Low Energy Electron Diffraction Spots Research Articles

Tin oxide thin films on Ag(111): Thickness and temperature dependent study of surface structure and electronic properties

Growth and structure of tin oxide films on Ag(111) from submonolayer to 10 monolayer equivalent thickness have been studied using low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy techniques for room temperature (RT) and high temperature (HT) (573 K). For RT growth, most of the tin is oxidised to form tin oxide (SnOx) as confirmed by XPS where no metallic component is noticed. However, the grown films are not epitaxial or ordered which can be confirmed by no spots in LEED for higher coverage. Metallic Sn along with two oxidation states of Sn are noticed for HT growth, with the surface mainly composed of SnO2. LEED pattern shows the presence of c(2 × 2) and (3 × 1) along with two different hexagonal multidomain type patterns. The electronic structure for lower coverage resembled the Sn/Ag(111) alloy structures for lower coverage. For higher coverages appearance of three bands below 2 eV binding energy was noticed.

Increasing the scan speed in high resolution, low energy electron diffraction measurements by presetting the gate time.

We report on a speed-up data acquisition routine for recording intensities in reciprocal space (k-space) with increased scan speed by a single point detector. It is designed for recording low energy electron diffraction (LEED) data with high resolution by a spot profile analysis LEED instrument. It counteracts the problem of long acquisition times that are encountered when larger areas in the reciprocal space are scanned. It exploits the fact that in typical LEED images of ordered surfaces, more than 90% of the data points in k-space belong to the low-intense background, which is often not of interest. Only about 10% of the data points are related to the relevant diffraction features, namely, the LEED spots. Often it is not necessary to measure the background with the same statistical significance as measuring those points that contain information. The data points belonging to the LEED spots can be discriminated from those of the background by their higher intensities. An acquisition routine that sets an increased gate time for the counting in response to higher intensities thus safeguards good statistics for data points of the LEED spots and saves measurement time when recording data points of the background with small gate times. For typical LEED images, a reduction of the total acquisition time by a factor of about 10 is obtained. We give examples of one- and two-dimensional scans from current experiments, recorded with and without the speed-up routine. We further discuss how the routine supports the measurement of energy dependent reciprocal space maps.

Influence of finite size and image charge screening on the antiferromagnetic ordering of CoO ultrathin films

The surface antiferromagnetic (AFM) ordering of ultrathin CoO(001) films grown on Ag(001) has been investigated using low energy electron diffraction (LEED) and valence band photoemission spectroscopy. We have observed coherent exchange scattered half-order spots in LEED with a periodicity of the magnetic unit cell of the CoO(001) surface. These spots show very low intensities (2–3% of the integer-order spots) and are visible only below a critical temperature for low electron beam energies (<70 eV), revealing its AFM origin. Significant enhancement of surface Nèel temperature (TN) of CoO(001) for different film coverages has been observed and it is exceeding the bulk TN even in the ultrathin limit. The enhancement of surface TN has been explained by the combined effects of finite size, surface strain, and polarizability of the underlying substrate. The critical behaviour near TN shows a coverage dependent evolution as the magnetic critical exponent (β) varies from 0.16 to 0.48 upon increasing film coverage from 0.5 ML to 10 ML, suggesting crossovers in the critical behaviour. Furthermore, upon transition from paramagnetic (PM) to AFM phase, minor modifications of the valence band electronic structure have been observed such as enhancement of spectral weight in the Co 3d band as well as widening of an energy gap relative to EF by ~150 meV, in agreement with the theoretical predictions.

Atomic adsorption of Sn on mechanically cleaved WS2 surface at room temperature

Room temperature (RT) adsorption of tin (Sn) under ultra-high vacuum (UHV) conditions on mechanically cleaved tungsten disulfide (WS2) surfaces is reported here. In-situ low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) studies indicate commensurate adsorption of Sn on the hexagonal surface of WS2. Bare WS2 displays an atomic periodicity of lattice constant 0.34 nm. Growth of Sn reveals monomer like a single atom or unpaired (U) and dimer like- or paired (P) adsorptions on the top (`T’) sulfur (S) sites of the trigonal prismatic sublattice within the honeycomb lattice of WS2. The lateral distance between two nearest adsorbed monomers is around 0.34 nm which is exactly similar to the atomic periodicity of the underneath WS2 substrate crystal. For the case of dimer like- adsorption, the separation between two adspecies is 0.293 nm. The bond length of the adsorbed Sn with the surface S atoms is around 40 (±10) pm. We do not find any evidence of Sn adsorption on the bridge (`B’) or hollow (`H’) site of the WS2 lattice. Analysis of the LEED spots and the FFT images of the STM micrographs after Sn adsorption discloses a stretched hexagonal surface which is most likely due to the overall consequence of a perfectly commensurate monomer and nearly commensurate dimer like- adsorptions of the Sn atoms on the WS2 surface.

