Nonequilibrium Structural Evolution of Q-Carbon and Interfaces

Q-carbon is a densely packed metastable phase of carbon formed by ultrafast quenching of carbon melt in a super-undercooled state. After quenching, diamond tetrahedra are randomly packed with >80% packing efficiency. This discovery has opened a pathway to fabricate various interesting heterostructures following the highly nonequilibrium route of nanosecond pulsed laser annealing. In the present work, we demonstrate the evolution of Q-carbon/α-carbon and Q-carbon/diamond heterostructures with atomically sharp interfaces, controlled via varying solidification rates of the undercooled C melt. This structure consists of ultrahard Q-carbon (∼80% sp³ and rest sp²) with an overlayer of soft α-carbon (∼40% sp³) on the inert c-Al₂O₃ substrate. Using high-resolution scanning transmission electron microscopy and Raman spectroscopy analysis, we present the formation of the highly dense Q-carbon/α-carbon bilayer structure with distinctly different atomic and electronic structures. The laser-solid interaction simulations coupled with atomistic ab initio modeling further confirm the conversion of C melt into Q-carbon by achieving maximum undercooling near the substrate and further into α-carbon with a decrease in regrowth velocity (<6 m/s) away from the substrate. We present details of the evolution of heterointerfaces formed from carbon melt for designing heterostructures far from equilibrium for various functional applications by using pulsed laser processing.