Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); studies by LEED,Auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I₂ vapor to form the Pt(111) ($\text{7}\text{×}\text{7}$)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.     

Ethylene and acetylene adsorption on Cu(111) and Pt(111) studied by Auger spectroscopy

High resolution, electron impact excited, carbon Auger spectra of ethylene and acetylene adsorbed on Cu(111) and Pt(111) are compared. The spectra of ethylene on the two metals provide the first example of the sensitivity of AES to the nature of metal-adsorbate bonding for molecular adsorbates. The acetylene spectra are identical on the two metals. The changes in the carbon Auger spectra resulting from thermal decomposition of the two adsorbates on Pt(111) are discussed in the context of results from electron energy loss spectroscopy.     

Adsorption of oxygen on Rh(110): a LEED, Auger electron spectroscopy and thermal desorption study

The adsorption of oxygen on the Rh(110) surface has been studied by a variety of techniques. Low-energy electron diffraction shows the following patterns: (2 × 1)p2mg at 1 ML coverage and temperatures between 125 and 300 K; (2 × 2)p2mg at 0.5 ML coverage after heating to above 470 K; c(2 × 8) and complex streaked c(2 × 2n) patterns at coverages above 0.5 after heating to 470 K. These results are in partial agreement with previous work. Models for the first two structures are suggested. In the (2 × 2) structure, the oxygen is found to be much less reactive with CO at room temperature than in the (2 × 1) structure, suggesting that it is subsurface. A metastable (1 × 2) structure was produced from the (2 × 2) by reduction of the oxygen by CO at 450 K, and is interpreted as a surface reconstruction.     

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(101̄0) 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.     

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.     

Surface diffusion of hydrogen on Ni(100) studied using laser-induced thermal desorption

The surface diffusion coefficient for hydrogen on Ni(100) at low coverage has been measured between 223 and 283 K. A pulsed laser is used to desorb hydrogen from a small, well defined, region on the surface without perturbing the ambient surface temperature. Hydrogen from nearby regions on the surface migrates into the vacant area and the time required to refill that area is determined by subsequent laser-induced-desorption measurements. The diffusion coefficient is obtained using an equation derived from Fick's Second law with non-stationary boundary conditions. The temperature dependence of the diffusion coefficients yields a value of 4.0 ± 0.5 kcal/mol for the energy barrier to diffusion. A value of roughly 3 × 10¹³ s^?1 for the site-to-site hopping frequency is derived when the pre-exponential for diffusion is fit to a random-walk mechanism.     

The adsorption of CO on Pt(111) studied by infrared-reflection-adsorption spectroscopy

The adsorption of CO on Pt(111) between 85K and 300K has been studied by infrared-reflection-absorption spectroscopy together with TPD and LEED. The intensity of the absorption band due to the CO stretch of the linear species shows a maximum at the formation of the (√3 × √3)R30° LEED pattern followed by a minimum at the c(4×2) structure during the adsorption of CO at low temperatures (150K). The absorption band due to the C-O stretch of the bridging species appears only after the formation of the (√3 × √3)R30° pattern and reaches maximum intensity at the c(4×2) structure. Adsorption of CO to higher coverages (corresponding to the compression structures) broadens and shifts this absorption band. At higher temperatures (150K) a third peak is observed at 40cm⁻¹ below the peak due to the bridging species and is attributed to adsorption in the three-fold sites. At 300K both peaks in this region are very broad. The intensity data differs from that measured with EELS (ref.1) and favors a “faultline” structure of the type proposed by Avery (ref.2). Together with the additional information from bandwidths it is possible to distinguish between the various structural models. The results obtained here may also be important in explaining data from other systems such as CO/Cu.     

Adsorption of oxygen and hydrogen on Pt(111) studied with second-harmonic generation

The adsorption of oxygen and hydrogen on a Pt(111) surface has been studied with phase-sensitive Second-Harmonic Generation (SHG) signal detection. The SHG-signal change measured with p-in, p-out polarization during the adsorption of oxygen and hydrogen was found to be different in amplitude and phase for the two adsorbates. At the wavelength used (1064 nm), only a localized interaction between adsorbate and substrate is seen, leading to a linear dependence of the susceptibility on the coverage. Sticking coefficients,s, and their coverage dependence were determined. For hydrogen, a linear decrease ins with coverage was found; the initial sticking coefficient beings ₀=0.06 at a temperature ofT=130 K. For oxygen,s whows a quadratic decrease with coverage, strongly dependent on temperature, withs ₀=0.05 atT=350 K.A method based on these results is proposed, which would allow the determination of adsorbate coverages of coadsorption systems with SHG using phase-sensitive signal detection.Presented at the 129th WE-Heraues-Seminar on Surface Studies by Nonliner Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

LEED crystallographic investigation of ultrathin films formed by deposition of Sn on the Pt(111) surface

Deposition of Sn on the Pt(111) surface followed by annealing at 1000 K leads to the formation of ordered phases showing (2 × 2 and ( LEED patterns, depending on the surface coverage of Sn. Both these phases were studied by LEED dynamical analysis. The best agreement between experimental and calculated I–V curves was obtained by means of models based on the formation of mixed Pt-Sn layers on the surface where Pt and Sn atoms are nearly coplanar with a slight upward buckling of Sn atoms. The structures of these phases are similar to those already observed for the Pt₃Sn(111) surface.     

