Field of Expertise: Advanced Material Science
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Field of Expertise: Advanced Material Science | |
Investigation of Electronic Properties of Self-Assembled Monolayers with Embedded Dipoles An efficient way of tuning the properties of interfaces is the inclusion of self-assembled monolayers. By using mid-chain ester functionalized alkanethiols, as studied in this work, one can efficiently shift the work function of substrate materials. This allows improving charge injection in devices. Here, we investigate the above-mentioned mid-chain ester functionalized alkanethiols adsorbed on gold{111} substrates using molecular dynamics simulations and density functional theory calculations. The extent of the alkyl segments above and below the carboxylate dipolar groups are systematically varied to study the impact of the molecular length on the order within the layers. The latter is quantified using various structural parameters, like tilt-, azimuthal- and twist-angles. Potential-energy shifts within the layers (as observed experimentally [1]) can be determined from an orientation histogram of the molecular dipoles or from the atomic charge distributions applying the Poisson equation. In this context, one, however, needs to consider that the electronic dipole moment of a molecule inside a SAM can be significantly smaller than that of the same molecule in vacuum [2,3]. This depolarization effect is due to Coulombic interactions of the individual molecular dipoles inside the monolayer. In this work we account for that effect by iteratively recalculating the atomic charges inside the electric field caused by the surrounding molecules. As depolarization can be expected to also impact the order within the layers we also compare molecular dynamics runs including and neglecting depolarization effects. |