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Research highlights

01.09.2017 Structure of the SnO2(110)−(4×1) Surface

Lindsay R. Merte, Mathias S. Jørgensen, Katariina Pussi, Johan Gustafson, Mikhail Shipilin, Andreas Schaefer, Chu Zhang, Jonathan Rawle, Chris Nicklin, Geoff Thornton, Robert Lindsay, Bjørk Hammer, and Edvin Lundgren, Phys. Rev. Lett. 119, 096102

Using surface x-ray diffraction (SXRD), quantitative low-energy electron diffraction (LEED), and density-functional theory (DFT) calculations, we have determined the structure of the (4×1) reconstruction formed by sputtering and annealing of the SnO2(110) surface. We find that the reconstruction consists of an ordered arrangement of Sn3O3 clusters bound atop the bulk-terminated SnO2(110) surface. The model was found by application of a DFT-based evolutionary algorithm with surface compositions based on SXRD, and shows excellent agreement with LEED and with previously published scanning tunneling microscopy measurements. The model proposed previously consisting of in-plane oxygen vacancies is thus shown to be incorrect, and our result suggests instead that Sn(II) species in interstitial positions are the more relevant features of reduced SnO2(110) surfaces.

22.11.2013 Structure and local variations of the graphene moire on Ir(111)

Sampsa K. Hämäläinen1, Mark P. Boneschanscher2, Peter H. Jacobse2, Ingmar Swart2, Katariina Pussi3, Wolfgang Moritz4, Jouko Lahtinen1, Peter Liljeroth1, and Jani Sainio, Phys. Rev. B 88, 201406(R) (2013).

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED I(V)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED I(V) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moiré structure: The corrugation varies smoothly over several moiré unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moiré symmetry sites and the underlying substrate. We also observe isolated outliers, where the moiré top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity.

6.2.2013 Acene adsorption on a Fibonacci-modulated Cu film

K. M. Young, J. A. Smerdon, H. R. Sharma, M. Lahti, K. Pussi, and R. McGrath, PHYSICAL REVIEW B 87, 085407 (2013).

The adsorption of pentacene (Pn) on the fivefold surface of an i-Al-Pd-Mn quasicrystal and on a one-dimensionally aperiodic Cu multilayer formed thereon is observed with scanning tunneling microscopy. The molecule has a strong interaction with the clean quasicrystal surface, leading to the formation of a disordered layer. On the Cu film, a molecular layer assembles, with the Cu rows acting as a template. At lower coverages, there is a repulsive interaction between the molecules, leading to a dispersed homogeneous arrangement. At higher coverages, the steric interaction of the Pn molecules counterbalances the aperiodic template, which results in a short-range periodic “checkerboard” arrangement of molecules. Density functional theory calculations using lower order acenes are used to probe the details of the interactions with the aperiodic Cu surface and it is found that adsorption with molecules parallel to Cu rows is preferred, in agreement with experimental results.

22.8.2011 Structure of the orthorhombic Al13Co4(100) surface using LEED, STM, and ab initio studies

Heekeun Shin, K. Pussi, ´E. Gaudry, J. Ledieu, V. Fournee, S. Alarc´on Villaseca, J.-M. Dubois, Yu. Grin,4 P. Gille, W. Moritz, and R. D. Diehl, PHYSICAL REVIEW B 84, 085411 (2011).

The aperiodic surfaces of quasicrystals have different physical properties from their periodic counterparts. For instance, they have lower coefficients of friction and lower adhesion energies against polar liquids. Because their unit cells are infinite, they are often modeled using structures that have the same local arrangements but which, unlike quasicrystals, are periodic over a longer range, so-called quasicrystal approximants. Due to the complexity of the structures, it can be difficult to ascertain in a quantitative way the degree to which the finite unit cell structures replicate the aperiodic crystals, i.e. are discrepancies due to the imperfect modelingmethods or due to themodel structure itself? It is also the case that the powders used for quasicrystalline coatings likely have a significant fraction of particles with periodic structures that are near to the quasicrystalline phase in the alloy phase diagram. For these reasons, it is desirable to study some examples of the quasicrystal approximants.

In a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study of the surface of Al13Co4(100), all techniques have found that after annealing to 1165 K, the surface structure is consistent with a dense Al-rich plane with surface Co atom depletion. Various structure models were considered, and in the LEED study, the best agreement was found with a model that consists of Al-rich terminating planes with no Co atoms, and otherwise a structure similar to the bulk puckered layers. This structure was also found to be stable in the DFT study. The best-fit structural parameters are presented for the two domains of this structure, which contain bipentagons that can be related to the pentagonal bipyramidal structures in the bulk, plus additional glue atoms between them. These domains are not strictly related to each other by symmetry, as they have different surface relaxations.

FIG. 1. The bulk structure of Al13Co4. The dark (blue) spheres are aluminum and the light (pink) spheres are cobalt. (a) Side view of the unit cell, composed of four layers of atoms. (b) Side view (left) of the pyramid structure, and a top view (right) of each layer. The registry of the center F layer is different for F1 and F2; both are shown. © Top view of F layer. (d) Top view of P layer.

