Nanoscale Properties of Low-Dimensional Crystalline Organic Semiconductor Films

Book excerpt: The self-assembly and optoelectronic properties of model crystalline organic semiconductor films was studied by atomic force microscopy (AFM) techniques. Small molecule organic semiconductors serve as model systems for the active materials in organic electronic devices. Applications such as organic solar cells and light-emitting diodes rely on organic polymers and small molecules for their properties but the performance of these organic devices could still yet be improved compared to the inorganic-based devices. The aim of this work is to study different structure-property relationships in model organic systems to gain a better understanding for designing organic electronic material. Other spectroscopic and structural techniques are used to complement the spatial mapping capability of AFM, providing a more comprehensive view of the fundamental processes governing organic semiconductor films. First, self-assembled oligothiophenes with different surface functionalization are studied for the role humidity has on the electronic properties of a monolayer film. In-situ AFM and x-ray photoelectron spectroscopy (XPS) show that the water vapor is found to change the electronic properties of films with hydrophilic surface termination groups leaving hydrophobic films unaffected. Next, different indigo small molecules are self-assembled at the air-water interface into crystalline structures. The role of intermolecular interactions is found to play a critical role in the indigo crystal morphology. The self-assembled indigo crystals are studied by photoluminescence (PL) spectroscopy revealing the presence of H-aggregate formation during self-assembly. Further studies of the electronic properties of the indigo crystal films are performed using electrical AFM techniques and field-effect transistors. Finally, a scheme for the fabrication of flat field-effect transistors using graphene photolithography is presented. Graphene field-effect transistors are fabricated and tested providing a platform to study more accurately thin organic semiconducting films. This dissertation demonstrates the advantage of studying model systems of organic semiconductors with nanoscale precision with the aim of designing better performing organic electronic devices.