Determination of the platelet structure in natural diamond by ADF ‐ STEM

Abstract

Electron microscopy is well known for its capability of determining the structure of many materials down to the atomic scale. A heavily studied material is natural diamond, but its rather small interatomic distance has made it a challenging material for TEM investigation. In this work we will demonstrate how novel TEM techniques can give fresh insights to this heavily studied material. Imperfections in natural diamond range from point lattice defects to dislocations and platelets. Platelet defects were first observed by means of TEM in the 1960s [1, 2] and nitrogen was detected making use of EELS in the 1980s [3]. Nevertheless, the exact crystalline structure of the platelets in natural diamond remains, perhaps surprisingly, still unclear. In this work, we demonstrate our approach to determine the structure of platelet defects making use of annular dark field scanning transmission electron microscopy (ADF‐STEM) combined with advanced image processing. Combining experimental data with image simulations we are able to offer a consistent crystallographic model of the defect. Furthermore, using monochromated energy loss spectroscopy technique we suggest the type of nitrogen embedding into the defect plane. TEM samples were prepared by a focused ion beam lift out procedure from natural type Ia diamond. ADF‐STEM and EELS experiments were carried out on FEI Titan microscope equipped with probe aberration corrector. We studied more than 50 platelets from different parts of the sample. In order to reduce scan distortions and damage to the defect plane a series of images with fast dwell time was obtained. The images were treated in the Smart Align software [4] to reduce the scan noise, correct the sample drift and a final summed image was obtained. Improved signal to noise ratio in this image allowed us to estimate the atomic positions and create a model of the platelet. Based on this model STEM image simulations were carried out. They are found to be in the good agreement with the experimental data. Using STEM‐EELS we prove the presence of nitrogen exactly in the platelet plane. Comparing DFT calculations of different nitrogen centres in diamond and the experimental spectra extracted from the defect plane we suggest the model for the nitrogen coordination in the platelet. Using this detailed information on the platelet defect, we will discuss the possible mechanisms of platelet formation.