A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

Theoretical study of the Cl-passivated Si(111) surface

The Cl-passivated Si(111) surface is studied using density functional theory, in conjunction with the B3LYP functional and the cluster model. We compute the Si–Cl frequency and the Si–Cl bond energy for R₃SiCl, and the abstraction barrier for the reaction R₃SiCl+H→R₃Si+HCl using the B3LYP approach. We calibrate the B3LYP bond energy and the abstraction barriers using the values obtained using the G2MP2, G2 and CCSD(T) approaches. Our computed B3LYP Si–Cl frequency of 555 cm⁻¹ is in good agreement with the experimental value of 588 cm⁻¹. The shift in the Si–Cl frequency as surface chlorine is added to the cluster agrees with experimental observations.     

XPS investigation of a-Si : H thin films after light soaking

Amorphous hydrogenated silicon thin films prepared by homogeneous chemical vapour deposition have been studied. The Si2p, O1s and C1s electron spectra have been recorded after different light soaking times using repeated 100 mW/cm² white light illumination and X-ray photoelectron spectroscopy (XPS) measurements. The change of the position and intensity of the Si2p peak has been observed after light soaking and is explained by the transformation of the Si–H bonds. The correlation between the micropore density in a-Si : H film and the binding energy of Si2p electrons is demonstrated.     

The structure of C60 and endohedral C60 on the Si{1 0 0} surface

The possible structures of C₆₀ on the Si{1 0 0} surface in the four dimer position over the dimer trench have been investigated using ab initio total energy minimisations. Four possible structures have been found. The fullerenes bond to the silicon surface by breaking carbon–carbon double bonds. One electron from the broken bond is contributed to the carbon–silicon bond. The second electron is involved in forming a new π-bond within the fullerene cage. The carbon–silicon bond is primarily covalent with some charge transfer. Some discussion of endohedral fullerenes is also given.     

NIR-FT-SERS of 4″-trimethylsilylethylsulfanyl-4,4′-di(phenyleneethynylene)benzenethiol on Au nanospheres

Oligo(phenyleneethynylene) (OPE) compounds have been identified as promising molecular electronic bridges. Self-assembled monolayers of 4″-trimethylsilylethylsulfanyl-4,4′-phenyleneethynylenebenzene thiol (OPE′) on Au were characterized by surface-enhanced Raman scattering (SERS). The FT-Raman spectrum of OPE′ shows three C–S bands at 834, 1086, and 1131 cm⁻¹. From the FT-Raman to the SERS spectra, the 1086 cm⁻¹ band exhibits a 9 cm⁻¹ red shift. Chemisorption of OPE′ to the gold surface occurs via oxidative cleavage of the disulfide bond and the formation of the Au–S bond. The Au–S vibration is visible in the SERS spectra at 257 cm⁻¹. Peaks due to the S–S and S–H stretch are observed at 544 and 2519 cm⁻¹, respectively, in the FT spectrum, but are unobserved in the surface-enhanced spectra. The C–H stretching region (2700–3350 cm⁻¹) in the spectrum of neat OPE′ shows three distinct bands, whereas the SERS spectra show a single broad band. Assignments of vibrational bands were based on DFT calculations performed at the B3LYP level with good agreement between theoretical and experimental values. An average percent difference of 2.52 was obtained for the non-CH stretching frequencies.     

XPS investigation of chemical states in monolayers: Recent progress in adsorbate redox chemistry on sulphides

The paper presents new results on the adsorption of 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzoxazole (MBO) on galena (PbS) and pyrite (FeS₂). Adsorption of MBT and MBO on galena and pyrite surfaces from 10⁻⁵ M aqueous solution results in monolayers of chemisorbed deprotonated molecules bound to the surface via the thiolate group. A secondary interaction between the lone pair of the nitrogen and the sulphide surface may be possible. From the unequal intensity of the two S2p emissions in MBT an upright orientation of the molecule can be assumed. Adsorption of MBT from 10⁻⁴ M aqueous solution leads to the formation of 2,2′-dithiobis(benzothiazole) as an oxidation product of MBT. Oxidation of MBO is not observed which may be the result of its 100 mV higher redox potential.

