CuPc molecules adsorbed on Au(1 1 0)-(1 × 2): growth morphology and evolution of valence band states

We present the growth morphology, the long-range ordering, and the evolution of the valence band electronic states of ultrathin films of copper phthalocyanine (CuPc) deposited on the Au(1 1 0)-(1 × 2) reconstructed surface, as a function of the organic molecule coverage. The low energy electron diffraction patterns present a (5 × 3) reconstruction from the early adsorption stages. High-resolution UV photoelectron spectroscopy data show the disappearance of the Au surface states related to the (1 × 2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

Determining the structure of Ru(0 0 0 1) from low-energy electron diffraction of a single terrace

While a perfect hcp (0 0 0 1) surface has threefold symmetry, the diffraction patterns commonly obtained are sixfold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180°. Here we use a low-energy electron microscope to acquire the threefold diffraction pattern from a single hcp Ru terrace and measure the intensity vs. energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(±0.9)% at 456 K.     

A surface X-ray diffraction study of TiO2(1 1 0)(3 × 1)-S

Surface X-ray diffraction has been used to investigate the structure of TiO₂(1 1 0)(3 × 1)-S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)-S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.     

The CuGaSe2(0 0 1) surface: A (4 × 1) reconstruction

We report on the formation of a stable (4 × 1) reconstruction of the chalcopyrite CuGaSe₂(0 0 1) surface. Using Ar⁺ ion-bombardment and annealing of epitaxial CuGaSe₂ films grown on GaAs(0 0 1) substrates it was possible to obtain flat, well-ordered surfaces showing a clear (4 × 1) reconstruction. The cleanliness and structure were analyzed in situ by AES and LEED. AES data suggest a slight Se-enrichment and Cu-depletion upon surface preparation. Our results demonstrate that (0 0 1) surfaces of the Cu-III-VI₂(0 0 1) material can show stable, unfacetted surfaces.     

Transformation to soluble model for structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface

The Ising model proposed previously for the structural phase transition from (4 × 1) to (8 × “2”) of In-adsorbed Si(1 1 1) surface, Hamiltonian of which is consisting of a two-spin interaction as well as a four-spin interaction is shown to be equivalent in thermodynamic properties to a soluble Ising model with two-spin interactions. Temperature dependence of the long range order and the transition temperature can now be determined from the exact formulae. Comparison between the simulation results and those from the exact formulae is made to see accuracy of the simulation.     

Structure analysis of W(0 0 1)2 × 1-O surface at room and liquid nitrogen temperatures

Surface structure of O-adsorbed W(0 0 1) surface after annealing to 1200 K has been analyzed by low energy electron diffraction at 77 K as well as at room temperature. The optimum structure has tungsten missing rows and oxygen double rows. Furthermore, the R-factor is minimized at the structure that O atoms are adsorbed on one of the two different threefold hollow sites of the (1 1 0) facet appearing on the W(0 0 1)2 × 1 with missing row. However, the results suggest that two domains of O atoms adsorbed on both the two different threefold hollow sites coexist. Then, I-V curves have been analyzed as a function of the mixing ratio of the two domains having different O adsorption sites at room and low temperatures. The energy difference between these two sites has been estimated to be 6.5 meV from the temperature dependence of the mixing ratio.     

Determination of a (4 × 4) structure formed on a Cu(0 0 1) surface by adsorption of calcium

The adsorption of calcium (Ca) atoms on a Cu(0 0 1) surface has been studied by low-energy electron diffraction (LEED) at 130, 300 and 400 K. It is found that a (4 × 4) was the only LEED pattern appeared at 400 K while a quasi-hexagonal structure was formed in a wide range of submonolayer coverage at 130 K. At 300 K, the (4 × 4) LEED spots were broad and weak. The (4 × 4) structure formed at 400 K was determined by a tensor LEED I-V analysis. It is a new-type of surface alloys consisting of five substitutional Ca atoms, nine surface Cu atoms, and two atomic vacancies in the unit cell. In spite of a quite large size-difference between Ca (3.94 Å) and Cu (2.55 Å) atoms, all Ca atoms are located at the substitutional sites. Among surface alloys so far reported, the atomic size ratio between Cu and Ca in the (4 × 4), 1.54, is the largest. Optimized structural parameters reveal that large lateral displacements of surface Cu atoms, being enabled by the appearance of the vacancies, allow the formation of the (4 × 4) structure.     

Structure and composition of the NiPd(1 1 0) surface

The NiPd(1 1 0) alloy surface was studied using low energy electron diffraction to measure the structure and composition of the first three atomic layers. The surface layer is highly enriched in Pd and has a significantly buckled structure. The second layer is also buckled, with displacements even larger than the surface layer. The second layer also exhibits intralayer segregation (chemical ordering), with alternate close-packed rows of atoms being Ni enriched and Pd enriched. The third layer has a structure and composition close to that of the bulk alloy. These results are compared with results for the other low index faces of NiPd, the extensive literature on NiPt alloy surfaces, and the growing body of theoretical literature for NiPd alloy surfaces.     

