Nanostructure engineering of superhard nano-polycrystalline diamond by compressing different fullerene precursors

Nano-polycrystalline diamond (NPD) is an important material with great application potential in various fields, including tool machining, high-pressure science, etc. Due to the strong covalent bonding structure, introducing controllable nanostructures, such as dislocations and twinned boundaries, into NPD to tune its properties remains challenging and our understanding of the underlying mechanism is also limited. In this work, we discovered a fundamentally important factor/mechanism that influences the formation of nanostructures in NPD, i.e., the reactivities of C–C bonds on fullerene cages affect the formation of intermediate phases and thus the final formed NPD. Our experiments and simulations reveal that the lower reactivity of C–C bonds on C₇₀ cages leads to more ordered graphitic carbon formation, while C₆₀ tends to amorphization under the same HPHT. This results in more complex twinning and stacking faults in the synthesized NPD from C₇₀ than C₆₀ through different transition mechanisms via the intermediate phases. The as-synthesized NPD samples from different fullerenes thus exhibit tunable hardness (85.5–101.7 GPa) and optical properties. Our findings provide new insights into the formation mechanism of diamond nanostructures and propose a new strategy to tune the nanostructures of the synthesized NPD for harder and stronger materials.