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

Abstract

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°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 onion-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 sp2-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 sp2 sites. This study unambiguously demonstrates the presence of boron substitution solely within an sp2-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.