The reduction of HAuCl 4 by Na 2 S has been reported to produce gold nanoparticles with an optical absorption in the near-infrared along with its characteristic absorption in the visible. The optical resonances in the visible are due to the gold surface plasma, which are a function of the geometry of the particles. The near-infrared absorption had been attributed to the formation of Au 2 S/Au core/shell structures. In this report we present new electronic absorption, electron microscopy, and X-ray absorption data in several systems to show that the near-infrared absorption does not involve core/shell structures. We further suggest that the near-infrared adsorption is most likely the result of the formation of aggregates of gold nanoparticles. The identification of the origin of the near-infrared resonance is critical in understanding the optical properties of metal nanoparticle systems.

We report the first direct observation of the ultrafast electronic relaxation and coherent vibrational oscillation of strongly interacting gold nanoparticle aggregates measured by femtosecond laser spectroscopy. The electronic relaxation, reflected as a fast decay component with a time constant of 1.5-2.5 ps, becomes faster with decreasing pump power, similar to earlier observations of isolated gold nanoparticles. Surprisingly, periodic oscillations have been observed in the transient absorption/bleach signal and have been attributed to the coherent vibrational excitation of the gold nanoparticle aggregates. The oscillation period has been found to depend on the probe wavelength. As the probe wavelength is varied from 720 to 850 nm, the period changes from 37 to 55 ps. This suggests that the broad extended plasmon band (EPB) contains contributions from gold nanoparticle aggregates with different sizes and/or different fractal structures. Each of the different probe wavelengths therefore interrogates one subset of the aggregates with similar size or structure. Interestingly, the observed oscillation period for a given aggregate size determined by dynamic light scattering is longer than that predicted based on a elastic sphere model. One possible explanation is that the actual size of the aggregates is larger than what was observed from dynamic light scattering. An alternative, perhaps more likely, explanation is that the vibration of the aggregates is "softer" than that of hard spherical gold nanoparticles possibly because the longitudinal speed of sound is lower in the aggregates than in bulk gold. Persistent spectral hole burning was performed and yielded a hole in the nanoparticle aggregate's extended plasmon band, further supporting that the near-IR band is composed of absorption subbands from differently sized/structured aggregates.

Mn 2+ -doped ZnSe nanoparticles were synthesized from molecular cluster precursors. Four ZnSe nanoparticle samples, one with low Mn 2+ concentration (A), one with an intermediate Mn 2+ concentration (B), one with a high Mn 2+ concentration (C), and one with no Mn 2+ , were prepared and characterized using UV-vis, luminescence, electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) techniques. The sample with no Mn 2+ had a sharp ZnSe band edge emission peak and a quantum yield of ∼2%. The samples with Mn 2+ had a significant decrease in band edge emission. Sample A had no Mn 2+ 4 T 1 f 6 A 1 emission but showed some ZnSe band edge emission and trap state emission. Sample B had Mn 2+ 4 T 1 f 6 A 1 emission and a further reduction in ZnSe band edge emission and trap state emission. Sample C showed an increase in the Mn 2+ 4 T 1 f 6 A 1 emission, a dramatic increase in trap state emission, and essentially no ZnSe band edge emission. The overall emission from all four samples was quenched with time. To better understand these observations, XAFS and ESR data were taken to characterize the local structural and chemical environment of the Mn 2+ ions. The XAFS data indicated that there was a reduction in the Zn and Mn first neighbor Se coordination from the bulk value but a lack of a reduction in the Se first neighbor coordination. This suggests that the core of the nanoparticles resembles that of bulk ZnSe, and the surface of the particle has a higher concentration of metal atoms. We propose that the surface Mn 2+ possessed an octahedral geometry due to significant OH -/O 2coordination and the interior Mn 2+ occupied the Zn 2+ tetrahedral site. The overall low Mn 2+ emission quantum yield (>0.1%) is primarily due to the presence of Mn 2+ on the particle surface, and the decrease in Mn 2+ emission overtime is attributed to the quenching of the luminescence by OH -/O 2coordinated to the surface metal ions. In sample C, which had the highest Mn 2+ concentration, the surface Mn 2+ enhanced the disorder of the nanoparticle surface structure, resulting in an increase in trap state emission.

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