Nonconfinement growth of edge-curved molecular crystals for self-focused microlasers

Abstract

Synthesis of single-crystalline micro/nanostructures with curved shapes is essential for developing extraordinary types of optoelectronic devices. Here, we use the strategy of liquid-phase nonconfinement growth to controllably synthesize edge-curved molecular microcrystals on a large scale. By varying the molecular substituents on linear organic conjugated molecules, it is found that the steric hindrance effect could minimize the intrinsic anisotropy of molecular stacking, allowing for the exposure of high-index crystal planes. The growth rate of high-index crystal planes can be further regulated by increasing the molecular supersaturation, which is conducive to the cogrowth of these crystal planes to form continuously curved-shape microcrystals. Assisted by nonrotationally symmetric geometry and optically smooth curvature, edge-curved microcrystals can support low-threshold lasing, and self-focusing directional emission. These results contribute to gaining an insightful understanding of the design and growth of functional molecular crystals and promoting the applications of organic active materials in integrated photonic devices and circuits.

Fig. 1.. Rational molecular modulation.

Molecular structures of (A) DPAVBi and (B) BDAVBi. The simulated growth morphology of (C) DPAVBi microcrystals and (D) BDAVBi microcrystals based on attachment energy. Optical microscopy images of (E) DPAVBi microcrystals and (F) BDAVBi microcrystals. Scale bars, 20 μm.

Fig. 2.. Solvent-antisolvent synergistic effect.

(A) Intermolecular interactions measured in eye-shaped BDAVBi microcrystals. (B) Molecular arrangement viewed perpendicular to the top face of eye-shaped BDAVBi microcrystals. (C) Morphological evolution of BDAVBi crystals predicted by molecular dynamics simulation based on the interaction between crystals and antisolvents. (D) Optical microscopy images of BDAVBi microcrystals grown using different antisolvents. Scale bar, 15 μm.

Fig. 3.. Preparation of eye-shaped microcrystals.

(A) SEM image of eye-shaped microcrystals. Scale bar, 20 μm. (B) TEM image and SAED patterns of a single eye-shaped microcrystal. Scale bar, 10 μm. (C) Fluorescence microscopy image of eye-shaped microcrystals. Scale bar, 50 μm. (D) AFM topography and height diagram of an eye-shaped microcrystal. (E) Fluorescence microscopy images from the top face and bottom face of an eye-shaped microcrystal.

Fig. 4.. Low-threshold lasing.

(A) Emission spectra of eye-shaped microcrystals as a function of pump energy. Inset: Corresponding PL image. Scale bar, 10 μm. a.u., arbitrary units. (B) Emission spectra of stadium-shaped microcrystals as a function of pump energy. Inset: Corresponding PL image. Scale bar, 10 μm. (C) PL intensity of eye-shaped (red squares) and stadium-shaped microcrystals (blue squares) around the mode peak 495 nm as a function of pump energy. (D) Plot and fitted curve of mode spacing around the mode peak 495 nm versus reciprocal of L value. (E) Q factors of eye-shaped microcrystals with various L values.

Fig. 5.. Self-focused directional emission.

(A) Optical microscopy image of the eye-shaped microcrystal with L/S = 3. (B) Simulated electric field distribution in the eye-shaped microcavity with L/S = 3 in the x-y plane. The dashed red curves in (A) and (B) indicate the same boundaries. (C) PL images from the top face and bottom face of the eye-shaped microcrystal excited uniformly with a pulsed laser. Scale bar, 5 μm. (D) Far-field emission pattern from the top face and bottom face of the eye-shaped microcavity with L/S = 3. (E) Angle-resolved PL spectrum of the eye-shaped microcavity. Excitation wavelength is 400 nm. (F) Far-field emission patterns of eye-shaped microcavities with various L/S values fitted by the Lorentzian function. (G) Laser divergence angle from eye-shaped microcavities with a similar L/S value of ~3.