End Group Dominated Molecular Order

Self-assembled monolayers of azobenzene on gold have been studied with atomic force microscopy in ethanol and with scanning tunneling microcopy at high gap impedance in air. These azobenzene functionalized self-assembled monolayers show two stable rectangular lattices having equal dimensions of 0.61 and 0.79 nm and and angle of 89 deg. We conclude that the lattices are end group dominated because they are incommensurate with Au(111), they extend over terrace edges with no phase shift, and they can also be observed on polycrystalline gold. The unti cells are populated with two molecules that have aromatic interactions and expose the upper phenyl ring edge of the azobenzene moiety to the surface. The anisotropic electronic shape of these phenyl rings allows distinct, isoenergetic geometries created by laterally shifting and tilting pairs of phenyl rings. The upper edge of these phenyl rings appears to be elliptical in STM images.

Figure 1: End groups in monolayers of azobenzene-functionalized thiols absorbed on gold(111) form an incommensurate rectangular lattice. The lattice extends without distortion across gold steps and is populated by elliptical objects. Dimensions of these elliptical objects and their pairing at the corners of the cell suggest that STM resolves upper parts of the phenyl ring edge of the azobenzenes, which are vertical in the monolayre and paired by aromatic interactions.

Bruno Michel <bmi@zurich.ibm.com>

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Last modified: Wed, 15 May 1996 11:34