摘要
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. Further 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. However, 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 electronic 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.
基金
Author to whom correspondence should be addressed. E-mail： zcdong@ustc.edu.cn, FAX： ＋86-551-3600103 This work was supported by the National Basic Research Program of China （No.2006CB922003 and No.2011CB921402）, the Chinese Academy of Sciences （No.KJCX2.YW.H06）, and the National Natural Science Foundation of China （No.91021004, No.10574117, and No.10974186）.
