Mapping orbital changes upon electron transfer with tunnelling microscopy on insulators

Abstract

Electron transfer plays a crucial part in many chemical reactions1,2, including photosynthesis, combustion and corrosion. But even though redox-state transitions change the electronic structure of the molecules involved, mapping these changes at the single-molecule level is challenging. Scanning tunnelling microscopy provides insights into the orbital structure3 of single molecules and their interactions4,5, but requires the use of a conductive substrate that keeps molecules in a given charge state and thereby suppresses redox-state transitions. Atomic force microscopy can be used on insulating substrates to obtain structural6 and electrostatic7,8 information but does not generally access electronic states. Here we show that when synchronizing voltage pulses that steer electron tunnelling between a conductive atomic force microscope tip and a substrate with the oscillation of the tip, we can perform tunnelling experiments on non-conductive substrates and thereby map the orbital structure of isolated molecules as a function of their redox state. This allows us to resolve previously inaccessible electronic transitions in space and energy and to visualize the effects of electron transfer and polaron formation on individual molecular orbitals. We anticipate that our approach will prove useful for the investigation of complex redox reactions and charging-related phenomena with sub-ångström resolution.

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Fig. 1: Working principle of single-electron AC-STM.
Fig. 2: Imaging of electronic transitions of pentacene.
Fig. 3: Jahn-Teller effect in CuPc.
Fig. 4: Electronic transition imaging of BDHN-TTF.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

We thank L. Gross, S. Fatayer, F. Giessibl, F. Evers, R. Huber and G. Meyer for discussions. Financial support from the Marie Curie Initial Training Network ‘MOLESCO’ (number 606728) and Deutsche Forschungsgemeinschaft (numbers RE2669/6-1 and GRK 1570) is gratefully acknowledged.

Reviewer information

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

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Contributions

All authors designed and performed the experiments and analysed the data. L.L.P. and J.R. were responsible for DFT calculations and co-wrote the paper. All authors revised the manuscript.

Corresponding authors

Correspondence to Laerte L. Patera or Jascha Repp.

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Extended data figures and tables

Extended Data Fig. 1 Scheme and detailed shape of a.c.

voltage pulses.

Extended Data Fig. 2 Orbital confinement upon electron transfer in pentacene.

a, b, Electronic transitions probed by the same metal tip apex (A = 1 Å): 0→1 (a; Vd.c. = 1.87 V, Va.c. = 1.00 Vpp, Δz = 2 Å); 1→0 (b; Vd.c. = 1.87 V, Va.c. = 1.50 Vpp, Δz = 2 Å). Δz is given with respect to an AFM setpoint of Δf = −3.2 Hz at Vd.c. = 0 V. Scale bar, 5 Å. c, Line profiles taken along the directions indicated in a and b.

Extended Data Fig. 3 Modulus square of calculated orbitals for neutral and negatively charged CuPc.

a, b, Constant-height cuts of the superposition of degenerate LUMOs (a) and of only one of the degeneracy-lifted LUMOs (b). Scale bar, 5 Å.

Extended Data Fig. 4 Dissipation signal as a function of tip–sample distance.

Spectrum acquired above the centre of an isolated pentacene molecule (Vd.c. = 1.76 V, Va.c. = 1.00 V, A = 1 Å, 0→1). Δz is given with respect to an AFM setpoint of Δf = −3.2 Hz at Vd.c. = 0 V. Signal saturation is observed at short tip–sample distances because not more than one electron per set pulse can be transferred.

Extended Data Fig. 5 Imaging of electronic transitions in constant-detuning mode.

a, AFM topography image (Δf = −28.5 Hz, Vd.c. = 1.60 V, Va.c. = 1.00 V, A = 1 Å) of an isolated pentacene molecule on NaCl. b, Simultaneously acquired dissipation channel, corresponding to the 0→1 transition.

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Patera, L.L., Queck, F., Scheuerer, P. et al. Mapping orbital changes upon electron transfer with tunnelling microscopy on insulators. Nature 566, 245–248 (2019). https://doi.org/10.1038/s41586-019-0910-3

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