6 research outputs found
Spectroscopic studies of fractal aggregates of silver nanospheres undergoing local restructuring
We present an experimental spectroscopic study of large random colloidal
aggregates of silver nanoparticles undergoing local restructuring. We argue
that such well-known phenomena as strong fluctuation of local electromagnetic
fields, appearance of "hot spots" and enhancement of nonlinear optical
responses depend on the local structure on the scales of several nanosphere
diameters, rather that the large-scale fractal geometry of the sample.Comment: 3.5 pages, submitted to J. Chem. Phy
Local anisotropy and giant enhancement of local electromagnetic fields in fractal aggregates of metal nanoparticles
We have shown within the quasistatic approximation that the giant
fluctuations of local electromagnetic field in random fractal aggregates of
silver nanospheres are strongly correlated with a local anisotropy factor S
which is defined in this paper. The latter is a purely geometrical parameter
which characterizes the deviation of local environment of a given nanosphere in
an aggregate from spherical symmetry. Therefore, it is possible to predict the
sites with anomalously large local fields in an aggregate without explicitly
solving the electromagnetic problem. We have also demonstrated that the average
(over nanospheres) value of S does not depend noticeably on the fractal
dimension D, except when D approaches the trivial limit D=3. In this case, as
one can expect, the average local environment becomes spherically symmetrical
and S approaches zero. This corresponds to the well-known fact that in trivial
aggregates fluctuations of local electromagnetic fields are much weaker than in
fractal aggregates. Thus, we find that, within the quasistatics, the
large-scale geometry does not have a significant impact on local
electromagnetic responses in nanoaggregates in a wide range of fractal
dimensions. However, this prediction is expected to be not correct in
aggregates which are sufficiently large for the intermediate- and
radiation-zone interaction of individual nanospheres to become important.Comment: 9 pages 9 figures. No revisions from previous version; only figure
layout is change
Nature of the Anomalous Size Dependence of Resonance Red Shifts in Ultrafine Plasmonic Nanoparticles
Plasmonic red shifts of nanoparticles are commonly used in imaging technologies to probe the character of local environments, and the understanding of their dependence on size, shape, and surrounding media has therefore become an important target for research. The red shift of plasmon resonances changes character at about 8-10 nm of size for spherical gold nanoparticles-above this value, the red shift progresses linearly with particle size, while below this size, the red shift changes nonlinearly and more strongly with size. Using an atomistic discrete interaction model, we have studied the special properties of the nanoparticle surface layers and discovered its importance for ultrafine plasmonic nanoparticles and their red shifts. We find that the physical origin for the specific properties inherent to the surface layer of atoms near the nanoparticle boundary is related to the anisotropy of the local environment of atoms in this layer by other atoms. The anisotropy changes the conditions for light-induced nonlocal interactions of neighboring atoms with each other and with the incident radiation compared to the atoms located in the particle core with isotropic nearest surroundings by other atoms. The local anisotropy of the nanoparticle crystal lattice is a geometric factor that increases toward its boundary and that is the most fundamental factor underlying the physical differences between the nanoparticle surface layer and the core material. It is shown that the inflexion point at 8-10 nm is due to a change in the dominant physical origin of the red shift -from chaotization of atomically light-induced dipoles within the surface layer in the case of ultrafine nanoparticles to retardation effects for large nanoparticles in which the relative volume of the surface layer decreases rapidly to a negligible value with increasing nanoparticle size. The patterns revealed are the basis for predicting the manifestation of surface layer effects in ultrafine plasmonic nanoparticles of different and of different materials