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.