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Nanoporous media exhibit structures significantly smaller than the wavelengths of visible light and can thus act as photonic metamaterials. Their optical functionality is not determined by the properties of the base materials, but rather by tailored, multiscale structures, in terms of precise pore shape, geometry, and orientation. Embedding liquid crystals in pore space provides additional opportunities to control light–matter interactions at the single-pore, meta-atomic scale. Here, we present temperature-dependent 3D reciprocal space mapping using synchrotron-based X-ray diffraction in combination with high-Resolution birefringence experiments on disk-like mesogens (HAT6) imbibed in self-ordered arrays of parallel cylindrical pores 17 to 160 nm across in monolithic anodic aluminium oxide (AAO). In agreement with Monte Carlo computer simulations we observe a remarkably rich self-assembly behaviour, unknown from the bulk state. It encompasses transitions between the isotropic liquid state and discotic stacking in linear columns as well as circular concentric ring formation perpendicular and parallel to the pore axis. These textural transitions underpin an optical birefringence functionality, tuneable in magnitude and in sign from positive to negative via pore size, pore surface-grafting and temperature. Our study demonstrates that the advent of large-scale, self-organised nanoporosity in monolithic solids along with confinement-controllable phase behaviour of liquid-crystalline matter at the single-pore scale provides a reliable and accessible tool to design materials with adjustable optical anisotropy, and thus offers versatile pathways to finetune polarisation-dependent light propagation speeds in materials. Such a tailorability is at the core of the emerging field of transformative optics, allowing, e.g., adjustable light absorbers and extremely thin metalenses.
The phase behavior and molecular ordering of hexakishexyloxy triphenylene (HAT6) DLC under cylindrical nanoconfinement is studied utilizing differential scanning calorimetry (DSC) and dielectric spectroscopy (DS), where cylindrical nanoconfinement is established through embedding HAT6 into the nanopores of anodic aluminum oxide membranes (AAO), and a silica membrane with pore diameters ranging from 161 nm down to 12 nm. Both unmodified and modified pore walls were considered, and in the latter case the pore walls of AAO membranes were chemical treated with n octadecylphosphonic acid (ODPA) resulting in the formation of a 2.2 nm thick layer of grafted alkyl chains. Phase transition enthalpies decrease with decreasing pore size, indicating that a large proportion of the HAT6 molecules within the pores has a disordered structure, which increases with decreasing pore size for both pore walls. In the case of the ODPA modification the amount of ordered HAT6 is increased compared to the unmodified case. The pore size dependencies of the phase transition temperatures were approximated using the Gibbs Thomson equation, where the estimated surface tension is dependent on the molecular ordering of HAT6 molecules within the pores and upon their surface. DS was employed to investigate the molecular ordering of HAT6 within the nanopores. These investigations revealed that with a pore size of around 38 nm, for the samples with the unmodified pore walls, the molecular ordering changes from planar axial to homeotropic radial. However, the planar axial configuration, which is suitable for electronic applications, can be successfully preserved through ODPA modification for most of the pore sizes.
The vibrational density of states of a series of homologous triphenylene-based discotic liquid crystals HATn (n = 5, 6, 8, 10, 12) depending on the length of the aliphatic side chain is investigated by means of inelastic neutron scattering. All studied materials have a plastic crystalline phase at low temperatures, followed by a hexagonally ordered liquid crystalline phase at higher temperatures and a quasi isotropic phase at the highest temperatures. The X-ray scattering pattern for the plastic crystalline phase of all materials shows a sharp Bragg reflection corresponding to the intercolumnar distance in the lower q-range and a peak at circa 17 nm-1 related to intracolumnar distances between the cores perpendicular to the columns as well as a broad amorphous halo related to the disordered structure of the methylene groups in the side chains in the higher q-range. The intercolumnar distance increases linearly with increasing chain length for the hexagonal columnar ordered liquid crystalline phase. A similar behaviour is assumed for the plastic crystalline phase. Besides n = 8 all materials under study exhibit a Boson peak. With increasing chain length, the frequency of the Boson peak decreases and its intensity increases. This can be explained by a self-organized confinement model. The peaks for n = 10, 12 are much narrower than for n = 5, 6 which might imply the transformation from a rigid system to a softer one with increasing chain length. Moreover the results can also be discussed in the framework of a transition from an uncorrelated to a correlated disorder with increasing n where n = 8 might be speculatively considered as a transitional state.