Recently, Gu Jianmin, etc. from the team of Professor Wang Desong from School of Environmental and Chemical Engineering of YSU, in cooperation with Institute of Chemistry of Chinese Academy of Sciences (ICCAS), have broken through the limitation of anisotropic growth of organic molecular crystals, realized the controllable synthesis ofedge-curvedmolecular single-crystal structure, filled the technical gap of curved single-crystal growth and detected the self-focusing characteristic of organic microlasers. The research results, titled "Nonconfinement growth of edge-curved molecular crystals for self-focused microlasers", were published online on October 21, 2022 in Science Advances, a sub-issue of Science. Paper link:https://www.science.org/doi/10.1126/sciadv.abn8106.
Microdisk lasers have attracted wide attention by virtue of their potential applications in laser display, high-throughput sensing and on-chip optical communication. By breaking the rotational symmetry of the microdisk laser, the edge-curved microcavity can effectively concentrate electromagnetic energy in a specific direction, and realize controllable and high-power output of optical signals. Most deformed microcavities are fabricated with the amorphous materials or inorganic semiconductors by high-precision etching techniques such as photolithography or electron beam etching. Compared with traditional optical gain materials, organic micro/nano crystals show not only the good solution processability and molecular designability, but also the excellent broadband gain performance due to their abundant energy level and excited state processes. However, due to the self-limiting growth of organic crystals, the crystals will spontaneously grow into symmetrical convex geometric polyhedrons, so the synthesis of high-quality edge-curved organic micro/nano crystals is still a technical barrier. The anisotropy of crystal shape is mainly determined by the inherent anisotropy of its stacking mode and the interaction between the basic units. Usually, more complex the structure of molecular units is, lower the geometric symmetry is. Therefore, most molecular crystals have stronger anisotropy than atomic crystals in macroscopic morphology. How to adjust the interaction between molecules and the stacking mode of molecules through molecular design and optimization of crystal synthesis conditions, break the growth anisotropy of molecular crystals, and realize the controllable synthesis of edge-curved molecular crystals has become an important scientific issue in the field of organic micro-nano materials.
For the efficient and controllable preparation of "eye-shaped" edge-curved organic single crystals, the research team proposed an "unrestricted" growth strategy in liquid phase to regulate the solvent-antisolvent synergistic effect with molecular structure. 4,4 '-Bis [4-(di-p-tolylamino) styryl] biphenyl (DPAVBi) and 4,4'-Bis [4-(diphenylamino) styryl] biphenyl (BDAVBi), two molecules with the same backbone but different substituents, are selected as model compounds owing to their outstanding excited-state gain properties. Compared with DPAVBi, the phenyl steric hindrance of BDAVBi molecule is lower, which can effectively avoid π-π interaction to inhibit the tight anisotropic stacking in the crystals, thus promoting the emergence of abundant high-index crystal planes at the crystal edge, and then forming a quasi-continuous curved surface. In the process of liquid self-assembly, controlling the supersaturation of molecules can further regulate the growth rate of crystal planes,making for the formation of smooth curved edges and the regulation of the curvature of edges. The "eye-shaped" microcrystals prepared by the research team can be used as deformed microcavities, and simultaneously realize the self-focusing emission of low-threshold laser and high-Q mode laser. These results are helpful to understand the design and synthesis of functional molecular crystals, and promote the application of organic active materials in integrated nano-photonic chips.
This research has obtained the financial assistance from the National Key R&D Plan (2018YFA0704802) and the Natural Science Foundation of Hebei (B2020203013). The co-first authors of the paper are Yin Baipeng and Liang Jie, and the correspondent authors are researcher Zhang Chuang (ICCAS), associate professor Gu Jianmin (YSU) and researcher Zhao Yongsheng (ICCAS).
The above picture shows the directional emission of edge-curved molecular crystal for the self-focusing laser. (A) Optical microscopic image of "eye-shaped" microcrystals. (B) Simulated electric field distribution of "eye-shaped" microcrystals in x-y plane. (C) PL images of the top and bottom faces of "eye-shaped" microcrystals. (D) Far-field emission diagram of the top and bottom faces of "eye-shaped" microcrystals. (E) Angle-resolved PL spectra of "eye-shaped" microcrystals. (F) Far-field emission diagrams of "eye-shaped" microcrystals with different length-width ratios fitted by Lorentzian function. (G) Laser divergence angle distribution of "eye-shaped" microcrystals.