Broadband optical absorption caused by the plasmonic response of coalesced au nanoparticles embedded in a TiO₂ matrix

Abstract

The effect of Au nanoparticles’ (NPs) concentration, size, and spatial distribution within a TiO₂ dielectric matrix on the localized surface plasmon resonance (LSPR) band characteristics was experimentally and theoretically studied. The results of the analysis of the Au NPs’ size distributions allowed us to conclude that isolated NPs grow only up to 5 to 6 nm in size, even for the highest annealing temperature used. However, for higher volume fractions of Au, the coalescence of closely located NPs yields elongated clusters that are much larger in size and cause a considerable broadening of the LSPR band. This effect was confirmed by Monte Carlo modeling results. Coupled dipole equations were solved to find the electromagnetic modes of a supercell, where isolated and coalesced NPs were distributed, from which an effective dielectric function of the nanocomposite material was calculated and used to evaluate the optical transmittance and reflectance spectra. The modeling results suggested that the observed LSPR band broadening is due to a wider spectral distribution of plasmonic modes, caused by the presence of coalesced NPs (in addition to the usual damping effect). This is particularly important for detection applications via surface-enhanced Raman spectroscopy (SERS), where it is desirable to have a spectrally broad LSPR band in order to favor the fulfilment of the conditions of resonance matching, to electronic transitions in detected species.

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ORCID for T. Polcar: orcid.org/0000-0002-0863-6287

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