Surface-Enhanced Light Emission from Single Hot Spots in Tollens Reaction Silver Nanoparticle Films: Linear versus Nonlinear Optical Excitation

The optical properties of fractal silver films grown using the Tollens silver mirror reaction enable highly reproducible single molecule surface-enhanced Raman scattering (SERS). These characteristics are a result of the nanostructure morphology which supports strongly localized surface plasmon polariton excitations that dramatically enhance the incident electromagnetic field in nanoscale regions called hot spots. Besides SERS, the local field amplification enhances an array of other linear and nonlinear optical effects such as silver luminescence and second-harmonic and continuum generation. By varying film growth conditions, we establish that the intrinsic linear and nonlinear responses qualitatively correlate with film morphology in the same manner as reported for SERS. We probe the polarization anisotropy, polarization memory, power dependence, emission spectra, excitation wavelength and temperature dependence, blinking, spectral diffusion, and emission decay dynamics of both hot spot species. Striking similarities and important differences exist between linear and nonlinear excitation: for example, the anisotropy distributions in excitation reveal a common surface enhancement process, whereas contrasting photodynamics clearly differentiates the two emission processes. Nonlinear hot spots do not blink, whereas linear hot spots exhibit strong blinking. We propose that the results can be understood by considering structure formation beyond the dimensions resolvable with scanning electron microscopy: the larger particles give rise to the surface enhancement phenomena, and smaller particles and clusters lead to the diverse emission properties. The existence of such silver clusters in a hot spot immediately impacts SERS since it effectively blocks all other surface amplification processes, such as SERS of an analyte molecule, leading to spatial anticorrelation between light generation and SERS. Applications of the intrinsic optical response for SERS as well as for high-resolution transmission microscopy are discussed.