Oxygen adsorption on Ge(111) surface: I. Atomic clean surface

Oxygen adsorption on a clean Ge(111) surface has been studied in the temperature range 300–560 K by means of Auger electron spectroscopy (AES), thermal desorption (TD), work function (WF) measurements, and electron energy loss spectroscopy (ELS). The adsorption and WF kinetics at 300 K exhibit a shape different from those observed at higher adsorption temperatures. At 300 K oxygen only removes the empty dangling bond surface state, whereas at higher temperature new loss transitions involving chemically shifted Ge 3d core levels appear. The findings imply that at 300 K only a chemisorption oxygen state exists on the Ge(111) surface whereas the formation of an oxide phase requires higher temperatures. The shapes of the TD curves show that the desorption of GeO follows $\text{1}\text{2}$ order desorption kinetics.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

The adsorption of nitric oxide on a silicon (100) 2 × 1 surface studied with Auger electron spectroscopy

We present an Auger electron spectroscopy (AES) study of the adsorption of nitric oxide (NO) on a clean Si(100)2 × 1 surface at 300 and 550 K. Accurate measurement reeveal well resolved fine structure at Auger SiL_2.3VV transitions at 62 and 83 eV. These peaks can be attributed to SiO and SiN bonds. Furthermore, it is argued that the broadening in the SiLi_2.3VV Auger transition at 83 eV at 300 K may be composed of two nearby peaks, which could be attributed to two different kinds of chemical bonding, SiN and SiO. The absence of a peak at 69 eV at room temperature strongly suggests the NO adsorption on a Si(100)2 × 1 surface to be molecular. Dissociation of NO on the Si(100)2 × 1 surface is observed at 550 K.     

Orientation of adsorbed ethylene oxide on Ni(110) by X-ray photoelectron diffraction

The adsorption of ethylene oxide on Ni(110) was studied at 95 K and monolayer coverage by angle-resolved X-ray photoelectron spectroscopy. A slow radiation-induced decomposition at hv = 1486.7 eV to most likely methoxy was noted. The orientation of the adsorbed ethylene oxide was determined by measuring forward scattering enhancements in the O 1s intensity distribution. Peaks in polar (θ) as well as azimuthal (φ) scans occurred at four angular positions in 2π above the surface: (θ = 54°, φ = 36°, 144°, 216°, 324°). These positions were evaluated to yield the tilt angle of the molecule at 48°_relative to normal, and the COC bond angle of adsorbed C₂H₄O of about 57°. The molecule is tilted towards the [001] and [001̄] directions (two domains), with a mirror plane in the [001] azimuth.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

Lateral interactions in the thermal desorption of H2 from clean Cu/Ni(110) alloy surfaces

The adsorption of hydrogen on a clean Cu10%/Ni90% (110) alloy single crystal was studied using flash desorption spectroscopy (FDS), Auger electron spectroscopy (AES), and work function measurements. Surface compositions were varied from 100% Ni to 35% Ni. The hydrogen chemisorption on a-surface of 100% nickel revealed strong attractive interactions between the hydrogen atoms in accordance with previous work on Ni(100). Three desorption states (β₁, β₂ and α) appeared in the desorption spectra. The highest temperature (α) state was occupied only after the initial population of the β₂-state. As the amount of copper was increased in the nickel substrate, desorption from the higher energy binding α-state was reduced, indicating a decrease in the attractive interactions among hydrogen atoms. The hydrogen coverage at saturation was not affected by the addition of copper to the nickel substrate until the copper concentration was greater than 25% at which a sharp reduction in saturation coverage occurred. This phenomenon was apparently due to the adsorption of hydrogen on Ni atoms followed by occupation of NiNi and CuNi bridged adsorption sites, while occupation of CuCu sites was restricted due to an energy barrier to migration.     

The adsorption of CO,O2, and H2 on Pt: II. Ultraviolet photoelectron spectroscopy studies

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the chemisorption of CO, O₂, and H₂ on platinum. Three single crystal surfaces ((111), 6(111) × (100), and 6(111) × (111)) and two polycrystalline surfaces were studied. These studies yielded three important results. First, the most dominant change in the Pt valence band upon gas adsorption was a decrease in the height of the peak immediately below the Fermi level. This decrease was nearly identical for all three gases studied. Second, CO adsorption resulted in the formation of a resonance state ～8 eV below the Fermi level which was attributed to CO molecular orbitals. In contrast, no dominant resonance states were observed for adsorbed O or H. The lack of an O resonance state on platinum is in contrast to the results observed for O adsorbed on Fe and Ni and suggests important differences between the OPt chemisorption bond and the OFe and ONi chemisorption bonds. Finally, adsorption of CO at steps or defects led to a decrease in work function while its adsorption on terraces led to an increase in work function. For H, adsorption at steps led to an increase in work function while adsorption on terraces led to a decrease in work function. The adsorption of O led to an increase in work function on all of the surfaces studied.     

