Nanosecond laser ablation of Ag-Au films in water for fabrication of nanostructures with tunable optical properties

In this study, we present results on nanosecond laser ablation of Ag-Au bimetallic thin films in double-distilled water. A standard on-axis pulsed laser deposition technology is used for the thin-film deposition. The targets used in the deposition stage were composed of two sections, each containing Au or Ag. Varying the area of the sections of the target, thin bimetallic films with different ratio of Ag:Au (72:28, 56:44, 50:50, and 34:66) were obtained. As-deposited thin films were immersed in double-distilled water and undergo irradiation with nanosecond laser pulses. The optical absorption spectra of the obtained bimetallic colloids show a single-surface plasmon resonance (SPR) band, positioned between SPR bands of the monometallic Ag and Au nanoparticles (NPs), proving the formation of Ag-Au alloyed NPs. A linear dependence of the SPR maximum of the resulting colloidal NPs on the metal concentration of the deposited thin films was established. The influence of the laser wavelength (the four harmonics of Nd:YAG laser), laser fluence and film thickness on the morphology, particle-size distribution, and optical properties of the obtained colloidal Ag-Au NPs was also studied. TEM analyses of the dried drops of the colloids reveal mainly two types of shapes of the produced nanostructures depending on the processing conditions: spherical NPs and network of nanowires. A characteristics size of the formed nanostructures of about a few nanometers was estimated in the cases of ablation at all the wavelengths used. However, a broader size distribution and presence of larger amount of bigger particles (diameters up to 50 nm) were observed in the case of ablation at 1064 nm. The laser fluence and film thickness are also found to influence the nanostructure size and morphology. The proposed technique can be a basis for the fabrication of complex nanostructure colloids with application in biophotonics, sensor design, and catalysis.