INFIQC   05475
INSTITUTO DE INVESTIGACIONES EN FISICO- QUIMICA DE CORDOBA
Unidad Ejecutora - UE
artículos
Título:
Cluster Size Effects in the Surface-Enhanced Raman Scattering Response of Ag and Au Nanoparticle Aggregates: Experimental and Theretical Insight
Autor/es:
J. C. FRAIRE; L.A. PÉREZ; E. A. CORONADO
Revista:
JOURNAL OF PHYSICAL CHEMISTRY C
Editorial:
AMER CHEMICAL SOC
Referencias:
Lugar: Washington; Año: 2013 vol. 117 p. 23090 - 23107
ISSN:
1932-7447
Resumen:
In this work we report the spectral
evolution of the surfaceenhanced Raman scattering (SERS) response of biotin
molecules that generate the assembly of Ag and Au nanospheres (NSs) into
clusters with controlled interparticle gaps during the
aggregation process. The experiments are analyzed using rigorous near-field electrodynamic calculations of the
enhancement produced in a close-packed cluster with a different number of NSs. Two kinetic mechanisms
during the NS aggregation were identified which give rise to a different cluster size distribution and as a consequence
to a different SERS response.
It is found that during NS aggregation there is an initial fast growth of the
SERS signals followed by a decay that reaches an almost constant value at relatively long times. These results are
qualitatively explained by the trends observed in the electrodynamics
calculations of the near-field enhancement as
a function of the cluster size, which highlights the critical role played by
radiation damping as the size of the cluster increases for small clusters
(dimers and trimers), in contrast with the behavior observed
for clusters with a relative greater number of particles where the decrease of
the near-field enhancement is
less significant. In addition,
a general procedure is presented that makes it possible to perform a suitable
comparison of the SERS response for tight-compact clusters, in the limit of
large clusters, as long as the wavelength range of the SERS response is almost
size independent. This procedure is tested for Ag and Au nanoaggregates
generated in solution formed by spheres of different size, at an almost constant interparticle
separation. It was found that even there are differences between the analytic enhancement factor and
the calculated enhancement predicted by the electrodynamic theory in the large cluster size limit, and the trends in
the SERS response of the biotin molecules are qualitatively explained by the
electromagnetic (EM) mechanism for both Ag and Au clusters. These discrepancies
could be indicative of a chemical contribution to the global SERS response, but
a rigorous assessment of this mechanism would require additional theoretical
calculations supported by experiments. Notwithstanding, the SERS enhancement
results obtained with noncovalent bonded dye molecules (Coumarin 440 and
Rhodamine B) that are randomly distributed in the clusters as Raman reporters,
using the same large clusters as substrates (linked by biotin molecules), are in
fairly good quantitative agreement with the electrodynamics model of the
average SERS enhancement.