Plasmonic excitation of light emission and absorption by porphyrine molecules in a scanning tunneling microscope

Light emission from a scanning tunneling microscope is investigated from Ag(111) surfaces using tips with a clean metallic apex and a molecule-covered shaft. The measurements are conducted such that no molecules are located in the tunneling gap. Nevertheless, spectra of the emission exhibit absorption and emission features of the tetraphenyl-H${}_2$-porphyrine molecules used. The experimental data show that molecules in the vicinity of the tunneling gap are excited through tip-induced plasmons and their radiative decay. The excitation of plasmons is due to inelastic tunneling and does not involve an external light source.

Figure 1

(a) Pseudo three-dimensional image of a single Ag adatom (left) and an impurity (right) on a Ag(111) surface imaged with a metallic tip with a molecule-covered shaft (“$m$-tip”, tunneling current $I=100\mathrm{pA}$, sample voltage $V=100\mathrm{mV}$). (b) Conductance (in units of the conductance quantum $G_0=2e^2/h$) acquired during the approach of the $m$-tip of (a) from tunneling to contact with the single Ag atom ($V=100\mathrm{mV}$).

Figure 2

Light emission spectra acquired with an $m$-tip at a current $I=1\mathrm{nA}$ and sample voltages 2.4, 2.8, 3.2, and 3.6 V over clean Ag(111) surface. The spectra are shifted vertically for clarity. Vertical dashed lines designate peak positions.

Figure 3

(a–c) Luminescence spectra acquired with different $m$-tips over a clean Ag(111) surface ($V=3.2\mathrm{V}$, $I=1\mathrm{nA}$). (d) Spectrum from a TPP multilayer on Ag(111) probed with a clean Ag tip ($V=3.4\mathrm{V}$, $I=20\mathrm{nA}$). The absorption and emission wavelengths of TBPP and TPP from Refs. 5, 22, and 31 are indicated by blue dotted and red dashed lines, respectively.