Â鶹ÒùÔº

There are various ways to create artificial diamonds, but a new method developed by researchers, including those at the University of Tokyo, yields some extra benefits.

By specially preparing samples for conversion to diamond by means of an electron beam, the team found their method can be used to protect organic samples from the damage usually caused by such a beam. This could lead to new and powerful imaging and analytical techniques.

The work has been published in the journal Science.

Diamond synthesis is a process that conventionally requires conversion from carbon sources under extreme conditions—pressures of tens of gigapascals and temperatures of thousands of Kelvin—where diamond is thermodynamically stable, or chemical vapor deposition techniques where it is unstable.

A team led by University Professor Eiichi Nakamura of the Department of Chemistry at the University of Tokyo explored an alternative low-pressure approach through controlled electron irradiation of a carbon cage molecule called adamantane (C₁₀H₁₆).

Diamond and adamantane share a tetrahedral symmetric carbon skeleton, with the carbon atoms arranged in the same spatial pattern, making adamantane an attractive precursor for the production of nanodiamonds.

Successful conversion, however, requires precise cutting of adamantane's C–H termination bonds to form new C–C bonds, while assembling the monomers into a three-dimensional diamond lattice. While this was common knowledge in the field, "The real problem was that no one thought it feasible," said Nakamura.

Previously, mass spectrometry, an analytical technique that sorts ions according to their differing mass and charge, has shown that single-electron ionization could be used to facilitate such C–H bond cleavage. Mass spectrometry, however, can only infer structure formation in the gas phase, and is unable to isolate products from intermolecular reactions.

The team was prompted to monitor electron-impact ionization of solid adamantane at atomic resolution using an analytical and imaging technique called transmission electron microscopy (TEM), irradiating submicrocrystals at 80–200 kiloelectron volts at 100–296 Kelvin in vacuum for tens of seconds.

Not only would the method reveal the evolution of the polymerized nanodiamond formation, but provide powerful ramifications for the potential of TEM as a tool to resolve the controlled reactions of other organic molecules.

For Nakamura, who has worked on synthetic chemistry for 30 years and computational quantum chemical calculations for 15 years, the study offered a breakthrough opportunity. "Computational data gives you 'virtual' reaction paths, but I wanted to see it with my eyes," he said.

"However, the common wisdom among TEM specialists was that organic molecules decompose quickly as you shine an electron beam on them. My research since 2004 has been a constant battle to show otherwise."

The process yielded defect-free nanodiamonds of cubic crystal structure accompanied by hydrogen gas eruptions, up to 10 nanometers in diameter under prolonged irradiation.

The time-resolved TEM images illustrated the passage of formed adamantane oligomers transforming into spherical nanodiamonds, moderated by the C–H cleavage rate. The team also tested other hydrocarbons, which failed to form nanodiamonds, highlighting the suitability of adamantane as a precursor.

The findings open a new paradigm for understanding and controlling chemistry in the fields of electron lithography, surface engineering and electron microscopy. Analysis of the nanodiamond conversion supports long-standing ideas that diamond formation in extraterrestrial meteorites and uranium-bearing carbonaceous sedimentary rocks may be driven by high-energy particle irradiation.

Nakamura also pointed to the basis it provides for synthesizing doped quantum dots, essential to the construction of quantum computers and sensors.

As the latest chapter in a 20-year-long research dream, Nakamura said, "This example of diamond synthesis is the ultimate demonstration that electrons do not destroy organic molecules but let them undergo well-defined chemical reactions, if we install suitable properties in molecules to be irradiated."

By forever changing the game in fields employing electron beams for research, his dream could now provide the vision for scientists to uncloud interactions under electron irradiation.