title: Direct observation of time-dependent photoluminescence spectral shift in CdS nanoparticles synthesized in polymer solutions.
abstract: 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.
authors.affiliation: Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India. mandaldeb@yahoo.com
authors.forename: Debabrata
authors.initials: D
authors.lastname: Mandal
authors.forename: Haruko
authors.initials: H
authors.lastname: Hosoi
authors.forename: Uma
authors.initials: U
authors.lastname: Chatterjee
authors.forename: Tahei
authors.initials: T
authors.lastname: Tahara
mesh_terms: D016880:Anisotropy; D019187:Cadmium Compounds / Q000737:chemistry*; D008823:Micelles; D046529:Microscopy, Electron, Transmission; D008956:Models, Chemical; D015233:Models, Statistical; D053758:Nanoparticles / Q000737:chemistry*; D036103:Nanotechnology / Q000379:methods*; D010777:Photochemistry / Q000379:methods; D011108:Polymers / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013053:Spectrophotometry / Q000379:methods*; D013440:Sulfides / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D019187:Cadmium Compounds; D008823:Micelles; D011108:Polymers; D012997:Solvents; D013440:Sulfides; C034939:cadmium sulfide