Structures of a film of the long-chain n-alkane n-C 44H 90 on a Cu [formula omitted] surface

Abstract

Structures of a film of a long-chain n-alkane, tetratetracontane ( n-C 44H 90; TTC), on a Cu(1 0 0) single-crystal surface were investigated by a combination of infrared reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). Vacuum vapor deposition was used to prepare epitaxially grown films. Measurements of the step-wise increase in film thickness revealed the structural changes in TTC film during film growth. IRRAS investigations revealed that three phases appear during film growth, as previously observed for TTC deposited on Au(1 1 1) and Ag(1 1 1) surfaces. In the first layer, which consists of TTC molecules directly adsorbed on a Cu(1 0 0) surface, the molecules assume a flat-on structure with the carbon skeleton plane of TTC parallel to the surface (flat-on phase). The next layer consists of slightly disordered molecules including a gauche conformation ( gauche phase). Finally, a film with molecular packing close to that in the crystalline state is formed with further deposition (quasi-crystalline phase). LEED investigations indicated that (1) the first layer forms a regular unit cell with a lamellar structure, (2) the molecular arrangement shows a transition upon the completion of the first layer, which can be ascribed to subdivision of the lamella into antiphase domains, and (3) the surfaces of the gauche and quasi-crystalline phases do not have two-dimensional order. Since the molecule itself has a one-dimensional periodic structure, it is necessary to consider both the intramolecular and intermolecular periodicity for detailed analysis by LEED. Simulation of the LEED pattern by a kinematic theory indicated that the intramolecular rather than the intermolecular periodicity dominates the LEED pattern.