Fluorination, and Tunneling across Molecular Junctions

This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form Ag(TS)O2C(CH2)n(CF2)(m)T//Ga2O3/EGaIn, where T is methyl (CH3) or trifluoromethyl (CF3). Alkanoate-based self-assembled monolayers (SAMs) having perfluorinated groups (R(F)) show current densities that are lower (by factors of 20-30) than those of the homologous hydrocarbons (R(H)), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)n(CH2)(-1)) and difluoromethylene (β = (1.15 ± 0.02)n(CF2)(-1)) are similar (although the value for (CF2)n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl- and -phenyl-alkanoates suggests that the C-F//Ga2O3 interface is responsible for the lower tunneling currents for CF3. The decrease in the rate of charge transport in SAMs with R(F) groups (relative to homologous R(H) groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga2O3 interface, and/or to weak van der Waals interactions at that interface.