Introduction
© 2006 National Academy of Sciences USA
The electromagnetic mechanism is one of two explanations describing the surface-enhanced Raman scattering (SERS) effect — amplification of the Raman signal arising from a local field generated by a metal surface. A challenge in characterizing this mechanism, however, is making SERS-active nanoparticles with well-defined chemical composition and controllable interparticle distances.
Now, researchers at Northwestern University in the US have used on-wire lithography to produce arrays of variously spaced gold nanodisks to investigate this phenomenon. The groups of George Schatz and Chad Mirkin made nanowires consisting of gold and nickel segments1. By carefully controlling the length of each segment, selective removal of the nickel portions resulted in gold nanodisk arrays with desired spacings. The SERS effects of 46 different gold nanodisk arrays were measured, allowing for comparisons of gap number, disk thickness and gap size to be made. Although thickness and gap were important, the largest enhancement did not occur for the smallest gaps, contrary to earlier reports.
Although there is no consensus on the explanation of SERS, this work provides new insights on the electromagnetic mechanism. Furthermore, Mirkin and Schatz speculate that the gold nanodisks will be useful in spectroscopic and biological/chemical sensing applications.
