Dynamical Low-energy Electron Diffraction Analysis Research Articles

Boron nanostructure formation on Mo(112) surface

Boron-based nanoscale structures are at the vanguard of current research due to multifarious applications in numerous fields. Here we report the formation of ordered boron structure on Mo(112), which was investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While Boron did not form ordered structures at room temperature, it showed a c(2 × 2) structure upon heating at 1500 K. The c(2 × 2) structure shows a regular array of boron clusters on the trench of Mo(112), and STM study does not indicate any alloying. Dynamical LEED analysis and density functional theory reveal that the c(2 × 2) structure is an array of boron clusters, B4. The formation of B4 cluster is also explained by favorable adsorption sites on the trench of Mo(112). The scanning tunneling spectroscopy study shows the metallic nature of this B nanostructure, which will be stimulating for practical applications. This novel network formation by boron clusters indicates strong inter-atomic as well as intra-cluster bonding.

Structural investigation and magnetic properties of oxygen adsorption on ultrathin Fe(110) film

We have investigated oxygen induced surface structures on Fe(110) thin film and their magnetic properties using low energy electron diffraction (LEED) and surface magneto-optic Kerr effect (SMOKE). We have determined two chemisorbed structures, p(2 × 2) at 0.25 monolayer (ML) and p(3 × 2) at 0.33 ML using a dynamical LEED analysis. We found that oxygen atoms adsorb in long bridge sites on p(2 × 2) and move to three fold hollow sites on p(3 × 2) due to a steric repulsion. During chemisorption, proceeding the formation of FeO, the topmost surface of Fe film is ferromagnetic and with further oxygen adsorption the surface loses ferromagnetism in accordance with the oxide formation.

Atomic and electronic structure of doped Si(111)(23×23)R30∘ -Sn interfaces

The hole doped Si(111)(2root3x2root3)R30(degrees)-Sn interface exhibits a symmetry-breaking insulator-insulator transition below 100 K that appears to be triggered by electron tunneling into the empty surface-state bands. No such transition is seen in electron-doped systems. To elucidate the nature and driving force of this phenomenon, the structure of the interface must be resolved. Here we report on an extensive experimental and theoretical study, including scanning tunneling microscopy and spectroscopy (STM/STS), dynamical low-energy electron diffraction (LEED) analysis, and density functional theory (DFT) calculations, to elucidate the structure of this interface. We consider six different structure models, three of which have been proposed before, and conclude that only two of them can account for the majority of experimental data. One of them is the model according to Tornevik et al. [C. Tornevik et al., Phys. Rev. B 44, 13144 (1991)] with a total Sn coverage of 14/12 monolayers (ML). The other is the 'revised trimer model' with a total Sn coverage of 13/12 ML, introduced in this work. These two models are very difficult to discriminate on the basis of DFT or LEED alone, but STS data clearly point toward the T\"ornevik model as the most viable candidate among the models considered here. The STS data also provide additional insights regarding the electron-injection-driven phase transformation as well as the critical role of valence band holes in this process. Similar processes may occur at other metal/semiconductor interfaces, provided they are non-metallic and can be doped. This could open up a new pathway toward the creation of novel surface phases with potentially very interesting and desirable electronic properties.

Epitaxial growth of Al9Ir2 intermetallic compound on Al(100): Mechanism and interface structure

The adsorption of Ir adatoms on Al(100) has been investigated under various exposures and temperature conditions. The experimental and theoretical results reveal a diffusion of Ir adatoms within the Al(100) surface selvedge already at 300 K. Above 593 K, two domains of a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.{6}^{\ensuremath{\circ}}$ phase are identified by low energy electron diffraction (LEED) and scanning tunneling microscopy measurements. This phase corresponds to the initial growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound at the Al(100) surface. The ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic domains are terminated by bulklike pure Al layers. The structural stability of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) grown on Al(100) has been analyzed by density functional theory based calculations. Dynamical LEED analysis is consistent with an Ir adsorption leading to the growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic compound. We propose that the epitaxial relationship ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}(001)\ensuremath{\parallel}\mathrm{Al}$(100) and ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}[100]\ensuremath{\parallel}\mathrm{Al}[031]/[013]$ originates from a matching of Al atomic arrangements present both on Al(100) and on pure Al(001) layers present in the ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound. Finally, the interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ precipitates and the Al matrix has been characterized by transmission electron microscopy measurements. The cross-sectional observations are consistent with the formation of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) compounds. These measurements indicate an important Ir diffusion within Al(100) near the surface region. The coherent interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ and the Al matrix is sharp.

The atomic structure of low-index surfaces of the intermetallic compound InPd.

The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ∼580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ∼660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (≥750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and "global" measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature.