Growth of Ag(1 1 1) on Si(1 1 1) with nearly flat band and abrupt interface

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 × 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 × 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 × 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.

Metastable phase of lead phthalocyanine films on graphite: Correlation between geometrical and electronic structures

The geometrical and electronic structures of a metastable phase of lead phthalocyanine (PbPc) films on graphite have been studied by combined use of low energy electron diffraction (LEED) and two-photon photoemission (2PPE) spectroscopy. In submonolayer (sub-ML) PbPc films on graphite, islands in a metastable phase are formed just after deposition, as we reported previously by use of photoelectron emission microscopy (PEEM) [I. Yamamoto, N. Matsuura, M. Mikamori, R. Yamamoto, T. Yamada, K. Miyakubo, N. Ueno, and T. Munakata, Surf. Sci. 602, 2232 (2008)]. On single crystalline graphite substrates, the metastable islands produce clearly discernible LEED spots. By comparing the unit cell with that of annealed 1 ML films, molecules in the metastable islands are standing upright with a molecular density 1.8 times higher than that in the well-ordered 1 ML films. The LEED spots for the sub-ML films disappear after annealing. The islands in the metastable phase are surrounded by areas of a two-dimensional (2D) gaslike phase composed of flat-lying molecules. The metastable islands melt into the 2D gas phase, consistent with the PEEM results. In 2PPE spectroscopy, the lowest unoccupied molecular orbital (LUMO) derived level of the metastable phase is clearly distinguishable from that of flat-lying molecules. By tracking the thermal annealing process of the films by 2PPE spectroscopy, we clarify the decay of the LUMO derived peak intensity, the work function shift, and the energy shifts of molecular states associated with the transition from the metastable phase to the 2D gas phase. With this, we demonstrate the complementary capabilities of LEED and 2PPE spectroscopy to probe phase transitions of organic films in a nondestructive manner.

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.

Ion bombardment of Ni(110) studied with inverse photoemission spectroscopy and low-energy electron diffraction

Inverse photoemission spectroscopy (IPES) performed on clean Ni(110) reveals an unoccupied electronic surface state with energy ~2.5eV above the Fermi level for emission near the Ȳ point of the Surface Brillouin Zone. Ion bombardment of the sample creates defects that reduce the intensity of the peak in IPES spectra. Sharp, intense diffraction spots in low-energy electron diffraction (LEED) patterns taken of the clean surface become dimmer after bombardment. Results of these measurements are compared to Monte Carlo simulations of the sputtering process to ascertain the approximate size of clean patches on the sample necessary to sustain the IPES and LEED features. At 170K, the IPES surface state peak appears closely associated with the population of surface atomic sites contained in clean circular patches of about 50 atoms. The LEED patterns persist to greater degrees of sputtering and are associated with smaller clean patches. Both measurements performed at 300K indicate significant self-annealing of the sputtering damage.

Specular reflection of spin-polarized electrons from the W(001) spin-filter crystal in a large range of scattering energies and angles

Extending previous work on the imaging spin-filter technique based on electron diffraction from W(001) in the specular (00)-LEED (low-energy electron diffraction) spot, we studied the scattering-energy and angle-of-incidence landscape of spin sensitivity $S$ and reflectivity ${I/I}_{0}$. The setup includes a spin-polarized GaAs electron source and a rotatable delay-line detector for spatially resolving detection. We measured the intensity and spin asymmetry of the specularly reflected beam for energies between 14 and 37 eV and angles of incidence between ${30}^{\ensuremath{\circ}}$ and ${60}^{\ensuremath{\circ}}$. Resulting energy-angular landscapes show rather good agreement with theory [relativistic layer KKR SPLEED (spin-polarized low-energy electron diffraction) code]. This provides a sensitive test of theory, in particular of details of the surface barrier. The results identify several regions of high asymmetry and figure of merit, in particular a region where the asymmetry can be ``switched'' between positive and negative values by just 4 eV energy variation.

Structural analysis of the intermetallic surface compoundCePt5/Pt(111)

We report on a detailed low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (STM) study of the intermetallic surface compound ${\mathrm{CePt}}_{5}$ on Pt(111). Depending on the thickness we observe various diffraction patterns and superstructures. In the low-thickness regime a slightly compressed $(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure is aligned along the $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ direction of the Pt(111) substrate. STM reveals another, much larger superstructure with a periodicity of ($9.02\ifmmode\pm\else\textpm\fi{}0.45$) nm presumably responsible for the strongly broadened LEED spots. At about 3 unit cells (u.c.) the surface is dominated by a $(3\sqrt{3}\ifmmode\times\else\texttimes\fi{}3\sqrt{3})R{30}^{\ensuremath{\circ}}$ pattern as revealed by LEED satellites and Fourier-transformed high-resolution STM images. It is interpreted as a moir\'e pattern between the film and the substrate. We precisely determine the superstructure of the intermetallic film to $(\frac{10}{9}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{10}{9}\sqrt{3})R{30}^{\ensuremath{\circ}}$ with respect to the Pt(111) substrate. Above 3 u.c. the satellites progressively disappear. A model is developed that consistently describes this thickness-dependent transition. For ${\mathrm{CePt}}_{5}$ films with a thickness between 6 and 11 u.c. the lattice of the compressed $(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure rotates back into the substrate's $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ directions.

