The reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature

Ultraviolet photoemission spectroscopy using hv = 21.2 eV and filtered 40.8 eV radiation as well as temperature programmed thermal desorption spectroscopy are used to investigate the chemical reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature. Striking crystallographic effects and several coexisting phases are observed and found to be coverage and temperature dependent. A methodology is described and used to predict the relative energy levels for a variety of adsorbed hydrocarbon fragments on Ni surfaces. Such levels together with the thermal desorption spectra are used to identify the observed species. In particular, CH and CCH species are isolated on Ni(100) and Ni(110) surfaces, respectively, via low temperature adsorption and subsequent pulsed sample warming experiments. The room temperature adsorption phases are deduced using these ionization levels together with those of chemisorbcd acetylene, atomic hydrogen and carbon. At room temperature on Ni(100), H, C, CH and C₂H₂ species form together below 2 L exposure while CH species form thereafter, up to a saturation exposure of ~10 L. On Ni(110), H and CCH species form below 1.5 L exposure followed by the formation of CH₂ and likely CH species. The relative stabilities of these species at elevated temperatures is: C₂H₂ < CCH ? CH < CH₂. A model for the bonding of acetylene and its reaction to form CCH species on Ni(110) is proposed.     

Mass spectrometry study of the kinetics of CdTe molecular-beam epitaxy: Part II. Cd, Te2, Te, and CdTe

The results of an in situ mass-spectrometric study of surface processes occurring during CdTe molecular beam epitaxy are presented. The measurements of kinetic parameters are performed with modulated Cd and Te₂ molecular beams with an intensity of 0.1–5.0 ML/s at a crystal temperature of 550–730 K. The experimental results are treated using a model in which condensation and evaporation proceed through adsorption and desorption steps. The desorption rates are 2–15 and 150 s^?1 for Te₂ and Cd, respectively. The activation energy of CdTe “evaporation” is found to be 1.2 eV; the desorption energies are E _d(Cd) = 1.0 eV and E _d(Te₂) = 0.3 eV. The adsobate coverage with cadmium atoms and tellurium molecules is estimated to be n(Cd) < 0.01, n(Te₂) = 0.02–0.20, and n(Te) = 0.2–1.0.     

Influence of an adsorbing gas on the layering transitions in crystal growth

The influence of an adsorbing gas on the layering transition of a film is studied. In agreement with an earlier publication, we find that the adsorbate lowers the critical temperature T_c(n) for the nth layer. We also find that a system which is dry near T = 0 may undergo an apparent wetting transition due to the adsorbate. Indirect evidence is given that the sequence T_c(n) ends at T_R, the roughening temperature, as conjectured by de Oliviera and Griffiths. Re-entrant behavior in the layering transitions is observed for sufficiently strong adsorption energy; similar behavior also appears in the roughening transition, for which there exists some experimental evidence.     

Reactivity screening of silica

As a screening of the chemical activity of silica [SiO₂/Si(1 0 0)], which is one of the most often used supports for nanostructures, thermal desorption spectroscopy data have been gathered for a variety of gases such as n-nonane, n-hexane, n-butane, iso-butane, ethane, CO₂, CO, O₂, and H/H₂. Whereas, the alkanes with chain lengths larger than three adsorb with large binding energies (E_d = 50-70 kJ/mol), the activity towards the other probe molecules is negligible (<24 kJ/mol) down to adsorption temperatures of 95 K. The adsorption of n- and iso-butane has additionally been studied by molecular beam scattering and follows standard precursor mediated adsorption dynamics.     

Simulation of kinetics of water temperature-programmed desorption from n-GaAs(100) and n-GaP(100) semiconductor surfaces

The energy spectra of adsorption centers on the n-GaAs(100) and n-GaP(100) surfaces are studied using temperature-programmed desorption of water. The desorption spectra are analyzed in terms of the model of discrete adsorption centers, which assumes the presence of a loosely bound precursor state. The values of frequency factors and activation energies of desorption are in good agreement with the frequency of electronic transitions and energies of surface electronic states in gallium arsenide and gallium phosphide. It is concluded that the water desorption kinetics is limited by slow electronic processes on the surface of the semiconductors.     

