Low-energy Electron Diffraction Observations Research Articles

Effect of embedded Cr on the structural phase transition of In/Si(111)4 × 1

Using scanning tunneling microscopy and low-energy electron diffraction observations accompanied with the density-functional theory calculations, adsorption of Cr atoms onto the Si(111)4 × 1-In surface and its effect on the 4 × 1-to-8 × 2 structural transition were investigated. It was found that in contrast to other metal adsorbates, such as Na, In and Pb, which remain adatoms, Cr atom becomes embedded below the level of In atoms constituting the In nanowire. But in other respects Cr produces effects similar to those of other metals, namely it also induces distortion in the In rows of its residence resulting in doubling of the periodicity along the rows and also reduces the critical temperature of the 4 × 1-to-8 × 2 transition almost linearly with amount of deposited metal.

Structure and phase transition of a uniaxially incommensurate In monolayer on Si(111)

We have studied the atomic structure and phase transition of a dense metallic monolayer of In on Si(111). Although the monolayer phase was previously considered to have a $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ periodicity, low-energy electron diffraction and scanning tunneling microscopy observations have revealed that the phase has an incommensurate structure with the In overlayer uniaxially contracted by 2% from $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$. The uniaxially contracted structure was found to be more stable than the commensurate $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ structure by first-principles calculations. We also observed a phase transition to a $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})$ phase at 250--210 K upon cooling. Angle-resolved photoelectron spectroscopy and macroscopic four-point-probe conductivity measurements demonstrated that the transition induces the disappearance of metallic surface states and a sharp drop in sheet conductivity, respectively. These results indicate an electronic metal-insulator transition.

(Tl, Sb) and (Tl, Bi) binary surface reconstructions on Ge(111) substrate

2D compounds made of Group-III and Group-V elements on the surface of silicon and germanium attract considerable attention due to prospects of creating III-V binary monolayers, which are predicted to hold advanced physical properties. In the present work, we have investigated two such systems, (Tl, Sb)/Ge(111) and (Tl, Bi)/Ge(111) using scanning tunneling microscopy, low energy electron diffraction observations and density-functional-theory calculations. In addition to the previously reported surface structures of 2D (Tl, Sb) and (Tl, Bi) compounds on Si(111), we found new ones, namely, 7 × 7 and 3 × 3. Formation processes and plausible models of their atomic arrangements are discussed.

Low-temperature one-atom-layer [formula omitted]-In phase on Si(111)

The Si(111)-hex-7×3-In reconstruction has been attracted considerable attention due to its superconducting properties occurring in the one-atom-layer metal film. However, the 7×3 periodicity is a characteristic feature of this surface only at room temperature. Upon cooling to low temperatures the 7×3 structure transforms reversibly to the 7×7 one that should not be ignored while considering superconductivity in this system. In the present study, atomic structure of the low-temperature one-atom-layer Si(111)7×7-In phase has been evaluated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and ab initio random structure searching (AIRSS) technique. Basing on the LEED observations, it has been found that the 7×7-In surface incorporates plausibly eight In atoms per 7×7 unit cell (i.e., ~1.14ML In). AIRSS demonstrates occurrence of a set of various surface structures with very close formation energies. Some of their counterparts can be found in the experimental STM images.

Electrical conductivity of reconstructed Si(111) surface with sodium-doped C60 layers

Electrical conductance of sodium-doped C60 ultra-thin layers (1–6 monolayers) grown on the Na-adsorbed Si(111)√3 × √3-Au surface has been studied in situ by four-point probe technique, combined with low-energy electron diffraction observations. Evidence of conductance channel formation through the C60 ultrathin layer is demonstrated as a result of Na dosing of 3 and 6 monolayers thick C60 layers. The observed changes in surface conductivity can be attributed to the formation of fulleride-like NaC60 and Na2C60 compound layers.

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.

