Diatomic Steps Research Articles

Phase transformation mechanism of nominal Si(001) surface driven by hydrogen thermal annealing

Herein, we report the phase transformation mechanism of the nominal Si(001) surface driven by hydrogen thermal annealing. The surface energies of H-terminated Si(001) surface with different phase structures were calculated by density functional theory. The results show that the surface phase with monoatomic steps can transform into the surface phase with diatomic steps under proper ranges of hydrogen chemical potential. Combining thermodynamic and kinetic factors, the phase transformation can’t occur when annealing temperature lower or higher than 800 °C. In addition, surface phases with different types of diatomic steps are alternately transformed through the intermediate phase with monoatomic steps and the imperfection of the transformation process gradually increases with the extension of annealing time. Finally, different experiments have been carried and the experimental results are in good agreement with the phase transformation mechanism. This study provides complete theoretical mechanism and process parameters for controlling the phase structures of the nominal Si(001) surface through hydrogen thermal annealing.

Studying the Formation of Si (100) Stepped Surface in Molecular-Beam Epitaxy

Experimental studies of the formation of a stepped surface structure during molecular-beam epitaxy of silicon on a Si (100) substrate have been carried out in wide ranges of variation of the substrate temperature and silicon growth rate. The conditions of the transition from a two-domain structure of the Si (100) surface to a single-domain structure associated with the formation of diatomic steps are determined using reflection high-energy electron diffraction. It is shown that the effect of an increase in the substrate temperature on the transition to a single-domain structure is non-monotonic: a single-domain surface forms in the region of relatively low temperatures, whereas a two-domain surface forms at high temperatures. The transition to a single-domain structure during the experiment is possible only, if the silicon growth rate is increased above a certain minimum value.

Interplay between Single and Cooperative H2 Adsorption in the Saturation of Defect Sites at MgO Nanocubes

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10–8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O3C) and Mg at inverse corners (MgIC) were identified to be the most reactive and to split H2 irreversibly already in the lowest pressure regime (PH2 < 10–3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm–1) in the intermediate range of pressures (10–3–1 mbar), along with the appearance of bands at 3605/1225 cm–1, was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At PH2 > 1 mbar, fully reversible adsorption was shown to occur at O4C (either on mono- or diatomic steps) and Mg3C sites. Another OH/MgH couple (3697/1030 cm–1) that became reinforced at high PH2 but remained stable upon pumping was correlated to O3C and MgIC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H2 and other hydrogen-containing molecules, such as CH4, on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 A/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 A/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Формирование ступенчатой поверхности Si(100) и ее влияние на рост островков Ge

AbstractThe transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I _2 × 1/ I _1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

The ordering of Ge islands on a stepped Si(100) surface

In the paper we investigate the Ge island nucleation on a Si(100) surface which was annealed to obtain the diatomic steps. It was observed that the islands tend to nucleate at the step edges.

Diatomic Steps in Pt(997) Surfaces Are Better Catalysts than Monatomic Steps for the CO Oxidation Reaction near Atmospheric Pressure

The oxidation of CO to CO2 has been investigated on Pt(997) single-crystal model catalysts with a flow reactor near ambient pressure with X-ray diffraction. Depending on the temperature and gas composition, the surface may consist of monatomic steps separated by 9 atom wide terraces or diatomic steps separated by 18 atom wide terraces. The diatomic stepped surface has been found to be significantly more active for CO2 production. The kinetics of the single- to double-step transition under reaction conditions has been investigated, as well as the dependence of the transition temperature with the gas composition.

In situ verification of single-domain III-V on Si(100) growth via metal-organic vapor phase epitaxy

Reflectance anisotropy spectroscopy (RAS) was used in situ for the quantification of antiphase domains on surfaces of thin GaP films deposited onto Si(100) by metal-organic vapor phase epitaxy (MOVPE). The preparation of a single-domain GaP∕Si(100) surface was determined via the analysis of RAS peak intensities in reference to the well-known P-rich surface reconstruction of homoepitaxially grown GaP(100). Both preprocessed Si(100) substrates and MOVPE as-grown GaP∕Si(100) films were also characterized ex situ by atomic force microscopy to identify the formation of mono- and diatomic surface steps and to analyze of the domain distribution, respectively.

