Designing, fabricating, and imaging Raman hot spots

Abstract

We have developed a probe of the electromagnetic mechanism of surface-enhanced Raman scattering via Au nanodisk arrays generated by using on-wire lithography. In this approach, disk thickness and interparticle gap are precisely controlled from 5 nm to many micrometers. Confocal Raman microscopy demonstrates that disk thickness and gap play a crucial role in determining surfaceenhanced Raman scattering intensities. Theoretical calculations also demonstrate that these results are consistent with the electromagnetic mechanism, including the surprising result that the largest enhancement does not occur for the smallest gaps.electromagnetic mechanism ͉ nanofabrication ͉ surface-enhanced Raman scattering ͉ templated synthesis ͉ discrete dipole approximation O n-wire lithography (OWL) allows one to fabricate unique one-dimensional structures that cannot be prepared via any other lithographic method (1-3). In particular, it allows one to make nanodisk arrays coated on one side with a thin silica sheath where the disk composition, thickness, and separation along the long axis can be controlled with nanometer precision. This ability enables the exploration of a variety of chemical and physical phenomena, including plasmon coupling and electromagnetic field enhancement in a very unique manner. Electromagnetic field enhancement is, in part, the basis behind surface-enhanced Raman scattering (SERS), a spectroscopic phenomenon discovered over 30 years ago but still perplexing to the scientific community both in terms of its potential and scientific origins (4-27). Two mechanisms are often mentioned in the literature, (6) the electromagnetic mechanism and the chemical mechanism. The latter involves charge transfer excitation (7, 9) between analyte molecules and the metal particles, whereas the former is dominated by plasmon excitation leading to hot spots around nano-sized metal particles (10,(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). A challenge in characterizing the electromagnetic mechanism is the difficulty associated with making nanoparticle structures with controllable interparticle gaps and doing so in a way that does not significantly change chemical composition. Because of this problem, it would be particularly useful to be able to make a series of nearly identical nanostructures with controllable gap size that can be probed simultaneously under one set of conditions in a SERS experiment. By designing the experiment in this manner, differences in chemical enhancement are minimized, and one can focus on the relationship between nanostructure and electromagnetic enhancement. Here, we show how OWL can be used to systematically prepare rows of Au disks with precisely controlled thicknesses and gaps, enabling a combinatorial format for identifying structures that provide maximum SERS enhancement.Nanodisk arrays fabricated by OWL are particularly useful for preparing SERS-active nanostructures for the following reasons. (i) Multiple features can be synthesized within a single nanowire, and, once functionalized wi...

Using theory and computation to model nanoscale properties

“…6 to quantitatively explain SERS and other measurements is still uncertain. However, the qualitative predictions have been verified in a recent study of methylene blue adsorbed onto pairs of gold disks, where the effects of disk spacing and thickness were examined, and good correspondence between theory and experiment was found (79).…”

Surface enhanced Raman spectroscopy on a flat graphene surface

“…Current efforts towards more reproducible SERS analyses are mainly focused on the nanotailoring or nanoassembly of substrates with uniform metal nanostructure arrays (15)(16)(17)(18)(19). However, a "uniform" SERS substrate is still not uniform on the nanoscale, the distribution of molecules and their states may be quite uncontrollable in the vicinity of the electromagnetic hot spots.…”

Development of Nanostructured Plasmonic Substrates for Enhanced Optical Biosensing

“…Obviously, the amplitude of plasmon waves decays rapidly when one moves further away from the nanostructure surface. Since locally enhanced plasmons called "hot spots" are distributed at very short distances from the surface [45,46], only a limited number of biomolecules within the local fields participate in the resonance shift of an LSPR biosensor, even if a large number of molecules are involved in the interaction.…”