Thaddeus J. Norman

Longitudinal Plasma Resonance Shifts in Gold Nanoparticle Aggregates

Recent interest in colloidal gold focuses on understanding the tunability of the longitudinal and transverse plasma resonance. It was reported that the reduction of HAuCl4 by Na2S produces gold nanoparticles with an optical absorption in the near infrared. This absorption blue shifts during the course of the reaction. X-ray photoemission spectroscopy (XPS) measurements on this system indicated that there was little sulfur present in the system. A small angle x-ray scattering (SAX) experiment was used to monitor the reaction while simultaneously the UV-VIS spectrum was measured. During the reaction the fractal dimension decreased from 4.154 ± 0.850 to 0.624 ± 0.146. The decrease in fractal dimension coincided with the blue shift in the longitudinal plasma resonance from the near IR to the visible. This suggests a change from reaction limited colloid aggregation (RLCA) to diffusion limited colloid aggregation (DLCA), caused the shift in the plasma resonance.

Optical Properties and Local Structure of Ag(I) Dopant in ZnSe:Ag Nanoparticles

Ag(I) doped ZnSe nanoparticles were synthesized using molecular cluster precursors. In the emission spectrum at 390 excitation, three emission bands, centered at 432 nm, 517 nm, and 484 nm, respectively were observed. The 432 nm and 517 nm bands can be assigned to ZnSe band-edge emission and donor-accepter emission from the vacancies and trap states in the ZnSe lattice to the Ag(I) dopant, respectively. Similarly the 484 nm band could be the result of ZnSe trap state or vacancies to Ag(I) acceptor emission or simply ZnSe trap state emission. X-ray Absorption Fine Structure (XAFS) data were collected at the Ag K-edge of the Ag(I) doped ZnSe nanoparticles. From these data it was concluded that the Ag(I) dopant occupied a variety of different environments in the ZnSe lattice.

Ultrafast electronic relaxation in colloidal gold (I) sulfide nanoparticles

Ultrafast electronic relaxation dynamics in Au2S colloidal nanoparticles have been studied using fs transient absorption spectroscopy. The electronic absorption spectrum of the nanoparticles exhibits a broad featureless absorption with increasing intensity from the near-IR into the visible and UV, indicating that Au2S is an indirect bandgap semiconductor. The electronic relaxation dynamics have been measured with 390 nm excitation and probing at 790 and 850 nm. The transient absorption decay profiles can be fit to a double exponential with time constants of 600 fs and 23 ps. The fast decay can be assigned to trapping of electrons from the conduction band to shallow trap states or from shallow traps to deep traps, while the long decay is assigned to recombination from shallow or deep trap states. The overall fast relaxation can be attributed to a high density of intrinsic or surface trap states. This fast decay is non-radiative and consistent with no observable luminescence at room temperature. EXAFS data show a 20% decrease in the first coordination shell for nanoparticles relative to bulk, which suggests a large number of surface dangling bounds that can contribute to a high density of surface trap states.