Perylene and its diimides : investigation of the influence of molecular diffusion on the thin film growth

• Perylen und seine Diimide : Untersuchung des Einflusses der molekularen Diffusion auf das Wachstum dünner Filme

Jodocy, Cathy; Wuttig, Matthias (Thesis advisor); Taubner, Thomas Günter (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Highly ordered organic thin films are required to realize novel electronics and optical applications for high-performance devices based on the class of organic materials. In this context, the present work focuses on the understanding of the processes involved in thin film growth. The aim is to control and optimize the formation of organic thin films. Thus, successful growth strategies are developed. The nucleation and growth of organic thin films is mainly examined with Atomic Force Microscopy (AFM). X-Ray Diffraction (XRD) methods are used to explore the crystalline structure.One way to improve the efficiency of devices with organic thin films as active layer, is to control the film growth by the application of dielectric surface modifications (DSMs). The energetics of the substrate surface are tailored by means of such organic molecules forming self-assembling monolayers (SAMs) or polymeric dielectrics. Often, the DSMs are chosen according to their tendency to change the surface free energy (SFE). Other parameters of the DSMs, affecting the morphology of the film on top, are not yet completely identified and understood. This insufficient comprehension hinders an elaborate choice of modifications. In the current thesis, the DSMs are chosen in a way that the SFE of the modified silicon substrates is analogously lowered. This offers the opportunity to precisely investigate the influence of parameters such as the morphology and roughness of the modified surface or a dipole at the interface, on the subsequent film growth. From previous growth studies it is known that lowering the surface free energy usually causes an enhancement of the molecular diffusivity on top of these surfaces. In addition, a high diffusivity is known to be correlated to the growth of large and well ordered islands, which results in good working devices. In this thesis, three different DSMs are investigated. On a polymeric modification with a homogeneous surface, highly ordered films are grown. On rough and irregular surfaces of another DSM, a certain diminution of the diffusion length of organic small molecules is clearly observed. Moreover, it is clearly shown, that the application of the investigated highly electronegative SAM inhibits an increase of the molecular diffusivity despite its low SFE. The interface dipole reduces the diffusion length of molecules. In addition to the investigation of surface modifications, an aim of this work is to understand the growth behavior of different small organic molecules. Therefore, perylene and some of its diimides are studied. The influence of the alkyl chain length of the N,N’-dialkyl perylene tetracarboxylic diimides (PTCDIs) is analyzed. For the different films formed, corresponding growth models are established. Perylene results as a reference system for 3-dimensional growth, also known as island growth. The diimides with the longest alkyl chain, PTCDI-C13, is a reference for highly 2-dimensional growth, which corresponds to layer-by-layer growth. It can be shown that for the PTCDIs, which are examined in this study, the alkyl chain length correlates with the growth type. The PTCDIs with long alkyl chains exhibit a clear layer-by-layer growth behavior with high structural order of the films, whereas, shorter molecules preferably grow 3-dimensionally. Increasing the substrate temperature during film deposition results in the increase in structural order within the thin films. Thus, the production of films with extremely high internal ordering has been successful.

Institutions

• Chair of Experimental Physics I A and I. Institute of Physics [131110]
• Department of Physics [130000]