  The coupling between molecular exciton and gap plasmons plays a key role in
single molecular electroluminescence induced by a scanning tunneling microscope
(STM). But it has been difficult to clarify the complex experimental phenomena.
By employing the nonequilibrium Green's function method, we propose a general
theoretical model to understand the light emission spectrum from single
molecule and gap plasmons from an energy transport point of view. The coherent
interaction between gap plasmons and molecular exciton leads to a prominent
Fano resonance in the emission spectrum. We analyze the dependence of the Fano
line shape on the system parameters, based on which we provide a unified
account of several recent experimental observations. Moreover, we highlight the
effect of the tip-molecule electronic coupling on the spectrum, which has
hitherto not been considered.