Optical activation of the MBT oxidation on CdS and FeS₂ in contrast to PbS is suggested from the increase of the S2p signals attributed to the bridging –S–S– group of BBTD during illumination. We conclude that a surface photovoltage reducing the band bending may be responsible for the higher oxidation rate at the pyrite/electrolyte interface during illumination with respect to the dark.

Oxidation of sulphide surfaces by UV light in air produces lead(II) sulphate soluble in aqueous solution. Dissolved Pb²⁺ ions lead to the formation and precipitation of a Pb(mbt)₂ complex in alkaline MBT solution.     

Electron-stimulated nitridation of Si(100) in pure ammonia

We present the results of an AES study of the Si(100) electron-stimulated nitridation at RT by ammonia gas. The influence of the gas pressure and electron beam density on the nitridation rate have been determined within the ranges 10⁻⁶–10⁻⁹ Torr and 5 × 10⁻³–5 × 10⁻² A/cm², respectively. The silicon nitride growth rate has been found to be proportional to the electron flux and is enhanced with increased ammonia pressure in the range 10⁻⁹–10⁻⁷ Torr. Beyond 10⁻⁷ Torr the Si nitride growth rate is constant and independent of ammonia pressure. A phenomenological model of electron-stimulated nitridation process is suggested, which is in good agreement with the experimental data. The rate of electron-stimulated nitridation has been deduced.     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

Corrosion resistance study of Fe–Mn–Al–C alloys using immersion and potentiostatic tests

The interaction of the Fe–32.7Mn–6.59Al–1.26Si–0.25C (wt.%) and Fe–32.3Mn–8.54Al–1.31Si–0.54C (wt.%) alloys with the environment was evaluated. Potentiostatic and total immersion tests, planned and analyzed by the statistic model of fixed effects were used for the evaluation of corrosion in gasoline, alcohol fuel, lactic acid solution (40 wt.%), sodium chloride solution (3 wt.%), and boiler water. Potentiostatic tests in 1N H₂SO₄ medium presented that the alloys showed a tendency towards passivation. The role that aluminum and silicon play in alloy corrosion mechanism was discussed.     

Ion conductive characteristics of cross-linked network polysiloxane-based solid polymer electrolytes

A series of cross-linked network polysiloxanes containing oligoethylene oxide units, (OCH₂CH₂)_n, as internal free chains have been synthesized by performing hydrosilylation of partially PEO-substituted polysiloxane precursor with , ω-diallyl terminated poly(ethylene glycol). The polymer electrolytes were formed by complexing with LiN(CF₃SO₂)₂ electrolyte salt and exhibited superior conductive property. The σ_RT of the network polymer electrolytes is in the range of 2.50×10⁻⁵ to 1.62×10⁻⁴ S/cm and depends on the cross-linking density (in terms of Si–H amount of the siloxane precursor), repeating unit number of internal oligoethylene oxide and chain length of the cross-linker. The significant enhancement of the conductivity was observed when low molecular weight dimethyl poly(ethylene glycol) was added as plasticizer. The temperature dependence of the ionic conductivity was also studied, following the Vogel–Tamman–Fulcher (VTF) equation.     

Covalent binding of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm⁻¹ in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π_C=C in binding is further supported by the absence of C=C stretching modes and the disappearance of the π_C=C photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di-σ bonding. The existence of one π_C=C bond in their chemisorbed states is confirmed by the observation of the C=C and (sp²)C---H stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.     

