Abstract
The structure and chemical composition are the key parameters influencing the properties of organic thin films deposited on inorganic substrates. Such films often display structures that substantially differ from the bulk, and the substrate has a relevant influence on their polymorphism. In this work, we illuminate the role of the substrate by studying its influence on para-benzoquinone on two different substrates, Ag(111) and graphene. We employ a combination of first-principles calculations and machine learning to identify the energetically most favorable structures on both substrates and study their electronic properties. Our results indicate that for the first layer, similar structures are favorable for both substrates. For the second layer, we find two significantly different structures. Interestingly, graphene favors the one with less, while Ag favors the one with more electronic coupling. We explain this switch in stability as an effect of the different charge transfer on the two substrates.
Highlights
Organic thin films are materials of increasing interest, mainly by virtue of their application to the field of organic electronics.In comparison to inorganic alternatives, they present advantages such as mechanical flexibility and low cost
By conducting a comparative molecular-orbital projected density of states (MODOS) analysis,[60,61] detailed in Figure 4b, we find that the lowest unoccupied molecular orbitals (LUMOs) and LUMO + 1 of the benzoquinone monolayer fall largely under the Fermi energy for Ag(111) but remain above it for graphene
It has been observed that, in analogous cases, one can find an inverse correlation between stability and the highest occupied molecular orbital (HOMO)−HOMO coupling, as a consequence of Pauli repulsion.[63−65] In our case, as we are interested in the response of the system to the introduction of additional electronic charge, we focus on the coupling between LUMOs
Summary
Organic thin films are materials of increasing interest, mainly by virtue of their application to the field of organic electronics.In comparison to inorganic alternatives, they present advantages such as mechanical flexibility and low cost. With a thickness ranging from less than a nanometer up to a few micrometers, organic thin films are commonly employed in the construction of organic field-effect transistors (OFETs),[1,2] organic light-emitting diodes (OLEDs),[3] and organic solar cells.[4] Of particular interest are films composed of molecules that form ordered structures with relatively high charge carrier mobilities.[5−7] the properties of molecular materials, and especially their charge carrier mobilities, depend drastically on the polymorph they assume, i.e., the relative arrangement of individual molecules in the thin film.[8,9]. Which polymorph a thin film forms depends on the fabrication conditions[10] and the nature of the substrate on which it grows has a decisive impact. The decisive role of the substrate is highlighted by reports, where even the same molecule forms different structures on different substrates.[16−18]
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