The role of the interface in organic thin film growth

If electronic devices based on organic materials are ever to have a significant impact in the marketplace there will be a need to control interface formation and film growth, as is the case at present with inorganic semiconductor devices. Unfortunately, the basic physical principles governing organic thin film growth and crystallisation needed to provide guidance for further development and optimisation are not well understood. The aim of this project is to provide an understanding of the role of the interface in organic film growth . We will investigate the balance between the layer-substrate interactions and the intermolecular interactions in growing crystalline films (i.e interface bonding versus packing energies). The novel approach which we will use will involve tailoring interfacial energies to manipulate the growth pattern. We have recently shown that this strategy is feasible in our preliminary investigations of sexiphenyl on Al(111). This will be achieved by controlled studies of growth in ultra-high vacuum conditions on clean and modified single crystal surfaces using a variety of modern surface science techniques. In this work we also wish to bridge the gap between conventional surface science studies of initial interface formation, which tend to stop at a few molecular layers (<100 Angstrom), and conventional thin film studies (> 1000 Angstrom).

In this project a deep understanding of the initial stages of organic on inorganic interface formation, and its role in the subsequent growth of organic films was achieved. This has allowed us to grow crystalline device relevant organic films of superior quality with different molecule and crystallite orientations. Technologically this is important, as if devices based on organic materials are ever to have a significant impact in the marketplace there will be a need to control interface formation and film growth, as is the case at present with inorganic semiconductor devices. The wider scientific importance of being able to control organic film growth is that it allows the unambiguous determination of the opto-electronic properties of the films one the one hand, while on the other it is also a necessary prerequisite for the controlled studies of organic-organic heteroepitaxy. In the project organic film growth from submonolayer coverages up to device relevant thickness for a number of molecules on a variety of controlled substrates with a large palette of state of the art techniques probing both electronic and geometric structure, molecular orientation and film morphology was investigated. Although the details of the film growth was found to be molecule-substrate specific the wide range of organic-inorganic substrate systems investigated allowed a number of fallacies prevalent in the literature to be exposed and more global rules regarding organic film growth to be developed. For instance, whether crystalline films of upright molecules or lying molecules grow was found to be princibly determined by the degree of long range atomic order of the substrate, rather than the reactivity of the substrate to the molecule, i.e. the growth was dominated more by growth kinetics than by thermodynamics. The subtle role of the molecule-substrate interaction and its influence on the resulting monolayer template film for further film growth of uniaxially oriented molecules lying parallel to the substrate was also investigated. A weak interaction to the substrate leads to attractive intermolecular interactions which yields highly ordered commensurate organic interfacial layers which are similar to layers in the bulk crystal. Stronger molecule-substrate interactions with charge transfer, result in repulsive intermolecular interactions that disturb the molecules ability to self-assemble in the first wetting monolayer. The resulting wetting monolayer is very different from any bulk plane and is a poor template for further growth. For optimal crystalline thin films, substrate surfaces are required that give the molecules a preferred diffusion direction without strongly perturbing their ability to organize into a near bulk crystal plane. Nevertheless small deviations from the bulk crystal structure can be adopted by the molecules to a certain extent due to their unique property to release stress by tilting.