Abstract
Localised energy loss spectroscopy (EELS) is carried out in order to extract information of the effect of extended defects on the electronic bandstructure in diamond. EELS is performed in a dedicated cold field emission scanning transmission electron microscope (STEM), equipped with a Gatan Enfina spectrometer, at high energy dispersions, allowing energy resolutions (FWHM of the zero loss peak) of around 0.3 eV to be achieved without monochromation, thus enabling low EELS to be carried out in the bandgap regime of diamond at high brightness. Experiments are carried out on single crystal CVD and brown and colourless natural diamonds. Experimental details and evaluation of spectra are discussed, and the effects of diffraction contrast on the EEL intensity in the vicinity of defects are critically examined. A general characteristic of extended defects in CVD diamonds appears to be a relative increase of the loss signal below 6 eV and a distinct feature at 6β7 eV, concomitant with sp^2^βbonds. It emerges that these states do not reside at cores of single dislocations, but near dislocation arrangements, in particular at nodes. A stark phenomenon in brown natural diamond is the omniβpresent increase in the EEL intensity below 6 eV over that of colourless samples, suggesting the existence of spatially dispersed, nonβdislocation related centres, with energies throughout the bandgap. Recent theoretical and experimental investigations have revealed strong links with vacancy clusters. In order to further support the vacancy cluster hypothesis high resolution electron transmission microscopy (HR TEM) imaging in combination with vacancy clusters simulations at high defocus values is performed. The simulations predict characteristic smallβscale contrast, which can be experimentally observed in brown, but not in colourless diamond. (Β© 2007 WILEYβVCH Verlag GmbH & Co. KGaA, Weinheim)