Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System

Hiroshi Imada, Kuniyuki Miwa, Miyabi Imai-Imada, Shota Kawahara, Kensuke Kimura, and Yousoo Kim

Phys. Rev. Lett. 119, 013901 – Published 5 July 2017

Abstract

We investigate the near-field interaction between an isolated free-base phthalocyanine molecule and a plasmon localized in the gap between an NaCl-covered Ag(111) surface and the tip apex of a scanning tunneling microscope. When the tip is located in the close proximity of the molecule, asymmetric dips emerge in the broad luminescence spectrum of the plasmon generated by the tunneling current. The origin of the dips is explained by energy transfer between the plasmon and molecular excitons and a quantum mechanical interference effect, where molecular vibrations provide additional degrees of freedom in the dynamic process.

Received 15 February 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.013901

Physics Subject Headings (PhySH)

Electroluminescence Excitons Optoelectronics Photonics

Scanning probe microscopy Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Hiroshi Imada¹, Kuniyuki Miwa¹, Miyabi Imai-Imada^1,2, Shota Kawahara^1,2, Kensuke Kimura^1,2, and Yousoo Kim^1,*

¹Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
²Department of Advanced Materials Science, Graduate School of Frontier Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8651, Japan

^*Corresponding author. ykim@riken.jp (Y.K.).

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Vol. 119, Iss. 1 — 7 July 2017

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Images

Figure 1
Emergence of “dip” structures in STL spectra. (a) A schematic illustration of the experiment. $μ$ represents the transition dipole moment of $H_{2} Pc$ , which is oriented in the molecular plane. (b) An STM topographic image of $H_{2} Pcs$ adsorbed on a three monolayer-thick NaCl(100) island grown on Ag(111) (sample bias voltage $V = - 2.5 V$ , tunneling current $I = 2 pA$ , $25 \times 25 {nm}^{2}$ ). (c) The measurement tip positions for the spectra shown in (d) and (e) and the definition of the coordinates around the molecule. An $H_{2} Pc$ (gray: C, blue: N, white: H) has two hydrogen atoms with a trans configuration at the molecular center. We follow the conventional definition of the molecular axes $x$ and $y$ , ( $x$ axis: parallel to N–H H–N bond) [32]. $θ$ is measured from the $x$ axis of $H_{2} Pc$ . The column on the right is the list of the distances $r$ measured from the molecular center. (d), (e) a series of STL spectra measured on and near an $H_{2} Pc / NaCl$ with different tip positions on the NaCl film ( $V = - 2.3 V$ , $I = 50 pA$ , exposure time $t = 3 \min$ ). The spectra are offset for clarity. A theoretical simulation corresponding to (d) is given in Fig. S2.
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Figure 2
Spectral analysis of the dip structures: comparison with theoretical calculations. (a) STL spectra measured on the NaCl film with $r = 2.2 nm$ (upper) and 4 nm (lower) ( $θ = 45 °$ , $V = - 2.5 V$ , $I = 250 pA$ , $t = 5 \min$ ). (b) An $I (r, θ) / I_{0}$ spectrum ( $r = 2.2 nm$ , $θ = 45 °$ ), generated by dividing the upper curve ( $I$ ) with the lower curve ( $I_{0}$ ) in (a). (c) Calculated luminescence spectra using the theory of STL [23, 24] with plasmon-exciton coupling $ℏ g = 0$ (lower) and 10 meV (upper). Two molecular excited states with 1.81 and 1.92 eV are assumed. (d) An $I / I_{0}$ spectrum using the two simulated spectra in (c). (e) A schematic diagram illustrating the dynamic processes arising from the plasmon-exciton coupling. $G$ and $E$ stand for the ground and excited states of the molecule, respectively.
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Figure 3
Angle dependency in the plasmon-exciton coupling. $I (r, θ) / I_{0}$ spectra measured at $θ = 0 °$ , 45º, 90º, and $r = 1.6 nm$ ( $V = - 2.2 V$ , $I = 500 pA$ , $t = 5 \sec$ , averaged 50 spectra). The corresponding STM images were obtained with $V = 0.6 V$ and $I = 5 pA$ . (b) Calculated molecular orbitals (HOMO, LUMO, and $LUMO + 1$ ) of $H_{2} Pc$ in the gas phase. The calculations were performed with gaussian09. The color (red or blue) represents the sign of the phase of the wave function. While HOMO is an odd function both in $x$ and $y$ , LUMO ( $LUMO + 1$ ) is odd in $x$ ( $y$ ) but even in $y$ ( $x$ ). All the three MOs are odd in $z$ .
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Figure 4
Fine structures in $I / I_{0}$ spectrum. (a) The same measurement as the top curve in Fig. 3 was performed with a different tip condition ( $r = 1.8 nm$ , $V = - 2.2 V$ , $I = 500 pA$ , $t = 5 \sec$ , averaged 100 spectra). The experimental data were fitted with a single Breit-Wigner-Fano function and was subtracted by the fitted curve to give the lower curve. (b) An energy diagram summarizing the observed transitions. $G$ , ground state; $E$ , excited state.
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