Dynamical Low-energy Electron Diffraction 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.

Atomic structure of “multilayer silicene” grown on Ag(111): Dynamical low energy electron diffraction analysis

We have investigated the atomic structure of the “multilayer silicene” grown on the Ag(111) single crystal surface by using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We measured the intensity of the LEED spot as a function of the incident electron energy (I–V curve) and analyzed the I–V curve using a dynamical LEED theory. We have found that the Si(111)(3×3)-Ag model well reproduces the I–V curve whereas the models consisting of the honeycomb structure of Si do not. The bias dependence of the STM image of multilayer silicene agrees with that of the Si(111)(3×3)-Ag reconstructed surface. Consequently, we have concluded that the multilayer silicene grown on Ag(111) is identical to the Si(111)(3×3)-Ag reconstructed structure.

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.

Formation of a Buffer Layer for Graphene on C-Face SiC{0001}

Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in a Si-rich environment are studied using low-energy electron diffraction (LEED) and low-energy electron microscopy (LEEM). Upon graphitization, an interface with rt(43) x rt(43)-R7.6 degree symmetry is observed by in situ LEED. After oxidation, the interface displays rt(3) x rt(3)-R30 degree symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like "buffer layer" that forms between the graphene and the SiC, similar to that observed on Si-face SiC. From a dynamical LEED structure calculation for the oxidized C-face surface, it is found to consist of a graphene layer sitting on top of a silicate (Si2O3) layer, with the silicate layer having the well-known structure as previously studied on bare SiC(000-1) surfaces. Based on this result, the structure of the interface prior to oxidation is discussed.

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

We have studied the incommensurate moir\'e 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 moir\'e structure: The corrugation varies smoothly over several moir\'e unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moir\'e symmetry sites and the underlying substrate. We also observe isolated outliers, where the moir\'e 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.

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.

Dynamical LEED analyses of the clean and the NO-adsorbed Ir(111) surface

The adsorption structure of nitric oxide (NO) on Ir(111) was studied by thermal desorption spectroscopy (TDS) and dynamical analyses of low-energy electron diffraction (LEED). At the saturation coverage at about 100K, a 2×2 pattern was observed by LEED and two peaks appeared at 365 and 415K in TDS. No change in the LEED I–V curves was observed by annealing at 280K, which means that the NO-saturated surface was retained at this temperature. On the contrary, partial desorption and changes of the LEED I–V curves were observed by annealing at 360K. Combined with previous vibrational studies, it is suggested that one adsorption species is not affected, while another species is partially desorbed and the rest of them are dissociated by annealing at 360K. Dynamical analyses of LEED were performed for the 280K-annealed and the 360K-annealed surfaces, which correspond to the NO-saturated and the NO-dissociated Ir(111) surfaces, respectively. These revealed that NO occupies the atop, fcc-hollow and hcp-hollow sites (atop-NO+fcc-NO+hcp-NO) for the NO-saturated Ir(111) surface with the saturation coverage of 0.75 ML. For the 360K-annealed surface, the atop-NO is not affected but the fcc-NO and the hcp-NO are partially desorbed as NO and partially dissociated to N and O, both of which occupy the fcc-hollow site on the surface.

C60on the Pt(111) surface: Structural tuning of electronic properties

The structure and electronic properties of the $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.{9}^{\ensuremath{\circ}}$ and $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R{30}^{\ensuremath{\circ}}$ ordered phases of C${}_{60}$ on the Pt(111) surface are investigated using combined dynamic low-energy electron diffraction and density functional theory (DFT) calculations. The two phases have the same local adsorption structure, while they are predicted by DFT calculations to exhibit very different electronic structures due to their different inter-C${}_{60}$ orientations and distances. This result demonstrates the structural tuning of electronic properties for molecular films or junctions composed of the same materials.

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.

Re-investigation of the Bi-induced Si(111)-( [formula omitted]) surfaces by low-energy electron diffraction

The atomic structures of the Bi/Si(111)-( 3 × 3 ) reconstructed surfaces obtained at different adsorbate coverages were studied by dynamical low-energy electron diffraction (LEED) I–V analysis. The experimentally obtained I–V curves of the β-Bi/Si(111)-( 3 × 3 ) surface, which is formed by a Bi coverage of 1 ML, showed good agreement with the curves calculated for the milk stool model, a model proposed in the literature. In contrast to this result, the I–V curves of the α-phase obtained at a coverage of 1/3 ML do not agree with those reported in early LEED studies. We found that the I–V curves reported in the former LEED studies resemble closely to the results obtained for the surface on which the β and α-phases coexist. Based on the obtained I–V curves, we propose a more reliable structural model for the α-phase, and present the way to determine the presence of a high quality α-phase by using LEED.

