Quantitative Low-energy Electron Diffraction Research Articles

Strain modulation in small molecule physisorption in two dimensions: LEED structure analysis and DFT modeling of the system.

The structure of the system was investigated experimentally by means of quantitative LEED I(V) analysis and computationally using dispersion corrected density functional theory (DFT-D). Three different structure models with four, five, and six molecules were considered. The lowest reliability factors and thus best agreement of measured and calculated I(V) curves was found for the structure model containing five molecules per surface unit cell. Essential features of the experimental best-fit adlayer structure are supported by DFT. A slight inclination and lateral shift of twofold coordinated molecules away from the on-top position over Na+ adsorption sites is interpreted as compensation of strain between substrate and adlayer.

Hybridization vs decoupling: influence of an h-BN interlayer on the physical properties of a lander-type molecule on Ni(111).

2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Structural analysis of clean LaB6(100), (111), and (110) surfaces via quantitative low-energy electron diffraction

LaB6 has very important features for electron emission, but its proposed surface structures of the fundamental planes contradict the work function values. In this study, we determined the surface structures of the LaB6 single crystal via quantitative low-energy electron diffraction analysis. On the (100) plane, the surface was terminated by a La atom, which relaxed toward the bulk crystal side. On the (111) and (110) planes, the surfaces were terminated by a La atom and a cluster of B, but the cluster had smaller number of B atoms than the B6 octahedron. Because the B6 octahedron is quite stable, it is likely that the surfaces are terminated by a B6 octahedron. In contrast, our structural analysis revealed that the surface of the (111) plane was terminated by a B3 triangle. The surface of the (110) plane has a c(2 × 2) periodicity, and we obtained three possible models according to our analysis. We cannot tell which of these models exists on the surface, and they can coexist on the surface. These three models have similar structures; however, model 1 has B4 termination, model 2 has B4 and B3 mixed termination, and model 3 has B3 termination. The determined structures for the (111) and (110) planes are very different from previously suggested models. Based on our surface analysis, the work function values of the above surface planes of the LaB6 single crystal can be reasonably explained.

Orbital-Driven Rashba Effect in a Binary Honeycomb Monolayer AgTe.

The Rashba effect is fundamental to the physics of two-dimensional electron systems and underlies a variety of spintronic phenomena. It has been proposed that the formation of Rashba-type spin splittings originates microscopically from the existence of orbital angular momentum (OAM) in the Bloch wave functions. Here, we present detailed experimental evidence for this OAM-based origin of the Rashba effect by angle-resolved photoemission (ARPES) and two-photon photoemission experiments for a monolayer AgTe on Ag(111). Using quantitative low-energy electron diffraction analysis, we determine the structural parameters and the stacking of the honeycomb overlayer with picometer precision. Based on an orbital-symmetry analysis in ARPES and supported by first-principles calculations, we unequivocally relate the presence and absence of Rashba-type spin splittings in different bands of AgTe to the existence of OAM.

Quantitative Multilayer Cu(410) Structure and Relaxation Determined by QLEED

Industrially relevant catalytically active surfaces exhibit defects. These defects serve as active sites; expose incoming adsorbates to both high and low coordinated surface atoms; determine morphology, reactivity, energetics, and surface relaxation. These, in turn, affect crystal growth, oxidation, catalysis, and corrosion. Systematic experimental analyses of such surface defects pose challenges, esp., when they do not exhibit order. High Miller index surfaces can provide access to these features and information, albeit indirectly. Here, we show that with quantitative low-energy electron diffraction (QLEED) intensity analyses and density functional theory (DFT) calculations, we can visualize the local atomic configuration, the corresponding electron distribution, and local reactivity. The QLEED-determined Cu(410) structure (Pendry reliability factor RP ≃ 0.0797) exhibits alternating sequences of expansion (+) and contraction (−) (of the first 16 atomic interlayers) relative to the bulk-truncated interlayer spacing of ca. 0.437 Å. The corresponding electron distribution shows smoothening relative to the bulk-determined structure. These results should aid us to further gain an atomic-scale understanding of the nature of defects in materials.

