Thin films molecular alignments

Thin-film technology is replete with acronyms and several of the main ones you will encounter are listed in Table 28.1. Molecular beam epitaxy (MBE) is the only acronym that assumes a crystalline structure and alignment of the thin film. Molecular-beam evaporation would be much better, especially when used to grow amorphous films ... 

S.A. Lee, Y. Yoshida, M. Fukuyama, and S. Hotta, Phenyl-capped oligothiophenes novel light-emitting materials with different molecular alignments in thin films, Synth. Met., 106 39-43, 1999. 

S. Hotta, Y. Ichino, Y. Yoshida, and M. Yoshida, Spectroscopic features of thin films of thiophene/ phenylene co-oligomers with vertical molecular alignment, J. Phys. Chem. B, 104 10316-10320, 2000. 

K. Aoki, T. Seki, M. Sakuragi, and K. Ichimura, Reversible alignment change of liquid crystals induced by photochromic molecular films, Part 15. Convenient methods to prepare command surfaces by surface-selective modifications of poly(vinyl alcohol) thin films with azobenzene units, Makromol. Chem. 193, 2163-2174 (1992). 

The investigation of the degree of molecular alignment in the thin films was performed by AFM measurements in contact as well as in non-contact mode. Figure 5.6a and b presents contact AFM images of the spin-coated DCNDBQT film on Ti02. 

In field-effect transistors based on the thin films of poly(3-alkyl)thiophene derivatives, high field-effect mobility has been observed because of the formation of crystal-like grains [116]. The mobility depends strongly upon the crystallinity of the thin films. Introduction of liquid crystallinity into conjugated polymers is expected to be effective for enhancing microscopic crystallization of conjugated polymers to increase their carrier mobilities. It can also cause anisotropy in various physical properties if macroscopic molecular alignment is achieved. 

The liquid crystallinity of poly(3-alkylthiophene) has been pointed out [121]. Enhancement of crystallinity by molecular alignment in a liquid crystal phase is prominent in poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b] thiophene) exhibiting a lamellar mesophase. When a thin film of the polymer fabricated by the spin-coating method is annealed at 100 °C, domain size remarkably extends from several tens of nanometers to a several micrometers, maintaining molecular alignment within domains because of the thermal motion of the polymer chains. The hole mobility of the thin film transistors based on this polymer reached 0.7 cm2 V-1 s 1 [122]. 

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