Direct imaging of quantum wires nucleated at diatomic steps

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place preferentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step.

Infrared Characterization of Hydroxyl Groups on MgO: A Periodic and Cluster Density Functional Theory Study

The infrared OH stretching frequencies of the various types of hydroxyl groups on MgO surfaces have been calculated by periodic (VASP) and cluster (Gaussian) DFT simulations. Surface irregularities (mono and diatomic steps, corners, step divacancies, and kinks) have been considered to model the IR spectra of hydroxylated MgO powders. A good correspondence between calculated and experimental frequencies is obtained with the B3LYP functional. Hydrogen-bonding is the parameter which influences most the IR frequency of OH groups, followed by location of OH groups in concave or convex areas of the surface and then oxygen coordination. The evolution of experimental IR spectra upon evacuation at increasing temperature can be rationalized on the basis of calculated thermal stabilities of each kind of OH groups. A new model is finally proposed to help assign the experimental bands, in terms of hydrogen-bonding, local topology of the hydroxylated sites, and coordination of oxygen.

Water on extended and point defects at MgO surfaces

The interaction of water with extended defects such as mono- and diatomic steps at the MgO(100) surface is investigated through first-principles simulations, as a function of water coverage. At variance with flat MgO(100) terraces, water adsorption is always dissociative on mono- and diatomic steps, as well as on MgO(110) surfaces. In most of the equilibrium configurations, the oxygen of the hydroxyl groups is two- or fourfold coordinated, but single-coordinated OH groups can be stabilized at diatomic step edges. The structural properties of the hydroxyl groups are discussed as a function of their coordination numbers and mutual interactions, as well as the surface defect morphology. It is shown that characteristics of water adsorption are primarily driven by the coordination number of the surface acid-base pair where the dissociation occurs. However, the OH groups resulting from water dissociation are also considerably stabilized by the electrostatic interaction with coadsorbed protons. At low coverage such an interaction, considerably stronger than hydrogen bonding, practically hinders any proton diffusion away from its neighboring hydroxyl. The computed adsorption energies allow us to discuss the onset of water desorption from flat MgO(100) terraces, diatomic and monoatomic steps, and from Mg-O divacancy.

Revisiting Acido-basicity of the MgO Surface by Periodic Density Functional Theory Calculations: Role of Surface Topology and Ion Coordination on Water Dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Brønsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.

Role of Hydroxyl Groups in the Basic Reactivity of MgO: a Theoretical and Experimental Study

Clean (i.e., carbonate- and hydroxyl-free) and hydroxylated MgO surfaces of various morphologies were used to study the correlation between thermodynamic Bronsted basicity (evaluated by the deprotonation of methanol and propyne followed by FTIR) and basic reactivity (evaluated by the conversion of 2-methyl-but-3-yn-2-ol (MBOH)). A correlation is evidenced for clean surfaces, but for hydroxylated surfaces, OH groups, though weak Bronsted bases, are much more active than oxide O2- ions. Some of the hydroxyl groups whose VOH infra-red stretching band is included in the narrow high-frequency OH band were shown to be responsible for such activity. In order to identify them, the irregularities of surfaces involving low coordination (LC) ions were modeled by DFT calculations (mono- and di-atomic steps for 4C ions, corners, kinks and divacancies for 3C ions). All of them dissociate water: except for kinks and divancancies where the OH formed can be considered as isolated, in all the other cases, OH groups are H-bonded. The i OH infra-red frequencies obtained by calculations are consistent with the experimental spectrum which exhibits two domains: the broad low-frequency () domain is assigned to H-bond donor OH groups (O3C H, O4C H and O5C H), whereas the high-frequency narrow band ( cm-1 ) corresponds to isolated OH (O2C H, O3C H and O4C H in kinks and divacancies) and H-bond acceptor OH groups (O1C H and O2C H). The latter are inferred to be implied in basic reactivity of hydroxylated surfaces toward MBOH.