Surface structure and composition of Pd 8Ni 92(1 1 1): A dynamic LEED study

The Pd 8Ni 92(1 1 1) surface was studied using low-energy electron diffraction (LEED). IV spectra collected close to normal incidence were analysed by dynamical tensor LEED. Composition and interlayer spacings were determined for first four atomic layers. The surface slab is found to be enriched in Pd (30% in total against 8% in bulk) but this extra palladium is irregularly distributed over the four topmost layers. Contrary to previous reports, the palladium amount, as compared to the bulk, is not significantly changed in the surface layer and it is enriched to 20% only in the third layer. The detected contractions of interlayer spacings are all but one ( d 34) within the error limits of no contraction at all. We also discuss some aspects of correlations between the fit parameters.

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.

Surface Geometry ofC60on Ag(111)

The geometry of adsorbed ${\mathrm{C}}_{60}$ influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a ${\mathrm{C}}_{60}$ monolayer, $\mathrm{Ag}(111)\mathrm{\text{\ensuremath{-}}}(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})30\ifmmode^\circ\else\textdegree\fi{}\mathrm{\text{\ensuremath{-}}}{\mathrm{C}}_{60}$, and related density functional theory calculations. The stable monolayer has ${\mathrm{C}}_{60}$ molecules in vacancies that result from the displacement of surface atoms. ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$ monolayers on close-packed metal surfaces.

Low-energy electron diffraction (LEED) study of an aperiodic thin film of Cu on 5-fold i-Al-Pd-Mn

Thin films of copper grown on five-fold i-AlPdMn at room temperature consist of domains that are rotationally aligned with the five primary symmetry directions of the substrate and which have one-dimensional aperiodic order. This aperiodic order is evident in scanning tunnelling microscopy images as wide and narrow rows that are spaced according to a Fibonacci sequence. A low-energy electron diffraction (LEED) study of this film indicates that the structure within the domains is periodic along the rows, with a repeat distance equal to the nearest-neighbour separation in bulk Cu. To determine the complete structure, a dynamical LEED experiment was performed for a five-layer Cu film at a sample temperature of 85 K. The analysis was performed using two different computational methods, one based on quasicrystalline slabs and the other on periodic approximants. Of the model structures tested, the film is found to be most consistent with a structure based on the Cu{100} surface structure, but having aperiodic displacements, both in-plane and out-of-plane, along a ⟨110⟩ direction.

Unveiling new systematics in the self-assembly of atomic chains on Si(111)

Self-assembled arrays of atomic chains on Si(111) represent a fascinating family of nanostructures with quasi-one-dimensional electronic properties. These surface reconstructions are stabilized by a variety of adsorbates ranging from alkali and alkaline earth metals to noble and rare earth metals. Combining the complementary strength of dynamical low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we recently showed that besides monovalent and divalent adsorbates, trivalent adsorbates are also able to stabilize silicon honeycomb chains. Consequently silicon honeycomb chains emerge as a most stable, universal building block shared by many atomic chain structures. We here present the systematics behind the self-assembly mechanism of these chain systems and relate the valence state of the adsorbate to the accessible symmetries of the chains.

Atomic and electronic structure ofTl∕Ge(111)−(1×1): LEED and ARPES measurements and first-principles calculations

We have investigated the atomic and electronic structure of the $\mathrm{Tl}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. The Tl adsorption site was determined to be the ${T}_{4}$ site by dynamical low-energy electron diffraction $I\text{\ensuremath{-}}V$ analysis. The surface electronic structure was measured using angle-resolved photoelectron spectroscopy. By comparing the observed band structure with the first-principles calculation, we revealed that only the $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6p$ electrons are involved in the Tl--Ge bonding and that hybridization of $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6{s}^{2}6p$ valence electrons is negligible. This indicates that the inert-pair effect plays a decisive role in the formation of the $(1\ifmmode\times\else\texttimes\fi{}1)$ surface. On the other hand, a surface band is not fully occupied but forms a small hole pocket around $\overline{\ensuremath{\Gamma}}$. We propose that it is caused by a small amount of remaining defects due to the small energy gap. Our calculation including spin-orbit interaction indicates the existence of a large spin-orbit splitting in the lowest unoccupied surface band of $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6{p}_{x}+6{p}_{y}$ character.

Nature of Xenon adsorption on graphite: On-top versus hollow site preference

We study the interaction between Xe adatoms and the graphite (0001) surface employing all-electron density-functional theory calculations. Xe adatoms prefer to adsorb in the hollow site, which is consistent with a recent dynamical low-energy electron diffraction intensity analysis. We explain the hollow site preference as a consequence of the greater polarization of Xe adatoms at the hollow site, as well as of the surface C atoms, and hence a greater induced dipole moment, which provides a larger contribution to the attractive interaction. As a consequence, Xe adatoms get closer to the surface in the hollow site. The above-mentioned mechanisms of the interaction between Xe atoms and graphite (0001) are, thus, very similar to the picture recently proposed for rare-gas adatoms on close-packed metal surfaces.