A research team from the University of Science and Technology of China has successfully visualized coherent intermolecular dipole-dipole coupling in real space using STM-based electroluminescence techniques.  

The work was published in Nature on March 31, and reviewers have commented, “The fact that such effects could be directly imaged at the level of the individual molecule is truly fascinating.” “I think this is a unique and exciting new way of studying dipole-dipole interactions between molecules with high significance for a broad community.” 

This advance was highlighted in the "News and Views" column of the same issue under the title, “A close-up view of coupled molecules.” 

A dipole describes the separation of positive and negative charges. For example, a pair of electric charges of equal magnitude but opposite sign and separated by some small distance forms the simplest dipole. Dipole-dipole interaction between molecules, or coherent intermolecular dipole-dipole coupling, underlies important energy-transfer and optical processes in a wide range of scenarios, such as molecular interaction, photosynthetic light-harvesting and quantum optics. However, it is highly challenging to visualize coherent dipole coupling in real space at the single-molecular level owing to the diffraction limit in conventional optics. 

As an old Chinese saying goes, one has to sharpen one’s tools first to get the job done. A scanning tunneling microscope (STM), with a very tiny metal tip used for position scanning, is a device that allows for imaging and manipulation of molecules at the atomic level. In 2010, the team developed a method of generating anomalous hot electroluminescence for porphyrin molecules confined inside a plasmonic nanocavity in an STM; this process can be applied to ultrahigh-resolution spectroscopy and sensing. Using this STM-induced electroluminescence method, the team successfully mapped the spatial distribution of the excitonic coupling in well-defined arrangements of a few zinc-phthalocyanine molecules. 

In the research, molecules on the substrate were innovatively “pushed together” under the STM. It takes imagination to come up with the unique idea of directly imaging intermolecular dipole interactions using state-of-the-art technology. Molecules on the substrate were “pushed” together by STM techniques and imaged using sub-nanometer resolved single-molecule electroluminescence techniques. The luminescence patterns obtained from the dimer reveal the local optical response of the system and its dependence on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. 

Furthermore, enhanced “single-molecule” superradiance was observed for in-line arrangements of up to four zinc-phthalocyanine molecules. “The findings we report clearly demonstrate that it is possible to visualize excitonic coupling in real space with sub-nanometer resolution,” said Prof. DONG Zhenchao, the group leader. “This capability should enable a greater understanding and rational engineering of light-harvesting structures and quantum light sources.” In fact, the experimental approach may go much further and open up new fronts for the study of molecular interaction and energy transfer.