A surface analysis of CuAu alloy by leed and Auger electron spectroscopy

Surface structures and compositions of the CuAu alloys have been investigated, which were prepared by depositing gold on (110) and (111) surfaces of copper and by subsequent heating. By this method the structure of alloy surfaces corresponding to different compositions can be observed by LEED. A series of the LEED patterns, streak, (1 × 2), (1 × 1)I, complex, c(3 × 1), (1 × 1)II, (2 × 2) and (1 × 1) have been observed on the (110) surface with decreasing gold composition. On the (111) surface (1 × 1) pattern, weak (2/√3 × 2/√3)R30° and (2 × 2) patterns are observed. The mean surface composition is determined by analysing the data of Auger electron spectroscopy. Most surface periodicities observed are different from those expected if one passes a mathematical plane through the crystal (unreconstructed surface).     

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

LEED-AES study of the AuSi(100) system

The system Au/Si(100) has been studied using LEED and AES. Au films grow as Au(111) | Si(100) having six azimuthally rotated orientations at low deposition temperatures below 50°C after the formation of intermediate gold suicide layers. Crystalline gold silicide thin layers are formed on the Au(111) film after heat treatment at 100–400°C. Two types of suicide LEED pattern observed seem to have no correlation with crystallographic data reported on quenched alloy films. Heat treatment over 450°C leads to agglomeration of the film, producing a series of Au-induced superstructures. Heat treatment of the Au film over 1000°C regenerates the clean Si surface accompanied with many etch pits.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Adsorption of oxygen on clean single crystal faces of aluminium

Auger electron spectroscopy and work function measurements have been used to study the interaction of clean Al(111) and Al(100) faces with oxygen at low pressure near room temperature. The results for the two faces differ strongly. Thus, the sticking probability of the (111) face decreases rapidly with coverage, while the work function increases slightly, by 0.1 eV at 200 L. In contrast, the sticking probability of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5–0.8 eV. The shape of the Al L_{2, 3}VV spectrum from oxidized Al(100) is independent of coverage, and it is in fact very similar to previously reported spectra from oxidized polycrystalline aluminium. The corresponding spectrum from Al(111) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 Å) islands of Al₂O₃-like oxide form on the (100) face.     

First stages of the deposition of bismuth on copper examined by LEED: I. The (100) substrate

The different stages in the formation of monolayers of chemisorbed bismuth on the (100) face of copper have been followed by low-energy electron diffraction and Auger spectroscopy. The various LEED patterns obtained after evaporation of bismuth were studied from a structural viewpoint and also for their thermal behaviour. Measurements were made of diffracted intensities as a function of temperature, I(T). These measurements are part of a general study of the use of I(T) plots for characterizing adsorbed layers. Four well ordered structures are observed. The final one is interpreted in terms of a compact dense monolayer which has a quasi-hexagonal close packed arrangement - the interpretation illustrates the usefulness of optical transform methods for certain LEED patterns from adsorbed overlayers. None of the structures exhibits a melting-type transformation in the temperature range up to 400°C, although there is evidence for a solid-solid transformation. The behaviour of this system is compared with that of lead on copper (sharp melting transformations) and lead on nickel [some evidence of melting, but only on the (111) face]. Observations on the first stages of growth and epitaxy of bismuth crystallites on the (100) face are also reported.     

Electronic properties of cleaved Si(111) upon room-temperature deposition of Au

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) measurements are performed on a set of 2 × 1 reconstructed silicon (111) surfaces with different bulk dopings as a function of gold coverage θ, from zero to a few hundred monolayers, obtained by UHV evaporation on a sample kept at room temperature. Our measurements show the formation of an Au-Si alloy with the first two monolayers of gold deposit which induces a decrease of the ionization energy Φ by about 0.15 eV while no variation of the work function is observed. In the effective density of states, the double structure related to the 2 × 1 reconstruction is then replaced by a single peak at ? 0.4 eV below the valence band edge. At larger coverages, the Au-Si alloy remains on top of a gold layer which forms an abrupt interface with the Si substrate.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

The surface composition of the gold-palladium binary alloy system

The surface composition of the AuPd alloy system has been determined by Auger electron spectroscopy. Measurements were performed on polished polycrystalline alloy foils. After cleaning, the intensities of the 71 eV and 2024 eV gold Auger transitions, and the intensity of the 330 eV palladium transition were measured, and then converted to atom concentrations in the surface layer. The surfaces of the annealed samples were found to be significantly enriched in gold with respect to the bulk. This result disagrees with the regular solution theory prediction. After extensive sputtering of the AuPd alloys with 1.5 keV Ar⁺ ions, a slight surface enrichment with palladium was found, as predicted by the simple theoretical model for sputtering.     

