Local surface potential characterization of nanostructures: Understanding interplay among charges, photons, and surface plasmons

Abstract

We fabricated ZnO/Ag nanogratings and P3HT-coated Si nanopillar (NP) arrays using nanoimprint and electron-beam lithography techniques, respectively. Experimental and calculated results revealed intriguing optical characteristics of these nanostructures. The grating structure could couple surface plasmon polaritons (SPPs) with photons, giving rise to strong light confinement at the ZnO/Ag interface. Mie-resonance strongly concentrated incoming light in the NPs. Such light confinement should affect the carrier generation behaviors as well as the optical response of the nanostructures. Spatial mapping of surface photovoltage (SPV), i.e., changes in the surface potential under illumination, using Kelvin probe force microscopy (KPFM) enabled us to visualize generation and subsequent redistribution of photogenerated carriers under illumination. Larger SPV under the SPP excitation and Mie resonance demonstrated that the concentrated light produced numerous carriers in the nanostructures. The SPV relaxation time of the ZnO/Ag nanogratings depended on the wavelength and polarization of the incident light, indicating that the SPP affected the recombination processes. In the P3HT-coated Si NPs, the SPV map clearly showed that the optical resonance modified the spatial distribution of photogenerated carriers in the sample. All of these results suggested that SPV measurement using KPFM should be very useful for studying the interplay among charges, photons, and surface plasmons in metal/semiconductor hybrid nanostructures.