Kinetic Studies of the Photogeneration of Silver Nanoparticles

Stable, intensely Raman active silver nanopartides are photogenerated by visible light from silver ions in a thin polymer film within a diffraction-limited focal area. The emission is resolved both spectrally and temporally to demonstrate that the source of the signal is surface-enhanced Raman scattering (SERS) from multiple silver nanoparticles generated in the diffraction-limited spot. The time evolution of the SERS signal is sigmoidal in shape and well described by Avrami phase transformation kinetics. The rate constant for the Avrami transformation depends linearly on illumination intensity, consistent with single photon photoreduction of the silver percholorate starting material as the limiting step to form silver nanoparticles. The asymptotic kinetic growth SERS signal exhibits a linear dependence on illumination intensity. Avrami analysis of the kinetics indicates that transformation is constrained to two dimensions, consistent with the similar to 10 nm thin film nature of the sample. The technique presented provides a novel route to large-scale periodic molecular sensor arrays with long-term stability, diffraction-limited resolution (<1 mu m), and laser-based spatial/temporal control of the formation kinetics.