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Single molecule perspectives of model organic semiconductors : energy level mapping by high-resolution scanning probe microscopy Cochrane, Katherine Anne


Organic semiconductors are a promising class of materials for many applications such as photovoltaics, light emitting diodes, and field-effect transistors. As these devices rely on the movement of charge at and near interfaces, understanding energy level alignment at these boundaries is essential to improve device performance. Differences in the local environment and surrounding molecular geometry have the potential to cause significant energy level shifts occurring on single molecule length scales, thus affecting device properties. Scanning Probe Microscopy is a family of techniques that allows investigation of materials on the molecular and submolecular level. Scanning Tunneling Spectroscopy (STS) allows for the mapping of electronic states with spatial and energetic resolution. Electrostatic Force Spectroscopic (EFS) mapping investigates the local charge distribution of surfaces even down to submolecular resolution. We utilize these techniques to investigate the prototypical semiconductors PTCDA and CuPc on NaCl(2ML)/Ag(111). Nanoislands of PTCDA were examined with STS, revealing strong electronic differences between molecules at the edges and those in the center, with energy level shifts of up to 400 meV. We attribute this to the change in electrostatic environment at the boundaries of clusters, namely via polarization of neighboring molecules. To further investigate the local electrostatics, we use EFS to probe the effect of adding charge to PTCDA molecules, both isolated and within clusters. We found that the charging energy depends on the initial local charge distribution by spatially resolving the charging events with sub-molecular resolution. In order to investigate the influence of interface geometry, we use pixel-by-pixel STS of the prototypical acceptor/donor system PTCDA/CuPc. We observe shifting of the donor and acceptor states in opposite directions, indicating an equilibrium charge transfer between the two. Further, we find that the spatial location of electronic states of both acceptor and donor is strongly dependent on the relative positioning of both molecules in larger clusters. The observation of these strong shifts illustrates a crucial issue: interfacial energy level alignment can differ substantially from the bulk electronic structure in organic materials. This has significant implications for device design, where energy level alignment strongly correlates to device performance.

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