Abstract
Many important energy-transfer and optical processes, in both biological and artificial systems, depend crucially on excitonic coupling that spans several chromophores1,2,3,4,5,6,7,8,9. Such coupling can in principle be described in a straightforward manner by considering the coherent intermolecular dipole–dipole interactions involved10,11. However, in practice, it is challenging to directly observe in real space the coherent dipole coupling and the related exciton delocalizations, owing to the diffraction limit in conventional optics. Here we demonstrate that the highly localized excitations that are produced by electrons tunnelling from the tip of a scanning tunnelling microscope, in conjunction with imaging of the resultant luminescence, can be used to map the spatial distribution of the excitonic coupling in well-defined arrangements of a few zinc-phthalocyanine molecules. The luminescence patterns obtained for excitons in a dimer, which are recorded for different energy states and found to resemble σ and π molecular orbitals, reveal the local optical response of the system and the dependence of the local optical response on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. We generate an in-line arrangement up to four zinc-phthalocyanine molecules, with a larger total transition dipole, and show that this results in enhanced ‘single-molecule’ superradiance from the oligomer upon site-selective excitation. These findings demonstrate that our experimental approach provides detailed spatial information about coherent dipole–dipole coupling in molecular systems, which should enable a greater understanding and rational engineering of light-harvesting structures and quantum light sources.
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Acknowledgements
We thank B. Wang for discussions. This work is supported by the National Basic Research Program of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities and Hefei Science Center of the Chinese Academy of Sciences.
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Contributions
Z.-C.D. and J.G.H. supervised the project. Yang Z., Yang L., Yao Z., Y.-J.Y., Y.-M.K., L.Z. and Q.-S.M. performed the experiments and analysed the data. Yang Z., Yang L., Yao Z., Yi L., J.-L.Y., Z.-C.D. and J.G.H. contributed to the interpretation of the data and theoretical simulations. Yang Z., Yi L., Z.-C.D. and J.G.H. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Figure 1 Schematic of our experimental set-up for a combined system of low-temperature ultrahigh-vacuum STM with optical detections.
The highly localized tunnelling electrons in a STM are used to excite light emission from the STM junction through inelastic tunnelling. This technique is called STM-induced luminescence (STML). In our system, photons emitted from the STM junction were collected by a two-channel double-lens system to increase the collection efficiency. The total hemisphere photon collection efficiency for the two-channel double-lens system is about 20%. The spectra were measured with a liquid-nitrogen-cooled charge-coupled device spectrometer (Princeton Instruments). Different gratings (150 grooves per mm, 600 grooves per mm and 1200 grooves per mm) and slits (100 μm and 25 μm) were used in spectral measurements for different requirements on wavelength ranges and spectral resolutions. All spectra presented in this paper are not corrected for the wavelength-dependent sensitivity of photon detection systems.
Extended Data Figure 2 Excitation mechanism in STML.
a, STML spectra acquired above the lobe of a single ZnPc molecule on the NaCl island at different excitation biases, as indicated (10 pA, 60 s). b, dI/dV spectra acquired at the ZnPc lobe (red line) using the lock-in technique. The tunnelling gap was set at −2.5 V and 50 pA. The bias modulation was 20 mV (r.m.s.) at 329 Hz. The molecular emission intensities over a different excitation bias is also plotted as a blue line and filled circles (integrated spectral range: 1.86–1.94 eV). The inset shows an STM image of a ZnPc molecule (3 nm × 3 nm; −1.7 V, 2 pA) with the ‘×’ marking the position for both the dI/dV and STML measurements in a. c–e, Sketch of a double-barrier junction showing the energy-level alignments of the HOMO and LUMO at different bias voltages: Vb = 0 V (c), Vb = −2.2 V (d) and Vb = +4.7 V (e). 
, Fermi energy of the tip/substrate.
