Electronic structure analyses of BN network materials using high energy‐resolution spectroscopy methods based on transmission electron microscopy

Abstract

Electronic structures of boron-nitride (BN) nanotubes and a BN cone-structure material were studied by using a high energy-resolution electron energy-loss spectroscopy (EELS) microscope. A trial of the whole electronic structure study of hexagonal BN (h-BN), which consists of flat BN honeycomb layers, was conducted by a combination of EELS and X-ray emission spectroscopy (XES) based on transmission electron microscopy (TEM) (TEM-EELS/XES). The pi and pi+sigma plasmon energies of BN nanotubes (BNT) were smaller than those of h-BN. The pi+sigma energy was explained by the surface plasmon excitation. The spectrum of a two-wall BNT of 2.7 nm in diameter showed a new spectral onset at 4 eV. The valence electron excitation spectra obtained from the tip region of the BN cone with an apex angle of 20 degrees showed similar intensity distribution with those of BNTs. The B K-shell electron excitation spectra obtained from the bottom edge region of the BN cone showed additional peak intensity when compared with those of h-BN and BNT. The B K-shell electron excitation spectra and B K-emission spectra of h-BN were compared with a result of a LDA band calculation. It showed that high symmetry points in the band diagram appear as peak and/or shoulder structures in the EELS and XES spectra. Interband transitions appeared in the imaginary part of the dielectric function of h-BN experimentally obtained were assigned in the band diagram. The analysis also presented that the LDA calculation estimated the bandgap energy smaller than the real material by an amount of 2 eV. Those results of TEM-EELS/XES analysis presented that high energy-resolution spectroscopy methods combined with TEM is a promising method to analyze whole electronic structures of nanometer scale materials.

Anti-wear and Friction Reducing Mechanisms of Carbon Nano-onions as Lubricant Additives

“…These significant differences are basically attributed to the curvature of sheets, the coupling of electrons on the spherical shells being well different from the coupling in the planar case. The same trend has been observed for boron nitride nanotubes and cones [20]. In our conditions, mainly r bonds are excited in the spectrum and this represents the stronger carbon bond contribution in nano-onions.…”

Excitons at the B K edge of boron nitride nanotubes probed by x-ray absorption spectroscopy

“…Specifically, the prominent peak of the B K edge at about 191.6 eV is due to the B 1s → π * (2p z ) transition. The sharpness of this peak and its shift of about 1.4 eV towards lower values with respect to the bottom of the conduction band [11] are a clear indication of the excitonic character of this transition. Several theoretical calculations performed on h-BN have confirmed the existence of this core exciton state [12,13].…”

Optimizing EELS acquisition

“…3(a) and the initial energy resolution was approximately 0.3 eV. It has been shown earlier with a dedicated monochromated system that the full-width at half-maximum of the 1s-p* peak has an intrinsic value of less than 0.2 eV [15] and was therefore used here to determine the energy resolution. Due to the relatively low signal intensity, the semi-manual spectral alignment needed to be done on the noisy edge onset; this results in a slight reduction of the resolution to 0.4 eV for binned gain averaging (plotted in black).…”