Synthesis and ultrafast study of cysteine- and glutathione-capped Ag2S semiconductor colloidal nanoparticles

A new synthetic method has been developed for preparing silver sulfide, Ag2S, nanoparticles capped with cysteine or glutathione. The average particle diameter has been determined to be around 9 nm using transmission electron microscopy. The ground-state electronic absorption spectra of the Ag2S nanoparticles show a continuous increase in absorption cross section toward shorter wavelengths starting from the red (600-800 nm). Ultrafast dynamics of photoinduced electrons in these nanoparticles have been measured using femtosecond transient absorption/bleach spectroscopy. In most cases studied, the early time transient profiles feature a pulse-width limited (<150 fs) rise followed by a fast decay (750 fs) and a slower rise (4.5 ps), The signal has contribution from both transient absorption and transient bleach. On longer time scales, three (Cys-1, Cys-2, and GSH-2) of the four samples studied show a recovery with 4.5 ps time constant that goes above the baseline and then decays gradually toward the baseline with a time constant of >1 ns. One sample (GSH-1) shows a bleach recovery that gradually approaches the baseline with a similar time constant (>1 ns) following the fast 4.5 ps rise. An interesting power dependence was observed for all the samples: the transient absorption contribution becomes more dominant over bleach with increasing excitation intensity. A simple four-state kinetic model developed to account for the main features of the dynamics suggests that initial photoexcitation populates the conduction band and depletes the valence band within the laser pulse (<150 fs). The conduction band electrons are first trapped in shallow trap states with a time constant of 500 fs and then further trapped into deep traps with a constant of 4 ps. The deep trapped electrons finally recombine with the hole with a time constant of >1 ns. This model suggests that the difference in dynamics observed between the different samples is due to different absorption cross sections of deep trap states. The observed excitation intensity dependence of the dynamics is attributed to shallow trap state saturation at high intensities.