Natural diamonds form in Earth’s mantle at high temperatures and pressures. The most popular method developed to make synthetic diamonds mimics those conditions by using anvil presses to achieve extreme pressures. A second approach, chemical vapor deposition, is used to grow diamonds layer by layer on a substrate in a vacuum. The process is more akin to the diamond growth that occurs in interstellar gas clouds. Now researchers have unveiled another route to diamond growth. The approach, which involves methane gas and a catalyst of molten metals, requires less energy and less advanced equipment than conventional methods, and it works at ambient pressure.
Rodney Ruoff (Ulsan National Institute of Science and Technology and the Institute for Basic Science Center for Multidimensional Carbon Materials, South Korea) was inspired to explore liquid-metal catalysis as a path to diamond growth after learning of experiments in which graphite—like diamond, another form of pure carbon—had been grown in liquid gallium. In early trials, he and his colleagues poured molten metals over diamond flecks on a silicon dioxide wafer in a chamber with carbon-bearing gases. The flecks didn’t grow—until a fortuitous spill brought a new element into the mix. In one experiment, the metal spilled over the wafer and touched pure silicon at its edge. Soon after, Yan Gong, a PhD student in Ruoff’s lab, saw evidence of new crystal growth on the diamond seeds.
In Gong and colleagues’ new paper, they show how a mixture of gallium, iron, nickel, and silicon in a graphite crucible can form diamonds: Once heated to 1025 °C in a chamber with methane and hydrogen gas, the mixture nucleates diamond nanocrystals after 15 minutes and forms a continuous diamond film after 150 minutes. And it was done without using any seed crystals. Although silicon’s role in the nucleation process is not fully understood, it does seem to be crucial to the experiment’s success. Silicon is incorporated into the lattice of the diamond, and slight variations of the silicon concentration produce different crystal sizes and densities. And when it is depleted from the mix, mineral growth comes to a halt.
The polycrystalline diamond film grown with the new method is far from the gem-quality single crystals that the jewelry industry prizes. But given diamond’s many unique and valuable qualities—extreme hardness, high thermal conductivity, and semiconduction, for example—diamond films have many existing and potential uses in electronic and industrial applications. Ruoff says he also sees potential in liquid-metal catalysis that could be applied to synthesizing materials made of other elements, like boron and nitrogen. (Y. Gong et al., Nature 629, 348, 2024.)
