Abstract:

Self-assembled monolayers (SAMs) of polar and polarizable organic molecules are widely used to tune semiconductors’ electronic properties for various applications. In the case of the dipoles’ arrangement in a dense and ordered SAM, intermolecular interaction between neighboring dipoles arises, inducing a change in the electrostatic properties of the polar SAM. These intermolecular long-range dipole interactions give rise to a molecular cooperative effect (MCE) through the layer, thus influencing the magnitude of the net surface dipole and suppressing the substrate contribution to dipole formation. Molecular engineering of the desired MCE could be a useful tool in various molecular electronics derived applications. In this work, we propose an experimental design to tailor the magnitude of the MCE through an organic monolayer. We constructed a mixed dipole monolayer containing parallel and antiparallel randomly organized dipoles. The creation of a mixed dipole monolayer enables controlling the MCE through the layer by giving rise to a smaller normal component to the surface coefficient and larger parallel component to the surface dipole coefficient. A deeper understanding of the MCE can be obtained by comparing the experimental and calculated values of such mixed dipole monolayers. The experimental values were obtained from contact potential difference measurements, and the calculated values were extracted by using the modified Helmholtz equation, based on the relative dipole contribution introduced in this work. This comparison enables analyzing the limit of the expected MCE, based on the evaluated surface dipole density along with its individual longitudinal molecular dipole.

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