Narrowing the dimensions of electrified interfacial systems from the microscopic to the nanometer scale leads to unexpected effects. The morphology of nanometer-scale thin films on a defined substrate is an important parameter as it strongly influences the transport properties of charge carriers in thin films. At an organic/metal or an organic/organic interface, the morphological phase and the manner how the molecules interact with the substrate are a determining factor for the energies needed to inject charge carriers into the thin film. Scanning probe microscopy is a powerful method for characterizing surface morphologies with nanometer-scale spatial resolution. Electronic bulk and interfacial properties are detectable when the tip of a scanning tunneling microscope in constant-current mode penetrates an organic thin film (z-V spectroscopy). In this way, the injection energies for positive charge carriers (holes) into the highest occupied molecular orbitals and of negative charge carriers (electrons) into the lowest unoccupied molecular orbitals can be determined. From z-V spectroscopy data, it is possible to derive the so-called single particle energy gap (E_gsp), which is a measure of the energy gap for bipolar charge injection at the interface. By combining E_gsp with the optical adsorption gap (E_a) of the material, the exciton binding energy (E_b) can be determined. The model for an organic/metal interface applied most often assumes the existence of a common vacuum level (CVL). Measurements were carried out on sublimated thin films of monomers on gold substrates. The results reveal differences to the CVL model, which are due to the formation of a dipole layer at the interface. This affects charge-carrier transport and the injection energy, as well as triggers charge injection into optically inactive molecular orbital states, which can result in a smaller E_gsp than optical band- gap measurements predict.
Keywords: Muller, Mueller
By: P. Müller, L. Rossi and S.F. Alvarado
Published in: RZ3481 in 2003
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