In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter

The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε=0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived.

Impact of a Mixed Oxide’s Surface Composition and Structure on Its Adsorptive Properties: Case of the (Fe,Cr)3O4(111) Termination of the α-(Fe,Cr)2O3(0001) Surface

Characterization of an α-(Fe0.75,Cr0.25)2O3(0001) mixed oxide single crystal surface was conducted using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). After sputter/anneal cleaning in ultrahigh vacuum (UHV), the mixed oxide surface became terminated with a magnetite-like (111) structure based on the presence of (2 × 2) spots in LEED and Fe2+ in XPS. The composition of the surface was close to that of M3O4 based on XPS, with the metal (M) content ratio of Fe2+/3+ and Cr3+ being close to 1.4:1, despite the film’s bulk being prepared with a 3:1 ratio of these cations. Enrichment of the surface with Cr was not altered by high temperature oxidation in UHV but could be partially reversed by exposure to air. Adsorption of various probe molecules (NO, O2, CO2, and H2O) was used in an effort to identify the active cation sites present on the (Fe,Cr)3O4(111)-terminated surface. Although both XPS an...

Broken symmetry induced band splitting in theAg2Gesurface alloy on Ag(111)

We report a study of the atomic and electronic structures of the ordered ${\mathrm{Ag}}_{2}\mathrm{Ge}$ surface alloy containing ⅓ monolayer of Ge. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES) data reveal a symmetry breaking of the expected \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 periodicity, which is established for other ${\mathrm{Ag}}_{2}$$M$ alloys ($M$ = Bi, Sb, Pb, and Sn). The deviation from a simple \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 structure manifests itself as a splitting of diffraction spots in LEED, as a striped structure with a 6\ifmmode\times\else\texttimes\fi{} periodicity including a distortion of the local hexagonal structure in STM, and as a complex surface band structure in ARPES that is quite different from those of the other ${\mathrm{Ag}}_{2}$$M$ alloys. These results are interesting in view of the differences in the atomic and electronic structures exhibited by different group IV elements interacting with Ag(111). Pb and Sn form \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 surface alloys on Ag(111), of which ${\mathrm{Ag}}_{2}\mathrm{Pb}$ shows a surface band structure with a clear spin-orbit split. Si and C form silicene and graphene structures, respectively, with linear band dispersions and the formation of Dirac cones as reported for graphene. The finding that ${\mathrm{Ag}}_{2}\mathrm{Ge}$ deviates from the ideal (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3) ${\mathrm{Ag}}_{2}\mathrm{Sn}$ and ${\mathrm{Ag}}_{2}\mathrm{Pb}$ surface alloys makes Ge an interesting ``link'' between the heavy group IV elements (Sn, Pb) and the light group IV elements (Si, C).

Reconstructions and phase transition of clean Ge(110)

The structure of the clean Ge(110) surface is characterized between room temperature and the Ge melting temperature using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) and microscopy (LEEM). Rapid cooling from high temperature (~800°C) to room temperature yielded a surface composed of the c(8×10) reconstruction, {17 15 1} facets, and a previously unreported (8×2) reconstruction. Heating from room temperature to above 430°C extinguishes some, but not all, high-order LEED spots, indicating the presence of ordering up to at least 650°C. LEED observations of the phase transition between the c(8×10) and a disordered phase differ from earlier work but are consistent with previously published STM data.

Measurement of microscopic surface deformation due to low energy ion bombardment on Si(111)

