Charge-transfer dipole low-frequency vibronic excitation at single-molecular scale.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 39441937.
- Also identified by DOI 10.1126/sciadv.ado3470 and PMC identifier 11639175.
- Licence recorded as CC BY-NC.
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Abstract
Scanning tunneling microscopy (STM) vibronic spectroscopy, which has provided submolecular insights into electron-vibration (vibronic) coupling, faces challenges when probing the pivotal low-frequency vibronic excitations. Because of eigenstate broadening on solid substrates, resolving low-frequency vibronic states demands strong decoupling. This work designs a type II band alignment in STM junction to achieve effective charge-transfer state decoupling. This strategy enables the successful identification of the lowest-frequency H<sub>g</sub>(ω<sub>1</sub>) (Raman-active H<sub>g</sub> mode) vibronic excitation within single C<sub>60</sub> molecules, which, despite being notably pronounced in electron transport of C<sub>60</sub> single-molecule transistors, has remained hidden at submolecular level. Our results show that the observed H<sub>g</sub>(ω<sub>1</sub>) excitation is "anchored" to all molecules, irrespective of local geometry, challenging common understanding of structural definition of vibronic excitation governed by Franck-Condon principle. Density functional theory calculations reveal existence of molecule-substrate interfacial charge-transfer dipole, which, although overlooked previously, drives the dominant H<sub>g</sub>(ω<sub>1</sub>) excitation. This charge-transfer dipole is not specific but must be general at interfaces, influencing vibronic coupling in charge transport.