Interdiffusion of adsorbed Pb and Bi on Cu(1 0 0)

The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(1 0 0) has been studied, at T = 513 K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consist of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features lead to the conclusion that Pb atoms are more strongly bound at surface alloyed sites in Cu(1 0 0) than Bi atoms. Once the total coverage (Pb + Bi) on the substrate reaches about one monolayer, Pb and Bi are dealloyed from the substrate, and the interdiffusion profiles become essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of ∼10⁻¹³ m²/s. This is considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion is different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction, leading to the conclusion that binding of Bi atoms to the Cu(1 0 0) substrate is stronger than that of Pb atoms in the highest-coverage surface dealloyed layers.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.     

Growth kinetics of metastable (3 3 1) nanofacet on Au and Pt(1 1 0) surfaces

A theoretical epitaxial growth model with realistic barriers for surface diffusion is investigated by means of kinetic Monte Carlo simulations to study the growth modes of metastable (3 3 1) nanofacets on Au and Pt(1 1 0) surfaces. The results show that under experimental atomic fluxes, the (3 3 1) nanofacets grow by 2D nucleation at low temperature in the submonolayer regime. A metastable growth phase diagram that can be useful to experimentalists is presented and looks similar to the one found for the stationary growth of the bcc(0 0 1) surface in the kinetic 6-vertex model.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

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.     

Growth of In nanocrystallite arrays on the Si(1 0 0)-c(4 × 12)-Al surface

Using scanning tunneling microscopy, growth of In nanoisland arrays on the Si(1 0 0)-c(4 × 12)-Al surface has been studied for In coverage up to 1.1 ML and substrate temperature from room temperature to 150 °C. In comparison to the case of In deposition onto the clean Si(1 0 0) surface or Si(1 0 0)4 × 3-In reconstruction, the In growth mode is changed by the c(4 × 12)-Al reconstruction from the 2D growth to 3D growth, thus displaying a vivid example of the Volmer-Weber growth mode. Possible crystal structure of the grown In nanoislands is discussed.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

Ga-induced superstructures on the Si(1 1 1) 7 × 7 surface

Monolayer Ga adsorption on Si surfaces has been studied with the aim of forming p-delta doped nanostructures. Ga surface phases on Si can be nitrided by N₂⁺ ion bombardment to form GaN nanostructures with exotic electron confinement properties for novel optoelectronic devices. In this study, we report the adsorption of Ga in the submonolayer regime on 7 × 7 reconstructed Si(1 1 1) surface at room temperature, under controlled ultrahigh vacuum conditions. We use in-situ Auger electron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction to monitor the growth and determine the properties. We observe that Ga grows in the Stranski-Krastanov growth mode, where islands begin to form on two flat monolayers. The variation in the dangling bond density is observed during the interface evolution by monitoring the Si (LVV) line shape. The Ga adsorbed system is subjected to thermal annealing and the residual thermal desorption studied. The difference in the adsorption kinetics and desorption dynamics on the surface morphology is explained in terms of strain relaxation routes and bonding configurations. Due to the presence of an energetic hierarchy of residence sites of adatoms, site we also plot a 2D phase diagram consisting of several surface phases. Our EELS results show that the electronic properties of the surface phases are unique to their respective structural arrangement.     

Surface structures formed by individual adsorption and coadsorption of Mn and Bi on Cu(0 0 1), studied by LEED

We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi₄ clusters are arranged in the top Cu layer instead of MgBi₄. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment.     

Nanostructure formation aided by self-organised Bi nanolines on Si(0 0 1)

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(0 0 1) surface. It is suggested that hydrogen-passivated single-domain Si(0 0 1) produced by the formation of defect-free, hundreds of nm long, and 1.2-1.5 nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge-Si dimer structures on the Si(0 0 1) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.     

