Plasmon-Assisted Resonant Electron Tunneling in a Scanning Tunneling Microscope Junction

Shuyi Liu, Martin Wolf, and Takashi Kumagai

Phys. Rev. Lett. 121, 226802 – Published 30 November 2018

Abstract

We report plasmon-assisted resonant electron tunneling from a Ag or Au tip to field emission resonances (FERs) of a Ag(111) surface induced by cw laser excitation of a scanning tunneling microscope (STM) junction at visible wavelengths. As a hallmark of the plasmon-assisted resonant tunneling, we observe a downshift of the first peak in the FER spectra by a fixed amount equal to the incident photon energy. STM-induced luminescence measurement for the Ag and Au tip reveals the clear correlation between the laser-induced change in the FER spectra and the plasmonic properties of the junction. Our results clarify a novel resonant electron transfer mechanism in a plasmonic nanocavity.

Received 10 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.226802

Physics Subject Headings (PhySH)

Plasmons

Metals Surfaces

Scanning tunneling microscopy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shuyi Liu¹, Martin Wolf¹, and Takashi Kumagai^1,2,*

¹Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
²JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

^*Corresponding author. kuma@fhi-berlin.mpg.de

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Issue

Vol. 121, Iss. 22 — 30 November 2018

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Images

Figure 1
(a) FER spectra (solid curves, left axis) of a Ag tip–Ag(111) surface junction with (red) and without (black) 633-nm excitation ( $h ν = 1.96 eV$ ). The junction is illuminated with a power density of $0.53 mW μ m^{- 2}$ . The spectra were recorded in the constant current mode at 0.1 nA. The tip-sample vertical displacements ( $Δ z$ ) are also plotted (dashed curves, right axis). (b) Incident power dependence of the FER spectra. The power density is varied from 0 to $0.53 mW μ m^{- 2}$ . The spectra are vertically offset for clarity. (c) FER spectra of a Ag tip-Ag(111) surface junction with (red) and without (black) 532-nm excitation ( $h ν = 2.33 eV$ ). The junction is illuminated at a power density of $0.84 mW μ m^{- 2}$ . The spectra were recorded in the constant current mode at 0.1 nA.
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Figure 2
(a) STML spectra measured over the Ag(111) surface with a Au (orange) and Ag (green) tip ( $V_{s} = 3 V$ , $I_{t} = 9 nA$ ). The vertical green broken line indicates the excitation wavelength in the FER measurement (532 nm). (b) FER spectra of a Au (orange) or Ag (green) tip-Ag(111) surface junction under 532-nm excitation with a power density of $0.10 mW μ m^{- 2}$ . The spectra were recorded in the constant current mode at 0.1 nA. (c) Polarization dependence of the FER spectra for a Ag tip-Ag (111) junction under 633-nm illumination with an incident power density of $0.09 mW / μ m^{2}$ . The red, blue, and black broken lines correspond to the $p$ polarization, $s$ polarization, and nonilluminated cases, respectively. The inset shows schematic of the experimental geometry.
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Figure 3
(a) FER spectra of a Au tip-Ag(111) surface junction with (red) and without (black) 633-nm excitation. The latter is vertically offset for clarity. The spectra were recorded in the constant current mode at 0.1 nA. The power density was set to $0.20 mW μ m^{- 2}$ . (b) Topographic image of the Ag(111) surface with a monoatomic step and an intrinsic defect ( $V_{s} = 1 V$ , $I_{t} = 0.1 nA$ , $10 \times 12 {nm}^{2}$ ). (c)–(e) FER mapping at different bias voltages (indicated in the figure) without laser excitation. (f)–(h) FER mapping at different bias voltages (indicated in the figure) with laser excitation (illuminated with a power density of $0.20 mW μ m^{- 2}$ ).
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Figure 4
Schematic energy diagram of the Ag or Au tip-vacuum-Ag(111) surface junction (a) without and (b) with illumination. The blue shaded area represents the schematic potential in the junction. Fermi level ( $E_{F}$ ), vacuum level ( $E_{vac}$ ), work function of the surface (tip) ( $Φ_{s (t)}$ ), sample bias ( $V_{s}$ ), gap distance ( $d$ ), index of the FERs [ $n (n^{'})$ ], FER voltage without (with) illumination [ $V_{n (n^{'})}$ ], schematic electron wave functions (red curves), incident photon energy ( $h ν$ ).
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