Single-molecule Electroluminescence and Beyond.

A scanning tunneling microscope (STM) can do more than atomic imaging and manipulation. Its tunneling current can also be used for the excitation of light, converting electron energy to photon energy. STM based single-molecule electroluminescence can be realized by adopting a combined strategy of both efficient electronic decoupling and nanocavity plasmonic enhancement. The emission intensity, upon optimized material combination for the molecule, spacer, tip, and substrate, can be strong and stable enough for performing second-order photon correlation measurements. The observation of an evident photon antibunching effect demonstrates clearly the nature of single-photon emission for single-molecule electroluminescence. Strikingly, the spectral peak of a monomer is found to split when a molecular dimer is artificially constructed through STM manipulation, which suggests that the excitation energy from tunneling electrons is likely to rapidly delocalize over the whole molecular dimer. The spatial distribution of the excitonic coupling for different energy states in a dimer can be visualized in real space through sub-nanometer resolved electroluminescence imaging technique, which correlates very well with the local optical responses predicted in terms of coherent intermolecular dipole-dipole coupling. Furthermore, a single molecule can also couple coherently with a plasmonic nanocavity, resulting in the occurrence of interference-induced Fano resonance. These findings open up new avenues to fabricate electrically driven quantum light sources as well as to study intermolecular energy transfer, field-matter interaction, and molecular optoelectronics, all at the single-molecule level.