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Selective triplet exciton formation in a single molecule

Abstract

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs)1,2,3,4,5,6,7. According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T1) excitons in a 1:3 ratio2,3,4. After the first report of a highly efficient OLED2 based on phosphorescence, which is produced by the decay of T1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage2,3,4,5,6. Because T1 excitons have lower energy than S1 excitons (owing to the exchange interaction), use of the energy difference could—in principle—enable exclusive production of T1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope8,9,10,11,12,13,14,15,16,17,18,19,20 and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule21,22,23,24,25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T1 excitons without creating their S1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.

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The data that support the findings of this study and associated codes are available from the corresponding authors (Y.K., M.G. and H.I.) on reasonable request.

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Acknowledgements

All experiments in this work were conducted at RIKEN in Japan. This work was supported by KAKENHI (grant numbers JP15H02025, JP21225001, JP18J11856, JP17H04796, JP17H05470, JP17K18766, JP26886013, JP16K21623) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, MEXT/JSPS Fellowships (numbers JP18J11856, JP15J03915) and the RIKEN Junior Research Associate Program. In addition, M.G. gratefully acknowledges support of the US Department of Energy (grant number DE-SC0018201). Some of the numerical computations were performed using the HOKUSAI system at RIKEN. We thank N. Takagi, C. Adachi, H. Nakanotani, R. Nagata, H. Walen and S. Yamamoto for discussions. In addition, we appreciate the work of Y. Hasegawa in supporting the preparation of Ag tips.

Reviewer information

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

Author information

K.K. and H.I. designed the experiments. K.K., M.I.-I. and S.K. performed the experiments. K.K. and H.I. analysed the data. K.M. and M.G. developed the calculation method. K.M. conducted the theoretical calculations. Y.K., M.K. and J.T. directed the project. All authors discussed the results and wrote the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Hiroshi Imada or Michael Galperin or Yousoo Kim.

Extended data figures and tables

Extended Data Fig. 1 STM measurements of the PTCDA/NaCl(3ML)/Au(111) system.

a, The dIt/dVs spectrum of the PTCDA/NaCl(3ML)/Au(111) system. PTCDA is neutral in this system, because the NaCl(3ML)/Au(111) surface has a larger work function than the NaCl(3ML)/Ag(111) surface. The resonant tunnelling peaks at Vs = −2.8 V and 0.9 V in the dIt/dVs spectrum originate from the HOMO and the LUMO, respectively. b, A constant-height STM image of PTCDA (4 nm × 4 nm, Vs = −3.0 V), where the tunnelling current was measured as a function of the tip position. c, d, Constant-current STM images of PTCDA (4 nm × 4 nm, It = 5 pA): c, Vs = 0.6 V; d, Vs = 1.0 V. A small eight-lobed structure is observed in the STM image at −3.0 V (b), and a double-lobed structure is observed at 1.0 V (d). Three black arrows in b indicate the voltages used for obtaining these STM images. In contrast to the PTCDA/NaCl(3ML)/Ag(111) system, different spatial distributions are obtained at negative and positive voltages in the case of the PTCDA/NaCl(3ML)/Au(111) system. e, DFT calculations of the spatial distribution of the HOMO (left, yellow spatial distribution) and the LUMO (right, blue spatial distribution). On the basis of the good agreements between the STM images and the spatial distribution of molecular orbitals, the small eight-lobed structure in b is attributed to the spatial distribution of the HOMO, and the double-lobed structure in d is attributed to the spatial distribution of the LUMO.

Extended Data Fig. 2 Proposed model for electron tunnelling in the PTCDA/NaCl(3ML)/Ag(111) system.

a, The dIt/dVs spectrum of the PTCDA/NaCl(3ML)/Ag(111) system (identical to the spectrum in Fig. 1c). b, c, Schematic energy diagrams of resonant tunnelling. Blue dots represent electrons. b, Vs < −0.5 V; c, Vs > 0.7 V. At negative sample voltage (Vs < −0.5 V), the pre-existing electron in the LUMO (open circle) first moves to the tip (movement indicated by green arrow), and the neutral state is formed. The neutral state returns to the initial state (grey arrow) via the supply of an electron to the LUMO from the substrate (‘from Ag(111)’; filled blue circle and green arrow). Similarly, at positive voltage (Vs > 0.7 V), the −2 charged state is formed by the injection of a second electron (filled blue circle and green arrow) into the singly occupied LUMO, and the molecule returns to the initial state (grey arrow) by supplying an electron to the substrate (‘to Ag(111)’; open blue circle and arrow). This mechanism was also proposed in previous reports dealing with the PTCDA/NaCl(2ML)/Ag(111) system21,22.