Extended Data Figure 3 Sub-nanometre spatial resolution of spectroscopic imaging.
a, STML spectrum (−2.5 V, 200 pA, 60 s) acquired at the lobe of an isolated ZnPc monomer on NaCl (top curve, red), indicated by the ‘×’ in the inset (−1.7 V, 2 pA; 4 nm × 4 nm). The NCP emission spectrum (bottom curve, black) acquired on the NaCl surface is shown as a reference. b, Energy-resolved photon imaging pattern of a single ZnPc molecule on NaCl by integrating the molecule-specific fluorescence signals over the energy range 1.89–1.91 eV, indicated by the yellow shading in a (4 nm × 4 nm, 30 × 28 pixels; −2.5 V, 200 pA, 5 s per pixel). c, STML intensity profile for the (purple dashed) line trace in b. The line-profile analysis has a spatial resolution of less than 1 nm (about 0.7 nm estimated within a 10%–90% contrast), showing the change of contrast from 0% to 100% over a distance as short as 0.9 nm.
Extended Data Figure 4 Spectral evolution with varied intermolecular distances.
a, STM images taken during the manipulation process (−1.7 V, 2 pA). There are three isolated ZnPc molecules adsorbed on NaCl marked as M1, M2 and M3. M3 is used as a reference molecule. M1 and M2 are pushed together, as indicated by the white arrows for manipulation directions. The intermolecular centre-to-centre distance r between M1 and M2 is marked. Scale bars, 2 nm. b, Corresponding electroluminescence spectra (−2.5 V, 200 pA, 60 s) collected from the lobes of the three ZnPc molecules, as indicated by the ‘×’s in a. c, Representative site-dependent STML spectra (−2.5 V, 200 pA, 60 s) for r = 2.0 nm acquired at the respective ‘×’ positions in the insets images (4.6 nm × 6 nm, −1.7 V, 2 pA). The shaded regions labelled 1–5 mark the possible peak positions induced by the coherent dipole–dipole interaction. d, Typical current–time and tip-position–time curves during the manipulation process. First, we placed the STM tip close to the edge of the lobe of a ZnPc molecule on NaCl. Second, we turned off the feedback and approached the tip to the molecule by about 200 pm (set point: +2.5 V, 2 pA). In this way, the ZnPc molecule was found to move away from the STM tip.
Extended Data Figure 5 Bias-dependent features of STM images and dI/dV spectra for isolated ZnPc monomers and ZnPc dimers on the NaCl surface.
a, STM images of an isolated ZnPc monomer acquired at different biases (4 nm × 4 nm; 2 pA). The cross-like feature is observed when the bias is set inside the HOMO–LUMO gap (for example, −1.7 V), revealing the characteristics of the molecular skeleton. The rotating behaviour of an isolated ZnPc monomer is evident at about −2.2 V, giving rise to a 16-lobe-like pattern instead of the 8-lobe-like pattern associated with the HOMO state for a immobilized ZnPc. b, STM images of an isolated ZnPc monomer and a ZnPc dimer at different biases (15 nm × 6 nm; 2 pA). The rotation of ZnPc is blocked when in contact with another ZnPc owing to the steric hindrance, as indicated by the appearance of an 8-lobe-like pattern of the component ZnPc monomer in the dimer. c, dI/dV spectra were acquired at the ZnPc lobes of M1, M2 and M3, as indicated in the inset (15 nm × 7 nm, −1.7 V, 2 pA). The dI/dV spectrum on the bare NaCl surface is also shown as a reference. The smoothed dI/dV curves are presented in colour; the raw data are represented as grey squares. The dI/dV signals were measured using the lock-in technique. The tunnelling gap was set at −2.5 V and 20 pA. The bias modulation was 20 mV (r.m.s.) at 329 Hz. The dI/dV spectra are offset for clarity. No noticeable difference in peak positions was observed between the dI/dV curves acquired from both the isolated ZnPc monomer and the ZnPc dimer. However, the STML spectral features from the dimer are markedly different to those of an isolated ZnPc monomer (Fig. 2).
