Gradual Change between\nCoherent and Incoherent Tunneling\nRegimes Induced by Polarizable Halide Substituents in Molecular Tunnel\nJunctions

This paper describes a gradual transition of charge transport\nacross\nmolecular junctions from coherent to incoherent tunneling by increasing\nthe number and polarizability of halide substituents of phenyl-terminated\naliphatic monolayers of the form S(CH<sub>2</sub>)<sub>10</sub>OPhX<sub><i>n</i></sub>, X = F, Cl, Br, or I; <i>n</i> = 0, 1, 2, 3, or 5. In contrast to earlier work where incoherent\ntunneling was induced by introducing redox-active groups or increasing\nthe molecular length, we show that increasing the polarizability,\nwhile keeping the organization of the monolayer structure unaltered,\nresults in a gradual change in the mechanism of tunneling of charge\ncarriers where the activation energy increased from 23 meV for <i>n</i> = 0 (associated with coherent tunneling) to 257 meV for <i>n</i> = 5 with X = Br (associated with incoherent tunneling).\nInterestingly, this increase in incoherent tunneling rate with polarizability\nresulted in an improved molecular diode performance. Our findings\nsuggest an avenue to improve the electronic function of molecular\ndevices by introducing polarizable atoms.