Mixed water/OH structures on Pd(1 1 1)

Chemisorbed O and water react on Pd(1 1 1) at low temperatures to form a mixed OH/H₂O layer with a (√3 × √3)R30° registry. Reaction requires at least two water molecules to each O before the (2 × 2)O islands are consumed, the most stable OH/water structure being a (OH + H₂O) layer containing 0.67 ML of oxygen, formed by the reaction 3H₂O + O → 2(H₂O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (√3 × √3)R30° − (OH + H₂O) phase cannot be formed by O/H reaction and is distinct from the (√3 × √3)R30° structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(1 1 1), preventing this phase catalyzing the low temperature H₂/O₂ reaction which only occurs at higher temperatures.     

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).     

Structural analysis of Si(1 1 1)-√21 × √21-Ag surface by reflection high-energy positron diffraction

The atomic structure of Si(1 1 1)-√21 × √21-Ag surface, which is formed by the adsorption of small amount of Ag atoms on the Si(1 1 1)-√3 × √3-Ag surface, was determined by using reflection high-energy positron diffraction. The rocking curve measured from the Si(1 1 1)-√21 × √21-Ag surface was analyzed by means of the intensity calculations based on the dynamical diffraction theory. The adatom height of the extra Ag atoms from the underlying Ag layer was determined to be 0.53 Å with a coverage of 0.14 ML, which corresponds to three atoms in the √21 × √21 unit cell. From the pattern analyses, the most appropriate adsorption sites of the extra Ag atoms were proposed.     

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.     

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.     

Adsorption site, core level shifts and charge transfer on the Pd(1 1 1)-I(√3 × √3) surface

We use core level photoelectron spectroscopy and density functional theory (DFT) to investigate the iodine-induced Pd(1 1 1)-I(√3 × √3) structure formed at 1/3 ML coverage. From the calculations we find that iodine adsorbs preferentially in the fcc hollow site. The calculated equilibrium distance is 2.06 Å and the adsorption energy is 68 kcal/mol, compared to 2.45 Å and 54 kcal/mol in the atop position. The adsorption energy difference between fcc and hcp hollows is 1.7 kcal/mol. Calculated Pd 3d surface core level shift on clean Pd(1 l 1) is 0.30 eV to lower binding energy, in excellent agreement with our experimental findings (0.28-0.29 eV). On the Pd(1 1 1)-I(√3 × √3) we find no Pd 3d surface core level shift, neither experimentally nor theoretically. Calculated charge transfer for the fcc site, determined from the Hirshfeld partitioning method, suggests that the iodine atom remains almost neutral upon adsorption.     

Eu- and Yb-induced reconstructions on a vicinal Si(1 0 0) surface

Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated.     

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.     

Structural analysis of Pt(1 1 1)c(√3 × 5)rect.-CO using photoelectron diffraction

Core level shift scanned-energy mode photoelectron diffraction using the two distinct components of the C 1s emission has been used to determine the structure of the Pt(1 1 1)c(√3 × 5)rect.-CO phase formed by 0.6 ML of adsorbed CO. The results confirm earlier assignments of these components to CO in atop and bridging sites, further confirm that the best structural model involves a 2:1 occupation ratio of these two sites, and provides quantitative structural parameter values. In particular the Pt-C chemisorption bondlengths for the atop and bridging sites are, respectively, 1.86 ± 0.02 Å and 2.02 ± 0.04 Å. These values are closely similar to those found in the 0.5 ML coverage c(4 × 2) phase, involving an atop:bridge occupation ratio of 1:1, obtained in earlier quantitative low energy electron diffraction studies. The results also indicate a clear tilt of the molecular axis of atop CO species in this compression phase, consistent with the finding of an earlier electron-stimulated desorption ion angular distribution investigation.     

Ethylene hydrogenation on Ni(1 0 0) surface

The hydrogenation of ethylene on Ni(1 0 0) surface has been studied by TDS. The decrease in the bonding energy with increasing coverage is revealed for both of adsorbed hydrogen and ethylene by the shift of desorption to lower temperatures. Ethane formation is only observed on the preadsorbed hydrogen coverage exceeding 0.5 monolayer (ML), coupled with the growth of H₂ shoulder peak at lower temperatures. Further increase of H coverage to saturation reduces the bonding energy of subsequently adsorbed ethylene by 15 kJ/mol and decreases the saturation coverage of ethylene to about one-third on the clean surface. This leads to the shift of ethane desorption from 250 to 220 K and an appearance of additional ethane peak at 180 K. The latter ethane formation coincides with the hydrogenation of surface ethyl species derived from ethyl iodide as a precursor. It indicates that the rate of ethyl formation on the surface would be comparable to that of subsequent hydrogen addition to the surface ethyl species in the hydrogenation of ethylene when the preadsorbed hydrogen coverage approaches 1.0 ML.     

