Publication: Mean-Field Theoretical Study of Bistability in Mixed Azobenzene-Alkylthiol Monolayers

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Title Mean-Field Theoretical Study of Bistability in Mixed Azobenzene-Alkylthiol Monolayers
Authors/Editors* C. R. L. Chapman, I. Paci
Where published* J. Phys. Chem. C
How published* Journal
Year* 2010
Volume 114
Pages 2645
Azobenzene derivatives exhibit a well-known cis-trans photoinduced isomerization. At the gold surface, scanning tunneling microscopy studies of the azobenzene derivative N-(2-mercaptoethyl)-4-phenyla- zobenzamide have revealed similar isomerization behavior under the influence of an electric field. The reversible conversion of this adsorbate between a low-current and a high-current isomer shows promise for applications in molecular switching. This paper employs a mean-field theoretical approach to study bistability at medium to high surface coverages of chemisorbed mixed monolayers of azobenzene and dodecanethiol on gold surfaces. We find that the trans isomer dominates the monolayer under most sets of conditions, stabilized both by interactions between its total dipole moment and the field, and by lateral interactions with matrix molecules. Our results also highlight two important effects, which are often overlooked in discussions of bistability in self-assembled monolayers. First, polarizability effects can overcome destabilizing coupling between the permanent dipole moment and the applied field when fields are strong. We find that, at strong negative sample biases, the highly polarizable trans azobenzenes become most prevalent in the monolayer even though they are hindered by unfavorable permanent dipole- field coupling. Second, the more favorable lateral interactions between trans isomers and the dodecanethiol matrix lead to almost full conversion to the trans isomer in the monolayer, at densities well below those for which one would expect the footprint of the molecule to become important.
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