Initial growth of platinum on oxygen-covered Ni(1 1 0) surfaces

The initial growth of Pt on the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) surfaces has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Prior to Pt deposition, the atomic structure of the near-surface regions of the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) structures were studied using CAICISS, finding changes to the interlayer spacings due to the adsorption of oxygen. Deposition of Pt on the Ni(1 1 0)-(3 × 1)-O surface led to a random substitutional alloy in the near-surface region at Pt coverages both below and in excess of 1 ML. In contrast, when the surface was treated with 1800 L of atomic oxygen in order to form a NiO(1 1 0) surface, a thin Pt layer was formed upon room temperature Pt deposition. XPS and LEED data are presented throughout to support the CAICISS observations.     

Characterisation of the interaction of glycine with Cu(1 0 0) and Cu(1 1 1)

Reflection-absorption infrared spectroscopy (RAIRS) has been used to characterise the interaction of standard and fully deuterated glycine with Cu(1 0 0) and Cu(1 1 1). RAIRS shows clearly that the surface interaction leads to formation of the adsorbed deprotonated glycinate (NH₂CH₂COO-) species, with some evidence for changes in orientation with coverage previously seen on Cu(1 1 0). Qualitative low energy electron diffraction observations were also conducted to characterise the long-range ordering, although effects of electron-beam-induced radiation damage limited the information obtained. Nevertheless, the results do suggest some subtle isotopic-mass-related structural variations. The results are discussed in the context of previously published scanning tunnelling microscopy and photoelectron diffraction measurements.     

Defects at the TiO2(1 0 0) surface probed by resonant photoelectron diffraction

We report photoelectron diffraction (PED) experiments of weakly sub-stoichiometric TiO₂(1 0 0) rutile surfaces. Apart from standard core-level PED from the Ti-2p3/2 line, we have studied valence band PED from the defect induced Ti-3d states in the insulating band gap. For maximum yield, the latter were resonantly excited at the Ti-2p absorption edge. The PED patterns have been analyzed within the forward scattering approximation as well as by comparison with simulated PED patterns obtained in multiple scattering calculations. The analysis shows that the defect induced Ti-3d charge is mainly located on the second layer Ti atoms.     

The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability

The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe₂O₃ (820 K). The Fe₂O₃ mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Fe-based spin-injection may play a vital role for ZnO-based spintronic devices.     

Study of the surface structure of Si(1 1 1)-6 × 1(3 × 1)-Ag using X-ray crystal truncation rod scattering

The structure of the Si(1 1 1)-6 × 1-Ag surface is investigated using crystal truncation rod (CTR) scattering along 00 rod. For the measurement, we developed a manipulator suitable for observing CTR scattering at large momentum transfer perpendicular to the surface. The heights of the silver and reconstructed silicon atoms from the substrate were determined. We also compared the obtained positions with those of the Si(1 1 1)-√3 × √3-Ag surface and found that the heights of those reconstructed atoms are almost the same.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Photoemission from the valence bands of Ce(1 1 1) on W(1 1 0)

We report a photoelectron spectroscopic study of the valence bands of epitaxial Ce(1 1 1) films grown on W(1 1 0) at room temperature. The evolution of γ → α → γ like phase transition of Ce is observed with increasing Ce coverage and the valence-band structures of γ-like Ce film are determined. The 4f and 5d photoemission cross sections in the photon energy region from 20 eV to 130 eV are presented and discussed. A 5d-like surface state and a 6s band bottom are identified.     

The structures of physisorbed and chemisorbed formic acid on Si(1 1 1)-7 × 7

The reaction of formic acid on Si(1 1 1)-7 × 7 was investigated using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The hydroxyl and carbonyl O 1s core levels of chemisorbed formic acid display chemical shifts of 2.4 and 0.2 eV respectively, compared with those of physisorbed molecules. The HREELS spectra of chemisorbed formic acid show the absence of stretching and bending modes of the O-H bond, the appearance of Si-H (2089 cm⁻¹) and the Si-O (680 cm⁻¹) stretching modes and the retained stretching mode of CO at 1703 cm⁻¹. Our results clearly suggest that formic acid dissociates to form monodentate formate species and H-atom on the adatom-rest atom pair of Si(1 1 1)-7 × 7.     

