Hydrogen adsorption and diffusion on Pd(1 1 1)

The adsorption, diffusion and ordering of hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy in the temperature range of 37-90 K. At low coverage isolated hydrogen atoms were observed. They formed √3×√3-1H islands as the coverage increased. Above 1/3 monolayer (ML) coverage areas of a new phase with √3×√3-2H structure were formed, with both structures coexisting between 1/3 and 2/3 ML. Finally a 1 × 1 structure was formed after high exposures of hydrogen above 50 K, with a coverage close to 1 ML. Atomically resolved images reveal that H binds to fcc hollow sites.     

Observation of double- to single-domain transition on the Eu/Si(1 0 0) surface by LEED and STM

The orientational phase diagram and morphology of the Eu-adsorbed Si(1 0 0) surface miscut by 0.4° have been studied by low-energy electron diffraction and scanning tunneling microscopy. We demonstrate that the original double-domain configuration with single-layer steps on the Si(1 0 0) substrate can be drastically broken at 0.4 monolayer (ML) of Eu. At this coverage, the ordered domain pattern formed by topographically non-equivalent terraces with Eu-induced 2 × 3 and “2 × 1” (so-called “wavy” structure) reconstructions is found, while no orthogonal 3 × 2 and “1 × 2” domains are observed. A model of the single-domain surface is proposed. The origin of the double- to single-domain transition found for the Eu/Si(1 0 0) system is discussed.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

Spectroscopic interpretation for Na-induced phase transitions on Na/Si(1 1 1)3 × 1

Na adsorption at room temperature causes the Na/Si(1 1 1)3 × 1 surface with Na coverage of 1/3 monolayer (ML) to transit into the Na/Si(1 1 1)6 × 1 surface at 1/2 ML and sequentially into the Na/Si(1 1 1)3 × 1 surface at 2/3 ML. The phase transition was studied by Si 2p core-level photoemission spectroscopy. The detailed line shape analysis of the Si 2p core-level spectrum of the Na/Si(1 1 1)3 × 1 surface (2/3 ML) is presented and compared to the Na/Si(1 1 1)3 × 1 surface (1/3 ML) which is composed of Si honeycomb chain-channel structures. This suggests that as additional Na atoms form atomic chains resulting in the Na/Si(1 1 1)3 × 1 surface (2/3 ML), the inner atoms of the Si honeycomb chain-channel structure is buckled due to the additional Na atoms.     

Reconstruction and de-reconstruction of the Ir(1 0 0) surface and ultrathin Fe/Ir(1 0 0) films

The structure, energetics and magnetic properties of the quasihexagonal reconstruction of the Ir(1 0 0) surface and nanostructures formed by Fe atoms on this surface have been investigated using first-principles density functional theory with generalized gradient corrections. We find the reconstructed (1 × 5) surface to be 0.10 eV/(1 × 1) area lower in energy than the unreconstructed surface and we demonstrate that first-principles calculations can achieve quantitative agreement with experiment even for such long-period and deep-going reconstructions. For Fe coverage of 0.4 monolayers (ML) we have studied the stripe-like structure with biatomic Fe rows placed in the troughs of the (1 × 5)-reconstructed surface. Results of nonmagnetic calculations agree well with the structure inferred from STM data. Higher Fe coverages lead to a de-reconstruction of the Ir substrate. At 0.8 ML coverage a surface compound with composition Fe₄Ir is formed, which shows an appreciable buckling. In this case, a ferromagnetic calculation leads to good agreement with the low-temperature LEED data. We predict that the (1 × 5) periodicity of the mixed interface layer will persist also in thicker films with a pure Fe surface. Films with 1-4 ML Fe are predicted to be tetragonally distorted and ferromagnetic, with an axial ratio corresponding well to an elastic distortion of the Fe lattice.     

STM observation of surface phases of Sn/Cu(0 0 1)

Geometrical structures of the Sn-adsorbed Cu(0 0 1) surfaces are studied with scanning tunneling microscopy. There are four phases in the Sn coverage range between 0.2 and 0.5 mono-atomic layer (ML). On the basis of the observed atomic images in this range, we propose structural models for the phases with 0.33 and 0.375 ML of Sn. All the phases consist of embedded Sn atoms in the Cu surface, forming two-dimensional surface alloy structures. On the surface with ∼0.4 ML of Sn, a novel one-dimensional structure is observed.     