Extended Data Figure 6 Origin of the emission peak at about 1.870 eV from the ZnPc dimer.
a, STML spectrum (−2.5 V, 200 pA, 60 s) acquired at a lobe of the isolated ZnPc molecule, as indicated in the inset (4 nm × 4 nm; −1.7 V, 2 pA), showing the Q-band emission containing a shoulder peak at 1.882 eV (about 659 nm). b, STML spectrum (−2.5 V, 200 pA, 60 s) acquired at the centre of the ZnPc dimer, as indicated in the inset (5 nm × 4 nm; −1.7 V, 2 pA), showing an additional peak at 1.870 eV (about 663 nm) labelled as mode 1′. c, Photon imaging pattern acquired by integrating the spectral range corresponding to the mode-1′ emission (−2.5 V, 200 pA, 5 s per pixel; frame size: 5 nm × 4 nm, 34 × 21 pixels; integration range: 1,870 ± 1.5 meV, shaded region in b).
Extended Data Figure 7 Theoretical simulation of real-space STML mapping.
a, Modelling the transition dipoles of the ZnPc monomer by pairs of opposite point charges. The top images show the distribution of two equivalent transition densities associated with the corresponding transition dipoles of a ZnPc molecule. Positive and negative transition densities are represented by the violet and cyan shading, respectively, which are superimposed on the ball-and-stick model of a ZnPc molecule (the H, C, N and Zn atoms are represented by the white, grey, blue and violet spheres, respectively). b, Arrangements of the orientations for the pair of point charges for selected examples of coupling modes (modes 1, 2 and 3) in our simulations. The distance between the two opposite charges in the schematic is exaggerated for clarity. The green dashed lines mark the centre-to-centre axial direction of the dimer. c, d, Schematic of the image-charge models when the tip is centred over the molecule (c) or off-centre (d).
Extended Data Figure 8 Analogy to the bonding/anti-bonding-like features of σ and π orbitals.
To better view the analogy, the experimental and simulated patterns in Fig. 3 are reorganized as ‘bonding-like’ and ‘anti-bonding-like’ pattern pairs. The left column shows the schematic diagrams of different modes corresponding to different coherent dipole–dipole coupling arrangements in terms of transition-dipole orientations and related phase relations. The corresponding experimental and simulated patterns are plotted in the middle and right columns.
Extended Data Figure 9 dI/dV mapping data for a ZnPc monomer, dimer, trimer and tetramer acquired with an open or closed feedback loop.
a, Sketch of ZnPc molecular configurations. b, c, Current images (b) and dI/dV images (c) for different ZnPc molecular configurations acquired simultaneously with an open feedback loop (−2.5 V). The bias modulation was 10 mV (r.m.s.) at 329 Hz. d, Corresponding normalized dI/dV images obtained by dividing by the tunnelling current pixel by pixel. e, f, STM images (e) and dI/dV images (f) for different ZnPc molecular configurations acquired simultaneously with a closed feedback loop (−2.5 V, 100 pA). The bias modulation was 10 mV (r.m.s.) at 329 Hz. For the measurements of dI/dV mapping and spectroscopic imaging with an open feedback loop, we positioned the tip above the NaCl surface (−2.5 V, 2 pA), switched off the feedback loop, lifted the tip up by 150 pm and started scanning over the selected area with molecules. Scale bars, 1 nm.
Extended Data Figure 10 Spectroscopic imaging of a ZnPc dimer with an open or closed feedback loop.
a, Sketch of a ZnPc dimer. b, c, Current image (b) and sum-up photon image (integrated over 1.86–1.94 eV) (c) acquired simultaneously with an open feedback loop (5.2 nm × 4 nm, 39 × 30 pixels; −2.5 V, 1 s per pixel). d, Corresponding normalized sum-up photon image obtained by dividing the sum-up image by the tunnelling current image pixel by pixel. e, Un-normalized photon images for each mode. f, Corresponding normalized photon images for each mode in (e). g, h, For comparison, a spectroscopic image on the same ZnPc dimer is acquired with a closed feedback loop. STM image (g) and sum-up photon image (integrated over 1.86–1.94 eV) (h) (5.2 nm × 4 nm, 39 × 30 pixels; −2.5 V, 200 pA, 2 s per pixel). i, Photon images for each mode.
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Zhang, Y., Luo, Y., Zhang, Y. et al. Visualizing coherent intermolecular dipole–dipole coupling in real space. Nature 531, 623–627 (2016). https://doi.org/10.1038/nature17428
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DOI: https://doi.org/10.1038/nature17428
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