Characterization of the (0001) cleavage surface of antimony single crystals using scanning probe microscopy: Atomic structure, vacancies, cleavage steps, and twinned interlayers

Atomically resolved scanning tunneling microscopy images of the unreconstructed hexagonal structure of surface atoms on Sb(0001) are presented. Lateral and vertical lattice parameters have been determined. The interatomic spacing of 4.31 \AA{} $(\ifmmode\pm\else\textpm\fi{}0.05\AA{})$ on the Sb(0001) surface corresponds to the known bulk data. Cleavage has been found to occur always between adjacent double layers, yielding at least diatomic cleavage steps of 3.75 \AA{} $(\ifmmode\pm\else\textpm\fi{}0.10\AA{})$ height. Different kinds of defect structures on the cleavage plane have been imaged with atomic resolution. Point defects, caused by a single or by three missing surface atoms, have been uncovered. Stable imaging of cleavage steps, which were found to be straight along the atomic rows, has been achieved. Twinned interlayers formed upon cleavage of Sb even at room temperature have been revealed by atomic force microscopy. The mean twinning angle of $2.42\ifmmode^\circ\else\textdegree\fi{}$ $(\ifmmode\pm\else\textpm\fi{}0.20\ifmmode^\circ\else\textdegree\fi{})$ is quantitatively in accord with the value of $2.45\ifmmode^\circ\else\textdegree\fi{}$ predicted by the model of twinning in Sb crystals. The observed features are discussed with respect to other layered materials and with regard to their relevance for the use of Sb(0001) as a support of nanostructures.

An AFM study of the thermal development of the [formula omitted] surface of KCl from room temperature up to the melting point

The topographical development of the (0 0 1) surface of KCl was investigated from room temperature up to the melting point by atomic force microscopy in UHV. Cleavage topographies of air-cleaved surfaces show extended growth hills formed by large numbers of rounded, mostly monatomic steps caused by the re-crystallization of crystal material dissolved in adsorbed water layers. Vacuum cleaved surfaces are more irregular. They exhibit among others sharp cleavage tips, which seem to be characteristic for ionic crystals. At moderately heated surfaces, 673 K, closed loop lamella systems as well as different types of spiral systems originating from edge and screw dislocations, respectively, are observed. High resolution images of spiral centers exhibit the emergence of mono and diatomic steps from the surface. The centers of closed loop step systems could not be visualized. After stronger heating, at 773 and 873 K, extended evaporation pits with macro-steps of heights between 20 and 35 nm are found. After shortly exceeding the melting point at about 1013 K, the surface is covered by macro-steps of heights between 3 and 5 nm and mono and diatomic steps running diagonally across the terraces between the high steps.

Structural characterization of ultra-thin (0 0 1) silicon films bonded onto (0 0 1) silicon wafers:: a transmission electron microscopy study

Ultra-thin (001) silicon films (thickness less than 25nm) directly bonded onto (001) silicon wafers have been investigated by transmission electron microscopy. Twist interfacial dislocations accommodate the twist between the two crystals, whereas tilt interfacial dislocations accommodate the tilt resulting from the residual vicinality of the initial surfaces. In low-twist angle grain boundaries, twist interfacial dislocations are dissociated and no precipitates are detected. In high-twist angle grain boundaries, there is no dissociation and a high density of silicon oxide precipitates is observed at the interface. Tilt interfacial dislocations are pinned by these precipitates, they are more mobile than precipitates. Without precipitates, their lines are straighter than those with precipitates, and this is especially when the bonded wafers are annealed at a high temperature. When no precipitates are present, tilt interfacial dislocations are associated by pairs, and we demonstrate that each tilt interfacial dislocations introduce a diatomic interfacial step at the interface.

