Electron reflectivity measurements of Ag adatom concentrations on W(110)

The density of two-dimensional Ag adatom gases on W(110) is determined by monitoring local electron reflectivity using low energy electron microscopy (LEEM). This method of adatom concentration measurement can detect changes in adatom density at least as small as 10$^{-3}$ ML for a $\mu$m size region of the surface. Using this technique at high temperatures, we measure the sublimation rates of Ag adatoms on W(110). At lower temperatures, where Ag adatoms condense into monolayer islands, we determine the temperature dependence of the density of adatoms coexisting with this condensed phase and compare it with previous estimates.Comment: Presented at the ECOSS 23 Conference (Berlin 2005

Practical limitations to indentation testing of thin films

A method that is becoming increasingly common for measuring the mechanical behavior of thin films is low-load indentation testing. However, there can be complications in interpreting the results as many factors can affect hardness and moduli measurements such as surface roughness and determination of the indentation contact area. To further the understanding, the mechanical properties of thin (50 nm) films of AlN on sapphire substrates were evaluated using a scanning force microscopy (SFM) based pico-indentation device to allow imaging of the surface and indentations. The primary emphasis was the types of problems or limitations involved in testing very thin, as deposited films in which properties are desired over indentation depths less than 50 nm

Determining the structure of Ru(0001) from low-energy electron diffraction of a single terrace

While a perfect hcp (0001) surface has three-fold symmetry, the diffraction patterns commonly obtained are six-fold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180 degrees. Here we use a Low-Energy Electron Microscope to acquire the three-fold diffraction pattern from a single hcp Ru terrace and measure the intensity-vs-energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(+-0.9) at 456 K.Comment: 10 pages, 4 figures. Corrected some typos, added more details. Accepted for publication in Surface Science (Letters

Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76 < y < 7.00)

In order to access the overdoped regime of the YBa_2Cu_3O_y phase diagram, 2% Ca is substituted for Y in YBa_2Cu_3O_y (y = 7.00,6.93,6.88,6.76). Raman scattering studies have been carried out on these four single crystals. Measurements of the superconductivity-induced renormalization in frequency (Delta \omega) and linewidth (\Delta 2\gamma) of the 340 cm^{-1} B_{1g} phonon demonstrate that the magnitude of the renormalization is directly related to the hole concentration (p), and not simply the oxygen content. The changes in \Delta \omega with p imply that the superconducting gap (\Delta_{max}) decreases monotonically with increasing hole concentration in the overdoped regime, and \Delta \omega falls to zero in the underdoped regime. The linewidth renormalization \Delta 2\gamma is negative in the underdoped regime, crossing over at optimal doping to a positive value in the overdoped state.Comment: 18 pages; 5 figures; submitted to Phys. Rev. B Oct. 24, 2002 (BX8292

Real-space study of the growth of magnesium on ruthenium

The growth of magnesium on ruthenium has been studied by low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). In LEEM, a layer-by-layer growth is observed except in the first monolayer, where the completion of the first layer in inferred by a clear peak in electron reflectivity. Desorption from the films is readily observable at 400 K. Real-space STM and low-energy electron diffraction confirm that sub-monolayer coverage presents a moiré pattern with a 12 Å periodicity, which evolves with further Mg deposition by compressing the Mg layer to a 22 Å periodicity. Layer-by-layer growth is followed in LEEM up to 10 ML. On films several ML thick a substantial density of stacking faults are observed by dark-field imaging on large terraces of the substrate, while screw dislocations appear in the stepped areas. The latter are suggested to result from the mismatch in heights of the Mg and Ru steps. Quantum size effect oscillations in the reflected LEEM intensity are observed as a function of thickness, indicating an abrupt Mg/Ru interface

Metastable misfit dislocations during thin-film growth: The case of Cu on Ru(0001)

8 pags., 7 figs.We have used low-energy electron microscopy (LEEM) to study how misfit dislocations evolve in the first atomic layer of Cu deposited on Ru(0001). At equilibrium the Cu islands are pseudomorphic and dislocation-free. However, the islands develop dislocation networks as they grow during deposition. These dislocations are metastable: after deposition ceases, the dislocated area shrinks as adatoms ejected from the network travel to the Cu-island edges. We analyze the dislocation decay rate and find no evidence of a significant energetic barrier for Cu adatoms to exchange with the dislocated phases. Instead, the metastable film's decay is consistent with adatom diffusion being the rate-limiting factor. A small barrier for changing misfit-dislocation density has a consequence – as a film's chemical environment changes during catalysis, its dislocation density can rapidly respond.This research was partly supported by the Office of Basic EnergySciences, Division of Materials Sciences, U.S. Department of Energy,and by the Spanish Ministry of Economy and Competitiveness ProjectNo. MAT2015-64110-C02-1-P. Sandia National Laboratories is a mul-timission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA000352

The structure of the alpha-Al2O3(0001) surface from low-energy electron diffraction: Al termination and evidence for large thermal vibrations

We have determined the surface structure of alpha-Al2O3(0001) using dynamical low-energy electron diffraction (LEED). Sapphire surfaces were prepared in three different ways, and the diffraction results were analyzed using an exhaustive search of possible models. For all sample processing conditions, the clearly favored structure has a single Al layer termination and a large first interlayer contraction. In addition, we find that the surface atoms have unusually large vibrational amplitudes at room temperature, suggestive of an anharmonic vibrational mode

The structure of the alpha-Al2O3(0001) surface from low-energy electron diffraction: Al termination and evidence for large thermal vibrations

We have determined the surface structure of alpha-Al2O3(0001) using dynamical low-energy electron diffraction (LEED). Sapphire surfaces were prepared in three different ways, and the diffraction results were analyzed using an exhaustive search of possible models. For all sample processing conditions, the clearly favored structure has a single Al layer termination and a large first interlayer contraction. In addition, we find that the surface atoms have unusually large vibrational amplitudes at room temperature, suggestive of an anharmonic vibrational mode

Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction

22 pags, 14 figs, 2 tabsPalladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2-6 monolayers (ML) thick on Ru(0001), and ∼20ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fee stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fee twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.This research was partly supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, US Department of Energy under Contract Number DEAC04-94AL85000, by the Spanish Ministry of Science and Technology under Project Numbers MAT2006-13149-C02-02 and MAT2007-66719-C03-02 and by the Research Grants Council of the Hong Kong Special Administrative Region, China (CityU3/CRF/08)

Determination of the surface structure of CeO2(111) by low-energy electron diffraction

We determine the atomic structure of the (111) surface of an epitaxial ceria film using low-energy electron diffraction (LEED). The 3-fold-symmetric LEED patterns are consistent with a bulk-like termination of the (111) surface. By comparing the experimental dependence of diffraction intensity on electron energy (LEED-I(V) data) with simulations of dynamic scattering from different surface structures, we find that the CeO2(111) surface is terminated by a plane of oxygen atoms.We also find that the bond lengths in the top few surface layers of CeO2(111) are mostly undistorted from their bulk values, in general agreement with theoretical predictions. However, the topmost oxygen layer is further from the underlying cerium layer than the true bulk termination, an expansion that differs from theoretical predictions. © 2013 AIP Publishing LLC.Peer Reviewe