Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions.

Direct observation of time-resolved emission spectra (TRESs) of cadmium sulfide nanoparticles in polymer solutions was carried out with picosecond resolution using a streak camera. The TRESs were found to undergo a pronounced time-dependent Stokes shift, eventually coinciding with the steady-state photoluminescence spectra within an approximately 40 ns delay time. Moreover, approximately 90% of the shift was complete within the first 1 ns after excitation, in contrast to the fact that overall photoluminescence involves very long time constants of 10-100 ns. The observed Stokes shift dynamics was very similar in CdS nanoparticles stabilized in two very different types of polymer solutions. Thus the solvent and/or polymeric stabilizer appeared to have a minimal effect on the shift. We propose that the relaxation proceeds through an internal mechanism involving the fast decay of high-energy traps into relatively slow-decaying low-energy traps. Time-dependent photoluminescence anisotropy experiments also revealed an approximately 1 ns decay component appearing only in the higher-energy end of the photoluminescence spectrum. Because this time constant is too short to represent rotational diffusion of the nanometer-sized particles, it was associated with the rapid relaxation of the high-energy trap states.