We developed a procedure for the fabrication of sub 1 nm gap Au electrodes via electromigration. Self-aligned nanogap formation was achieved by applying a bias voltage, which causes electromigration during metal evaporation. We also demonstrated the application of this method for the formation of nanogaps as small as 1 nm in width, and we found that the gap size can be controlled by changing the magnitude of the applied voltage. On the basis of the electric conductance and surface-enhanced Raman scattering (SERS) measurements, the fabricated gap size was estimated to be nearly equal to the molecular length of 1,4-benzenedithiol (BDT). Compared with existing electromigration methods, the new method provides two advantages: the process currents are clearly suppressed and parallel or large area production is possible. This simple method for the fabrication of a sub 1 nm gap electrode is useful for single-molecule-sized electronics and opens the door to future research on integrated sub 1 nm sized nanogap devices.
We have investigated the conductance and atomic structure of single ethylene and acetylene molecule junctions on the basis of the conductance measurement and vibration spectroscopy of the single molecule junction. Single molecule junctions have a conductance comparable to that of metal atomic junctions (around 0.9G 0 : G 0 = 2e 2 /h) due to effective hybridization between metal and the π molecular orbital. The ethylene molecules are bound to Pt electrodes via a di-σ bond, while the acetylene molecules are bound to Pt electrodes via di-σ and π bonds. By using the highly conductive single molecule junctions, we investigated the characteristics of vibration spectroscopy of the single molecule junction in an intermediate regime between tunneling and contact. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction.
Surface enhanced Raman scattering (SERS) is a highly sensitive vibrational spectroscopy that allows for structural determination of analytes with utmost resolution. SERS remarkable sensitivity and non-destructive nature has been quickly embraced by the scientific community having a great impact in a number of fields such as: analytical chemistry; surface science; biochemistry and medicine among others. Here we present a contemporary review of SERS with a focus on studies conducted in metallic nano-sized cavities: junctions within nanoparticle assemblies, nano-engineered substrates, sphere-plane structures, electrodes, and molecular junctions. A brief introduction to the field and the underlying mechanisms responsible for the enhanced Raman scattering is provided. The most recent developments in the use of SERSactive nanogaps for single-molecule SERS studies are discussed in detail. Finally, the future prospects of the field are briefly discussed.
The characterization of a single molecular junction is essential to investigate and utilize the single molecular junction in single molecular devices. Vibration spectroscopy is a promising technique for characterizing the atomic structure of the single molecular junction. In this review paper, we describe the surface-enhanced Raman scattering (SERS) as a vibration spectroscopy of a single molecular junction. A strong electric field is formed in the nanogaps in the single molecular junction, which enhances the intensity of the Raman signal. The Raman signal from a single molecule in the nanogap is selectively observed thanks to the strong electric field. Simultaneous SERS and conductance measurements provide information of the geometric structure of the single molecular junction, which can clarify the single molecular dynamics.
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