Harnessing honeycombs

The driving force of the research was to enable the miniaturisation of electronic circuits. Currently, small electronic circuitry (such as that found on microchips) is made by a process called lithography, where light is used to shape and alter the materials deposited on a wafer of silicon. However, the size of the resulting circuit elements cannot be smaller than the wavelength of light. Alternatives to light and UV lithography use X-ray or electron beams, but such processes are cumbersome and expensive.

Scientists from Martin Luther University Halle-Wittenberg in Germany, the University of Sheffield, and Zhejiang Sci-Tech University in China came together to solve this problem from a different angle: by making molecules that could form their own patterns. In this way, miniature circuits could be built up using liquid crystals that aggregate in certain ways to confer unusual properties.

Models showing the organisation of the aromatic cores of the molecules in three different liquid crystal superstructures (lower) with the molecular models of their corresponding compounds (upper).

Rings and chains

The team synthesised a series of amphiphilic molecules that comprised aromatic rings with flexible side chains that varied in length and form. These molecules self-assembled to form liquid crystal superstructures that were analysed at Diamond with the application of X-ray diffraction at I22 (the Small Angle Scattering and Diffraction beamline) and grazing-incidence diffraction at I16 (the Materials and Magnetism beamline).

Professor Ungar of the study team explained how the synchrotron was essential for the research: “The structures were very large, so they had a very small angle of diffraction. We needed to use a very high resolution technique to visualise our complex structures.”

Surprisingly, the team saw that the aromatic rods stacked up to form the walls of unique honeycomb structures and the side chains sought to occupy the space inside the honeycomb channels. The side chains were the key players in this process. By adjusting the volume of the side chains, the resulting shapes of the honeycombs could be fine-tuned to generate channels with hexagonal, square, triangular or other shapes.

Unique liquid crystals

The idea behind the current work was to make the hexagonal honeycomb less ‘comfortable’, to introduce ‘packing frustration’, and thus force the liquid crystal to adopt a more complex structure. Therefore instead of using linear side chains, branched chains were attached with the same total volume but with shorter length. This meant that such chains could not reach the centre of the channels, and so the molecules had to find another solution to avoid creating empty space in their structure. When the compounds with branched side chains were analysed at Diamond, two new structures were discovered: an unusual complex honeycomb with a mixed pattern of pentagonal and hexagonal channels and, for even shorter side chains, a mixed pentagonal-octagonal honeycomb. The latter came as a real surprise, until it became apparent that secondary bundles of rods formed within the centres of the large octagonal channels. This intriguing observation meant that the rods spanned the structure in multiple directions.

For the first time ever, a liquid crystal had formed that contained rods oriented both perpendicular and parallel to the direction of the axis. With this unique organisation, it could be possible to fine-tune the liquid crystal to adjust its optical, magnetic or electrical properties, which would mean it might be used as a bespoke nanosensor, microchip, or electrode. This discovery alone is of great importance, but it also demonstrates a novel method for complex soft-matter self-assembly. The strategy of enforced packing frustration could open up a realm of future liquid crystal innovations.

Prof Ungar explained how the group intend to advance these discoveries: “We haven’t yet investigated the addition of ‘dopant’ molecules, which could enter the channels and adjust the properties of the liquid crystals. We could add guest substances to really expand the potential of these compounds, e.g. for use as highly selective sensors, or chemically switchable nanopatterning agents.”

To find out more about using I22 or I16, please contact the respective Principal Beamline Scientists: Prof Nick Terrill on I22 (nick.terrill@diamond.ac.uk), or Prof Steve Collins on I16 (steve.collins@diamond.ac.uk).

Poppe S et al. Zeolite-like liquid crystals. Nat Communications (2015). DOI:10.1038/ncomms9637