LEED Techniques Research Articles

Phase transition and thermal stability of epitaxial PtSe2 nanolayer on Pt(111).

This work relates to direct synthesis of the two-dimensional (2D) transition metal dichalchogenide (TMD) PtSe2 using an original method based on chemical deposition during immersion of a Pt(111) surface into aqueous Na2Se solution. Annealing of the sample induces significant modifications in the structural and electronic properties of the resulting PtSe2 film. We report systematic investigations of temperature dependent phase transitions by combining synchrotron based high-resolution X-ray photoemission (XPS), low temperature scanning tunnelling microscopy (LT-STM) and low energy electron diffraction (LEED). From the STM images, a phase transition from TMD 2H-PtSe2 to Pt2Se alloy monolayer structure is observed, in agreement with the LEED patterns showing a transition from (4 × 4) to (√3 × √3)R30° and then to a (2 × 2) superstructure. This progressive evolution of the surface reconstruction has been monitored by XPS through systematic de-convolution of the Pt4f and Se3d core level peaks at different temperatures. The present work provides an alternative method for the large scale fabrication of 2D transition metal dichalchogenide films.

Chemical order and magnetic anisotropy in alternate Fe/Co monolayers on Cu3Au(001)

In this work, we have investigated the magnetic anisotropy of Fe/Co layers epitaxially grown on Cu3Au(001). The ordered Fe/Co thin films were prepared by deposition of alternated monolayers of Fe and Co. The stoichiometry, structure, morphology and magnetic properties of the films were characterized in-situ using XPS, LEED, RHEED, STM and MOKE techniques respectively. Measuring the film properties after the deposition of each monoatomic layer, we have shown that the chemical order and the surface layer also play an important role in establishing the film magnetic anisotropy. Our results show that the effective magnetic anisotropy can be tuned from in-plane to out-of-plane depending on which material (Fe or Co) the alternated film is terminated. These results were compared to codeposited films that presented in-plane magnetic anisotropy for all film thicknesses. In addition, the spin and orbital magnetic moments of Fe and Co atoms in a 6 ML film were extracted applying XMCD sum rules.

Hf dissolution and surface alloy formation in Hf/W(011) adsorption system

The system Hf/W(011) was investigated using the STM, LEED, AES and CPD techniques. For low coverages (about 1 ML), dissolution of Hf in W and alloying on the W(011) surface was observed. For temperatures above 1200 K, mixing of the adsorbate and substrate atoms was observed. This process occurred at the edges of atomic steps. A surface alloy appeared at a temperature in excess of 1500 K. The alloy was being formed as large islands occupying one terrace each, and separated by atomic step bunchings of the substrate. At 1850 K the surface alloy decayed. The decay was followed by solution of the Hf in the subsurface layer of substrate.

Behaviour of the 3C–SiC(100) c(2 × 2) (C‐terminated) and 3 × 2 (Si‐rich) surface reconstructions upon initial H2/CH4 microwave plasma exposures

AbstractWe present a comparative study of C‐terminated and Si‐rich 3C–SiC(100) surfaces exposed to a H2/CH4 microwave plasma used in diamond growth. Each surface is characterized before and after short plasma exposures using LEED, XPS and STM techniques. In both cases, a 5 minutes plasma exposure led to a 1 × 1 LEED pattern associated with a slight oxygen contamination as evidenced by XPS. However, upon additional annealings, the two surfaces exhibit very different behaviors. For the Si‐terminated 3 × 2 surface, a 1 × 1 → 2 × 1 transition occurring at 1173 K is observed while the initial c(2 × 2) surface reconstruction is restored at 1323 K for the C‐terminated surface. Furthermore, the XPS measurements show a correlation between oxygen surface release and LEED transitions. An atomic arrangement is observed by STM for the C‐terminated surface (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of antimony 1D-nanostructures on Si (5 5 12) surface

AbstractOf late, high index Si surfaces like, (5 5 12) are being explored for the formation of 1D nanostructures in the form of nanowires or chains. The surface topography of Si (5 5 12) presents an unique template for the growth of 1D nanostructures. In this paper we report the adsorption-desorption studies of Sb onto (2×1) reconstructed surface of Si (5 5 12). Motivated by our earlier studies of adsorption/desorption of Sb on low index surfaces (001) and (111), studying the interaction on high index surface like (5 5 12) is found to be interesting. The experiments have been performed in UHV with in-situ growth and probed by using AES, LEED and EELS techniques. The uptake curve shows initially a simultaneous multilayer growth of Sb up to 4 ML and then at 20 ML the entire Si surface is covered by Sb. The LEED patterns show a disorder during growth at RT above 1 ML. EELS studies at 250 eV, primary beam energy show that as the adsorption proceeds the features of bulk Sb start appearing above 4ML coverage. However, the bulk Sb features are not complete until a high enough coverage of Sb (˜20ML) which suggests the existence of the primary rows up to a higher coverage. Annealing studies have also been performed on the RT adsorbed system, to study the residual thermal desorption characteristics. Annealing the system to about 7900C results in Si (337) facets at a low coverage of 0.2 ML. The anisotropy in the LEED spots suggests an ordered Sb adsorption along (1 10) and a local disorder in the (665 ) direction. Thus we provide evidence for the formation of a zig-zag 1D-nanowire of Sb grown on the (5 5 12). The new phase is also accompanied by forming a dangling bond state at 3 eV in EELS.

