▎ 摘　　要

We report on the fabrication of various nanoscale carbon structures (Q-carbon, diamond, alpha-carbon, and reduced graphene oxide) by controlling the quenching rates of undercooled molten carbon. Laser annealing thin amorphous carbon films at energy densities above a threshold results in the formation of molten carbon. The photon-solid interactions of a nanosecond laser with amorphous carbon are utilized to control the heating of Al2O3 substrate, by limiting the thickness of deposited amorphous carbon. On annealing ultrathin carbon films, extensive substrate heating occurs, which reduces the quench rates of molten carbon, resulting in liquid-phase regrowth of reduced GO. The thicker amorphous carbon lowers Al2O3 heating, providing enough undercooling to form microdiamonds, nanodiamonds, and Q-carbon films with a subsequent rise in undercooling. From the high-resolution FESEM imaging, Raman spectroscopy, and EBSD investigations, the first-order phase transformation of amorphous carbon into single-crystalline < 110 > oriented nanodiamonds is established, which is associated with ultrafast unseeded crystallization. The laser-solid interaction modeling confirmed the phase transformation of molten carbon into Q-carbon by achieving maximum undercooling near the substrate, and subsequently into alpha-carbon, and diamond on lowering the regrowth velocity (<6 m/s) away from the substrate. These findings shall open up the pathway toward selective amorphous-carbon-phase transformation into nanoscale Q-carbon, nanodiamonds, and graphene materials for radiofrequency device applications.