Adsorption and reaction of oxygen and deuterium on a Pd(110) surface, studied by Δφ and TDS

The co-adsorption of oxygen and deuterium at 100 K on a Pd(110) surface has been studied by measurements of the change in work function (Δφ) and by thermal desorption spectroscopy (TDS). When the surface with co-adsorbed species is heated, the adsorbates O and D react to form D₂O which desorbs from the surface at T > 200 K. The D₂O desorption peaks shift continuously to lower temperatures as the surface D coverage (θ_D) increases. The maximum production of D₂O is estimated to be 0.26 ML (1 ML = 9.5 × 10¹⁴ atoms cm⁻²), resulting from reaction in a layer containing 0.65 ML D and 0.3 ML O. The maximum work function increase caused by adsorption of D to saturation onto oxygen precovered Pd(110) decreases almost linearly with Δφ_O of the oxygen precovered surface. On a surface with pre-adsorbed D however, the maximum Δφ increase contributed by oxygen adsorption decreases abruptly at Δφ_D > 200 mV. This sharp change occurs at θ_D > 1 ML and is believed to be associated with the development of the reconstructed (1 × 2) phase of D/Pd(110).     

Decomposition mechanism of triethylindium (TEI) on a GaP(0 0 1)-(2×1) surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of triethylindium (TEI: (C₂H₅)₃In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C₂H₅ to form P–C₂H₅ bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface.     

Oxygen atom-induced D2 and D2O desorption on D/Si(1 1 1) surfaces

We studied reaction of oxygen atoms with D-terminated Si(1 1 1) surfaces from a desorption point of view. As the D (1 ML)/Si(1 1 1) surface was exposed to O atoms D₂ and D₂O molecules were found to desorb from the surface. The desorption kinetics of D₂ and D₂O molecules exhibited a feature characterized with a quick rate jump at the very beginning of O exposure, which was followed by a gradual increase with a delayed maximum and then by an exponential decrease. The O-induced D₂ desorption spectra as a function of T_s appeared to be very similar to the H-induced D₂ desorption spectrum from the D/Si(1 1 1) surfaces. Possible mechanisms for the O-induced desorption reactions were discussed.     

Effect of surface orientation and sulfur coverage on the oxidation of carbon on Ni surfaces

The reaction between adsorbed oxygen and segregated carbon on a cylindrical nickel single-crystal has been examined with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD), for a range of surface orientation, oxygen exposure, and sulfur coverage. It was found that for small oxygen exposures, surface carbon and surface oxygen react during TPD to form a CO desorption peak, labeled β₁. The β₁ CO peak temperature and peak shape vary with orientation. At higher oxygen coverages, the CO desorption peak split into low-temperature and high-temperature peaks. The behavior of the β₁ CO desorption peak for large oxygen exposures is consistent with a model of the carbon-oxygen recombination reaction in which the morphologies of the initial carbon and oxygen phases change during oxygen exposure as a result of repulsive lateral interactions. High oxygen exposures result in the formation of large regions of contact between the two phases; this is believed to produce the low-temperature β₁ CO desorption peak. Small segregated-sulfur coverages, and low oxygen exposures, caused the β₁ CO peak to shift to lower temperatures for all orientations. Sulfur is believed to cause more frequent contact between carbon and oxygen for small oxygen exposures because it compresses the adsorbed oxygen and segregated carbon into the sulfur-free areas of the surface. Large coverages of segregated sulfur inhibited carbon segregation on some, and oxygen adsorption on most, orientations. The absence of reactant species explains the disappearance of the β₁ CO peak during TPD from orientations which had a high sulfur coverage.     

Reversible H2 passivation of Si ≡ Si3 interface defects in (1 1 1)Si/SiO2

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 1]P_b centers (^*Si ≡ Si₃ defects with unpaired sp³ hybrid [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO₂ interface. This has been enabled by the perfectly reversible H₂ passivation of P_b, which affects the defect's spin state. Sequential hydrogenation at 253–353°C and degassing treatments in high vacuum at 743–835°C allowed to vary the P_b density in the range 5 × 10¹⁰ < [P_b] (1.14 ± 0.06) × 10¹³ cm^-2. With increasing [P_b] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO₂ interfaces, dry thermally grown at ≈920°C, naturally comprise a ^*Si ≡ Si₃ defect density — passivated or not — of 1.14 × 10¹³ cm^-2.     

Isothermal desorption of hydrogen from polycrystalline diamond films

The kinetics of isothermal H₂ desorption from polycrystalline diamond are studied in real time. The surface H coverage (θ_H) is measured by mass analyzing the recoiled H₊ ion signal during the desorption. We find that the H₂ desorption is 1st order in θ_H with an activation energy of 69 ± 6 kcal/mol and a prefactor of 10^{10.5 ± 0.9} s⁻¹. We suggest that formation of a C---C π-bond on the clean surface plays a key role in H₂ desorption from diamond, a view consistent with previous theoretical calculations of H₂ desorption from diamond.     

