Dynamic magnetic field alignment and polarized emission of semiconductor nanoplatelets in a liquid crystal polymer

ARTICLE https://doi.org/10.1038/s41467-022-30200-2 OPEN Dynamic magnetic field alignment and polarized emission of semiconductor nanoplatelets in a liquid crystal polymer 1234567890():,; Dahin Kim1, Dennis Ndaya2,3, Reuben Bosire2, Francis K. Masese2, Weixingyue Li4, Sarah M. Thompson5, Cherie R. Kagan 4,5,6, Christopher B. Murray4,6, Rajeswari M. Kasi2,3 & Chinedum O. Osuji 1 ✉ Reconfigurable arrays of 2D nanomaterials are essential for the realization of switchable and intelligent material systems. Using liquid crystals (LCs) as a medium represents a promising approach, in principle, to enable such control. In practice, however, this approach is hampered by the difficulty of achieving stable dispersions of nanomaterials. Here, we report on good dispersions of pristine CdSe nanoplatelets (NPLs) in LCs, and reversible, rapid control of their alignment and associated anisotropic photoluminescence, using a magnetic field. We reveal that dispersion stability is greatly enhanced using polymeric, rather than small molecule, LCs and is considerably greater in the smectic phases of the resulting systems relative to the nematic phases. Aligned composites exhibit highly polarized emission that is readily manipulated by field-realignment. Such dynamic alignment of optically-active 2D nanomaterials may enable the development of programmable materials for photonic applications and the methodology can guide designs for anisotropic nanomaterial composites for a broad set of related nanomaterials. 1 Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA. 2 Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA. 3 Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA. 4 Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA. 5 Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA. 6 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA. ✉email: NATURE COMMUNICATIONS | (2022)13:2507 | https://doi.org/10.1038/s41467-022-30200-2 | www.nature.com/naturecommunications 1 ARTICLE C NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-30200-2 omposite materials constituted by nanoparticles (NPs) dispersed in liquid crystals (LCs) have attracted interest as a new type of stimuli-responsive functional material1–3. This interest is driven by the potential to use the stimuliresponsive director field of LCs to control the positional and orientational order of NPs with useful properties4–6. Positional order can be programmed by carefully exploiting interparticle interactions driven by director field distortion, and the tendency of NPs to localize at topological defects in the LC7–9. Orientational order likewise is imposed by the director field coupling to the structural anisotropy of the NP10–16. The anchoring condition of LC mesogens at the surface of NPs plays a critical role in dictating the resulting interactions and director field coupling that lead to positional and orientational control, respectively. In practice, however, the realization of such stimuli-responsive nanocomposites is hampered greatly by the difficulty of producing stable dispersions of NPs in LCs above vanishingly small volume fractions. Dispersion stability is diminished by energetic penalties associated with any director field distortion due to the NP, and the excess free energy that arises due to chemical incompatibility of the NP surface with the LC medium. If there is director field distortion, the LC host expels NPs to minimize the total elastic energy17–19, and unfavorable surface interactions result in phase separation of NPs unless the interactions are offset entropically20. Minimizing or eliminating director field distortion requires that the anchoring condition is matched to the symmetry of the NP. Homeotropic anchoring (i.e. perpendicular to the surface) of rod-like mesogens at any curved surface mandates the formation of a topological defect, and splay deformation of the mesophase. Planar anchoring (i.e. parallel to the surface) on a sphere, or along the circumference of a cylinder or rod, likewise induces distortion of the LC. Homeotropic and planar anchoring at flat surfaces, and planar anchoring parallel to the long axis of a cylindrical or rodlike NP inclusion, can be accomplished with little or no director field distortion. Minimizing the energetic penalty associated with the chemical incompatibility of the LC and the NP can be accomplished by modifying the NP surface chemistry using appropriate ligands. The director distortion and surface energy contributions to dispersion stability are coupled as the mesogen anchoring, particularly at smooth surfaces, is strongly influenced if not completely dictated by NP surface chemistry. Surface conditions to satisfy both together are often restricted as the surface anchoring and elastic energetic costs are competing each other. Prior reports have demonstrated spatial localization of spherical NPs in highly dilute dispersion in nematic fluids21–25 and orientational order in dispersions of anisotropic nanomaterials, particularly nanorods10–16. This limited success to date in the development of functional NP-LC materials is properly viewed in the context of the challenging dispersion problem discussed above. Here, our focus is centered on controlling orientational order of two-dimensional (2D) nanomaterials, and specifically, CdSe nanoplatelets (NPLs). CdSe NPLs present unique anisotropic optoelectronic features due to strong quantum confinement along their thickness direction, and their atomically uniform thickness enables narrow photoluminescence linewidth compared to other CdSe nanomaterials26,27. The development of CdSe NPLs assemblies with controlled orientation paves the way to harnessing their orientation-dependent optical properties in useful ways; their properties of interest include linearly polarized emission in the “edge-on” orientation and enhanced light extraction in the “faceon” configuration28–30. Native ligands with long alkyl tails (i.e. oleic acid) of CdSe NPLs can provide a strong perpendicular boundary condition at the flat NPL-LC interface and the anchoring condition could 2 minimize director field distortion by spontaneously aligning NPLs’ normal parallel to the director31. However, in spite of the relatively weaker director distortion at flat surfaces, there are few examples of 2D NP-LC composites including high contents of particles, and no reports on CdSe NPLs6,10,32. 2D nanomaterials present additional challenges regarding dispersion in LCs as strong interparticle interactions (e.g. van der Waals, hydrophobic, and depletion attractions) between extended flat faces easily lead to aggregation33. Destabilization factors in a 2D NP-LC system needs to be further studied and the processing pathway should be desi (...truncated)