R. Stumpf

How metal films de-wet substrates?identifying the kinetic pathways and energetic driving forces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation.

Structure and magnetism of ultra-thin chromium layers on W(110)

We have investigated the structural and magnetic properties of ultra-thin Cr films on W(110) by means of low-energy electron diffraction intensity-versus-voltage (LEED IV) data acquired using selected-area diffraction within a low-energy electron microscope (LEEM), spin-polarized scanning tunneling microscopy (SP-STM), and ab initio calculations. The interlayer distances as obtained from LEED IV data are compared to ab initio calculations. The first-principles calculations predict very different interlayer spacings depending on whether the Cr films are antiferromagnetic or non-magnetic. Only antiferromagnetic spin ordering leads to interlayer spacings similar to the experimental spacings determined by LEED IV. This strongly suggests that films of one, two and three atomic layers of Cr on W(110) have antiferromagnetic short-range order. SP-STM data confirm this finding: the Cr monolayer on W(110) shows characteristic stripes along the (001) direction due to the antiferromagnetic order of nearest-neighbor Cr atoms. Additionally, the SP-STM data of the Cr monolayer reveal a periodically varying magnetic amplitude that peaks every 7.7± 0.5 nm. On thick Cr(110) films the signature of an

The electronic structure of Be(101̄0)

Abstract We measured the surface state dispersions on Be(1010) by angle-resolved photoemission (ARUPS) and calculated the electronic structure for both possible terminations of the bulk crystal from first principles. The experimental data agrees only with the prediction for the termination with the shorter first interlayer spacing. This is also supported by a comparison between our calculation of the surface core-level shift spectrum with the experimental data from Johansson et al. However, in contrast to their interpretation, our calculation predicts a larger surface core level shift for second-layer Be atoms than for first-layer atoms.

Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110).

We observe using low-energy electron microscopy the self-assembly of monolayer-thick stripes of Au on W(110) near the transition temperature between stripes and the nonpatterned (homogeneous) phase. We demonstrate that the amplitude of this Au-stripe phase decreases with increasing temperature and vanishes at the order-disorder transition (ODT). The wavelength varies much more slowly with temperature and coverage than theories of stress-domain patterns with sharp boundaries would predict, and maintains a finite value of about 100 nm at the ODT. We argue that such nanometer-scale stripes should often appear near the ODT.

Ab-Initio Pseudopotential Calculations of Boron Diffusion in Silicon

In this work we investigate boron diffusion as a function of the Fermi-level position in crystalline silicon using ab-initio calculations and the nudged elastic band method to optimize diffusion paths. Based on our results, a new mechanism for B diffusion mediated by Si self-interstitials is proposed. We find a two-step diffusion process for all Fermi-level positions, which suggests a kick-out with a directly following kick-in process without extensive B diffusion on interstitial sites in-between. Our activation energy of 3.47 – 3.75 eV and diffusion-length exponent of -0.55 to -0.18 eV are in excellent agreement with experiment.

Hydrogen storage in sodium aluminum hydride.

Sodium aluminum hydride, NaAlH{sub 4}, has been studied for use as a hydrogen storage material. The effect of Ti, as a few mol. % dopant in the system to increase kinetics of hydrogen sorption, is studied with respect to changes in lattice structure of the crystal. No Ti substitution is found in the crystal lattice. Electronic structure calculations indicate that the NaAlH{sub 4} and Na{sub 3}AlH{sub 6} structures are complex-ionic hydrides with Na{sup +} cations and AlH{sub 4}{sup -} and AlH{sub 6}{sup 3-} anions, respectively. Compound formation studies indicate the primary Ti-compound formed when doping the material at 33 at. % is TiAl{sub 3} , and likely Ti-Al compounds at lower doping rates. A general study of sorption kinetics of NaAlH{sub 4}, when doped with a variety of Ti-halide compounds, indicates a uniform response with the kinetics similar for all dopants. NMR multiple quantum studies of solution-doped samples indicate solvent interaction with the doped alanate. Raman spectroscopy was used to study the lattice dynamics of NaAlH{sub 4}, and illustrated the molecular ionic nature of the lattice as a separation of vibrational modes between the AlH{sub 4}{sup -} anion-modes and lattice-modes. In-situ Raman measurements indicate a stable AlH{sub 4}{sup -} anion that is stable at the melting temperature of NaAlH{sub 4}, indicating that Ti-dopants must affect the Al-H bond strength.