Nanocrystal Fabrication of Highly Nonlinear Optical Organic Materials and Evolution of Polar Morphology

Abstract

3-Methyl-4-methoxy-4′-nitrostilbene (MMONS) exhibits pronounced second-order nonlinear optical effects, with its bulk crystals demonstrating efficient second-harmonic generation. Bulk MMONS crystals exhibit unidirectional growth in the [001] direction when grown from solution of polar solvents─such as methyl ethyl ketone, ethyl acetate, and acetone. This asymmetric growth may be ascribed to the differential attachment of the polar solvent molecules to the (001) and (001̅) surfaces, which is facilitated by the alignment of the dipoles of the solvent molecules at the solution-crystal interface along the Coulomb electric field emanating from both polar surfaces. Highly perfect MMONS nano- and microcrystals were fabricated using the reprecipitation technique coupled with various stirring methods, including magnetic stirring, vortex mixing, sonication, and piezoelectric nozzle application. Among these, the piezoelectric nozzle yielded the smallest particle size of approximately 3 nm, with a narrow size distribution. MMONS nanocrystals of up to 400 nm in size exhibited an isotropic morphology with a truncated rectangular parallelepiped shape, featuring eight slanted trapezoidal {111} faces at the corners of a cuboid, along with four {100} and {001} faces. Isotropic and polar morphologies coexist in crystals sized 1-2 μm, with the polar morphology becoming predominant in crystals larger than 3 μm. This transition was marked by unidirectional growth, which is due to the poisoning of the (001̅) face by solvent molecules, resulting in a truncated angular cone shape along the [001̅] direction. This morphological transformation converges toward a perfect angular cone shape in centimeter-long crystals while maintaining a normal growth morphology in the [001] direction. This research provides new insights into the morphological evolution of polar organic crystals, and is particularly relevant for applications in biological detection, pharmaceutical manufacturing, and nonlinear photonics.