Role of surface bonding on liquid-crystal alignment at metal surfaces

The alignment of a nematic liquid crystal, 4-n-pentyl-4-cyanobiphenyl (5CB), on several smooth metal (Cr, Cu, Ag, and Au) surfaces was observed experimentally. Perpendicular (homeotropic) induced alignment was observed for Cu and Ag boundary layers, whereas parallel alignment was observed for Cr and Au. These differences are discussed in relation to the differences in chemical bonding interactions at the surfaces as described by theoretical calculations performed for the cases of Cu, Cr, and Au. Several differences in orbital interactions, as well as a sensitivity to bonding configurations, are illustrated with the aid of extended Ḧckel calculations on hypothetical organometallic complexes and extended-Ḧckel"tight-binding calculations of the interface for parallel versus perpendicular adsorbate orientations. © 1989 The American Physical Society.