The adsorption of water on Pt(111) studied by irreflection and UV-photoemission spectroscopy

IR-reflection spectroscopy (IRS) under grazing incidence and UV-photoemission (UPS) with Hel and Hell radiation were used to study the adsorption of D₂O and H₂O, respectively, on Pt(111) under UHV conditions. In IRS three different vibrational bands in the OD stretch region yield information about the orientation of hydrogen-bonded water molecules and the formation of water clusters. Initially formed water multimers grow to clusters and finally build up multilayers. From UPS the adsorption of molecular water in the whole coverage range is confirmed. Both UPS and IRS show that the water molecules in the “first layer” are bound through their oxygen end to the surface.     

Electron-stimulated defect formation in single-walled carbon nanotubes studied by hydrogen thermal desorption spectroscopy

Defects in single-walled carbon nanotubes introduced by low-energy electron irradiation at 8 K were sensitively detected by cryogenic thermal desorption of hydrogen molecules H₂ in the temperature range of 10-40 K. The thermal desorption spectrum was found to change significantly with sample annealing at temperatures as low as 40-70 K. Experimental results suggest that the H₂ physisorption sites responsible for the ‘defect’ peak at 28 K are interstitial channel space between nanotubes closely packed in bundles which becomes more easily accessible on damage. It is also suggested that the disordering provides groove sites for H₂ physisorption with smaller binding energy causing the damage-induced spectral component around 16 K, slightly lower than the desorption peak at 20 K that is observed in undamaged samples. The spectral change at 40-70 K could be interpreted by migration of adatoms at the low temperatures.     

Adsorption and decomposition of HNCO on Cu(111) surface studied by auger electron,electron energy loss and thermal desorption spectroscopy

No detectable adsorbed species were observed after exposure of HNCO to a clean Cu(111) surface at 300 K. The presence of adsorbed oxygen, however, exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociative adsorption of HNCO with concomitant release of water. The adsorption of HNCO at 300 K produced two new strong losses at 10.4 and 13.5 eV in electron energy loss spectra, which were not observed during the adsorption of either CO or atomic N. These loses can be attributed to surface NCO on Cu(111). The surface isocyanate was stable up to 400 K. The decomposition in the adsorbed phase began with the evolution of CO₂. The desorption of nitrogen started at 700 K. Above 800 K, the formation of C₂N₂ was observed. The characteristics of the CO₂ formation and the ratios of the products sensitively depended on the amount of preadsorbed oxygen. No HNCO was desorbed as such, and neither NCO nor (NCO)₂ were detected during the desorption. From the comparison of adsorption and desorption behaviours of HNCO, N, CO and CO₂ on copper surfaces it was concluded that NCO exists as such on a Cu(111) surface at 300 K. The interaction of HNCO with oxygen covered Cu(111) surface and the reactions of surface NCO with adsorbed oxygen are discussed in detail.     

Halogen adsorption on Fe(100): I. The adsorption of Cl2 studied by AES,LEED, work function change and thermal desorption

The initial sticking probability of chlorine on Fe(100) at room temperature is calculated to be 0.13, and there is evidence to suggest that the chlorine adsorbs into a short lived mobile precursor state above the surface. The work function change, Δφ, is proportional to coverage and reaches a maximum value of 1.43 eV at saturation. At this coverage a c(2 × 4) LEED pattern is formed. On heating, chlorine is lost from the surface, but the mechanism is such that no detectable loss is incurred at a constant elevated temperature. The c(2 × 4) pattern is shown to be a coincidence structure formed from a $\text{(}\text{1}\text{2}\text{3}\text{?1}\text{2}\text{3}\text{)}$ net of chlorine atoms on the Fe(100) substrate. This structure is a special case of the more general $\text{(}\text{1}\text{2}\text{tan}\text{α}\text{?1}\text{2}\text{tan}\text{α}\text{)}$ structure formed at lower concentrations of chlorine. The c(2 × 4) is formed when α = 56.31°, which gives the chlorine atoms a hard sphere diameter of 0.345 nm and a concentration of 0.75 atoms per four-fold site.     

KrF-excimer-laser-induced native oxide removal from Si (100) surfaces studied by Auger electron spectroscopy

The mechanism of KrF-excimer-laser cleaning of Si(100) surfaces was studied by Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) spectroscopy. The dependence of the cleaning efficiency on the laser fluence was investigated by using a mildly focused laser beam and carefully measuring the energy density distribution of the laser spot impinging on the sample. These values were compared with the AES spectra measured in different points of the irradiated area and with the morphology observed by optical microscopy. Samples as received from the manufacturer were first investigated. It was found that desorption of weakly bonded organic adsorbates occurs at energy densities as low as 0.3 J/cm², whereas significant oxide removal takes place only at an energy density above 0.8 J/cm², which produces damaged surface morphologies. The experimental findings, in agreement with the temperatures calculated for the laser-induced Si heating, indicated that a large fraction of the oxide film is dissolved in the molten silicon, leading to oxygen concentration below the AES detection limit only when the melted depth was of the order of several hundred nanometers. Atomically clean, damage-free Si(100) surfaces were obtained after irradiation of samples pre-etched for 1 min in a HF: H₂O (5%) solution, which had only a thin SiO _x (x < 2) layer and F, C and O containing adsorbed species. Complete contaminant elimination was achieved in this case with 15 pulses at 0.8 J/cm² without any damaging of the surface.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.