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.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

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.     

Modification by H-termination in growth process of titanium silicide on Si(0 0 1)-2 × 1 observed with scanning tunneling microscopy

Formation processes of titanium silicide on hydrogen-terminated H/Si(0 0 1)-2 × 1 surface are studied at the atomic scale with a scanning tunneling microscopy (STM). Square-shaped nanoislands were observed on the Ti/H/Si(0 0 1) surface after annealed at 873-1073 K. These are the epitaxial nanoislands moderately grown due to the local orientation relationship between C49-TiSi₂ and Si(0 0 1), because passivation by surface hydrogen on Si(0 0 1) suppresses active and complex bond formation of Ti-Si.     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

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.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Hydrogen adsorption on GaAs(0 0 1)-c(4 × 4)

Exposure of the As-terminated GaAs(0 0 1)-c(4 × 4) reconstructed surface to atomic hydrogen (H) at different substrate temperatures (50-480 °C) has been studied by reflection high-energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM). Hydrogen exposure at low temperatures (∼50 °C) produces a disordered (1 × 1) surface covered with AsH_x clusters. At higher temperatures (150-400 °C) exposure to hydrogen leads to the formation of mixed c(2 × 2) and c(4 × 2) surface domains with H adsorbed on surface Ga atoms that are exposed due to the H induced loss of As from the surface. At the highest temperature (480 °C) a disordered (2 × 4) reconstruction is formed due to thermal desorption of As from the surface. The results are consistent with the loss of As from the surface, either through direct thermal desorption or as a result of the desorption of volatile compounds which form after reaction with H.     

First-principles study of oxygenated diamond (0 0 1) surfaces with and without hydrogen

The structural characterization of oxygenated diamond (0 0 1) surface with and without H-presence has been investigated by the density-functional theory within the generalized gradient approximation and the plane-wave pseudopotential method. It is shown that, without the H, the O-bridge site has lower energy than the O-on-top site for both half and fully monolayer (ML) oxygen coverage. For hydrogenated diamond (0 0 1) surfaces, we found for half ML O coverage, a more stable (2 × 2):0.5H&0.5(OH) phase than the previously reported (2 × 1):0.5H&0.5(OH) phase [H. Tamura, H. Zhou, K. Sugisako, Y. Yokoi, S. Takami, M. Kubo, K. Teraishi, A. Miyamoto, A. Imamura, M.N. Gamo, T. Ando, Phys. Rev. B 61 (2000) 11025]. Furthermore, at one ML O coverage, a (2 × 2):(OH) phase is the most stable structure than the previously reported (2 × 1):(OH) phase. With the increase of the (OH) coverage, the importance of the hydrogen bonds increases in terms of stabilizing these structures.     

Experimental deduction of In/Si(1 1 1) 2D phase diagram and ab initio DFT modeling of 2√3 phase

We have carried out adsorption and residual thermal desorption experiments of Indium on Si (1 1 1) 7 × 7 reconstructed surface, in the submonolayer regime, in Ultra High Vacuum (UHV) using in situ probes such as Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). The coverage information from AES and the surface symmetry from LEED is used to draw a 2D phase diagram which characterizes each observed superstructural phases. The different superstructural phases observed are Si(1 1 1)7 × 7-In, Si(1 1 1)√3 × √3R30°-In, Si(1 1 1)4 × 1-In, Si(1 1 1)2√3 × 2√3R30°-In and Si(1 1 1)√7 × √3-In in characteristic temperature and coverage regime. In addition to the 1/3 ML, √3 × √3-In phase, we observe two additional √3 × √3-In phases at around 0.6 and 1 ML. Our careful residual thermal desorption studies yields the Si(1 1 1)2√3 × 2√3R30°-In phase which has infrequently appeared in the literature. We probe theoretically the structure of this phase according to the LEED structure and coverage measured by AES, assuming that the model for Si(1 1 1)2√3 × 2√3R30°-In is very proximal to the well established Si(1 1 1)2√3 × 2√3R30°-Sn phase, using ab initio calculation based on pseudopotentials and Density Functional Theory (DFT) to simulate an STM image of the system. Calculations show the differences in the atomic position and charge distribution in the Si(1 1 1)2√3 × 2√3R30°-In case.