Dynamical LEED Research Articles

Crystal structure reconstruction in the surface monolayer of the quantum spin liquid candidate α-RuCl3

α-RuCl3, a layered 2D material, was recently identified as a promising candidate for realizing a Kitaev quantum spin liquid. However one fundamental property, the crystal structure, has not been well resolved yet due to difficulty of bulk diffraction techniques caused by layer stacking faults. Furthermore, the surface relaxation of monolayer-level thin films is completely unknown yet. In this report, surface sensitive low energy electron diffraction technique with µm selectivity (µ-LEED) combined with dynamical LEED analysis were used to reveal the detailed crystal structure of the surface monolayer of α-RuCl3. A surface structural distortion that breaks the inversion symmetry of the ideal bulk structure was revealed. To be specific, we found the surface Cl sub-lattice is buckled with one Cl atom approximately 0.16 Å below the other two Cl atoms, in the unit cell. The Ru atomic layer shows an even larger buckling of approximately 0.31 Å. Through density functional theory (DFT) calculations, we suggested that this surface distortion may be induced by Cl-vacancies that are inevitable in this material system. Inversion symmetry breaking in this material could have a significant impact on the 2D Kitaev interaction for both, interfaces with other 2D materials, such as graphene and future monolayer devices.

Interaction of copper with the rhenium surface

We have studied the adsorption of copper on the clean Re surface between 300 and 900 K by means of low- and medium-energy electron diffraction (LEED and MEED) and temperature-programmed thermal desorption (TPD). The persistence of a (1 × 1) LEED pattern during Cu deposition suggests the formation of pseudomorphic Cu islands. Accordingly, the intensity–voltage behaviour of the (1 × 1) LEED beams can be quantitatively superimposed by the coverage-weighed fractions of the I(V)-curves of uncovered Re areas and of Cu-covered (1 × 1) islands. At a coverage of 1.625 × 1019 Cu atoms m−2 dynamical LEED I(V) calculations suggest a full hcp-oriented Cu bilayer (BL). Within this first BL, Cu wets the Re surface completely, while all following layers exhibit remnant roughness due to small Cu nuclei, as confirmed by in situ grazing-incidence MEED experiments. The completion of the first BL coincides with the saturation of a single β TPD state at 1180 K, whilst higher coverages produce an additional zero-order α state, which peaks at 1080 K at 2.6 BL. The energy of desorption rises from 320 kJ mol−1 at small coverages to ∼360 kJ mol−1 for a bilayer Cu film, pointing to attractive lateral Cu–Cu interactions. An analysis of the leading edge of the multilayer α state yields a desorption energy of ∼305 kJ mol−1, somewhat lower than the sublimation enthalpy of bulk Cu. Our data are discussed and compared with previous results on related systems.

Dynamical LEED analysis of Ni 2P (0 0 0 1)-1 × 1: Evidence for P-covered surface structure

Nickel phosphide (Ni 2P) is an emerging catalyst for hydrodesulfurization and other important environment- and energy-related catalytic reactions. To understand its high performance, the surface structure of a Ni 2P (0 0 0 1) single crystal surface was investigated using dynamical LEED analysis. The obtained surface structure for Ni 2P (0 0 0 1)-1 × 1 is a P-covered Ni 3P 2 structure (Ni 3P_P structure) as opposed to the expected bulk terminated surface structures. This Letter discusses the driving force for the formation of the Ni 3P_P surface, which involves the minimization of dangling bonds.

Ultrathin silver films on Ni(111)

The growth and atomic structure of ultrathin silver films on Ni(111) was investigated by low-energy electron microscopy and diffraction (LEEM/LEED) as well as intensity-voltage $[I(V)]$-LEEM in the growth temperature range between 470 and 850 K. We find that silver grows in a Stranski-Krastanov mode with a two monolayer thin wetting layer which takes on a $p(7\ifmmode\times\else\texttimes\fi{}7)$ reconstruction at temperatures lower than 700 K and a $(\sqrt{52}\ifmmode\times\else\texttimes\fi{}\sqrt{52})R13.9\ifmmode^\circ\else\textdegree\fi{}$ reconstruction at higher temperatures. The occurrence of the two distinct reconstructions is shown to have profound implications for the growth characteristics of films exhibiting thicknesses of one and two monolayers. The nanoscale $I(V)$ characteristics of the films were analyzed by means of multiple-scattering calculations based on dynamical LEED theory. Furthermore, the vertical interatomic spacing at the interface between the Ag film and the Ni substrate was determined to $(2.8\ifmmode\pm\else\textpm\fi{}0.1)\text{ }\text{\AA{}}$ for all film thicknesses $(<13\text{ }\text{ML})$ while the uppermost silver layer relaxes by about $(4\ifmmode\pm\else\textpm\fi{}1)\mathrm{%}$ toward the crystal.

