2 research outputs found
Silver-Overgrowth-Induced Changes in Intrinsic Optical Properties of Gold Nanorods: From Noninvasive Monitoring of Growth Kinetics to Tailoring Internal Mirror Charges
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
hexadecyltrimethylÂammonium 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
Macroscopic Strain-Induced Transition from Quasi-infinite Gold Nanoparticle Chains to Defined Plasmonic Oligomers
We
investigate the formation of chains of few plasmonic nanoparticlesî—¸so-called
plasmonic oligomersî—¸by strain-induced fragmentation of linear
particle assemblies. Detailed investigations of the fragmentation
process are conducted by <i>in situ</i> atomic force microscopy
and UV–vis–NIR spectroscopy. Based on these experimental
results and mechanical simulations computed by the lattice spring
model, we propose a formation mechanism that explains the observed
decrease of chain polydispersity upon increasing strain and provides
experimental guidelines for tailoring chain length distribution. By
evaluation of the strain-dependent optical properties, we find a reversible,
nonlinear shift of the dominant plasmonic resonance. We could quantitatively
explain this feature based on simulations using generalized multiparticle
Mie theory (GMMT). Both optical and morphological characterization
show that the unstrained sample is dominated by chains with a length
above the so-called infinite chain limitî—¸above which optical
properties show no dependency on chain lengthî—¸while during
deformation, the average chain length decrease below this limit and
chain length distribution becomes more narrow. Since the formation
mechanism results in a well-defined, parallel orientation of the oligomers
on macroscopic areas, the effect of finite chain length can be studied
even using conventional UV–vis–NIR spectroscopy. The
scalable fabrication of oriented, linear plasmonic oligomers opens
up additional opportunities for strain-dependent optical devices and
mechanoplasmonic sensing