Single-molecule electroluminescence and its relevant latest progress

Research on the interaction and interconversion between electrons and photons on an individual molecular scale can provide scientific basis for the future developing of information and energy technology. Scanning tunneling microscope(STM) can offer abilities beyond atomic-resolution imaging and manipulation, and its highly localized tunneling electrons can also be used for exciting the molecules inside the tunnel junction, generating molecule-specific light emission, and thus enabling the investigation of molecular optoelectronic behavior in local nano-environment. In this paper, we present an overview of our recent research progress related to the single-molecule electroluminescence of zinc phthalocyanine (ZnPc) molecules. First, we demonstrate the realization of single-molecule electroluminescence from an isolated ZnPc by adopting a combined strategy of both efficient electronic decoupling and nanocavity plasmonic enhancement. By further combining the photon correlation measurements via the Hanbury-Brown-Twiss interferometry with STM induced luminescence technique, we demonstrate and confirm the single-photon emission nature of such an electrically driven single-molecule electroluminescence. Second, by developing the sub-nanometer resolved electroluminescence imaging technique, we demonstrate the real-space visualization of the coherent intermolecular dipole-dipole coupling of an artificially constructed non-bonded ZnPc dimer. By mapping the spatial distribution of the photon yield for the excitonic coupling in a well-defined molecular architecture, we can reveal the local optical response of the system and the dependence of the local optical response on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. Third, by using a single molecular emitter as a distinctive optical probe to coherently couple with the highly confined plasmonic nanocavity, we demonstrate the Fano resonance and photonic Lamb shift at a single-molecule level. The ability to spatially control the single-molecule Fano resonance with sub-nanometer precision can reveal the coherent and highly confined nature of the broadband nanocavity plasmon, as well as the coupling strength and the anisotropy of the field-matter interaction. These results not only shed light on the fabrication of electrically driven nano-emitters and single-photon sources, but also open up a new avenue to the study of intermolecular energy transfer, field-matter interaction, and molecular optoelectronics, all at the single-molecule level.