Tuning the nanoscale morphology and optical properties of porous gold nanoparticles by surface passivation and annealing

We synthesized the arrays of porous gold nanoparticles attached to a fused silica and SiO2/Si substrates employing a combination of thin film solid state dewetting and dealloying methods. We demonstrated that the surface of porous gold nanoparticles can be passivated with a thin and conformal alumina layer produced by the plasma-enhanced atomic layer deposition method. This passivation results in significant improvement of the thermal stability of the nanoporous morphology. Whereas as-produced porous gold nanoparticles start to coarsen at the temperature as low as 160 °C, their passivated counterparts are thermally stable up to 800 °C. At higher temperatures, the solid gold nanoparticles are formed on the outer surfaces of the passivated porous gold nanoparticles, leaving behind partially emptied alumina shells. We correlated the optical absorbance spectra of the as-produced and annealed particles with their morphology and microstructure. A broad absorption peak in the infrared region was associated with thin gold ligaments of 10-15 nm in diameter, and coarsening of these ligaments in the unpassivated particles has led to the attenuation of the peak. The passivated particles demonstrated the stability of the infrared absorption intensity up to the formation of solid gold nanoparticles at 900 °C, and the red shift of the visible-range plasmon resonance peak above 600 °C. We conclude that surface passivation and heat treatments represent efficient tools for tuning surface plasmon resonance properties of porous gold nanoparticles.

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