Raman spectroscopy of buried interfaces in molecular electronic devices

Carbon based molecular junctions consisting of 1-10 nm thick layers of aromatic molecules between conducting carbon surfaces have proven commercially viable for use in audio processing circuits, with approximately 3000 devices currently in consumer hands. Characterization of molecular structure in completed and operational molecular components is challenging, since the important interfaces are buried under conducting contacts, and the required sensitivity is beyond commercial Raman spectrometers. A custom, line focused, f/2 Raman spectrograph designed by Dr. Jun Zhao permitted acquisition of spectra of carbon/molecule interfaces both during fabrication and operation of molecular tunnel junctions.(1) Spectra in operating junctions revealed structural changes resulting from redox behavior of aromatic molecules under bias which underlie conductance changes possibly useful for memory devices.(2) Raman spectral monitoring during growth of nitroazobenzene layers up to 40 nm thick demonstrated weak electronic coupling between oligomer subunits and permitted proposal of a new, long range, activationless charge transport mechanism unique to molecular junctions.(3,4) These and other valuable scientific results were enabled by the sensitivity and low power density of the Zhao-designed spectrometer. 1. Ramsey, J. D.; Ranganathan, S.; Zhao, J.; McCreery, R. L., Performance Comparison of Conventional and Line-Focused Surface Raman Spectrometers, Appl. Spectrosc. 2001, 55, 767-773 2. Nowak, A. M.; McCreery, R. L., In Situ Raman Spectroscopy of Bias-Induced Structural Changes in Nitroazobenzene Molecular Electronic Junctions, J. Am. Chem. Soc. 2004, 126, 16621-16631. 3. Supur, M.; McCreery, R. L., A Simple Mechanism for Activationless, Long Range Charge Transport in Molecular Junctions, ECS Journal of Solid State Science and Technology 2022, 11, 045009 4. McCreery, R. L., Carbon-Based Molecular Junctions for Practical Molecular Electronics, Acc. Chem. Res. 2022, 55, 2766-2779.