Black phosphorus (BP) is widely applied to semiconductor technology, optoelectronics, field-effect transistors, and flexible electronic devices due to its tunable bandgap, high carrier mobility, excellent switching ratio, and perfect thermal conductivity. As an allotrope of phosphorus, BP has a layered structure and is more stable than white phosphorus (WP) and red phosphorus (RP). Currently, the primary method for synthesizing BP is chemical vapor transport (CVT). The CVT method has strong controllability and the ability to synthesize multiple materials and is thus widely employed in the synthesis of semiconductor materials and the manufacturing of optoelectronic devices. Although the orthogonal BP preparation is relatively mature, the CVT method introduces mineralizers, which results in the presence of impurities in the synthesized BP. These impurities may affect the performance and application of BP. Therefore, further purification processes are needed. We combine the CVT method with seed crystal technology to investigate the growth of high-quality and large-size BP.

The high-quality and large-size BP is successfully prepared by changing the seed crystal number. The specific experimental steps are as follows. RP, Sn, and SnI₄ are weighed at a ratio of 50∶2∶1 in a glove box to prepare the experimental materials, followed by vacuum sealing. The sealed quartz tube is horizontally placed in a muffle furnace and heated at around 620 ℃. The temperature is maintained for five hours to completely melt RP, then slowly drops to 500 ℃ within six hours, and is maintained at 500 ℃ for six hours. The rise and fall programs of the temperature are ended when the temperature slowly lowers to 450 ℃. After the muffle furnace is cooled to room temperature, the quartz tube is removed. Then the obtained BP crystals are cleaned by ultrasonic treatment with anhydrous ethanol and then dried in a vacuum drying oven. A small piece of dried BP with a flat size of 0.5 mm×0.2 mm is adopted as the seed crystal. Subsequently, the influence of seed crystal quantity on the growth of BP is investigated. The annealing temperature of BP is set to 595 ℃. RP, Sn, and SnI₄ are placed at the bottom of the quartz tube, while different numbers of seed crystals are placed at the other end. The sealed quartz tube is horizontally positioned in the muffle furnace, which ensures that the RP end is close to the thermocouple and the seed crystal end is far from the thermocouple.

BP without seed crystals has a lateral size of 1.2 cm and a darker metallic luster. However, the lateral size of BP with seed crystals is 1.8 cm. Meanwhile, BP with seed crystals has a brighter metallic luster, which is attributed to the fact that seed crystals can induce BP growth on the surface and increase BP crystallinity. When the seed crystal numbers are 0, 1, 2, and 3, the lateral sizes of BP are 1.2 cm, 1.8 cm, 0.5 cm, and 0.3 cm respectively. The shape of the prepared BP is irregular when the number of seed crystals is greater than 2. Therefore, the size and shape of BP vary with the number of seed crystals. Fewer seed crystals can lead to larger BP sizes, while more seed crystals can form smaller BP. The BP morphology can be controlled to a certain extent by adjusting the number of seed crystals. However, too many seed crystals can result in growth competition between crystals, forming uneven crystal sizes or irregular shapes. When the crystal seed number is 1, BP has the largest size. Furthermore, TEM, SEM, XRD, Raman spectrometer, and XPS analyses of BP with and without seed crystals show that BP prepared by the seed crystal method does not change its orthogonal structure, with fewer impurities and higher crystallinity. Finally, the absorption spectrum and electrochemical impedance measurement show that BP with seed crystals has better light absorption ability and lower electrochemical impedance in the visible light range than BP without seed crystals. This indicates that BP prepared by the seed crystal method has higher crystallinity and fewer defects, with enhanced light absorption efficiency and electron transport capability.

We employ a combination of the CVT method and seed crystal method to prepare BP crystals with a length of 1.8 cm and a width of 1.1 cm. The obtained BP is analyzed for morphology and phase. BP with seed crystals has a larger lateral size than that without seed crystals. The absorption spectrum and electrochemical impedance tests indicate that BP with seed crystals has higher light absorption efficiency and better electron transport capability. When the crystal seed number is 1, BP has the best performance. Finally, we provide references and guidance for the preparation of high-quality, large-size BP crystals.