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Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging

Abstract

Watching a single molecule move on its intrinsic timescale has been one of the central goals of modern nanoscience, and calls for measurements that combine ultrafast temporal resolution1,2,3,4,5,6,7,8 with atomic spatial resolution9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30. Steady-state experiments access the requisite spatial scales, as illustrated by direct imaging of individual molecular orbitals using scanning tunnelling microscopy9,10,11 or the acquisition of tip-enhanced Raman and luminescence spectra with sub-molecular resolution26,27,28. But tracking the intrinsic dynamics of a single molecule directly in the time domain faces the challenge that interactions with the molecule must be confined to a femtosecond time window. For individual nanoparticles, such ultrafast temporal confinement has been demonstrated18 by combining scanning tunnelling microscopy with so-called lightwave electronics1,2,3,4,5,6,7,8, which uses the oscillating carrier wave of tailored light pulses to directly manipulate electronic motion on timescales faster even than a single cycle of light. Here we build on ultrafast terahertz scanning tunnelling microscopy to access a state-selective tunnelling regime, where the peak of a terahertz electric-field waveform transiently opens an otherwise forbidden tunnelling channel through a single molecular state. It thereby removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window shorter than one oscillation cycle of the terahertz wave. We exploit this effect to record approximately 100-femtosecond snapshot images of the orbital structure with sub-ångström spatial resolution, and to reveal, through pump/probe measurements, coherent molecular vibrations at terahertz frequencies directly in the time domain. We anticipate that the combination of lightwave electronics1,2,3,4,5,6,7,8 and the atomic resolution of our approach will open the door to visualizing ultrafast photochemistry and the operation of molecular electronics on the single-orbital scale.

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Figure 1: Ultrafast THz-induced tunnelling of a single electron out of a single molecule.
Figure 2: Ultrafast THz-STM imaging of a pentacene molecular orbital.
Figure 3: Ultrafast dynamics of a single molecule.
Figure 4: Influence of substrate surface and molecular species on ultrafast single-molecule dynamics.

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Acknowledgements

We thank A. Pöllmann, F. Albrecht, R. Kronfeldner, M. Eisele and M. Furthmeier for assistance, and F. Evers, F. A. Hegmann, V. Jelic, P. C. M. Planken, M. Grifoni and K. Richter for discussions. We acknowledge financial support from the Volkswagen Foundation (Lichtenberg program), the European Research Council through grant number 305003 (QUANTUMsubCYCLE), and the Deutsche Forschungsgemeinschaft (DFG) through GRK 1570 and research grants HU1598/3 and CO1492/1. T.L.C. thanks the A. v. Humboldt Foundation.

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Authors

Contributions

T.L.C., D.P., P.Y., J.R. and R.H. conceived, set up and carried out the experiments. T.L.C., D.P., J.R. and R.H. analysed the data and wrote the manuscript.

Corresponding authors

Correspondence to Jascha Repp or Rupert Huber.

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The authors declare no competing financial interests.

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Nature thanks H. Shigekawa and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Spatial resolution of THz-STM imaging.

a, b, Line-scan cuts (a) through a constant-height, zero-bias THz-STM image (b), where the THz-induced current ITHz is shown in units of rectified electrons per THz pulse. The pixel size in the image is 0.25 Å × 0.25 Å and the white scale bar is 1 Å long. The image contains edges where the signal rises from the background level to a maximum within 0.75 Å. The signal rise from 10% to 90% in these cases occurs over approximately 0.6 Å, which provides an upper bound for the spatial resolution of the image. This estimate agrees with simulations of the spatial distribution of the rectified current (Fig. 2d, i). There, we calculate the square of the matrix element between the pentacene HOMO and the tip wavefunction (Bardeen model). The tip wavefunction, which ultimately determines the attainable spatial resolution, is modelled as an s-wave (as in the Tersoff–Hamann approach) and the spatial decay of this wavefunction is used as a fitting parameter, as is the tip height. The best agreement between the experimental and simulated images is found for s-waves with decay lengths of ≤ 0.5 Å, consistent with our spatial resolution estimate of 0.6 Å based on the line scans in a.

