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  • Open Access

Bipolar single-molecule electroluminescence and electrofluorochromism

Tzu-Chao Hung, Roberto Robles, Brian Kiraly, Julian H. Strik, Bram A. Rutten, Alexander A. Khajetoorians, Nicolas Lorente, and Daniel Wegner
Phys. Rev. Research 5, 033027 – Published 14 July 2023
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  • INTRODUCTION
  • EXPERIMENTAL DETAILS
  • RESULTS
  • DISCUSSION
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    Understanding the fundamental mechanisms of optoelectronic excitation and relaxation pathways on the single-molecule level has only recently been started by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced luminescence (STML). In this paper, we investigate cationic and anionic fluorescence of individual zinc phthalocyanine (ZnPc) molecules adsorbed on ultrathin NaCl films on Ag(111) by using STML. They depend on the tip-sample bias polarity and appear at threshold voltages that are correlated with the onset energies of particular molecular orbitals, as identified by STS. We also find that the fluorescence is caused by a single-electron tunneling process. Comparing with results from density functional theory calculations, we propose an alternative many-body picture to describe the charging and electroluminescence mechanism. In this paper, we provide aspects toward well-defined voltage selectivity of bipolar electrofluorochromism as well as fundamental insights regarding the role of transiently charged states of emitter molecules within organic light-emitting diode devices.

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    • Received 15 March 2023
    • Accepted 13 June 2023

    DOI:https://doi.org/10.1103/PhysRevResearch.5.033027

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    ElectroluminescenceExcitonsTrions
    1. Physical Systems
    Molecular junctions
    1. Techniques
    Fluorescence spectroscopyLuminescenceScanning tunneling microscopyScanning tunneling spectroscopySingle molecule techniquesTime-dependent DFT
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Tzu-Chao Hung1, Roberto Robles2, Brian Kiraly1, Julian H. Strik1, Bram A. Rutten1, Alexander A. Khajetoorians1, Nicolas Lorente2,3, and Daniel Wegner1,*

    • 1Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands
    • 2Centro de Física de Materiales, CFM/MPC (CSIC-UPV/EHU), Paseo de Manuel de Lardizabal 5, 20018 Donostia-San Sebastián, Spain
    • 3Donostia International Physics Center (DIPC), Paseo de Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain

    • *Corresponding author: d.wegner@science.ru.nl

    Article Text

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    Vol. 5, Iss. 3 — July - September 2023

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    • Figure 1
      Figure 1

      ZnPc fluorescence on 2- vs 3-monolayer (ML) NaCl/Ag(111). (a) Constant-current scanning tunneling microscopy (STM) image of ZnPc molecules adsorbed on 2- and 3-ML NaCl/Ag(111) (VS=−2.5 V, It=10 pA). To acquire STM-induced luminescence (STML) spectra at positive sample bias, the ZnPc molecules were assembled into dimers by STM manipulation. (b) STML spectra of the ZnPc dimers on 2- and 3-ML NaCl/Ag(111) at opposite sample bias polarities (VS is indicated in the figure, It=100 pA, acquisition time t=120 s) measured at the positions marked in (a). All spectra were offset for clarity.

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    • Figure 2
      Figure 2

      Bias-dependent scanning tunneling microscopy-induced luminescence (STML) spectra of ZnPc dimer on 3-ML NaCl/Ag(111). (a) Negative bias-dependent STML spectra taken with decreasing sample bias. The emission of X+ is observed when the VS≤−2.65 V. (b) Positive bias-dependent STML spectra taken with increasing sample bias. The emissions of X0 and X− are both observed when VS≥+2.20 V. All STML spectra in (a) and (b) are raw data and were taken in constant-current mode at the tip position as indicated in Fig. 1 (It=100 pA, t=120 s).

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    • Figure 3
      Figure 3

      Characterization of the molecular orbitals (MOs). (a) Scanning tunneling spectroscopy (STS) of the ZnPc dimer at the center (blue) and lobe (red) of the molecule [tip position indicated by dots in (b), feedback loop opened at VS=−3.0 V, It=50 pA (blue) and It=100 pA (red), respectively]. (b) Constant-current scanning tunneling microscopy (STM) image of ZnPc dimer adsorbed on 3-ML NaCl/Ag(111) (VS=−2.5 V, It=10 pA). (c)–(g) The constant-height differential conductance maps show (c) the lowest unoccupied MO (LUMO)/LUMO + 1, (d) LUMO +x, (e) highest occupied MO (HOMO), (f) HOMO − 1 and (g) HOMO − x [VS as stated in the figures, lock-in parameters Vrms=8 mV at fmod=819 Hz, feedback loop was opened at the center of molecule at It=50pA, and sample bias (c) VS=+0.8 V, (d) VS=+2.3 V, (e) and (f) VS=−2.5 V, and (g) VS=−3.3 V].

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    • Figure 4
      Figure 4

      Molecular orbital mapping and density functional theory (DFT) calculation of an anchored ZnPc adsorbed on 2-ML NaCl/Ag(111). (a) Constant-current scanning tunneling microscopy (STM) image of ZnPc molecule anchored against 3-ML NaCl step edge (VS=−2.4 V, It=10 pA). (b), (c), and (g)–(i) Constant-height differential conductance maps. Sample biases are indicated in the figures. The feedback loop was opened at set current It=100 pA and sample bias (b) VS=−3.1 V, (c) and (g) VS=−2.4 V, (h) VS=+0.9 V, and (i) VS=−3.1 V above the center of the molecule. (d) Stick-ball model of the anchored molecule. (e), (f), and (j)−(l) DFT simulated constant-height differential conductance maps at voltages as stated.