Increase of Pd surface area by treatment in dioxygen

The surface area of Pd(111) and Pd(110) single crystals increased after oxidation in O 2 transformed them to PdO. The surface area of the oxide decreased after complete reduction in H 2. The techniques of STM, TPD, XPS, AES, and LEED were employed to study the Pd(111) and Pd(110) single-crystal surfaces after they were subjected to O 2 oxidation, methane combustion, and H 2 reduction. The surface area of the treated palladium single crystals was measured by 18O isotope exchange and by direct measurements using the STM image. These two methods showed agreement within 20%. After oxidation in O 2 (100 and 150 Torr) at 600 K, the surface area for both Pd(111) and Pd(110) single crystals increased by a factor of approximately two. The effect was more pronounced on the Pd(111) surface. The oxidized surfaces were covered with 3- to 4-nm semispherical oxide agglomerates that formed a “cauliflower-like” structure 10–20 nm in size. Similar surface structures were observed after exposure of the Pd single crystals to a lean O 2 and CH 4 reaction mixture (O 2:CH 4=10:1). Thus, the oxidized single crystal becomes amorphous. Reduction in H 2 decreased the surface area of the preoxidized Pd(111) and Pd(110) crystals. An amorphous metallic surface was produced after H 2 reduction at 373 K, whereas a smooth surface with characteristic single-crystal features was observed after reduction at 673 K. These experiments suggest that oxidizing a Pd metal catalyst or reducing an oxidized Pd catalyst, for example, before palladium metal surface area measurement, will affect the surface area of the sample. It also shows that the increase in surface area on Pd catalysts after oxidation treatment is caused by surface roughening.

Thermal stability of the Ta(1 1 1) surface covered with chemisorbed metal layer. Part II: Ag

The growth mechanism of Ag layers on Ta(1 1 1) face and thermal stability of this surface covered with Ag layers has been investigated using AES, LEED, Δ φ and STM techniques. It has been found that the layer growth is Stransky–Krastanov type and the transition non-wetting/wetting occurs when the adsorbate layer thickness θ>2 ML. No facets appear due to the annealing.

Interaction of oxygen with palladium deposited on a thin alumina film

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O 2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ≈5–7 kJ mol −1 for 1 and 3–5 nm Pd particles studied.

Stoichiometry determination of VO x thin films by 18O-RBS spectrometry

This paper reports on the stoichiometry determination of epitaxially grown vanadium monoxide (VO x ) thin films on MgO(100) substrates. The epitaxial growth was confirmed by RHEED, LEED and XRD techniques. The oxygen content of VO x thin films, as a function of oxygen flux, was determined using Rutherford backscattering spectrometry. The 18O isotope was used for film growth, in order to distinguish between the oxygen of film and substrate. The upper and lower stoichiometry limit found are consistent with the ones known for bulk material.

Study of the transition from the ideal Si( [formula omitted])–H(1×1) surface to the (7×7) reconstruction by HREELS, UPS and LEED

The temperature-induced evolution of the Si(1 1 1)–H(1×1) surface towards the (7×7)-reconstruction has been studied by means of UPS, HREELS and LEED techniques. We found that H atom desorption occurs at about 500 °C, and the full development of the (7×7)-phase, checked by both UPS and LEED, occurs around 700 °C. In the valence band spectra, the (7×7)-related restatom peak is present already for annealing temperature of 465 °C, while the adatom-related peak starts to appear only around 600 °C and fully develops at 700 °C. A band bending is detected and its behavior discussed in relation to the surface state evolution. In the first annealing steps, the desorption-induced defects cause the shift of the surface Fermi level and the consequent upwards band bending, which reaches the maximum value of 0.47 eV for annealing to 465 °C. The subsequent evolution of the dangling-bond states into the (7×7) surface states determines the band bending decrease till 0.1 eV.

