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Anomalous coarsening driven by reversible charge transfer at metal-organic interfaces.

ABSTRACT: The unique electronic properties and functional tunability of polycyclic aromatic hydrocarbons have recently fostered high hopes for their use in flexible, green, portable, and cheap technologies. Most applications require the deposition of thin molecular films onto conductive electrodes. The growth of the first few molecular layers represents a crucial step in the device fabrication since it determines the structure of the molecular film and the energy level alignment of the metal-organic interface. Here, we explore the formation of this interface by analyzing the interplay between reversible molecule-substrate charge transfer, yielding intermolecular repulsion, and van der Waals attractions in driving the molecular assembly. Using a series of ad hoc designed molecules to balance the two effects, we combine scanning tunnelling microscopy with atomistic simulations to study the self-assembly behavior. Our systematic analysis identifies a growth mode characterized by anomalous coarsening that we anticipate to occur in a wide class of metal-organic interfaces and which should thus be considered as integral part of the self-assembly process when depositing a molecule on a conducting surface.

SUBMITTER: Della Pia A

PROVIDER: S-EPMC4278690 | biostudies-literature | 2014 Dec

REPOSITORIES: biostudies-literature

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Anomalous coarsening driven by reversible charge transfer at metal-organic interfaces.

Della Pia Ada A Riello Massimo M Floris Andrea A Stassen Daphne D Jones Tim S TS Bonifazi Davide D De Vita Alessandro A Costantini Giovanni G

ACS nano 20141204 12

The unique electronic properties and functional tunability of polycyclic aromatic hydrocarbons have recently fostered high hopes for their use in flexible, green, portable, and cheap technologies. Most applications require the deposition of thin molecular films onto conductive electrodes. The growth of the first few molecular layers represents a crucial step in the device fabrication since it determines the structure of the molecular film and the energy level alignment of the metal-organic inter ...[more]

PMID: 25419580

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Project description:In any physicochemical process in liquids, the dynamical response of the solvent to the solutes out of equilibrium plays a crucial role in the rates and products: the solvent molecules react to the changes in volume and electron density of the solutes to minimize the free energy of the solution, thus modulating the activation barriers and stabilizing (or destabilizing) intermediate states. In charge transfer (CT) processes in polar solvents, the response of the solvent always assists the formation of charge separation states by stabilizing the energy of the localized charges. A deep understanding of the solvation mechanisms and time scales is therefore essential for a correct description of any photochemical process in dense phase and for designing molecular devices based on photosensitizers with CT excited states. In the last two decades, with the advent of ultrafast time-resolved spectroscopies, microscopic models describing the relevant case of polar solvation (where both the solvent and the solute molecules have a permanent electric dipole and the mutual interaction is mainly dipole-dipole) have dramatically progressed. Regardless of the details of each model, they all assume that the effect of the electrostatic fields of the solvent molecules on the internal electronic dynamics of the solute are perturbative and that the solvent-solute coupling is mainly an electrostatic interaction between the constant permanent dipoles of the solute and the solvent molecules. This well-established picture has proven to quantitatively rationalize spectroscopic effects of environmental and electric dynamics (time-resolved Stokes shifts, inhomogeneous broadening, etc.). However, recent computational and experimental studies, including ours, have shown that further improvement is required. Indeed, in the last years we investigated several molecular complexes exhibiting photoexcited CT states, and we found that the current description of the formation and stabilization of CT states in an important group of molecules such as transition metal complexes is inaccurate. In particular, we proved that the solvent molecules are not just spectators of intramolecular electron density redistribution but significantly modulate it. Our results solicit further development of quantum mechanics computational methods to treat the solute and (at least) the closest solvent molecules including the nonperturbative treatment of the effects of local electrostatics and direct solvent-solute interactions to describe the dynamical changes of the solute excited states during the solvent response.

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Anomalous coarsening driven by reversible charge transfer at metal-organic interfaces.

Publications

Anomalous coarsening driven by reversible charge transfer at metal-organic interfaces.

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