Sublimation-Induced Vapor Deposition of Cyanuric Acid-Melamine Supramolecular Single Crystals on Surfaces

Herein, a low-temperature sublimation-based vapor deposition (SVD) method is developed to synthesize hexagonal crystal plates of cyanuric acid-melamine (CAM) with outstanding crystallinity. Through meticulous design of the reaction apparatus and careful selection of source materials, substrate-confined SVD in a tube furnace is explored to grow single crystals of CAM in hexagonal shapes. Additionally, the orientation preference of the (202) facet is revealed, corresponding to the 2D arrangement of the H-bonded network, of single-crystalline plates on surfaces using selected area electron diffraction and X-ray diffraction techniques. By employing atomic force microscopy and scanning electron microscopy for topography characterization, a mechanism of three-stage step-growth crystallization is proposed, including nucleation, in-plane expansion, and out-of-plane growth. Furthermore, it is found that the interactions among melamine molecules in CAM synthesized via SVD are more intense compared to those in CAM synthesized via water-based methods, as evidenced by infrared and photoluminescent spectra studies. Subsequent nanoindentation tests on the (202) facet of CAM single-crystalline plates reveals a reduced modulus and hardness of 12.8 and 0.82 GPa, respectively. This methodology addresses the longstanding challenge of synthesizing hexagonal CAM single crystals and provides valuable insights for the fabrication of functional organic crystalline materials.