Energy-level and structural arrangement of organic/metal and organic/organic interfaces

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Physics, School Of Engineering and Mathematical Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Organic molecules provide unique electronic properties that offer immense possibilities to future electronic device industries. Semiconductor materials consisting of molecular building blocks can be customised with distinct functional properties and have already found their way into state-of-the-art devices for applications such as optoelectronics and photovoltaics. Further advancements in the performance and efficiencies of these devices necessitates a thorough understanding of the interfacial energy level alignment and interfacial structure resulting from surface transfer doping. The fluorofullerenes are very efficient molecular acceptors on account of their high electron affinity, providing a model molecular system for the study of surface doping of underlying materials because the doping process may proceed with submonolayer fluorofullerene coverages.The thesis presents synchrotron based photoemission investigations that elaborate the energy level alignment of fluorofullerene molecules on zinc-tetraphenylporphyrin (ZnTPP) and copper-phthalocyanine (CuPc) films. High-resolution core level photoelectron spectroscopy was used to identify and to follow the evolution with coverage of two distinct charge states of adsorbed fluorofullerene molecules: charged molecules that participate in the charge transfer process and neutral molecules that do not. The relative intensity of the two molecular species was used to determine the molecular doping efficiency and applied to develop an integer charge transfer model to quantify the doping process and explore the organic energy level alignment. These experiments and the resulting doping model were further extended to consider the energy level structure at the interface of C60F48 and graphene. Here, intrinsically n-doped epitaxial graphene on Si(0001) and neutral ’quasi-free standing’ graphene were shown to develop p-type conductivity upon fluorofullerene deposition. Complementing the knowledge of the energy level structure at fluorofullerene interfaces, low temperature scanning tunnelling microscopy (STM) was used to explore the structural behaviour of C60F48-ZnTPP and C60F48-Au(111) interfaces. For the case of the C60F48-ZnTPP interface, the fluorofullerene adlayer was found to be randomly arranged and the underlying ZnTPP film ordered for the case of multilayer films. For the C60F48-Au(111) interface, ordered adlayers of C60F48 were identified, including a new, distinct orientationally ordered ’floral’ state of C60F48.