Paramagnetic Defects in e-Beam Irradiated and Annealed Polycrystalline Nanodiamonds Produced by External Shock-Wave Detonation Synthesis

PolyCrystalline NanoDiamonds (PCNDs), manufactured by an external shock-wave detonation (the DuPont method), are among directly synthesized diamond nanoparticles which attract growing interest to their use in biomedical and quantum sensing application. In contrast to detonation nanodiamonds (DNDs), origin and evolution of paramagnetic defects in PCNDs induced by various treatments such as thorough purification, irradiation, annealing were beyond the scope of diamond community. We report on results of room temperature electron paramagnetic resonance (EPR) experiments done on a series of PCND samples undergone purification and e-beam irradiation at increasing fluences up to 1×10¹⁹ e^-/cm² followed by standard annealing at 800 ^oC. The paramagnetic defects’ pool in all samples consists of high abundant primary defects (mostly of <i>S</i> = 1/2) and low abundant triplet (<i>S</i> = 1) defects [1]. The total content of primary defects in the pristine PCND sample reaches 1280 ppm whereas the content of triplet NV^- centers does not exceed 35 ppb. Boiling acid treatment of the non-irradiated and non-annealed sample PCND-BA causes significant reduction of primary defects (down to ~780 ppm) but does not really affect triplet centers. Annealing of that sample retains the content of all defects untouched, however causes prolongation of spin-lattice relaxation times of primary defects. Deconvolution of the intense singlet quasi-Lorentzian line of primary defects into broad and narrow components allowed attributing these components to dangling bonds (broad) and exchange coupled P1 centers (narrow) [2]. The content of dangling bonds in the annealed PCND-BA sample was estimated as ~620 ppm and that of P1 of ~160 ppm. E-beam irradiation (fluences 1×10¹⁸, 3×10¹⁸, 5×10¹⁸ and 1×10¹⁹ e^-/cm²) increases the total content of primary defects at that contribution of the broad component practically does not change, the increase occurs due to the narrow one (up to ~400 – 500 ppm). At the same time content of the triplet centers in e-beam irradiated PCND-BA samples remains at the same low level. The PCND-BA sample irradiated with 1×10¹⁹ e^-/cm² fluence was further undergone the annealing treatment. It was found that annealing reduces total amount of primary defects (to ~400 ppm) due to reduction of both broad (~310 ppm) and narrow (~90 ppm) components. Some insignificant growth of NV^- content (up to 50 ppb) was observed as well. The observed findings allow attributing the increase of the content of defects contributed to the narrow component in e-beam irradiated samples to appearance of radiation-induced paramagnetic V^-centers. Annealing of these samples causes diffusion and recombination of V^-centers accompanying by formation of a certain amount of radiation induced NV^- centers. Surprisingly, the effect of NV^- centers formation in PCND samples by e-beam irradiation and annealing was found to be at least on the order of magnitude weaker than that in both NDs obtained by nanonization of HPHT diamonds and DNDs. In strong contrast, DNDs show an NV^- concentration of ~100 ppb in pristine sample, which increases up to ~500 ppb under identical electron irradiation and annealing condition [3]. This specificity of PCND samples is discussed within the framework of models recently proposed in the Ref. [3].<br/><br/>[1] A. I. Shames et al., Phys. Status Solidi A 212 (2015) 2400<br/>[2] A. I. Shames et al., Physica E 146 (2023) 115523<br/>[3] F. T.-K. So et al, J. Phys. Chem. C 126 (2022) 5206−5217