Simultaneous measurements of electronic conduction and Raman response in molecular junctions

Abstract: Electronic conduction through single molecules is affected by the molecular electronic structure as well as by other information that is extremely difficult to assess, such as bonding geometry and chemical environment. The lack of an independent diagnostic technique has long hampered single-molecule conductance studies. We report simultaneous measurement of the conductance and the Raman spectra of nanoscale junctions used for single-molecule electronic experiments. Blinking and spectral diffusion in the Raman response of both para-mercaptoaniline and a fluorinated oligophenylyne ethynylene correlate in time with changes in the electronic conductance. Finite difference time domain calculations confirm that these correlations do not result from the conductance modifying the Raman enhancement. Therefore, these observations strongly imply that multimodal sensing of individual molecules is possible in these mass-producible nanostructures.

• The paper demonstrates concurrent conductance and Raman measurements, providing insights into the orientation and environment-dependent behavior of single molecules.
• The authors observe that about 11% of the junctions show correlated fluctuations, linking Raman spectral changes directly to electronic conductance variations.
• FDTD simulations confirm that the combined signals originate from the same molecular source, supporting potential advances in single-molecule sensing.

Simultaneous Measurements of Electronic Conduction and Raman Response in Molecular Junctions

The study presented examines the interconnectedness of molecular electronic conduction and the Raman spectral response within molecular junctions. Specifically, it tackles the complexities in measuring electronic transport through single molecules, which are affected by often elusive factors like bonding geometry and surrounding chemical environments. Traditional techniques have not adequately resolved these issues due to the lack of combined diagnostic capabilities.

The authors report simultaneous measurements of conductance and Raman spectra within nanoscale junctions, particularly focusing on molecules such as para-mercaptoaniline (pMA) and fluorinated oligophenylene ethynylene (FOPE). A noteworthy observation is the correlation between fluctuations in the electronic conductance and Raman response, highlighting the potential for multimodal sensing in nanostructures that are mass-producible. The use of nanoscale gaps between extended electrodes as enhanced SERS (Surface-Enhanced Raman Spectroscopy) hotspots provides new modalities in evaluating and understanding molecular junctions.

Several key findings can be drawn from this study:

1. Simultaneous Measurements: By leveraging SERS, it is feasible to perform simultaneous Raman and conductance measurements. This approach provides detailed insight into the orientation and environment-dependent behavior of molecules during electron transport.
2. Temporal Correlations: Approximately 11% of the measured junctions showed strong correlations between conductance variations and Raman spectral changes. These changes manifest as both intensity modifications across specific Raman modes ("blinking") and spectral diffusion.
3. Finite Difference Time Domain (FDTD) Calculations: FDTD simulations confirmed that the observed correlations between conductance and Raman response are unlikely to be caused by conductance-induced modifications to the Raman enhancements. This points to a direct link where both Raman and conductance signals originate from the same molecular sources.
4. Potential for Single-Molecule Sensing: The results support the hypothesis that such measurement techniques could facilitate single-molecule sensing, given the substantial Raman enhancements achievable. Moreover, they open avenues for examining vibrational pumping and local temperature variations in single-molecule electronic transport.

The implications of this research are significant, as they lay groundwork for future technological applications in molecular electronics. The potential for non-invasively assessing molecular orientation, bonding, and conformation in real-time is particularly valuable for developing nanoscale electronic devices. Additionally, this methodology could address fundamental questions concerning SERS, such as the mechanisms of chemical enhancement and spectral diffusion.

Looking ahead, enhancements in SERS-based sensing methods, perhaps incorporating techniques like single-molecule transistors, could provide more profound insights into molecular dynamics and characteristics at the nanoscale. This could also prompt further exploration into the crossover effects of electrical and optical properties in molecular junctions.

This study represents a methodological advancement in detecting and analyzing single-molecule phenomena, providing a comprehensive platform for deeper theoretical and practical investigations in the field of molecular electronics.