Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals

This paper reports two fundamental observations on the size-dependent optic al properties of colloidal gold nanoparticles. First, faceted gold nanocrys tals in the size range of 63 +/- 3 nm have been found to be highly efficien t for surface-enhanced Raman scattering (SERS). These nanocrystals are iden tified from a heterogeneous population for large optical enhancement at 647 -nm laser excitation. Second, spatially isolated single gold particles emit Stokes-shifted Raman photons in an intermittent on-and-off fashion. In con trast to population-averaged studies, blinking surface-enhanced Raman scatt ering is a signature of single-particle (or even single-molecule) behavior. By directly measuring optical enhancement and time-resolved emission on si ngle nanoparticles, this work opens new possibilities in studying the mecha nisms of SERS, in developing metal tips for near-field optical microscopy, and in designing new nanostructured materials.