On the way to mass-scale production of perfect bulk diamonds.

D iamonds are hard. Humans have known that for ages. This hardness encompasses not only mechanical hardness, but also chemical, electrical, optical, and structural hardness (1). Diamonds are hard to cut, and shaping them requires a complex technology. Diamonds are chemically inert and, because of this, they are not processed chemically. Diamonds are resistant to electrical fields and, as an electronic material, they can work in the highest electric fields that solids can withstand. They are hard structurally and, because of this, they do not allow diffusion, they are immune to radiation, and they possess a stable defect structure. This hardness is a blessing when making ‘‘diamonds forever,’’ but it turns into a real curse when considered from the technological viewpoint. However, there is one physical parameter against which diamonds are not hard—temperature. Notwithstanding their many hardnesses, diamonds are a metastable solid and this metastability makes diamonds transform into graphite at high temperatures. This graphitization temperature ranges from 1,650 to 1,750 °C (2, 3) at normal pressure and may be as high as 3,000 °C at a pressure of 50 kbar. Ironically, this only weakness makes diamonds an even more difficult material technologically because it does not allow diamonds to anneal properly. Indeed, to modify a diamond in a desirable way, it is necessary to change its defect structure. Impurity doping during growth, ion implantation, and irradiation are the most common methods to do this (4). However, in many cases, to complete the activation of the useful defects and/or to remove the unwanted ones, it is necessary to perform subsequent heating (annealing) (5). Usually the annealing process is performed in an inert gas at normal pressure or in a vacuum at a temperature 1,700 °C to avoid graphitization. However, a temperature of 1,700 °C is not high enough to activate atomic motion in the diamond lattice. For instance, full activation of the ion-implanted boron as acceptors (6), or transformation of nitrogen aggregates, which strongly inf luence the electronic and optical properties of diamonds, require much higher temperatures (7). To increase the temperature to 1,700 °C the heating must be performed at a high stabilizing pressure. This high-pressure/high-temperature (HPHT) annealing is not a simple technique. It requires expensive highpressure equipment and experienced personnel to run the process. It is not an easily reproducible technique and it does not allow processing of large samples. As a result, HPHT annealing remains a unique technique available in a few laboratories and some companies for research and small-scale production. In this issue of PNAS, Meng et al. (8) reveal a way to anneal diamonds to a temperature of 2,200 °C without the application of stabilizing pressure. This low-pressure/high-temperature (LPHT) annealing was run in hydrogen plasma and was demonstrated on chemical vapor deposition (CVD)-grown single crystals in temperatures ranging from 1,400 to 2,200 °C. Comparative spectroscopic studies of the annealed diamonds were performed to reveal the action of high temperature. It was shown that 1,700 °C is a critical temperature to make radical structural changes in CVD diamonds and to improve their optical and structural quality. Although the article largely confirms the results already obtained with the HPHT technique (9– 12), its methodological and technological importance is significant. Low-pressure annealing at temperatures 2,000 °C is a real breakthrough in diamond research and technology, because it overcomes one of the diamond hardnesses and provides a novel tool for diamond processing. This new research raises questions about diamond annealing, the most intriguing of which are whether the new LPHT heating in a plasma is a decisive factor, and whether the superheating without stabilizing pressure is applicable only to CVD diamonds, which, unlike natural and HPHT diamonds, are saturated with hydrogen. However, even in the most restricted way of ‘‘plasma heating only’’ and ‘‘CVD diamond only,’’ LPHT annealing opens up the opportunity for broadscale studies of the high-temperature modification of diamonds and the fabrication of high-quality single-crystal diamonds. LPHT plasma heating is a simpler, cheaper technique than HPHT, and it could be routinely run in any laboratory where it is needed. LPHT annealing, when considered in combination with the high-growth-rate technique of CVD diamonds [also developed by the authors (13)], seems to be a starting point of mass-scale production of perfect diamond material at a low price. A profound impact of this innovation on industry (electronics, optics, thermal management, precise cutting), medicine (diamond scalpel surgery), and jewelry (cheap large brilliants) is difficult to overestimate. In conclusion, I would like to mention one more important advantage of LPHT annealing—the opportunity for controllable high-temperature treatment of diamond surfaces, which is of crucial importance for the microand nanostructuring of diamonds (14, 15). For example, Fig. 1 shows a carbon nanodot array on a diamond surface, processed by focused ion beam irradiation. Such arrays, when made on diamond surfaces terminated in a hydrogen atmosphere at high temperatures, reveal a hydrophobic-