Extended Data Figure 2 Ultrafast THz-STM imaging of the lowest unoccupied orbital of pentacene.

a, When Au(110) is replaced with Cu(100) as the substrate, the HOMO and LUMO levels realign with respect to the Fermi level of the substrate in such a way that the LUMO can be probed by THz-induced currents. This situation is depicted schematically here, in analogy to Fig. 1b. Left, steady-state differential conductance measured on a pentacene molecule adsorbed on NaCl/Cu(100). Right, THz electric-field waveform measured by electro-optic sampling. Although the THz waveform is the same as that in Fig. 1b, it now induces sequential tunnelling into the pentacene LUMO at the most intense positive half-cycle, while the negative half-cycle is too weak to allow for HOMO tunnelling. b, Steady-state constant-current STM image of three pentacene molecules side-by-side at VDC = 100 mV and I = 1.4 pA. Greyscale range = 2.4 Å. c, Constant-height THz-STM image of the same sample area as in b, with VDC = 0 mV and the maximum positive THz voltage set to  ≈ +1.3 V. The spatial distribution of the THz-induced current closely resembles the LUMO density for each molecule, indicating state-selective THz-induced LUMO tunnelling. The onset field for this process agrees with the modified alignment of the orbitals. Specifically, the peak THz voltage in the positive direction that is predicted based on the field enhancement determined from HOMO tunnelling is consistent with the new LUMO transport level voltage observed in the steady-state dI/dV curve here. This agreement lends further support to the proposed mechanism for THz-STM in this regime.

Extended Data Figure 3 Modelling ultrafast terahertz-induced tunnelling out of a pentacene HOMO.

a, Terahertz voltage waveforms used in the simulations. We approximate the time trace of the THz voltage with the THz electric-field waveform measured in the far field by electro-optic sampling. In all plots, the pink curve corresponds to a waveform with a peak of −2.05 V and the navy-blue curve corresponds to a waveform with a peak of −2.22 V. The scaling factor and resulting peak voltages are determined by fitting the shape of the experimental autocorrelation (Fig. 3a) as follows. We simulate the autocorrelation by taking the sum of two THz pulses and calculating the total rectified current from the resulting transient as a function of the delay time between the pulses. b, The dI/dV characteristic needed for the simulation is obtained by modelling the HOMO and LUMO molecular-resonance peaks in the experimental dI/dV curve (Fig. 2b, centre) with two Gaussians. This dI/dV curve is then scaled up to account for the significantly lower tip height for THz-STM imaging. Note that the scaling factor does not affect the shape of the autocorrelation. Blue line, centre of HOMO Gaussian, −1.93 V (0.62 V full width at half-maximum, FWHM). Red line, centre of LUMO Gaussian, 2.06 V (0.58 V FWHM). c, Resulting current response induced by the THz voltage waveform in a when applied to a junction with a dI/dV relation defined by b. The asymmetry of the THz voltage pulse leads to a much larger current response in the negative bias direction than in the positive bias direction. Inset, induced current response during the negative crest of the THz voltage waveform. Pink curve, 120-fs FWHM; navy-blue curve, 140-fs FWHM. d, Total number of electrons that have tunnelled across the junction, calculated from the integral of c. Negative numbers refer to the negative bias direction, that is, electron tunnelling from the HOMO to the tip. Inset, rise of the rectified electron signal during the negative crest of the THz voltage waveform. The rise time from 10% to 90% of the maximum signal is 115 fs for the pink curve and 130 fs for the navy-blue curve. The simulated autocorrelation for the −2.22 V peak (navy-blue curve) provides the best fit to the shape of the measured autocorrelation. The dI/dV curve is scaled such that the autocorrelation peak at −2.22 V matches the measured peak of approximately −0.75 electrons per THz pulse. The peak voltage of the THz pulses used in the imaging configuration is then determined by finding the THz peak for which the simulations yield this number of rectified electrons per THz pulse. For example, in Fig. 2e the maximum observed signal is approximately −0.58 electrons per THz pulse, and the best agreement is found for −2.05 V (pink curve). We note that the simulations are based on the assumption that the THz pulse modulates the bias of the junction quasi-instantaneously. In our simulations we disregard any blocking of tunnelling or other effects resulting from the finite time until an electron from the substrate refills the molecular state. This blocking is expected to become important if the THz current reaches or even exceeds one electron per pulse.

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Cocker, T., Peller, D., Yu, P. et al. Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging. Nature 539, 263–267 (2016). https://doi.org/10.1038/nature19816

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