Deterministic topological defects and quantum dot assembly in a nematic liquid crystalline medium

Abstract

Liquid crystal (LC) materials exhibit interesting electro-optic switching and molecular ordering properties. Furthermore, the addition of chemically compatible active emitters such as core-shell quantum dots (QD) in a LC medium allows optical as well as dielectric tuning in an anisotropic, reconfigurable ordered medium. Order in a nematic LC phase is characterized by an orientational order parameter. In this work, we demonstrate the use of patterned substrates to generate arrays of integer topological defects in a nematic LC medium doped with cadmium selenide (core) cadmium sulfide (shell) core–shell QDs. We demonstrate the formation of metastable air-pillar-induced integer topological defects (TDs) in relatively thinner 9 μm LC sandwich cells, and the formation of field-induced TDs in thicker (25 μm) cells. Simultaneously, the self-assembly of core-shell QDs into square arrays on the patterned substrates is discussed, highlighting potential electro-optic device applications. The surfactant hexadecyltrimethylammonium bromide (CTAB) is found to play a significant role in LC TD formation as well as QD spatial organization at the optimized concentration. Self-assembly and ordering of single- and multi-component LCs within structured devices is a highly relevant problem for modern optoelectronic devices. This work opens new possibilities for classical as well as quantum light sources which require spatially ordered optical emitters in a reconfigurable dielectric medium at a micron-scale.

Electro‐Optic Topological Defect Devices Utilizing Nematic Liquid Crystal Binary Mixtures

“…The defect array formation is a result of multiple contributing factors including a difference of electric field magnitude above and below the SU-8 well walls due to dielectric shielding, field-induced drift of ionic dopant CTAB and boundary conditions imposed by the SU-8 patterns. [14] As the concentration of CTAB is increased, TD arrays (four dark brushes) become well defined and regular as seen in the 5CB LC samples doped with 0.5 wt% (Figure 2b) and 1 wt% (Figure 2c) doped CTAB. The presence of CTAB induces EHDI as the charge carriers move rapidly under the influence of the AC field and LC molecules attain poly-domain orientation as the field intensity is increased.…”

“…[24,25] Under an external electric field, the LC molecules experience a torque due to dielectric anisotropy and simultaneously the motion of free ionic species through this realigned medium can lead to a frustrated texture sustained by EHDI. [26] Previous work [14] demonstrated that the application of an oscillating electric field to 5CB-filled, CTAB-coated patterned devices leads to the formation of regular TD arrays. However, the exact role of CTAB was not clear due to its incorporation as a spincoated layer on the substrate, primarily added as an alignment layer to generate homeotropic alignment boundary conditions at the surface.…”

“…Previous work [ 14 ] demonstrated that the application of an oscillating electric field to 5CB‐filled, CTAB‐coated patterned devices leads to the formation of regular TD arrays. However, the exact role of CTAB was not clear due to its incorporation as a spin‐coated layer on the substrate, primarily added as an alignment layer to generate homeotropic alignment boundary conditions at the surface.…”

“…[10] LC systems in particular provide an excellent test bed for studying as well as manipulating TDs in matter. [11][12][13] It has been demonstrated that TDs in LCs can be used to control molecular self-assembly, nanoparticles, colloidal particles, [14][15][16] and the motion of active matter such as swimming bacteria. [13] Furthermore, arrays of electrically tunable LC TDs can be applied in next-generation spatial light modulators, diffraction gratings, and soft photonic devices.…”

“…[20] In a previous study, 5CB LC was filled into patterned substrates that were spin-coated with the surfactant hexadecyltrimethylammonium bromide (CTAB). The application of an external electric field generated a regular array of the TDs, [14] that were hypothesized to be a result of ion-induced electrohydrodynamic instabilities (EHDI) effects in the patterned regions. However, there was little understanding of the active role of surface adhered CTAB as well as effect of LC properties on the observed TD arrays.…”