Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methodshigh pressure high temperature (HPHT) growth and chemical vapour deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g. by implantation and/or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamondalone, and in association with vacancies, hydrogen and transition metal atoms. Amongst these, the negatively charged nitrogen-vacancy (NV −) defect in diamond is attracting particular current interest on account of the many new and exciting opportunities it offers for, e.g., quantum technologies, nanoscale magnetometry and biosensing. 2 Laboratory Based Synthesis of Diamond and Nitrogen-Containing Diamond 2.1 High Pressure High Temperature (HPHT) Methods. Nature was the inspiration for the HPHT method, by which diamond growth was demonstrated by Swedish company ASEA in 1953 (though not reported at that time) and subsequently by US company General Electric in 1955. 14 , 15 Most present-day HPHT synthesis exploits the temperature-gradient growth (TGG) method developed later in that decade. 16 However, it took many further years before the design and control of HPHT reactors yielded diamonds of

Density functional theory was used to determine selected experimentally observable properties of the N 2 V H and N 2 V H 2 point defects in diamond. We report that the symmetry of the defects are C 1h and C 2v retrospectively with the hydrogen(s) saturating carbon radicals that are produced when the vacancy is formed. N 2 V H has an accessible negative charge state if the Fermi energy lies above around 2.4 eV above the valence band top, as N 2 V H 2 contains only species with their valence satisfied, only the neutral charge state of this species is thermodynamically stable. We predict that N 2 V H 0 would be detectable in infrared, with a C-H stretch mode around 3050 cm −1 for the neutral species, shifting to 2700 cm −1 in the negative charge state. N 2 V H 2 is also expected to be visible in infrared, with the presence of two, sterically interacting H atoms resulting in peaks with higher wave numbers, being around 3370 and 3540 cm −1 for the anti-symmetric and symmetric stretch modes, respectively. Similarities can be drawn between the bandstructures of N 2 V H and the isoelectronic N 3 V centre, from which we predict the likely electronic optical transition for this centre. N 2 V H 2 is expected to not give rise to any sharp electronic transitions.

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Hydrogen is grown into CVD diamond and occurs in point defects also involving a lattice vacancy, V . Complexes involving V , H and nitrogen, or silicon have been identified by experiment, and in some cases the microscopic structure has been identified with the use of quantum-chemical simulations. In this study, we present the results of density functional simulations of the primitive vacancy-hydrogen defect in diamond. We find that the symmetry of the V H defect is C 3v , with the H atom strongly bonded to one of the four C radicals that are formed when the vacancy is created. The defect is expected to occur in both the neutral and negatively charged forms, with the possibility of both positive and −2 charge states. For V H 0 , S = 3/2 and S = 1/2 spin states are found to be indistinguishable in energy, with the quartet not expected to yield sharp optical transitions, unlike the doublet. V H −1 in the S = 1 ground-state is predicted to have an optical transition that is broadly similar to that of NV − (S = 1), although it is important to note that the non-degenerate band involved in the transitions arises from a different origin in V H −1 as there are no lone-pairs present in this case. We have also made predictions for the C-H stretch mode frequencies, noting a general trend with charge state. Combinations of optical spectroscopy, paramagnetic resonance and vibrational mode spectroscopy are therefore required to fully experimentally resolve V H in its various charge and spin states.