The role of electron spin resonance in the study of both natural and synthetic diamond is reviewed. A brief survey of the physical significance of the constants in the spin Hamiltonian, as well as experimental technique, is given. The various nitrogen centres are discussed treating exchange-interaction, Jahn-Teller and relaxation effects associated with these centres. The spin Hamiltonian parameters of these centres are tabled and the results are discussed within the framework of the defect molecule approach. In conclusion, the correlation between optical effects and the ESR measurements in the case of four defect centres are discussed in some detail as this seems to be a powerful method of testing the various models suggested for the observed defects.

https://iopscience.iop.org/article/10.1088/0034-4885/41/8/002/pdf

Electron spin resonance in the study of diamond

This review outlines some relevant aspects of theoretical physics-molecular orbital and other calculation of the properties of point defects, symmetry and group theory, zero-phonon lines and the configuration coordinate diagram. The experimentally observed spectra can conveniently be divided into infrared (<1 eV), which are observed in absorption and are mainly vibrational, and 'visible' (1-5.5 eV), which are vibronic and are observed in both absorption and luminescence. 'Visible' spectra include those present naturally, those induced by irradiation, and those induced by irradiation plus annealing. This review covers all the important point defects observed optically in diamond, and includes some results which are not generally available. The relevance of the experimental results to the current controversy about how to calculate the properties of point defects is brought out.

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Optical absorption and luminescence in diamond

Solid-state spin systems including nitrogen-vacancy (NV) centers in diamond constitute an increasingly favored quantum sensing platform. However, present NV ensemble devices exhibit sensitivities orders of magnitude away from theoretical limits. The sensitivity shortfall both handicaps existing implementations and curtails the envisioned application space. This review analyzes present and proposed approaches to enhance the sensitivity of broadband ensemble-NV-diamond magnetometers. Improvements to the spin dephasing time, the readout fidelity, and the host diamond material properties are identified as the most promising avenues and are investigated extensively. Our analysis of sensitivity optimization establishes a foundation to stimulate development of new techniques for enhancing solid-state sensor performance.

Sensitivity optimization for NV-diamond magnetometry

Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1--$5.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on ${111}$ planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

Origin of brown coloration in diamond

We present a study of the spin properties of dense layers of near-surface nitrogen-vacancy (NV) centers in diamond created by nitrogen ion implantation. The optically detected magnetic resonance contrast and linewidth, spin coherence time, and spin relaxation time, are measured as a function of implantation energy, dose, annealing temperature, and surface treatment. To track the presence of damage and surface-related spin defects, we perform in situ electron spin resonance spectroscopy through both double electron-electron resonance and cross-relaxation spectroscopy on the NV centers. We find that, for the energy ($4--30$ keV) and dose ($5\ifmmode\times\else\texttimes\fi{}{10}^{11}--{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{ions}/\mathrm{cm}}^{2}$) ranges considered, the NV spin properties are mainly governed by the dose via residual implantation-induced paramagnetic defects, but that the resulting magnetic sensitivity is essentially independent of both dose and energy. We then show that the magnetic sensitivity is significantly improved by high-temperature annealing at $\ensuremath{\ge}1100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Moreover, the spin properties are not significantly affected by oxygen annealing, apart from the spin relaxation time, which is dramatically decreased. Finally, the average NV depth is determined by nuclear magnetic resonance measurements, giving $\ensuremath{\approx}10--17$ nm at 4--6 keV implantation energy. This study sheds light on the optimal conditions to create dense layers of near-surface NV centers for high-sensitivity sensing and imaging applications.

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Spin properties of dense near-surface ensembles of nitrogen-vacancy centers in diamond

The ESR spectrum of an electron irradiated, type IIa diamond has been studied after it had been annealed at various temperatures up to 1400°C Six sets of lines were identified following anneals

Electron spin resonance in electron irradiated diamond annealed to high temperatures

The effect of an electron irradiation near helium temperature on the E.S.R. spectrum of type IIa diamond has been measured. The unirradiated stones showed a small isotropic line with a g value within 0·2% of the free-spin value. It is known as the ‘a’ line. Marked changes in the intensity of the ‘a’ line occurred during exposure to light and ionizing radiation and during annealing. They can be attributed to charge redistribution. Careful control of the experimental conditions was therefore necessary to establish the intrinsic effect of the low-temperature irradiation. The size of the ‘a’ line remained unchanged during the irradiation at 17 K for doses up to 1017 electrons/cm2. On annealing the diamond to 80 K the line was enhanced. It decreased to its pre-irradiation value on annealing to 300 K. Structure was observed at about 60 gauss either side of the ‘a’ line. In one case it was seen during the irradiation at 17 K and appeared to grow linearly with dose. In another case it was only observed a...

A. M. A. Wild

Papers

Electron spin resonance in electron irradiated diamond annealed to high temperatures

E.S.R. in diamond electron-irradiated at low temperature