Methods to induce a preferred molecular alignment in the epitaxial growth of n-alkane thin films

Abstract

The development of general techniques to define and control the molecular orientation at the nanoscale can improve the efficiency of some functional materials, such as organic electronic devices, waveguides and liquid crystal based devices. The main aim of this project is to develop methods to define and control the orientation and ordering of molecules in the epitaxially grown organic thin films. There are a number of factors such as field effects (magnetic and electric), temperature, and substrate crystal lattice orientation, which can be tailored to achieve the molecular orientation control. The first method to control the molecular orientation control was based upon the concept of liquid crystal imprinting (LCI) technique developed by the Patrick group (Western Washington University, USA) in which an oriented monolayer n-alkane film was grown by epitaxy on a liquid crystal (LC) film on a graphite substrate, where the LC film was initially oriented by using a magnetic field. The first goal of this research work was to reproduce this LCI work for oriented n-alkane multilayer films. A LC film can act as a sacrificial template and the magnetic field can initially align LC thin films on the graphite substrate. Then, n-alkanes deposited by physical vapour deposition can diffuse through the LC and replace the oriented LC molecules while preserving their orientation, leading to oriented n-alkane films. The second goal of research project is to develop a new method for controlling the molecular orientation of n-alkane films. The idea of LCI was modified and in the new method LC films are oriented with an electric field. Then, thin films of n-tetracontane (C40, C40H82) are subsequently deposited on the electrically oriented LC by physical vapour deposition. In this process, the oriented LC film acts as a sacrificial template and highly oriented n-alkane films are obtained. In this research work, the results of Patrick et al.1 were reproduced and the idea of LCI was extended to electric field alignment for obtaining highly oriented n-alkane films.