Carbon nanocrystals with unusual symmetry shed light on the true structure of nanodiamonds and open up new possibilities for their application – says a scientist from the Institute for Geological and Geochemical Research in their paper published with his international colleagues in the journal Diamond and Related Materials.
Nanodiamonds are infinitesimally small crystals that are 10,000 to 20,000 times smaller than a human hair. Because of their importance, they are the focus of several disciplines. They can be found in meteorites, in the vicinity of young stars (HD 97048, Elias), in interstellar dust (comets), and have also been described in certain sedimentary layers that can be linked to asteroid impacts, so their appearance is being studied by both earth and planetary scientists. Nanodiamonds are also at the heart of materials science research. Due to their unique physical and mechanical properties, they are suitable for the production of hard and resistant coatings, abrasive powders and motor oil additives, as well as polymer and metal composites with exceptional properties. They are also candidate nanomaterials as drug carriers and can therefore be used in medical treatments, especially in cancer medicine.
Despite their importance, understanding the internal structure of nanodiamonds has continued to pose a research challenge. Although it is widely accepted that their structure generally corresponds to that of diamonds, with a three-dimensional arrangement of covalently bonded carbon atoms, features of their electron microscopic, diffraction and spectroscopic characteristics differ significantly from those of the well-known diamond. According to the latest research results  of Péter Németh and his international colleagues, a significant fraction of the nanodiamond particles is not diamond, but corresponds to nanosized diaphite structures that consists of crystallographically intergrown diamond and graphite units that are bonded together. This new family of diamond-related materials has already been identified by the research team in their previous research conducted on asteroid impact and artificial shock-wave produced samples [2, 3]. On this occasion, the researchers examined nanodiamonds from the Orgueil and Murchison meteorites (Fig. 1) and an artificial sample produced by chemical vapor deposition using state-of-the-art electron microscopy and microbeam Raman spectroscopy and synchrotron X-ray diffraction . The results were modeled using ab initio computational calculation techniques.
The scientists identified nanosized carbon particles with six- and 12-fold crystallographic symmetries (Fig. 2), which cannot be explained in terms of the structure of normal diamond. The authors associated the appearance of the unusual symmetry with the presence of diaphite units. Structural modeling showed that the six- and 12-fold symmetry appears only in certain projections of the grains but remains hidden in most of the possible crystal orientations relative to the viewing direction, so that recognizing the diaphite structure represents a serious challenge. This finding may explain why the existence of the nanosized diaphite domains may have been overlooked in previous research.
The research team determined the spectroscopic properties of diaphites using energy calculations and found that they match well with the experimental results for nanodiamonds. The authors concluded that the unusual features observed in nanodiamonds that are incompatible with those of normal diamond can be perfectly associated with differently bonded carbon atoms of the diaphite structure (Fig. 3).
The study not only provides a convincing explanation for the contradictory microscopic and spectroscopic observations to date, but it also sheds light on the application possibilities inherent in the diaphite structure. The researchers point out that diaphites have excellent mechanical and electrical properties due to the carbon atoms occurring in different bonding states. Controlling the experimental synthesis conditions or using lasers to write features within specific areas of diamond layers would provide an excellent opportunity to consciously produce diaphite structures with specifically designed characteristics, thereby developing nanomaterials with tunable electrical properties that can be candidates for the semiconductor industry.
The research was supported among others by the NKFI FK126502 project, the ÚNKP-20-5-PE-7 New National Excellence Program of the Ministry for Innovation and Technology, and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.
Németh P, McColl K, Garvie LAJ, Salzmann CG, Pickard J, Corà F, Smith RL, Mohamed M, Howard CA, McMillan PF Diaphite-structured nanodiamonds with with six- and twelve-fold symmetries. Diamond and Related Materials 2021, 119,108573.
Németh P, McColl K, Garvie LAJ, Salzmann CG, Murri M, McMillan PF Complex nanostructures in diamond. Nat. Mater., 2020, 19, 1126-1131
Németh P, McColl K, Murri M, Smith RL, Garvie LAJ, Alvaro M, Pécz B, Jones AP, Corà F, Salzmann CG, McMillan PF Diamond-graphene nanocomposite structures. Nano Letts., 2020, 20(5), 3611–3619.