We have developed a new technology that makes it possible to make diamonds under normal pressure!
Natural diamonds are mainly formed in the metallic melts of the Earth's mantle, which require a high temperature of 900-1400°C and a high-pressure environment of 5-6 GPa. According to the phase diagram of carbon, it is under such high pressure and high temperature conditions that a diamond can reach a thermodynamically stable state.
Currently, up to 99% of synthetic diamonds are manufactured by the high-pressure, high-temperature (HPHT) method. This technique was first developed in 1955 by scientists at General Electric in the United States, who synthesized diamonds using molten iron sulfide at about 7 GPa and 1600°C. HPHT technology facilitates the diffusion of carbon by using metal solvents at higher temperatures and pressures. However, this method is limited by the substances used in the manufacturing process and can often only produce diamonds with a size of about one cubic centimeter, as the equipment that achieves such high pressures can only operate in a relatively small space. Here, Prof. Rodney S. Ruoff from the Ulsan Institute of Science and Technology, South Korea, together with Won Kyung Seong and Da Luo, describes the growth of diamond crystals and polycrystalline diamond films using liquid metal at 1 atm pressure and 1,025 °C, breaking the limitations of high temperature and pressure in the above method. Through the catalytic activation of methane and the diffusion of carbon atoms into the subsurface region, diamonds grow in the subsurface layer of a liquid metal composed of gallium, iron, nickel, and silicon. The authors found that the supersaturation of carbon in the subsurface layer of liquid metal led to the nucleation and growth of diamonds, and that silicon played an important role in stabilizing the tetravalent bond carbon clusters, which played a role in the nucleation process. The growth of (transferable) diamonds in liquid metal at moderate temperatures and pressures of 1 atm opens up many possibilities for further basic scientific research and the scaling up of such growth. The study broke with this traditional paradigm and demonstrated that it is possible to grow diamond under standard atmospheric pressure and milder temperature conditions. The results were published in Nature under the title "Growth of diamond in liquid metal at 1 atm pressure", and the first author was Gong Yan, an alumnus of Fuzhou University (undergraduate)/Xiamen University (master's).
Yan Gong is a Ph.D. student in the Department of Chemistry, Korea Institute of Basic Sciences/Ulsan Institute of Science and Technology, supervised by Prof. Rodney S. Ruoff, B.S. from Fuzhou University in 2017, and M.S. from School of Materials Science and Technology, Xiamen University in 2020. (Picture from the website of the research group)
Diamonds are grown in 1atm liquid metalThe author utilizes liquid metals containing gallium, nickel, iron, and silicon to grow diamonds in a custom cold-wall vacuum system. The system heats and cools quickly, and the graphite crucible is heated by electric joules with precise temperature control (Fig. 1a, b). In 760 Torr of methane and hydrogen, the temperature is maintained at 1175°C, and nickel, iron, and silicon are completely dissolved in liquid gallium. At 15 to 30 minutes of growth, the diamond crystals were partially buried in solidified Ga-Fe-Ni-Si alloy sheets, partially exposed (Fig. 1d, e), and at 60 minutes, micron-sized islands were observed due to the increased density of the diamond crystals (Fig. 1f). After 150 minutes, an almost continuous film of diamonds with multiple colors (Figure 1g) was formed, and over 150 minutes there was no significant change in film thickness and morphology.
Figure 1Characterization of a liquid metal surface synthetic diamond diamond at the interface with graphite Using hydrochloric acid to dissolve the metal alloy sheet, the growing diamond film can be layered and transferred to other substrates. On a copper TEM grid coated with a Quantifoil porous amorphous carbon film, the transferred diamond film was highly transparent (Figure 1i). AFM images and 2D XRD analysis by synchrotrons show that these diamond films are polycrystalline with a cubic structure (Figure 1k) and exhibit diffraction Debye rings of (111), (220), and (311). In the experiment, the authors replaced ordinary methane with 13CH4 labeled with 99% 13C (Figure 2a). Samples grown over 150 minutes showed that the Raman peaks of diamond and graphite were mainly derived from 13CH4 (Fig. 2b), indicating the importance of carbon sources. In particular, in experiments using 13CH4, both the resulting diamonds and graphite showed characteristics related to methane carbon, but not to the carbon in the crucible, suggesting that methane is a more efficient carbon source than crucible carbon. In addition, the zero phonon line (ZPL) intensity of the SiV color center showed significant homogeneity in the photoluminescence spectrum excited by the 532 nm laser, emphasizing the critical role of methane in the growth of high-quality diamonds.
