Surface composition and structure of palladium ultra-thin films deposited on Ni(1 1 1)

Ultra-thin palladium films deposited on the Ni(1 1 1) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). For low coverage, LEED shows a (1 × 1) pattern similar to that of the substrate. For intermediate coverage, the LEED pattern displays extra spots around the main (1 × 1) spots, resembling a Moiré coincidence pattern, probably associated with the formation of Pd bi-dimensional islands oriented in different directions on the Ni(1 1 1) surface. The results obtained by XPS and XPD corroborate this finding. The LEED pattern displays this structure up to 500 °C. Annealing at 650 °C brings back the (1 × 1) pattern, which is associated with a Pd island coalescence and alloy formation by Pd diffusion in the first atomic layers of the Ni(1 1 1). In this paper we present a detailed study of this surface structure via a comparison between XPD experiment and theory.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction.     

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(1 0 0) 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(1 0 0) 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(1 0 0) with 0.1°, 2° and 6° off-cut in [0 1 1] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(1 0 0) 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(1 0 0) 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.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

Substitutional adsorption geometry for Pb(1 1 1)-(√3 × √3)R30°-K

Intermixed structures for alkalis (larger than Li) on close-packed substrates have previously been observed only on Al(1 1 1). This study shows that K forms an ordered intermixed structure on Pb(1 1 1). The structures of clean Pb(1 1 1) and Pb(1 1 1)-(√3 × √3)R30°-K were studied using dynamical low-energy electron diffraction (LEED). The clean Pb(1 1 1) surface at 47 K was found to be a relaxed version of the bulk structure, in agreement with an earlier study of the same surface [Y.S. Li, F. Jona, P.M. Marcus, Phys. Rev. B 43 (1991) 6337]. At room temperature, adsorption of K on this surface results in a (√3 × √3)R30° structure, which was shown using dynamical LEED to consist of K atoms substituted in surface vacancies. The K-Pb bond length was found to be 3.62 ± 0.3 Å, with no significant change to the Pb interlayer spacings.     

Quasi-one-dimensional structures on the Si(1 1 1) surface induced by Ba adsorption

Ba-induced quasi-one-dimensional reconstructions of the Si(1 1 1) surface have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the 3 × ‘2’ surface shows double-periodicity along the stripes in STM images consistent with half-order streaks observed in LEED patterns, no sign of the double-periodicity along the chain direction was detected for the 5 × 1 surface. The 5× stripes in STM images show internal structures with multiple rows. The two rows comprising the boundaries of a 5× stripe in the filled-state STM image are found to have 3a × √3/2 spacing across the stripe. The observation of the successive 3× and 2× spacings between the boundary rows supports a structural model proposed for the Ba-induced 5 × 1 Si reconstruction composed of honeycomb chains and Seiwatz chains. The highest coverage 2 × 8 surface does not reveal a quasi-1D row structure in STM images.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

Epitaxial growth of Bi thin films on Ge(1 1 1)

We investigated Bi thin film growth on Ge(1 1 1) by using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). In the submonolayer regime, adsorbed Bi atoms form patches of the (2×1) structure. However, the structure does not grow to a long-range order. Following the formation of a (1×1) monolayer (ML) film, two-dimensional (1 1 0)-orientated Bi islands grow. The film orientation changes from (1 1 0) to (1 1 1) at 6-10 ML. The (1 1 0)-oriented Bi film shows a six-domain LEED pattern with missing spots, associated with a glide-line symmetry. The hexagonal (1 1 1) film at 14 ML has a lattice constant 2% smaller than bulk Bi(1 1 1).     

Initial growth of platinum on oxygen-covered Ni(1 1 0) surfaces

The initial growth of Pt on the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) surfaces has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Prior to Pt deposition, the atomic structure of the near-surface regions of the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) structures were studied using CAICISS, finding changes to the interlayer spacings due to the adsorption of oxygen. Deposition of Pt on the Ni(1 1 0)-(3 × 1)-O surface led to a random substitutional alloy in the near-surface region at Pt coverages both below and in excess of 1 ML. In contrast, when the surface was treated with 1800 L of atomic oxygen in order to form a NiO(1 1 0) surface, a thin Pt layer was formed upon room temperature Pt deposition. XPS and LEED data are presented throughout to support the CAICISS observations.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.     

Growth and crystalline structure of Co layers on Cu(0 0 1)

The crystalline structure of Co layers deposited on the Cu(0 0 1) surface was investigated with the use of the directional elastic peak electron spectroscopy (DEPES). For clean Cu(0 0 1) the experimental DEPES profiles obtained for different energies of the primary electron beam exhibit intensity maxima corresponding to the close packed rows of atoms. The Auger peak kinetics recorded during continuous Co deposition suggest the layer-by-layer growth mode. The DEPES profiles recorded for 10 monolayers (ML) of Co on Cu(0 0 1) reflect a short-range order in the adsorbate. Intensity maxima observed in the DEPES profiles for Co along [1 0 0], [0 1 0], and [1 1 0] azimuths of Cu(0 0 1) are characteristic of the face centered cubic (fcc) Co(0 0 1) layers. Low-intensity reflections and considerable background intensities were found in the low energy electron diffraction (LEED) patterns recorded from 10 ML of Co, which indicates a weak long-range order in the adsorbate. The adsorption of about 20 ML of Co results in considerable background contribution to DEPES. No reflections but a large background were observed with the use of LEED for this layer. The heating of the Co/Cu(0 0 1) system at T = 770 K leads to an increase of the short- and long-range order in the overlayer, observed in the DEPES profiles and LEED patterns, respectively. The theoretical DEPES profiles were obtained with the use of a multiple scattering approximation. A very good agreement between experimental and theoretical scans was found for the clean and covered copper substrate. The latter proves the epitaxial growth of Co layers on Cu(0 0 1).     

