Interaction of aluminum with iridium surface: Adsorption, desorption, dissolution, and formation of surface compounds

The interaction of aluminum with an iridium (111) surface was studied in ultrahigh vacuum by Auger electron spectroscopy over the broad temperature range 300–2000 K. At room temperature, layer-by-layer growth of an aluminum film was observed, with a monolayer forming in coherent relation to the substrate. Deposition at 1100–1300 K gives rise to the formation of surface aluminide Ir₄Al with an adatom concentration N _Al = (4.20 ± 0.15) × 10¹⁴ cm^?2. It was shown that aluminum escapes out of the surface aluminide by thermal desorption in the 1300–1700-K temperature interval, with the desorption activation energy changing from ～4.5 to ～5.7 eV as the coverage decreases from the value corresponding to the surface aluminide (taken for unity) down to zero.     

The Pt(110) phase transitions: A study by rutherford backscattering,nuclear microanalysis,LEED and thermal desorption spectroscopy

The adsorbate induced (1×2) (1×1) (2×1)p1g1 phase transitions on Pt(110) have been studied by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED and thermal desorption spectroscopy. RBS data indicate that any displacement of the surface atoms from their expected bulk-like lattice sites in the (1×2) phase is ? 0.002 nm laterally and ? 0.007 nm vertically. This contraint eliminates models for the reconstruction which involve significant lateral displacements (e.g., the paired-atom or hexagonal overlayer models). The RBS data are consistent with both the rumpled model with up/down displacements not exceeding ~0.007 nm and the missing row model with an unrelaxed surface in which the out-of-plane vibrational amplitude is slightly enhanced. A c(8×4) phase, produced by CO (or NO) exposure at T?250 K, has also been characterized by RBS which demonstrated that 0.92×10¹⁵ Pt cm^?2 move on average by ~0.017 nm laterally out-of-registry with the bulk upon formation of this phase. The values of the saturation adsorbate coverages at T?200K were determined by NMA to be 0.92 ± 0.05×10¹⁵, 1.0 ± 0.06×10¹⁵ and 1.07 ± 0.10×10¹⁵ CO molecules, NO molecules and D atoms, respectively, per cm². The value of the saturation coverage by CO (θ = 1.0) supports recent models of the (2×1)p1g1 overlayer. The isosteric heat of adsorption of CO is 160 ± 15 kJ mol^?1 in the range 0.2?θ?0.5.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Adsorption of sodium and its effect upon some electric properties of clean germanium (111) surfaces

The effect of adsorbed Na on the surface conductivity, Δσ, and surface recombination velocity, S, of a clean (114)Ge surface is studied. The surface conductivity is a complicated function of the surface Na concentration, N_Na; at N_Na ≈ 1.5 × 10¹³ atoms/cm², it has a minimum; at ca. (3–5) × 10¹⁴atoms/cm², it has a maximum. For a monolayer coverage (ca. 7.2 × 10¹⁴atoms/cm²) the values of Δσ are not much different from those of a clean Ge surface. The surface recombination velocity is a three-valued function of the surface potential, U_S (calculated from the Δσ values), depending on the Na overlayer coverage and heat treatment of the sample. Three different surface structures (LEED data) were found to correspond to the three S versus U_S curves reported here. Thermal desorption studies show that Na is desorbed in a wide temperature interval. Two peaks have been isolated, studied and discussed. At low coverages a single peak is found to exist, which obeys the first-order desorption kinetics, with a desorption energy of (52 ± 3)kcal/mol. This peak is attributed to the surface defects. For coverages close to$\text{1}\text{4}$ monolayer a new peak was observed in the spectrum. The desorption energy of this binding state exceeds that of all the other states. When the overlayer coverage is increased, this peak is shifted to higher temperatures, as predicted for a half-order desorption kinetics. By comparing also with LEED data, it may be concluded that this most tightly bound sodium has formed on the Ge(111) surface patches of an ordered structure in which one Na atom is bonded to three Ge atoms.     

