DIAMOND PLATELET STRUCTURE

# Resolving the controversy

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The structure of the platelet defect in diamond has been determined by transmission electron microscopy, distinguishing the best-matched atomic model that settles a long-standing debate.

Diamonds are supposed to be highly transparent for visible light as well as parts of the infrared and ultraviolet spectrum, but in many cases they are not. It is defects in the diamond that make them absorb and emit photons in this spectral range, and also it’s these same defects that make them interesting for a variety of scientific and technological applications. More than 100 different types of optically active defects are known in diamond, yet many of which have an unknown atomic configuration1,2. One of these defects is the so-called platelet defect, a planar structure that lies on the {100} planes of the crystal and has a typical diameter of tens to hundreds of nanometres (Fig. 1). It is optically active in the infrared, with a frequency that depends on the size of the platelet1,3. The platelet defects exist in many natural and synthetic diamonds, while the exact atomic configuration of this defect has been subject to a significant controversy for more than half a century. Following X-ray diffraction studies in 1940 and subsequent transmission electron microscopy studies, several models have been proposed. Although to some extent the later models can be seen as a refinement of the earlier ones based on new experimental evidence, so far the experiments could not clearly distinguish between these theoretical structure models. Now, reporting in Nature Materials, Ezra Jaco Olivier and colleagues4 have settled very important aspects of this debate. Using atomically resolved scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), and electron energy-loss spectroscopy (EELS), they identified which, out of the set of candidate structures, agrees with reality. They concluded that the ‘zigzag’ model (Fig. 2) proposed in ref. 5 is the experimentally favoured configuration, and that the other candidates can be ruled out. In this model, interstitials of carbon or nitrogen are arranged at alternating (zigzag) sites, as indicated by the blue arrows in Fig. 2 for a platelet viewed along the $$[1\bar{1}0]$$ direction. The spectroscopic analysis also confirmed earlier studies that the platelet is primarily composed of carbon, but does also contain a small fraction of nitrogen.

The work of Olivier and co-workers was made possible by recent advances in aberration-corrected electron microscopy, which enabled imaging of the platelets with sub-ångström spatial resolution and a sufficiently high contrast transfer. Imaging carbon materials by TEM or STEM is particularly challenging because the light atoms have a low contrast, are easily displaced by the energetic electron irradiation, and are particularly ‘small’ — that is, the interatomic distances that need to be resolved are shorter than those of most other elements. Electron microscopy is now sufficiently powerful to resolve the fine details in the extended defects in diamond, such as the platelet4,6. Figure 2 highlights some aspects of the images that were obtained by Oliver and colleagues: not only does the spatial resolution allow resolving atomic columns with a projected distance of only 0.89 Å, but also the much weaker features within the defect can be separated and detected above the experimental noise level, in order to distinguish different models. In this context it is important to reiterate that along with higher spatial resolution, aberration correction in TEM or STEM also results in more contrast for weakly scattering features like the ones indicated by blue arrows in Fig. 2, which otherwise are hidden in the proximity of strongly scattering ones. Last but not least, aberration correction reduces ‘nonlocal’ effects in the images (contrast of the structure appearing away from its true location, due to contrast delocalization in TEM7 or probe tails in STEM8), which is often critical for an unambiguous interpretation. All of these were essential elements for the successful identification of the platelet defect structure.

Most of the knowledge about defects in diamond stems from the interplay of various spectroscopic methods and extensive modelling, and it will be interesting to see what role electron microscopy can play in the future. Nitrogen is the most common impurity in diamond, and probably the most well-known point defects in diamond are the nitrogen centre and nitrogen–vacancy centre — the latter has gained considerable attention recently due to its optical transition being extremely sensitive to external perturbations and its potential applications in quantum computing2. The platelets, in contrast, are an extended defect structure that are thought to arise from an agglomeration of nitrogen-containing defects into a planar geometry3,9, although most of the nitrogen appears to escape in the process. The resulting chemical composition with a small amount of nitrogen among predominant carbon was also confirmed by the spectroscopic analysis of Olivier and colleagues4. Unfortunately, the formation process and the role of nitrogen in it still remain somewhat unclear, and further microscopic studies at different stages of the platelet formation might shed light on this point. Now that the platelet can be imaged so clearly, a most intriguing question is whether other defects in diamond, especially point defects, will become accessible by electron microscopy. While nitrogen centres or nitrogen–vacancy centres have been imaged in graphene10,11, such structures would be masked if they are embedded in a thick three-dimensional sample that possibly also has an amorphous cover layer. Hence, going from the extended to the point defects still entails some formidable challenges lying ahead — for example, in sample preparation and radiation damage prevention. Uncovering the vast zoo of defects with unknown structures would open new doors for tailoring optical and electronic properties of diamond. The work of Olivier and colleagues is a remarkable step forward for characterizing this challenging material, and it also highlights a promising avenue to study defects in diamond where the identification of the platelet structure can be seen as a starting point.

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Correspondence to Jannik Meyer.

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Meyer, J. Resolving the controversy. Nature Mater 17, 210–211 (2018). https://doi.org/10.1038/s41563-018-0026-4

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