University of Tokyo achieves successful nanodiamond synthesis in mild conditions

A research group led by University Professor Eiichi Nakamura of the Graduate School of Science at the University of Tokyo succeeded in synthesizing nanoscale spherical diamonds (NDs) by irradiating crystals of adamantane (Ad), which has a diamond framework, with electron beams using atomic-resolution transmission electron microscopy. While conventional diamond synthesis required harsh conditions, in this study the researchers pioneered a new reaction pathway utilizing radical cations of Ad molecules, realizing synthesis in a short time under mild conditions of -173℃ to room temperature and 0.0001 Pa. Through in-situ observation and reaction rate analysis, they were able to synthesize NDs with uniform sizes in the range of 2-8 nanometers with nearly 100% yield. They also revealed that C-H bond cleavage is involved in the rate-determining step and that the ND surfaces are ultimately terminated with hydrogen. This demonstrates new possibilities for organic synthetic chemistry as a precise organic synthesis method using electron beam irradiation. The study was published in Science.

Artificial diamond synthesis generally requires harsh conditions, including high temperatures (approximately 1500℃) and pressure (approximately 10 GPa), making control of the size of NDs difficult and structural defects unavoidable. While it is widely imagined that removing all the C-H bonds from Ad, an organic molecule with a 10-carbon ring structure that is a component of the diamond framework, would result in diamond formation, this had not been achieved in practice.

The research group utilized high-speed, high-resolution single-molecule, atomic-resolution, time-resolved observation using transmission electron microscopy (SMART-EM) to perform in-situ observation of the process of ND formation via the electron beam irradiation (80-200 keV) of Ad crystals. They performed recording with sub-angstrom spatial resolution and millisecond-level time resolution how spherical, defect-free NDs with cubic crystal structures are formed from Ad oligomers accompanied by hydrogen gas evolution.

As a result of detailed analysis combining kinetic analysis of this reaction with electron energy loss spectroscopy (EELS), it was found that Ad molecules are first ionized by the electron beam, C-H bonds are selectively cleaved, and the resulting radicals undergo repeated dimerization, leading to Ad polymerization and diamond framework growth. By applying this knowledge to set conditions, the group achieved 100% yield ND synthesis under low temperature and low pressure in the short time of 1-20 seconds. Unlike conventional methods, precise reaction control is possible, realizing synthesis of single-crystal, perfectly spherical NDs with narrow size distributions of approximately 2-8 nanometers. When the reaction proceeds further, single crystals can fuse with each other to synthesize polycrystalline spherical NDs.

Carbonaceous chondrites that reach Earth from space sometimes contain nanodiamonds, but their origin has long been unknown. This research showed that NDs can be formed from Ad crystals by 80-200 keV electron beam irradiation. Since these electron beams have similar energy scales to high-energy electrons existing in space (part of cosmic rays), this suggests the possibility that cosmic ray electrons were involved in the formation of NDs in meteorites.

Journal Information
Publication: Science
Title: Rapid, low-temperature nanodiamond formation by electron-beam activation of adamantane C-H bonds
DOI: 10.1126/science.adw2025