The interaction of Ag with Si(111)

Deposits of Ag on Si(111), at room temperature, have yielded a linear Auger signal-time characteristic to a gradient break point at (7.6 ± 0.9) × 10¹⁴ atoms ofAg cm^?2, which is very close to the Si surface state density of (8–10) × 10¹⁴ cm^?2, and which supports a Stranski-Krastanov growth mechanism. Analysis of the Auger spectra at the monolayer end point revealed a new peak at 82 ± 1 eV. This peak is believed to arise from an Auger process involving an induced Ag-Si interface state. A model is proposed for this state arising from the chemisorption of Ag on Si.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

The adsorption of Xe and CO on Au(100)

The adsorption of Xe and CO on Au(100) has been studied by LEED, Auger electron spectroscopy, electron energy loss spectroscopy (EELS) and surface potential measurements. The physical adsorption of xenon showed successive stages preceding the completion of a monolayer. The heat of adsorption was 22 (±2) kJ mol^?1 and the maximum surface potential was 0.45 V. Carbon monoxide gave a surface potential of 0.85 V at the highest coverage reached. The heat of adsorption showed a continuous fall from an initial value of 58 (±3) kJ mol^?1 as the coverage increased. Ordered adsorption structures were not observed in LEED for either Xe or CO. The EEL spectrum of clean Au(100) agreed well with spectra of polycrystalline gold. New loss features observed with adsorbed Xe and CO are discussed.     

Chemisorption of nitrogen on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-absorption infrared spectroscopy has been combined with thermal desorption and surface coverage measurements to study nitrogen adsorption on a {111}-oriented platinum ribbon under ultrahigh vacuum conditions. Desorption spectra show a single peak (at 180 K) after adsorption at 120 K, giving a coverage-independent activation energy for desorption'of ～40 kJmol^?1. The initial sticking probability at this temperature is 0.15, and the maximum uptake was ～1.1 × 10¹⁴ molecule cm^?2. The adsorbed nitrogen was readily displaced by CO, h₂ and O₂. An infrared absorption band was observed with a peak located at 2238 ± 1 cm^?1, and a halfwidth of 9 cm^?1, with a molecular intensity comparable to that reported for CO on Pt{111}. The results are compared with data for chemisorption on other group VIII metals. An earlier assignment of infrared active nitrogen to B₅ sites on these metals is brought into question by the present results.     

The reactive adsorption of CCl4 on GaAs(100) surfaces at 300 K

Auger electron spectroscopy has been used to monitor the adsorption of CCl₄ on an As-rich GaAs(100) surface at 300 K. Intensities of the Ga (55 eV), As (34 eV), C (270 eV) and Cl (181 eV) transitions have been used to estimate surface number densities at saturation and relative C : Cl stoichiometry of the surface species. Number densities of (4.3 ± 0.2) × 10¹⁴ and (2.0 ± 0.2) × 10¹⁴ cm ⁻² are obtained for carbon and chlorine respectively, suggesting that coverage saturates near one theoretical monolayer and that the C : Cl stoichiometry is approximately 2:1. These data are discussed in terms of a reactive adsorption mechanism.     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

L'adsorption du bismuth sur des films minces d'or etudiée par variation de résistance electrique

Adsorption of bismuth on gold thin films is studied by electrical resistance variations. At low coverage the increase of gold resistivity due to Bi adatoms is 2.3 ± 0.1 μΩ cm/at% and is independent of temperature and thickness of the Au layers. At 20° C and ?150°C, the shape of the resistance variation curve for increasing Bi coverage indicates that the adatoms form a first monolayer having a higher density at lower temperature. At 85°C two Bi monolayers are made successively: the first is adsorbed on the free surface of the gold film; the second grows on the other face, between the gold and the glass substrate, after migration of the Bi atoms through the grain boundaries or other defects. These results are verified by Auger electron spectroscopy. They are used to show that the initial reflectivities for conduction electrons on the two surfaces are similar in recrystallized gold films; the specularity coefficients P and Q have been evaluated using the Fuchs theory. For clean surface, the electron reflection is large specular (P ? Q ? 0.75 ± 0.05) and becomes entirely diffuse when a monolayer of bismuth is adsorbed on each face of the gold film.     

