Evaluation of microstructural evolution of glassy carbon induced by helium implantation and annealing

The effects of helium ion (He⁺²) implantation into glassy carbon (GC) were systematically investigated. He⁺² ions with an energy range of 17 keV were implanted into GC to fluences of 10¹⁶, 10¹⁷ and 10¹⁸ cm⁻² at room temperature (RT). The as-implanted GC samples were subsequently vacuum annealed at 300 °C, 500 °C, and 800 °C for 1 h. Structural evolution of GC was characterized using Raman spectroscopy and transmission electron microscopy (TEM). A fluence-dependent trend in displacement per atom (dpa) and He concentration was observed. Raman spectroscopy revealed progressive structural disorder and amorphization at fluences 10¹⁷ and 10¹⁸ cm⁻², marked by merging and redshifts of the D and G peaks, indicating tensile strain in the carbon matrix. Partial recovery of D/G peak separation and crystalline order was observed, especially at 800 °C for the 10¹⁶ cm⁻² fluence. TEM micrographs showed a confined damaged region of about 130 nm, with distinct defect aggregation towards the bulk for fluences of 10¹⁶ cm⁻² and 10¹⁷ cm⁻², whereas the defect aggregation appeared in two regions for the fluences of 10¹⁸ cm⁻². At this high fluence, bubble-like structures were observed upon annealing, indicating He accumulation and pressurisation within the carbon matrix. This observation reveals a nonlinear dispersion and saturation effect. The bubbles contributed to the localized distribution of the lattice structure. Overall, annealing at 800 °C facilitated partial microstructural recovery, particularly for samples implanted to fluences of 10¹⁶ cm⁻² and 10¹⁷ cm⁻².