Liquid crystals for fast switching devices

An international team has investigated a newly synthesized liquid-crystalline material that promises applications in optoelectronics. Simple rod-shaped molecules with a single center of chirality self-assemble into helical structures at room temperature. Using soft X-ray resonant scattering at BESSY II, the scientists have now been able to determine the pitch of the helical structure with high precision. Their results indicate an extremely short pitch at only about 100 nanometres which would enable applications with particularly fast switching processes.

Liquid crystals are not solid, but some of their physical properties are directional – like in a crystal. This is because their molecules can arrange themselves into certain patterns. The best-known applications include flat screens and digital displays. They are based on pixels of liquid crystals whose optical properties can be switched by electric fields.

Some liquid crystals form the so-called cholesteric phases: the molecules self-assemble into helical structures, which are characterised by pitch and rotate either to the right or to the left. “The pitch of the cholesteric spirals determines how quickly they react to an applied electric field,” explains Dr. Alevtina Smekhova, physicist at HZB and first author of the study, which has now been published in Soft Matter.

Read more on the HZB website

Image: The photo shows the cells on the modified sample holder which was used in the real experiment. This modified sample holder is mounted within the ALICE chamber at BESSY II.

Credit: © A. Smekhova/HZB

A kappa diffractometer for intermediate X-ray energies at APS beamline 29-ID

An ultra-high vacuum, non-magnetic kappa geometry diffractometer has been designed and commissioned for the resonant soft x-ray scattering (RSXS) branch of the X-ray Science Division (XSD) Intermediate Energy X-ray (IEX) beamline 29-ID at the Advanced Photon Source (APS). Beamline 29-ID is managed by the XSD Magnetic Materials Group; the APS is an Office of Science user facility at Argonne National Laboratory. There were three main design goals for this diffractometer: kappa geometry, non-magnetic, and high-precision. The kappa geometry was chosen to allow for a large q-range and space for a sample environment (electric or magnetic fields). Non-magnetic components were used for all the components above and including the κ-arm to avoid disturbing magnetic or electric fields during experiments. Lastly, the diffractometer precision requirement of a sphere of confusion (SOC) of less than 50 µm was a key driving factor for this instrument in terms of rotation stages and machining precision.

The complete diffractometer can be seen in Fig. 1(a), shown installed into the RSXS UHV vacuum chamber at 29-ID. The precise SOC (< 50 µm) requirement drove the design method. In order to reach this goal, it was decided that a combination of precision machining, Finite element analysis, and stage precision would be used instead of calibrating an error-correction table. This has the advantage that the upper bound of the SOC requirement can be achieved without any control hardware, making the device more robust.

Read more on the Advanced Photon Source Website

Image: Fig1. (a) Image of the commissioned kappa diffractometer inside the RSXS vacuum chamber on the APS 29-ID beamline with the main components identified. (b) A close up model of the components above the f axis. The model also shows the new thermal break and thermal strap.