Structural transformations in Pb/Si(111) phases induced by C60 adsorption

Structural transformations at the Pb/Si(111) surface occurring upon C60 adsorption onto Pb/Si(111)1 × 1 phase at room temperature and Pb/Si(111) at low temperatures between 30 and 210 K, have been studied using scanning tunneling microscopy and low-energy electron diffraction observations. Typically, C60 fullerenes agglomerate into random molecular islands nucleated at the surface defects. C60 island formation is accompanied by expelling Pb atoms to the surrounding surface area where more dense Pb/Si(111) phases form. Productivity of C60-induced expelling of Pb atoms is controlled by surface defects and is suppressed dramatically when regular (‘crystalline’) C60 islands self-assemble at the defect-free Pb/Si(111) surface. When Pb atoms are ejected by the random C60 islands, extended structural transformations involving reordering of numerous Pb atoms are fully completed at the surface within the shortest possible time (a few dozen seconds) to reapproach and image the surface after C60 deposition. Estimations show that the observed transformations cannot be controlled by random walk diffusion of Pb adatoms, which implies a highly correlated motion of the Pb atom displacements within the layer.

Influence of Au and TiO2 structures on hydrogen dissociation over TiO2/Au(100)

We performed H2–D2 exchange reactions over TiOx/Au(100) and compared the observed reaction kinetics with those reported for TiOx/Au(111) in order to clarify the influence of the Au and TiO2 structures on dissociation of H2 molecules. Low energy electron diffraction observations showed that the TiO2 produced on Au(100) was disordered, in contrast to the comparatively ordered TiO2 structure formed on Au(111). The activation energies and the turnover frequencies for HD formation over TiO2/Au(100) agreed well with those for TiO2/Au(111), clearly indicating that the hydrogen dissociation sites created over TiO2/Au(100) were the perimeter interface between stoichiometric TiO2 and Au, as was previously concluded for TiO2/Au(111). We concluded that the creation of active sites for hydrogen dissociation was independent of the Au and TiO2 structures consisting perimeter interface, and that local bonds that formed between Au and O atoms of stoichiometric TiO2 were essential for the creation of active sites.

Formation of Graphene Nanofin Networks on Graphene/SiC(0001) by Molecular Beam Epitaxy

A graphene nanofin consists of few-layer graphene and sticks out from the surface like a fin. To understand what graphene nanofin is and gain insight into its formation mechanism, we grew graphene at various substrate temperatures by gas-source molecular beam epitaxy (MBE) on previously prepared epitaxial graphene formed on a SiC(0001). Then, the surfaces were observed using atomic force microscopy, transmission electron microscopy, and low-energy electron microscopy (LEEM). We found the graphene nanofin formation at the growth temperature from 600 to 915 °C. The LEEM and low-energy electron diffraction observations reveal that the crystal axes of the graphene layer are completely random, although their (0001) axes are aligned. This experimentally supports a formation mechanism where the nanofins are formed by the collision between incommensurate domains of graphene at their boundaries.

Surface physical property of the CrO2 thin films prepared using a closed chemical vapor deposition method

ABSTRACTWe have examined the intrinsic surface physical property of a CrO2 thin film by means of surface sensitive photoemission spectroscopy. Epitaxial thin film of CrO2(100) has been grown on TiO2(100) by a closed chemical vapor deposition method using a Cr8O21 precursor. Low-energy electron diffraction (LEED) observations find that epitaxial growth of rutile-phase CrO2 occurs to the top monolayer of the film. Surface sensitive x-ray photoemission spectroscopy (XPS) measurements show a finite intensity in the region of the Fermi energy. The result evidences that the physical nature of near topmost layer of CrO2 thin film is metallic. Progress of understanding of the surface physical property of CrO2 thin film helps not only perform a reliable photoemission study to understand the physics of ferromagnetic metal in CrO2, but also develop the CrO2-based devices using a half-metallic nature for spintronics applications.

