We have measured the boron and nitrogen 1s photoabsorption cross section from hexagonal, cubic, rhombohedral, wurtzite BN powders and from an incoherent boron nitride thin-film. From the near edge resonances in these measurements, we observe unique cross section features that can be associated with either sp2 or sp3 bonding. The hBN and rBN exhibit sp2 bonding (as determined by the presence of an intense B 1s - p* resonance), while cBN and wBN NEXAFS indicate that only sp3 bonding is present in these phases. All four observations are consistent with first-principles, electronic-structure calculations for each phase. The identification of sp2 or sp3 bonding in these samples using B and N 1s NEXAFS enable us to evaluate the bonding in an incoherent film of BN. This BN thin film was grown on a Si substrate using ion assisted laser ablation and was shown to have predominantly sp2 bonding, like hBN. However, there was additional evidence in the core-level photoabsorption that there was sp3, or cubic-like, bonding as well. This work demonstrates that NEXAFS can provide valuable structural information from fine-grained, or amorphous, BN films.
Acknowledgments
The authors would like to thank F. Corrigan (General Electric Superabrasives) for providing the wurtzite BN powder used in this study. This work was conducted under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48, and was performed at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the Department of Energy (Division of Materials Sciences and Division of Chemical Sciences of Basic Energy Sciences) under Contract No. DE-AC02-76CH0016, and at the Stanford Synchrotron Radiation Laboratory, which is supported by the Department of Energy, Office of Basic Energy Sciences.