Energy-loss Near-edge Structure Spectra Research Articles

Theoretical Study of Carbon K‐Edge Energy‐Loss Near‐Edge Structure Spectra in the Ordered Mo2TiAlC2 MAX and Mo2TiC2 MXene

By means of density functional theory, the energy‐loss near‐edge structure (ELNES) of carbon K‐edge of Mo2TiAlC2 and corresponding MoTiC2 Mxene at orientational‐independent condition is dealt with. Compared to the MAX (M is transition metal, A is an elment from group 13–16, X is C or N) phase, the energy separations increase between the main spectral features at the C K edge of Mo2TiC2 MXene owing to the structural change and decreased bond length. The dispersions of the C K edge in both systems are similar to p‐symmetry densities of states. It is indicated that the source of the first fine structure at the C 1s edge in both phases mainly comes from the electron transfer to px + py‐like character. The other fine structures result from the transition to hybridization of pz and px + py states with the prominent contribution of px + py‐like character. Moreover, the comparison of C K‐edge ELNES spectra in three Mo‐based compounds reveals that, ongoing from Mo2TiAlC2 to Mo2TiC2 and then to Mo2C, the energy position of the fine structures is shifted to higher energies (blueshifted), due to the quantum confinement effects and the change of the chemical environment around the excited carbon.

Energy-loss near-edge structure changes with bond length in carbon systems

We show that when the graphene planes of graphite are uniformly expanded, thereby increasing the CC bond length to $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, the ${\ensuremath{\sigma}}^{*}$ edge onset of the energy-loss near-edge structure (ELNES) spectrum shifts to lower energies by almost $5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, meanwhile the ${\ensuremath{\pi}}^{*}$ edge shifts by less than $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The shift of the ${\ensuremath{\sigma}}^{*}$ edge demonstrates that for bond lengths which are typical of some carbon systems such as amorphous carbon, it is possible to find ${\ensuremath{\sigma}}^{*}$ features in the ELNES spectra at energies as low as $286--288\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Calculations on 64-atom amorphous carbon $(a\text{\ensuremath{-}}\mathrm{C})$ and amorphous carbon nitride model structures characterized by a wide range of bond lengths confirm this. Most of the $s{p}^{2}∕s{p}^{3}$ quantification techniques that are available overlook this issue of ${\ensuremath{\sigma}}^{*}$ contamination of the ${\ensuremath{\pi}}^{*}$ region and assume that all features within this energy range are entirely of ${\ensuremath{\pi}}^{*}$ origin. We show that the effect of bond length variation on the ${\ensuremath{\pi}}^{*}$ spectrum of graphite and $a\text{\ensuremath{-}}\mathrm{C}$ is minor, thereby supporting the reliability of the former spectrum for $s{p}^{2}∕s{p}^{3}$ quantification purposes, as was recently demonstrated [see J. T. Titantah and D. Lamoen, Phys. Rev. B 70, 075115 (2004)].

Chemical bonding state at grain boundaries in BaTiO3 doped with a small amount of cation

The chemical bonding state at grain boundaries was examined with electronenergy-loss spectroscopy in BaTiO3 doped with a small amount of cations at a level of 0.001. In contrast with A-site excess compounds, the oxygen K-edge electron energy-loss near-edge structure spectra were different for grain interiors and for grain boundaries in B-site excess compounds. The difference is explained in terms of the linkage of TiO6 octahedra at grain boundaries in the B-site excess BaTiO3.