Near-edge x-ray absorption spectroscopy is well-known as a method for characterizing the bonding and orientation of organic molecules adsorbed on thin-film surfaces.[1] Subsequent work has shown that core-level x-ray absorption is also a useful technique for characterizing the unoccupied electronic states of low-atomic-number solids. A number of x-ray absorption studies have focussed on graphite[2] and diamond,[3] the two common crystalline phases of carbon. Other researchers have compared the spectra of amorphous carbon and hydrogenated diamond-like carbon thin films to those of the bulk crystalline phases.[4]
Boron nitride is isoelectronic to carbon and has both hexagonal and cubic phases analogous to graphite and diamond. In hexagonal BN (hBN), B and N atoms in alternate layer planes lie directly on top of one another in an AAprimeA stacking arrangement, as opposed to the staggered ABA stacking in graphite, which causes C atoms in neighboring layers to be offset. Rhombohedral BN (rBN) differs from hBN only in its staggered ABCA stacking. Previous NEXAFS studies have compared the B and N K-edge spectra of well-ordered hexagonal, cubic, wurtzite and rhombohedral BN powders.[5] The insight obtained through analysis of the powder spectra can be used to interpret similar data taken on thin BN films, which are likely to find use as both tribological coatings and as wide-gap semiconducting materials.