Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.
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Data presented in this work are available upon reasonable request from Y. Kim.
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This work was financially supported in part by the Grant-in-Aid for Scientific Research (KAKENHI) nos 15H02025 (Y.K.), 15H03569 (N.H.), 17H04796 (H.I.), 17H05470 (H.I.), 17K18766 (H.I.), 18K14153 (R.B.J.), 16K21623 (K.M.) and 17K14428 (T.I.), and Grant-in-Aid for JSPS Fellows no. 15J03915 (K.M.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. R.B.J. acknowledges the Special Postdoctoral (SPDR) program of RIKEN. Parts of the numerical calculations were performed with the aid of the HOKUSAI supercomputer system at RIKEN. We thank H. Ueba, K. Kimura and M. Balgos for helpful discussions, D. Miyajima and T. Aida for the vacuum purification and room temperature absorbance measurements and F.C. Catalan and R. Wong for carefully reading the manuscript.
The authors declare no competing interests.
Peer review information Nature Nanotechnology thanks Patrick (Z.) El-Khoury and the other, anonymous, reviewers for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 1 Comparison between STM-TERS spectra with micro-Raman spectra and DFT calculations.
The observed peaks in the STM-TERS correspond well to DFT calculated vibrational modes and the powder Raman spectrum taken at room temperature (see Table S1). STM-TERS spectrum was taken using 738 nm laser with 1 mW and 30 s exposure time at Vb = 1 V and It = 50 pA. Powder Raman spectrum was taken using a homebuilt Raman microscope with 532 nm excitation at 1 mW power and 30 s exposure time.
Figure shows the STM topography (a) and (b) of the CuNc molecule and the DFT calculated molecular orbitals (c)–(e). The STM topography at HOMO a, was taken at Vb = -1.85 V and It = 10 pA while LUMO image b, was taken at Vb = 0.8 V and It = 10 pA. The STM image of the HOMO level is identical in shape to the DFT calculated HOMO c, Meanwhile, the DFT calculated LUMO is degenerated into LUMOα and LUMOβ ((d) and e, respectively). Considering that the STM can image the degenerated LUMO, of the molecule simultaneously, the STM image the STM image correspond well to the spatial distribution of the density ρ = |LUMOα|2 + |LUMOβ|2 shown in f. Since the STM image of CuNc on NaCl/Ag(111) mimics the DFT calculated molecular orbitals of a free molecule, we can argue that the NaCl is effective in electronically decoupling the CuNc molecule from the Ag(111) surface.
a, STA of the molecule showing the different excitation wavelengths used as coloured lines according to the colours in (b) and (c), STA shown is the same as Fig. 2c of the main text. In (b) and (c), the labels correspond to the wavelength used for the particular spectrum. b, Wavelength dependence of the Raman spectrum with wavelength as x-axis. The spectra were clipped from 755 nm for the data taken using 720 - 739 nm pump and from 790 nm for the those taken using 742 nm and above to account for the long pass filter response. The small lines at the left part of each spectrum correspond to the position of the excitation laser per spectrum. c, Wavelength dependence of the Raman spectrum with wavenumber as x-axis. Raman data were taken using 1 V, 50 pA, 30 s exposure time, and 1 mW laser power. The spectra in (b) and (c) were shifted vertically for clarity.
Figure shows the intensity plots of individual vibrational modes taken at 1 s integration time, Vb = 1 V, It = 50 pA and 1 angstrom / pixel step size (40 pixels × 40 pixels). A complete Raman spectrum is taken per pixel simultaneously with the STM topography also shown here. Apart from one peak (1211 cm-1, see Supplementary Table 1), all experimental TERS peaks can be assigned unambiguously to calculated resonance Raman vibrational modes. A table comparing the DFT and experimental results is shown in Supplementary Table 1. Three image profiles can be seen and their appearance are due to the symmetry of the assigned vibrational modes as discussed in the main text.
Figure shows the calculated molecular orbital of the CuNc at the x-y (a–c) and x-z (d–f) planes. For a centrosymmetric molecule like CuNc, HOMO-LUMO transitions are only allowed when there is a change in the parity as invoked by the parity rule. As viewed from the x-y plane, the molecular orbital show odd parity for the HOMO a, and an even parity for the LUMO b, c. This implies that HOMO-LUMO transitions with excitation along the x-y plane are parity allowed according to the Laporte rule. On the other hand, as viewed from the x-z plane, the molecular orbitals shows an odd parity for both the HOMO (d) and the LUMO e, f, suggesting that, HOMO-LUMO transitions with excitation along the z-axis are parity forbidden according to the Laporte rule.
Figure shows the direction of the induced dipole moment vector represented by the blue arrow for different vibrational symmetries and tip positions. The tip positions are at the phenyl ring (a) and (c) and in between two phenyl rings b, as indicated by the red dot. The dipole moment vector was obtained using pρ(ωL± ων) = αρσEσ(ωL) where pρ, Eσ, ωL and ων are the dipole moment vector, incident electric field vector of the plasmonic nanocavity, frequency of the laser and the frequency of the vibrational mode respectively, and αρσ is the Raman tensor given by equation 1 in the main text. The cartesian coordinate axes x, y, and z are represented by the subscripts ρ and σ. The vector of the plasmonic nanocavity at three different tip positions (red dot) is represented by the grey arrow.
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Jaculbia, R.B., Imada, H., Miwa, K. et al. Single-molecule resonance Raman effect in a plasmonic nanocavity. Nat. Nanotechnol. 15, 105–110 (2020). https://doi.org/10.1038/s41565-019-0614-8
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