Simulation of surface plasmon resonance of Au–WO3−x and Ag–WO3−x nanocomposite films

Surface plasmon resonance (SPR) has been efficiently employed for chemical- and bio-sensing in numerous fields such as molecular biology, medicine, biotechnology, drug and food monitoring, and environmental monitoring. Nanocomposite thin films formed by noble metal nanoparticles embedded in a dielectric matrix show attractive SPR phenomenon due to collective excitations of conduction electrons in metal nanoparticles when photons are coupled to the metal particle–dielectric interface. In this paper, the SPR responses of Au–WO3−x and Ag–WO3−x nanocomposite films with either stoichiometric (x = 0) and non-stoichiometric (x > 0) structures were simulated using effective medium theory and Macleod's general characteristic matrices method for various metal percentages and film thicknesses in the Kretschmann configuration at the wavelength of 632.8 nm. Our simulation results predicted that the most suitable Au percentage and film thickness when using non-stoichiometric Au–WO3−x films for SPR gas sensing were 60–80 vol.% of Au with 30–50 nm thickness, while for stoichiometric Au–WO3−x films, Au percentage higher than 40% will lead to a narrow SPR dip for high resolution SPR gas sensing. For Ag–WO3−x nanocomposite films, higher metal percentage and thinner thickness are required to achieve similar sharpness of the SPR dips as that for Au–WO3−x composite films. Our results indicated that the Au–WO3−x and Ag–WO3−x nanocomposite thin films with 40–80% metal fractions and film thickness of 30–50 nm are applicable for optical gas sensing.