Optical properties of segmented Ag–Au wire at single particle level studied with a home-built micro-spectrometer

Abstract

Nanomaterials having hetero-metallic junctions are being explored for sensing, catalytic, and biomedical applications. Insight into the bimetallic junction at nanoscale is important from both fundamental and application perspectives. In this study, we synthesized segmented Au–Ag wire by sequentially electroplating Au and Ag in cylindrical pores in anodic alumina membrane filters. We probed the one photon, two photon, and second harmonic signal in Au–Ag wire at single particle level by focusing laser beam to a diffraction limited spot using a home-built sample scanning type micro-spectrometer. When exciting the Au–Ag junctions into the mid-UV range using two-photon excitation, we observed two luminescence peaks at 455 and 520 nm, respectively, possibly because of the surface plasmon resonances supported by the sharp boundary and granular nanostructures on the Au–Ag interface. Interestingly, we also observed intense second harmonic generation (SHG) signal from the junction with a yield more than two orders of magnitude higher than that from the Au tips. The SHG intensity as a function of excitation wavelength showed a similar trend to the two-photon excited luminescence emission spectrum, indicating that the SHG signal was enhanced in the presence of optical resonances. The observation of enhanced radiative properties of the bimetallic junction in the suggested that the Au–Ag wire may serve as an excellent imaging probe or single particle sensors.