Temperature- and coverage-dependent evolution of the Au/Pd(1 1 0) surface structure

The morphology and the atomic scale structure of thin gold films (up to 2.5 ML) on Pd(1 1 0) were studied by means of scanning tunneling microscopy and surface X-ray diffraction. At room temperature the films exhibit a multilayer growth mode accompanied by the formation of highly anisotropic islands. Annealing above 500 K significantly increases the smoothness of the gold films, which are in registry with the substrate. Above a critical threshold of two monolayers a (1 × 2) missing-row reconstructed film is found. This reconstructed surface is well ordered after annealing at temperatures above 580 K. The specific gold film morphology is envisaged as a way to relax the strain caused by the mismatch between gold and palladium.     

Surface pre-melting and surface flattening of Bi nanofilms on Si(1 1 1)-7 × 7

We report on the in situ observation of temperature-driven drastic morphology evolution and surface pre-melting of the Bi(0 0 1) nanofilm deposited on the Si(1 1 1)-7 × 7 surface by use of spot-profile-analyzing low-energy electron diffraction (SPA-LEED). Surface step density of the single-crystalline, epitaxial Bi(0 0 1) film decreases above 350 K in a critical manner. On annealed Bi(0 0 1) films, we have detected surface pre-melting with a transition temperature of 350 K, which yields reversible diffraction intensity drop in addition to the harmonic Debye-Waller behavior. The observed surface flattening of the as-deposited film is driven by the increased amount of mobile adatoms created through the surface pre-melting.     

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.     

Study of phase transitions within alumina grown on top of CoAl (1 0 0) surface

The oxidation of CoAl(1 0 0) was investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). In the case of alumina grown after oxidation with 15,000 L at room temperature, the characteristic EEL spectrum, along with the sharp (2 × 1) LEED pattern observed after annealing at 1000 K, showed that a stable well-ordered θ alumina was formed. The intermediate phase was found after heat treatment at 1150 K. At higher temperatures, the formation of the α-like alumina was observed. The comparative study of as-oxidation versus subsequent annealing of amorphous alumina at high-temperatures revealed a close similarity between the structures of alumina.     

The adsorption geometry of the (2 × 1)-2O oxygen phase formed on the Co(10 0) surface

The bond geometry of the (2 × 1)-2O-p2mg overlayer on Co(10 0) was determined by analyzing low-energy electron diffraction (LEED) intensity data. Oxygen occupies the three-fold coordinated hcp site along the densely packed rows on the unreconstructed surface. The O atoms are attached to two atoms in the first Co layer and to one Co atom in the second layer. The strong interaction between O and Co is indicated by the bond lengths of 1.83 ± 0.10 Å and 1.99 ± 0.10 Å to the top-layer Co and the Co atoms in the second layer, respectively. The most striking result of our work is that oxygen adsorption causes a marked expansion (by 25%) of the first Co layer spacing (0.90 Å) with respect to the bulk value of 0.72 Å. This strong expansion might offer diffusion channels for O atoms to penetrate further into the subsurface region.     

Observation of a (√3 × √3)-R30° reconstruction on GaN(0 0 0 1) by RHEED and LEED

A new reconstruction of √3 × √3-R30° has been observed on a GaN film grown on a 6H-SiC (0 0 0 1)-√3 × √3 surface using RHEED and LEED experimental techniques. The experimental LEED PF shows that the GaN film is Ga-terminated hexagonal. The surface is a mixture of two structures with a single bilayer height difference between them. One is a √3 × √3-R30° reconstruction with Ga-adatoms occupying the T4 sites. Another is a Ga-terminated 1 × 1 with no extra Ga on top. The area ratio of the √3 × √3 part to the 1 × 1 part is slightly larger than 1. The first principle total energy calculations and Tensor-LEED I-V curves simulations further confirm this structure model.     

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.     

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.     

Methanethiolate structural phases on Cu{1 1 1} observed using a novel fibre-optic low-energy electron diffraction instrument

We have used a novel fibre-optic low-energy electron diffraction (FO-LEED) instrument, capable of low flux measurements that minimise electron beam damage to surface overlayers, to study methanethiolate (CH₃-S-) structural phases formed on Cu{1 1 1} at temperatures between 110 and 300 K. Three structural phases were seen: a (√3 × √3)R30° phase that forms at 110-140 K; a (4 × 4) phase which was observed transiently at 110 K; and a pseudo-{1 0 0} reconstructed phase which forms at room temperature. We discuss these in the context of previous studies of this system, and demonstrate the ability of the FO-LEED instrument to record high-quality LEED patterns and intensity data from a strongly beam-sensitive surface.     

Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3)R30° unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this phase. Finally, at a slightly higher coverage of 0.53 ML, the unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains.The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(1 1 1) surface is the main driving force for the different phases.     

Reversible electromigration of thallium adatoms on the Si(1 1 1) surface

By means of low-energy electron diffraction (LEED), we found a reversible structural change of on thallium (Tl) adsorbed Si(1 1 1) surfaces by switching the polarity of applied DC voltage for heating the sample. It was shown in the literature that Tl adatoms are located on the T₄ sites of the bulk-terminated surface both in the (1 × 1) and . It is clarified that the structural change is caused by the electromigration of the Tl adatoms. Tl atoms migrate towards the cathode, being induced by the electric field (10-20 V/cm). We discussed an atomic process of the electromigration.