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    • Figure 5
      Figure 5

      Schematic many-body energy diagram for exciton formation of ZnPc adsorbed on 3-ML NaCl/Ag(111). The light emission of X+ (X−) originates from the transition of excited doublet state D1+ (D1−) to doublet ground state D0+ (D0−). To reach the D1+ (D1−) state, tunneling into/out of higher energy molecular orbitals (MOs) is required, which leads to excitation into Dm+ (Dn−). Relaxation to D1+ (D1−) occurs via Auger-like intramolecular transitions. The excited singlet state (S1) can be reached by discharging from D1± and/or D0+ but not from the lower-lying D0−. From there, X0 emission occurs by radiatively decaying into singlet ground state (S0). Note that the energy axis is not to scale, i.e., the positions of the levels are only qualitative in nature.

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    • Figure 6
      Figure 6

      Tip dependence of X0 and X± emission thresholds for ZnPc dimers on 3-ML NaCl/Ag(111). (a) Peak intensity of the X0 emission vs sample bias for VS<0. For all tips, it becomes visible at VS≲−1.9 V but has a strongly increased intensity from VS≤−2.20 V that depends on the tip. (b) Peak intensity of the X+ emission for VS<0. The observed threshold varies with different tips between −2.65 and −2.90 V. (c) Peak intensity of the X0 emission and (d) X− emission for VS>0. In both cases, an almost tip-independent onset of the threshold is observed between +2.05 and +2.15 V. All measurements were acquired in constant-current mode using stabilization currents as stated in the legends.

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    • Figure 7
      Figure 7

      Power-law analysis of ZnPc. (a)–(c) Log-log graph of current-dependent intensity of X0 and X+. (d)–(f) Log-log graph of current-dependent intensity of X0 and X−. The sample biases are (a) VS=−3.2 V, (b) VS=−3.0 V, (c) VS=−3.0 V, (d) VS=+2.5 V, (e) VS=+2.5 V, and (f) VS=+2.4 V. The intensities in (a) and (d) were acquired from the spectra by integrating the total photon counts in the range ±2 meV around the peak position. The intensities shown in (b), (c), (e), and (f) were acquired from the spectra at the maximum of the peak position.

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    • Figure 8
      Figure 8

      Scanning tunneling microscopy-induced luminescence (STML) spectra of monomer (anchored) vs dimer ZnPc molecules on 3-ML NaCl/Ag(111). STML spectra acquired with positive bias of anchored ZnPc monomer (red) and ZnPc dimer (dark red; VS=+2.4 V, It=50 pA, acquisition time t=300 s) as well as negative bias (light and dark green, respectively; VS=−3.2 V, It=200 pA, acquisition time t=120 s). All spectra were acquired using the same tip and are offset for clarity. For the positive bias STML spectra, the raw data (gray) was 20-point Savitzky-Golay filtered.

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    • Figure 9
      Figure 9

      Lateral distance-dependent scanning tunneling microscopy (STM)-induced luminescence (STML) spectra of ZnPc dimer on 3-ML NaCl/Ag(111). (a) STML spectra acquired at VS=−3.0 V. Lateral positions are color coded and indicated by the dots in the inset constant-current STM image (VS=−2.5 V, It=5 pA, scale bar =2 nm), the separation between each position being 250 pm. (b) STML spectra acquired at VS=+2.4 V. All STML spectra were acquired in constant-current mode (It=100 pA, acquisition time t=120 s).

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    • Figure 10
      Figure 10

      Differential conductance spectrum vs the thresholds of X0 and X– emission of ZnPc dimer adsorbed on 2-ML NaCl/Ag(111). The threshold of X0 (red circle) and X− (red triangle) is found at sample bias VS=+2.1 V. The onset of the second negative ion resonance (NIR) is located around VS=+2.1 V (blue curve). Scanning tunneling microscopy-induced luminescence (STML) spectra were acquired with different sample bias [taken in constant-current mode, tip position is indicated in Fig. 1, It=100 pA, t=120 s]. The intensities are extracted from the spectra by integrating the total photon counts in the range ±2 meV around the peak position. For the scanning tunneling spectroscopy (STS), the feedback loop was opened at the center of molecule at It=100 pA, and sample bias VS=+2.4 V.

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    • Figure 11
      Figure 11

      Unit cells and projected density of states (PDOS) for density functional theory (DFT) calculations of ZnPc on NaCl/Ag(111). (a) Top and (b) side views of the unit cell used in the calculation of a ZnPc molecule on NaCl/Ag(111). In (c) and (d), the same is shown for a ZnPc molecule anchored to a ribbon of NaCl atoms. (e) PDOS for ZnPc with and without a ribbon.

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    • Figure 12
      Figure 12

      Molecular orbital mapping with increment bias steps. Constant-height differential conductance maps at sample biases as indicated. Feedback loop was opened at (a)–(f) VS=−2.8 V, It=50 pA; (g)–(j) VS=+1.2 V, It=50 pA; and (k)–(l) VS=+2.3 V, It=50 pA.

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    • Figure 13
      Figure 13

      Time-dependent density functional theory (TDDFT) calculation results. The plots show spin-dependent orbital energies of anionic and cationic ZnPc in the ground state. Black solid lines refer to occupied states, red dashed lines to empty states. Orbital labels (blue) are with respect to the neutral molecule. TDDFT-calculated excitation energies for the lower-energy transitions are also given, and the double arrows show in which orbitals the electron-hole pair is (mainly) located. For the anion, the first excitation with nonzero oscillator strength is that at 2.221 eV; for the cation, it is the excitation at 2.353 eV.

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