Abstract
Organic semiconductor devices rely on the movement of charge at and near interfaces, making an understanding of energy level alignment at these boundaries an essential element of optimizing materials for electronic and optoelectronic applications. Here we employ low temperature scanning tunneling microscopy and spectroscopy to investigate a model system: two-dimensional nanostructures of the prototypical organic semiconductor, PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) adsorbed on NaCl (2 ML)/Ag(111). Pixel-by-pixel scanning tunneling spectroscopy allows mapping of occupied and unoccupied electronic states across these nanoislands with sub-molecular spatial resolution, revealing strong electronic differences between molecules at the edges and those in the centre, 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 neighbouring molecules. 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.
Highlights
Organic semiconductor devices rely on the movement of charge at and near interfaces, making an understanding of energy level alignment at these boundaries an essential element of optimizing materials for electronic and optoelectronic applications
Recent studies using scanning tunnelling microscopy (STM) and spectroscopy (STS), alongside complementary techniques, on mixed donor–acceptor monolayers on metallic and semi-metallic surfaces have sought to characterize their electronic structure as model interfaces for organic photovoltaic materials[22,23,24,25]
Studies of the electronic structure of PTCDA monolayers and thin films on metal substrates have shown that molecular energy levels can shift by 100s of meV due to a variety of effects including small differences in the hydrogen bond (H-bond) lengths between inequivalent adsorption sites[16,35,36,37,38,39], and stabilization of charge by polarization of the surroundings as a function of film thickness and distance from an interface[13]
Summary
Organic semiconductor devices rely on the movement of charge at and near interfaces, making an understanding of energy level alignment at these boundaries an essential element of optimizing materials for electronic and optoelectronic applications. Pixel-bypixel scanning tunneling spectroscopy allows mapping of occupied and unoccupied electronic states across these nanoislands with sub-molecular spatial resolution, revealing strong electronic differences between molecules at the edges and those in the centre, 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 neighbouring molecules. Recent studies using scanning tunnelling microscopy (STM) and spectroscopy (STS), alongside complementary techniques, on mixed donor–acceptor monolayers on metallic and semi-metallic surfaces have sought to characterize their electronic structure as model interfaces for organic photovoltaic materials[22,23,24,25] In these examples, energy level shifts in the electronic states of both species arise from a combination of intermolecular and molecule-substrate interactions. Pixel-by-pixel STS allows us to probe the energy levels of electronic states with sub-molecular spatial resolution, revealing the influence of the abrupt change in the local chemical and electrostatic environment at the edges of nanoislands
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