Color Modification and Defect Evolution of Type IIa CVD-Grown Diamonds During HPHT, Irradiation, and Annealing Treatments

With the rapid expansion of laboratory-grown diamond production, post-growth color modification has become an essential route to enhance the value of CVD-grown diamonds. In this study, five faceted type IIa CVD-grown diamonds (round brilliant cut) were processed through HPHT treatment (1600 °C, 5.5 GPa), 3 MeV electron irradiation, and subsequent vacuum annealing at 800 °C, with defect evolution tracked step by step by FTIR spectroscopy, low-temperature photoluminescence (PL) spectroscopy, and DiamondViewTM fluorescence imaging. HPHT treatment eliminated the CVD-typical 3123 cm−1 (NVH) absorption, near-infrared hydrogen-related bands, and the 389, 563.5, and 596/597 nm PL centers, while promoting nitrogen aggregation (N3 at 415 nm; N3VH at 3107 cm−1) and shifting the NV charge state toward neutral. Electron irradiation introduced GR1, ND1, 3H, and the 489 and 523.5 nm centers, turning CVD-1-3 deep blue and CVD-4 yellowish green. Annealing removed most irradiation-induced centers except GR1 and 3H and drove vacancy trapping by isolated nitrogen, markedly enhancing NV emission and yielding pink-to-purplish hues. The 505 nm center, the 595 nm center (observed in some samples), and the H1a absorption indicate irradiation followed by low-temperature annealing, providing a spectroscopic basis for the color enhancement and identification of treated CVD diamonds.