Low-energy Electron Diffraction Results Research Articles

Structural investigation of flat overlayer and surface alloy of Sn on Mo(110)

Atomic structures of Sn on Mo(110) are investigated using low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. Sn atoms occupy substitutional sites at elevated temperatures in (3 × 1) structure, corresponding to a coverage of 0.33 ML. The substitutional model agrees with previous LEED, workfunction, and STM results. For (1 × 3) structure at 0.67 ML, Sn atoms adsorb in the nearest three-fold hollow sites. With increasing Sn coverage, the (1 × 4) structure at 0.75 ML, Sn atoms start to fill four-fold hollow sites. On both overlayer phases, Sn structures are flat without significant buckling. Compared with a two-dimensional tin, stanene, which is a stacking of α-Sn(111), Sn overlayer on Mo(110) is not related to α-Sn(111) but to β-Sn(100), which is normal metallic

Germanene structure enhancement by adjacent insoluble domains of lead

Growth of Xene, referred to as the analog of graphene made of atoms other than carbon, with higher lattice quality and more intrinsic electronic structures, is an important challenge to the development of two-dimensional (2D) materials. We report an approach using Pb-atom deposition to striped-phase (SP) germanene grown on Ag(111) to trigger it to a quasifreestanding phase (QP). Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with QP germanene, not only driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moir\'e patterns. Originally oriented at 30\ifmmode^\circ\else\textdegree\fi{} to Ag(111), the QP germanene transits first to a narrow distribution of rotations then to two sharp domains at \ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from 30\ifmmode^\circ\else\textdegree\fi{}, while the monoatomic Pb domains twist from \ifmmode\pm\else\textpm\fi{}4.3\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}9.5\ifmmode^\circ\else\textdegree\fi{} with respect to Ag(111). The analysis of higher-order coincidence for both layers on Ag(111) is in complete agreement with LEED results and reveals a picture of two insoluble monatomic layers driving each other to a more optimal configuration. Moreover, the corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy \ensuremath{\sim}0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation.

Nontrivial central-atom dependence in the adsorption of M-TPP molecules (M = Co, Ni, Zn) on Fe(001)-[formula omitted

Metal-tetraphenyl-porphyrin (M-TPP) molecules typically self-assemble forming square-like superlattices, as dictated by their shape. The dependence of the adsorption properties on the central atom is systematically studied for Co-, Ni-, and Zn-TPP adsorbed on oxygen passivated Fe(001), namely the Fe(001)-p(1×1)O surface. It is found by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) that despite the weak molecule–substrate interaction, preserving many features of quasi-free molecules, the self-assembled structure switches from the (5×5)R37° superlattice of Co-TPP and Ni-TPP to the plain (5×5) of Zn-TPP. Ab initio calculations based on density functional theory (DFT) are used to investigate the adsorption properties of the different molecules and the possible overlayers formed. Adsorption energies, structures, and electronic properties are reported, discussing the bonding mechanisms and the magnetic character. Only moderate energy differences are found, suggesting that subtle effects may steer the selection of the structure among overlayers with similar properties although differing substantially as for the LEED and STM experimental results.

Growth of well-ordered iron sulfide thin films.

In this paper a growth recipe for well-ordered iron sulfide films and the results of their characterisation are presented. The film was studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). XRD data reveal that the film has a NiAs-like structure with Fe vacancies, similar to iron sulfides such as pyrrhotite and smythite, although no indication of any ordering of these vacancies was observed. LEED and STM results show that the film exhibits a 2 × 2 surface reconstruction. XPS data provide additional evidence for a large number of Fe vacancies, and the oxidation states of the Fe and S in the film are analysed.

Structural and electronic properties of atomically thin Bismuth on Au(111)

Atomically thin Bismuth (Bi) films are predicated to be candidates for high temperature quantum spin hall materials. Here, we reported the structural evolutions and the corresponding electronic properties of Bi ultrathin films on Au(111) at initial stage as a function of coverage using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and photoemission spectroscopy (PES). The small window for a stable two dimensional (2D) Bi in Kagome and honeycomb lattice on Au(111) has been optimized. LEED and STM results show that Bi atoms self-assemble into dimered (5 × 5) superdots, (37×37)R25.3∘ in a Kagome lattice and (p × √3) stripes sequentially in the first monolayer regime and into bi-layered Bi (110) islands afterwards. UPS results show the disappearance of Au(111) surface states at coverage of 0.18 ML, revealing the Van der Waals interactions between Au(111) and Bi adatoms. The repulsive interactions between Bi atoms prevent the 2D Bi structure from islands forming at initial stage and benefit large domains in (37×37)R25.3∘ Kagome lattice. Such 2D Bi may be interesting because of its relevance in terms of novel physical properties.

