Chlorine chemisorption on Cu(0 0 1) by surface X-ray diffraction: Geometry and substrate relaxation

We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 × 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu-Cl interlayer spacing of 1.60 Å and a slightly expanded Cu-Cu first interlayer spacing of 1.85 Å (1.807 Å for bulk Cu) was determined by LEED. The resulting Cu-Cl bond length, 2.41 Å, is very similar to the SEXAFS value of 2.37 Å. Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu-Cl interlayer spacing of 1.60 Å, no first Cu-Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)-c(2 × 2)-Cl system using surface X-ray diffraction technique with synchrotron radiation. We find a Cu-Cl interlayer spacing of 1.584(5) Å and confirm the expansion of the first Cu-Cu interlayer, with an average spacing of 1.840(5) Å. In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu-Cu interlayer expansion.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

Core-level shifts of the c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) surfaces

We have studied In-stabilized c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III–V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the ζ and ζa models, which have been previously established to describe the atomic structures of the III–V(1 0 0)c(8 × 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III–V(1 0 0)c(8 × 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated ζ and ζa SCLS values shows that the InSb spectra are reproduced by the ζ SCLSs better than by the ζa SCLSs. Interestingly, the ζa fits agree better with the InAs spectra than the ζ fits do, indicating that the ζa model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III–V(1 0 0)c(8 × 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.     

D2 layers on MgO(0 0 1): Simulation study

In response to recent helium atom scattering (HAS) and neutron scattering results, Monte Carlo simulations and perturbation theory calculations have been performed for D₂ on MgO(0 0 1). Monte Carlo simulations predict that D₂ molecules form a series of interesting structures, p(2 × 2) → p(4 × 2) → p(6 × 2), with coverages Θ = 0.5, 0.75, 0.83 respectively, and followed by a formation of a top layer of p(6 × 2) unit cell symmetry. The three types of mono-layers are stable up to 13 K, whereas the top layer still exists up to 10 K. This is in partial agreement with the neutron scattering and HAS results that report c(2 × 2), c(4 × 2) and c(6 × 2); they agree in terms of coverage and stability, but disagree in terms of symmetry. A quantum mechanical examination of the D₂ molecules’ rotational motion shows the molecular axes are azimuthally delocalized and hence the simulated structures are c-type rather than p-type. These calculations also indicate that ortho-D₂ and helicoptering para-D₂ prefer cationic sites, while cartwheeling para-D₂ prefers anionic sites.     

Magnetic ordering of Vn/Mo(0 0 1) systems: Ab-initio calculations

A density-functional theory (DFT) study is performed using a full-potential linearized-augmented-plane-waves (FP-LAPW) method to investigate the magnetic structure of vanadium-molybdenum systems (V_n/Mo(0 0 1), n = 1, 2). The topmost V layers relax inward in both systems with a larger contraction in V₂/Mo(0 0 1) system. A p(1 × 1) in-plane ferromagnetic ordering with appreciable magnetic moments is obtained on V overlayers, which is found to be the ground state in both systems. The layers below the surface exhibit induced magnetism with antiferromagnetic interlayer coupling.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Study of c(2 × 2)-MnAu(0 0 1) layers on Mn(0 0 1) by means of scanning tunneling microscopy/spectroscopy

Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (0 0 1) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2 × 2)-MnAu(0 0 1) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5 × 23)-like Au reconstruction appears instead of the c(2 × 2) structure and local density of states peaks obtained on the c(2 × 2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer.     

Spin-polarized scanning tunneling microscopy through an adsorbate layer: Sulfur-covered Fe/W(1 1 0)

Spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) has been performed on clean and sulfur-covered three-dimensional Fe islands on W(1 1 0). Upon dosing with H₂S the island surface is covered with 1/3 ML S leading to a c(3 × 1) reconstruction. The characteristic magnetic vortex structure is observable before and after dosing, even though the electronic structure of the surface is modified as is shown by SP-STS.     

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.     

Cyclic cluster study of water adsorption structures on the MgO(1 0 0) surface

Cyclic cluster calculations were performed with the quantum chemical method MSINDO to elucidate the relative stabilities of c(4 × 2), p(3 × 2) and (1 × 1) overlayer structures of water molecules on the MgO(1 0 0) surface. For the c(4 × 2) and p(3 × 2) structures both molecular adsorption and partially dissociated adsorption were considered. In agreement with earlier theoretical studies partial dissociation was found to be more stable than molecular adsorption. For the c(4 × 2) structure both monolayer and double layer coverage were studied. Adsorption was found to be more stabilized with increasing degree of dissociation until 50% of the water molecules were dissociated. In the case of 50% dissociated water molecules we found that one quarter of the Mg atoms were pulled out of the MgO surface when surface relaxation was taken into account. A new structure for the fully dissociated (1 × 1) water monolayer was found which is considerably more stable than previously studied arrangements. In all cases surface relaxation was found to be important. The most stable structures of c(4 × 2), p(3 × 2) and (1 × 1) symmetry have adsorption energies which differ by no more than 13 kJ/mol. This offers an explanation of phase transitions of overlayer structures found by experiments between 180 and 300 K.     

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.     

Low-frequency noise and inelastic tunneling spectroscopy in Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) epitaxial magnetic tunnel junctions

We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm². The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm²) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure.     

