The existence of sub-nanometer plasmonic hot-spots and their relevance in spectroscopy and microscopy applications remain elusive despite a few recent theoretical and experimental evidence supporting this possibility. In this Letter, we present new spectroscopic evidence suggesting that Angstrom-sized hot-spots exist on the surfaces of plasmon-excited nanostructures. Surface-enhanced Raman scattering (SERS) spectra of 4,4′-biphenyl dithiols placed in metallic junctions show simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes were found to vary widely between junction sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at the specific molecular sites. Furthermore, the model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks, further confirming that the sources of the hot-spots are located on the metallic surfaces. This result not only provides compelling evidence for the existence of Angstrom-sized hot-spots but also opens up the new possibilities for the vibrational and electronic control of single-molecule photochemistry and real-space visualization of molecular vibration modes.
Bibliographical noteFunding Information:
The authors thank Javier Aizpurua and Yao Zhang for the assistance in modeling of SERS spectra and stimulating discussion on sub-nm hot-spots. This work is supported by the BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea as Global Frontier Project, H-GUARD_2013M3A6B2078950, and by the NRF grant no. 2016R1A2B2011160 and NRF-2016M3A7B4909776, Nano R&D program.
This work is supported by the BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea as Global Frontier Project, H-GUARD-2013M3A6B2078950, and by the NRF grant no. 2016R1A2B2011160 and NRF-2016M3A7B4909776, Nano R&D program.
© 2017 American Chemical Society.
- Surface-enhanced Raman scattering
- vibrational spectroscopy