Layer speciation and electronic structure investigation of freestanding hexagonal boron nitride nanosheets

Abstract

Chemical imaging, thickness mapping, layer speciation and polarization dependence have been performed on single and multilayered (up to three layers and trilayered nanosheets overlapping to form 6 and 9 layers) hexagonal boron nitride (hBN) nanosheets by scanning transmission X-ray microscopy. Spatially-resolved XANES directly from freestanding regions of different layers has been extracted and compared with sample normal and 30° tilted configurations. Notably a double feature σ* excitonic state and a stable high energy σ* state were observed at the boron site in addition to the intense π* excitonic state. The boron projected σ* DOS, especially the first σ* exciton, is sensitive to surface modification, particularly in the single layered hBN nanosheet which shows more significant detectable contaminants and defects such as tri-coordinated boron/nitrogen oxide. The nitrogen site has shown very weak or no excitonic character. The distinct excitonic effect on boron and nitrogen was interpreted to the partly ionic state of hBN. Bulk XANES of hBN nanosheets was also measured to confirm the spectro-microscopic STXM result. Finally, the unoccupied electronic structures of hBN and graphene were compared.

Angle-resolved electron energy loss spectroscopy in hexagonal boron nitride

“…The core hole is on the nitrogen atom, but the electron in the conduction band is still concentrated on the boron atoms. Excitonic effects and oscillator strengths are therefore expected to be weaker, which is the case: The measured EELS signals are much weaker and therefore less accurate, and the spectra show broader and less "atomiclike" features, in agreement with previous studies [34,35,37,40,42].…”

“…It can be noticed that the π * peak at low energy is separated from a quasicontinuum starting with the σ * peak at higher energy which is typical of excitonic behavior. The splitting of the main σ * peak apparent in the ω-q plot probably also has an excitonic origin [40]. The corresponding EEL spectrum forq along the hexagonal axis is shown in Fig.…”

Peculiar bond characters of fivefold coordinated octet compound crystals

“…41 As discussed in the ESI, † under the dipole approximation, 42,43 angle-resolved near-edge spectra are closely related to densities of states projected onto p z -and p x,y -like symmetries, whereby transition peaks are identied as originating from p* (p z ) and s* (p x,y ) states. [43][44][45][46][47] For 2D and layered materials such as graphene, graphite, and h-BN, both angle-resolved ELNES and polarization-resolved XANES have experimentally revealed p* and s* transitions, as predicted by density functional theory (DFT) calculations with core hole effects properly considered. 44,47,48 Fig.…”

Influence of point defects on the near edge structure of hexagonal boron nitride

“…Significant differences in the ratio of peak intensities for the σ * doublet feature are evident between samples. Although the σ * doublet feature is routinely observed in bulk, monolayer and nanotube hBN samples, there is debate over its origin[24,31,55]. In the case of the NK edge [Fig.2(b)], the peak at 401.8 eV is attributed to 1s to π * transitions, while a shoulder at 406.4 eV (labeled γ ) and the 408.7-eV peak and above are mainly due to 1s to σ * transitions.…”