Abstract
In this article, some fundamental topics related to the initial steps of organic film growth are reviewed. General conclusions will be drawn based on experimental results obtained for the film formation of oligophenylene and pentacene molecules on gold and mica substrates. Thin films were prepared via physical vapor deposition under ultrahigh-vacuum conditions and characterized in-situ mainly by thermal desorption spectroscopy, and ex-situ by X-ray diffraction and atomic force microscopy. In this short review article the following topics will be discussed: What are the necessary conditions to form island-like films which are either composed of flat-lying or of standing molecules? Does a wetting layer exist below and in between the islands? What is the reason behind the occasionally observed bimodal island size distribution? Can one describe the nucleation process with the diffusion-limited aggregation model? Do the impinging molecules directly adsorb on the surface or rather via a hot-precursor state? Finally, it will be described how the critical island size can be determined by an independent measurement of the deposition rate dependence of the island density and the capture-zone distribution via a universal relationship.
Highlights
Thin films of organic semiconductors have attracted considerable interest in the recent past due to their promising applications in organic electronics
Some frequently discussed issues in the context of the nucleation and growth of films composed of rod-like organic molecules will be addressed: Which parameters define whether the island-like film is composed of lying or upright standing molecules? Is there a wetting layer between and underneath the islands? Can the nucleation and growth be described by the classical diffusion-limited aggregation model? The discussion of these issues is mainly based on experimental work performed in my group at the Graz University of Technology and on theoretical work carried out by Alberto Pimpinelli at the Rice University, Texas
We proposed that the experimental result should be described by another aggregation model rather than by diffusion-limited aggregation (DLA), namely via attachment-limited aggregation (ALA). [50]
Summary
Thin films of organic semiconductors have attracted considerable interest in the recent past due to their promising applications in organic electronics. Winkler physical vapor deposition techniques, many of the fundamental physical processes taking place during adsorption and organic thin film formation are still poorly understood The aim of this short review article is to summarize some of the most recent findings regarding the nucleation and growth of films composed of the rod-like organic molecules pentacene (C22H14), p-quaterphenyl (C24H18) and p-hexaphenyl (C36H26) on gold and mica substrates. These molecules are commonly used and increasingly relevant for applications in organic electronics. Some frequently discussed issues in the context of the nucleation and growth of films composed of rod-like organic molecules will be addressed: Which parameters define whether the island-like film is composed of lying or upright standing molecules? Is there a wetting layer between and underneath the islands? Can the nucleation and growth be described by the classical diffusion-limited aggregation model? The discussion of these issues is mainly based on experimental work performed in my group at the Graz University of Technology and on theoretical work carried out by Alberto Pimpinelli at the Rice University, Texas
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