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

Abstract

Thin films containing monometallic (Ag,Au) and bimetallic (Ag-Au) noble nanoparticles were dispersed in TiO2, using reactive magnetron sputtering and post-deposition thermal annealing. The influence of metal concentration and thermal annealing in the (micro)structural evolution of the films was studied, and its correlation with the Localized Surface Plasmon Resonance (LSPR) and Surface Enhanced Raman Spectroscopy (SERS) behaviours was evaluated. The Ag/TiO2 films presented columnar to granular microstructures, developing Ag clusters at the surface for higher annealing temperatures. In some cases, the films presented dendrite-type fractal geometry, which led to an almost flat broadband optical response. The Au/TiO2 system revealed denser microstructures, with Au nanoparticles dispersed in the matrix, whose size increased with annealing temperature. This microstructure led to the appearance of LSPR bands, although some Au segregation to the surface hindered this effect for higher concentrations. The structural results of the Ag-Au/TiO2 system suggested the formation of bimetallic Ag-Au nanoparticles, which presence was supported by the appearance of a single narrow LSPR band. In addition, the Raman spectra of Rhodamine-6G demonstrated the viability of these systems for SERS applications, with some indication that the Ag/TiO2 system might be preferential, contrasting to the notorious behaviour of the bimetallic system in terms of LSPR response.

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

“…These features are determined by the Au/Ag ratio in the bimetallic nanostructure, which leads to different optical properties. From the alloy formation of Au-Ag bimetallic nanoparticles, only one LSPR band results between the peaks of the constituting monometallic nanoparticles, while a mixed system originates two plasmonic bands, as reported in different works [3,22,23,27].…”

“…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].…”

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

“…The microstructure of Au-TiO2 thin film is also relatively dense, though more granular and thicker, while the Ag-TiO2 thin film reveals a more porous microstructure. This behavior is not only related to the higher content of plasmonic metal dispersed in the TiO2 matrix, but also due to the growth of more disordered microstructures associated to the presence of Ag instead of Au [57]. Using the preferential growth orientation of the noble metals (Au or Ag), the grain sizes were estimated for the thin films annealed from 300 to 700 ˚C.…”

“…Nevertheless, at 700 °C, where the crystallization of Ag presents a diffraction peak of high intensity, and the matrix also is crystallized in the rutile phase, the film has an almost constant absorption throughout almost the whole spectra. This optical response is a consequence of the diffusion of Ag towards the surface of the films, aggregating in large crystals or even fractal structures [57].…”

“…From 500 °C to 700 °C of annealing temperature, the surface roughness decreases, from 109.6 to 60.9 nm, as the matrix crystallizes in the rutile phase and crystallization of Ag increases in intensity. This lower surface roughness could also be due to the formation of large Ag crystals in the surface of the thin film, in contrast with the film's surface, as seen by Borges et al, due to the coalescence of silver[57]. In the AFM 3D…”