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Orbital-resolved visualization of single-molecule photocurrent channels

Abstract

Given its central role in utilizing light energy, photoinduced electron transfer (PET) from an excited molecule has been widely studied1,2,3,4,5,6. However, even though microscopic photocurrent measurement methods7,8,9,10,11 have made it possible to correlate the efficiency of the process with local features, spatial resolution has been insufficient to resolve it at the molecular level. Recent work has, however, shown that single molecules can be efficiently excited and probed when combining a scanning tunnelling microscope (STM) with localized plasmon fields driven by a tunable laser12,13. Here we use that approach to directly visualize with atomic-scale resolution the photocurrent channels through the molecular orbitals of a single free-base phthalocyanine (FBPc) molecule, by detecting electrons from its first excited state tunnelling through the STM tip. We find that the direction and the spatial distribution of the photocurrent depend sensitively on the bias voltage, and detect counter-flowing photocurrent channels even at a voltage where the averaged photocurrent is near zero. Moreover, we see evidence of competition between PET and photoluminescence12, and find that we can control whether the excited molecule primarily relaxes through PET or photoluminescence by positioning the STM tip with three-dimensional, atomic precision. These observations suggest that specific photocurrent channels can be promoted or suppressed by tuning the coupling to excited-state molecular orbitals, and thus provide new perspectives for improving energy-conversion efficiencies by atomic-scale electronic and geometric engineering of molecular interfaces.

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Fig. 1: Atomic-scale measurement of photocurrent generation in a single molecule.
Fig. 2: Voltage dependence of photocurrent channels through a single molecule.
Fig. 3: Mechanism of photocurrent generation in an FBPc molecule.
Fig. 4: Control of the quantum efficiencies of photoelectron energy conversion.

Data availability

All experimental data shown in Figs. 14, Extended Data Figs. 19 and Extended Data Tables 1, 2 can be found at Zenodo, https://doi.org/10.5281/zenodo.5553643, and are also available from the corresponding authors upon reasonable request.

Code availability

The code used to calculate the results shown in this work is available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank B. Yang, E. Kazuma, Y. Hasegawa and Y. Shimizu for supporting the preparation of the Au tips. Some of the numerical calculations were carried out using the HOKUSAI system at RIKEN. All experiments were conducted at RIKEN in Japan. This work was financially supported in part by JST PRESTO (grant no. JPMJPR186225 (H.I.)) and by Japan Society for the Promotion of Science (JSPS) KAKENHI: grant nos JP 19K23591 (M.I.-I.), JP19H04681, JP 17H04796 (H.I.), JP 20K22488 (K.K.), JP 18K14153 (R.B.J.), JP 18H05257 (Y.K.), JP17H06173, JP17H05430 (M.U.), JP20H02728 (A.M.).

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Contributions

M.I.-I, H.I. and Y.K. conceived the project and constructed the experimental set-up with the help of Y.K. M.I.-I., H.I., K.K., I.Z., R.B.J. and H.Y. performed the experiment using STM and analysed the data. Y.T., A.M. and M.U. synthesized and characterized the D2Pc molecule. K.M. conducted the theoretical calculations. Y.K. directed the project. All authors discussed the results and wrote the manuscript.

Corresponding authors

Correspondence to Hiroshi Imada or Yousoo Kim.

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Extended data figures and tables

Extended Data Fig. 1 Trace of tip–height variation under laser irradiation.

The laser power was 80 μW, and the energy was 1,816 meV. During the measurement, the tip was placed on the Ag(111) with the closed STM feedback loop.

Extended Data Fig. 2 Voltage dependence of dark-current images over a single molecule.

Current images over the FBPc molecule at −2.0 and 0.0 V (image size, 3.0 nm × 3.0 nm). While the tip was scanning, the laser was turned off, and the STM feedback loop was open. The tip–molecule distance was maintained to be the same as the laser-on image in Fig. 2b. Shades of blue and red indicate current flow from the substrate to the tip and vice versa, respectively. The line profiles along the dashed lines are shown below the dark-current images. The red dots show the raw data at each pixel and the black line shows a smoothed profile by a moving average of five adjacent data points. At −2.0 V, the dark current slightly exceeds the margin of error around the centre of the molecule, whereas it does not at 0.0 V. The maximum absolute value of the detected current at −2.0 V was about 0.03 pA. Because this value was only 0.3% of the maximum detected current value under illumination (Fig. 2b), the contribution of the dark current to the photocurrent image in Fig. 2b is negligibly small.

Extended Data Fig. 3 Tautomerization reaction of an FBPc molecule under laser irradiation.

a, Z(t) spectra were measured on a D2Pc molecule on the NaCl(4ML)/Ag(111) with the STM feedback loop closed (Vs = +1.0 V, It = 3 pA). The black and blue spectra were measured under laser-off and laser-on conditions, respectively. The incident laser energy was tuned at the resonance of the S0–S1 transition shown in Fig. 1d, with the power at 1 μW. Both spectra were measured with the same tip placed at the same distance and angle from the centre of the molecule, and the Z values for both spectra refer to the identical position. b, The current images of a D2Pc molecule (Vs = +0.75 V) under laser-on (left) and laser-off (right) conditions, respectively. The power of the laser was 2 μW. c, The orientation of the molecule in the images shown in b. Under the laser-on condition, tautomerization reactions cause the molecular axes to rotate through 90°, whereas under laser-off condition it does not.

Extended Data Fig. 4 Molecular structure and the spatial distribution of frontier molecular orbitals.

