Effect of annealing on Raman spectra of monolayer graphene samples gradually disordered by ion irradiation

Raman scattering spectra (RS) of two series of monolayer graphene samples irradiated with various doses of C⁺ and Xe⁺ ions were measured after annealing in a high vacuum and in forming gas (95%Ar + 5%H₂). It is shown that annealing below 500 °C leads to a significant decrease in both the D-line, associated with defects, and the 2D-line, associated with the intact lattice structure. This can be explained by annealing-induced enhanced doping. Further annealing in a vacuum up to 1000 °C leads to a significant increase in the 2D-line together with a continuous decrease in the D-line. This gives evidence for the partial removal of the defects and restoration of the damaged lattice. Annealing in forming gas is less effective in this sense. A blue shift of all lines is observed after annealing. It is shown that below 500 °C, unintentional doping is the main origin of the shift. At higher annealing temperatures, the blue shift is mainly due to lattice strain arising because of mismatch between the thermal expansion coefficients of graphene and the substrate. Inhomogeneous distribution of stress and doping across the samples lead to the correlated variation of the height and peak position of RS lines.