2.6.2011 Tailoring the Structure of Water at a Metal Surface: A Structural Analysis of the Water Bilayer Formed on an Alloy Template

Fiona McBride, George R. Darling, Katariina Pussi, and Andrew Hodgson, PRL 106, 226101 (2011).

Recent studies show that structures based on the traditional “icelike” water bilayer are not stable on flat transition metal surfaces and, instead, more complex wetting layers are formed. Here we however show that an ordered bilayer can be formed on a SnPt(111) alloy template.

A possible way to stabilize an ordered commensurate ice layer is to form a suitable template by modifying the surface so that different adsorption sites are chemically distinct. Here we describe water adsorption on a Pt(111) surface alloy containing Sn atoms arranged in a root3xroot3-R30 arrangement. By combining helium atom scattering (HAS), work function change, and low energy electron diffraction (LEED), we show that water forms a simple commensurate wetting layer. Water adopts the H-down bilayer structure, with one water bonding flat atop Sn and the other oriented ‘‘H-down’’ above Pt, allowing us to make a quantitative comparison between the structure of a water layer determined by LEED IV and that predicted by density functional theory (DFT).

FIG. 1 Comparison of structure calculated by DFT (left) and that obtained from the LEED IV fit (right). Interlayer and O-metal separations in Ångstrom relative to the bulk position.

21.8.2009 Surface Geometry of C60 on Ag(111)

H. I. Li, K. Pussi, K. J. Hanna, L.-L. Wang, D. D. Johnson, H.-P. Cheng, H. Shin, S. Curtarolo, W. Moritz, J. A. Smerdon, R. McGrath, and R. D. Diehl, PRL 103, 056101 (2009).

The geometry of adsorbed C60 influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a C60 monolayer, Ag(2√3×2√3)R30-C60, and related density functional theory calculations. The stable monolayer has C60 molecules in vacancies that result from the displacement of surface atoms. C60 bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C60 monolayers on close-packed metal surfaces.

We have demonstrated that a monolayer of C60 on Ag(111) induces a substrate reconstruction, producing vacancies that are occupied by C60. This phenomenon is relatedated to the relative energies of vacancy formation and chemisorption. Our DFT calculations show that on both Ag(111) and Au(111), the balance favors reconstruction. A previous DFT study indicated the same for C60 on Al(111). Until now, although reconstructions were often observed for C60 adsorption on other surfaces, they were thought to be uncommon for C60 adsorption on close- packed noble metal substrates, where the bonding is regarded as mainly ionic, even though reconstructions were reported for C60 on Cu(111). This study demonstrates utility of LEED for the determination of large molecule adsorption geometries. In light of these results, it seems probable that the observed preference for C60 adsorption along the zigzag steps may also involve vacancy reconstructions.

FIG. 1 (a) Vacancy site structure, showing three layers of Ag. The mirror plane of the molecule is parallel to the mirror plane of the substrate (dashed line). There are two such parallel orientations of the molecules, one as shown, and one rotated 180. The top hexagon is surrounded by 3 hexagons and 3 pentagons–180 rotation interchanges these (as well as some others that are not visible). (b) Side view of the vacancy structure, viewed from the bottom of (a), along the dashed line, including the atoms in the box on (a). The buckling is magnified and the atoms are shown with a reduced size for clarity.

11.3.2009 Stainless Steel Alloys

H. Pitkänen, A. Puisto, M. Alatalo, M. Ropo, K. Kokko, M. P. J. Punkkinen, P. Olsson, B. Johansson, and L. Vitos, PRB 79, 024108 (2009).

Stainless steels form the largest group of maintenance free and safe engineering materials. The main stainless groups are austenitic and ferritic, but today, a third important group is developing around the so called duplex grades. These steels have better corrosion resistance compared to conventional steel grades, and, since they are also mechanically stronger, material costs can be significantly reduced with this class of materials.

Duplex steels are, basically, materials which consist of a macro-scale mixture of BCC and FCC alloys. They are formed when is is energetially more favourable for a material to break into grains of BCC and FCC, rather than assuming only either of these phases. Because of the complexity of the problem, a unique methodological approach is used. Here, we employed a method called the Coherent Potential Approximation (CPA), in which the alloy components are embedded in an effective medium, which is constructed in such a way that it represents, on the average, the scattering properties of the alloy. Thus, we can avoid performing numerous calculations with huge supercells, as we would have to do when using a conventional method.

In figs 1 and 2, we have plotted the mixing enthalpies which were calculated with the above mentioned method, and several entropy/energy contributions which were also taken into account. When the three different entropy/energy contributions are added to the mixing enthalpy, we arrive to the end result, the Gibbs free energy of formation. The graphs showed in Figs 1 and 2 are calculated with the Cr content fixed to 16%, and shown as a function of varying Ni content. The same results were calculated with various Cr concentraions also, and, by comparing the Gibbs free energies of BCC and FCC phases to each other using the common tangent plane method, we arrive at our final result, shown in Fig. 3. The contour plot shows the Gibbs free energy of formation as a function of Cr and Ni composition, and the dashed lines mark the approximated phase boundaries between ferritic, duplex, and austenitic grades.

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