A model is proposed to explain the irradiation-induced amorphization of graphite. This model assumes that the single-vacancy concentration saturates to Cvs(T) at irradiation temperature T; that the single-vacancy contained regions gradually transform to disordered regions, of which accumulation leads to amorphization; and that the transformation rate is proportional to the single-vacancy concentration. Utilizing Cvs(T) as a fitting parameter, this model explains the dose and the temperature dependencies of the Raman spectra of 25 keV He+-irradiated highly oriented pyrolytic graphite, Also, the Arrhenius plot of the obtained values of Cvs, which indicates the critical doses of amorphization, corresponds well to a previous TEM study, which showed two activation energies of 0.036 eV below -573 K and 0.25 eV above -573 K. By analyzing the chemical kinetics of the steady state, activation energies for single and di-interstitial migration of 0.14 and 0.86 eV, respectively, are obtained. The stored energy is attributed to a significant accumulation of di-interstitials below -573 K, which originated in the reduced annihilation with vacancies due to a barrier. By extending the theory to the quasisteady state with the collapsed line formation and the loop growth, the dimensional changes before amorphization can be also explained. This model should provide insight into the graphitization process and the formation mechanism of various carbon clusters.

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We report the development of Raman spectroscopy as a powerful tool for quantitative analysis of point defect and defect clusters in irradiated graphite. Highly oriented pyrolytic graphite (HOPG) was irradiated by 25 keV He+ and 20 keV D+ ions. Raman spectroscopy and transmission electron microscopy revealed a transformation of irradiated graphite into amorphous state. Annealing experiment indicated a close relation between Raman intensity ratio and vacancy concentration. The change of Raman spectra under irradiation was empirically analyzed by “disordered-region model,” which assumes the transformation from vacancy-contained region to disordered region. The model well explains the change of Raman spectra and predicts the critical dose of amorphization, but the nature of the disordered region is unclear. Then, we advanced the model into “dislocation accumulation model,” assigning the disordered region to dislocation dipole. Dislocation accumulation model can simulate the irradiation time dependencies of Raman intensity ratio and the c-axis expansion under irradiation, giving a relation between the absolute concentration of vacancy and Raman intensity ratio, suggesting an existence of the barrier on the mutual annihilation of vacancy and interstitial.

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