Nanogap structures: combining enhanced Raman spectroscopy and electronic transport

Abstract

Surface-enhanced Raman spectroscopy (SERS) is an experimental tool for accessing vibrational and chemical information, down to the single molecule level. SERS typically relies on plasmon excitations in metal nanostructures to concentrate the incident radiation and to provide an enhanced photon density of states to couple emitted radiation to the far field. Many common SERS platforms involve metal nanoparticles to generate the required electromagnetic enhancements. Here we concentrate on an alternative approach, in which the relevant plasmon excitations are supported at a truly nanoscale gap between extended electrodes, rather than discrete subwavelength nanoparticles. The ability to fabricate precise gaps on demand, and in some cases to tune the gap size in situ, combined with the additional capability of simultaneous electronic transport measurements of the nanogap, provides access to information not previously available in standard SERS. We summarize the rich plasmonic physics at work in these extended systems and highlight the recent state of the art including tip-enhanced Raman spectroscopy (TERS) and the application of mechanical break junctions and electromigrated junctions. We describe in detail how we have performed in situ gap-enhanced Raman measurements of molecular-scale junctions while simultaneously subjecting these structures to electronic transport. These extended electrode structures allow us to study the pumping of vibrational modes by the flow of tunneling electrons, as well as the shifting of vibrational energies due to the applied bias. These experiments extend SERS into a tool for examining fundamental processes of dissipation, and provide insight into the mechanisms behind SERS spectral diffusion. We conclude with a brief discussion of future directions.

Voltage tuning of vibrational mode energies in single-molecule junctions

“…The vibrational modes curve slightly toward lower energies at larger jVj, and the anti-Stokes intensities increase at high biases. The latter effect indicates current-driven heating of vibrational degrees of freedom, as reported previously (22,23). The spectral shifts are more difficult to resolve in the anti-Stokes case because of this evolution of anti-Stokes intensity.…”

“…Plasmonic junctions between extended electrodes (19)(20)(21)(22)(23)(24)(25) show correlations of Raman response and conductance implying single-or few-molecule sensitivity, and enable studies of vibrational physics as a function of electrical bias. Spectral diffusion often is observed in singlemolecule SERS experiments (18,25,26), and there is some preliminary evidence of bias-driven mode shifts in such junctions (23), with the mechanisms of these phenomena remaining unclear.…”

Structural, optical and electrostatic properties of single and few-layers MoS 2 : effect of substrate

“…Furthermore, we also observe an increased Raman signal at the channel edge in the bare substrate not associated with the flake, which could be an indication of SERS-like enhancement, as many nanoscale-textured metallic surfaces have been found capable of producing strong electromagnetic fields that give rise to SERS enhancements. The enhancement could be caused by the polarization of the incident laser light possibly resonating with the narrow Au channel to give an enhancement of the absorption and scattering processes cross sections [47,48].…”

Modeling and optimization of Au-GaAs plasmonic nanoslit array structures for enhanced near-infrared photodetector applications

“…3(b) shows that as electrode spacing decreases for a constant w, average optical enhancement increases nearly exponentially. This is due to plasmonic gap hotspots 54 that increase in field intensity with decreasing s. At each value of s, the peak average enhancement was found at approximately w ¼ 160 nm for a constant thickness of t Au ¼ 15 nm. At the optimal width and electrode spacing, the enhancement was nearly 25 times greater than for geometries with electrode spacing larger than 15 nm and electrodes larger than 500 nm.…”