A low energy electron diffraction spot profile analysis of the Si(111) surface, after argon ion bombardment at an elevated temperature, finds a continuous, low amplitude distribution of surface height, in addition to the atomic-step-and-terrace structure. With an amplitude of tenths of an Angstrom or less in height, correlated laterally over tens of Angstroms, this microscopic surface deformation is measured versus ion dose and for various sample temperatures during bombardment and annealing. For 230eV argon ion doses increasing in the range of 1015−1016cm−2 with the sample held at 580K and 800K, the amplitude of surface deformation, measured as the standard deviation wz of the continuous height distribution, increases steadily in the range 0.06−0.10Å. For higher ion doses in the range of 3×1016−1×1017cm−2, saturation-like behavior with wz≈0.14−0.15Å is measured at 800K. After an ion dose of 1017cm−2, the surface deformation shows a lateral correlation length of 20Å, implying an average lateral feature size of ≈40Å, slightly less than the average terrace width of ≈60Å. On the other hand, after the same ion dose at 300K followed by annealing at 800K, the surface deformation has a similar correlation length but a much smaller height amplitude of wz=0.06Å. It is notable that wz depends quite differently on sample preparation conditions overall, compared to the average terrace width and to the degree of (7×7) order indicated by diffraction intensities, both of which are found to evolve much more slowly with ion dose in conditions for which the height amplitude increased in the range of 0.06−0.14Å. The surface height deformation due to strain around buried, subsurface defects is estimated as a function of defect concentration using a simple, order-of-magnitude theoretical model, in which a distribution of subsurface defects is modeled as small inclusions in a continuous elastic medium. This approximate model is consistent with the measured growth of the deformation amplitude wz, for a range of possible parameter choices for small defect clusters located in a shallow layer below the surface. The surface deformation is partially removed by annealing above 870K.

Monolayer formation of hexagonal boron nitride on Ag(001)

The formation of monolayers from hexagonal boron nitride, h-BN, on the square Bravais lattice of a Ag(001) surface via chemical vapor deposition of borazine (HBNH)3 was investigated by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Due to the weak interaction between the h-BN monolayer and the Ag surface, there is no preferred orientation of the h-BN lattice. The h-BN-related hexagonal distributions of LEED spots transform into nearly homogeneous rings. The growth of h-BN on Ag(001) is the same as previously demonstrated for Ag(111), giving evidence that for weakly interacting transition metal surfaces the surface symmetry has no distinct impact on the h-BN growth.

Thin Mn germanide films studied by XPS, STM, and XMCD

Thin Mn germanide films with nanoscale thicknesses on Ge(111) have been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), core-level spectroscopy (CLS), and x-ray magnetic circular dichroism. The 260 ${}^{\ensuremath{\circ}}$C annealing of 16 monolayers of Mn deposited on Ge(111)$c$($2\ifmmode\times\else\texttimes\fi{}8$) resulted in a uniform film with intense threefold split $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ LEED spots and Moir\'e patterns in the STM images. This ultrathin film shows a clear ferromagnetism with a Curie temperature of $\ensuremath{\sim}$250 K. High-resolution Ge 3$d$ CLS spectra were recorded with photon energies between 50 and 90 eV at normal and 60${}^{\ensuremath{\circ}}$ emission angle. To achieve a consistent fit over the energy and angular range three components were used in the line-shape analysis. The low temperature (260 ${}^{\ensuremath{\circ}}$C) annealed film shows significant differences in terms of electronic structure and magnetism in contrast to the high temperature (330 ${}^{\ensuremath{\circ}}$C or above) annealed ones. Our results indicate that the annealing temperature and the Mn coverage play important roles in the formation of a thin magnetic Mn germanide film.

Structural transformations of Cu(110) surface induced by adsorption of molecular chlorine

The adsorption of molecular chlorine on Cu(110) has been studied by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) in the combination with density functional theory (DFT) calculations. We have found that at early stage of chlorine adsorption the reconstruction of the Cu(110) surface takes place. At coverages 0.4<θ<0.5ML (monolayer), an array of the striped light domain walls separating the c(2×2) domains forms on the surface. According to DFT calculations, each light domain wall contains an additional row of copper atoms in the middle. As the coverage increases to 0.5ML, all the light domain walls disappear from the adlayer and a simple c(2×2) structure forms. Further chlorine dosing results in the uniaxial splitting of the overlayer spots in LEED indicating the compression of the chlorine layer in the 〈110〉 direction. At coverages close to the saturation level, the adsorption of chlorine leads to formation of 1D facets consisting of two {210} planes and running along the [001] direction.

Mn5Ge3film formation on Ge(111)c(2×8)

Thin manganese germanide films with different thicknesses on Ge(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and core-level spectroscopy (CLS). Annealing of the deposited Mn on Ge(111)$c$($2\ifmmode\times\else\texttimes\fi{}8$) between 330--450${}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$C resulted in well-ordered Mn${}_{5}$Ge${}_{3}$ surfaces as seen by intense $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ LEED spots. Up to a coverage of 24 monolayers (ML), island formation is favored. At a coverage of 32 ML, a well-ordered Mn${}_{5}$Ge${}_{3}$ film was found to fully cover the surface. High-resolution Ge 3$d$ CLS spectra were recorded with photon energies between 50 and 110 eV at normal and 60${}^{\ensuremath{\circ}}$ emission angles. In contrast to earlier results, three components have been used in the line-shape analysis to achieve a consistent fit over the energy and angular range. In addition, three components have been identified for the Mn $2p$ CLS spectra. The two major components fit well with a layered Mn germanide structure suggested in the literature.

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.