Deposition of nanocomposite coatings based on Al2O3 and gold nanoparticles exhibiting surface plasmon resonance

Nanoparticles exhibit unique properties due to their small size, which are absent in bulk materials. These properties can be employed for innovative applications, making them highly promising. Nanocomposite coatings containing metal nanoparticles, particularly their ability to support Localized Surface Plasmon Resonances (LSPRs), have garnered significant attention from both researchers and industries. The precise control over the morphology and distribution of nanoparticles in the matrix material becomes crucial for tailoring the LSPR signal, making it imperative for successful applications. In this Thesis, alumina and gold were chosen as a dielectric and metal nanoparticle.
The growth of Al2O3 films in an oxidizing atmosphere was investigated using reactive magnetron sputtering. Challenges such as process instability and the formation of insulating layers on the target leading to arcing events were addressed by implementing an active feedback reactive sputtering control for the deposition of Al2O3 thin films. These films were deposited using two sputtering sources: high-power impulse magnetron sputtering (HiPIMS), mid-frequency (MF) and a combination of both (MF + HiPIMS) at varying powers and deposition temperatures. X-ray diffraction (XRD) analysis indicated the formation of polycrystalline y-Al2O3 films, except for those deposited at low temperature, which were amorphous. Additionally, energetic depositions (higher power, HiPIMS) yielded films with higher grain density and refractive index. However, HiPIMS deposition resulted in higher compressive stress due to increased ion energy and lower deposition rate.
Prior to Au deposition, an optimization process was conducted for alumina films. It was observed that alumina films produced at room temperature exhibited decreased roughness, lower likelihood of delamination and ensured no thermal impact on the subsequent Au deposition.
The alumina surface underwent a plasma pre-treatment involving argon and hydrogen to evaluate its impact on the following growth of Au nanoparticles. The findings revealed that such treatment had advantageous effects on depositing Au nanoparticles; thus, this treatment led to a decrease in the formation of a continuous film (percolation) over time and higher levels of absorbance compared to untreated surfaces. This effect was analysed using AFM and XPS techniques. Both techniques revealed that treated surfaces exhibited greater height/roughness and lower coverages/correlation length, which retarded the percolation effect and enhanced the growth of nanoparticles when comparing with untreated surfaces. The effect of Ar/H2 plasma treatment was studied using continuous and pulsed DC sources for deposition of Au. It was observed that the use of both sources yielded the same optical behaviour for plasmonic properties, indicating that the effect is independent of the source but dependent on the surface treatment.
Finally, it is worth mentioning that samples prepared with Ar/H2 plasma proved effective for Surface-Enhanced Raman Scattering (SERS) applications for detecting low concentrations of Rhodamine-6G, in contrast to the much lower effect observed for the Au grown on untreated alumina surfaces.