Well-ordered (1 0 0) InAs surfaces using wet chemical treatments

Atomic ordering of HCl-isopropanol (HCl-iPA) treated and vacuum annealed (1 0 0) InAs surfaces was studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflectance anisotropy spectroscopy (RAS). On the as-treated surface, a diffused (1 × 1) pattern is observed, which successively evolves to the β₂(2 × 4)/c(2 × 8) and (4 × 2)/c(8 × 2) ones after annealing to 330 °C and 410 °C, respectively. At the intermediate temperature of 370 °C, an ₂(2 × 4)/(4 × 2) mixed reconstruction is observed. Reflectance anisotropy spectra are compared with those of the corresponding reconstructions observed after As-decapping and found to be quite similar. Therefore we conclude that high-quality (1 0 0) InAs surfaces can be obtained by wet chemical treatment in an easy, inexpensive and practical way.     

Improved constants for the 7820 Å and 7883 Å bands of CO2

High-dispersion spectra of Venus are used to obtain line positions and band constants for the (105)_I and (105)_II bands of CO₂. An improved method of analysis is used to obtain very accurate results. Assuming B″ = 0.390218 and D″ = 13.3 × 10^-8 for the ground state, we find for 7820: ω₀ = 12774.727 cm^-1 ±0.002, B' = 0.374540 ±0.000006, D' = 10.9 × 10^-8 ±0.4, for 7883: ω₀ = 12672.274 cm^-1 ±0.004, B' = 0.375657 ±0.000014, D' = 17.2 × 10^-8 ±1.7,

The values for ω₀ and B' are at least an order of magnitude more accurate than those given by Herzberg and Herzberg in 1953, and the D' values are new.     

Effect of branching in base polymer on ionic conductivity in hyperbranched polymer electrolytes

Novel hyperbranched polymer, poly[bis(diethylene glycol)benzoate] capped with a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group (poly-Bz1a), was prepared, and its polymer electrolyte with LiN(CF₃SO₂)₂, poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte, was all evaluated in thermal properties, ionic conductivity, and electrochemical stability window. The poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte exhibited higher ionic conductivity compared with a polymer electrolyte based on poly[bis(diethylene glycol)benzoate] capped with an acetyl group (poly-Ac1a), and the ionic conductivity of poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte was to be 7×10⁻⁴ S cm⁻¹ at 80 °C and 1×10⁻⁶ S cm⁻¹ at 30 °C, respectively. The existence of a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group as a branching unit present at ends in the base polymer improved significantly ionic conductivity of the hyperbranched polymer electrolytes. The polymer electrolyte exhibited the electrochemical stability window of 4.2 V at 70 °C and was stable until 300 °C.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Observation of triangular terraces and triangular craters of CaF2 film on Si(1 1 1) substrate

We have investigated the growth mode and surface morphology of CaF₂ film on Si(1 1 1)7×7 substrate by reflection high-energy electron diffraction (RHEED) using very weak electron beam and atomic force microscopy (AFM). It was found by RHEED intensity oscillation measurements and AFM observations that three-dimensional (3D) islands grow at RT; however, rather flat surface appears with two-dimensional (2D) islands around 300 °C. Especially, at high temperature of 700 °C, characteristic equilateral triangular terraces (or islands) with flat and wide shape grow with the tops directed toward [1 1 −2] of substrate Si(1 1 1). On the other hand, the desorption process of the CaF₂ film due to electron stimulated desorption (ESD) was also examined. It was found that the ESD process at 300 °C forms characteristic equilateral triangular craters on the film surface with the tops (or corners) directed toward [−1 −1 2] of substrate Si(1 1 1), provided that the film was grown at 700 °C.     

A LEED, TPD and HREELS investigation of NO adsorption on Sn/Pt(111) surface alloys

The adsorption of NO on Pt(111), and the (2 × 2)Sn/Pt(111) and (√3 × √3)R30°Sn/Pt(111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 × 2) LEED pattern on Pt(111) and a (2√3 × 2√3)R30° LEED pattern on the (2 × 2)Sn/Pt(111) surface. The initial sticking coefficient of NO on the (2 × 2)Sn/Pt(111) surface alloy at 100 K is the same as that on Pt(111), S₀ = 0.9, while the initial sticking coefficient of NO on the (√3 × √3)R30°Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of β-state NO is reduced by 8–10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm⁻¹ in the low frequency range and only one N-O stretching frequency at 1698 cm⁻¹. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm⁻¹ was also observed on the (2 × 2)Sn/ Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (√3 × √3)R30°Sn/Pt(111) surface alloy at any NO coverage studied.     