Ytterbium on vicinal Si(1 0 0): Growth and properties of the 2D wetting layer and the Yb silicide phase

The Yb growth on a vicinal Si(1 0 0) surface has been studied by scanning tunneling microscopy and low energy electron diffraction in the coverage range of 1-4 ML. Two different methods of the Yb/Si(1 0 0) interface formation are applied, leading to a remarkable modification of structural and morphological properties of two-dimensional (2D) wetting layer and Yb silicide phase. In particular, the switchover of the 2D layer orientation, similar to the case of the Bi nanolines on Si(1 0 0) [J.H.G. Owen, K. Miki, D.R. Bowler, J. Mater. Sci. 41 (2006) 4568], is observed depending on the growth procedure. Moreover, the structure and morphology of the Yb silicide phase is found to depend critically on the growth conditions, and the ability to grow very long, unidirectional Yb silicide nanowires is demonstrated. The results obtained are discussed in the context of the previous studies of 1D nanowires and 3D islands of rare-earth silicides on Si(1 0 0).     

Tl/Ge(1 0 0) system: Phase formation and phase transitions

Using scanning tunneling microscopy, phase formation and temperature-driven phase transitions in Tl/Ge(1 0 0) system have been studied. Evolution of Tl overlayer structure has been considered for three temperature ranges, including around room temperature (RT), high-temperature (HT) (350-450 K) and low-temperature (LT) (20-100 K) ranges. Upon RT growth, a 2 × 1-Tl phase develops in submonolayer range and is completed at around 1 ML of Tl. Cooling of the RT-deposited Tl overlayer results in formation of a set of various LT structures. These are 1D chains, 5 × 4-Tl and “stroked” phases observed in submonolayer range and a long-period c(12 × 14)-Tl phase developed at around 1 ML. All transitions between these RT and LT structures are reversible. At doses beyond 1 ML, RT deposition of Tl onto Ge(1 0 0) leads to the growth of second-layer Tl stripes, forming arrays with a 1 × 4 periodicity. Meanwhile, structure of the first layer also changes and it displays a set of various reconstructions, c(2 × 8), c(10 × 6) and c(10 × 7). All these structures remain unchanged upon cooling to LT. Growth at HT as well as heating of RT-deposited Tl overlayer irreversibly produces 3 × 2-Tl phase whose rows become decorated by second-layer Tl stripes at prolonged Tl deposition.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

Growth morphology of ultra-thin Ni films on Pd(1 0 0)

A series of thin Ni films, with thicknesses between 0.2 ML to 13 ML, were deposited on a Pd(1 0 0) substrate (a = 3.89 Å) at room temperature (RT). The growth morphology was investigated using scanning tunneling microscopy (STM). STM images indicate the existence of three different growth modes as a function of increasing coverage. Up to 6.5 ML, the films grow pseudomorphically, consistent with a face-centered tetragonal (fct) structure. From 6.5 ML to 10.5 ML a new apparent interlayer distance of 1.0 ± 0.1 Å is established. The new structure is accompanied by the appearance of an arrangement of filaments on the top layer surface. These filaments are presumably related to a strain relief mechanism of the fct films. Finally above 10.5 ML the Ni films recover the face-centered cubic (fcc) lattice constants. The filaments evolve, as a function of coverage, to form a net-like structure over the whole surface.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

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.     

Adsorption of gold on hydrogen terminated Si(0 0 1): Formation of chain structure

Possible formation of stable Au atomic wire on the hydrogen terminated Si(0 0 1): 3×1 surface is investigated under the density functional formalism. The hydrogen terminated Si(0 0 1): 3×1 surface is patterned in two different ways by removing selective hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different sub-monolayer coverages. At 4/9 monolayer (ML) coverage, zigzag continuous Au chains are found to be stable on the patterned hydrogen terminated Si(0 0 1): 3×1 surface. The reason for the stability of the wire structures at 4/9 ML coverage is explained. It is to be noted that beyond 4/9 ML coverage, the additional Au atoms may introduce clusters on the surface. The continuous atomic gold chains on the substrate may be useful for the fabrication of atomic scale devices.     

