We studied the adsorption of organic molecules, growth behavior, and physical properties on silver, gold, and HOPG surfaces by using ultra-high-vacuum low-temperature scanning tunneling microscopes. Combined with low energy electron diffraction and first-principles density functional theory calculations, we tried to understand the self-assembly mechanism of molecular structures, and then to find the key parameters which can modulate their structures for obtaining functional molecular thin films. The following three factors are found to be useful for the modulation. 1) The electric filed can induce molecular structural transition due to the molecular polarity, which is helpful for designing functional molecules of electronic devices. 2) The alkyl chains of quinacridone derivatives (QA) determine the orientation of molecular overlayers on an Ag(110) substrate. The interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to specific positions at the silver substrate, determining the molecular orientation and preferred adsorption site. However, the intermolecular arrangement can be adjusted via the length of attached alkyl chains. We are able to fabricate uniform QA films with very well controlled physical properties. 3) By thermal and chemical control, we are able to self-assemble three dimensional molecular nanostructures, e.g. ordered PTCDA structures exclusively on flat Ag(111) facets, or DMe-DCNQI structures exclusively on stepped Ag(221) facets. It is demonstrated that bonding, the key factor for selectivity, occurs via the end-atoms, while the molecule's mid-region arches away from the substrate. Theoretical results obtained by high-level theoretic calculations are consistent with the experimental observations.
1 Z.T. Deng et al., Phys. Rev. Lett. 96, 156102(2006).
2 W. Ji et al., Phys. Rev. Lett. 97, 246101(2006).
3 D.X. Shi et al., Phys. Rev. Lett. 96, 226101(2006).
4 S.X. Du et al., Phys. Rev. Lett. 96, 156105(2006).
In collaboration with D.X. Shi, S.X. Du, Z.T. Deng, W. Ji, L. Gao, Z.H. Cheng, Q. Liu, N. Jiang (Institute of Physics, Chinese Academy of Sciences, China), W.A. Hofer (The University of Liverpool, Britain), H. Fuchs (University of Muenster, Germany), and S.T. Pantelides (Vanderbilt University, USA).
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