Tunneling across SAMs Containing Oligophenyl Groups

This paper reports rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga2O3/EGaIn as the top electrode. It compares the attenuation constant, β, and the pre-exponential parameter, J0, of the simplified Simmons equation across oligophenyl groups (R = Phn; n = 1, 2, 3) with three different anchoring groups (thiol, HSR; methanethiol, HSCH2R; and acetylene, HC≡CR) that attach R to the template-stripped gold or silver substrate. The results demonstrate that the structure of the molecular linker between the anchoring group (−S– or −C≡C−) and the oligophenyl moiety significantly influences the rate of charge transport. SAMs of SPhn and C≡CPhn on gold show similar values of β and log |J0| (β = 0.28 ± 0.03 Å–1 and log |J0| = 2.7 ± 0.1 for Au/SPhn; β = 0.30 ± 0.02 Å–1 and log |J0| = 3.0 ± 0.1 for Au/C≡CPhn). The introduction of a single intervening methylene (CH2) group between the anchoring sulfur atom and the aromatic units generates SAMs of SCH2Phn and increases β to ca. 0.66 ± 0.06 Å–1 on both gold and silver substrates. (For n-alkanethiolates on gold, the corresponding values are β = 0.76 ± 0.03 Å–1 and log |J0| = 4.2 ± 0.2). Density functional theory calculations indicate that the highest occupied molecular orbitals (HOMOs) of both SPhn and C≡CPhn extend beyond the anchoring group and onto the phenyl rings; SAMs composed of these two groups of molecules result in indistinguishable rates of charge transport. The introduction of the CH2 group, to generate SCH2Phn, disrupts the delocalization of the orbitals, localizes the HOMO on the anchoring sulfur atom, and results in the experimentally observed increase in β to a value closer to that of a SAM of n-alkylthiolate molecules.