Mass spectrometry study of the kinetics of CdTe molecular-beam epitaxy: Part I. Cd, Te2, and CdTe

Surface processes in CdTe molecular-beam epitaxy were studied using in situ mass spectrometry. Modulated molecular Cd and Te₂ beams were used for measuring kinetic parameters. The experiments were performed at crystal temperatures of 600–730 K. The results were processed within a model in which condensation and evaporation occur through adsorption and desorption stages. The desorption rate was 2–10 s^?1 for Te₂ and more than 30 s^?1 for Cd. The CdTe evaporation activation energy and desorption energies were determined as E _ev = 1.1 eV, E _d (Cd) = 1.0 eV, and E _d (Te) = 0.6 eV. The adsorbate coverage was estimated as n(Cd) < 0.01 and n(Te) = 0.1–1 Te.     

A density functional theory study on the H2S molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation on Au_nH₂S (n = 1-13) clusters has been performed by using density functional theory with the generalized gradient approximation at PW91 level. The small gold cluster would like to bond with sulfur in the same plane and the H₂S molecule prefers to occupy the on-top and single fold coordination site in the cluster. The Au_n structures and H₂S molecule in all Au_nH₂S clusters are only slightly perturbed and still maintain their structural integrity. After adsorption, the S-H, H-H bond-lengths and most Au-Au bond-lengths are elongated, only a few Au-Au bond-lengths far from H₂S molecule are shortened. The reactivity enhancement of H₂S molecule is obvious and the strong gold-sulfur bond is observed expectedly. The most favorable adsorption takes place in the case that the H₂S molecule is adsorbed by an even-numbered Au_n cluster and becomes Au_nH₂S cluster with even number of valence electrons. It is believed that the strong scalar relativistic effect is favorable to H₂S molecule adsorption onto small gold clusters and is also one of the important reasons for the strong gold-sulfur bond.     

Theoretical study of the coadsorption of CO and O2 on doped cationic gold clusters MAun + (M = Ti, Fe, Au; n = 1, 6, 7)

We present a study based on first-principles calculations of the adsorption of CO on selected equilibrium configurations of MAu_nO₂ ⁺ (M = Ti, Fe; n = 1, 6, 7) complexes resulting from the adsorption of O₂ on doped cationic gold clusters MAu_n ⁺. Empirical rules for the formation of CO₂-MAu_nO⁺ complexes are outlined. The desorption energy of CO₂ is calculated. The adsorption of a second CO molecule on the residual MAu_nO⁺ complex leads in some cases to the formation of CO₂-MAu_n ⁺. The desorption of a second CO₂ molecule brings back to the initial doped gold cluster MAu_n ⁺.     

Étude par spectrométrie auger et par spectrométrie de masse de la chimisorption de l'oxyde de carbone sur le molybdène polycristallin

A combination of Auger spectrometry and mass spectrometry was employed to study CO chemisorption on polycrystalline Mo surfaces at room temperature. We observed five adsorption states and calculated the binding parameters (E,n₀,τ₀) for the three important states. The results obtained by the two methods are in accord but we pointed out the occurence of electronic desorption in Auger experiments. Contamination effects by C atoms in such studies were investigated by repeated cycles of adsorption-desorption and a characteristic evolution of flash desorption peaks was observed. The results are discussed in this point of view enhancing the importance of a control of the adsorption surface cleanness by a method of great sensibility like Auger spectrometry.     

Quantum statistical theory of adsorption and desorption of a gas at a solid surface

The non-equilibrium initial value problem of phonon-mediated adsorption and desorption of a gas at a solid surface is formulated explicitly in a quantum-statistical theory and adsorption, flash desorption and isothermal desorption times are calculated using a non-local separable surface potential. We find that the flash desorption time is a function of both initial and final temperatures T_g and T_s and can, for fixed T_g, be approximated by Frenkel's formula over a limited range of flash temperatures T_s. We fit our theory to flash desorption data for He desorbing from Constantan and find excellent agreement. Predictions are given for the H/NaCl system. Isothermal and flash desorption times are correlated, and experiments are suggested to demonstrate the differences.     

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir_n/TiO₂ samples prepared by size and energy-selected deposition of Ir_n⁺ (n=1, 2, 5, 10, 15) on rutile TiO₂(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO₂ surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir_n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C¹⁶O and C¹⁸O doses, facile C¹⁶O → C¹⁸O exchange is observed for Ir₅ and larger clusters, but not for Ir₂. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO_x layer.     