Figure 2: Characterization of diamondsCross-sectional TEM analysis was used to study the atomic-scale structure and elemental composition of primary diamonds, primary graphite, and the regions where they come into contact with solidified liquid metal sheets. A large cross-sectional TEM image of the D150 reveals a thin film of diamond on the surface of the liquid metal (Figure 3a). High-resolution TEM analysis showed that the metal region contained two distinct structures: amorphous M1 and crystalline M2, with the M2 region showing clear and defect-free lattice fringes (Fig. 3b, c). Through the TEM energy-dispersive X-ray spectral line profile, it was found that there was a large amount of carbon in the M1 region, and the carbon concentration decreased significantly from 26.5at% at the surface to 5.0at% at a depth of 40 nm, while the carbon concentration in the M2 region stabilized at about 3.0-5.0at%. This high concentration of carbon may be responsible for the amorphous structure of the M1 region. Electron energy loss spectroscopy (EELS) showed that the diamond region was a homogeneous diamond film, showing only one major σ* peak (Figure 3e). Atomic-resolution TEM images viewed from the [110] direction reveal that the diamond region is aligned along the (110) plane, showing a typical C-C dumbbell cell structure. The authors also studied the interface between the diamond and the liquid metal surface at the beginning of growth and found that the isolated diamond crystals were in direct contact with the amorphous M1 region and the crystallized M2 region (Fig. 3g, h). The alignment angles of the lattice edges of these crystals are inconsistent with the M1 surface, further revealing complex interfacial interactions.
Figure 3: TEM data for cross-sectional samples prepared by SEM-FIB Diamond growth mechanism The authors performed four growth experiments using 13CH4 and H2 for 5 min, 10 min, 15 min, and 150 min, respectively, and then investigated the carbon concentration in the surface region of the solidified liquid metal sheet by TOF-SIMS depth profiling technique. The results showed that in all four experiments, the surface region still contained a large amount of 13C even at depths of 100 nm, which was consistent with the carbon-rich region M1 observed by transmission electron microscopy. In particular, the carbon concentration in the central region of the bottom of the solidified liquid metal sheet was much higher than in the 15-minute growth experiment. The authors speculate that at 10 minutes the surface concentration of carbon atoms is so high that it almost reaches a supersaturated state for the formation of diamond nuclei, and that the nucleation and rapid growth of diamonds can occur between 10 and 15 minutes. In addition, theoretical simulations have shown that methane is activated on the surface of the liquid metal, and that silicon atoms play an important role in the growth of diamonds, especially in the authors' previous studies on the epitaxial growth of single-crystal diamonds. Growth experiments using different concentrations of silicon have shown that higher silicon concentrations promote high-density growth of diamonds with smaller crystal sizes, which may be related to the role of silicon in the formation of diamond prenuclei. The authors found a method to grow diamonds using liquid metal alloys at 1 atm and moderate temperatures, which is unprecedented in traditional growth modes that previously required 5-6 GPa high pressures and high temperatures. The grown diamond film can be transferred to any other substrate. The authors suggest that it is possible to grow diamonds over a larger area by enlarging the surface or interface, configuring heating elements to cover a larger potential growing area, and distributing carbon to the growing zone in new ways. Methods using liquid metal may accelerate and promote the growth of diamonds on a variety of surfaces, or they may facilitate the growth of diamonds on small diamond (seed) particles. Considering the multiple possibilities of liquid metals and their co-crystals, as well as the potential to add different catalysts, combined with a variety of possible carbon precursors, the prospect of exploring such methods to grow diamonds looks very promising. Source: Frontiers of Polymer Science