Artificial Grain Alignment of Organic Crystalline Thin Films
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1150-RR06-05
Artificial Grain Alignment of Organic Crystalline Thin Films
Toshihiro Shimada Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
ABSTRACT
It is important to obtain single crystalline organic thin films for electronics and optics applications. Due to the mismatching in the crystal symmetry, it is difficult to align the crystalline grains of organic molecular films even on single crystalline surfaces. We have developed several techniques for the artificial grain alignment in organic epitaxial growth. (1) Use of nanoscale-textured surfaces prepared by step bunching of vicinally-cut single crystalline surfaces, in which the height of the steps is critically important. (2) Application of external electric field. (3) Optical excitation of the molecules which can be applied to the polar semiconducting molecules. These techniques might be applicable to other materials including ionic materials and ferroelectrics.
INTRODUCTION
For the application of the organic materials to electronics and photonics, the crystallinity and the alignment of grains will become more and more important. It is difficult to achieve highly ordered grains of organic crystals because the crystal structure of the organic materials has low symmetry and because there are many crystal polymorphs with similar thermodynamic stability. There are many reports of epitaxial growth of organic molecules on inorganic single crystals but it is not enough in most cases. Re-evaporation of the films on organic crystals with low cohesive energy is also a great problem because the annealing process in vacuum for the grain evolution, which is frequently used for the inorganic materials, cannot be used for the organic materials.
In this paper, we summarize the various attempts for the artificial grain alignment of organic thin films focusing on the results of our groups. The first technique is the use of the nanoscale-textured surfaces prepared by step bunching of vicinally-cut single crystalline surfaces. The height of the steps is critically important: it must be greater than the height of one molecular layer. The mechanism of the grain alignment cannot be explained only by thermodynamic considerations, and kinetic effects revealed by molecular beam experiments must be involved. The second technique is the application of external electric field. We found that there is threshold field strength for the grain alignment. The third one is the optical excitation of the molecules which can be applied to the polar semiconducting molecules. Some of them are specific to organic molecular materials, but we hope that there are hints for the grain alignment of other materials.
ALIGNMENT BY SUBSTRATE SURFACES WITH NANOSCALE TEMPLATES
First, we will discuss the effect of the morphology of the substrate surface. There are reports on the alignment of organic crystals or polymers by modifying the surface morphology, i.e. rubbing of the surface for liquid crystal devices, friction transfer of polymers [1] and deposition of small mole
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