ReaxFF Grand Canonical Monte Carlo simulation of adsorption and dissociation of oxygen on platinum (111)

Atomic-level Grand Canonical Monte Carlo (GCMC) simulations equipped with a reactive force field (ReaxFF) are used to study atomic oxygen adsorption on a Pt(111) surface. The off-lattice GCMC calculations presented here rely solely on the interatomic potential and do not necessitate the pre-computation of surface adlayer structures and their interpolation. As such, they provide a predictive description of adsorbate phases. In this study, validation is obtained with experimental evidence (steric heats of adsorption and isotherms) as well as DFT-based state diagrams available in the literature. The ReaxFF computed steric heats of adsorption agree well with experimental data, and this study clearly shows that indirect dissociative adsorption of O2 on Pt(111) is an activated process at non-zero coverages, with an activation energy that monotonically increases with coverage. At a coverage of 0.25ML, a highly ordered p(2×2) adlayer is found, in agreement with several low-energy electron diffraction observations. Isotherms obtained from the GCMC simulations compare qualitatively and quantitatively well with previous DFT-based state diagrams, but are in disagreement with the experimental data sets available. ReaxFF GCMC simulations at very high coverages show that O atoms prefer to bind in fcc hollow sites, at least up to 0.8ML considered in the present work. At moderate coverages, little to no disorder appears in the Pt lattice. At high coverages, some Pt atoms markedly protrude out of the surface plane. This observation is in qualitative agreement with recent STM images of an oxygen covered Pt surface. The use of the GCMC technique based on a transferable potential is particularly valuable to produce more realistic systems (adsorbent and adsorbate) to be used in subsequent dynamical simulations (Molecular Dynamics) to address recombination reactions (via either Eley–Rideal or Langmuir–Hinshelwood mechanisms) on variously covered surfaces. By using GCMC and Molecular Dynamics simulations, the ReaxFF force field can be a valuable tool for understanding heterogeneous catalysis on a solid surface. Finally, the use of a reactive potential is a necessary requirement to investigate problems where dissociative adsorption occurs, as typical of many important catalytic processes.

Ground state of theSn/Ge(111)−3×3surface and its electron-beam-induced disordering

Low-energy electron diffraction (LEED) studies at 14 K revealed that the ground state of the $\text{Sn}/\text{Ge}(111)\text{\ensuremath{-}}3\ifmmode\times\else\texttimes\fi{}3$ surface is the so-called one-Sn-up and two-Sn-down (1U2D) structure. The electron beam in the LEED observation was found to disorder the 1U2D structure below $\ensuremath{\sim}40\text{ }\text{K}$. Cross sections for the structural change suggest that an interband transition between Sn-derived surface state bands becomes the trigger for the structural change. A mechanism of the excitation-induced structural change activated below $\ensuremath{\sim}40\text{ }\text{K}$ is discussed.

Structure and magnetic properties of iron nitride thin films on Cu(001)

The structural and magnetic properties of iron nitride thin films on a Cu(001) surface were investigated by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). In the STM and LEED observations of 1 and 2 monolayer (ML) films, the nitride films exhibit $p4gm(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed smooth surfaces without significant amounts of dislocations or islands. The stoichiometry of the 1 ML film was found to be ${\text{Fe}}_{2}\text{N}$, while that of 2 ML was ${\text{Fe}}_{4}\text{N}$, consisting of a topmost layer with the ${\text{Fe}}_{2}\text{N}$ composition and a second layer containing Fe atoms only. The XMCD measurements were performed for nitride films up to 4 ML. The angle-dependent XMCD spectra indicate that the films are ferromagnetic, with easy axes along the surface parallel direction. In addition, according to the results of the sum-rule analysis, strong magnetic anisotropy appears in the 1 ML film, where the orbital magnetic moment perpendicular to the surface is almost zero. The total magnetic moment, which is a combination of the spin and orbital magnetic moments, increases with the increase in film thickness up to 3 ML. The magnetic moment for more than 2 ML is $\ensuremath{\sim}2.1\text{ }{\ensuremath{\mu}}_{\text{B}}$, which is nearly equal to that of bulk ${\ensuremath{\gamma}}^{\ensuremath{'}}{\text{-Fe}}_{4}\text{N}$ of $2.2\text{ }{\ensuremath{\mu}}_{\text{B}}$.

