Photoluminescence properties and excited state dynamics of monolayer perylene on graphite (0001)

Takashi Yamada

Phys. Rev. B 108, 205422 – Published 22 November 2023

Abstract

The correlation between the photoluminescence properties and excited state dynamics of perylene $(C_{20} H_{12})$ formed on a graphite (0001) substrate was investigated at the monolayer limit. Time-resolved two-photon photoemission spectroscopy was used to evaluate the lifetime of the excited state on the order of nanoseconds. On the molecular monolayer films, this unexpectedly long lifetime at the interface is significantly different from those typically observed for adsorption-induced electronic states, where ultrafast decays on the order of femtoseconds are dominant with electron/hole scattering. On the graphite (0001) surface, the standing molecular orientation of perylene indicates that the excited states are electronically decoupled from the substrate, which results in the suppression of the ultrafast nonradiative decay. As a result, orange light luminescence (610 nm) is observed with visible strength, which is ascribed to the deexcitation from the excited state of the dimeric molecular arrangement at the interface. Understanding the photoluminescence properties at the organic/electrode interface could be a key to the realization of organic optoelectronic thin-film devices with only a few molecular layers.

Received 17 April 2023
Revised 4 September 2023
Accepted 30 October 2023

DOI:https://doi.org/10.1103/PhysRevB.108.205422

Physics Subject Headings (PhySH)

Surfaces

Low-energy electron diffraction Two-photon photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Takashi Yamada

Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan

tyamada@chem.sci.osaka-u.ac.jp

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Issue

Vol. 108, Iss. 20 — 15 November 2023

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Images

Figure 1
(a) Photoluminescence (PL) spectra of perylene on single-crystalline graphite as a function of deposition time. Perylene films were excited by a UV light at a photon energy of 4.70 eV (263 nm). The substrate was kept at room temperature (298 K) during the deposition and measurements. The inset shows the molecular structure of perylene. (b) Deposition time dependent PL area intensities integrated from the energy window between 1.6 and 2.7 eV. The sigmoid function (red curve) is overplayed to guide the eye.
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Figure 2
(a)Temperature-dependent PL spectra for a 1 ML perylene film. As the temperature increases, the peak at around 2.0 eV is weakened and broadened toward the higher-energy side. The black arrow indicates the spectroscopic feature known as the Y (yellow) emission. (b) Contrast-enhanced LEED pattern for a 1 ML perylene film acquired at 90 K. The primary energy of the electron beam $(E_{p})$ was 37.0 eV. The reciprocal unit cells along with the three crystallographic directions are shown and some simulated spots are highlighted with red points. Circles indicate absent diffraction spots due to the glide plane symmetry. Black lines indicate the directions of one of the reciprocal lattice vectors of the substrate. (c) LEED pattern of a 1 ML perylene film acquired at 298 K and $E_{p} = 36.9$ eV. Less-ordered molecular distribution was observed, as characterized by the disk-shaped background with diffuse spots. (d) Photographs taken during the temperature-dependent PL measurements. As the temperature increased, a change in the PL color was observed from orange to yellow-green.
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Figure 3
Photon energy dependent 2PPE spectra of 1 ML perylene films on HOPG acquired at room temperature [(a1,a2), at 298 K] and low temperature [(b1,b2), at 90 K]. The vertical dotted lines are shown to highlight the peaks derived from unoccupied states. The traces in the right-hand panels are expanded by factors of 3 (298 K) and 300 (90 K) relative to those in the left-hand panel. H0 indicates the peak derived from the HOMO. L0 and L1 indicate the peaks derived from LUMO and the next LUMO $(LUMO + 1)$ , respectively. Ly and Lz are other unoccupied molecular orbitals. $IP S_{perylene} (n = 1)$ indicates the first $(n = 1)$ image potential states on the 1 ML perylene film. EX indicates the peak derived from the excited states of dimers. It should be noted that the EX peak was observable at (b1) low temperature but not at (a1) room temperature. F1 and F2 indicate the final states, i.e., unoccupied states above the vacuum level. (a3,b3) represent schematic energy diagrams of PL at 298 and 90 K, respectively.
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Figure 4
(a) Time-resolved 2PPE spectra for a 1 ML perylene film acquired at 90 K and at a photon energy of 4.77 eV. The spectrum acquired at 100 ps delay time was subtracted from the raw 2PPE spectrum. (b–d) Pump-probe delay time dependence of intensities for (b) LUMO, (c) $LUMO + 1$ , and (d) $IP S_{perylene} (n = 1)$ . The pulse width of the laser light was 110 fs, determined on a clean graphite substrate. In (b–d), open circles indicate experimental data points. Black, blue, and red lines indicate the components of autocorrelation (AC) of the pump-probe pulses, exponential decay, and the summation of these two components (black and blue lines), respectively. For (b–d), the estimated lifetimes were on the order of 50–60 fs, close to the detection limit in the current experimental setup.
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Figure 5
(a) Time-resolved 2PPE spectra of the EX peak for a 1 ML perylene film. The spectra were acquired at 90 K using a photon energy of 4.77 eV. (b) Delay time dependent intensities of the EX peak (red open circles) and estimated changes of EX peak intensities with specific lifetimes (exponential decay curves shown with solid lines). Unlike the time-resolved traces shown in Fig. 4, the intensities are almost constant with the small fluctuation of the data, which indicates the formation of an apparent steady state within the measured timescale.
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