Defects and inhomogeneities in Fe3O4(111) thin film growth on Pt(111)

Growth and surface termination of a Fe${}_{3}$O${}_{4}$(111) thin film on a Pt(111) surface were examined by a combination of low-energy electron microscopy, selected area low-energy electron diffraction (LEED), and x-ray-induced photoemission electron microscopy. The film exhibits the predominance of one out of two possible rotational domains, independent of film thickness. The morphology strongly depends on preparation conditions, e.g., at high oxidation temperature FeO/Pt(111) domains are formed that prevent the closure of the thin film. Dynamical LEED analysis and spot-profile analysis LEED (SPA-LEED) show that the surface exposes \textonequarter{} monolayer of Fe over a close-packed oxygen layer only when the sample is subsequently annealed in ultrahigh vacuum at 900 K. In contrast, the as-prepared films grown by oxidation at 1000 K and subsequent cooling down in oxygen, additionally exhibit small FeO${}_{x}$ agglomerates that rest upon the canonical surface termination. Their formation as a function of the various preparation conditions of the thin film is discussed.

Structure determination of Pb/Ge(111)- by dynamical low-energy electron diffraction analysis and first-principles calculation

We have determined the atomic structure of the Pb/Ge(111)- surface, which was shown to exhibit a large Rashba spin splitting in a metallic surface state by dynamical low-energy electron diffraction analysis. The Pb coverage for the optimized atomic structure is 4/3 with one Pb atom located at every third H3 site of the bulk-truncated Ge(111) surface and the other three near the T1 sites but slightly displaced towards the T4 sites. The determined atomic structure agrees well with the energetically optimized one obtained from the first-principles calculation. The calculation also revealed that the potential for the Pb atoms on the H3 sites is very soft along the surface normal, suggesting that their vertical position is distributed within a range of about 0.2–0.3 Å. The previously proposed phase transition associated with the surface melting is discussed.

Structure determination of Bi/Ge(111)- by dynamical low-energy electron diffraction analysis and scanning tunnelingmicroscopy

We have determined the atomic structure of the Bi/Ge(111)- surface by dynamical low-energy electron diffraction (LEED) analysis and scanning tunnelingmicroscopy (STM). The optimized atomic structure consists of Bi atoms which are adsorbed nearthe T1 sites of the bulk-truncated Ge(111) surface and form triangular trimer units centered at theT4 sites. The atomically resolved STM image was consistent with the LEED result. Thestructural parameters agree well with those optimized by a first-principles calculationwhich supports the interpretation of the electronic band splitting on this surface in terms ofthe giant Rashba effect.

Location of hydrogen adsorbed on Rh(111) studied by low-energy electron diffraction and nuclear reaction analysis

The structures of clean and hydrogen-adsorbed Rh(111) surfaces were investigated by dynamical low-energy electron-diffraction (LEED) analysis. Exposure of ${\mathrm{D}}_{2}$ induced no additional LEED patterns except for $(1\ifmmode\times\else\texttimes\fi{}1)$. Surface-layer relaxation occurs vertically on both clean and D-saturated surfaces. On the clean surface, the interlayer distance between the first and second layers $({d}_{12})$ is smaller by $1.2(\ifmmode\pm\else\textpm\fi{}0.6)%$ than the corresponding bulk distance of $2.194\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. On the other hand, the contraction of ${d}_{12}$ is removed on the D-saturated surface. Detailed LEED analysis demonstrates that the D atoms are adsorbed on the fcc threefold hollow sites. The absolute saturation coverage of H on Rh(111) was determined to be 0.84 ML by nuclear reaction analysis (NRA). Moreover, the zero-point vibrational energy of H was derived from the analysis of the NRA resonance profile, which is discussed in comparison with the results of high-resolution electron-energy-loss spectroscopy.

Surface atomic structure determination of three-dimensional yttrium silicide epitaxially grown on Si(111)

The surface atomic structure of thin layers of three-dimensional yttrium silicide epitaxially grown on Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ has been investigated by means of dynamical low-energy electron diffraction analysis. We determine the interlayer distances as well as the lateral and/or vertical relaxations of the atoms in the superficial planes. The epitaxial silicide consists of stacked hexagonal rare-earth planes and graphitelike Si planes with an ordered arrangement of Si vacancies. The ordered net of Si vacancies in the inner planes is responsible for the lateral relaxations of the surrounding Si atoms. The topmost layer does not present a graphitelike structure, forming a buckled Si layer with no vacancies. One of the three Si atoms in the lower plane of this bilayer is closer to the yttrium layer due to the presence of the vacancy in the last Si plane just below. This produces vertical relaxation in the termination layer.

