  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.