X-ray photoemission study of physically absorbed SF6

The physical adsorption of octahedral SF₆ on Ru(001) has been studied with X-ray photoelectron spectroscopy (XPS) in an attempt to see effects on the energy levels resulting from the conformation of the molecule on the surface. Near 80 K surface coverages up to a monolayer have been studied at various steady state pressures of SF₆. Kinetic studies, core level binding energies, and peak areas indicate that the surface species studied was a physically adsorbed monolayer of sf₆. The sticking coefficient of SF₆, at ? 80 K is approximately unity. Also, a multilayer structure was observed at the highest pressures of SF₆. The binding energy of the F(ls) peak for monolayer coverage is centered at 688.2 ± 0.2 eV relative to the Ru Fermi level. while the multilayer F(ls) peak is shifted more than 3.5 eV to higher binding energy. The F(ls) linewidth for one monolayer has a full width at half maximum of 1.75 ± 0.1 eV. The F(ls) linewidth of the multilayer peak narrows with increasing coverage. Its narrowest observed linewidth was 1.35 eV ± 0.1 eV or approximately the same as that found in the gas phase. One of the mechanisms which may account for the F(ls) linewidth with monolayer coverage is a difference in F(ls) binding energy between those F atoms in contact with the substrate and those further away. This may be due to the variation in chemical environment and relaxation effects as a function of distance from tlie substrate. A classical image force calculation including finite screening effects of the substrate indicates that there is a differential binding energy, ΔW. between the F ligands; ΔW = 0.85 ± 0.25 eV, for realistic ranges of adsorption distances from the substrate and screening lengths in the substrate. The observed broadening of the monolayer F(ls) level is consistent with a ΔW of 0.7 ± 0.1 eV, indicating the possible existence of such a mechanism. Adsorption of a monolayer of SF₆ onto the Ru covered with a monolayer of oxygen shifts the F(ls) peak to lower binding energy by 0.8 eV. Similar effects due to oxygen have been observed previously in the physical adsorption of Xe on W(111).     

Adsorption of lead on low-index planes of tungsten

Using probe-hole field emission microscopy the effect of adsorbed lead on the work function of the 100 and 110 planes of tungsten hasbeen studied and compared with the findings of Bauer et al. who studied the same system using LEED/Auger techniques. The effect of lead on the average work function $\text{}\text{?}$gf and that of (211) is also reported. Sub-monolayer lead increases φ(211) and this is ascribed to formation of a lead-tungsten dipole, the lead being negatively charged, with dipole moment 0.035 × 10^?30 C-m and polarizability 2.0 ų. On (110) lead reduces φ and behaves as a dipole with positively charged lead of moment 0.15 × 10^?30 C-m and polarizability 2.5 ų. φ(100) is also observed to decrease at low coverages equilibrated at low temperatures. This contrasts with Bauer's findings but is considered to result from failure of the Fowler—Nordheim model. With increasing lead coverage on all planes φ(hkl) tends to a constant value φ_sat. By comparison with Bauer et al. we can identify φ_sat on (110) as a compressed monolayer of lead. Likewise φ_sat produced by low temperature (～450 K) spreading on (100) is also associated with a compressed (1 × 1) structure. The lower value of φ(100) produced at higher temperatures (～850 K) is identified with the microfacetted surface observed by Bauer et al. Lead is observed to be absent from (110) when mean adatom densities as high as 8 × 10¹⁴ atoms cm^?2 are thermally equilibrated, and this is shown to result from the relatively low binding energy of lead on (110). The general agreement between the present findings and those of Bauer lends confidence to the belief that both techniques can detect the same behaviour despite the very large (10¹⁰) difference in the size of the area examined.     

