We provide insights pertaining the dependence of undercooling in the formation of graphite, nanodiamonds, and Q-carbon nanocomposites by nanosecond laser melting of diamond-like carbon (DLC). The DLC films are melted rapidly in a super-undercooled state and subsequently quenched to room temperature. Substrates exhibiting different thermal properties—silicon and sapphire, are used to demonstrate that substrates with lower thermal conductivity trap heat flow, inducing larger undercooling, both experimentally and theoretically via finite element simulations. The increased undercooling facilitates the formation of Q-carbon. The Q-carbon is used as nucleation seeds for diamond growth via laser remelting and hot-filament chemical vapor deposition.