Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Angular distributions of atoms sputtered from NiAl{1 1 0} and Ni{1 0 0}

Using the laser post-ionization surface analysis technique, I have for the first time studied angular distributions of Ni and Al atoms sputtered from NiAl{1 1 0}. Emission angular distributions from Ni{1 0 0} have also been measured. I have observed preferential emissions of Ni and Al atoms along 〈1 1 1〉 and 〈1 0 0〉crystallographic directions for NiAl{1 1 0} and of Ni atoms along 〈1 1 0〉 and 〈1 0 0〉 directions for Ni{1 0 0}. The observed preferential ejections can be explained in terms of the theory of focusing-collision sequences. Because of the difference in surface binding energy between Al and Ni atoms, preferential ejection angles of Ni atoms are slightly different from those of Al atoms along the 〈1 1 1〉 ejections. For NiAl, the 〈1 1 1〉 preferential ejections were less prominent than the 〈1 0 0〉 preferential ejections and this can be related to the low efficiency of momentum transfer in Ni-Al collision sequences along 〈1 1 1〉 lattice directions. The low efficiency of momentum transfer due to the mass mismatch can also be responsible for the experimental observation that the preferential ejections in the alloy were less prominent than those in the Ni metal.     

Scattering of F atoms and anions on a TiO2(1 1 0) surface

Results of a study of energy losses and electron transfer processes for grazing scattering of fluorine atoms and anions scattering along different azimuthal orientations of the TiO₂ crystal are presented. We observe strong variations in the overall intensity of scattered particles which are due to channelling effects. The energy losses do not show strong variations as a function of crystal azimuth except for the case of scattering along the (0 0 1) direction between the bridging oxygen atom rows, where we also observe differences in the energy losses of scattered ions and neutrals. We attribute this to the fact that larger F⁻ survival occurs for trajectories staying farther from the surface, when also the energy losses remain small. The overall characteristics of energy losses are attributed mainly to trajectory effects due to scattering in regions of different electron density. Measurements of the ratio of scattered ions to the total scattered flux, i.e. the ion fractions which reflect electron capture and loss processes, show that these are not the same for incident anions and atoms. A strong difference for scattering along the (0 0 1) direction is observed, where at low incident energies a strong survival of incident ions occurs. These results are tentatively discussed in terms of non resonant electron capture at lattice O⁻ sites and electron loss into the conduction band or by collisional detachment with bridging O atoms.     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(0 0 1) via ion beam triangulation and spin-polarized electron capture

The structure and magnetism of thin epitaxial Fe layers grown on Cu(0 0 1) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation with protons. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during scattering of He atoms. For the formation of bcc(1 1 0)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Nano- and micro-scale patterning of Si (1 0 0) under keV ion irradiation

Evolution of Si (1 0 0) surface under 100 keV Ar⁺ ion irradiation at oblique incidence has been studied. The dynamics of surface erosion by ion beam is investigated using detailed analysis of atomic force microscopy (AFM) measurements. During an early stage of sputtering, formation of almost uniformly distributed nano-dots occurs on Si surface. However, the late stage morphology is characterized by self-organization of surface into a regular ripple pattern. Existing theories of ripple formation have been invoked to provide an insight into surface rippling.     

STM observation of initial growth of Sn atoms on Ge(0 0 1) surface

We have studied initial growth of Sn atoms on Ge(0 0 1) surfaces at room temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto the Ge(0 0 1) substrate at room temperature, the Sn atoms form two kinds of one-dimensional structures composed of ad-dimers with different alignment, in the 〈3 1 0〉 and the 〈1 1 0〉 directions, and epitaxial structures. For Sn deposition onto the substrate at 80 K, the population of the dimer chains aligning in the 〈3 1 0〉 direction increases. The diffusion barrier of the Sn adatom on the substrate kinetically determines the population of the dimer chain. We propose that the diffusion barrier height depends on surface strain induced by the adatom. The two kinds of dimer chains appearing on the Ge(0 0 1) and Si(0 0 1) surfaces with adatoms of the group-IV elements are systematically interpreted in terms of the surface strain.     

