecond pump–probe laser spectroscopy. In AgI the transient absorption signal features a laser pulsewidth limited rise followed by a double exponential decay with time constants of 2.5 ps and >0.5 ns. The decay dynamics were found to be independent of pump power, indicating that the decays are not second order kinetic processes. The dynamics were also independent of the probe wavelengths, suggesting that the absorption spectrum of the photogenerated electrons is fully developed within ∼150 fs. The fast 2.5 ps decay is attributed to trapping and nonradiative electron–hole recombination mediated by a high density of trap states while the slow decay is attributed to reaction of deep trapped electrons with silver ions to form Ag atoms. For core/shell structured cocolloids of AgI/Ag2S, similar decay dynamics were observed. The only interesting difference is that the fast decay becomes faster with increasing concentration of Ag2S. This is due to direct excitation of Ag2S, which has a faster initial decay (800 fs) than in AgI. This is supported by the results of AgBr/Ag2S for which the same 800 fs decay was observed while no signal was observed for AgBr colloids alone, indicating clearly that the signal is from Ag2S. These findings on the early time dynamics of photo-induced excitons in AgI, AgI/Ag2S, and AgBr/Ag2S are important in better understanding the photographic process involving silver halides.