Correlation between the atomic and electronic structures in Dy/Mo(1 1 2) and Gd/Mo(1 1 2) adsystems

The atomic and electronic structures of Dy and Gd submonolayers adsorbed on the Mo(112) face have been investigated with the use of the LEED, EELS and AES techniques. The one-dimensional compression of the Dy films is accompanied by the appearance of a series of medium energy extra losses, whereas they are absent under analogous conditions for Gd. The effects indicate good correlation with the shape of the concentration dependences of the low energy loss for both adsorbates. The atomic and electronic structures of the Dy/Mo(112) and Gd/Mo(112) adsorption systems are compared and possible reasons for their correlation are discussed.

Interlayer exchange coupling in Fe/SiFe/Fe sandwiches

The interlayer exchange coupling in Fe/c-FeSi/Fe has been theoretically investigated in the framework of the ab initio tight-binding linear-muffin-tin-orbitals method. The system is modeled according to recent structural analysis performed with LEED techniques, which show that the Si spacer transforms into an ordered Si 0.5Fe 0.5 alloy in the CsCl structure (c-FeSi). We obtain antiferromagnetic coupling between the Fe layers for spacer thicknesses smaller than about 17 A ̊ with non-monotonous decaying behavior. The density of states of the FeSi spacer at the Fermi level is very small, and displays a sharp peak at about 0.5 eV above the Fermi level. Our results are discussed and compared with available experimental data.

Surface studies of single-crystalline refractory metal low-dimensional structures

We present the results of surface investigations of novel heteroepitaxial metallic low-dimensional structures fabricated on the basis of high-quality single-crystalline refractory-metal films (W(100) with thickness of 30–2000Å on r-plane sapphire).Films were grown by laser ablation deposition technique. Procedure consisting of soft ion bombardment and annealing at 1200–1300K in UHV allowed to obtain atomically clean (100) film surface (without oxygen and carbon contamination) with four-fold symmetry LEED pattern typical for refractory metals. Study of atomic and electronic structures and composition of the film surface was performed by LEED, AES, and UPS techniques. UPS spectra of these low-dimensional metallic systems are presented and compared with electronic structure of bulk single crystals.

Atomic scale observation of iodine layer compression on Cu(1 1 1)

The atomic structure of the iodine covered Cu(1 1 1) surface was studied by STM in combination with LEED, AES and TDS techniques. It was established that iodine adsorption on Cu(1 1 1) leads to the uniaxial compression of chemisorbed iodine layer. The structure of the saturated iodine coverage on Cu(1 1 1) was described in terms of formation of the striped super heavy domain walls, with the interatomic distances in the wall being smaller than van der Waals diameter of iodine.

Self-organization of monolayer (Pb, Sn) adsorbates at Si(111) surface recoil-implanted by keV ion impact

The concentration changes of Pb and Sn at the Si(111)- 7 × 3 -(Pb, Sn) surface by 5 keV Ar + ion bombardment have been studied by means of XPS, AES and RBS techniques with LEED and STM techniques, in order to determine their cross-sections for desorption and recoil-implantation of them. The potential barrier heights for desorption and recoil-implantation of Pb at the Si(111)- 7 × 3 -(Pb, Sn) surface have been determined to be 1.4 ± 0.2 and 200 ± 140 eV, respectively, from the obtained cross-sections, while those of Sn to be 3.2 ± 0.7 and 4.7 ± 2.8 eV, respectively. These data are found to be very similar to those for Si(111)-1×1-Pb and Si(111)- 2 3 ×2 3 + 3 × 3 -Sn surfaces. The extremely high potential barrier heights for recoil-implantaiton of Pb are explained in terms of self-recovery of Pb atoms, which have been once recoil-implantated into the subsurface layers by the Ar + ion, to the original surface before the next Ar + ion comes again to the original surface. Co-existence of Sn atoms at the Si(111)- 7 × 3 -(Pb, Sn) surface is shown to shorten the characteristic time for the self-recovery of Pb, which is concluded to give an evidence for the self-organization.

Adsorption of Ho on Mo(1 1 2) crystal face

Growth and atomic structure of Ho submonolayers deposited on the Mo(112) crystal face were investigated with the use of AES and LEED techniques. It is well known that various species of alkali metal, alkaline-earth and rare-earth adatoms adsorbed on the furrow faces form linear chain structures across the atomic rows of Mo(112) and W(112) surfaces. In contrast to this case Ho deposited on the Mo(112) face does not reveal an ordered structure in the annealed adlayers at low coverage. Upon further deposition of Ho, the adlayer undergoes one-dimensional compression in the 〈1̄11〉 direction and incommensurate (1×4) and (1×5) structures are formed. This behavior of Ho adlayers is similar to that of Gd adlayers adsorbed on the Mo(112) face and essentially different from that for Gd-, and Ho-W(112) adsystems. This fact is connected with the different nature of the indirect interaction between atoms adsorbed on Mo and W substrates at low coverages.