Coordination-dependent valence state of Sm adsorbed on Cu(1 0 0) and (1 1 0) studied by STM and XPS

Geometric and electronic structures of Sm adlayers on Cu(1 0 0) and (1 1 0) were studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The present results, in addition to the previous results for Sm/Cu(1 1 1) [Y. Nakayama, H. Kondoh, T. Ohta, Surf. Sci. 552 (2004) 53], indicate that the valence state of Sm adsorbed on Cu surfaces is determined by Sm-Sm and Sm-Cu coordination numbers. We propose that the valence state of the adsorbed Sm atoms can be explained by a simple thermodynamic equation including the coordination numbers.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

Geometric and electronic structure of positively and negatively poled LiNbO3 (0 0 0 1) surfaces

The effect of ferroelectric poling direction on the structure and electronic properties of the LiNbO₃ (0 0 0 1) surface was characterized. Low energy and reflection high energy electron diffraction indicated that both the positively and negatively poled surfaces were (1 × 1) with no evidence of longer range periodic reconstructions. Low energy ion scattering spectra from both surfaces were dominated by scattering from oxygen atoms. X-ray and ultraviolet photoelectron spectra also showed little difference between the positively and negatively poled surfaces, with the exception of a high binding energy shoulder on the O 1s core level of the negative surface. Exposure of the surfaces to atomic hydrogen caused reduction of the surface Nb rather than an increase in intensity on the high binding energy side of the O 1s peak, indicating that the shoulder on the O 1s peak on the negative surface was not due to surface hydroxyl groups. Temperature programmed desorption measurements indicated that the nearly stoichiometric LiNbO₃ samples were susceptible to loss of Li₂O starting at temperatures as low as 500 K, independent of the poling direction. An adatom/vacancy model is proposed in which oxygen ad-anions accumulate on one side of the crystal while oxygen anion vacancies are created on the opposite surface. This model can explain the apparent oxygen termination of both surfaces and the observed (1 × 1) periodicity of the surfaces, and also effectively screens the thickness dependent electric field associated with the polar orientation of the crystal.     

Temperature dependent quantum well state on Cu(1 1 0)(2 × 1) striped surface studied by angle resolved photoelectron spectroscopy

We have investigated the temperature dependence of angle resolved photoelectron spectroscopy for the lateral quantum well states (QWS) on the striped Cu(1 1 0)(2 × 1)O surface. For the striped surface with oxygen coverage of 0.25 ML, we have successfully observed two discrete levels along a perpendicular direction to the stripes in the surface Brillouin zone, which is generated by quantization of the Shockley surface state. We have found that the relative photoelectron intensity of the two discrete levels depends on the temperature. The photoelectron intensity tends to concentrate on the ground level of the QWS with decreasing temperature. Our investigation indicates that the electron population of each quantum well level depends on the temperature.     

XPS and UPS study on band alignment at Pt-Zn-terminated ZnO(0 0 0 1) interface

The interface between Pt and Zn-terminated ZnO(0 0 0 1) was investigated by X-ray and ultra violet photoelectron spectroscopy in order to examine electronic band alignment. An angle-resolved X-ray photoelectron spectroscopy measurement of the clean ZnO(0 0 0 1) surface has revealed a downward band bending by 0.25 eV. The results of the valence band analysis show that the work function and the valence band maximum of clean ZnO(0 0 0 1) were 4.49 eV and 2.79 eV, respectively. Platinum was then deposited in several deposition steps onto a clean ZnO(0 0 0 1) surface up to 0.6 nm in thickness. After the deposition, the binding energy of Zn 2p doublet peak was shifted towards lower value by 0.77 eV, and the measured work function changed to 5.51 eV. As a result, the Schottky barrier height of Pt/ZnO(0 0 0 1) interface was 1.11 eV.     

