Novel gold nanoparticle aggregates have been synthesized using simple colloidal chemistry techniques. The electronic absorption spectra of the aggregates can be manipulated by controlling the synthetic conditions. The aggregates have been demonstrated for the first time to exhibit strong activity for surface-enhanced Raman scattering (SERS). SERS studies were performed using rhodamine 6G (R6G), a molecule which normally does not show SERS enhancement on gold surfaces, showed an enhancement factor on the order of 10<sup>9</sup>, which is similar to or better than most ensemble averaged SERS enhancement factors reported to date. The results demonstrate that these gold nanoparticle aggregates are promising for SERS applications in detection and analysis of molecules.
Ultrafast electronic relaxation dynamics in Au<sub>2</sub>S 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 Au<sub>2</sub>S 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.
Recent interest in colloidal gold focuses on understanding the tunability of the longitudinal and transverse plasma resonance. It was reported that the reduction of HAuCl<sub>4</sub> by Na<sub>2</sub>S 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.