Dynamical low-energy electron diffraction analysis of the structure of nitric oxide on Pt( [formula omitted

The adsorption structure of NO on the Pt(1 1 1) surface at low temperature (90–130 K) was studied by dynamical analysis of low-energy electron diffraction and reflection absorption infrared spectroscopy. Three different 2×2 patterns were identified and their experimental intensity vs. voltage curves ( I– V curves) were compared with dynamically calculated I– V curves for the models including the tilted (bent) NO geometry. The differences of I– V curves can be recognized by virtue of the wider incident electron energy range than the previous dynamical analysis. The most promising interpretation is that NO occupies three sites sequentially depending on the NO coverage. In this interpretation, NO first adsorbs at the threefold fcc hollow site, next at the atop site and last at the threefold hcp hollow site. The NO saturation coverage is 0.75 ML ( 1 ML=1.5×10 −15 molecules/cm 2). The NO species at the atop site is tilted and those at both hollow sites are upright. Our new model agrees well with the recent density functional theory calculation and explains the experimental results better than the previous model, in which NO transits from the bridge site to the atop site with increasing coverage.

Hexagonal heterolayers on a square lattice: A combined STM and LEED study of FeO(111) on Pt(100)

Formation of an epitaxial iron oxide monolayer on a Pt(100)-hex substrate was studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). High-resolution STM images reveal a sinusoidal height modulation of the top atomic rows along the [011] direction of the original Pt(100)-hex substrate. This modulation is assigned to the buckling of the top oxygen layer due to the interaction with Pt substrate atoms. Two superstructures, described as $\mathrm{FeO}(111)/\mathrm{P}\mathrm{t}(100)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}10)$ and $(2\ifmmode\times\else\texttimes\fi{}9)$ coincidence structures, coexist on the surface. The latter structure results in a much lower Pendry R factor in dynamical LEED analysis than earlier reported for a $c(2\ifmmode\times\else\texttimes\fi{}10)$ structure. Numerous islands with the same surface structure as the terraces develop on the dense FeO overlayer. They are assigned as the $\mathrm{Pt}(100)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ islands formed during the $\mathrm{hex}\ensuremath{\rightarrow}(1\ifmmode\times\else\texttimes\fi{}1)$ reconstruction of the Pt substrate underneath the FeO(111) bilayer. The islands are rectangular and elongated in the direction of hex reconstruction on the original Pt(100). Combined STM and LEED data clearly indicate that anisotropy in the substrate reconstruction leads to anisotropy of the oxide overlayer.

Evidence ofc(2×2)antiferromagnetic order of Mn in an ideal monolayer on Ag(001)

We present a dynamical low-energy electron diffraction analysis of the atomic and magnetic structure obtained by deposition of 0.9 Mn monolayers on Ag(001) held at 100 K. We find that the film grows in an essentially flat ideal monoatomic layer with the Mn located in fourfold hollow sites of the Ag(001) substrate that undergoes no sizeable reconstruction of its topmost layers. The Mn-Ag interlayer distance ${d}_{12}=2.00\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ is found to be much larger than \ensuremath{\sim}1.85 \AA{} expected from simple addition of atomic radii in elemental \ensuremath{\alpha}-Mn and face-centered-cubic (fcc) Ag. This is direct evidence of the strong reduction in Mn $3d$ contribution to cohesion due to the atomiclike high-spin state of the Mn in the monolayer structure. Taking into account the exchange scattering by means of a local von Barth-Hedin exchange-correlation potential, we demonstrate that the weak intensity of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure reflection intensities $I(E)$ observed at low energies only $(El~120\mathrm{eV})$ can be quite well explained in terms of a two-dimensional antiferromagnetic arrangement of the Mn moments. Nonmagnetic origins of these reflections such as out of plane (buckling) or in plane displacive reconstructions of the Mn monolayer as well as surface alloying with Ag or impurity related superstructures can be safely ruled out. Evidence of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic to $p(1\ifmmode\times\else\texttimes\fi{}1)$ paramagnetic phase transition is also presented.

Low-energy electron diffraction analysis of the buried-heteroatom type Pd(100)-p(2×2)-p4g-Al surface

The structure of the p(2×2)-p4g ordered bimetallic phase formed on aluminum-deposited Pd(100) is determined by dynamical low-energy electron diffraction analysis. The best-fitted structure among 29 different model types examined consists of the Pd top layer clock-reconstructed to the hard sphere limit, and a buckled c(2×2)-PdAl second layer. The buried-heteroatom scheme, a new mechanism of in-plane atom movements, is proposed to interpret the yielded structure; Al atoms migrate beneath the topmost Pd layer and replace half of the second-layer Pd atoms to demand optimum coordination. Pd–Al bonding interaction forces top-layer Pd atoms to move systematically towards the buried Al. Half of the fourfold hollows of top layer are opened and the other half are deformed as a result of the movements. The top and second layers of the reconstructed surface can be regarded as a surface-limited compound of Pd3Al composition supported on the Pd(100) substrate, because a reduced d-state overlap between the top two layers and the Pd substrate is expected.