In situ observation of change in surface atomic arrangement of Sc–O∕W(100) system during phase transition at high temperature

In situ observation of low-energy electron diffraction (LEED) patterns during the phase transition of the Sc–O∕W(100) surface at high temperature was performed to investigate the correlation between the surface atomic arrangement, coverage of surface atoms, and work function during the phase transition. For this purpose, a LEED system enabling LEED observation at high temperatures of 1500–1900K was developed. The temperature dependence of the intensity of diffraction spots was measured, and the Debye temperatures were estimated for relevant surfaces. The changes in the intensity of diffraction spots during the phase transition were found to correlate with the changes in the coverage of surface atoms and work function. The present results revealed that surface phenomena such as surface segregation, oxidation, oxygen desorption, and diffusion of ScO complexes predominate the surface properties of the Sc–O∕W(100) system.

Effects of Si capping layers on the properties of ultrathin Co/Ir(111) films

Surface compositions and annealing effects for ultrathin Co/Ir(111) films influenced by Si capping layers have been investigated. On the surface of Co/Ir(111) films, Si adatoms are chemically active in the formation of Co–Si interfaces. Low-energy electron diffraction observations reveal that Si–Co interfaces is disordered in the long range. From sputtering profiling measurements, alloy formations occur at the interfacial layers, while pure cobalt remains on the Ir(111). After depositing submonolayer silicon on Co/Ir(111), the relative easy axis of magnetization in the polar geometry is mainly attributed to the positive interface anisotropy of the Co/Ir interface. The formation of nonmagnetic Co–Si compounds plays a dominant role in reducing the effectiveness of magnetic layers. Below 500 K, the interdiffusion of Si into the underlying Co layer occurs. Further interaction of Si atoms with Co layers and interdiffusion at the Co–Ir interface are observed at higher temperatures. This causes no detectable magnetic hysteresis after annealing up to 800 K.

Adsorption of Di- and Tripeptides on Au(110) under Ultrahigh Vacuum Conditions. 1. Polarization Modulation Reflection Absorption Infrared Spectroscopy and X-ray Photoelectron Spectroscopy Characterization

Two tripeptides, insulin-like growth factor I (IGF) (1−3) and gluthation (GSH), only differing by their central fragment, and one IGF-related dipeptide, Gly-Pro, were adsorbed on a Au(110) surface under UHV conditions. Studies of the kinetics of adsorption, as well as the chemical and structural characterization of the adlayers, were achieved by combining polarization modulation reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy techniques. Low-energy electron diffraction observations are also reported. The adsorption behavior varies drastically with the size and chemical composition of these three molecules. Gly-Pro, constituted of two of the three amino acids of IGF, adsorbs on the Au(110) surface, giving rise to a 2D organization and rapidly forming a full monolayer. The two tripeptides only differ by their central fragment and this modifies their growth mode drastically: IGF tends to form homogeneous, likely continuous, layers, while GSH adsorption results in heterogeneous layers with dimers or patches of molecules, leaving fractions of the gold surface bare or covered with a very thin layer of organics. Moreover, the average thickness of the IGF layer of on Au (110) increases faster than that of GSH with exposure time. The adsorption kinetics is faster for IGF than for GSH.

Relative stabilities of adsorbed versus substitutional Al atoms in submonolayerAl/SixGe1−x(111)

Using scanning tunneling microscopy, low-energy electron-diffraction observations, and density-functional calculations, the effect of adding Ge to Si(111) substrate on the reconstructions induced by Al adsorption has been studied. It has been found that Ge incorporation alters the relative stability of the reconstructions. In particular, while in the ``pure'' Al/Si(111) system the magic cluster array ($\ensuremath{\alpha}\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ phase) is less stable than the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ reconstruction (to which it irreversibly converts upon heating above $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$), in the $\text{Al}/{\text{Si}}_{x}{\text{Ge}}_{1\ensuremath{-}x}(111)$ system magic clusters possess an enhanced thermal stability and persist almost up to the Al desorption temperature of $\ensuremath{\sim}800\ifmmode^\circ\else\textdegree\fi{}$. The results of calculations allow us to track the sequential stages of the substitution of Si atoms by Ge in the $\ensuremath{\alpha}\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ phase. The general trend found is that adding Ge to Si(111) makes the Al substitutional configuration more preferable than the Al adatom configuration, which is opposite to the relative stabilities of the configurations in the pure Al/Si(111) system.