Adsorption and epitaxial growth of benzene on ZnO(1010) studied by thermal desorption spectroscopy,LEED and UPS

The adsorption and condensation of benzene on ZnO(101&#x0304;0) was investigated by thermal desorption spectroscopy and LEED. The first monolayer shows an ordered c(2 × 2) super-structure. First order desorption is observed. The desorption energy and frequency factor decrease from 73 to 56 kJ mole^?1 and from ~10¹⁵ to ~10¹² s^?1, respectively, with coverage increasing to 0.85. The second layer is more weakly bound. Two-dimensional (2D) island formation is deduced from peak shape analysis. Near completion, the second layer converts to a more tightly bound configuration as deduced from a sudden shift of the desorption peak and the formation of an additional c(4 × 3) LEED pattern. This pattern which can be identified as a property of bulk benzene is preserved upon epitaxial growth of the 3D benzene crystal. Angular resolved UPS measurements indicate the benzene molecules of the first layer to be arranged in an oblique position of low symmetry.     

Absolute coverages for the various surface phases of CO and O adsorbed on Pd(110)

The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the ¹²C(d, p)¹³C and ¹⁶O(d, p₁)¹⁷O¹ reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 10¹⁴ CO molecules cm⁻²) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML.     

A field emission study of silver on rhenium

Field emission measurements of the change in average work function ?f of rhenium with adsorbed silver indicate that a rhenium-silver dipole forms with silver positive, of moment μ₀=5.2±1.5 ×10^?30 C m and polarizability $\text{α=29±12}\text{A}\text{?}{}^3$. Measurement of the rate of thermal desorption yields a mean binding energy of 2.31 ± 0.04 eV for sub-monolayer silver and 2.69±0.04 eV for a 2.5 monolayer deposit. Changes in work function induced by adsorption of silver on low-index rhenium plane surfaces are characterised by the formation of well-defined states and in this, silver resembles gold. These states are thought to result from a relatively large difference between the binding energy of adatoms on the low-index planes and on the surrounding surfaces, and this differnce is maintained when the surfaces are covered with silver. At the lowest coverage, silver is believed to be absent from all four observed planes and the measured rise in work function is thought to be apparent and to result from a decrease in field strength on the plane due to extension of the plane area by surrounding adsorbed silver. The structures adopted by silver overlayers are not known, but it is argued that on (101?0) and (101?1?) the final state at high coverage has the Ag(111) surface structure. On (112?0) and (112?2?) the silver layer at high coverage is thought to have either Ag(110) or Ag(100) surface structure. The structures of intermediate states found on all four low-index planes remain unkown. Field emission spectroscopy shows that emission from clean (101?0) is free-electron like and confirms earlier observations that emission from (202?1) is not. Spectroscopy also reveals a feature in the spectrum from silver on (101?0) which may be identified with a known surface state on Ag(111), thus providing some support for the assignment of Ag(111) to the surface structure of thick silver layers (> 3 monolayers) on (101?0).     

The kinetics of D2 desorption from Ni(110) (2 × 1)C in the presence of reactive intermediates

A kinetic study of D₂ formation from HCOOD decomposition on Ni(110) (2 × 1)C was performed using the flash desorption technique. The surface structure and surface composition were monitored by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Flash curves were obtained using initial coverage and heating rate variations. D₂ formation exhibited a single second-order rate-determining step. Three different techniques were employed in obtaining the activation energy, two of which did not require the assumption of reaction order. Using an average value of 12.6 kcal/mole for the activation energy the pre-exponential factor was calculated to be 2.7 × 10^?4 cm² molecules^?1 sec^?1. Good agreement was achieved with the theoretically generated second-order flash curves only up to the peak temperature. The discrepancy on the high temperature side was explained using the model proposed by Clavenna and Schmidt utilizing a coverage dependent pre-exponential factor.