title: Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering.
abstract: Unambiguous chemical identification of individual molecules closely packed on a surface can offer the possibility to address single chemical species and monitor their behaviour at the individual level. Such a degree of spatial resolution can in principle be achieved by detecting their vibrational fingerprints using tip-enhanced Raman scattering (TERS). The chemical specificity of TERS can be combined with the high spatial resolution of scanning probe microscopy techniques, an approach that has stimulated extensive research in the field. Recently, the development of nonlinear TERS in a scanning tunnelling microscope has pushed the spatial resolution down to ∼0.5 nm, allowing the identification of the vibrational fingerprints of isolated molecules on Raman-silent metal surfaces. Although the nonlinear TERS component is likely to help sharpen the optical contrast of the acquired image, the TERS signal still contains a considerable contribution from the linear term, which is spatially less confined. Therefore, in the presence of different adjacent molecules, a mixing of Raman signals may result. Here, we show that using a nonlinear scanning tunnelling microscope-controlled TERS set-up, two different adjacent molecules that are within van der Waals contact and of very similar chemical structure (a metal-centred porphyrin and a free-base porphyrin) on a silver surface can be distinguished in real space. In addition, with the help of density functional theory simulations, we are also able to determine their adsorption configurations and orientations on step edges and terraces.
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Song
authors.identifier: http://orcid.org/0000-0002-2223-8517
authors.initials: S
authors.lastname: Jiang
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Yao
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Rui
authors.initials: R
authors.lastname: Zhang
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Chunrui
authors.initials: C
authors.lastname: Hu
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Menghan
authors.initials: M
authors.lastname: Liao
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Luo
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Jinlong
authors.initials: J
authors.lastname: Yang
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: Zhenchao
authors.initials: Z
authors.lastname: Dong
authors.affiliation: Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information &Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
authors.forename: J G
authors.initials: JG
authors.lastname: Hou