Optical properties of Al2O3 thin films doped with hollow monometallic and core/shell bimetallic gold, silver nanoparticles

The incorporation of Au, Ag and (Ag/Au) core/shell nanoparticles with thin layers of Al2O3 was frequently sought for the realization of hybrid materials with superior optical, photoelectric and photocatalytic performances. A crucial aspect for these applications is how the surface plasmon resonance (SPR) of metallic nanoparticles is modified after assembly with Al2O3. The finite element method is used to study the SPR and optical properties of hollow gold, silver, and Ag/Au core/shell alloy nanoparticles embedded in Al2O3 thin-film structures. The absorbance, transmittance and reflectance of this nano-plasmonic thin films were determined from the effective dielectric permittivity. The calculations show that the SPR-peak position lambda(max) of Au and Ag-HNPs (Hollow nanoparticles) are shifted towards the infrared band when the thickness of the shell decreases. The enhancement electric field at the SPR is very important for thin Au-shells and for thick Ag-shells ones. For the three investigated nanostructures Au-HNPs, Ag-HNPs and Ag/Au, the absorbance, transmission and reflectance spectra show that at the SPR: the absorbance is maximum, i.e., the medium is completely absorbent, the transmission is minimal (high opacity) and the reflectance spectrum is behaving as a bimodal Gaussian distribution showing a minimum value at lambda(max).