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Rappoport, Dmitrij

Junctions with the structure AgTS/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = −OH, −SH, −CO2H, −CONH2, and −PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm2, that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = −CO2H, and T = −CH2CH3−two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)−suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.

This paper describes 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 injection current, J0, and the attenuation constant, β, 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 and silver substrates. The results demonstrate that the structure of the molecules between the anchoring group (-S- or -C≡C-) and the oligophenyl moiety significantly influences 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 to generate SAMs of SCH2Phn, increases β to ~0.6 Å-1 on both gold and silver substrates. (For n-alkanethiolates on gold the corresponding values are β = 0.76 Å-1 and log|J0| = 4.2). As a generalization, based on this and other work, it seems that increasing the height of the tunneling barrier in the region of the interfaces increases β, and may decrease J0; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J0.