Substrate-induced phases: transition from a liquid-crystalline to a plastic crystalline phase via nucleation initiated by the substrate

Abstract

The presence of substrate-induced phases in thin films is an intriguing phenomenon with the physical and chemical factors responsible for their formation are not yet clearly understood. In this article, we present the structure and morphological changes associated with a substrate-induced phase in a discotic liquid crystal. The thin films of the discotic molecule are characterised by a combination of various X-ray diffraction methods to investigate the structural properties. Atomic force microscopy and polarised optical microscopy are used to determine the thin film morphologies. This is the first experimental proof of the presence of a substrate-induced phase in discotic liquid crystal, although they are known in thin films of molecular crystals. Moreover, we sought to decipher how the substrate-induced phase behaves with the evolution of time, temperature and changes at the interfaces. This work gives a unique example where the two-dimensional liquid-crystalline phase converts to a three-dimensional crystal plastic phase because of nucleation caused by the solid substrate over a time scale of a month or longer.

Keywords:

• liquid crystals
• plastic crystal
• subtrate-induced phase
• X-ray diffraction
• discotics

Acknowledgements

Financial support from the Marie Curie IIF Scheme of the 7th EU Framework Program for the DISCO-project (project no. 298319) and from the ARC program of the Communauté Française de Belgique (Grant No. 20061) is kindly acknowledged. We thank Dr I. Salzmann for his help in GIXD measurements and analysis. We thank Dr S. Napolitano and B. Vanroy for providing the Al-substrates.