Energetics of oxygen embedment into unreconstructed and reconstructed Cu(1 0 0) surfaces: Density functional theory calculations

Atomic oxygen embedment into a Cu(1 0 0) surface is studied by density functional theory calculation and the nudged elastic band method. As the oxygen coverage increases on the unreconstructed surface from 0.25 monolayer (ML) to 0.75 ML, the energy barrier for oxygen embedment decreases and an energetically favorable sub-surface site is found at 0.75 ML coverage. At a fixed oxygen coverage of 0.5 ML, the oxygen embedment energetics vary with the surface morphology and the embedment is found to be more probable for reconstructed structures compared to the bare surface. On the missing-row reconstructed surface, we find that the energy barrier for atomic oxygen embedment is smaller through the missing-row compared to other paths, suggesting a mechanism for the formation of sub-surface oxygen structures that are consistent with a recent experiment. The energy barrier for sub-surface oxygen diffusion is predicted to be less than that for on-surface diffusion.     

Halogen chemisorption, the pairwise diffusion of I, and trapping by defects on Si(1 0 0)

Halogen molecules dissociatively chemisorb on Si(1 0 0)-(2 × 1), and the bonding structures that they adopt can be elucidated with scanning tunneling microscopy. Of the Cl, Br, and I group, Cl has the highest single atom diffusion barrier, and both single and paired adatoms are observed at 295 K. The barrier is smaller for Br, and the adatoms can interrogate the surface until they form pairs, which are then immobile, or are trapped at C-type defects. The barrier is smallest for I, allowing the formation of pairs and trapped states, but the pairs are mobile at ambient temperature. Their motion is thermally activated, the events are random, and the diffusivities along and across the dimer row are ∼0.42 and ∼0.17 Å²/s at 295 K. The respective energy barriers for pairwise diffusion are ∼0.76 and ∼0.82 eV, assuming an attempt frequency of 10¹² s⁻¹. Studies over long times reveal that pairwise diffusion at low coverage is ultimately quenched by the increasing density of C-type defects, i.e. the increasing amounts of dissociated H₂O.     

Adsorption induced faceting and superstructural phase diagram of the Sb/Si(5 5 12) interface

The planar high index Si(5 5 12) surface consists of trenches formed by the several proximal surface planes, that can be employed as templates for the adsorption of low dimensional nanostructures. This paper reports the results of an extensive UHV study of the adsorption of Sb, in the sub-monolayer coverage regime, onto the Si(5 5 12) surface. The evolution of the surface phases, surface morphology and electronic structure is monitored by Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS). A careful control of substrate temperatures and Sb coverages formed at a low flux rate of 0.06 ML/min enable us to extract a complete adsorption phase diagram of the important interface, for the first time. The phase diagram clearly demonstrates the conversion of the large Si(5 5 12) unit cell into facets of planes of smaller (2 2 5), (3 3 7) and (1 1 3) base units. The study also reveals the formation of various superstructural phases formed by steering the kinetic parameters.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(1 0 0) and Pt(1 0 0) electrodes in 0.1 M HClO₄, revealing the formation of well-ordered adlattices of Au(1 0 0)-(√2 × √5) between 0.2 and 0.9 V and Pt(1 0 0)-(√2 × √2)R45° between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(1 0 0)-(√2 × √5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(1 1 1). In comparison, the coverage of Pt(1 0 0)-(√2 × √2)R45° - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that benzenethiol admolecules on Pt(1 0 0) could stand even more upright than those on Au(1 0 0). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(1 0 0) and Pt(1 0 0) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(1 0 0) electrode resulted in irreversible desorption of admolecules at E ? 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E ? 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(1 0 0) at potentials as negative as the onset of hydrogen evolution. Raising the potential rendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V.     

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).     

Sulphur overlayers on the Au(1 1 0) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)-(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)-(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S₂ molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal.     

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.     

Configuration of pentacene (C22H14) films on Si(1 0 0)-2 × 1 studied by NEXAFS

Pentacene films on Si(1 0 0)-(2 × 1) surface at 300 K were investigated using near edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. NEXAFS spectra show that pentacene molecules are chemisorbed on the Si(1 0 0)-(2 × 1) surface for monolayer with flat-laying and predominantly physisorbed on the Si(1 0 0)-(2 × 1) surface for multilayer films with an upright molecular orientation. Absorption angle of pentacene molecules were measured through π^∗ transition. The angles between the double bond and the silicon surface were 35-55°, 65° and 76° at monolayer, 24 and 48 nm pentacene deposited on the Si(1 0 0) surface, respectively. We observed that the intermediate flat-laying phase is favored for monolayer coverage, while the films of molecules standing perpendicular to the Si(1 0 0) surface are favored for multilayer coverage.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.