College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China.
Proc Natl Acad Sci U S A. 2022 Apr 19;119(16):e2201451119. doi: 10.1073/pnas.2201451119. Epub 2022 Apr 11.
It is a key challenge to prepare large-area diamonds by using the methods of high-pressure high-temperature and normal chemical vapor deposition (CVD). The formation mechanism of thermodynamically metastable diamond compared to graphite in low-pressure CVD possibly implies a distinctive way to synthesize large-area diamonds, while it is an intriguing problem due to the limitation of in situ characterization in this complex growth environment. Here, we design a series of short-term growth on the margins of cauliflower-like nanocrystalline diamond particles, allowing us to clearly observe the diamond formation process. The results show that vertical graphene sheets and nanocrystalline diamonds alternatively appear, in which vertical graphene sheets evolve into long ribbons and graphite needles, and they finally transform into diamonds. A transition process from graphite (200) to diamond (110) verifies the transformation, and Ta atoms from hot filaments are found to atomically disperse in the films. First principle calculations confirm that Ta-added H- or O-terminated bilayer graphene spontaneously transforms into diamond. This reveals that in the H, O, and Ta complex atmosphere of the CVD environment, diamond is formed by phase transformation from graphite. This subverts the general knowledge that graphite is etched by hydrogen and sp3 carbon species pile up to form diamond and supplies a way to prepare large-area diamonds based on large-sized graphite under normal pressure. This also provides an angle to understand the growth mechanism of materials with sp2 and sp3 electronic configurations.
采用高温高压法和常压化学气相沉积（CVD）方法来制备大面积金刚石是一个关键挑战。与低压 CVD 中的石墨相比，热力学亚稳金刚石的形成机制可能暗示了一种独特的方法来合成大面积金刚石，但由于在这种复杂的生长环境中，原位表征的局限性，这仍然是一个有趣的问题。在这里，我们设计了一系列在菜花状纳米金刚石颗粒边缘的短期生长，使我们能够清楚地观察金刚石的形成过程。结果表明，垂直石墨烯片和纳米金刚石交替出现，其中垂直石墨烯片演变成长丝带和石墨针，最终转化为金刚石。从石墨（200）到金刚石（110）的转变过程验证了这种转变，并且从热灯丝中发现 Ta 原子在薄膜中原子分散。第一性原理计算证实，添加 Ta 的 H 或 O 终止双层石墨烯会自发转化为金刚石。这表明在 CVD 环境中的 H、O 和 Ta 复合气氛中，金刚石是通过石墨的相变形成的。这颠覆了一般的认识，即石墨是被氢蚀刻的，sp3 碳物种堆积形成金刚石，并为在常压下基于大尺寸石墨来制备大面积金刚石提供了一种方法。这也为理解具有 sp2 和 sp3 电子构型的材料的生长机制提供了一个角度。
