Jody Yang

SERS and the Single Molecule

Surface Enhanced Raman spectroscopy (SERS) was discovered in 1978 and has grown to become a significant surface diagnostic and analytical technique. It has also launched a wide variety of investigations into the electromagnetic, and especially the optical, properties of nanostructured disordered materials. A number of phenomena contribute to SERS including adsorbate resonances as well as new resonances (such as metal to molecule charge transfer transitions) that result form the formation of the adsorbate-to-surface bonds, or other adsorbate-metal interactions. Chief among the contributions to SERS, however, is the enhancement of the optical fields in the vicinity of the nanoparticles constituting the SERS-active system. The field enhancement is especially high when highly localizable resonances such as surface plasmons are excited. Aggregates and assemblies of nanoparticles (of appropriate materials) can, in turn, manifest unusually enhanced SERS by virtue of particle-particle interactions. For example, while the SERS enhancement in the vicinity of single silver nanoparticles rarely exceed 104, the Raman spectrum of molecules located in the interstitial volume between two closely-spaced nanoparticles can be enhanced some 10 orders of magnitude when the two particles approach each another to within molecular dimensions and the system is excited at an appropriate wavelength. Other aggregates can Show similar levels of enhancement at special locations within the aggregate. Large fractal aggregates form a special class of enhancing aggregates. Illuminating such aggregates, in general, results in a highly inhomogeneous distribution of enhancement over the body of the aggregate with electromagnetic hot Spots where the Raman enhancement can reach or slightly exceed 10 orders of magnitude. Moreover, such hot Spots can be excited with a broad range of wavelengths (although the pattern of hot spots is critically wavelength dependent). Recently, reports have been published suggesting SERS enhancements upwards of 1014, sufficient for single molecule SERS detection. Although the cause of such huge enhancements was at first mysterious, we suggest that these observations result from the aforementioned electromagnetic effects in aggregates combined with either intramolecular or metal-to-molecule (or molecule-tometal) resonances. We also Show that the purported optical pumping of vibrationally excited states by such intense SERS transitions is spurious.

Developing 1D nanostructure arrays for future nanophotonics

There is intense and growing interest in one-dimensional (1-D) nanostructures from the perspective of their synthesis and unique properties, especially with respect to their excellent optical response and an ability to form heterostructures. This review discusses alternative approaches to preparation and organization of such structures, and their potential properties. In particular, molecular-scale printing is highlighted as a method for creating organized pre-cursor structure for locating nanowires, as well as vapor–liquid–solid (VLS) templated growth using nano-channel alumina (NCA), and deposition of 1-D structures with glancing angle deposition (GLAD). As regards novel optical properties, we discuss as an example, finite size photonic crystal cavity structures formed from such nanostructure arrays possessing highQ and small mode volume, and being ideal for developing future nanolasers.

Adsorbate Alignment in Surface Halogenation: Standing Up is Better than Lying Down**

Bromine atom transfer to a silicon surface as a function of physisorbed adsorbate alignment (see picture: left, vertical 1-bromopentane; right, horizontal 1-bromopentane) of 1-bromopropane and 1-bromopentane on Si(111)-7×7 has been studied by STM. In both thermal and electron-induced bromination reactions, the vertical alignment is more reactive.

SERS and the single molecule: near-field microscopy and spectroscopy

Recent results suggest that surface-enhanced Raman Spectroscopy (SERS) of single adsorbate molecules is possible under appropriate circumstances. We propose that this phenomenon is associated with very intense enhancements available at interstitial sites (hot spots) of nanoparticle assemblies (either colloid particle aggregates or rough surfaces) illuminated with light of an appropriate wavelength so as to excite surface plasmons, coupled with additional resonance enhancements due to a judicious choice of ad-molecule. The former contribution, known as electromagnetic (EM) enhancement, has been known for years to be capable of producing EM hot spots where the enhancement can top 1011. This fact seems to have been rediscovered recently. It is also known that the fields at the surface of fractal aggregates commonly show hot spots. These are also, at times, capable of such high local enhancements. On fractals, the location of these hot spots are, however, highly dependent on parameters such as the excitation wavelength. In contrast, small compact clusters (when properly designed) have the benefit of a wavelength-independent hot spot where a small number of molecules could be (chemically) directed and detected. This insight suggests an eventual optimally engineered single-molecule SERS system with predictable enhancement capabilities and optimal adsorption (i.e. chemical) characteristics at the hot spot.