Kinetics and mechanisms of ultrasonic degradation of volatile chlorinated aromatics in aqueous solutions

Ultrasonic decompositions of chlorobenzene (ClBz), 1,4-dichlorobenzene and 1-chloronaphthalene were investigated at 500 kHz in order to gain insight into the kinetics and mechanisms of the decomposition process. The disappearance of ClBz on sonication is almost simultaneously accompanied by the release of chloride ions as a result of the rapid cleavage of carbon–chlorine bonds with a concomitant release of CO, C₂H₂, CH₄ and CO₂. The intermediates resulting from attack of HO^√ radicals were detected but in a quite low yield (less than 2 μM). The generation of H₂O₂ on sonolysis is not significantly affected by the presence of aqueous ClBz while the generation of NO₂⁻ and NO₃⁻ is inhibited initially due to the presence of ClBz which diffuses into the gas–bubble interfaces and inhibits the interactions between free radicals and nitrogen. Moreover, brown carbonaceous particles are present throughout the ultrasonic irradiation process, which are consistent with soot formation under pyrolytic conditions. These important features suggest that, at the relatively high initial substrate concentrations used in this study, ultrasonic degradation of ClBz takes place predominantly both within the bubbles and within the liquid–gas interfaces of bubbles where it undergoes high-temperature combustion. Under these conditions, the oxidation of ClBz by free radical HO^√ outside of bubbles is a minor factor (though results of recent studies suggest that attack by HO^√ is more important at lower initial substrate concentrations). The sonochemical decomposition of volatiles follows pseudo-first-order reaction kinetics but the degradation rates are affected by operating conditions, particularly initial substrate concentration and ultrasonic intensity.     

Si–SiO2 interface formation in low-dose low-energy separation by implanted oxygen materials

The evolution of the Si–SiO₂ interface morphology of low-dose low-energy separation by implanted oxygen materials was investigated by transmission electron microscopy and atomic force microscopy. The Si–SiO₂ interface morphology and the RMS roughness are strongly affected by the implantation conditions and the annealing process. Three main types of the domains including round, square, and pyramid shapes with the step-terrace structure were observed on the buried SiO₂ surface. Round domains are observed in the early stage of the annealing process, while the square and pyramid domains are observed after the high temperature annealing. The mean RMS roughness decreases with increasing time and annealing temperature, while in the 1350 °C 4-h annealed samples, the mean RMS roughness decreases with either increasing the implantation dose or decreasing implantation energy. The scaling analysis shows that the Si–SiO₂ interfaces were found to be self-affine on the short length scales with a roughness exponent above 0.50. Qualitative mechanisms of Si–SiO₂ surface flattening are presented in terms of the variations of morphological features with the processing conditions.     

Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation

Irradiation of SiO₂ with soft X-ray photons (hν>100 eV) produces a variety of defects, of which E₁′ centers and neutral Si–Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO₂ film diminishes the number of defects which are assigned to E₁′ centers by about half. The competing channels for annihilation of E₁′ centers are the recovery of the Si–O–Si bonding configuration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si–Si bond units and precipitates as nanocrystalline particles of Si.     

An infrared spectroscopic investigation of thin alumina films: measurement of acid sites and surface reactivity

The surface chemical activity of an alumina films grown on Mo(100) by oxidation of aluminum evaporated onto the surface and oxidized using water is examined using Auger, X-ray photoelectron and reflection/absorption infrared spectroscopies. The formation of alumina is confirmed using Auger and X-ray photoelectron spectroscopy from the positions and intensities of the aluminum features and using reflection-absorption infrared spectroscopy from the longitudinal optical modes of the Al–O bonds measured at 935 cm⁻¹. The presence of surface hydroxyls is monitored by forming films using D₂O which are evidenced by a feature at 2700 cm⁻¹. Ammonia adsorption on a dehydroxylated surface yields a single peak at 1260 cm⁻¹ due to ammonia adsorbed at a surface Lewis site where the principle symmetry axis of ammonia is oriented perpendicularly to the surface plane. Ammonia also appears to adsorb at Lewis sites on a hydroxylated surface with a slightly different adsorption geometry from that on a dehydroxylated surface. Finally, the chemistry of trimethyl aluminum adsorbed on the planar hydroxylated alumina surface is compared with that found on high-surface-area γ-alumina where the spectra and the chemistry found in both régimes is exactly identical except that the low-frequency methyl bending modes (at 769 and 718 cm⁻¹) are not obscured on the thin film by the intense substrate whereas they are on the high-surface-area support.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Surface tension and density measurements for indium and uranium using a sessile-drop apparatus with glow discharge cleaning

