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State-of-the-art electron microscopy offers clarity on diamond defects

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.

Research into the nature of diamond defects has uncovered more than a hundred different optically-active types, many of which have an unknown atomic configuration. For more than 50 years, researchers have been investigating the platelet defect. A number of theoretical structure models have been proposed, but imaging techniques were unable to give experiments the atomic-level resolution necessary to confirm or discard them.

Now a team of researchers have made use of recent advances in electron microscopy, at the state-of-the-art electron Physical Science Imaging Centre (ePSIC), to solve the mystery of the platelet’s structure. They used a combination of atomically-resolved scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) to uncover the structural details of a natural diamond, and confirm the correct structural model. Further studies could shed light on how the platelet defect is formed, or structures of other types of defect, and this remarkable step forward could bring new opportunities for tailoring optical and electronic properties of diamond.

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.

Over 100 different types of optically-active defects have been identified, and in a natural diamond the presence and structure of defects plays an important role in defining colour and quality, and can be used to accurately determine the geological environment in which it formed.

Nitrogen is a common impurity in diamond, forming several different types of defect². The presence and quantity of nitrogen-related defects are fundamental to diamond classification (Fig. 1). Type I diamonds have a high concentration of nitrogen, and related absorption bands in their spectra. They are further classified depending on whether nitrogen is present in an aggregated form (type Ia), or in isolated substitutional sites (type Ib). Diamonds with a low concentration of nitrogen, which do not show the absorption bands, are classified as type II.

In natural type 1a diamonds, nitrogen aggregates occur with and within extended defects on {100} planes, called platelets. Platelets have a typical diameter of tens to hundreds of nanometers. They are optically active in the infrared, with a frequency that depends upon the size of the platelet. Platelet defects are common in both natural and synthetic diamonds, but their exact atomic configuration has been a mystery for more than 50 years.

In 1940, Raman and Nilakantan used X-ray diffraction to make the first observations of platelets in diamonds, and these defects were characterised by Evans and Phaal in 1962 using TEM. A number of theoretical structural models were proposed, but until now these could not be tested experimentally.

: Figure 1: Image characteristics of platelets in diamond. (a) Bright-field TEM image of platelets in diamond. The platelets on (100)p and (010)p planes are viewed edge-on and loops on inclined {111} planes are also visible. (b) High-resolution TEM image showing a single platelet. The beam direction is parallel to <110> (c) HAADF STEM image of diamond viewed along an beam direction showing the 89 pm {004} separation (indicated by red arrows). [Reprinted by permission from Springer Nature: Olivier EJ et al. doi:10.1038/s41563-018-0024-6 (2018)].

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.

Previous work found evidence of a strong link between nitrogen and platelets, suggesting that nitrogen may play an important role in the formation and structure of these defects. EELS spectra from the core of several platelet defects revealed the presence of a nitrogen K-edge, and subsequent studies (also predominantly using EELS) confirmed the detection of measurable quantities of nitrogen in platelets. However, the quantities detected are generally too low to suggest a nitrogen-only platelet structure; the presence of carbon is also necessary.

The new study presents aberration-corrected high-resolution STEM images, which show the atomic arrangement in the platelet core. Its findings are supported by EELS data from the core, which enable a complete description of the platelet structure.

The team used aberration-corrected STEM imaging in high-angle annular dark field (HAADF) mode, together with EELS spectrum imaging, to study the atomic structure and chemical composition of platelets in greater detail (Fig. 2). They also recorded high-resolution TEM (HRTEM) images, and then qualitatively compared HRTEM and STEM images to simulated images generated from the various theoretical structural models.

: Figure 2: High-resolution HAADF STEM images of two platelets viewed edge-on along <110> directions showing the asymmetric nature of the platelet structure. (a) Image of platelet A. (b) Image of platelet B. [Reprinted by permission from Springer Nature: Olivier EJ et al. doi:10.1038/s41563-018-0024-6 (2018)].

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.

The experimental results favour one particular structural model, the ‘zigzag’ model, in which interstitials of carbon or nitrogen are arranged at alternating (zigzag) sites. They also confirm earlier studies showing that the platelet is primarily composed of carbon, with a small fraction of nitrogen.

It is still unclear how these platelets form and the role of nitrogen in their formation, and further studies at different stages of platelet formation could further our understanding. However, this is a remarkable step forward in characterising a challenging material, and it is hoped that these new imaging capabilities will also shed light on other diamond defects with unknown structures. A new level of clarity on the atomic structure of these defects will offer up new opportunities for tailoring the optical and electronic properties of diamond.

References

Bosak, A. et al. Symmetry of platelet defects in diamond: new insights with synchrotron light. Acta Cryst B66, 493-496, doi: 10.1107/S0108768110031071 (2010).
Goss, J.P. et al. Interstitial nitrogen and its complexes in diamond. Phys Rev B 70, 235208, doi:10.1103/PhysRevB.70.235208 (2004).

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 protected]

Related publication

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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State-of-the-art electron microscopy offers clarity on diamond defects

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