Researchers Probe Banana Helical Nanofilaments with Resonant X-Rays (December 2015)

In a collaboration with Center scientists, researchers at the Advanced Light Source in Berkeley have made the first in situ measurement of the pitch of helical nanofilaments (HNFs) in the B4 phase of bent-core liquid crystals using linearly polarized, resonant soft X-ray scattering at the carbon K-edge. A strong, anisotropic scattering peak corresponding to the half-pitch of the twisted smectic layer structure was observed. The equilibrium helical half-pitch of NOBOW is found to be 120 nm, essentially independent of temperature. However, the helical pitch can be tuned by mixing guest organic molecules with the bent-core host, followed by thermal annealing.

Previous attempts to probe the helical structure of the B4 HNF phase using conventional hard X-ray scattering met with limited success because such techniques are only sensitive to electron-density modulation, which is minimal along the axis of a continuous helix. To address this challenge, researchers applied linearly polarized, resonant soft X-ray scattering in which the scattering contrast is not only sensitive to X-ray energy near absorption edges but, because of the coupling between the linear polarization of the X-rays and the molecular bond orientations, also depends on the orientation of the molecule with respect to the X-ray polarization. The measured pitches are in reasonable agreement with the results of freeze-fracture transmission electron microscopy.

resonantB4a

resonantB4be

(a) NOBOW molecular structure, phase sequence, and the simplified structural model of an HNF, where the gray surfaces are the smectic layer surfaces. The smectic layers stack together and twist into an HNF. (b-e) Resonant soft X-ray scattering from drop-cast films of NOBOW in chlorobenzene. Experiments were performed with horizontally and vertically polarized X-rays (red arrows), at two different X-ray energies. The intensities are shown on a log scale. Cleary, the scattering contrast increases dramatically near the carbon K-edge (284.2 eV). The sample was at room temperature.

This work was published in C. Zhu, C. Wang, A.T. Young, F. Liu, I. Gunkel, D. Chen, D. Walba, J. Maclennan, N. Clark, and A. Hexemer, "Probing and Controlling Liquid Crystal Helical Nanofilaments," Nano Lett. 15, 3420 (2015).

This experiment is a collaboration between scientists at the Advanced Light Source at the Lawrence Berkeley Laboratory and the SMRC. Lead author Chenhui Zhu is a Center graduate.

Read the LBL press release.

 

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