The origin of color in brown diamonds has attracted much attention in recent years, and resulted in many attempts to characterize the responsible defect. These years of focused study have only ruled out possible solutions, including the popular hypothesis of dislocation states. Most recently, positron annihilation studies have verified the existence of large clusters of vacancies in type-IIa brown diamond that were significantly higher in concentration than in the corresponding treated colorless diamond. Also, theoretical calculations of large clusters of vacancies showed {111} π-bonded surfaces caused the same broad featureless optical absorption as brown diamond. This bonding was also shown to produce an intensity at 5 to 7eV in low loss electron energy loss spectroscopy. Comparison of electron energy-loss spectroscopic analyses of brown and colorless diamond revealed a relative increase in intensity, at 5 to 7eV, in the brown diamond. Using electron energy-loss spectroscopy, with an energy resolution of between 0.25 and 0.4eV, a π-bonding intensity was shown to be present as a bulk feature in brown diamond. This π-bonding was found across all regions of brown diamond regardless of the position relative to dislocations. On closer inspection this π-bonding intensity exhibited variations on a scale that could be related to vacancy clusters or several clusters, however more investigation was needed. In colorless diamond, any intensity in the π-bonding region could be attributed to surface contamination.

Investigating Large Vacancy Clusters in Type IIa Diamond with Electron Energy Loss Spectroscopy (EELS). R.Barnes, U.Bangert, A.Scott: Physica Status Solidi A, 2007, 204[9], 3065-71