Extended Data Fig. 3 Comparison of the positions of the vibrational satellites in the fluorescence spectra.

a, Comparison of the fluorescence STL spectrum (S1 → S0) and the PL spectrum25. The blue spectrum shows fluorescence from PTCDA (Vs = −3.5 V, It = 30 pA, t = 180 s). The vertical dashed lines show the peak positions of the vibrational satellites with respect to the 0–0 peak position in the PL spectrum of PTCDA on a thick KCl film (about 10 ML) grown on a Ag(100) surface25. The long dashed lines show the position of the vibrational satellites in the PL spectrum that were also observed in the STL spectrum. b, Table of the positions of the vibrational satellites in the PL and STL spectra. This comparison shows that the positions of the small peaks near the 0–0 transition peak are almost identical to those in the previous PL data25.

Extended Data Fig. 4 Comparison between vibrational satellites observed in phosphorescence and in fluorescence.

a, Comparison of the experimental (Expt., from STL) and the calculated (Calc.) spectra of fluorescence and phosphorescence from PTCDA. Blue lines, fluorescence; red lines, phosphorescence. The solid lines show experimental STL spectra (blue; Vs = −3.5 V, It = 30 pA, t = 180 s: red; Vs = −2.5 V, It = 30 pA, t = 300 s). The dashed lines show the results of calculations using TD-DFT. The positions of the 0–0 transition peak are fixed to the origin. The positions of the small peaks (2, 5, 6 and 7) in the STL phosphorescence spectrum (T1 → S0) agree with those of the fluorescence peaks (S1 → S0). In both the fluorescence and phosphorescence spectra, a small peak (labelled ) was observed 30 meV higher in energy than the 0–0 transition peak. This luminescence peak was attributed to hot luminescence43, which is defined as a transition from the vibrational excited states of S1 (or T1) to the vibrational ground states of S0. b, Table summarizing the positions of the vibrational satellites in the experimental PL25 and STL spectra and in the results of calculations.

Extended Data Fig. 5 An STL spectrum of PTCDA/NaCl(3ML)/Ag(111) using a tip with a different plasmon resonance.

We intentionally changed the condition of the tip to modify the plasmon resonance13, and obtained the STL spectrum (Vs = −3.5 V, It = 50 pA, t = 60 s) shown here. Both a phosphorescence signal at 1.33 eV and a fluorescence signal at 2.45 eV are observed. However, the relative intensity of these two peaks is substantially different from that in the STL spectrum shown in Fig. 2b, despite the same tunnelling conditions. Compared to the spectrum in Fig. 2b, the plasmon resonance around 2.45 eV is weak in this spectrum. This result clearly indicates that the presence of plasmon resonance at the energy of the molecular luminescence amplifies its photon intensity, and the relative intensity of the phosphorescence and fluorescence peaks varies, depending on the plasmon resonance of the tip.

Supplementary information

Supplementary Information

This file contains theoretical analysis for the single molecule electroluminescence within the molecular many-body states description, including Supplementary Table 1, Supplementary Figs 1-4 and additional references.

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Fig. 1: STM measurements of the PTCDA/NaCl(3ML)/Ag(111) system.
Fig. 2: STL fluorescence and phosphorescence spectra of the PTCDA/NaCl(3ML)/Ag(111) system.
Fig. 3: Voltage dependence of STL spectra of the PTCDA/NaCl(3ML)/Ag(111) system.
Fig. 4: Phosphorescence STL map of the PTCDA/NaCl(3ML)/Ag(111) system.
Extended Data Fig. 1: STM measurements of the PTCDA/NaCl(3ML)/Au(111) system.
Extended Data Fig. 2: Proposed model for electron tunnelling in the PTCDA/NaCl(3ML)/Ag(111) system.
Extended Data Fig. 3: Comparison of the positions of the vibrational satellites in the fluorescence spectra.
Extended Data Fig. 4: Comparison between vibrational satellites observed in phosphorescence and in fluorescence.
Extended Data Fig. 5: An STL spectrum of PTCDA/NaCl(3ML)/Ag(111) using a tip with a different plasmon resonance.

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