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.
Keywords: chemical vapor deposition; graphite-diamond phase transformation; nanocrystalline diamond films.