Monte Carlo calculation of thermal desorption spectra: Application to CO/Ru(001)

The thermal desorption spectrum and phase diagram for the triangular lattice gas model with strong repulsive 1NN and weak attractive 2NN interaction are calculated using Monte Carlo method and the transition state rate theory. Starting from an extreme case of the hard hexagon model, the magnitudes of the interactions are varied to reach a good coincidence with the experimental desorption spectrum of CO/Ru(001). The calculated spectrum describes well the characteristic zero-order (-like) behavior and asymmetric sharp peak observed in experiment, which are known to arise from the critical point in the model. Also, calculation of the phase diagram confirms that the tricritical temperature appears in the same magnitude with the 2NN interaction, kT_t ≈ ¦ε₂¦. The determined 1NN and 2NN interaction energies, ε₁ ≈ 8 to 10 kJ/mol, ε₂ ≈ −1.6 to −1.2 kJ/mol, are about t than those estimated previously by the isosteric Arrhenius analysis of the experimental desorption spectrum.     

Chlorine/silicon surface reactions under heating

Reactions on Cl-adsorbed Si(111) and Si(100) surfaces—(Cl/Si(111) and Cl/Si(100))—under heating in ultrahigh vacuum (UHV) and in a Cl₂ atmosphere were studied. Auger electron spectroscopy (AES) and low-energy electron energy loss spectroscopy (LEELS) were used for examination of surface changes. Heating in UHV at 820°C for 30 s successfully removed almost all Cl atoms, both on Cl/Si(111) and Cl/Si(100). Variance in LEELS spectra shows that decomposition of SiCl_x (x > 1), a small amount of which was present on Cl/Si(111), occurs under heating on Si(111) both in UHV and in Cl₂ and desorbs reaction products, leaving the Si---Cl bonds on the surfaces. Such Si---Cl bonds specific to those on Cl/Si(111) are formed also on Cl/Si(100) heated in C1₂ at 820°C. On Cl/Si(100) heated in C1₂, there are various surface changes: relaxation of the 2 × 1 structure remaining on the Cl/Si(100), desorption of reaction products, and formation of Si---Cl bonds specific to those on Cl/Si(111). The Si---Cl bonds, both on Cl/Si(111) and Cl/Si(100), decomposed under longer heating and under heating at higher temperatures in UHV.     

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10⁻¹ to 10⁻⁶ ML s⁻¹. From their temperature dependence between 780 and 1000 K, activation energies of E_A=3.4±0.2 eV and E_A=4.0±0.3 eV and pre-exponential factors of ν₀=10^16±1 s⁻¹ and ν₀=10^20±1 s⁻¹ for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.     

Growth, structure and oxidation of ordered silicon layers on nickel (001)

Thin silicon films deposited in ultra-high vacuum on Ni(001) have been studied, as a function of annealing and/or O₂ exposure, using Auger spectroscopy and low energy electron diffraction. Annealing at temperatures as low as 200 ° C causes rapid intermixing, with only about one monolayer of Si (θ ≈ 1) remaining in a disordered surface layer. Annealing at successively higher temperatures causes further intermixing and the appearance of a c(2 × 2) ordered overlayer (θ = 0.5). The rate of O chemisorption and the structure of the oxide are found to depend on the structure of the adsorbed Si layer. For the c(2 × 2) layer, O₂ exposure leads to the rapid formation of multiple Si---O bonds at all surface Si sites, as in the case of bulk Ni₃Si. This effect is less pronounced (and the saturation O coverage lower) for a surface having a somewhat higher Si coverage but in the form of a disordered layer. The results are interpreted in terms of the involvement of underlayer Ni atoms in promoting O₂ dissociation.     

Stimulated desorption from CO chemisorbed on Cr(110)

Electron stimulated desorption (ESD) experiments using a time-of-flight pulse counting method are reported for molecular CO chemisorbed on the Cr(110) surface at 90 K. Consistent with previous qualitative observations, negligible CO⁺ and O⁺ desorption signals were measured from the ₁CO overlayer which saturates at 1/4 monolayer. For θ_CO > 0.25, a terminally-bonded (₂CO) binding mode is populated in addition to the existing ∝₁CO binding mode and the ion yield increases sharply. For ₂CO, both O⁺ and CO⁺ ions are observed; the CO⁺ ions desorb with characteristically lower kinetic energies than O⁺ ions. Near saturation coverages of CO(ads), an observed decrease in the O⁺ yield is attributed to adsorbate-adsorbate interactions which reduce the ion desorption probability, as seen in ESD studies of terminally-bonded CO on other metals. These results are considered in the context of two possible models proposed for the ₁CO binding state and related ESD observations for CO chemisorbed on Fe(001) and potassium-promoted Ru(001).     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Measurement of electro-optic coefficients of low-symmetry crystal YbCa4O(Bo3)3

In this paper, we report on all the electro-optic coefficients of the low-symmetry crystal YbCa₄O(BO₃)₃ measured by the interferometric method. The new sample orientations, which have not been reported so far, have been used for measuring the skew electro-optic coefficients γ₅₁, γ₅₃, γ₄₂ and γ₆₂ independently. The results obtained are γ₁₁=0.6, γ₂₁=0.4, γ₃₁=0.3, γ₁₃=0.3, γ₂₃=0.2, γ₃₃=2.2, γ₅₁=0.9, γ₅₃=4.1, γ₄₂=0.8 and γ₆₂=0.4×10⁻¹² m/V.