Evidence for the solubility of boron in graphite by electron energy loss spectroscopy

We present the results of transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) studies on two carbon-boron alloys both prepared by chemical vapour deposition at ca. 1000 degrees C and differing in their [Boron]/[Carbon] atomic ratio as well as in their morphology. In both samples, impurity concentrations, principally oxygen and nitrogen, were found to be low relative to boron dopant levels, For low boron contents, typically around 5-10 at.%, the sample consisted of union-like spherical particles approximately 10 nm in diameter which exhibited a non-homogeneous distribution of boron, concentrated at a level of 5-6 at.% in the centre. For this sample, studies of the B-K- and C-K-ELNES (electron energy loss near-edge structure) together with associated modelling of the unoccupied density of electronic states, indicate a substitution of boron atoms on threefold coordinated spl-sites within the graphite network. For higher boron doping levels, typically 25 at.%, the sample consisted of homogeneous thin films. In this case, the change in shape of the B-K-ELNES indicates that boron has higher coordinations than planar trigonal together with possibly some residual sp(2) sites. This study unambiguously demonstrates the presence of boron substitution solely within an sp(2)-bonded graphite network in the case of low boron contents and, when combined with other studies, gives an indication of the solubility limit for boron in graphite for the chemical vapour deposition (CVD) process. (C) 2000 Elsevier Science Ltd. All rights reserved.