High-resolution imaging of LaAlO3(100)–(1 × 4) reconstructed surface using non-contact atomic force microscopy

High-spatial-resolution imaging of an insulating LaAlO3 film is performed with non-contact atomic force microscopy. The LaAlO3 film was grown homogeneously on a LaAlO3 substrate using pulsed laser deposition, and it was imaged without breaking the vacuum. The sample preparation method for obtaining atomically flat surfaces suitable for atomic resolution imaging is presented. The LaAlO3 surface shows a reconstruction with a (1 × 4) structure, which is consistent with low-energy electron diffraction results. We believe that the method presented in this manuscript is an important advancement in the fabrication of insulating thin films used for atomic-level devices.

Adsorption study of copper phthalocyanine on Si(111)(√3 × √3)R30°Ag surface

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)(√3×√3)R30°Ag surface is studied at room temperature under ultra high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results indicate that after one monolayer deposition the molecular layer is highly ordered with a flat lying adsorption configuration. The corresponding pattern reveals the coexistence of three symmetrically equivalent orientations of molecules with respect to the substrate. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate; however there is evidence of Fermi level movement. During the growth, the work function was found to decrease from 4.90eV for the clean substrate to 4.35eV for the highest coverage (60 monolayers). Within a thickness of two monolayer deposition an interface dipole of 0.35eV and a band bending of 0.2eV have been found. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.55eV. The changes in the work function, in the Fermi level position and in the HOMO state have been used to determine the energy level alignment at the interface.

Thickness-Dependent Air-Exposure-Induced Phase Transition of CuPc Ultrathin Films to Well-Ordered One-Dimensional Nanocrystals on Layered Substrates

Highly ordered organic crystals are important building blocks for future high-performance organic nanodevices. Here we report a feasible way to produce arrays of well-ordered 1D copper phthalocyanine (CuPc) nanocrystals by using molybdenum disulfide (MoS2) or highly oriented pyrolytic graphite (HOPG) as substrates. The growth behaviors of CuPc on MoS2(0001) as well as on HOPG and corresponding effects of air exposure were systematically investigated by means of in situ photoemission spectroscopy (PES) and low-energy electron diffraction (LEED), combined with ex situ atomic force microscopy (AFM), surface X-ray diffraction (SXRD), and Raman spectroscopy. PES and LEED results show that CuPc molecules adopt a face-on configuration at thickness up to 4.8 nm, while AFM and SXRD results show that they adopt an edge-on configuration to form 1D nanocrystals in films thicker than 2.4 nm. Detailed analyses show that the formation of these 1D nanocrystals is closely related to air exposure, thicknesses, and growth t...

Oxygen-induced nano-faceting of Re(112¯1)

Abstract We report results from a detailed study of oxygen-induced faceting of Re( 11 2 ¯ 1 ) using low energy electron diffraction (LEED). Depending on conditions for reaction with oxygen, the initially planar Re( 11 2 ¯ 1 ) surface evolves sequentially to form zigzag chains consisting of ( 01 1 ¯ 0 ) and ( 10 1 ¯ 0 ) facets that coexist with stepped ( 11 2 ¯ 1 ) plateaus; three-sided pyramids exposing ( 01 1 ¯ 0 ), ( 10 1 ¯ 0 ) and ( 33 6 ¯ 4 ) facets; a faceted surface with five facets including ( 01 1 ¯ 0 ), ( 10 1 ¯ 0 ), ( 11 2 ¯ 2 )-(2 × 1), ( 01 1 ¯ 1 )-(2 × 1) and ( 10 1 ¯ 1 )-(2 × 1); and four-sided pyramids exposing { 01 1 ¯ 0 } and { 01 1 ¯ 1 }-(2 × 1) facets. We have observed reversible morphological transitions between the faceted surfaces and discovered that each intermediate facet in the evolutionary chain contains microscopic structural elements of the preceding and following facets, which may provide a natural explanation for the microscopic mass transport pathways in the morphological evolution. Scanning tunneling microscopy (STM) data confirm the morphology of faceted surfaces and give complementary information to LEED results.

