A density matrix approach for the electroluminescence of molecules in a scanning tunneling microscope

TL;DR

This paper introduces a density matrix-based theoretical framework to understand electroluminescence in molecules within a scanning tunneling microscope, emphasizing the role of surface plasmons and matching experimental spectra.

Contribution

It provides a novel, comprehensive model combining electron tunneling and surface plasmon effects to explain electroluminescence phenomena at the molecular level.

Findings

Reproduces experimental spectra of porphyrin derivatives.

Explains large variations in emission spectral profiles.

Highlights the influence of local surface plasmons.

Abstract

The electroluminescence of molecules confined inside a nanocavity in the scanning tunneling microscopy possesses many intriguing but unexplained features. We present here a general theoretical approach based on the density matrix formalism to describe the electroluminescence from molecules near a metal surface induced by both electron tunneling and local surface plasmon excitations simultaneously. It reveals the underlying physical mechanism for the external bias dependent electroluminescence. The important role played by the local surface plasmon on the electroluminescence is highlighted. Calculations for porphyrin derivatives have reproduced corresponding experimental spectra and nicely explained the observed unusual large variation of emission spectral profiles. This general theoretical approach can find many applications in the design of molecular electronic and photonic devices.