Anti-Kasha Emissions of Single Molecules in a Plasmonic Nanocavity

Result of the Month

Author: Hiroshi Imada, Miyabi Imai-Imada, Xingmei Ouyang, Atsuya Muranaka, and Yousoo Kim Institute: ''RIKEN, Japan'' The Journal of Chemical Physics
Date: 4/2023 Instruments: LT STM Lab

Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (H2Nc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunnelling microscope (STM) and metal substrate. A narrow-line tuneable laser was employed to precisely reveal the excited-state levels of a single molecule located under the tip and to selectively excite it into a specific excited state, followed by obtaining STM-photoluminescence (STM-PL) spectrum to reveal energy relaxation from the state. The excitation to higher-lying states of H2Nc caused various changes in the emission spectrum, such as broadening and the appearance of new peaks, implying the breakdown of Kasha's rule. These observations indicate emissions from the vibrationally excited states in the first singlet excited state (S1) and second singlet excited state (S2), as well as internal conversion from S2 to S1. Moreover, we obtained direct evidence of electronic and vibronic transitions from the vibrationally excited states, from the STM-PL measurements of MgNc. The results obtained herein shed light on the energy dynamics of molecular systems under a plasmonic field and highlight the possibility of obtaining various energy-converting functions using anti-Kasha processes.

In this study, we employ a recently developed STM-photoluminescence (STM-PL) technique with a narrow-line tunable laser16 to investigate anti-Kasha emissions. The experimental setup is shown in Fig. 1(a). The monochromatic and tunable plasmonic field driven by an externally irradiated laser allows state-selective excitation of a single molecule in the plasmonic nanocavity. The PL emission spectra were obtained to reveal the energy relaxation processes initiated from a specific excited state (Fig. 1(b)).

Fig. 2. (a, b) Molecular structure of H2Nc and MgNc, respectively (white: hydrogen, grey: carbon, blue: nitrogen, and red: magnesium). (c) Wide area STM topographic image of the sample (sample bias voltage Vs = 1 V, tunnelling current I = 3 pA, 200 × 200 nm2). (d) Magnified STM image of H2Nc on 3ML-NaCl film on Ag(111) (Vs = 0.8 V, I = 3 pA, 7.5 × 7.5 nm2). (e) Magnified STM image of MgNc on the same substrate (Vs = 0.9 V, I = 3 pA, 7.5 × 7.5 nm2).

Experiment: STM and optical system

All experiments were performed using low-temperature STM (Scienta Omicron), which was operated at 4.6 K under ultrahigh vacuum (UHV). We designed an STM stage that is equipped with two optical lenses for laser illumination and light detection. Each lens covered a solid angle of ~0.5 sr. For the STM-PL measurements, a tunable continuous-wave (CW) laser (Sacher Lasertechnik) was used for excitation. The laser is an external cavity semiconductor diode laser (ECDL) in a Littman/Metcalf configuration with a motorized tuning mechanism, which has a narrow linewidth (<1 MHz corresponding to < 4 neV), and its wavelength can be quickly tuned by the mechanical rotation of a grating in the laser cavity. Two lasers with different tuning ranges of 1.78–1.83 eV (677–695 nm) and 1.59–1.67 eV (744–779 nm) were used. In this study, the laser power was approximately 5–10 Μw and p-polarization was applied.

Fig.3. (a) Schematic depiction of STM-EL and STM-PL measurements. For STM-EL, the tunnelling current I of STM was used for excitation, while a plasmonic field driven by a laser was used for STMPL. (b) STM-PL (top) and STM-EL (bottom) spectra of H2Nc on 3ML-NaCl/Ag(111). The measurement conditions for STM-PL: excitation energy Eex = 1.618 eV, laser power = 8.2 μW, Vs = - 2 V, I = 3 pA, and measurement time 1 = s; the measurement was performed at the blue dot in c. For STM-EL: Vs = -2 V, I = 20 pA, measurement time = 60 s; the measurement was performed at the red dot in c. (c) An STM image of H2Nc with tip positions for the spectra shown in b.

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AUTHORS:

Hiroshi Imada, Miyabi Imai-Imada, Xingmei Ouyang, Atsuya Muranaka, and Yousoo Kim

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INSTITUTE:

RIKEN, Japan

https://www.riken.jp/en/

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CORRESPONDING AUTHORS:

Yousoo Kim: ykim@riken.jp

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