Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

Surface structure and morphology of InAs(1 1 1)B with/without gold nanoparticles annealed under arsenic or atomic hydrogen flux

We have investigated the structure and morphology of the InAs(1 1 1)B surface using Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM) and Scanning Electron Microscopy (SEM). The surface was prepared by annealing in the presence of an arsenic or atomic hydrogen pressure. A (2 × 2) reconstruction that changes into a (1 × 1) unreconstructed surface after prolonged annealing was observed irrespective of preparation method, while the surface morphology was distinctly different in the two cases. Detailed atomic scale models are proposed to explain the behavior. Deposition of Au aerosol nanoparticles on the sample prior to annealing was found to have no effect on the surface reconstruction. The Au particles were found to sink into the surface.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Bonding geometry of Mn-wires on the Si(100)(2 × 1) surface

The bonding geometry of monoatomic Mn-wires, which form on the reconstructed Si(100)(2 × 1) surface at room temperature, was investigated with scanning tunneling microscopy (STM). The Mn-wire structures are always perpendicular to the Si-dimer rows and the images exhibit a strong modulation of their apparent height as a function of bias voltage. The Mn-wire structures appear as depressions in the empty state images for bias voltages around 0.7 V, and as protrusions for all other bias voltages. It is suggested that the wire-images are defined by mixed Mn-Si states, either through a hybridization between the Mn d-states and the Si-p states, or backbonding from Mn-d electrons into the broken Si-dimer bond. The dominant bonding geometry shows that the Mn-wire maxima are positioned in between the Si-dimer rows, and a small percentage of about 20% is in registry with the Si-dimer rows, and might be described as defective wires. The experimental STM images cannot currently be described in a satisfactory manner with theoretical bonding models from the literature.     

Metallization of Ge(0 0 1)-p(2 × 1) surface as result of thermal fluctuations

We present a theoretical study of the metallization of Ge(0 0 1)-p(2 × 1) surface which is observed in experimental data. We have considered the connection between thermal fluctuation of this surface structure and its metallic properties. To this end we have performed long-time MD-DFT simulations. The obtained results show that thermal fluctuation of the Ge(0 0 1)-p(2 × 1) structure may cause its metallization which in not necessary connected with a flip-flop motion of dimer atoms. It was shown that the metallization of the Ge(0 0 1)-p(2 × 1) surface takes place when the dimer buckling angle is reduced to around 11°. In the case of our simulations the considered surface system remained in the metallic state for 25% of the simulation time. We have also found that the metallic state of the fluctuating Ge(0 0 1)-p(2 × 1) surface is built up by dangling bonds of the dimer atoms shifted up (D_up) and down (D_down).     

Edge and terrace structure of CoTPP on Au(1 1 1) investigated by ultra-high vacuum scanning tunnelling microscopy at room temperature

Edge adsorption and terrace molecular domain structures of Cobalt(II) tetraphenylporphyrin (CoTPP) on Au(1 1 1) were investigated using STM at room temperature. Two different terrace domain structures were observed. These two arrangements were found to be enantiomorphous arrangements of the molecular assemblies, where the molecular rows rotate ±16° with respect to the [1 2 1] direction of Au(1 1 1). In both arrangements, most of the CoTPP molecules were imaged as one bright dot with four legs, corresponding to a planar conformation of the macrocycle. A small proportion of the CoTPP molecules appear as two bright dots, corresponding to a saddle shape of the macrocycle. Our results show that most of the saddle-deformed CoTPP molecules are distributed in the vicinity of the bridging sites of the reconstructed gold surface. Besides terrace domains, we found that several edge adsorption structures of CoTPP are also stable enough to be imaged and analysed in detail. Furthermore, the relationship between edge structures and terrace domains was revealed.     

h-BN on Rh(1 1 1): Persistence of a commensurate 13-on-12 superstructure up to high temperatures

We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(1 1 1). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830 °C. Surface X-ray diffraction measurements up to 830 °C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.     