Temperature-dependent carbon incorporation into the Si1−yCy film during gas-source molecular beam epitaxy using monomethylsilane

Coverage and adsorption state of hydrogen atoms on the growing surface of Si_1−yC_y film using monomethylsilane has been investigated by using temperature-programmed desorption (TPD) and multiple-internal-reflection Fourier-transform infrared spectroscopy (MIR-FT-IR). The surface hydrogen coverage decreases with the growth temperature T_g until it disappears at 800 °C. All the H₂-TPD spectra are well resolved into six SiH-related and one CH_n-related hydrogen desorption peaks. The SiH-related FT-IR peak showed a blue shift with increasing T_g, which, in conjunction with the TPD, is related to enhanced C incorporation at backbonds of SiH.     

A covalent model for the bonding of adsorbed hydrocarbon fragments on the (111) face of platinum

A systematic molecular orbital study of the location of CH_n and C-CH_n (n = 1, 2, 3) species on a Pt(111) single-crystal surface has been performed by using both a cluster model and a band structure calculation within the framework of the Extended Hückel Theory (EHT). These species may be involved in the hydrogenation/dehydrogenation processes of hydrocarbons. The observed dependence of the adsorption site on the number of hydrogens in the CH_n fragments suggests that any C-H bond breaking in CH_n species must involve a change of adsorbate bonding site. The carbon is found to be located on the surface in such a way as to complete its tetravalancy. Thus, CH occupies a three-fold coordinated hollow site, CH₂ a two-fold coordinated bridge site and CH₃ a one-fold coordinated top site. C-CH₃ is found to be perpendicular to the surface in a three-fold hollow site in agreement with experimental observations. It is also found that a displacement of C-CH₂-R to a top site makes a β C-R cleavage easier.     

Triple point of the first monolayer of nitric oxide adsorbed on the cleavage face of magnesium bromide

This paper is the first of three articles devoted to the CO/Mo(110) chemisorption. The experimental study of adsorption and desorption kinetics was performed by several methods: thermal desorption, low energy electron diffraction and Auger electron spectroscopy. The adsorption of CO on Mo(110) presents two different states. For these two states the desorption kinetics are first order ones, the desorption energies and frequency factors have been determined (E₁ = 99 kcal mole^?1, E₂ = 50 kcal mole^?1, v₁ = 10¹⁹ s^?1, v₂, = 5 × 10¹⁰ s^?1). The dependence of sticking coefficient on surface coverage θ was investigated and was found different for the two states of adsorption. LEED shows that the adsorption is not ordered. AES investigation suggests that in the two states C and O have different positions with respect to MO atoms.     

Paramagnetic resonance of some three-nuclear clusters of iron

ESR of exchange-coupled iron triads in carboxilates [Fe₃O(RCOO)₆(H₂O)₃·(RCOO)_n where R = ClCH₂, FCH₂, Cl₂CH or Cl₃C is studied at 4.2 K (microwave frequency v = 9.5 kMc/sec). ESR spectra and observed values of g-factors are discussed according to the model of a symmetrical cluster. Fine structure δ splittings of the triad ground state are determined on bases of the theory and ESR data: δ ≈ 0.03-0.16 cm^?1.     

The effect of surface modification with silane coupling agent on suppressing the photo-catalytic activity of fine TiO2 particles as inorganic UV filter

The effect of surface modification with 3-aminopropyltriethoxysilane (APTES) and n-propyltriethoxysilane (PTES) on photo-catalytic activity and UV-shielding ability of fine TiO₂ particles were investigated. The number of surface groups (N_R) [nm⁻²] which shows the density of modifier on TiO₂ surface was calculated from the results of elemental analysis and BET measurement. The modified samples of which N_R are different were obtained by changing the concentration of modifier. When the photo-catalytic activity and UV-shielding ability of modified samples were evaluated, it was found that APTES was more effective modifier than PTES to obtained samples with low photo-catalytic activity and high UV-shielding ability. This is probably because the adsorption mechanisms on TiO₂ surface between modifiers were different and N_R is a key factor to control the performances of fine TiO₂ particles. The result of zeta potential showed that surface character of modified samples was varied by changing N_R. It suggested from these results that N_R affected the photo-catalytic activity and UV-shielding ability because N_R changed surface character of modified samples.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