Surface structure and stacking of the commensurate (13×13)R13.9∘ charge density wave phase of 1T−TaS2(0001)

By quantitative low-energy electron diffraction (LEED) we investigate the extensively studied commensurate charge density wave (CDW) phase of trigonal tantalum disulphide (1T-TaS$_2$), which develops at low temperatures with a $\left(\sqrt{13}\times\sqrt{13}\right)$R13.9{\deg} periodicity. A full-dynamical analysis of the energy dependence of diffraction spot intensities reveals the entire crystallographic surface structure, i.e. the detailed atomic positions within the outermost two trilayers consisting of 78 atoms as well as the CDW stacking. The analysis is based on an unusually large data set consisting of spectra for 128 inequivalent beams taken in the energy range 20-250 eV and an excellent fit quality expressed by a bestfit Pendry R-factor of R=0.110. The LEED intensity analysis reveals that the well-accepted model of star-of-David-shaped clusters of Ta atoms for the bulk structure also holds for the outermost two TaS$_2$ trilayers. Specifically, in both layers the clusters of Ta atoms contract laterally by up to 0.25 $\r{A}$ and also slightly rotate within the superstructure cell, causing respective distortions as well as heavy bucklings (up to 0.23 $\r{A}$) in the adjacent sulphur layers. Most importantly, our analysis finds that the CDWs of the 1$^{\text{st}}$ and 2$^{\text{nd}}$ trilayer are vertically aligned, while there is a lateral shift of two units of the basic hexagonal lattice (6.71 $\r{A}$) between the 2$^{\text{nd}}$ and 3$^{\text{rd}}$ trilayer. The results may contribute to a better understanding of the intricate electronic structure of the reference compound 1T-TaS$_2$ and guide the way to the analysis of complex structures in similar quantum materials.

Structure determination of ultra-flat stanene on Cu(111) using low energy electron diffraction

The 2D hexagonal structure of tin atoms which is termed stanene is structurally determined on a Cu(111) substrate using low energy electron diffraction (LEED). The structural analysis of Cu{111}-p(2 × 2)-Sn at a coverage of 0.5 at low temperature reveals the relaxation of underlying Cu atoms, which stabilizes the formation of almost zero-buckled Sn atoms forming a honeycomb structure. Our results using quantitative LEED conclusively reveals an ultra-flat stanene structure that complements well with the previous calculations. The detailed structural analysis presented in this article is expected to give in-depth information for characterizing the properties of as-grown stanene.

Structure of the SnO_{2}(110)-(4×1) Surface.

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 SnO_{2}(110) surface. We find that the reconstruction consists of an ordered arrangement of Sn_{3}O_{3} clusters bound atop the bulk-terminated SnO_{2}(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 SnO_{2}(110) surfaces.

Crystallographic structure and energetics of the Rh(1 0 0)-(3 × 1)-2O phase

In this study we investigate the crystallographic structure of the Rh(1 0 0)-()-2O phase by quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy as well as the energetics of the system applying density functional theory calculations (DFT). The () structure forms upon exposing the clean Rh(1 0 0) surface to 1200 L of oxygen at 520 K. A full-dynamical LEED intensity analysis (Pendry R-factor ) reveals an oxygen-induced shifted row-reconstruction of the rhodium top layer where every third Rh-row is displaced by half a surface lattice parameter along the [0 1 1]-direction. There are two oxygen atoms within the unit cell which assume threefold coordinated sites on both sides of the shifted Rh-row with one bond to the shifted and two bonds to the unshifted rows. DFT calculations yield a total energy gain of 0.27 eV per oxygen atom compared to adsorption on the unreconstructed surface. This by far overcompensates the energetic penalty of 0.10 eV per oxygen atom for shifting the Rh-row and thus drives the substrate reconstruction. A coadsorption of oxygen at remaining regular sites of the substrate is not observed in experiment and is found to be energetically unfavorable.

Monatomic Co, CoO2 , and CoO3 nanowires on Ir(100) and Pt(100) surfaces: Formation, structure, and energetics

In this study we investigate the structural and chemical changes of monatomic CoO$_2$ chains grown self-organized on the Ir(100) surface [P. Ferstl et al., PRL 117, 2016, 046101] and on Pt(100) under reducing and oxidizing conditions. By a combination of quantitative low-energy electron diffraction, scanning tunnelling microscopy, and density functional theory we show that the cobalt oxide wires are completely reduced by H$_2$ at temperatures above 320 K and a 3x1 ordered Ir$_2$Co or Pt$_2$Co surface alloy is formed. Depending on temperature the surface alloy on Ir(100) is either hydrogen covered (T < 400 K) or clean and eventually undergoes an irreversible order-disorder transition at about 570 K. The Pt$_2$Co surface alloy disorders with the desorption of hydrogen, whereby Co submerges into subsurface sites. Vice versa, applying stronger oxidants than O$_2$ such as NO$_2$ leads to the formation of CoO3 chains on Ir(100) in a 3x1 superstructure. On Pt(100) such a CoO$_3$ phase could not be prepared so far, which however, is due to the UHV conditions of our experiments. As revealed by theory this phase will become stable in a regime of higher pressure. In general, the structures can be reversibly switched on both surfaces using the respective agents O$_2$, NO$_2$ and H$_2$.

