Spherulitic growth and morphology control of lithium carbonate: the stepwise evolution of core-shell structures

Bio-Inspired Crystalline Core-Shell Guanine Spherulites

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture is discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent (Δ n ≈ 30 % $\Delta n \approx \ 30\%$ ) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, the synthesis of core-shell spherulites composed of guanine crystal platelets ((Δ n ≈ 25 % $\Delta n \approx 25\%$ ) is presented in a two-step emulsification process in which a water/oil/water emulsion and induced pH changes are used to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, β-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, it is found that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenic particles.

Polymer-directed crystallization of HMX to construct nano-/microstructured aggregates with tunable polymorph and microstructure

Multi-scale structural control of the high explosive HMX was achieved by a polymer-directed crystallization strategy. For the first time, HMX spherulites that exhibited a “Maltese cross” extinction pattern under crossed polarizers were obtained.

Growth mechanism of the spherulitic propylthiouracil–kaempferol cocrystal: new perspectives into surface nucleation

A growth mechanism of spherulitic cocrystals shows three stages of single crystal growth, non-crystallographic branching and surface nucleation. Low supersaturation forms loose spherulites, and high temperature causes “spherulites on spherulites”.