Morphology and atomic structure of hydrogen-terminated Si(110)-(1 × 1) surfaces prepared by a wet chemical process

We have measured the macroscopic and microscopic surface morphology of hydrogen-terminated Si(110)-(1×1) [H:Si(110)-(1×1)] surfaces prepared by an improved wet chemical process. The observations were performed by high-resolution electron-energy-loss spectroscopy (HREELS), low-energy-electron diffraction (LEED), and scanning tunneling microscopy (STM). At macroscopic scale, it was found that the surface is a mono-hydride terminated surface with a two-dimensional p2mg space group, thus, being a well-defined H:Si(110)-(1×1) surface. At microscopic scale, elongated terraces were observed along the [11¯0] direction reflecting surface anisotropy. The terraces extend in frames with sizes up to a few micrometers. We discussed the macroscopic and microscopic surface morphology by combining our LEED and STM results.

Atomic structure of the Au(110)c(2 × 2)–Sb system: A combined LEED and DFT study

In this paper we present a combined low-energy electron diffraction (LEED) and a DFT study of the Au(110)c(2×2)–Sb surface in order to determine its atomic structure. The DFT calculations, using both LDA and GGA approaches, have indicated that the adsorption of antimony atoms on the Au(110) surface hollow sites is energetically more favored as compared with other possible adsorption sites. The LEED analysis also showed a Sb-overlayer termination with the Sb atoms segregating to the hollow sites instead of forming an Au–Sb surface alloy. This overlayer results to be contracted of about 0.16Å with the first gold layer presenting a small expansion (0.04Å) with respect to the Au bulk interlayers distance. The agreement between the LEED and DFT results is very good.

Correlation of electron self-energy with geometric structure in low-energy electron diffraction

In low-energy electron diffraction (LEED) studies of surface geometries where the energy dependence of the intensities is analyzed, the in-plane lattice parameter of the surface is usually set to a value determined by x-ray diffraction for the bulk crystal. In cases where it is not known, for instance in films that are incommensurate with the substrate, it is desirable to fit the in-plane lattice parameters in the same analysis as the perpendicular interlayer spacings. We show that this is not possible in a conventional LEED I(E) analysis because the inner potential, which is typically treated as an adjustable parameter, is correlated with the geometrical structure. Therefore, without having prior knowledge of the inner potential, it is not possible to determine the complete surface structure simply from LEED I(E) spectra, and the in-plane lattice parameter must be determined independently before the I(E) analysis is performed. This can be accomplished by establishing a more precise experimental geometry. Further, it is shown that the convention of omitting the energy dependency of the real part of the inner potential means geometrical LEED results cannot be trusted beyond a precision of approximately 0.01 Å.

Influence of Intermolecular Interactions on the Structure of Copper Phthalocyanine Layers on Passivated Semiconductor Surfaces

The surface structures of copper phthalocyanine (CuPc) thin films deposited on sulphur-passivated and plane perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)-covered InAs(100) surfaces have been studied by low energy electron diffraction (LEED) and van der Waals (vdW) intermolecular interaction energy calculations. The annealing to <TEX>$300^{\circ}C$</TEX> and <TEX>$450^{\circ}C$</TEX> of <TEX>$(NH_4)_2S_x$</TEX>-treated InAs(100) substrates produces a (<TEX>$1{\times}1$</TEX>) and (<TEX>$2{\times}1$</TEX>) S-passivated surface respectively. The CuPc deposition onto the PTCDA-covered InAs(100) surface leads to a ring-like diffraction pattern, indicating that the 2D ordered overlayer exists and the structure is dominantly determined by the intermolecular interactions rather than substrate-molecule interactions. However, no ordered LEED patterns were observed for the CuPc on S-passivated InAs(100) surface. The intermolecular interaction energy calculations have been carried out to rationalise this structural difference. In the case of CuPc unit cells on PTCDA layer, the planar layered CuPc structure is more stable than the <TEX>$\alpha$</TEX>-herringbone structure, consistent with the experimental LEED results. For CuPc unit cells on a S-(<TEX>$1{\times}1$</TEX>) layer, however, the <TEX>$\alpha$</TEX>-herringbone structure is more stable than the planar layered structure, consistent with the absence of diffraction pattern. The results show that the lattice structure during the initial stages of thin film growth is influenced strongly by the intermolecular interactions at the interface.

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III–V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(100) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(100) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(100) with 0.1°, 2° and 6° off-cut in [011] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(100) surfaces indicated two-domain (2×1)/(1×2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III–V layers. However, double-layer steps developed on 6° misoriented Si(100) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.

One-dimensional electronic states inGa2Se3on Si(001):As

Gallium selenide Ga2Se3 is a III-VI chalcogenide material whose intrinsic cation vacancy ordering into one dimensional chains has been predicted to result in a one dimensional band at the valence-band maximum VBM. The electronic structure of Ga2Se3 thin films on Si001:As is studied with angle-resolved photoemission spectroscopy. The integrated density of states and the dispersion exhibit good agreement with that predicted by published density-functional theory results. Consistent with a one dimensional state, the electronic states at the VBM show no dispersion perpendicular to the vacancy chains. The Se-Ga bond length from extended x-ray absorption fine structure is 2.34 A. Low-energy electron-diffraction results indicate that the surface is characterized by nanometer-scale 111 facets, consistent with previous scanning tunneling microscopy results.