Dependence of the superconducting transition temperature on electron per atom ratio and lattice deformation in noble metal alloys

Superconducting transition temperature measurements of the α-phase alloys CuSi, CuGe, AgGe, AgAl, AuGe, and AuZn are reported. It is found that for alloys with the same electron per atom ratio, T_c values are highest when the pure metal lattice is deformed the least by the solute.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

The influence of potassium and the role of coadsorbed oxygen on the chemisorptive properties of Pt(100)

The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θ_K = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the $\text{2π}{}^1$ orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θ_K = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.     

Electron energy-loss spectroscopy of alkali-metal-covered Ge(111) surfaces

Electron energy-loss spectra of a Ge(111) surface covered with Na, K or Cs in the submonolayer range have been measured. The presence of alkali metal (a.m.) causes the empty dangling-bond surface states to vanish and results in the creation of new interface states. The filling of the latter is a decreasing function of the ionicity of the a.m.—Ge bond.It was found that the energy shift of transitions involving a.m. s resonance as the final state is a linear function of a.m. coverage.     

A study of SO2 adsorption on CaO (100) surfaces using X-ray photoelectron spectroscopy

The adsorption of SO₂ on CaO (100) at 300 K has been studied using X-ray photoelectron spectroscopy. Under ultrahigh-vacuum conditions, the surface was exposed to 0–500 Langmuirs of SO₂. The resulting adsorption yields a single SO surface species with an S 2p peak at 168.2 eV and an O 1s_{$\text{sol1}\text{2}$} peak at 531.7 eV. Subsequent heating of the exposed surface to 673 K indicated no desorption or changes in the binding energies of the S 2p and O 1s_{$\text{1}\text{2}$} peaks. On the basis of these data and binding-energy data for standard compounds, the adsorbed species is identified as SO₄^2?. The surface coverage due to the SO₄^2? species was also measured as a function of SO₂ exposure. From these data, the initial adsorption is found to be first-order in surface coverage, and the initial sticking probability is found to have a value of 0.4.     

Oxidation mechanism of hydrogen-terminated Ge(1 0 0) surface

Control of the surface chemistry to prepare a robust termination on the Ge surface is crucial for the development of high-end Ge devices. In this study, oxidation of a H-terminated Ge surface was studied in air ambient and H₂O using a multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR) technique. Ge surface treated in less diluted HF exhibited a stronger Ge-H peak intensity, and the surface was easily oxidized in the air ambient. Therefore, it is believed that the treatment of the Ge surface in highly diluted HF solution has an advantage in suppressing the oxidation of Ge in the air ambient. For the oxidation of Ge(1 0 0) surface in air ambient, the Ge surface is attacked by oxidizing agents to break Ge-H and Ge-Ge bonds, and the transition GeO_x layer is first formed, followed by a layer-by-layer GeO₂ formation with the increase in exposure time. When the H-terminated Ge surface was treated in H₂O, GeO_x was mainly formed, the thickness of the oxide layer was not changed with an increase in treatment time, and the Ge surface was maintained in a suboxide state, which exhibits a different oxidation mechanism from that in air ambient.     

The interference of an electron beam with the surface reaction between oxygen and germanium

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10^?7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm^?2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10^?7 Torr min, the maximum oxide thickness was about 18 Å. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 Å), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.     

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.     

Interaction of Co with an Fe(111) surface

Adsorption of CO on Fe(111) below 300 K causes the appearance of three different non-dissociated species as distinguished by their CO stretch frequencies of about 1530 cm^-1 (a), 1800 cm^-1 (b), and 2000 cm^-1 (c). At T ? 220 K the b-state is first filled up and saturates after 1.5 L exposure; upon increasing the temperature it partly desorbs around 400 K and partly dissociates. Recombination of the C and O atoms followed by CO desorption takes place at about 800 K. Above 1.5 L exposure the a- and c-states are occupied simultaneously; in the thermal desorption spectrum in turn they show up as a relatively broad shoulder at ～ 340 K, which indicates similar adsorption energies for these two species. Saturation of the surface is reached after about 6 L exposure, which is paralleled by a continuous work function increase of up to Δφ = 1.6 eV. A high background intensity in the LEED pattern suggests substantial disorder in the adlayer. Evaluation of the TDS data yields about 2:1 population of the b- and (c + a)-states. The unusual low CO frequency of the a-state finds its analogues in reports on CO adsorption at stepped surfaces, as well as with complex compounds where the π-orbitals of the ligand directly interact with a neighboring metal atom. This species is therefore identified with adsorption in the “deep hollow” sites on the rather open Fe(111) surface. The b-state is tentatively attributed to the “shallow hollow” sites, and the c-state to adsorption on the “on top” sites.     

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH₂CH₂COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH₂CH₂COO⁻). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al-O layer.     

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: II. CO adsorption onto Al

Adsorption of carbon monoxide on aluminium is studied by the combination of in situ evaporation of Al and the Auger electron spectroscopy. New Auger signals which are attributed to interatomic Auger transitions in Al₄C₃ type AlC bonds are found. It is revealed that two adsorption stages exist in the adsorption process, i.e., that after the formation of the AlC bond takes place, the adsorbed layer consisting of AlC and AlO bonds is formed.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.