Thin films of non-glassforming liquid crystal: relaxation and vibrational dynamics

The progressive miniaturization of electronic and photonic devices has catalyzed growing scientific interest in the structural and functional behavior of ultrathin liquid crystal (LC) films. In this study, we present the first successful fabrication of ultrathin films of 4-hexyl-4’-isothiocyanatobiphenyl (6BT), a non-glassforming liquid crystal, using organic molecular beam deposition (OMBD) under room temperature conditions. This solvent-free, vacuum-based deposition technique enables precise control over film growth and molecular organization at the nanoscale.

Quantitative thickness measurements were performed using spectroscopic ellipsometry and X-ray reflectometry, allowing nanometer-resolution characterization of film morphology. Fourier-transform infrared (FTIR) spectroscopy revealed a distinct evolution of molecular ordering with increasing film thickness. At minimal thicknesses, we observe initial self-organization dominated by π–π stacking of aromatic biphenyl cores and van der Waals interactions among alkyl chains. As the film grows thicker, a significant degree of orientational ordering emerges among the isothiocyanate (-NCS) terminal groups, suggesting enhanced intermolecular cooperativity.

Complementary broadband dielectric spectroscopy (BDS) was employed to probe the dynamic response of the films, uncovering relaxation processes and vibrational dynamics that progressively shift toward bulk-like behavior with increasing thickness. These findings provide fundamental insight into structure–property relationships in confined liquid crystalline systems.

Our results offer a new platform for tailoring liquid crystal alignment, dynamics, and interfacial interactions in ultrathin geometries, opening promising avenues for the integration of anisotropic organic materials into next-generation nanoelectronic, photonic, and sensing technologies.