Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties
of Gold Nanorods: From Noninvasive
Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges
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Abstract
We investigate the effect of surfactant-mediated,
asymmetric silver overgrowth of gold nanorods on their intrinsic optical
properties. From concentration-dependent experiments, we established
a close correlation of the extinction in the UV/vis/NIR frequency
range and the morphological transition from gold nanorods to Au@Ag
cuboids. Based on this correlation, a generic methodology for <i>in situ</i> monitoring of the evolution of the cuboid morphology
was developed and applied in time-dependent experiments. We find that
growth rates are sensitive to the substitution of the surfactant headgroup
by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with
hexadecyltrimethylammonium chloride (CTAC). The time-dependent
overgrowth in BDAC proceeds about 1 order of magnitude slower than
in CTAC, which allows for higher control during silver overgrowth.
Furthermore, silver overgrowth results in a qualitatively novel optical
feature: Upon excitation inside the overlap region of the interband
transition of gold and intraband of silver, the gold core acts as
a retarding element. The much higher damping of the gold core compared
to the silver shell in Au@Ag cuboids induces mirror charges at the
core/shell interface as shown by electromagnetic simulations. Full
control over the kinetic growth process consequently allows for precise
tailoring of the resonance wavelengths of both modes. Tailored and
asymmetric silver-overgrown gold nanorods are of particular interest
for large-scale fabrication of nanoparticles with intrinsic metamaterial
properties. These building blocks could furthermore find application
in optical sensor technology, light harvesting, and information technology