Commensurate growth and diminishing substrate influence in a multilayer film of a tris(thieno)hexaazatriphenylene derivative on Au(111) studied by scanning tunneling microscopy

Abstract

Layer-by-layer growth of the electron-transport material tris 2,5-bis 3,5-bis-trifluoromethyl-phenyl -thieno 3,4-b,h,n -1,4,5,8,9,12-hexaazatriphenylene THAP on Au 111 is probed by scanning tunneling microscopy STM . A relative of discotic liquid crystalline molecules, THAP is shown to grow in commensurate ordered planes from the first to fourth monolayers. The four monolayers all show a concordant ordered structure in which the molecules arrange parallel to the substrate in a hexagonal close-packed lattice with a herringbone pattern defined by alternating rows of molecules with antiparallel orientation. The unit cell is rectangular with two molecules per cell and is nearly equivalent for each layer. The spatial broadening of the local density of states due to the metallic substrate is appreciably diminished in upper layers, as expected and as evidenced by the localization of states seen in STM. There is good agreement between the highest occupied molecular orbital obtained in density functional theory calculations for a single molecule and STM images of the upper layers, in accord with the localized nature of electronic states on molecules under minimal substrate influence.