Self-assembly of α-6T Molecule on Ag(100) and Related STM Induced Luminescence

We have investigated the self-assembly and light emission properties of organic α-sexithiophene (α-6T) molecules on Ag(100) under different coverage by scanning tunneling microscopy (STM). At very low coverage, the α-6T molecules form a unique enantiomer by grouping four molecules into a windmill supermolecular structure. As the coverage is increased, α-6T molecules tend to pack side by side into a denser stripe structure. Fur-ther increase of the coverage will lead to the layer-by-layer growth of molecules on Ag(100) with the lower-layer stripe pattern serving as a template. Molecular fluorescence for α-6T molecules on Ag(100) at a coverage of five monolayers has been detected by light excitations,which indicates a well decoupled electronic states for the top-layer α-6T molecules. How-ever, the STM induced luminescent spectra for the same sample reveal only plasmonic-like emission. The absence of intramolecular fluorescence in this case suggests that the elec-tronic decoupling is not a sufficient condition for generating photon emission from molecules. For intramolecular fluorescence to occur, the orientation of the dynamic dipole moment of molecules and the energy-level alignment at the molecule-metal interface are also important so that molecules can be effectively excited through efficient dipolar coupling with local plasmons and by injecting holes into the molecules.