Electronic Control of Chemistry and Catalysis at the Surface of an Individual Tin Oxide Nanowire

Tin oxide single nanowires configured as field effect transistors were shown to be operable and tunable alternately as gas sensors or as catalysts under a gaseous atmosphere that simulated realistic ambient conditions. The unusually large surface-to-volume ratio available with nanowires causes adsorption or desorption of donor or acceptor molecules on the nanowire's surface to greatly alter its bulk electron density at relatively small values of the gate voltage. This process can be sensitively monitored as changes in the nanowire's conductivity. The potentially radical change in carrier density can lead to significant changes in the nanowire's sensitivity as a sensor or reciprocally as a catalyst in reactions that involve charge exchange across the nanowire's surface. This leads to the prospect of tuning catalysis or other surface reactions entirely through electronic means

Quantitative Determination of the Raman Enhancement of Ag₃₀(CO)₂₅ and Ag₅₀(CO)₄₀ Matrix Isolated in Solid Carbon Monoxide

Size-selected Ag clusters in the range Ag₃–Ag₅₀ were prepared by sputtering a silver substrate, mass-selecting Ag cation clusters using a Wien filter, neutralizing and matrix-isolating them at cryogenic temperatures in solid CO. The Raman spectra of the resulting silver-cluster carbonyls were recorded using excitation wavelengths in the range 457.9 to 514.5 nm. For Ag₃₀ and Ag₅₀, the “adsorbed” carbon monoxide (which we estimated to number ∼25 and ∼40, respectively) gave rise to broad Raman bands centered at ∼2110 cm^–1. Because both the metal cluster and the CO density were measured quantitatively, a good estimate was computed for the increase in the Raman scattering cross-section per CO molecule adsorbed on the silver particle. For Ag₅₀ and 457.9 nm laser excitation, an enhancement of ∼1850 was measured, which dropped to ∼1350 at 514.5 nm excitation. For Ag₃₀ (and 457.9 nm excitation) the enhancement was ∼530. The enhancements of Ag₃, Ag₅, and Ag₉ were too low to measure accurately (i.e., <10). Extrapolating the enhancements obtained with blue and green wavelengths to the “plasmonic” band center, which for an Ag₅₀ cluster is expected to be at ∼370 nm, and assuming the excitation band to be a Lorentzian with a fwhh of 0.8 eV, the maximum Raman enhancement per CO ligand in Ag₅₀CO₄₀ was estimated to be ∼12000, in good agreement with computed results using a time-dependent density functional quantum calculation, carried out on a Ag₂₀ cluster complex by Jensen et al. (Jensen et al. Size-Dependence of the Enhanced Raman Scattering of Pyridine Adsorbed on Ag_<i>n</i> (<i>n</i> = 2–8, 20) Clusters. <i>J. Phys. Chem. C</i> <b>2007</b>, <i>111</i>, 4756–4764)