Laser-induced graphitized periodic surface structure formed on tetrahedral amorphous carbon films

Abstract

Femtosecond laser-induced periodic surface structure (LIPSS), graphitization and swelling observed on ultra-hard, hydrogen-free tetrahedral amorphous carbon (ta-C) films are examined and compared with those on hydrogenated amorphous carbon (a-C:H) films, nitride films, and glassy carbon plates. The threshold fluence for LIPSS formation on ta-C is approximately twice as high as that for other specimens, and the LIPSS period Λ near the threshold is very fine at ca. 80 nm. Λ gradually increases with increasing fluence, and rapidly increases to ca. 600 nm at a high fluence. The ablation rate also increases rapidly at this fluence. In addition, ta-C and a-C:H are graphitized by irradiation and expand in volume. The surface layer of ta-C film changes to nanocrystalline graphite as the fluence increases and the crystallinity is improved; however, at higher fluence, the crystallinity deteriorates suddenly similar to that at low fluence. At high fluence, the rapid increase in Λ and the ablation rate, and the sudden deterioration in crystallinity are determined as common phenomena for these disordered carbons. LIPSS formation and swelling over a large area by scanned spot irradiation produces submicron height flat hills with conductivity and surface functionality on the insulating surface.