Methylthiolate-induced reconstruction of Ag(1 1 1): A medium energy ion scattering study

Medium energy ion scattering (MEIS), using 100 keV H⁺ incident ions, has been used to investigate the structure of the Ag(1 1 1)(√7 × √7)R19° -CH₃S surface phase. The results provide the first direct evidence that this structure does involve substantial reconstruction of the Ag surface layer. The measured absolute scattered ion yields and blocking curves are in generally good agreement with a specific structural model of the surface based on a reconstructed layer containing 3/7 ML Ag atoms, previously suggested on the basis of scanning tunnelling microscopy (STM) and normal incidence X-ray standing wave (NIXSW) studies. However, the MEIS data indicate that any rumpling of the thiolate layer, is small, and probably ?0.2 Å. This value is smaller than the amplitude suggested in the STM and NIXSW studies, but could be entirely consistent with the earlier experimental data.     

Novel mesoscale defect structure on NiO(1 0 0) surfaces by atomic force microscopy

Cleaved NiO(1 0 0) surfaces were imaged with atomic force microscopy (AFM) to determine defect concentrations and morphology. Random 〈0 1 0〉 and 〈0 0 1〉 oriented steps, which have been previously characterized, were the most common defect observed on the cleaved surface and formed with step heights in multiples of 2.1 Å, the Ni-O nearest-neighbor distance, and terrace widths in the range of 25-100 nm. In addition, the surface showed novel mesoscale (∼0.5-2 μm) square pyramidal defects with the pyramid base oriented along 〈1 0 0〉 symmetry related directions. Upon etching, the pyramidal defects converted to more stable cubic pits, consistent with (1 0 0) symmetry related walls. The square pyramidal pits tended to cluster or to form along step edges, where the weakened structure is more susceptible to surface deformations. Also, a small concentration of square pyramidal pits, oriented with the base of the pyramid along 〈0 1 1〉, was observed on the cleaved NiO surfaces. For comparison purposes, chemical mechanical polished (CMP) NiO(1 0 0) substrates were imaged with AFM. Defect concentrations were of comparable levels to the cleaved surface, but showed a different distribution of defect types. Long-ranged stepped defects were much less common on CMP substrates, and the predominant defects observed were cubic pits with sidewalls steeper than could be accurately measured by the AFM tip. These defects were similar in size and structure to those observed on cleaved NiO(1 0 0) surfaces that had been acid etched, although pit clustering was more pronounced for the CMP surfaces.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

In situ X-ray analysis of solid/electrolyte interfaces: electrodeposition of Cu and Co on Si(1 1 1):H and GaAs(0 0 1) and corrosion of Cu3Au(1 1 1)

Electrochemical deposition of Cu and Co in monolayer amounts on hydrogen terminated Si(1 1 1) was studied ex situ and in situ by X-ray techniques. The X-ray beam was found to have a strong effect on the deposit causing desorption under the beam. Cu deposition on GaAs(0 0 1) from UHV is compared with electrodeposited Cu on the same surface, elucidating similarities and differences of electrochemical and UHV deposition. Roughening due to corrosion of Cu₃Au(1 1 1) is observed by crystal truncation scattering. The observed behaviour of passivation of this surface is explained by the formation of Au clusters, increasingly covering the surface at higher oxidation potential.     