Magnetic properties of self-assembled Co nanorods grown onCu(110)−(2×3)N

Structural and magnetic properties of self-assembled Co nanorods on a $\text{Cu}(110)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}3)\text{N}$ surface have been investigated by low-energy electron diffraction (LEED), Auger-electron spectroscopy (AES), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD). The LEED observation confirms that the Co nanorod grows epitaxially along the $[1\overline{1}0]$ axis and its interval exhibits the $(1\ifmmode\times\else\texttimes\fi{}6)$ periodicity. The AES clarifies that the N atom locates always at the surface even after 5 monolayer (ML) Co deposition. Angle dependent magnetization curves of the Co nanorods recorded by MOKE and XMCD show that the magnetic easy axis is perpendicular to the rod within the substrate plane, irrespective of the Co thickness down to 0.8 ML. This implies that the magnetic anisotropy is not dominated by the shape anisotropy but by the magnetocrystalline anisotropy. The XMCD sum-rule analysis reveals significant enhancement of the orbital magnetic moment along the easy axis compared to the hard axes. The magnetocrystalline anisotropy is found to be directly related to the anisotropy of the orbital magnetic moment.

Sum Frequency Generation Spectroscopic Investigation of TiOx/Pt(111): Surface Active Sites and Reaction Paths Probed by Formate

The catalytic behavior of ultrathin titanium oxide films deposited on platinum (111) surfaces is investigated through sum frequency generation (SFG) spectroscopy of the adsorption and decomposition of formate. TiOx films of 7 Å or less in thickness are prepared on Pt(111) substrates by evaporation of titanium under oxygen, and samples are examined as-prepared or after annealing at 873 or 973 K. Formate is adsorbed to the samples by exposure to 1000 langmuir of formic acid at 250 K. The frequency of the stretching vibration (ν(C−D)) of the deuterated formate C−D bond is found to be sensitive to the interaction between the adsorption sites and formate. SFG observations under different polarization combinations reveal that most formate is adsorbed to the surface with a bridge configuration, and that only a small amount of monodentate formate is present on the surface of TiOx/Pt(111) annealed at 973 K. Investigation of the effect of annealing temperature, time, and atmosphere on formate ν(C−D) frequency and intensity, in combination with low-energy electron diffraction (LEED) observations, reveals three kinds of surface active sites: defect sites, disordered titanium ion sites, and ordered 5-fold coordinated Ti4+ sites. These adsorption sites are responsible for ν(C−D) responses at frequencies of 2134−2184, 2195, and 2210−2226 cm−1, respectively. The defect sites are suggested to be located at the boundary between the disordered phase and the ordered phase of titanium oxide. Upon heating, formate adsorbed to defect sites undergoes decomposition at temperatures below 250 K, while formate adsorbed to ordered 5-fold coordinated Ti4+ sites decomposes via reaction paths: dissociation via monodentate formate as an intermediate, and diffusion onto defect sites before dissociation. The temperature of direct decomposition of bridge formate adsorbed to Ti4+ sites appears to be related to the formate ν(C−D) frequency, with lower frequencies corresponding to higher decomposition temperatures.

Pd-Induced Surface Reconstructions of Si(110) Studied by Scanning Tunneling Microscopy and Low-Energy Electron Diffraction

Pd-induced surface reconstructions of Si(110) have been studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). After sub-monolayer deposition of Pd on Si(110) at room temperature, Pd clusters are formed and cover the up-and-down terrace structure of the Si(110) surface. After annealing to 870–1070 K, the up-and-down terrace structure disappears and a new structure with two equivalent domains is formed. From LEED observation, the structure is found to be a (4,2)×(-1,3) superstructure. On the basis of high-resolution STM images, a possible structural model of the (4,2)×(-1,3) reconstruction is proposed.