Multilayer Relaxation of Ru(0001)-(2 × 2)-O Studied by Surface X-ray Diffraction

The topmost and inner layer structures of Ru(0001)-(2 × 2)-O have been determined by surface X-ray diffraction. The periodical peaks near integer positions were observed along the fractional order rod. Adsorbed oxygen causes relaxation and rumpling in the deeper layer as well as in the topmost layer. Atomic displacement from the bulk position decreases exponentially with depth below the second layer. The topmost Ru−Ru interlayer spacing and Ru−O bond length are consistent with those obtained from DFT calculations and dynamical LEED measurements reported previously.

Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite

Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali metals with surfaces. The geometries of the $(2\ifmmode\times\else\texttimes\fi{}2)$ structure of potassium on both single-crystal graphite (SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of $\mathrm{K}$ atoms in the hollow sites on top of the surface. The $\mathrm{K}$-graphite average perpendicular spacing is $2.79\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, corresponding to an average $\mathrm{C}\text{\ensuremath{-}}\mathrm{K}$ distance of $3.13\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the spacing between graphite planes is consistent with the bulk spacing of $3.35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite.

Adsorption geometry of Cu( [formula omitted])-(12 × 2)-14Xe

Low-energy electron diffraction was used to study the adsorption geometry of Xe on Cu(1 1 0). At low temperatures, a higher-order commensurate (12 × 2) structure can be formed that consists of rows of Xe atoms aligned along the direction of the rows on Cu(1 1 0), and having a spacing between the rows equal to the Cu row spacing. A dynamical LEED study of this structure at 20 K indicates that the rows of Xe atoms are localized on top of the Cu substrate rows. The lateral registry of the Xe rows along the Cu rows includes 2 Xe atoms per (12 × 2) unit cell in atop positions above Cu atoms, with the others in lower-symmetry sites. The average Xe–Cu interlayer spacing is 3.3 ± 0.1 Å and there is a small degree (0.1 Å) of vertical rumple in both the overlayer and the top substrate layer. The substrate surface with adsorbed Xe has essentially the same relaxation as the clean surface. The analysis of the adatom vibrations in this structure indicates that the isotropically-averaged rms vibration amplitude is 0.2 Å for Xe, while that of the top-layer Cu atoms is 0.04 Å.

HOLOGRAPHIC ANALYSIS OF KIKUCHI ELECTRON DIFFRACTION DATA FROM ${\rm Si}(111)(\sqrt{3}\times\sqrt{3}){\rm R}30^\circ{\rm -Al}$

Holographic analysis was applied to Kikuchi electron diffraction data collected from the [Formula: see text] structure to produce real-space images of the surface structure. Methods for calibrating the initial diffraction data are fully described. The images clearly show features caused by the Al atoms in the T4 absorption site. Measurements of the Al–Si bond length and the height of the Al above the top layer of Si agree with the values known from dynamical LEED studies.

Bonding mechanism and atomic geometry of an ordered hydroxyl overlayer on Pt(111).

Exposing water to a (2 x 2)-O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (square root 3 x square root 3)R30 degrees overlayer. The structure of this overlayer is determined by DFT and full dynamical LEED calculations. There are two O containing groups per (square root 3 x square root 3)R30 degrees unit cell and both occupy near on-top positions with a Pt-O bond length of (2.11 +/- 0.04) A. DFT calculations determined the hydrogen positions of the OH species and clearly indicate hydrogen bonds between the neighboring adsorbed OH groups whose interaction is mainly of electrostatic nature. A theoretical comparison with H(2)O shows the hybridization of OH on Pt(111) to be sp(3).

Low-energy electron diffraction study of the surface geometry of Ni(100)–(3×3)-K+4O

A dynamical LEED study of the (3×3) structure which forms upon coadsorption of K and O on Ni(100) indicates that this structure is comprised of one K atom and four O atoms per (3×3) unit cell. The K atom is adsorbed in a substitutional site, and surrounded by four O atoms in hollow sites. The four O atoms are displaced by 0.35±0.08 Å from the centers of the hollows, and the average ONi distance is 1.92±0.17 Å. The KO distance is 2.93±0.15 Å and the KNi distance is 3.33±0.12 Å. We propose that the primary driving force for the creation of the vacancies is the reduction of energy when the K atoms move closer to the surface. At a later stage of oxidation, these vacancies may facilitate the formation of NiO by helping to alleviate the strain in the lattice mismatch between the NiO structure and Ni(100).

