Practical spatial resolution of electron energy loss spectroscopy in aberration corrected scanning transmission electron microscopy

The resolution of electron energy loss spectroscopy (EELS) is limited by delocalization of inelastic electron scattering rather than probe size in an aberration corrected scanning transmission electron microscope (STEM). In this study, we present an experimental quantification of EELS spatial resolution using chemically modulated 2×(LaMnO(3))/2×(SrTiO(3)) and 2×(SrVO(3))/2×(SrTiO(3)) superlattices by measuring the full width at half maxima (FWHM) of integrated Ti M(2,3), Ti L(2,3), V L(2,3), Mn L(2,3), La N(4,5), La N(2,3) La M(4,5) and Sr L(3) edges over the superlattices. The EELS signals recorded using large collection angles are peaked at atomic columns. The FWHM of the EELS profile, obtained by curve-fitting, reveals a systematic trend with the energy loss for the Ti, V, and Mn edges. However, the experimental FWHM of the Sr and La edges deviates significantly from the observed experimental tendency.     

Instability,intermixing and electronic structure at the epitaxial LaAlO3/SrTiO3(001) heterojunction

The question of stability against diffusional mixing at the prototypical LaAlO₃/SrTiO₃(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO₃ or SrTiO₃, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface—electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO₃ below the interface as being the cause of the observed conductivity.     

Hydrogen encapsulation in a silicon clathrate type I structure: Na5.5(H2)2.15Si46: synthesis and characterization

A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300 degrees C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23Na, 1H, and 29Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23Na NMR shows that sodium completely fills the small cage and is deficient in the larger cage. The 1H NMR spectrum shows a pattern consistent with mobile hydrogen in the large cage. 29Si NMR spectrum is consistent with phase pure type I clathrate framework. Elemental analysis is consistent with the stoichiometry Na5.5(H2.15)2Si46. The sodium occupancy was also examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the Na occupancy of the large cage, 6d sites, is less than 2/3, consistent with the NMR and elemental analysis.     

Visualizing the Stoichiometry of Industrial‐Style Co‐Mo‐S Catalysts with Single‐Atom Sensitivity

The functional properties of transition metal dichalcogenides (TMDs) may be promoted by the inclusion of other elements. Here, we studied the local stoichiometry of single cobalt promoter atoms in an industrial‐style MoS₂‐based hydrotreating catalyst. Aberration‐corrected scanning transmission electron microscopy and electron energy loss spectroscopy show that the Co atoms occupy sites at the (?100) S edge terminations of the graphite‐supported MoS₂ nanocrystals in the catalyst. Specifically, each Co atom has four neighboring S atoms that are arranged in a reconstructed geometry, which reflects an equilibrium state. The structure agrees with complementary studies of catalysts that were prepared under vastly different conditions and on other supports. In contrast, a small amount of residual Fe in the graphite is found to compete for the S edge sites, so that promotion by Co is strongly sensitive to the purity of the raw materials. The present single‐atom‐sensitive analytical method therefore offers a guide for advancing preparative methods for promoted TMD nanomaterials.     

Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

Energy loss spectra from fluorite-structured ZrO(2), CeO(2), and UO(2) compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT?+?U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT?+?U and hybrid computational approaches.