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.     

Asymmetric adsorption-site of potassium atoms in the (3 × 2)-p2mg structure formed on Cu(0 0 1)

The adsorption of K atoms on Cu(0 0 1) has been studied by low-energy electron diffraction (LEED) at room temperature (RT) and 130 K. At RT, a (3 × 2)-p2mg LEED pattern with single-domain was observed at coverage of 0.33, whereas the orthogonal two-domain was found at 130 K. At 130 K, a c(4 × 2) pattern with orthogonal two-domain was observed at coverage 0.25. Both the (3 × 2)-p2mg and c(4 × 2) structures have been determined by a tensor LEED analysis. It is demonstrated that K atoms are adsorbed on surface fourfold hollow sites in the c(4 × 2), while in the (3 × 2) structure two K atoms in the unit cell are located at an asymmetric site with a glide-reflection-symmetry. The asymmetric site is at near the midpoint between the exact hollow site and bridge-site but slightly close to the hollow site. A rumpling of 0.07 Å in the first Cu layer was confirmed, which might stabilize K atoms at the asymmetric site. Surface structures appearing in a coverage range 0.25-0.33 are discussed in terms of the occupation of the asymmetric site with increase of coverage.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

Oxidation of Pt(1 0 0)-hex-R0.7° by gas-phase oxygen atoms

We utilized temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), and low energy electron diffraction (LEED) to investigate the oxidation of Pt(1 0 0)-hex-R0.7° at 450 K. Using an oxygen atom beam, we generated atomic oxygen coverages as high as 3.6 ML (monolayers) on Pt(1 0 0) in ultrahigh vacuum (UHV), almost 6 times the maximum coverage obtainable by dissociatively adsorbing O₂. The results show that oxidation occurs through the development of several chemisorbed phases prior to oxide growth above about 1 ML. A weakly bound oxygen state that populates as the coverage increases from approximately 0.50 ML to 1 ML appears to serve as a necessary precursor to Pt oxide growth. We find that increasing the coverage above about 1 ML causes Pt oxide particle growth and significant surface disordering. Decomposition of the Pt oxide particles produces explosive O₂ desorption characterized by a shift of the primary TPD feature to higher temperatures and a dramatic increase in the maximum desorption rate with increasing coverage. Based on thermodynamic considerations, we show that the thermal stability of the surface Pt oxide on Pt single crystal surfaces significantly exceeds that of bulk PtO₂. Furthermore, we attribute the high stability and the acceleratory decomposition rates of the surface oxide to large kinetic barriers that must be overcome during oxide formation and decomposition. Lastly, we present evidence that structurally similar oxides develop on both Pt(1 1 1) and Pt(1 0 0), therefore concluding that the properties of the surface Pt oxide are largely insensitive to the initial structure of the Pt single crystal surface.     

Surface structure of K/Pd(1 0 0) and the effect of H coadsorption: Density functional theory calculations

We have performed density-functional theory calculations to study the atomic structure of the K/Pd(1 0 0)-p(2 × 2) and -c(2 × 2) surfaces formed at 0.25 ML and 0.5 ML, respectively. We find that K atoms prefer the hollow site with the K adsorption height 2.44 Å for p(2 × 2) and 2.50 Å c(2 × 2). The first interlayer spacing (d₁₂) of the Pd(1 0 0) substrate appears slightly contracted from the bulk value as Δd₁₂ = −0.8% and −0.3% for p(2 × 2) and c(2 × 2), respectively. The calculated contraction Δd₁₂ = −0.3% for c(2 × 2) is not in accord with the expansion Δd₁₂ = +1.3% reported by a low-energy electron diffraction (LEED) study. As the origin of this difference, a possibility of hydrogen contamination of the surface sample used in the LEED study is suggested: Our calculations show that the d₁₂ of K/Pd(1 0 0)-c(2 × 2) increases linearly with the coverage of H coadsorption, which leads to an estimation for the H coverage of the surface sample as 0.1-0.4 ML.     

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.