LEED AND STM STUDY OF Cs ON Cu(211)

A low-temperature (25 K) STM study of Cs adsorption on Cu(211) indicates that Cs forms variable-density structures which align along the step edges of the Cu(211) surface. The density of the overlayer increases with Cs coverage, forming a quasihexagonal c(2×2) structure at a coverage of 0.17. A dynamical LEED study of that structure at 130 K indicates that the Cs atoms are adsorbed on top of the Cu atoms in the center of the terraces, with a Cs–Cu nearest-neighbor distance of 3.56± 0.04 Å. This structure is accompanied by a significant rumpling of the Cu(211) surface.

Face-dependent segregation and relaxation in Mo 0.75Re 0.25 random alloy surfaces

The structure and composition of the three low index surfaces (100), (110) and (111) of the substitutionally disordered alloy Mo 0.75Re 0.25 was investigated by full dynamical LEED intensity analyses. Strong face-dependencies are found for the composition of the surface layers as well as for the relaxation of their spacings. The (110) orientation turns out to be almost bulk-like terminated geometrically as expected for a quasi-close packed surface as well as chemically. The more open (100) and (111) surfaces, however, exhibit heavily oscillating layer stoichiometries and strong relaxations of interlayer distances both extending deep below the surface. The face-dependent segregation behaviour can be consistently explained by a surface induced tendency to stabilize an ordered intermetallic compound with the order parameter rapidly decaying with depth.

The surface structure of a β-Al(Cu1−xFex)-(110) film formed on an AlCuFe quasicrystal substrate, analyzed by dynamical LEED

In order to understand the unique properties of quasicrystalline surfaces, it is paramount to compare quasicrystalline and crystalline surfaces of similar chemical composition. To that end, the fivefold surface of an AlCuFe quasicrystal substrate was transformed, by ion bombardment and annealing, into a film with the β-Al(Cu1−xFex) bulk structure. This is a CsCl structure with Fe and Cu atoms randomly occupying the body-center sites, and Al atoms at the corner sites. The film exposes a (110) free surface: its external structure was analyzed by dynamical low-energy electron diffraction in terms of atomic positions and layer-wise composition. The best fit between theory and experiment slightly favors a copper-rich composition in the surface layers (x=0), although the sensitivity to the value of x is low, because Cu and Fe are very similar electron scatterers. The Al atoms in the surface layer buckle outward by 0.12±0.04Å relative to the Cu or Fe atoms in the same layer, while other relaxations from the bulk structure are small.

Water monolayers on MgO(100): structural investigations by LEED experiments, tensor LEED dynamical analysis and potential calculations

LEED experiments and tensor LEED dynamical analysis of diffracted intensities are combined with semi-empirical potential calculations to determine the detailed structure of water monolayers adsorbed on MgO(100) single-crystal surfaces. The densities of the two experimentally observed solid phases are evaluated from LEED kinetic isotherms. A numerical procedure based on energy minimization at 0 K gives the positions and the orientations of the water molecules in various commensurate phases. The calculations show that the perfect MgO surface accommodates very stable flat monolayers with the oxygen atoms of the water molecules adsorbed nearly above the cation sites. These structures, which are energetically very close, differ only by the mutual orientations of the molecules in a plane parallel to the surface. The most stable configuration corresponds to a (1×1) geometry, with an adsorption energy per molecule of −945 meV, a value in fair agreement with both experiments and ab initio calculations. The other geometries including the experimental p(3×2) phase correspond to slightly higher energy values. The results of the calculations on the positions of the oxygen atoms in the p(3×2) water monolayer are used as reference structure for a dynamical tensor LEED analysis and it is shown that the agreement is good.

Vertical layer distribution and atomic step height of ultrathin Co films on Cu(001) determined by lattice G-factor analysis

The molecular beam epitaxial growth of ultrathin Co films on the Cu(001) surface at room temperature has been monitored in situ through the angular profile measurement of integer order beams using the high resolution low energy electron diffraction (HRLEED) technique. A simple kinematic lattice G-factor analysis of energy dependent angular profiles at a fixed coverage gives the vertical layer distribution of Co films and atomic step height. The vertical layer distribution is consistent with the nearly layer-by-layer growth mode. The atomic step height ∼ 1.77 Å is consistent with the structure determined from dynamical LEED calculations. A precaution in the application of the lattice G-factor analysis in heteroepitaxial systems is discussed.

Surface segregation profile for Ni 50Pd 50(100)

A recent dynamical LEED study [G.N. Derry, C.B. McVey, P.J. Rous, Surf. Sci. 326 (1995) 59] reported an oscillatory surface segregation profile in the Ni 50Pd 50(100) system with the surface layer enriched by Pd. We have performed ab-initio total-energy calculations for the surface of this alloy system using the coherent potential approximation and obtain an oscillatory segregation profile, in agreement with experiments. We discuss the energetic origin of the oscillatory segregation profile in terms of effective cluster interactions. We include relaxation effects by means of the semi-empirical effective medium theory, and find that this cannot explain the remaining differences between theory and experiment.