Surface phase transition and electronic structure of c(5√2 × √2)R45°-Pb/Cu(1 0 0)

The c(5√2 × √2)R45°-Pb/Cu(1 0 0) surface phase is investigated by means of angle resolved ultraviolet photoemission and low energy electron diffraction in the temperature range between 300 and 550 K. We identify and characterize a temperature-induced surface phase transition at 440 K from the room temperature c(5√2 × √2) R45° phase to a (√2 × √2)R45° structure with split superstructure spots. The phase transition is fully reversible and takes place before the two-dimensional melting of the structure at 520 K. The electronic structure of the split (√2 × √2)R45° phase is characterized by a metallic free-electron like surface band. This surface band is backfolded with c(5√2 × √2)R45° periodicity phase at room temperature, giving rise to a surface band gap at the Fermi energy. We propose that a gain in electronic energy explains in part the stability of the c(5√2 × √2)R45° phase.     

First-principles study of oxidized Nb(1 0 0) surface structures

The atomic positions of the oxygen-induced c(2 × 2)-O, (3 × 1)-O and (4 × 1)-O surface structures on Nb(1 0 0) are determined by first-principles electronic structure calculations within the density functional theory comparing experimentally observed scanning tunneling microscopy (STM) images. STM images of these surfaces are calculated on the basis of the theory of Tersoff and Hamann. The theoretical and experimental STM images of the oxygen-chemisorbed c(2 × 2)-O structural model agree well. However, only the oxide-covered (3 × 1)-O and (4 × 1)-O structural models with two layers of NbO and contraction of the unit length along longitudinal 〈1 0 0〉 direction by 10% result in the theoretical STM images that agree with the experimental ones.     

A perturbation theory study of H2 on LiF(0 0 1)

In response to recent helium atom scattering (HAS) results, Monte Carlo (MC) simulations and perturbation theory have been performed for H₂ on LiF(0 0 1). MC simulations predict that H₂ molecules form a series of structures, p(2 × 2), p(8 × 2), p(4 × 2) with coverages Θ = 0.5, 0.625 and 0.75, respectively, that are stable up to 8 K. This is in partial agreement with the HAS results that report c(2 × 2) and c(8 × 2) structures; they agree in terms of coverage and stability, but disagree in terms of symmetry. To reconcile the results of the simulations and experiments, the orientation of the adsorbed H₂ molecules was studied using perturbation theory. These calculations show that the adsorbed H₂ molecules are azimuthally delocalized and that the structures are c-type rather then p-type. The calculations also indicate that p-H₂ and helicoptering o-H₂ prefer cationic sites, while cartwheeling o-H₂ prefers anionic sites.     

Theoretical study of magnetism and electronic structure of Fe3/Crn(1 1 0) superlattices

The electronic structure and magnetism of Fe₃/Cr_n(1 1 0) (n=1, 3, 5) superlattices (SL) with varying layer thickness have been studied using the full-potential linearized augmented plane-wave (FLAPW) method within the first-principle formalism. The results show that the ferromagnetic state is the preferable phase in the ground state. The magnetic moments of the Fe layers are slightly modified by the presence of the Cr layers. The Cr magnetic moments alternate direction from layer to layer, and an antiferromagnetic coupling between Fe and Cr at the interfacial layer is seen. The magnetic moments of the Cr layers are suppressed because there is a strong hybridization between d-states of both Fe and Cr atoms. Only a small moment is found in the Cr layer. The Cr moment alignment is determined by a delicate balance between the different magnetic interaction.     

New reconstruction-stoichiometry correlation for GaAs(0 0 1) surface treated by atomic hydrogen

The structure, stoichiometry and electronic properties of the GaAs(0 0 1)-(2 × 4)/c(2 × 8) surface treated by cycles of atomic hydrogen (AH) exposure and subsequent annealing in UHV were studied with the aim of preparing the Ga-rich surface at low temperatures. Low energy electron diffraction showed reproducible structural transformations in each cycle: AH adsorption at the (2 × 4)/c(2 × 8) surface led to the (1 × 4) structure at low AH exposure and to a (1 × 1) surface at higher AH exposure with subsequent restoration of the (2 × 4)/c(2 × 8) structure under annealing at 450 °C. The cycles of AH treatment preserved the atomic flatness of the GaAs(1 0 0) surface, keeping the mean roughness on to about 0.15 nm. The AH treatment cycles led to the oscillatory behavior of 3dAs/3dGa ratio with a gradual decrease to the value characteristic for the Ga-rich surface. Similar oscillatory variations were observed in the work function. The results are consistent with the loss of As from the surface as a result of the desorption of volatile compounds which are formed after reaction with H. The prepared Ga-rich GaAs(0 0 1) surface showed the stability of the (2 × 4)/c(2 × 8) structure up to the annealing temperature of 580 °C.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

SrTiO3(0 0 1) reconstructions: the (2 × 2) to c(4 × 4) transition

Scanning tunneling microscopy (STM) is used to investigate the (0 0 1) surface structure of Nb doped SrTiO₃ single crystals annealed in ultra high vacuum (UHV). Atomically resolved images of the (2 × 2) reconstructed surface are obtained after annealing a chemically etched sample. With further annealing dotted row domains appear, which coexist with the (2 × 2) reconstruction. The expansion of these domains with further annealing gives rise to the formation of a TiO₂ enriched c(4 × 4) reconstruction.