Upon quenching, the crystallization process immediately starts in the top relatively cold part of the cell resulting in a disordered layer of ZnO polycrystals. Further, these polycrystals act as seeds for the crystal growth. Upon further temperature decrease, the ZnO crystals continue to grow along the temperature gradient forming a batch of needle-like crystals. ZnO crystals were determined from powder and single-crystal X-ray diffraction data (Table ).