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Optical and Chemical Response of Nanostructured Films


When illuminated with visible light, nanostructured noble metals exhibit a strong

plasmon resonance at wavelength, p, that has been shown to be sensitive to its size,

structure, the dielectric properties of the surrounding medium, and charge density. The

tunability of the plasmon resonance has allowed metal nanosystems to be fabricated with

resonances matching the solar spectrum for us in plasmon promoted catalysis, plasmonic

photovoltaics, and surface-enhanced raman spectroscopy. Here we use UV-Visible spectroscopy

to track the shifts of the plasmon resonances from an array of gold nanoparticles

buried under metal oxide layers of varying thickness when in contact with one of two bulk

metals: aluminum or silver. By assuming the array of gold nanoparticles and metal-oxide

layers to be an optically homogenous lm of core-shell particles on a substrate, we developed

a Maxwell-Garnett effective medium approximation to extract reliable optical

parameters for the gold nanoparticles, yielding their charge state before and after contact

with the bulk metal.

Based on the optical parameters extracted from our model, we nd the magnitude of

charge transfer from the bulk metal to the gold nanoparticle is independent of the work

function of the bulk metal. Furthermore, when gold is used as the bulk layer in contact

with the gold nanoparticles, we measured an appreciable amount of charge transfer to the

gold nanoparticles, failing to support the well-established model for electrostatic contact

electrication. Instead, we attribute the charge transfer to the so called plasmoelectric

effect, an optically induced charge transfer mechanism, in which the gold nanoparticle

modifes its charge density to allow its resonant wavelength to match that of the incident

light. We show, however, that in our devices the Schottky barriers between the metals

and the metal oxide layers create a rectication effect that favors electron transfer from

the bulk metal to the nanoparticles over the reverse effect.

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