1. Could you explain the significance of your article to the non-specialist?

Some years ago, one of us was fortunate enough to obtain a binary mixture composed of a metallomesogen (a metal-containing LC) and TNF (2,4,7-trinitrofluoren-9-one) exhibiting a SmAb phase - a rather rare smectic-A modification with in-plane biaxiality (T. Hegmann et al., Angew. Chem. Int. Ed., 2001, 40, 887). We chose this particular mixture to demonstrate that scanning transmission x-ray microscopy (STXM) can contribute to and enhance our understanding of LC film textures by providing fundamental information on the ordering of the LC molecules in a SmAb phase at a spatial resolution not accessible by optical microscopy. The present study using STXM (and near edge x-ray absorption fine structure, NEXAFS) not only enabled us to confirm the phase assignment made previously based on polarized optical microscopy and x-ray diffraction, but allowed us to confirm the exclusive formation of one particular type of line defect (so-called ±½ ? disclinations) as predicted theoretically, and estimate an in-plane order parameter as a measure of the degree of in-plane biaxiality.

2. What has motivated you to conduct this work? 

Pierre-Gilles de Gennes (Nobel Prize in Physics, 1991) in his book The Physics of Liquid Crystals (3rd ed.) back in 1982 suggested the existence of a new lamellar liquid crystalline (LC) phase with molecules aligned orthogonal to the layer planes, in which the board-like shape of the constituent molecules would hinder rotation around the long molecular axis leading to phase biaxiality - a biaxial smectic-A phase (SmAb). However, in the over 20 years since its theoretical prediction, the SmAb phase has only been claimed for very few LC compounds or mixtures. In addition, some of these claims resulted in controversies mainly about the influence of boundary conditions (i.e. effect of surfaces), the nature and type of defects observed in thin film textures (so-called disclination), and in some instances about the phases' biaxiality itself. It occurred to us that STXM is capable of solving some of these issues.

3. Where do you see this work developing in the future? 

First off, as researchers working at Canadian Universities, we are looking forward to conducting STXM experiments at the recently commissioned Canadian Light Source's soft x-ray spectromicroscopy beamline, since all experiments in the present work were performed at the Advanced Light Source in Berkeley (to which we are grateful for granting beamtime). There are likely three important future directions for research on biaxial nematic (Nb) and smectic-A phase (SmAb) using STXM: 1) We are interested in studying the origin of and the molecular organization responsible for the intriguing stripe patterns that have been reported (by almost all groups) at the transition to the SmAb phase on cooling from a high-temperature phase, as this may answer important questions on SmAb phase formation. 2) We will try to take advantage of recent instrumental developments that will allow us to study the distribution of the anchoring of the molecules to the Si3N4 surfaces (windows) used for STXM. 3) Most importantly, we are looking forward to applying our methodology to other compounds or mixtures showing Nb or SmAb phases. Practically speaking, this is also a call for samples going out to researchers with potentially interesting samples having no access to synchrotron sources and STXM beamlines.

4. Are there any particular challenges facing future research in this area? 

It is expected that future instrumental developments will further contribute to an enhanced spatial resolution of this technique (currently ca. 50 nm) - so, for research interested in very small features such as LC nanoscience and nanotechnology as a whole, the resolution might still be a limiting factor. Considering the wealth of information STXM can provide such as alignment of mesogenic molecules on surfaces (e.g., rubbed alignment layers used in display applications) as well as molecular orientation in biaxial and other unique LC phases, future challenges (or factors limiting the use of STXM) are likely to occur only from limited access to synchrotron sources and soft x-ray beamlines, facilities requiring significant amounts of funding from government and the private sector.