Surface Structure of Nickel in Acid Solution Studied by In Situ Scanning Tunneling Microscopy

The structure of Ni(111) single‐crystal surfaces in (pH 3) has been investigated in situ by scanning tunneling microscopy after cathodic reduction and after passivation at potentials ranging from 0.45 to 1.15 V/SHE. The metallic surface has monoatomic and diatomic step edges and an unreconstructed Ni(111)‐(1 × 1) atomic lattice. The passivated surface has two different structures corresponding to the two chemical layers of the passive film. The hydroxide outer layer has a granular and amorphous structure. The lack of crystalline order is assigned to the incorporation of water molecules. Aging under polarization favors the hydration of this layer and the increase of the passivation potential favors its thickening. The oxide inner layer of the passive film has a crystalline structure with a stepped surface independent of the passivation potential. The average tilt angle between the terraces of the oxide layer and the substrate terraces is 3 ± 1°. The lattice parameters correspond to NiO(111) in parallel or antiparallel epitaxy with the substrate. The (111)‐(1 × 1) unreconstructed orientation of the oxide layer is stabilized by surface hydroxylation. The local orientation of the step edges of the oxide is assigned to its dissolution in the passive state. © 2000 The Electrochemical Society. All rights reserved.

Surface structural transitions induced by repetitive underpotential deposition of Ag on Au(111)

Abstract The underpotential deposition (upd) of Ag on a Au(111) electrode in aqueous sulphuric acid has been studied by electrochemical scanning tunneling microscopy (ECSTM). The Au(111) surface morphology is substantially altered during repeated cycles in the Ag upd region. Prior to Ag upd, the surface consists of large, flat terraces (50–200 nm in width) with monatomic and diatomic steps. After several cycles of Ag deposition and dissolution, the surface is characterised by the appearance and growth of a high density of monolayer deep ‘pits.’ As cycling is continued, the pits expand rapidly to leave isolated Au islands accompanied by receding terrace edges. This process is followed by a slow lateral growth of Au islands which apparently fuse with terrace edges or aggregate with each other to form extended, atomically flat terraces. The surface topography is dominated by straight-edged, narrow terraces (15–50 nm in width) running parallel to or crossing each other to form a mesh, in which the terrace edges meet at an angle of 120 or 60°, characteristic of a (111) close-packed plane. The slow transformation of a high quality Au(111) surface in comparison to the faster transformation of a defect rich surface suggests that the kinetics of the transition process are related to the initial number of surface defects. The surface morphological changes accompanying Ag upd cycling are reflected in the cyclic voltammograms via dramatic changes in both peak shape and potential. The effect of variation of the supporting anion on this transition has also been investigated. Repetitive cycling of Ag upd on the Au(111) electrode in the presence of perchlorate anions results in similar alterations in voltammetric peak shape and potential. This weakly adsorbing anion appears to enhance the rate of structural transformation of the Au(111) surface.

Influence of the step height of the vicinal surface of germanium on the formation of antiphase boundaries in a gallium-arsenide-germanium-gallium-arsenide(001) system

An investigation was made of the formation of antiphase boundaries in a GaAs/Ge/GaAs(001) system using accurately oriented substrates and substrates misoriented by 3° and 5° in the [110] direction. It was shown that growth of germanium on a misoriented gallium arsenide surface leads to the formation of diatomic steps of height a0/2 and therefore results in the absence of any antiphase boundaries in a GaAs film grown on this surface. Conditions required to obtain a vicinal Ge surface consisting of monatomic steps of height a0/4, whose presence leads to the formation of antiphase boundaries during GaAs growth, are determined.

Molecular Landers

ABSTRACT: Molecular landers are molecules designed for the study of electron transport in a tip‐molecule‐surface experimental set‐up. They comprise a fully aromatic and rigid main board linked to several spacers (legs). When the molecular landers are adsorbed on a metallic surface, the spacers maintain the main board at such a distance from the surface that the aromatic part is electronically decoupled from the surface. One end of this board extends beyond the last spacer and can be chimisorbed by docking along a diatomic step. Therefore in this conformation, the current can flow between the STM tip at one position of the molecular board and the surface at the other end, and only through the aromatic part.