Screening of strongly charged macroparticles in liquid electrolyte solutions

A modified Poisson-Boltzmann model has been proposed which makes it possible to describe the screening of strongly charged macroparticles in liquid electrolyte Z: Z solutions in the case when parameter B= ZeQ₀/εRT?1(Q₀ is the surface electric charge, T is the temperature, ε is the solution permittivity, and Z is the valence of ions) provided that the solution is dilute: κR ≡ (8πZ²e²n_i0/εT)^1/2R?1 (n_i0 is the equilibrium number density of ions). It is assumed that the charge Q₀ of a macroparticle appears as a result of adsorption of ions of a certain polarity on its surface. Quantitative criteria of division of dissolved ions into capable and incapable of adsorption are formulated. For aqueous solutions, the adsorption mechanism always leads to values of B ? 1. It is shown that the charge inversion effect predicted by other authors on the basis of different models must be observed for such solutions for all Z ≥ 1. The effect of Brownian movement of macroparticles on their screening is considered. It is shown that viscous forces emerging during such movement lead to peripheral destruction (“washing out”) of the screening ionic shell of macroparticles and, as a result, to violation of their electroneutrality. This results in the emergence of two types of oppositely charged compound particles with small radii close to R and with radii much larger than R, the charge polarity of the latter being opposite to the polarity of Q₀. It is found that both types of ions of compound particles obey the “law of distribution” of the mean energy of their electric field, expressed by formula (29). The problem of ionic screening of gas bubbles accompanied by the formation of bubstons (bubbles stabilized by ions) is considered separately. It is shown that the bubston radius R in pure water and in aqueous solutions of electrolytes is equal to 14 nm irrespective of the ion number density n_i0. The value of n_i0 determines the number density n _b of bubstons themselves, which are formed spontaneously under equilibrium conditions.     

Study on selective adsorption of deuterium on boron nitride using photon-stimulated ion-desorption

Adsorption behavior of atomic deuterium on a hexagonal boron nitride (h-BN) thin film is studied by photon-stimulated ion desorption (PSID) of D⁺ and near edge X-ray absorption fine structure (NEXAFS) at the B and N K-edges. After the adsorption of atomic deuterium, D⁺ desorption yield η(hν) shows clear enhancement at the B K-edge and almost no enhancement at the N K-edge. NEXAFS spectra show a large change in the B K-edge and a small change in the N K-edge after the adsorption. We propose selective adsorption of atomic deuterium on the h-BN thin film based on the experimental results, and mention the effectiveness of applying the PSID method with X-ray to study hydrogen storage materials.     

Micropatterning of perfluoroalkyl self-assembled monolayers for arraying proteins and cells on chips

Organosilane self-assembled monolayers (SAMs) with perfluoroalkyl groups (R_f) on glass surfaces were used for arraying proteins and cells on chips. Quartz crystal microbalance measurements confirmed the inhibition of protein adsorption on R_f-SAM-modified surfaces and showed efficient adsorption on hydroxyl-, carboxyl-, and amino group-modified surfaces. The characteristics of R_f-modified surfaces were evaluated using solvent contact angle measurement and Fourier transform infrared (FTIR) spectroscopy. The R_f surface was highly water- and oil-resistant, as inferred from the contact angles of water, oleic acid, and hexadecane. Specific peaks of IR spectra were detected in the region from 1160 to 1360 cm⁻¹. Etching with dry plasma completely exfoliated the R_f-SAM, exposing the underlying intact glass surface. Modification conditions were optimized using contact angle and FTIR measurements. After dry plasma processing, the contact angles of all solvents became undetectable, and the IR peaks disappeared. Micrometer scale protein and cell patterns can be fabricated using the proposed method. Protein adsorption on micropatterned R_f-SAM-modified chips was evaluated using fluorescence analysis; protein adsorption was easily controlled by patterning R_f-SAM. PC12 and HeLa cells grew well on micropatterned R_f-SAM-modified chips. Micropatterning of R_f-SAM by dry plasma treatment with photolithography is useful for the spatial arrangement of proteins and cells.