Investigating large vacancy clusters in type IIa diamond with electron energy loss spectroscopy (EELS)

The origin of colour in brown diamonds has attracted much attention in recent years, and resulted in many attempts to characterise the responsible defect. These years of focused study have only ruled out possible solutions, including the popular hypothesis of dislocation states (B. Willems, PhD Thesis, University of Antwerp (2006) [1]). Most recently, positron annihilation studies have verified the existence of large clusters of vacancies in type Ha brown diamond (J. M. Mäki et al., presented at SBDDX (2004) [2]) that are significantly higher in concentration than in the corresponding treated colourless diamond. Also, theoretical calculations of large clusters of vacancies show {111} π-bonded surfaces cause the same broad featureless optical absorption as brown diamond (L. Hounsome et al., phys. stat. sol. (a) 202, 2182 (2005) [3]). This bonding is also shown to produce intensity at 5-7 eV in low loss electron energy loss spectroscopy (EELS). Comparing EELS analysis of brown and colourless diamond reveals a relative increase in intensity at 5-7 eV in the brown diamond. Using EELS with energy resolution between 0.25 and 0.4 eV (U. Bangert et al., Ultramicroscopy 104, 46 (2003) [4]) on the NW-STEM and SuperSTEM, π-bonding intensity is verified to be present as a bulk feature in brown diamond. This π-bonding is found across all regions of brown diamond regardless of position relative to dislocations. On closer inspection this π-bonding intensity shows variations on a scale that could be related to vacancy clusters or several clusters, however more investigation is needed. In colourless diamond, any intensity in the π-bonding region can be attributed to surface contamination.