J. Ciston

Water-driven structural evolution of the polar MgO (111) surface: An integrated experimental and theoretical approach

We report an experimental and theoretical analysis of the root(3)xroot(3)-R30 and 2x2 reconstructions on the MgO (111) surface combining transmission electron microscopy, x-ray photoelectron spectroscopy, and reasonably accurate density functional calculations using the meta-GGA functional TPSS. The experimental data clearly shows that the surfaces contain significant coverages of hydroxyl terminations, even after UHV annealing, and as such cannot be the structures which have been previously reported. For the 2x2 surfaces a relatively simple structural framework is detailed which fits all the experimental and theoretical data. For the root(3)xroot(3) there turn out to be two plausible structures and neither the experimental nor theoretical results can differentiate between the two within error. However, by examining the conditions under which the surface is formed we describe a kinetic route for the transformation between the different reconstructions that involves mobile hydroxyl groups and protons, and relatively immobile cations, which strongly suggests only one of the two root(3)xroot(3) structures.

Why the case for clean surfaces does not hold water: Structure and morphology of hydroxylated nickel oxide (1 1 1)

We report an experimental and theoretical analysis of the sqrt(3)x sqrt(3)-R30 and 2x2 reconstructions on the NiO (111) surface combining transmission electron microscopy, x-ray photoelectron spectroscopy, and reasonably accurate density functional calculations using the meta-GGA hybrid functional TPSSh. While the main focus here is on the surface structure, we also observe an unusual step morphology with terraces containing only even numbers of unit cells during annealing of the surfaces. The experimental data clearly shows that the surfaces contain significant coverage of hydroxyl terminations, and the surface structures are essentially the same as those reported on the MgO (111) surface implying an identical kinetically-limited water-driven structural transition pathway. The octapole structure can therefore be all but ruled out for single crystals of NiO annealed in or transported through humid air. . The theoretical analysis indicates, as expected, that simple density functional theory methods for such strongly-correlated oxide surfaces are marginal, while better consideration of the metal d-electrons has a large effect although, it is still not perfect.

Diffraction refinement of localized antibonding at the Si(111) 7×7 surface

We report an experimental refinement of the local charge density at the Si (111) 7x7 surface utilizing a combination of x-ray and high energy electron diffraction. By perturbing about a bond-centered pseudoatom model, we find experimentally that the adatoms are in an anti-bonding state with the atoms directly below. We are also able to experimentally refine a charge transfer of 0.26(4) e- from each adatom site to the underlying layers. These results are compared with a full-potential all-electron density functional DFT calculation.

Optimized conditions for imaging the effects of bonding charge density in electron microscopy

We report on the observability of valence bonding effects in aberration-corrected high resolution electron microscopy (HREM) images along the [010] projection of the mineral Forsterite (Mg₂SiO₄). We have also performed exit wave restorations using simulated noisy images and have determined that both the intensities of individual images and the modulus of the restored complex exit wave are most sensitive to bonding effects at a level of 25% for moderately thick samples of 20-25 nm. This relatively large thickness is due to dynamical amplification of bonding contrast arising from partial de-channeling of 1s states. Simulations also suggest that bonding contrast is similarly high for an un-corrected conventional electron microscope, implying an experimental limitation of signal to noise ratio rather than spatial resolution.

Cone‐angle Dependence of Ab‐initio Structure Solutions Using Precession Electron Diffraction

Precession electron diffraction (PED) is a technique which is gaining increasing interest due to its ease of use and reduction of the dynamical scattering problem in electron diffraction, leading to more direct structure solutions. We have performed a systematic study of the effect of precession angle for the mineral andalusite on kinematical extinctions and direct methods solutions where the semiangle was varied from 6.5 to 32 mrad in five discrete steps. We show that the intensities of kinematically forbidden reflections decay exponentially as the precession semiangle (φ) is increased and that the amount of information provided by direct methods increases monotonically but non‐systematically as φ increases. We have also investigated the zeolite‐framework mineral mordenite with PED and have found a direct methods solution where the 12‐ring is clearly resolved for the first time.

Fitting valence charge densities at a crystal surface

A procedure is reported for obtaining a much better initial parameterization of the charge density than that possible from a neutral atom model. This procedure involves the parameterization of a bulk charge density model in terms of simple variables such as bond lengths, which can then be transferred to the problem of interest, for instance a surface. Parameterization is accomplished through the fitting of density functional theory calculations for a variety of crystal distortions. The details of the parameterization are discussed for the specific case of silicon. This parameterized model can then be applied to surfaces or to other problems where an initial higher-order model is needed without the addition of any extra fitted parameters. The non-convexity of the charge density problem is also discussed.

Experimental measurements of bond density at the Si(111)-7x7 surface

coupled technique, PArallel Recording Of Dark-field Images (PARODI). It is a new breed of convergent beam electron diffraction developed at Brookhaven to accurately determine structure factors of low-order reflections that are sensitive to valence electron distribution. The synchrotron based single-crystal x-ray diffraction was used to determine the structure factors of high-order reflections that are sensitive to atomic positions and Debye-Waller factors. The two sets of experimental data were combined and refined, and then compared with DFT calculations. Examples on charge density studies including CaCu3Ti4O12 oxide that exhibits extremely high dielectric constant (~104) over a wide range of frequencies and temperatures will be given [1,2]. Our recent work on electron scattering amplitudes involving non-spherical orbitals (p and d orbitals) of transition-metal elements will also be reported. We demonstrate that it is possible to accurately measure valence electron distribution, electron orbitals and bonding characteristics of complex functional materials using quantitative electron and x-ray diffraction. Collaborations with J.C. Zheng, L. Wu, J. Hanson, P. Northrup and W. Ku are acknowledged. This work is supported by U.S. DOE under Contract No. DE-AC02-76CH00016. [1] Y. Zhu, J.C Zheng et al Phys. Rev. Lett., 99, 037602 (2007). [2] L. Wu, Y. Zhu, et al Phys. Rev. B 69, 064501 (2004).