Iron multilayers on Cu(100) - a case of complex reconstruction investigated by quantitative LEED

Epitaxial growth of iron on Cu(100) leads to the formation of fcc iron whereby as function of film thickness a variety of different structures is observed. As detected by full dynamical LEED structure analyses using tensor LEED these structures are rather complex. In the coverage regime below 5 monolayers all iron layers are reconstructed both vertically and in-plane. Above 5 monolayers only the top layer is reconstructed until a martensitic transformation at about 10 monolayers leads to the formation of a bcc(110) surface. The structures found correlate nicely with known magnetic properties. Surprisingly, the in-plane lattice parameter of the fcc iron layers was found to be 2.52 Å, i.e. smaller than the value of 2.55 Å known for bulk copper layers. However, a re-investigation of clean Cu(100) also yields a 1% in-plane contraction of copper surface layers compared to bulk layers, i.e. the clean copper surface - at least after conventional preparation procedures - relaxes under its surface stress. In conclusion, the growth of Fe on Cu(100) has to be regarded as pseudomorphic.

Approach to surface structure determination with the scanning tunneling microscope: Multiple-gap imaging and electron-scattering quantum-chemistry theory.

We have successfully developed and tested a method of quantitative surface structure determination using scanning tunneling microscopy (STM). Image simulations of the c(2\ifmmode\times\else\texttimes\fi{}2) S on Mo(100) chemisorption system were calculated as a function of surface and tip structure using electron-scattering quantum-chemistry STM theory. STM images with a wide range of tunneling gap resistance values were acquired in a ``multiple-gap'' mode which preserves information on the z separation and lateral registry between the images under different tunneling conditions. The best fit of a numerical comparison of the image simulations with experimental data simultaneously determined two structural parameters of the surface. The STM results differ from those of dynamical LEED by approximately 0.1 \AA{}, which we estimate to be the level of accuracy obtainable with the present implementation of the method.

A LEED- IV determination of the Ru(001)-p(2 × 2) (O + CO) structure: A coadsorbate-induced molecular tilt

A detailed LEED- IV analysis of the coadsorbate system p(2× 2)( O + CO) Ru(001) has been performed by conventional dynamical LEED calculations and by Tensor LEED. We find that in this coadsorbate layer the oxygen atoms remain in the threefold hcp sites as they occupy in the pure p(2 × 2)O structure with minimally changed distances, in contrast to the behaviour of the p(2 × 2)(2O + CO) structure where a structural rearrangement of the oxygen atoms has been observed before. The CO molecules adsorb in the top site in the center of three O atoms. Their molecular bond length is 1.16 ± 0.06 A ̊ , i.e., insignificantly longer than in the free molecule; the bond length to the underlying Ru atom is d( C− Ru) = 1.93 ± 0.06 A ̊ . Buckling of the top Ru layer similar to that for the p(2 × 2)(2O + CO) system is found. A dramatic reduction of the best-fit R-factors (for the Pendry R-factor of 0.33–0.17, with particularly strong effect on the integral-order beams) can be achieved by the introduction of a tilt angle for the CO molecules, without any other structural changes. The resulting tilt lies in the three equivalent symmetry planes through each of the O atoms; the molecule is tilted away from the O atom in this plane. At 120 K, the best-fit tilt angle is 12.6°. Different domain averaging procedures do not change these conclusions. The physical meaning of these findings and the possible reasons for the tilt and for the high sensitivity of the structure determination to it are discussed.

Oxygen induced missing row reconstruction of substitutionally disordered Mo0.75Re0.25(100)

Adsorption of oxygen on Mo0.75Re0.25(100) with exposures in the range 0.5–100 L and subsequent annealing at or above about 1400 K produces a 2 × 1 superstructure as observed by low energy electron diffraction. The superstructure spots are very bright indicating a strong oxygen induced substrate reconstruction. By full dynamical LEED intensity analysis this is proved to be of the missing row type in agreement with earlier results by ion scattering. As usual, the missing of rows induces row pairing and buckling in the second and third substrate layers, respectively, by amplitudes of the order of 0.1 Å. Substrate interlayer distances change drastically. Oxygen atoms reside in 3-fold coordinated sites binding to both first and second layer atoms with bond lengths of 2.02 Å and 2.04 Å (oxygen hard core radii 0.66 Å and 0.68 Å), respectively. The MoO bonds being stronger than ReO bonds seems to cause and dominate an oxygen induced surface segregation process activated by the high annealing temperature which results in a considerable enrichment of Mo in the surface. If one views the oxygen coordinated atoms as belonging to a composite first layer, both the stoichiometry and distances of deeper layers are very similar to those found for the clean surface.