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Electrofluorochromism at the single-molecule level

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Science  20 Jul 2018:
Vol. 361, Issue 6399, pp. 251-255
DOI: 10.1126/science.aat1603
  • Fig. 1 STM-induced fluorescence of neutral and charged ZnPc molecules.

    (A) Sketch of the experiment, in which the light emission (squiggly arrow) of a single ZnPc molecule separated from a Au(111) surface by three layers of salt is excited with the tunneling current traversing a STM junction. E, energy. (B) dI/dV spectrum acquired on a single ZnPc adsorbed on a trilayer of NaCl on Au(111). Insets show STM images (2.9 nm by 2.9 nm, I = 30 pA) acquired at V = −1 V (left) and 1.85 V (right). (C and D) STM-LE spectra acquired at (C) positive voltage (V = 2 V, I = 300 pA, acquisition time t = 300 s) and (D) negative voltage (V = −2.5 V, I = 300 pA, t = 300 s) for the STM tip located at the positions marked by black crosses in the STM images in (B). (E and F) Sketches of the luminescence mechanisms for the (E) neutral and (F) charged ZnPc. The molecular states reflect the optical gap of the neutral and charged molecules. In (E), the energy lost by an inelastic tunneling electron (vertical dashed arrow) is transfered (wave-line) to the ZnPc molecule, which returns to its ground state by emitting a photon (squiggly arrow). In (F), the molecule is first driven in a positively charged state (labeled 1), before the excitation-emission process (labeled 2) takes place.

  • Fig. 2 Vibronic fingerprints of neutral and charged ZnPc molecules.

    (A and B) STM-LE spectra (black lines) revealing the vibronic spectroscopic fingerprint of (A) a single ZnPc radical cation deposited on a trilayer of NaCl on Au(111) (V = −2.5 V, I = 300 pA, t = 180 s) and (B) a neutral ZnPc [adapted with permission from (20)] belonging to a molecular tetramer deposited on a trilayer of NaCl on Ag(111) (31) (V = −2.5 V, I = 750 pA, t = 300 s). The colored bars correspond to theoretical vibronic intensities for the A1g (black), B1g (cyan), and B2g (red) vibrational modes. “×25” and “×50” indicate multiplication factors, and arb. units are arbitrary units.

  • Fig. 3 Tuning the luminescence intensities.

    (A) STM-LE spectra acquired, with the same STM tip, on the bare Au(111) surface (labeled 0, V = −2.5 V, I = 300 pA, t = 60 s) and on single ZnPc molecules (V = −2.5 V, I = 300 pA) separated from the Au(111) surface by an increasing number of NaCl layers [1 ML (t = 300 s), 2 ML (t = 180 s), 3 ML (t = 120 s), and 4 ML (t = 10 s) labeled 1 to 4, respectively; see sketch]. The spectra labeled 0 to 4 have been vertically shifted for clarity. “×2” and “×15” indicate multiplication factors. (B) Two STM-LE spectra [black lines, V = −2.5 V, I = 300 pA, t = 300 s (top panel) or t = 60 s (middle panel)] acquired on single ZnPc on 3 ML of NaCl on Au(111) with two different STM tips, whose plasmonic response is deduced from STM-LE spectra acquired in front of the NaCl-Au(111) surface (red lines, V = −2.5 V, I = 300 pA, t = 120 s; green lines, V = −2.5 V, I = 300 pA, t = 60 s). The bottom panel displays the molecular emission spectra normalized by the respective plasmonic responses of the junction.

  • Fig. 4 Distance-dependent measurements of the luminescence contributions.

    (A) Sketch of the experiment, in which the light emission from the junction is monitored as a function of the lateral position of the tip with respect to a ZnPc molecule. The black wave-line illustrates the coupling between the localized plasmon and the molecular exciton. (B) STM image (with magnification on right) acquired at I = 30 pA and V = −2.5 V. (C and D) STM-LE spectra acquired for subnanometric variations of the lateral position of the STM tip with respect to a single ZnPc molecule [sketched in (A)] for (C) negative voltage (V = −2.5 V, I = 180 pA, t = 180 s) and (D) positive voltage (V = 2 V, I = 60 pA, t = 180 s). The position of the tip for each spectrum is marked by a colored dot in the STM image in (B).

Supplementary Materials

  • Electrofluorochromism at the single-molecule level

    Benjamin Doppagne, Michael C. Chong, Hervé Bulou, Alex Boeglin, Fabrice Scheurer, Guillaume Schull

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • Supplementary Text
    • Figs. S1 to S4 
    • References 

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