Giant Faceting of Vicinal Si(001) Induced by Au Adsorption

4° vicinal Si(001) shows perfectly ordered terraces with a width of 4 nm which are separated by double steps. Adsorption of Au at 800°C results in a dramatic change of the step morphology: the surface decomposes into areas which are perfectly flat with a (001) orientation and (119) facets, which compensate for the macroscopic miscut. Extremely straight superterraces with a length limited only by the size of the sample (here 4 mm) and a width ranging from 400 nm to 4 μm are formed by massive Si mass transport. The extreme aspect ratio of 1:10 000 of this submicron structure is attributed to a heterogeneous nucleation process. SPA-LEED reveals a new, Au-induced incommensurate 5×3.2 reconstruction above a critical coverage as the driving force for the formation of large elongated (001) terraces. LEEM shows the strongly anisotropic nucleation process in vivo. Dark field imaging and microspot LEED techniques have been used to determine the influence of the different 5×3.2 domain orientations on the growth behavior of the (001) superterraces. The majority of domain terraces grow with a speed of more than 10 μm per second. The width and area of the (001) terraces increase proportionally to the Au coverage. The steps of the vicinal surface are accumulated in irregular step bunches. With further increasing Au coverage the step bunches are transformed into well-defined facets with a (119) orientation, as determined by SPA-LEED. The kinetics of the faceting process have been studied with SPA-LEED, REM, STM, and light diffraction using a HeNe laser, because the typical size of the superterraces is of the order of the wavelength of visible light: the resulting structure is visible to the bare eye. The parallel arrangement of superterraces acts as an irregular optical phase grating: illumination with white light results in stripes of all possible diffraction colors.

Atomic structure of saturated chlorine monolayer on Ag(111) surface

STM and LEED techniques were applied to study the structure of saturated chlorine monolayer formed on Ag(111) as a result of Cl 2 adsorption at room temperature. We present for the first time atomic resolution STM images of chlorinated Ag(111) surface obtained in ultra-high vacuum conditions. The structure of chlorine layer was identified by means of a Fourier analysis of the STM images as Ag(111)–(17 × 17)-Cl. The structure of saturated chlorine monolayer remains unchanged for Cl 2 exposures up to 100 000 L and coexists with AgCl islands on Ag(111) surface.

Determination of site-occupancy pair correlation functions of CO adsorbed on Pt(111) adapted for DLEED intensity calculations

The aim of this paper is to determine the site-occupancy pair correlation functions of a partially ordered layer of CO adsorbed on Pt(111). These functions are needed to calculate the structure factor appearing in the diffuse LEED intensity expression. CO molecules can be adsorbed at on-top sites and at three kinds of bridge sites obtained from each other by rotation of 120°. Pair correlation functions Γ ij ( p, p′) = 〈 n i ( p) n j ( p′)〉 − 〈 n i ( p)〉〈 n j ( p′)〉 are obtained by a Monte Carlo simulation at constant coverage ϑ m and temperature T. The model involves adsorption energy and repulsive pair interaction potentials up to third neighbours, depending only on the distance. We deal with a N × N fcc (111) 2D lattice with periodic boundary conditions. We study the dependence of correlation functions on pair interaction potentials and coverage ϑ m. The determination of correlation functions is based on the experimental observations coming from the HREELS, IRAS and LEED techniques. In accordance with experiment, a diffuse (√3 × √3)R30° superstructure, involving only on-top sites, appears at low coverage. On increasing the coverage, more and more bridge sites are occupied and a c(4 × 2) superstructure grows progressively.

Adsorption of gadolinium on the Mo(112) crystal face

Structure and electron-adsorptive properties of Gd films adsorbed on Mo(112) surface have been investigated using LEED and CPD techniques. For the coverage range 0< θ<0.6ML (monolayer) the Gd deposit does not reveal an ordered structure. At θ > 0.6 the structures of type c(2 × 1 θ ) occur jointly with the type p(1 × 1 θ ) (at a very small region of θ〉 and show features of one-dimensional compression of the adlayers with increasing coverage. During the process of further Gd deposition the adsorbed layer undergoes one-dimensional compression in the 〈111〉 direction and incommensurate structures (1 × 4) and (1 × 5) are formed. The one-dimensional compression vanishes at θ = 1.25. With increasing θ, the work function change and binding energy are correlated with the phase transition in the adlayer structure. The peculiarities of indirect interaction between adsorbed Gd atoms have been revealed at low coverage.