Water adsorption and dissociation on BeO(0 0 1) and (1 0 0) surfaces

Plateaus in water adsorption isotherms on hydroxylated BeO surfaces suggest significant differences between the hydroxylated (1 0 0) and (0 0 1) surface structures and reactivities. Density functional theory structures and energies clarify these differences. Using relaxed surface energies, a Wulff construction yields a prism crystal shape exposing long (1 0 0) sides and much smaller (0 0 1) faces. This is consistent with the BeO prisms observed when beryllium metal is oxidized. A water oxygen atom binds to a single surface beryllium ion in the preferred adsorption geometry on either surface. The water oxygen/beryllium bonding is stronger on the surface with greater beryllium atom exposure, namely the less-stable (0 0 1) surface. Water/beryllium coordination facilitates water dissociation. On the (0 0 1) surface, the dissociation products are a hydroxide bridging two beryllium ions and a metal-coordinated hydride with some surface charge depletion. On the (1 0 0) surface, water dissociates into a hydroxide ligating a Be atom and a proton coordinated to a surface oxygen but the lowest energy water state on the (1 0 0) surface is the undissociated metal-coordinated water. The (1 0 0) fully hydroxylated surface structure has a hydrogen bonding network which facilitates rapid proton shuffling within the network. The corresponding (0 0 1) hydroxylated surface is fairly open and lacks internal hydrogen bonding. This supports previous experimental interpretations of the step in water adsorption isotherms. Further, when the (1 0 0) surface is heated to 1000 K, hydroxides and protons associate and water desorbs. The more open (0 0 1) hydroxylated surface is stable at 1000 K. This is consistent with the experimental disappearance of the isotherm step when heating to 973 K.     

Scattering effects and damping of electrons in Pt(1 1 1) and Cu(1 1 1)

In this work an analysis of experimental and theoretical data associated with the scattering and attenuation of electrons in the Pt(1 1 1) and Cu(1 1 1) crystalline samples is presented. The information about the crystalline structure of the first few atomic layers was obtained by the directional elastic peak electron spectroscopy (DEPES) at the primary electron beam energies E_p from 1.5 keV to 2.0 keV. The comparison of the experimental and theoretical DEPES distributions indicates a qualitative agreement between experiment and theory. The relative signal values associated with the intensity maxima were found to be different. The latter effect suggest that the electron attenuation in the crystalline samples can have an anisotropic character. A qualitative analysis of the characteristic pattern around the [1 1 1] direction concerning the calculation of the scattering factors was performed. The collective scattering of electrons by atoms located around the threefold symmetry axis resulting in the so called ring focusing effect is discussed.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

Morphological change of D2O layers on Ru(0 0 0 1) probed with He atom scattering

The morphological change of D₂O layers on a Ru(0 0 0 1) surface has been investigated on the basis of He atom scattering. With the increase of D₂O exposure on Ru(0 0 0 1) at 111 K, the intensity of a specularly reflected He beam continuously decreases up to the exposure of 1.0 L (Langmuir). At the D₂O coverage of 1.0 adsorbed layer (∼1.5 L), which is characterized by temperature-programmed desorption measurements, the formation of the (√3 × √3)R30° superstructure as a result of the diffusion of D₂O on the surface was confirmed by He atom diffraction. With the further increase of D₂O exposure, at 2-3 adsorbed layers, the disordered structure was found to be on the surface at 111 K. The morphological change of the disordered layers was observed during annealing, and discussed in detail.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Performance of combined 〈1 0 0〉-〈1 1 0〉 ZnTe crystals in an amplified THz time-domain spectrometer

In this work we report on the performance of combined index-matched ZnTe 〈1 1 0〉-〈1 0 0〉 THz emitters and detectors for reduction of etalon effects in a nonlinear crystal-based THz time-domain spectrometer, driven by an amplified femtosecond laser. We demonstrate that the application of a combined electrooptic detector crystal reduces the strength of the first etalon and the average spectral dynamic range oscillations by a factor of two. We also show that even in a simple emitter configuration the emitter-related etalon is significantly reduced due to the transient dielectric effects of carriers created by two-photon absorption in the emitter.     

Studies on scattering of fast H and He atoms from Fe films grown on Cu(0 0 1)

The structure of ultrathin epitaxial Fe films grown on Cu(0 0 1) at room temperature is investigated by grazing scattering of fast H and He atoms. By making use of ion beam triangulation, direct information on the atomic structure of the film and substrate surfaces are obtained. We discuss a new variant of this method based on the detection of the number of emitted electrons. The data is analyzed via computer simulations using classical mechanics which provides a quantitative analysis with respect to projectile trajectories.