Average Stoichiometry Research Articles

Calibration-Free Determination of Copper, Zinc, Cadmium and Lead in Tap Water Using Coulometric Stripping Potentiometry

Coulometric stripping potentiometry has been used to achieve calibration-free determinations of Cu(II), Zn(II), Cd(II) and Pb(II) in tap water samples. With the coulometric approach, interference from the formation of Cu-Zn intermetallic compounds (CuZnx) on the quantification of Zn(II) was circumvented and the average stoichiometry of CuZnx could be determined. For Zn to Cu concentration ratios below 1, CuZn is quantitatively formed. At higher ratios amalgamated Zn coexists with CuZn, CuZn2 and CuZn3. Both mercury soluble and mercury insoluble CuZnx exist. The former dissociates, with a half time of the order of one second, at potentials between the oxidation potentials for amalgamated Zn and that for CuZnx, while formation of the latter is irreversible. Due to interference from surface active groups on the glassy carbon surface, used as a substrate for the mercury film, determination of Zn(II) was not possible at Zn to Cu concentration ratios below approximately 0.1.

Surface hydroxylation and local structure of NiO thin films formed on Ni(111)

X-ray photon spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been combined to study the surface hydroxylation and the structure of NiO thin films formed on Ni(111) at 300 and 500 K. After oxidation at 300 K, the thin film consists of NiO(111) grains (3 ± 1 nm large) in parallel epitaxy with the substrate and homogeneously distributed on the substrate. The average thickness is three to four NiO(111) layers with local variations measured on different grains [ca 1 NiO(111) plane] and at boundaries between grains (up to 0.6 nm). The film is terminated by 0.85 ± 0.1 monolayer (ML) of hydroxyl groups (OH − according to XPS) stabilizing the unreconstructed (1 × 1) oxide surface structure. Exposing this film to 150 L (1 Langmuir = 10 −6 Torr s −1) of water leads to the formation of a second layer of hydroxyl groups with a total coverage of 1.5–2 ML with no significant variation of the structure of the film. The XPS data are well fitted with a terminal sequence (OHNiOH) equivalent to one layer of β-Ni(OH) 2 with partial filling of the topmost OH plane. After oxidation at 500 K, the surface is covered at 93 ± 3% by extended and atomically flat NiO(100) areas. Triangular NiO(111) grains (8 ± 2 nm large) in parallel epitaxy are present within the NiO(100) layer and cover 7 ± 3% of the surface. They are covered by a full monolayer of OH − groups also stabilizing the (1 × 1) structure. The NiO(100) layer is characterized by (2 × 2), (2 × 3), (3 × 2) and (3 × 3) local superstructures assigned to local deviations from the average NiO stoichiometry in the topmost plane. These areas do not adsorb hydroxyl groups on regular sites but possibly on defect sites (steps). Exposing this film to 150 L of water leads to the lateral extension of the triangular NiO(111) grains. A preferential reorientation of hydroxyl-free NiO(100) to OH −-covered NiO(111) upon water exposure is observed. This reorientation affects the NiO(100) matrix only at the perimeter of the NiO(111) grains, showing that the hydroxyl groups do not induce nucleation of NiO(111) in the NiO(100) layer but cause the growth of the existing NiO crystallites.

Surface modification of InAs(110) surface by low energy ion sputtering

Sputter-induced electronic states and stoichiometry at the low ion-energy sputter-annealed InAs(110) surface compared to cleaved InAs(110) have been studied by Auger and UV photoelectron spectroscopy. Sputtering modifies the surface electronic structure even with 0.5 keV Ar + ions. In particular, InAs(110) surface states are quenched, while sputter-induced electronic states in the band gap close to the Fermi level are present. After the sputtering process a variation of the In bonding coordination is observed in the surface and subsurface region while maintaining the correct average stoichiometry. Subsequent annealing induces a reordering process and a light accumulation layer in the subsurface region.

Surface chemistry of 6H-SiC(000) after reactive ion etching

Abstract X-Ray (Al Kα 1486.6 eV) induced core level photoemission spectra of C-terminated 6H-SiC surfaces are reported with the intention to elucidate some of the surface chemistry responsible for the reactive ion etching process in gas mixtures of CHF3/O2 as a function of gas composition. After etching, the SiC surface exhibits a carbonaceous surface layer that is enriched in fluorine solely bonded to C, such that an average stoichiometry of CF1.6 is maintained. Despite a dramatic increase in etch rate with the addition of oxygen this layer remains essentially unchanged in stoichiometry while the thickness decreases from 60 A (at 5% O2) to 25 A (20% O2). Fluorine could be quantitatively removed by annealing at 850°C leaving a graphitic layer on the surface. Oxygen is observed at the SiC-CFy interface in the form of SiOz (z = 1…2).