Potential-induced structure transitions in self-assembled monolayers: II. Propanethiol on Au(1 0 0)

The structure of propanethiol self-assembled monolayers (SAMs) on Au(1 0 0) in 0.1 M H₂SO₄ has been investigated as a function of electrode potential by in situ scanning tunnelling microscopy (STM). These studies reveal a potential-induced, reversible transition between an in essence quadratic and a distorted hexagonal structure. We suggest that a thiolate-driven surface reconstruction, similar to Au(1 0 0)-(hex), is responsible for the hexagonal SAM structure, whereas the quadratic one refers to the unreconstructed substrate.     

Oxidation of ultra-thin zinc films on Rh(100) surface

The oxidation of submonolayer zinc films on Rh(100) surface by O₂ gas has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). With a zinc coverage of 0.8 ML, an atomically flat ultra-thin zinc oxide film formed at an oxygen partial pressure of 2 × 10^? 8 mbar and a temperature of 150 °C. The zinc oxide film showed a c(16 × 2) LEED pattern. The high resolution STM image of the zinc oxide film showed single dotted spots and double dotted spots arranged linearly and periodically along the [01¯1] direction. We propose an atomic arrangement model of the film accounting for the LEED pattern, the STM image, and the atomic arrangement of the bulk ZnO(0001) surface.     

Sulphur adsorption on Au{1 1 0}: DFT and LEED study

The adsorption of sulphur on clean reconstructed Au{1 1 0}-(1 × 2) surface was studied using density functional theory (DFT) and quantitative low energy electron diffraction (LEED) calculations. The results show that the sulphur atoms form a (4 × 2) ordered structure which preserves the missing row reconstruction of the clean surface. The sulphur atom is found to adsorb on threefold hollow sites, on the {1 1 1} microfacets that border the trenches of the missing rows.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Observation of hydrogen adsorption on 6H-SiC(0 0 0 1) surface

We have used coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) to investigate the adsorption of atomic hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface. It has been found that the saturation coverage of hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface is about 1.7 ML. Upon saturated adsorption of atomic hydrogen, the √3×√3 surface structure changes to the 1×1 structure. The data of the CAICISS measurements have indicated that as a result of the hydrogen adsorption, Si adatoms on the √3×√3 surface move from T₄ to on-top sites.     

Structural and electronic characterization of the MgO/Mo(0 0 1) interface using STM

The nucleation and growth of ultrathin MgO films on Mo(0 0 1) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.     

Interaction of NO with the O-rich RuO2(1 1 0) surface at 300 K

The interaction of NO with the O-rich RuO₂(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with ¹⁸O. During exposure to NO an O···N–O surface group (NO₂-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO₂-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.     

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Oxygen adatoms at SrTiO3(0 0 1): A density-functional theory study

We present a density-functional theory study addressing the energetics and electronic structure properties of isolated oxygen adatoms at the SrTiO₃(0 0 1) surface. Together with a surface lattice oxygen atom, the adsorbate is found to form a peroxide-type molecular species. This gives rise to a non-trivial topology of the potential energy surface for lateral adatom motion, with the most stable adsorption site not corresponding to the one expected from a continuation of the perovskite lattice. With computed modest diffusion barriers below 1 eV, it is rather the overall too weak binding at both regular SrTiO₃(0 0 1) terminations that could be a critical factor for oxide film growth applications.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Study of CO diffusion on stepped Pt(1 1 1) surface by scanning tunneling microscopy

We have used a time-dependent tunneling current mode based on scanning tunneling microscopy/spectroscopy (STM/STS) to study the tracer diffusion of CO molecules along steps and on terraces of Pt(1 1 1). The results show that the hopping rate of CO molecules along steps is about 10 times faster than that on terraces in the measured temperature range. The diffusion activation energies are 5.1 kcal/mol and 3.8 kcal/mol on terraces and along steps, respectively. The lower activation energy and faster hopping rate for CO molecules diffusing along steps provide evidence that steps provide fast diffusion channels for CO molecules on stepped Pt(1 1 1) surfaces.