4 research outputs found
Surface Plasmon Resonance Damping in Spheroidal Metal Particles: Quantum Confinement, Shape, and Polarization Dependences
A key
parameter for optimizing nanosized optical devices involving
small metal particles is the spectral width of their localized surface
plasmon resonances (LSPR). In the small size range the homogeneous
LSPR line width is to a large extent ruled by the spatial confinement-induced
broadening contribution which, within a classical description, underlies
the popular phenomenological limited mean free path model. This unavoidable
contribution to the LSPR line width is basically a quantum finite-size
effect rooted in the finite extent of the electronic wave functions.
This broadening reflects the surface-induced decay of the coherent
collective plasmon excitations into particleāhole (<i>p</i>ā<i>h</i>) excitations (Landau damping),
the signature of which is a size-dependent fragmented LSPR band pattern
which is clearly evidenced in absorption spectra computed within the
time-dependent local density approximation (TDLDA). In this work we
analyze the spatial confinement-induced LSPR damping contribution
in the framework on an exact Hamiltonian formalism, assuming for convenience
a jellium-type ionic density. In resorting to the harmonic potential
theorem (HPT), a theorem stating that in the case of a harmonic external
interaction the electronic center-of-mass coordinates separate strictly
from the intrinsic motions of the individual electrons, we derive
a simple approximate formula allowing to (i) quantify the size dependence
of the LSPR damping in spherical nanoparticles (1/<i>R</i> law, where <i>R</i> is the sphere radius) and (ii) bring
to the fore the main factors ruling the confinement-induced LSPR broadening.
Then the modeling is straightforwardly generalized to the case of
spheroidal (prolate or oblate) metal particles. Our investigations
show that the LSPR damping is expected to depend strongly on both
the aspect ratio of the spheroidal particles and the polarization
of the irradiating electric field, that is, on the natureīøtransverse
or longitudinalīøof the collective excitation. It is found that
the magnitude of the damping is tightly related to that of the LSPR
frequency which rules the number of <i>p</i>ā<i>h</i> excitations degenerate with the plasmon energy. Qualitative
analysis suggests that the results are quite general and probably
hold for other nonspherical particle shapes. In particular, in the
case of elongated particles, as rods, the enlargement of the longitudinal
LSPR band by the confinement effects is predicted to be much smaller
than that of the transverse LSPR band
Optical Properties of a Particle above a Dielectric Interface: Cross Sections, Benchmark Calculations, and Analysis of the Intrinsic Substrate Effects
We show that the optical properties of a particle above a plane dielectric interface differ dramatically from those of the same particle embedded in a homogeneous matrix. Calculations for gold and silver spheres have been carried out in using the exact multipole expansion method, providing thus benchmark results for testing the accuracy of the available numerical methods. For silver spheres, the dependence of the extinction cross-section has been studied in detail as a function of the parameters characterizing the particle/interface system, namely, the radius of the sphere, the particle-surface distance, and the dielectric index of the substrate, as well as those characterizing the light excitation, that is, the angle of incidence and the polarization. Throughout this study we have separated the effects arising from the inhomogeneity of the applied field (interference between the incoming and reflected plane waves) from the intrinsic substrate effects resulting from the interaction with the induced surface charges on the surface. These last effects are, in the present formalism, encoded in the reflected scattered field impinging on the particle. For particles close to the interface, a rich multipolar plasmonic structure is observed, which can be described in the frame of a hybridization scheme similar to that developed for dealing with layered particles or dimers. Comparison with approximate models is also provided
Photo-Oxidation of Individual Silver Nanoparticles: A Real-Time Tracking of Optical and Morphological Changes
Absolute extinction measurements on individual silver
nanoparticles
under illumination show a steady evolution of their localized surface
plasmon resonance. Their progressive transformation during light exposure
and the influence of various parameters such as the nature of stabilizers,
the local environment (oxygen rate), the spectral range of the incident
light, and the shape of the nanoparticle (spheres or nanocubes) have
been carefully investigated in correlation with transmission electron
microscopy imaging. A combination of optics and electron microscopy
gives evidence that photoaging mainly consists of the progressive
formation of an oxide shell around a metallic silver core during light
illumination. Moreover, in the case of nanocubes, the metallic core
not only decreases in volume but also changes morphologically since
edges and corners are rounded off during the photo-oxidation process.
The generalized Mie theory and finite element method, used to calculate
the optical extinction cross-section of core/shell Ag@Ag<sub><i>x</i></sub>O nanoparticles, well account for the observed time
evolutions of the absolute extinction spectra of the silver nanospheres
and nanocubes. Furthermore, the calculated electromagnetic field at
the nanocube surface, enhanced on edges and corners, can explain the
higher efficiency of the photo-oxidation on edges and corners and
the rounding increase under illumination
Plasmon Spectroscopy and Chemical Structure of Small Bimetallic Cu<sub>(1ā<i>x</i>)</sub>Ag<sub><i>x</i></sub> Clusters
The
optical properties of small CuāAg bimetallic clusters
have been experimentally and theoretically investigated in relation
to their chemical structure analyzed by high resolution transmission
electron microscopy (HRTEM). Cu <sub>(1ā<i>x</i>)</sub>Ag<sub><i>x</i></sub> clusters of about 5 nm in diameter
are produced in a laser vaporization source with a well-defined stoichiometry
(<i>x</i> = 0, 25, 50, 75, and 100%) and dispersed in an
alumina matrix. Absorption spectra are dominated by a broad and strong
surface plasmon resonance whose shape and location are dependent on
both cluster composition and sample aging. Detailed modeling and systematic
calculations of the optical response of pure and oxidized mixed clusters
of various chemical structures have been carried out in the framework
of classical and semiquantal formalisms. Optical and HRTEM measurements
combined with theoretical predictions lead to the conclusion that
these bimetallic clusters are not alloyed at the atomic scale but
rather present a segregation of chemical phases. Most likely, they
adopt a Cu@Ag coreāshell configuration. Moreover, the nanoparticle
oxidation process is consistent with the formation of a copper oxide
layer by dragging out inner copper atoms to the cluster surface