Structure of Low and High Coverage Phases of Bromine on Pd(110)

Halogens can be used to steer the catalytic selectivity of transition metal surfaces. In the present work the structures of bromine on Pd(110) in the low-coverage c(2 × 4) and the high-coverage c(4 × 2) adlayers are investigated by density functional theory calculations and quantitative low-energy electron diffraction. Excellent agreement is achieved between theory and experiment. Both find the high-coordinated hollow site to be favored at low coverage, while in the high coverage phase also adsorption close to long-bridge sites occurs due to the Br–Br repulsion. This results in an almost uniform distribution of adatoms in the troughs of the Pd(110) surface as in the majority of halogen fcc(110) transition metal adsorption systems, but not on Pt(110). The different coordination of the hollow and quasi long-bridge adsorption sites is reflected by different distances to the top-layer Pd atoms, 271 pm for the high-coordinated hollow site and 248 pm for the low-coordinated quasi long-bridge site. With increasing bromine coverage the contraction of the first interlayer distance d12 of clean Pd(110) is reduced and inverted to an expansion of ∼3% at high bromine coverage. As shown by DFT, this change in substrate relaxation depends markedly on the adsorption site. A population analysis reveals a non-negligible ionic contribution for the Br–Pd bonding. Finally, we discuss the, at first sight, surprising DFT prediction of pure long-bridge adsorption at intermediate coverages.

Structure and stability of ultrathin Fe films on W(110)

The growth of one and two atomic layers of iron on a W(110) substrate was followed by low-energy electron microscopy. The near-surface structural properties of the perfectly flat pseudomorphic films were studied by quantitative low-energy electron diffraction analysis from areas of uniform thickness as well as by the density functional theory. A strong relaxation of the outermost atomic layers was found in Fe mono- and bilayers on W(110). By calculating the phonon dispersion relations and phonon density of states, the stability of the pseudomorphic iron bilayer on a tungsten substrate has been addressed. To complete the physical picture, an iron trilayer has also been analyzed in order to identify the source of instability for its pseudomorphic phase. Our results show that the surface instability originates from the softening of the in-plane surface modes along the $[1\overline{1}0]$ direction, although the soft modes were not observed. The enhanced magnetic moments calculated within the density functional theory are in good agreement with experimental findings reported for these systems.

Structure determination of the ordered (2 × 1) phase of NiSi surface alloy on Ni(111) using low-energy electron diffraction

The (2 × 1) structure of the two-dimensional nickel silicide surface alloy on Ni(111) was investigated using quantitative low-energy electron diffraction analysis. The unit cell of the determined silicide structure contains one Si and one Ni atom, corresponding to a chemical formula of NiSi. The Si atoms adopt substitutional face-centered cubic hollow sites on the Ni(111) substrate. The Ni–Si bond lengths were determined to be 2.37 and 2.34 Å. Both the alloy surface and the underlying first layers of Ni atoms exhibit slight corrugation. The Ni–Si interlayer distance is smaller than the Ni–Ni interlayer distance, which indicates that Si atoms and underlying Ni atoms strongly interact.

Electronic Structure of Pd Multilayers on Re(0001): The Role of Charge Transfer

Understanding the origin of the properties of metal-supported metal thin films is important for the rational design of bimetallic catalysts and other applications, but it is generally difficult to separate effects related to strain from those arising from interface interactions. Here we use density functional (DFT) theory to examine the structure and electronic behavior of few-layer palladium films on the rhenium (0001) surface, where there is negligible interfacial strain and therefore other effects can be isolated. Our DFT calculations predict stacking sequences and interlayer separations in excellent agreement with quantitative low-energy electron diffraction experiments. By theoretically simulating the Pd core-level X-ray photoemission spectra (XPS) of the films, we are able to interpret and assign the basic features of both low-resolution and high-resolution XPS measurements. The core levels at the interface shift to more negative energies, rigidly following the shifts in the same direction of the valence d-band center. We demonstrate that the valence band shift at the interface is caused by charge transfer from Re to Pd, which occurs mainly to valence states of hybridized s-p character rather than to the Pd d-band. Since the d-band filling is roughly constant, there is a correlation between the d-band center shift and its bandwidth. The resulting effect of this charge transfer on the valence d-band is thus analogous to the application of a lateral compressive strain on the adlayers. Our analysis suggests that charge transfer should be considered when describing the origin of core and valence band shifts in other metal / metal adlayer systems.

