Optical properties of CdS quantum dots, capped with different surface-active materials and suspended in transparent silicate glasses (prepared by a low-temperature sol/gel process), have been examined by photoluminescence and picosecond laser spectroscopy. Glasses prepared with different surface-active materials [poly(vinyl alcohol), PVA; hexametaphosphate, HMP] display different charge carrier dynamics, demonstrating the importance of the CdS particle/silicate glass interface. Excitation of the glasses at high laser power (2-3 mJ/pulse; ca. 30-ps pulses) generate several excitons (bound electron/hole pairs) in the same CdS particle to give multiexcitonic states. Upon relaxation, these excited states lead to Auger recombination which ejects an electron that gets trapped on the particle surface as cadmium atom(s), Cdn0, or on the silicate glass network (photodarkening), leaving excess positive charge on the CdS particle to give S•- radicals. Transient absorption and emission spectra show variations in the different time domains examined from -20 ps to 10 ns. From the congruence between transient absorption rise times and decay times with those from double-exponential fits of the emission decay, we argue that the charge carrier dynamics have photochemical origins. Charge carrier migration to the glass host occurs in several stages, initiated first by rapid carrier trapping into very shallow energy traps (ca. a few millielectronvolts), followed by slower trap to trap diffusion until a deeper energy trap is encountered. Photochemical processes are identified that lead to cathodic and anodic corrosion of the CdS particles and to photodarkening of the glass matrix.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry