Single molecule dynamics at a mechanically controllable break junction in solution at room temperature.

Area of Science:

• Molecular Electronics
• Surface Science
• Spectroscopy

Background:

• In situ observation of single molecule junctions is crucial for molecular electronics.
• Sensitivity limits hinder direct observation of geometrical and electronic dynamics.
• Mechanically controllable break junction (MCBJ) electrodes offer a platform for such studies.

Purpose of the Study:

• To investigate the geometrical and electronic structural dynamics of a single 4,4'-bipyridine molecule junction.
• To utilize surface-enhanced Raman scattering (SERS) for in situ observation under current flow.
• To correlate Raman spectral changes with molecular junction behavior.

Main Methods:

• Fabrication of nano-fabricated, mechanically controllable break junction (MCBJ) electrodes.
• In situ current flow measurements through single 4,4'-bipyridine molecule junctions in solution.
• Surface-enhanced Raman scattering (SERS) spectroscopy to monitor molecular vibrations.

Main Results:

• Nontotally symmetric b(1) and b(2) modes of 4,4'-bipyridine were significantly enhanced in single molecule junctions compared to bulk samples.
• Changes in SERS intensity, energy, and selectivity correlated with current fluctuations, indicating distinct electronic and geometric states.
• Observed molecular motion between vertical and tilting configurations via b(1) and b(2) mode switching.
• Increased Raman mode energies and decreased conductance indicated a slight increase in the molecule's tilting angle.

Conclusions:

• SERS is a powerful tool for probing the dynamics of single molecule junctions.
• Real-time observation of molecular structural changes under current flow is achievable.
• This study provides insights into the behavior of molecular junctions, crucial for advancing molecular electronics.