Role of molecular orientation in vibration, hopping, and electronic properties of single pyridine molecules adsorbed on Ag(1 1 0) surface: A combined STM and DFT study

A combination of low-temperature scanning tunneling microscopy and density functional theory calculations was used to determine the adsorption characteristics of single pyridine molecules on Ag(1 1 0) surfaces. Pyridine was chemisorbed onto Ag(1 1 0) surfaces at 13 K in either the stand-up or flat-lying configuration. The fractional ratio of the two configurations depended on the molecular coverage: at very low coverage, the flat-lying pyridine predominated on the surface; however, the molecules stood up with increasing surface coverage until the stand-up configuration was favored. The adsorption configurations were characterized by distinct C-H stretching vibrational energies, hopping barriers, and binding characteristics: (i) The C-H stretching energy for the flat-lying configuration was lower than that of the stand-up configuration. (ii) The hopping barrier for the stand-up configuration along the [1 1¯ 0] direction was the lowest. (iii) The electrostatic interaction was the dominant contribution to the binding of pyridine on Ag surfaces in both configurations, although the interaction was especially strong for the flat-lying configuration.