Effect of Tilt-Angle on Electron Tunneling through Organic Monolayer Films

This work explores how the geometry of alkanethiol molecules in organic thin films impacts the tunneling of electrons from an electrode to a redox species in solution. Film thicknesses for C 8 H 17 SH (C8), C 12 H 25 SH (C12), and C 16 H 33 SH (C16) self-assembled monolayers (SAMs) on InP(100) were determined by angle-resolved X-ray photoelectron spectroscopy to be 6.4 ± 0.7, 11.1 ± 0.6, and 14.9 ± 1.2 Å, respectively. These thicknesses correspond to tilt-angles of 62 ± 4° for C8, 53 ± 3° for C12, and 51 ± 4° for C16 SAMs, which gives an average tilt-angle of 55 ± 6°. The decay constants for electron tunneling through alkanethiols on InP(100), Au(111), and Hg(liquid) are shown to correlate with the geometry, “tilt” angle, of the alkane chains on the surface. This work suggests that superexchange coupling between the alkane chains, which comprise the insulating film, can play an important role in defining electron tunneling barriers, especially for highly tilted chains.