Femtosecond laser-induced graphite structural patterning and surface gasification: a molecular dynamics study

Femtosecond lasers are superior to nanosecond lasers in graphite material manufacturing, due to their advantages in precise micro-nano-scale engraving. However, so far, most of the research studies have focused on monolayer graphene on hetero-substrates (such as silicon and glass). Few studies on laser absorption and propagation between graphene layers in a graphite material have been reported. Here, based on a two-temperature equation model inserted molecular dynamics (TTM-MD) simulation framework, we systematically studied the propagation process of femtosecond laser energy in a graphite material. The results show that, when the pulse energy density increases in the range of 1.6-3.2 J cm^-2, the absorption mechanism of the light intensity in the transverse direction (X- and Y-axes) is relative to the depth (Z-axis), and the morphology of the ablation cavity in the irradiated area changes from vertebral to hemispherical. During ablation above 4.8 J cm^-2, the energy spreads directly to a fixed layer, and the atoms in the irradiated region start to vaporize and discharge upward, driving the entire graphene layer to sublimate and peel off. Furthermore, we analyzed the gasification products at the graphite surface. As the pulse energy density increases in the range of 1.6-6.4 J cm^-2, the main vaporization products were transformed from single-carbon particles, dimers, and trimers to tetramers and pentamers. Eventually, during ablation above 4.8 J cm^-2, facilitated by the laser ablation plume, carbon chains comprising a dozen carbon atoms directly detached from the graphene layer and formed long carbon chains above C18. Our results could provide insights for elaborate micro-nano manufacturing of graphite and may stimulate research on laser-induced vapor phase deposition of carbon films such as graphene.