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ACS Nano. 2018 Jul 24;12(7):6492-6503. doi: 10.1021/acsnano.7b08224. Epub 2018 Jun 28.

Electron Transport Across Plasmonic Molecular Nanogaps Interrogated with Surface-Enhanced Raman Scattering.

Lin L1, Zhang Q2,3, Li X1, Qiu M4, Jiang X1, Jin W4, Gu H1, Lei DY3,5, Ye J1,6,7.

Author information

1
State Key Laboratory of Oncogenes and Related Genes, School of Biomedical Engineering , Shanghai Jiao Tong University , Shanghai 200030 , China.
2
School of Materials Science and Engineering, Shenzhen Graduate School , Harbin Institute of Technology , Shenzhen 518055 , China.
3
Department of Applied Physics , The Hong Kong Polytechnic University , 999077 , Hong Kong, China.
4
Department of Electrical Engineering , The Hong Kong Polytechnic University , 999077 , Hong Kong, China.
5
Shenzhen Research Institute , The Hong Kong Polytechnic University , Shenzhen 518057 , China.
6
Shanghai Key Laboratory of Gynecologic Oncology, Ren Ji Hospital, School of Medicine , Shanghai Jiao Tong University , Shanghai 200030 , China.
7
Shanghai Med-X Engineering Research Center, School of Biomedical Engineering , Shanghai Jiao Tong University , Shanghai 200030 , China.

Abstract

Charge transport plays an important role in defining both far-field and near-field optical response of a plasmonic nanostructure with an ultrasmall built-in nanogap. As the gap size of a gold core-shell nanomatryoshka approaches the sub-nanometer length scale, charge transport may occur and strongly alter the near-field enhancement within the molecule-filled nanogap. In this work, we utilize ultrasensitive surface-enhanced Raman spectroscopy (SERS) to investigate the plasmonic near-field variation induced by the molecular junction conductance-assisted electron transport in gold nanomatryoshkas, termed gap-enhanced Raman tags (GERTs). The GERTs, with interior gaps from 0.7 to 2 nm, are prepared with a wet chemistry method. Our experimental and theoretical studies suggest that the electron transport through the molecular junction influences both far-field and near-field optical properties of the GERTs. In the far-field extinction response, the low-energy gap mode predicted by a classical electromagnetic model (CEM) is strongly quenched and hence unobservable in the experiment, which can be well explained by a quantum-corrected model (QCM). In the near-field SERS response, the optimal gap size for maximum Raman enhancement at the excitation wavelength of 785 nm (633 nm) is about 1.35 nm (1.8 nm). Similarly, these near-field results do not tally with the CEM calculations but agree well with the QCM results where the molecular junction conductance in the nanogap is fully considered. Our study may improve understanding of charge-transport phenomena in ultrasmall plasmonic molecular nanogaps and promote the further development of molecular electronics-based plasmonic nanodevices.

KEYWORDS:

charge transfer; electron transport; gap-enhanced Raman tags; molecule junction conductance; quantum plasmonics

PMID:
29924592
DOI:
10.1021/acsnano.7b08224

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