Pierre Nacke | Searcharxiv

arXiv2025

Revealing Nanoscale Molecular Organization in Liquid Crystals via Cryogenic Atom Probe Tomography

While liquid crystals (LCs) have been extensively studied, obtaining a comprehensive nanoscale picture of their molecular organization remains challenging, as conventional techniques face an intrinsic trade-off between spatial and chemical resolution. Here, cryogenic atom probe tomography (cryo-APT) is introduced as a new analytical approach for LC materials, using 4'-Pentyl-4-cyanobiphenyl (5CB) and 4'-Octyl-4-cyanobiphenyl (8CB) as representative model compounds. This was enabled by a tailored cryogenic focused ion beam (cryo-FIB) protocol optimized for small organic molecules. The method enables controlled field evaporation of both intact molecules and diagnostic fragments, achieving over 90% molecular retention while preserving four characteristic dissociation patterns. By spatially correlating these fragmentation profiles with the local electric field derived from the tip geometry, we reveal field-directed dissociation pathways of CB molecules. In parallel, the distribution of intact molecular ions enables nanoscale visualization of material structure: we resolve homogeneous mixing of 5CB and 8CB in the nematic phase and directly observe the sub-nanometer crystalline layering in a supercooled 8CB sample, with contrast to the surrounding amorphous matrix suggesting the presence of a solid-liquid interface. This work establishes cryo-APT as a new powerful analytical platform for LC research and reveals its broad potential for application in soft matter systems.

cond-mat.mtrl-sci↗

arXiv2022

Observation of a uniaxial ferroelectric smectic A phase

We report the smectic $A_F$, a new liquid crystal phase of the ferroelectric nematic realm. The smectic $A_F$ is a phase of small polar, rod-shaped molecules which form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the $\sim 10$ Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field of about $2 \times 10^5$ V/m is observed. The smectic $A_F$ phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic ($N$) -- smectic $Z_A$ (Sm$Z_A$) -- ferroelectric nematic ($N_F$) -- smectic $A_F$ (Sm$A_F$) phase sequence; and 7N/DIO, exhibiting an $N$ -- Sm$Z_A$ -- Sm$A_F$ phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.

cond-mat.soft↗