The sessile-drop method is used to measure the surface tension and density of liquid indium and uranium under high vacuum. Measurements are made over the temperature range 156–500°C for In and at the melting point for U. Surface oxides are efficiently removed with a glow discharge system. Drop profiles are captured by photograph and processed using nonlinear regression to yield the surface tension and density. In this regression procedure, normal distances from calculated profiles to data points are minimized. For indium, the density and surface tension measurements yield _mp = 7.05 × 10³kg/m³, d/dT = −0.776 kg/m³·°C, and γ_mp = 0.568 N/m, dγ/dT = −9.45 × 10⁻⁵ N/m·°C. The results for uranium at the melting point are _mp = 17.47 × 10³ kg/m³ and γ_mp = 1.653 N/m.     

One- and two-dimensional solid-state magic angle spinning NMR studies on the hydration process of layered sodium disilicate SKS-6

Three different kinds of silanols, which include isolated silanol, silanol I (with the hydroxyl proton bonded to an oxygen atom in the adjacent layer) and silanol II (with the hydroxyl proton bonded to the non-bridging oxygen at the same silicon atom), are generated during the hydration process of SKS-6 (δ-Na₂Si₂O₅). ¹H–¹H nuclear Overhauser enhancement spectroscopy reveals that the proton of silanol I has an effective chemical exchange or spin diffusion with the proton of hydrogen-bonded water, while the proton of silanol II is likely far away from the other proton-containing species. ²⁹Si magic angle spinning, ¹H→²⁹Si CP/MAS NMR and ¹H–²⁹Si phase-modulated Lee–Goldburg decoupled correlation experiments demonstrate that the local environments of the silicon sites in the final hydrated sample are mainly composed of Q² [(SiO)₂Si(OH)O⁻Na⁺], Q³ [(SiO)₃Si(OH) and (SiO)₃SiO⁻Na⁺] and Q⁴ [Si(OSi)₄] groups.     

Microstructural analysis of hard amorphous carbon films deposited with high-energy ion beams

Hard amorphous carbon films produced using high-energy (ca. 30 keV) ion beam deposition of CH₃⁺ and CH₄⁺ on silicon wafers, have been investigated by Positron Annihilation Spectroscopy (PAS), the results are correlated with Raman Spectroscopy and Electrical Resistivity measurements. The microstructural modifications of the films as a function of the annealing temperature in the 300–600°C range have been studied. The evolution of the fractions of sp² and sp³ bonds is described and related to the changes of the open volume defect distribution and the graphitization process.     

HREELS of H/GaN(0001): evidence for Ga termination

Two issues relevant to the growth and processing of GaN are the termination of the GaN(0001) surface and its reaction with hydrogen. We have used high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES) to study the adsorption of hydrogen on MOCVD-grown GaN(0001). LEED of the sputtered and annealed surface shows evidence of facetting. No adsorbate vibrations are observed on the clean surface by HREELS, only Fuchs–Kliewer phonons at intervals of 700 cm⁻¹. Following exposure of the clean GaN surface to hydrogen atoms, HREEL spectra show adsorbate loss peaks at 2580, 3280, and 3980 cm⁻¹. The Ga–H stretching vibration at 1880 cm⁻¹ becomes evident when the HREEL spectrum is deconvoluted to remove the phonon multiple-loss peaks. We assign the 2580, 3280, and 3980 cm⁻¹ peaks to combination modes of the Ga–H stretch and phonon(s). Upon dosing with deuterium, the Ga–D bending mode is observed at 400 cm⁻¹. No vibrational peaks due to N–H (N–D) species are observed after H (D) exposure. We conclude that sputtered and annealed GaN(0001) is Ga-terminated.