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.     

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).     

Role of final states in photoemission from Al(1 1 1)

Enhancement of surface state peaks in angle resolved ultraviolet photoelectron spectra (ARUPS) from the Al(1 1 1) surface is studied experimentally and theoretically within the one-step model of photoemission. The resonant enhancement of the surface state emission is explained by the crucial role of elastic scattering of the outgoing electron. Dipole transitions to evanescent states in the final bands of the crystal are shown to determine photoemission at the resonant photon energy. The band structure based explanation is confirmed by the measurements of electron reflectivity and of the fine structure of valence band spectra. The surface sensitivity of ARUPS is shown to depend strongly on the complex band structure of the crystal and to be finely tunable by the choice of photoemitted electron energy.     

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.     

Epitaxial SiC formation induced by medium energy ions on Si(1 1 1) at room temperature

In the search for silicon technology compatible substrate for III-nitride epitaxy, we present a proof-of-concept for forming epitaxial SiC layer on Si(1 1 1). A C/Si interface formed by ion sputtering is exposed to 100-1500 eV Ar⁺ ions, inducing a chemical reaction to form SiC, as observed by core-level X-ray photoelectron spectroscopy (XPS). Angle dependent XPS studies shows forward scattering feature that manifest the epitaxial SiC layer formation, while the valence band depicts the metal to insulator phase change.     

FeO(1 1 1) formation by exposure of Fe(1 1 0) to atomic and molecular oxygen

The growth, structure, and morphology of ultrathin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, low energy electron diffraction, and grazing ion scattering. For Fe oxidation by atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude because surface sticking and dissociation are not limiting the growth process. A well-ordered FeO(1 1 1) film with low defect density is only obtained with atomic oxygen. Compared to the bulk, the FeO lattice is laterally compressed by about 5-6% resulting in an in-plane oxygen (Fe) nearest-neighbor distance of 2.87 Å. Independent of the preparation method, long-range structural order is poor if the oxide film thickness is increased to 3-5 layers. This is attributed to the relatively large lattice mismatch between FeO(1 1 1) and Fe(1 1 0).     

Enhanced reactivity and selectivity in oxidation of Cu(1 0 0) and α-Cu-Al(5 at.%)(1 0 0) surfaces studied by electron and ion spectroscopies

In this study we investigate the influence of alloying on the reactivity and bonding of oxygen on α-Cu-Al(5 at.%)(1 0 0) oriented single crystal surfaces by X-ray photoelectron spectroscopy (XPS), ultra-violet spectroscopy (UPS) and low energy ion scattering (LEIS) spectroscopy, at room temperature. It was found that alloying results in an enhanced reactivity of both Cu and Al sites in comparison with the pure metals. According to adsorption curves calculated from XPS, saturation of the alloy surface occurs for exposures of ∼15 L. At saturation the total amount of adsorbed oxygen is similar for the alloy and pure copper surfaces. It was determined that first mostly Al sites are oxidized, followed by simultaneous oxidation of Cu and Al sites. At saturation the amount of oxygen bonded to Cu sites is ∼1.7 larger then that bonded to Al sites. From a comparison of the XPS and LEIS data analysis as a function of oxygen exposure it was found that oxidation of α-Cu-Al(5 at.%)(1 0 0) alloy is a multi-stage process with fast and slow stages. These stages involve an interplay of chemisorption, sub-surface diffusion of oxygen and Al segregation. UPS measurements show an increase in the work function of the alloy surface with oxygen adsorption. This is a contrast to pure Cu surfaces where the work function decreases at the initial stages of oxidation followed by an increase with oxygen exposure. Annealing to 400 °C drives the oxidized alloy surface into its thermodynamic state resulting in the formation of an aluminum oxide layer. Possible mechanisms to explain the enhanced reactivity of the alloy surface compared to that of pure copper are suggested and discussed.