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 TiO 2 dielectric matrix on the Localized Surface Plasmon Resonance (LSPR) band characteristics, were experimentally and theoretically studied. The results of the analysis of the Au NPs' size distributions allowed to conclude that isolated NPs grow only up to 5-6 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, which 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 fulfillment of the conditions of resonant matching, to electronic transitions in detected species.

Development of biocompatible plasmonic thin films composed of noble metal nanoparticles embedded in a dielectric matrix to enhance Raman signals

“…Finally, when the annealing temperature reaches 700 ºC, the LSPR band became broadened, and the band minimum reached zero transmittance, extending from 550 nm of wavelength to 800 nm. This is in accordance with the coalescence of the gold nanoparticles into large spheroidal shaped clusters, as predicted in theoretical studies of this plasmonic system[64].…”

Thin films of Ag–Au nanoparticles dispersed in TiO2: influence of composition and microstructure on the LSPR and SERS responses

“…This behaviour was previously observed in similar systems and may be attributed to the crystallization and growth of large and well-separated Au nanoparticles inside the TiO2 matrix during the heattreatment [5]. In addition, the position of the LSPR peak is red-shifted about 30 nm (from nearly 625 nm to 655 nm) with the increase of the annealing temperature from 300 ºC to 600 ºC, probably due to an increase of the refractive index of host the matrix [4]. This is in agreement with the XRD results (Figure 4) that revealed a progressive crystallization of the TiO2 matrix into its anatase phase (which refractive index is higher than that of amorphous TiO2) thereby increasing the wavelength at which the resonance occurs [5].…”

“…To produce these nanocomposite films, Physical Vapor Deposition (PVD) techniques have been frequently used. Magnetron sputtering is an example, and its versatility allows to tailor the chemical and (micro)structural properties of these nanocomposite films towards the above mentioned applications [4]. Nevertheless, when using this method to prepare plasmonic thin films, a second step may be required, especially in the case of low preparation temperatures [5,33,34].…”

“…Beyond the work developed by F. Vaz et al [4,35,36], few studies addressed the synthesis of Ag/TiO2 [37,38] and Au/TiO2 [39,40] thin films by magnetron sputtering followed by a heat treatment, as an efficient and versatile way to control the morphology, particle growth and optical properties of these nanocomposites. Therefore, the application of this method to prepare bimetallic Ag-Au nanoparticles on a TiO2 matrix, seems to be a very promising way to achieve different Ag/Au atomic ratios in a dielectric matrix such as TiO2, which, in principle, will allow to tune the LSPR peak position between those of the corresponding monometallic counterparts.…”

Gas Sensing with Nanoplasmonic Thin Films Composed of Nanoparticles (Au, Ag) Dispersed in a CuO Matrix

“…Surface plasmons are coherent oscillations of free electrons excited by an electromagnetic field at the boundaries between a metal and a dielectric. They can propagate along the surface of the conductor, which are designated by surface plasmon polaritons, or be confined to metallic nanoparticles or nanostructures, in which case, are denominated as localized surface plasmons [3][4][5]. LSPR can give rise to strong absorption bands, the enhancement of the electromagnetic field near the nanoparticles, and the appearance of scattering to the far field [6][7][8][9][10].…”