Study Of The Mechanism Of Organic Molecular Film Growing On Metal Surfaces

The past decades witness the fast developing in the study of organic materials, which are believed to bear potential applications in electronic devices that are presently dominated by their inorganic counterpart, Si and Ge. Presently, the organic materials have been used in field effect transistor (OFET) and light-emitting device (OLED) as the active layer due to their unique electronic characters. One problem that hinders the application of organic materials in electronic devices is the low mobility of charge carriers in the film as well as between film and metal contact. It has been widely accepted that the mobility of charge carrier in the film can be significantly improved by improving the ordering of the film. It is therefore urgently important to get deeper understanding about the factors that affecting the self-assembly (SA) structure of organic molecules on metal surfaces, namely intermolecular interaction and molecule-substrate interaction. What are their roles and how they driving the SA process. In addition, the interface electronic structures of organic/metal system were also studied due to their importance in determining the injection barrier (hole and electron injection barrier) between organic film and metal contact.The main effort of this thesis is focused on the growth properties of planar aromatic molecules on metal surfaces and their interfacial electronic structures, aiming to understand the roles of interactions mentioned above in the film growth process. Especially, the intermolecular interaction was emphasized. However, the molecule-substrate interaction was also studied since the SA process is driven by both interactions. The role and its importance of intermolecular interaction in the SA process were investigated by series competitive studies by depositing molecules onto surfaces with different surface reaction activities.The first chapter is an introduction about the organic film growth and the interfacial electronic structures between adsorbed molecules and metal substrate. Except the general review regarding to the organic film growth on metal surface, the methods to determine the adsorbed structures and the factor affecting the interfacial electronic structures are also presented. The second chapter is a detailed description about the experimental instruction and calculation method, including scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS) and the first-principle calculation based on density functional theory (DFT).The experimental and theoretical results were presented in the following four chapters, in which two aspects regarding with the film growth process were discussed, namely SA structures and interfacial electronic structures. Three kinds of molecules were selected as candidates for these purposes, which are tetracene, perylene and fluorinated derivatives of perylene diimides.In the third chapter, the electronic structure of tetracene film on Cu (110) surface was investigated by a combination study of UPS and DFT calculation. The photoemission features originated fromπcharacterized molecular orbitals suffered obvious shift in energy position, indicating the strongπ-d interaction between tetracene and Cu (110) surface. The growth beyond the first layer undergoes a structural phase transition from flat-lying mode to upright-standing mode, due the weakening of molecule-substrate interaction and gradually dominating intermolecular interaction in the multilayer. This work showed strong evidence that the previous conclusion about tetracene aligning geometry on the Cu (110) surface is wrong, in which the structure with molecular plane perpendicular to the substrate surface is actually multilayer structure.The fourth chapter focuses on the structural study of tetracene adsorbed on Cu (100) surface, which is a four-fold symmetry surface. The molecule was observed to adopt lying-down mode rather than the upright-standing mode at monolayer regime if the molecules were deposited by keeping the substrate at room temperature. The long axis of the molecules were found to be aligned randomly along two identical high symmetry direction of<011> with the counting ratio of the molecules aligning along the two directions closes to unity. Since the intermolecular interaction in this system is much weaker with respect to the molecule-substrate interaction, no long-range ordered structures were observed. However, the ordered structure in some local region was observed showing the possibility of ordering growth motivated by intermolecular interaction.In the fifth chapter, the weak interacting system was concerned, namely perylene/Ag (110), which is turned out to be a proper candidate to study the role of intermolecular interaction in the SA process due the moderate molecule-substrate interaction. By a combination study of STM and DFT calculation, the competition between intermolecular interaction and molecule-substrate interaction was found to be dominated in the SA process, and the stable structure is a balance between these interactions.Finally in the sixth chapter, the electronic structure of three kinds of Fluorinated Derivatives of Perylene Diimides on Cu (110) surface had been studied by using UPS as well as DFT calculations. The difference in electronic structures of these molecules on metal surface was found to be attributed to the different aligning geometry with respect to the substrate surface, which is determined by different intensity of molecule-substrate interaction for these molecules.