Flawless diamonds are extremely rare and expensive. Most diamonds contain defects, and for scientific and industrial applications, the defects are very important. In a naturally-formed diamond, the presence and structure of defects can be used to accurately determine the geological environment in which it was formed. The defects also shape the electronic and optical properties of diamonds, and exploiting these properties in synthetic diamonds could lead to next-generation optical storage devices, high-frequency field-effect transistors (FETs) and high-power switches.
The nature of defects in diamonds has attracted considerable scientific interest, mainly due to their role in shaping electronic and optical properties1. They may well hold the key to next-generation devices, including optical storage, high-frequency field effect transistors and high-power switches.
Using STEM and TEM to image carbon materials is particularly challenging, as the light atoms show a low contrast and are easily displaced by electron irradiation. The interatomic distances that need to be resolved are shorter than most other elements, and it is only now that recent advances in aberration-corrected electron microscopy at low voltage have made it possible to image platelets with sub-Ångström spatial resolution and sufficiently high contrast.
Using atomically-resolved scanning STEM, TEM and EELS, a team of researchers from the UK and South Africa have imaged the atomic structure and local chemistry of platelets in a natural type Ia diamond.
High spatial-resolution EELS spectrum imaging was used to investigate the presence, distribution and coordination environment of nitrogen in the platelets, taking care to use sufficient flux for accurate signal detection while avoiding progressive electron-beam-induced damage of the platelet structure. The results showed clear evidence for the presence of nitrogen within the defect.
The change in the EELS structure of the carbon K-edge at the defect core, and its similarity to the nitrogen K-edge, indicates that the defect core contains both atomic species in an interstitial arrangement. The presence of a pre-edge feature in both carbon K- and nitrogen K-edges indicates trigonal distortion of the lattice with the consequent coordination environment of the interstitial atoms being three-fold rather than four-fold.
Funding acknowledgment: E.J.O., J.H.N., R.E.K. and S.R.N. acknowledge the financial support of the NRF and DST in South Africa and the DST-NRF Centre of Excellence in Strong Materials at the University of the Witwatersrand. A.I.K. acknowledges financial support from EPSRC and the Royal Society. We thank Diamond Light Source for access and support in use of the electron Physical Science Imaging Centre during part of this work.
Corresponding author: Prof Angus Kirkland, ePSIC and Oxford University, email@example.com
Olivier EJ, Neethling JH, Kroon RE, Naidoo SR, Allen CS, Sawada H, van Aken PA, Kirkland AI. Imaging the atomic structure and local chemistry of platelets in natural type Ia diamond. Nature Materials 17, 243-248, doi:10.1038/s41563-018-0024-6 (2018).
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