Surface-enhanced Raman scattering
We present an introduction to surface-enhanced Raman
scattering (SERS) which reviews the basic experimental
facts and the essential features of the mechanisms which
have been proposed to account for the observations. We then
review very recent fundamental developments which include:
SERS from single particles and single molecules;
SERS from fractal clusters and surfaces; and new insights
into the chemical enhancement mechanism of SERS.
1 Introduction
Surface-enhanced Raman scattering (SERS) was discovered
twenty years ago. The field developed aggressively, even
explosively, for the first decade or so and then settled down a bit
as it entered its teenage years. Interest shifted from fundamentals
to applications and a steady stream of papers was
published in diverse fields which included electrochemistry,
analytical chemistry, chemical physics, solid state physics,
biophysics and even medicine, for example. As SERS enters its
third decade there has been a renewed interest in fundamentals,
especially in the short-range or chemical mechanism, and single
molecule detection has been achieved. Few scientists 25 years
ago would have bet that Raman spectroscopy, notorious for its
difficulty and insensitivity at that time, would have joined the
ranks of single molecule spectroscopies before the end of the
century.
We present here a contemporary review of SERS with two
objectives in mind. First, we wish to provide an introduction to
the field for scientists (and for students, in particular) who may
wish to conduct research in the area or use SERS techniques in
their own research in other fields. Second, we wish to highlight
new areas of SERS research which we feel are particularly
exciting and show promise for further development. Our goal is
to provide the reader with sufficient background, orientation
and perspective to permit him or her to read the primary reviews
and the original literature more easily. In keeping with the spirit
of the Journal only the briefest historical development will be
presented and the reader is referred to other review articles,
rather than the original literature, for elaboration and further
discussion. We acknowledge here the important contributions
made by an enormous number of scientists from around the
world over the past twenty years and regret that the format of
this review does not permit us to recognize many individual
contributions.
The review is organized as follows. (We assume that the
reader is familiar with the elementary aspects of Raman
scattering; if not, the monograph by Long1 is recommended.) A
short history of the discovery of SERS is followed by a
summary of the key experimental facts. Two classes of SERS
mechanisms, electromagnetic and chemical, are introduced and
it is shown how these mechanisms account for the experimental
observations. Most of the features of the electromagnetic
mechanism can be understood by examining the electrostatics
of a polarizable metal sphere in a uniform external electric field;
that model, therefore, will be discussed in some detail. Evidence
that the electromagnetic mechanism is not the whole story will
be presented to motivate a discussion of the chemical mechanism.
Three areas of contemporary interest and activity are then
reviewed in some detail. Recent advances in microscopy have
made it possible to use single particles as SERS substrates and
to obtain the Raman spectra of single molecules adsorbed on
them; an introduction to this field and selected examples are
presented. The importance of interparticle interactions is
illustrated nowhere better than by fractal clusters. Recent
advances in theory and computational techniques have provided
a quantitative understanding of the localization of electromagnetic
energy and its effects on both linear and nonlinear
spectroscopies. Finally, very recent studies on systems which
show chemical enhancement without electromagnetic enhancement
have provided a new and more detailed level of
understanding about this mechanism. We conclude with
mention of two areas of application which we find particularly
promising and our assessment of the future of the field.
2 History and fundamentals
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