Single Defect Imaging Studies in Diamond

Our knowledge of synthetic diamond growth has improved vastly over recent years, such that it is possible to engineer almost defect-free diamond^[1,2]. However, with our current understanding the distinction between natural and synthetic diamond is becoming increasingly more challenging, especially so for the gem trade. Hence there arises a need to be able to improve our detection methods and be able to correctly certify a type IIa diamond as synthetic or natural.

Diamond can host over 500 defects known as colour centres, where each one can give an indication as to the material synthesis or treatment. As the name suggests, the colour centre emits at a particular wavelength unique to it’s architecture making it a fingerprint for the diamond. Nitrogen is known to be used in synthetic growth due to the reduction in synthesis time^[2], and so detection of nitrogen-related defects can be characteristic of synthetic growth. Furthermore, silicon-related defects and even missing lattice atoms can also indicate synthetic growth and treatment. Whilst these defects are common with synthetic diamond, natural diamonds can also play host to these defects.

A simple study of defect orientation is one such step forward to understanding if a diamond is synthetic or natural. So much so that it has been shown experimentally that for a natural diamond containing single nitrogen vacancy (NV) centres, the orientation of defects are randomly distributed across the sample. For a synthetic diamond grown on a known orientation substrate, it can be shown that these centres exhibit preferential orientation, shown in Figure 1 below;

Figure 1: (a) polarisation image of single NV centres in natural diamond, there are 4 colours present indicating NV randomly aligned in 4 directions. (b) synthetic diamond investigated by the same polarisation method, the single colour therefore indicates one, preferential orientation of the NV centre. Image taken from ^[3].

By assessing the spatial distribution, any multi-colour centre correlation and the preferential alignment, it may be possible to identify whether a high purity type IIa diamond is synthetic or natural with very high certainty. During the PhD project it is hoped that more can be established about synthetic diamond growth and provide industry the tools to deal with low defect concentration diamonds.

In addition, the optical and spin properties of defects will be investigated in an attempt to find systems that can outperform (in one or more ways) or compliment the nitrogen-vacancy and silicon-vacancy defects in a range of different applications.

References

[1] H. Sumiya and K. Tamasaku. (2012) Jpn. J. Appl. Phys. 51 090102

[2] B. Willems, A. Tallaire, and J. Achard (2014) Diam. Rel. Mat. 41 25-33

[3] A. M. Edmonds, U. F. S. D’Haenens-Johansson, R. J. Cruddace, M. E. Newton, K.-M. C. Fu, C. Santori, R. G. Beausoleil, D. J. Twitchen, and M. L. Markham (2012) Phys. Rev. B 86, 035201