Molecular structure of FBPc and the spatial distributions of the HOMO, LUMO and nearly degenerated LUMO+1 of FBPc in the gas phase as predicted by density functional theory (DFT) calculations. The orientation of molecule and the position of the arrows, which show the position 45° from the axes, correspond to those in Fig. 2b, d.

Extended Data Fig. 5 Voltage dependence of the photocurrent image in the transition range.

Photocurrent images over the FBPc molecule at −0.4, −0.3, −0.25, −0.2 and −0.1 V (Image size: 2.4 nm × 2.4 nm) acquired with the STM feedback loop open. The laser energy was set at the resonance of the S0–S1 transition, with a power of 77 μW. Shades of blue and red represent the current flow from the substrate to the tip and vice versa, respectively. The line profiles along the dashed line in each photocurrent image are shown below the respective images. The vertical dashed lines in the line profiles are for comparing the same tip positions. It is revealed that positions showing the local maximum of the positive photocurrent at −0.1 V correspond to where the negative photocurrent value shows a local minimum at −0.4 V. This result is indicative of the competition between the negative and positive photocurrent channels in the molecule.

Extended Data Fig. 6 Line profiles of photocurrent and photoluminescence signals.

a, Photocurrent image of an FBPc molecule on NaCl(4ML)/Ag(111) measured at Vs = −2.0 V and the STM tip position during the measurement in Fig. 4d, e (black circles). b, The line profile of the photocurrent along the dashed line in a (45° from the molecular axes). Photocurrent signals were measured at lateral tip positions ranging from 0.6 nm to 2.4 nm. The size of the photocurrent signal agrees well with the molecular size observed in the STM image, indicating that photocurrent generation occurs when the tip is placed on the molecule. c, The line profile of photoluminescence (PL) along the dashed line in a. Photoluminescence signals appear at all lateral tip positions between 0.0 nm and 3.0 nm. The intensity is minimal at the centre of the molecule (~1.5 nm), and increases with distance from the centre. The local maximum values appear at ~0.5 and ~2.6 nm, and the intensity further turns to decrease with distance. These tendencies correspond to the tip–position dependence of the coupling between the molecule and the localized plasmon which drives the molecular luminescence36. Because the photoluminescence is driven by the localized plasmon, which is several nanometres long24, photoluminescence can be observed even when the tip is outside the molecule. The STM tip positions during the measurement in Fig. 4d, e are shown by the arrows. Although both photocurrent and photoluminescence signals were observed on the molecule (at the position of the left arrow), only the photoluminescence signal was detected outside the molecule (at the right arrow). Using this difference, we investigated the influence of photocurrent generation on photoluminescence quenching (Fig. 4d, e).

Extended Data Fig. 7 Comparison of the populations of increasing electrons and decreasing photons.

The increase of electrons (red area in Fig. 4d) and the decrease of photons (blue area in Fig. 4d) are plotted against Ztip–mol. The number of electrons was obtained from the detected photocurrent value divided by the elementary charge e. The number of quenched photons (\({I}_{{\rm{ph}}}^{{\rm{quench}}}\)) was obtained by the following equation. \({I}_{{\rm{ph}}}^{{\rm{quench}}}=({I}_{{\rm{ph}}}^{{\rm{\det }}}-\,{I}_{{\rm{ph}}}^{{\rm{fit}}})/{\eta }_{{\rm{\det }}}\). Here, \({I}_{{\rm{ph}}}^{{\rm{\det }}}\) is the number of the photons (counts per second) detected in the experiment, and \({I}_{{\rm{ph}}}^{{\rm{fit}}}\) is the photon numbers (counts per second) deduced by the fitting curve Iph = 1.09 × 105 × exp(−6.77Ztip–mol) for the Ztip–mol range between 1.1 nm and 0.55 nm shown in Fig. 4d. By extrapolating the fitted curve into the Ztip–mol value less than 0.53 nm, we estimated the expected photon intensity without the photoluminescence quenching in this region. ηdet is the detection efficiency determined by the experimental set-up. By considering the collection solid angle of the lens, detection quantum efficiency of the detector, and the reflection, diffraction and transmission of the optics, ηdet was estimated to be 4.9 × 10−4 for this measurement (see Methods). It was revealed that the number of electrons flowing as photocurrent and the number of quenched photons at each Ztip–mol were comparable. These results suggest that the observed photoluminescence quenching mainly originated from the photocurrent generation. The slightly large value of the quenched photons might come from another nonradiative recombination pathway or underestimation of ηdet.

Extended Data Fig. 8 Determination of the tip–molecule distance.

a, The conductance (G) curve for the bare Ag(111) (black) and the linear fitting curve (red). The tip displacement Z refers to the initial tip position at Vs = −2.5 V and It = 3 pA. The G is normalized by the quantization conductance G0 which is defined by 2e2/h with the electron charge e and Planck’s constant h. b, A schematic illustration of the various relevant distances in the photocurrent measurement.

Extended Data Fig. 9 Simulation results of the IV curve in the single-molecule junction under laser irradiation.

af, Results are given for various values of electron–phonon coupling parameter λ.

Extended Data Table 1 Total energy Etotal (summation of the electronic and zero-point vibrational energies) for each electronic state
Extended Data Table 2 The correction \({{\boldsymbol{E}}}_{{\boldsymbol{N}}{\boldsymbol{a}}}^{{\bf{Img}}}\) of the total energy for each molecular many-body state owing to the image-charge effects from the electrodes

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Imai-Imada, M., Imada, H., Miwa, K. et al. Orbital-resolved visualization of single-molecule photocurrent channels. Nature 603, 829–834 (2022). https://doi.org/10.1038/s41586-022-04401-0

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