Gd adsorption on the Mo(2 1 1) surface

The adsorption of Gd thin layers on the Mo(2 1 1) face was investigated by using Auger electron spectroscopy (AES), low electron energy diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and measurements of the work function changes (Δφ). It was found that at 300 K Gd does not form any dilute chain structures and from the very beginning of the adsorption process Gd forms a densely packed layer. The dilute p(4 × 1) chain structure was observed by LEED after annealing thin layers (θ < 1 ML) to temperatures above 770 K. STM images confirm the existence of the p(4 × 1) structure islands. The intermixing of the substrate and adsorbate atoms takes place.     

Me-PTCDI thin film deposition on InSb(1 1 1)A

The evolution of N,N′-dimethylperylene-3,4,9,10-dicarboxyimide (Me-PTCDI) thin films formed by vapour deposition on InSb(1 1 1)A substrates has been studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED). XPS studies of the Me-PTCDI covered surface indicate that no significant interaction occurs at sub-monolayer coverage when compared to multilayer Me-PTCDI films. HREELS studies suggest only a weak interaction as evidenced by very small changes in the frequencies of several molecular vibrational modes. LEED patterns show the Me-PTCDI overlayer adopts a structure commensurate with the underlying InSb(1 1 1)A substrate surface and that can be rationalised by van der Waals intermolecular energy calculations for the Me-PTCDI unit cell. The results are consistent with a weak interaction at the Me-PTCDI/InSb interface, the formation of the commensurate structure being sufficient to overcome the small energetic penalty associated with deviation from the calculated intermolecular interaction energy minimum.     

The epitaxial growth of Pd electrodeposition on Au(1 0 0) studied by LEED and RHEED

The epitaxial growth of Pd adlayers electrochemically deposited onto Au(1 0 0) has been studied by LEED, RHEED and AES. For the first 6 ML, the Pd deposits grow pseudomorphically on Au(1 0 0) with a lateral expansion of 4.5% with respect to bulk Pd. The strain in the expanded commensurate (1 × 1) Pd layers on Au(1 0 0) begins to be relieved at the Pd coverage between 6 and 9 ML range by formation of a compressed Pd film with respect to Au(1 0 0) surface and the compression increases continuously with thickness. At ca. 20 ML Pd the lattice constant of the film approaches to the bulk Pd and three-dimensional Pd islands develop since around 30 ML coverage. No superstructure due to the Pd-Au surface alloy can be found for coverages from monolayer up to 30 ML Pd on Au(1 0 0). A c(2 × 2) phase has been observed on the Pd-deposited Au(1 0 0) electrodes, which is ascribed to an ordered Cl adlayers adsorbed on Pd adlayers rather than a Pd-Au surface alloy.     

Structural analysis of the c(4 × 2) reconstruction in Si(0 0 1) and Ge(0 0 1) surfaces by low-energy electron diffraction

The c(4 × 2) structures in (0 0 1) surfaces of Si and Ge have been studied by low-energy electron diffraction (LEED). Using a proper cleaning method for the Si surface, we were able to observe clear c(4 × 2) LEED patterns up to incident energy of ∼400 eV as well as the Ge surface. Extensive experimental intensity-voltage curves allowed us to optimize the asymmetric dimer model up to the eighth layer (including the dimer layer) in depth in the dynamical LEED calculation. Optimized structural parameters are almost the same for the Si and Ge except for the height of the buckled-up atom of the asymmetric dimer. For the Ge surface, the structural parameters are in excellent agreement with those obtained by a previous theoretical calculation. The tilt angle and bond length of the dimer are 18 ± 1 (19 ± 1)° and 2.4 ± 0.1 (2.5 ± 0.1) Å for the Si(0 0 1) (Ge(0 0 1)), respectively.     

Oxygen induced facet formation on Rh(2 1 0) surface

Oxygen induced nanometer-scale faceting of the atomically rough Rh(2 1 0) surface has been studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The Rh(2 1 0) surface completely covered with nanometer-scale facets when annealed at ≥550 K in the presence of oxygen. LEED studies reveal that the pyramidal faceted surface is characterized by three-sided nanoscale pyramids exposing (7 3 1), (7 3 −1) and (1 1 0) faces. A clean faceted surface was prepared through the use of low temperature surface cleaning method using the reaction with H₂ while preserving (“freezing”) the pyramidal facet structure. The resulting clean faceted surface remains stable for T ∼ 600 K and for higher temperatures; the faceted surface irreversibly relaxes to the planar surface. STM measurements confirms the formation of nanopyramids with average pyramid size ranging from 12 to 21 nm depending upon the annealing temperature. The nanopyramidal faceted Rh surface may be used as a potential template for the growth of metallic nanoclusters and for structure sensitive reactions.