Structural and electronic model of negative electron affinity on the Si/Cs/O surface

Structural and electronic models are proposed which correlate Goldstein's LEED, Auger, photo-emission, plasmon, and desorption data for negative electron affinity (NEA) on Si(100) surfaces. In the structural model, the surface Si atoms group into adjacent rows of surface “pedestals” and surface “caves”. Their density is 3.4 × 10¹⁴ cm^?2 each, as inferred from the LEED 2 × 2 reconstruction pattern and other data. Adsorbed Cs resides in fourfold coordination with Si atop the pedestals. Adsorbed oxygen is completely submerged in the caves of aperture 2.98Å to give a Cs-O dipole length of 2.9Å. Similar structural arguments show why Cs must be adsorbed before O₂, and why Si(111) does not exhibit NEA. In the electronic model, the surface dielectric constant, 5.3. obtained from the surface plasmon energy, 7 eV, is used to compute the dipole length from the final work function, 0.9 eV. It is 2.8Å in excellent agreement with the dipole length computed from the above structural model. Some properties of the “induced” surface states in the presence of Cs and O are also described.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

On the rotation of non-epitaxy crystallites on single crystal substrate

The experimental results of Masson et al. [Surf. Sci. 27 (1971) 463] were further interpreted with regard to the rotation behavior of non-epitaxy Au crystallites on a KCl(100) substrate, which developed into primary epitaxy ([111]_Au 6 [100]_KCl; [11&#x0304;0]_Au 6 [010]_KCl), and “second” epitaxy (in fact [100]_Au 6 [100]_KCl; [010]_Au 6 [010]_KCl and [100]_Au 6 [100]_KCl; [010]_Au 6 [011]_KCl) upon annealing at 94°C. These epitaxy orders were clarified to be nonsequential events on the basis of the development of electron diffractions (220), (111) and (200) of Au. The “second” epitaxy can be attributed to the later formation of a (100)_Au|(100)_KCl contact in comparison with the preferred (111)_Au|(100)_KCl contact in the as-deposited state. The Au crystallites with irrational contact plane should have rotated and changed shape before bifurcating into (111)_Au|(100)_KCl and (100)_Au|(100)_KCl contact, upon which viscous rotation was still feasible, until the primary and “second” epitaxy were reached, respectively.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

Optical properties of high-Al-content crack free AlxGa1−xN (x<0.67) films grown on Si(111) by molecular-beam epitaxy

We report on the optical properties of high-Al-content crack free Al_xGa_1−xN (x<0.67) films grown by molecular-beam epitaxy on Si(111) substrates using ammonia as nitrogen source. The energetic position of the A free exciton as a function of the Al content is determined from photoluminescence and reflectivity measurements at low temperature. A bowing parameter of b=1 eV is deduced from these measurements. The excitonic linewidth increases as a function of Al concentration. The observed variation agrees very well with the one calculated using a model in which the broadening effect is assumed to be due to alloy compositional disordering.     

Catalytic activity of Pd ensembles incorporated into Au nanocluster for CO oxidation: A first-principles study

We investigated the catalytic activity of Pd atoms incorporated into Au(111) facet through first-principles calculations, and found that the Pd monomer, dimer, and trimer are highly reactive for the reaction of CO+O₂→CO₂+Ovia association mechanism, in which an intermediate state (OOCO) is formed. Significantly, a low energy barrier (0.19-0.32 eV) was found for the formation of OOCO. The atomic oxygen left by CO+O₂→CO₂+O reaction can be removed by another CO on Pd-decorated Au cluster via Langmuir-Hinshelwood or Eley-Rideal mechanism. Our studies indicate Pd ensembles incorporated into Au(111) facet markedly improve the catalytic activity of gold nanocluster.     

The chemisorption of sulphur on W(100)

The interaction of sulphur vapour with a W(100) surface is studied in detail with Auger Electron Spectroscopy (AES), LEED, work function difference (Δ?) measurements and thermal desorption spectroscopy (TDS). The dissociative adsorption of S occurs on the W surface without reconstruction. Several LEED structures are observed which indicate repulsive adatom interactions. TDS shows that the desorption energy of atomic S decreases from about 8 eV at θ = 0.1 ML to about 3 eV near saturation in close vicinity of 1 ML. Above $\text{θ =}\text{3}\text{4}$ ML, S₂ desorbs in addition to S in a high temperature peak which saturates at about 1 ML. Sulphur in excess of about 1 ML is desorbed in two low temperature peaks of which the lower one consists not only of S and S₂ but also of S₃ and S₄.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

Segregation at CuAu alloy surfaces

Experimental results for Auger measurements on clean evaporated CuAu alloy films having (111)-orientation are presented. Signals from Auger transitions at 72 eV, 239 eV, and 2024 eV from Au in the alloys were normalized to signals from pure Au references. The experimental data were converted to atomic layer compositions using a model which allowed the first two atomic layers to differ in composition from the bulk and using estimates of the energy dependence of the electron attenuation length derived from published results. Significant enrichment of the first layer with Au was found over the entire range of composition studied.