Low-energy electron diffraction analysis of the Pd(100)-p(2 × 2)-p4g-Al surface: a buried-heteroatom structure

Dynamical low-energy electron diffraction analysis reveals that the p(2 × 2)-p4g ordered bimetallic phase formed on aluminum-deposited Pd (100) consists of a clock-reconstructed Pd top layer and a strongly buckled c(2 × 2)-PdAl second layer. A buried-heteroatom model is proposed to interpret the obtained structure. The Pd Al heteroatomic bonding interaction forces the top-layer Pd atoms to move systematically to make Pd Al Pd units. The four-fold hollows of the top layer open up only as a result of the systematic unit formation, in contrast to the squeezed-adatom type p(2 × 2)-p4g C/Ni(100) surface.

COMPLETION OF THE DETERMINATION OF COMPLEX SURFACE STRUCTURES FORMED ON LOW-INDEX PLANES OF COPPER SINGLE CRYSTAL BY Li DEPOSITION: Cu(110)-(4×1)-3Li

With increasing Li coverage on Cu(110), (n×1)(4≤n≤8) structures follow the (1×2) missing-row structure at room temperature. We determine the (4×1) structure by a dynamical low-energy electron diffraction (LEED) analysis with symmetrized automated tensor LEED. This completes the solution of the complex surface structures formed on low-index planes of copper single crystal by Li deposition. Every fourth Cu row in the [001] direction is substituted by Li atoms and the remaining Cu rows are covered by Li adatoms. Features of this structure, denoted as Cu (110)-(4×1)-3 Li , are very similar to those of the previously determined Cu (001)-(3×3)-5 Li , Cu (001)-(4×4)-10 Li and Cu (111)-(2×2)-3 Li structures.

Structure of Physisorbed Molecules on an Oxide Surface from Potential Calculations and Dynamical Low-Energy Electron Diffraction Analysis: Acetylene on MgO(100)

Semiempirical potential calculations have been combined with a dynamical low-energy electron diffraction analysis to determine the complete structure of molecules physisorbed on an oxide surface, namely, acetylene (${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$) adsorbed on MgO(100) at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}88\mathrm{K}$. This allows determination of the adsorption sites and the molecule-molecule and molecule-substrate distances. The results are consistent with molecules that lie almost parallel to the surface in a herringbone structure. Neither molecule nor substrate exhibits significant distortions, due to the weak physisorption.

Adsorption mode change from adlayer- to restructuring-type with increasing coverage, evidenced by structural determination of c(2 × 2)→(4 × 4)→(5 × 5) sequence formed on Ni(001) by Li deposition

We determined surface structures in a structural sequence c(2 × 2)→(4 × 4)→(5 × 5) formed on Ni(001) at 370 K with increasing Li coverage by a dynamical low-energy electron diffraction analysis. The (4 × 4) and (5 × 5) are complex surface-structures involving restructuring of substrate surface atoms, and are analogous to the previously determined (3 × 3) and (4 × 4) structures formed for Li Cu(001) . The c(2 × 2) at low coverages is a Li adlayer, so a change of the adsorption mode from adlayer- to restructuring-type is evidenced in the course of increasing coverage within a monolayer range.

Structural Analysis of the Fivefold Symmetric Surface of the Al70Pd21Mn9Quasicrystal by Low Energy Electron Diffraction

The atomic structure of the fivefold symmetric quasicrystal surface of A${\mathrm{l}}_{70}$P${\mathrm{d}}_{21}$M${\mathrm{n}}_{9}$ was investigated by means of a dynamical low energy electron-diffraction analysis. Approximations are developed to address the high structural complexity of quasicrystals, yielding average layer-by-layer structural and compositional information. A mix of several relaxed bulklike terminations is found, consistently favoring dense Al-rich outermost atomic layers. These results can be understood in terms of principles known to govern surface structures of simpler, crystalline metals.

Completion of the structural determination of and rationalization of the surface-structure sequence (2 x 1)-->(3 x 3)-->(4 x 4) formed on Cu(001) with increasing Li coverage.

With increasing Li coverage, the surface structure of Cu(001) evolves as (2\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\rightarrow}(3\ifmmode\times\else\texttimes\fi{}3)\ensuremath{\rightarrow}(4\ifmmode\times\else\texttimes\fi{}4) at room temperature. We here report the detailed structure of the most complex (4\ifmmode\times\else\texttimes\fi{}4) phase determined by a dynamical low-energy electron diffraction (LEED) analysis with symmetrized automated tensor LEED. This completes the solution of these structures and allows a rationalization of their formation. The unit cell of the (4\ifmmode\times\else\texttimes\fi{}4) structure consists of four Li adatoms forming a square cluster located on top of islands of nine Cu atoms; the islands are separated and joined by rows of substituting Li atoms.