We report on systematic studies of size-dependent alloy formation of silver-coated gold nanoparticles (NPs) in aqueous solution at ambient temperature using X-ray absorption fine structure spectroscopy (XAFS). Various Au-core sizes (2.5-20 nm diameter) and Ag shell thicknesses were synthesized using radiolytic wet techniques. The equilibrium structures (alloy versus core-shell) of these NPs were determined in the suspensions. We observed remarkable size dependence in the room temperature interdiffusion of the two metals. The interdiffusion is limited to the subinterface layers of the bimetallic NPs and depends on both the core size and the total particle size. For the very small particles (< or =4.6 nm initial Au-core size), the two metals are nearly randomly distributed within the particle. However, even for these small Au-core NPs, the interdiffusion occurs primarily in the vicinity of the original interface. Features from the Ag shells do remain. For the larger particles, the boundary is maintained to within one monolayer. These results cannot be explained either by enhanced self-diffusion that results from depression of the melting point with size or by surface melting of the NPs. We propose that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other. Molecular dynamics calculations correctly predict the activation energy for diffusion of the metals in the absence of vacancies and show an enormous dependence of the rate of mixing on defect levels. They also suggest that a few percent of the interfacial lattice sites need to be vacant to explain the observed mixing.

duced self-assembly (EISA) technique [25]. The crystals were mechanically stabilized by vapor-phase growth of a silica coating by alternating exposure to SiCl 4 and H 2 O vapors [26]. Transfer-printing of the PDMS-supported PE multilayer was performed by putting a drop of water on the colloidal crystal, applying the PDMS under pressure, and drying the sample for ∼ 15 min at 80°C. The PDMS was then carefully peeled off, leaving the PE multilayer on the crystal. A second colloidal crystal of identical diameter silica spheres was then grown on top of the PE defect by another EISA step.Redox Cycling: Oxidation of the ferrocene repeat units in the PFSdefect CPC was performed by immersing the sample in a solution of iodine dissolved in hexane (5 mM) followed by a thorough washing in hexane. Reduction was accomplished by immersing the oxidized sample in decamethylferrocene dissolved in THF (9 mM) and subsequently washing with THF.Characterization: Transmission spectra were recorded using an Ocean Optics SD 2000 fiber-optics spectrometer interfaced to an Olympus BX-41 optical microscope. Ellipsometry measurements were performed with a SOPRA GES-5 variable angle spectroscopic ellipsometer. Cross-sectional SEM images were obtained using a Hitachi S-5200 field emission scanning electron microscope by first coating the samples with a ∼ 5 nm carbon film by arc-discharge. [1-3] The improvement in conductivity and charge transfer at the electrode interface has made them excellent nanostructure supports for fuel-cell electrodes. [4][5][6][7][8] Enhanced performance of the SWCNT-based fuel-cell electrodes highlights the role of SWCNTs in decreasing charge-transfer resistance.[8]Another area in which SWCNTs show promise is in the development of light-harvesting assemblies. In a recent study we explored the semiconducting properties of SWCNTs in generating photocurrent. [9] SWCNT films cast on optically transparent electrodes (OTEs) respond to visible-light excitation. The low photocurrent generation efficiency of these films was attributed to ultrafast recombination of photogenerated charge carriers.[10] One way to improve charge separation is to develop composite nanostructures. Using this strategy in the past we COMMUNICATIONS 2458

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Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron-hole pair densities above 10(19) cm(-3) the dominant process for carrier cooling is the "bimolecular" Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton-exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron-hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.

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