Structure determination of the ordered [formula omitted] phase of Ni2Si and Ni2Ge surface alloys on Ni(111) via low-energy electron diffraction

The 3×3R30° structures of 2-D nickel silicide and nickel germanide surface alloys on Ni(111) were investigated using quantitative low-energy electron diffraction analysis. The unit cells of the determined silicide and germanide structures contain two Ni atoms and one Si or Ge atom, corresponding to the chemical compositions Ni2Si and Ni2Ge, respectively. Both the Si and Ge atoms adopt substitutional face-centered cubic hollow sites to sit on the Ni(111) substrates, and the alloy surfaces exhibit a slight corrugation. The NiSi and NiGe distances were 2.44 and 2.56Å, respectively, and corresponded well to their respective empirical radii. It was also found that the Ni atoms in the second and third layers moved toward the Si or Ge atom sites from their bulk crystal positions. These results indicate rather strong interactions of the Ni atoms with both the Si and Ge atoms at the surface, leading to trapping of the Si and Ge atoms at the substitutional sites without diffusion into the bulk.

Surface structure of V_{2}O_{3}(0001) revisited.

In a recent paper [A. J. Window etal., Phys. Rev. Lett. 107, 016105 (2011)], it was proposed that V_{2}O_{3}(0001) is terminated by the so-called O_{3} termination, a reconstruction with a terminating distorted hexagonal oxygen layer. We show that the surface is terminated by vanadyl (V═O) groups instead. This conclusion is based on quantitative low-energy electron diffraction combined with scanning tunneling microscopy, fast atom scattering, and density functional theory employing the Heyd-Scuseria-Ernzerhof functional. New insights into the subsurface sensitivity of ion beam triangulation show that results previously interpreted in favor of the O_{3} termination are reconcilable with vanadyl termination as well.

Surface relaxation of Cu(5 1 1)

The multilayer relaxation of the stepped Cu(5 1 1) surface has been studied by quantitative low-energy electron diffraction and analyzed using the CLEED program package. Relaxations with respect to the bulk interlayer spacing of 0.6934 Å are −9.5%, −10.4%, +8.2% and −1.8% for the first four interlayer spacings, respectively (negative sign corresponds to contraction). The relaxation sequence (− − + −…) is thus in agreement with the theoretical prediction. The deeper relaxations are damped in a non-uniform manner and the lateral relaxations are smaller than 2% of the lateral spacing. This result agrees well with theoretical studies of the same surface. The Pendry R-factor for the favored structure is 0.21.

The Adsorption and Decomposition of Formic Acid on Cu{100} and Cu{100}Pt Surfaces using Temperature Programmed Reaction Spectroscopy

The structures formed by adsorbing thin-film platinum, formic acid and oxygen on Cu{100} single crystal are investigated by quantitative low-energy electron-diffraction (LEED) and Temperature Programmed Reaction Spectroscopy (TPRS). Symmetrized Automated Tensor LEED (SATLEED) calculations are used to determine the structure of the formed surface alloys and overlayers. TPRS was used to probe the surface reactivity of the systems studied while surface composition was obtained using Auger Electron spectroscopy (AES). The decomposition of a formate intermediate from a clean Cu{100} surface has been monitored through the use of TPD Spectroscopy. It has also been evidenced that platinum has a destabilising effect on the formate intermediate. The peak temperature (Tp ) for the CO2 desorption spectra from copper-platinum model surfaces, appear around 40 K lower than those from clean copper. This suggests a much less stable surface alloy compared to the clean surface. In activation energy terms, this destabilisation can be expressed as a 13% decrease in the energy required for the formate to decompose. It was also observed that desorption is much more rapid from the copper-platinum than from clean copper surfaces. Keywords: Cu{100}, Cu{100}Pt, Formic Acid, TPRS

The structure of carbon monoxide adsorbed on the NaCl(100) surface—a combined LEED and DFT-D/vdW-DF study.

The structure of the first layer CO adsorbed on NaCl(100) is investigated experimentally by means of quantitative low-energy electron diffraction at 25 K, and theoretically by means of density functional theory. Consistent with earlier helium atom diffraction results, the monolayer structure has p(2×1) symmetry with a glide-plane along the longer axis of the unit cell. The structure analysis confirms the binding of CO via the carbon end to the NaCl(100) surface. The vertical distance of carbon above Na(+) is 2.58 ± 0.08 Å, in good agreement with geometry optimizations based on dispersion-corrected density functional theory, and 0.15 Å lower than predicted in calculations based on the nonlocal van der Waals density functional.

Subsurface cation vacancy stabilization of the magnetite (001) surface.

Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (√2 × √2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace.