Electroluminescence of molecules in a scanning tunneling microscope: Role of tunneling electrons and surface plasmons

We study electroluminescence from molecules confined in a scanning tunneling microscope based on a recently proposed density-matrix approach. The molecule is treated by a two-state model with each state consisting of a set of vibrational energy levels. The interband transition probabilities are described by Franck-Condon factors as calculated from a harmonic model. The role played by the tunneling electron as well as by the surface plasmon resonance is investigated. The dependence of the electron-tunneling-induced electroluminescence on the temperature, the bias voltage, the coupling strength between the molecule and the electrodes, and the radiative decay rate of the excited state has been systematically studied. It is found that under high temperature, due to the thermally assisted electron tunneling, photon emissions can still be detected when the bias voltage is less than the excitation energy of the molecule. We also find that when the molecule is asymmetrically coupled to the electrodes, electroluminescence does not strictly follow the Franck-Condon distribution. Our simulations also show that the increase of the radiative decay rate of the excited state cannot lead to the hot luminescence from higher vibrational levels. The involvement of the surface plasmon can drastically alter the spectral profiles, resulting in hot luminescence from molecules, when the surface plasmon in a scanning tunneling microscope is of high strength and ultrashort duration. The influence of the strength and the duration of the plasmon pulse on the electroluminescence spectra has been discussed.