Carbon monoxide adsorption on Co deposited Pt(1 0 0)-hex: IRRAS and LEED investigations

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm −1, respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co 0.3 nm /Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm −1. For Co 0.3 nm /Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co 0.3 nm /Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm −1, which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co 0.3 nm /Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt–Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co x /Pt(1 0 0)-hex are discussed.

Growth of oxygen-induced nanoscale-pyramidal facets on Rh(210) surface

Oxygen-induced nanometer scale faceting of the atomically rough Rh(210) surface has been studied using Auger electron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). When the Rh(210) surface is annealed at temperature $\ensuremath{\ge}550\text{ }\text{K}$ in oxygen (pressure $\ensuremath{\ge}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{Torr}$), it becomes completely covered with nanometer-scale facets. LEED studies reveal that the faceted surface is characterized by three-sided nanoscale pyramids exposing one reconstructed (110) and two {731} faces on each pyramid. STM measurements confirm the LEED results and show that the average facet size ranges from 12 to 21nm when changing annealing temperature from 800 to 1600 K. Moreover, atomically resolved STM images show that the (110) face of faceted Rh(210) exhibits various reconstructions ($1\ifmmode\times\else\texttimes\fi{}n$, $n=2--4$) depending on oxygen coverage. Faceted Rh(210) is a potential template for studies of structure sensitive reactions.

Geometry of the Au(1 1 0)-(1 × 2) missing-row clean surface: A New LEED and DFT study

In this work we report on a very accurate low-energy electron diffraction (LEED) and a DFT study of Au(1 1 0)-(1 × 2) in order to determine its surface geometry. The LEED results indicate a surface missing row reconstruction with a large contraction of ( - 0.29 ± 0.02 ) Å , a pairing in the second layer ( 0.06 ± 0.05 ) Å and in the fourth layer ( 0.07 ± 0.04 ) Å , and a buckling in the third layer of 0.30 Å and in the fifth layer of 0.01 Å. These LEED results are in very good agreement with those found by the DFT calculations.

Comment on “Computer Simulation of Coaxial Impact-Collision Ion Scattering Spectroscopy Spectra of Clean Pt(111) Surface and Pt(111)–p(2×2)-O Surface”

The clean Pt(111) and Pt(111)-p(2×2)-O surfaces have been investigated using the computer simulation of coaxial impact-collision ion scattering spectroscopy (CAICISS). The computer simulations employing the atomic collisions in a crystalline target (ACOCT) program code, which treats the atomic collisions three-dimensionally and is based on the binary collision approximation (BCA), were carried out for the case of 3 keV He+ ions incident along the [211] azimuth of the clean Pt(111) and Pt(111)-p(2×2)-O surfaces. The comparisons between ACOCT results and experimental CAICISS data show that the experimental results on the clean Pt(111) surface are relatively well reproduced by the ACOCT simulations for no surface relaxation, and that the ACOCT simulations for the Pt(111)-p(2×2)-O surface appear the oxygen occupancy in the fcc-hollow sites over the hcp-hollow sites. The ACOCT results for the clean Pt(111) and Pt(111)-p(2×2)-O surfaces are consistent with the analyzed results of low-energy electron diffraction (LEED).

Multiple scattering x-ray photoelectron diffraction study of the SrTiO3(100) surface

The atomic surface structure of SrTiO3(100) after annealing at 630 °C in vacuum is investigated by x-ray photoelectron diffraction (XPD) using the Sr 3d5/2 core level. The photoelectron diffraction peaks are successfully assigned by considering the forward scattering of photoelectrons by the atomic potential near the emitter atom in the lattice. The strongest diffraction peaks are aligned along the single crystal internuclear axes. We compare the results of photoelectron multiple scattering calculations (MSC) of SrO and TiO2 terminated SrTiO3(100) surfaces, including surface relaxation and rumpling, with the experimental data. For TiO2 and SrO terminated SrTiO3(100) surfaces, all top-layer cations relax inward, whereas second-layer atoms relax outward. The surface rumpling for SrO- and TiO2-terminated surfaces agrees well with low-energy electron diffraction results. Using a genetic algorithm the best agreement of MSC to the experimental XPD data is obtained for a SrO terminated surface with a 30% coverage of 3 ML SrO(100) islands.