Ion-assisted etching and profile development of silicon in molecular chlorine

An ion beam etching study designed to characterize the kinetic and transport processes important in the ion-assisted etching of silicon in molecular chlorine was performed. Monoenergetic argon ion beams were directed normal to a silicon wafer that was simultaneously exposed to a background of molecular chlorine, thereby simulating a low temperature rf plasma etching process. The ion-induced etching yield scaled with the square root of the ion energy for a surface saturated with adsorbed chlorine. Moreover, the yield was found to depend strongly on the ion to neutral flux ratio which was varied over two orders of magnitude. A kinetic model in which the dissociative sticking probability and the yield scale linearly with surface coverage was developed and was found to be consistent with the yield data in the ion energy range of 90–300 eV. Its applicability to etching topography was tested with additional experiments where patterned silicon wafers (with oxide masks and with a range of features and linewidths) were etched at saturation in the apparatus. Minimal lag effects and some microtrenching and sidewall sloping were encountered. The etching selectivity of silicon over oxide at 100 eV was greater than 30. Computer simulations of the etching process and profile development were performed using the kinetic model and a line-of-sight re-emission model for the chlorine transport. Knudsen diffusion was found to be important in the transport of neutrals to the base of the feature, with the aspect ratio dependence of the etch rate becoming significant only at high values of the ion to neutral flux ratio. The agreement between simulation and experiment was good, with the initial sticking probability on a clean surface and the average product stoichiometry as parameters comparing favorably to published results. However, the observed microtrenching and sidewall sloping were not predicted by these simulations.

Formation of a copper-butyl ethoxycarbonyl thiourea complex

Butyl ethoxycarbonyl thiourea (BECTU) reacts readily with cupric solution to form an insoluble Cu-BECTU complex in a reaction which increases with pH. The selectivity of BECTU for copper ions was confirmed as no reaction was observed between BECTU and ferrous, ferric or lead ions. The complex was characterised by elemental analysis, nuclear magnetic resonance and fast atom bombardment mass spectroscopies as well as infrared and X-ray photoelectron spectroscopies. These analyses indicate that the complex has an extended structure with an average stoichiometry derived from a mixture of cuprous. CuBECTU, and cupric, CuBECTU 2, species. It is suggested that the reaction probably involves an electrochemical mechanism in which cupric ion is reduced to cuprous ion to form a copper chelate in the same way as xanthate collectors form stable, cuprous complexes. Infrared and X-ray photoelectron analyses suggest that BECTU acts as a bidentate ligand bonding with the copper ion through the carbonyl oxygen and the thiol sulfur atoms to form a six membered ring structure. Furthermore, these results support the earlier assertion that a proton is removed from the collector during the reaction with the copper ions.

Phase stability and stress relaxation effects of cubic boron nitride thin films under 350 keV ion irradiation

Abstract To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) films, samples either prepared by r.f. magnetron sputtering or ion beam-assisted sputter deposition (IBAD) were irradiated at room temperature with 350 keV inert ions (Kr + ). FTIR spectra taken before and after each irradiation step clearly demonstrate that c-BN is stable under this medium energy bombardment. Furthermore, additional AES measurements show that the average film stoichiometry is not affected by the ion fluences used in the present experiments. While the observed broadening of the different lines in the IR spectra with increasing ion fluences points to a build-up of disorder and/or a decreasing average grain size due to the bombardment, the additionally found significant shift of the line related to the c-BN TO-mode towards smaller wave numbers, indicates a strong relief of the compressive stress present in the as-prepared films. This irradiation-induced stress relief could be independently confirmed by profilometer measurements yielding post-bombardment values of 5.1 GPa as compared to 20.5 GPa before irradiation.

The applicability of implanted -sources to thickness and stoichiometry measurements of thin films

A method for determining both the thickness and the average stoichiometry of thin films is presented. The method is based on implanting radioactive -sources in the substrate prior to layer growth and measuring the energy loss of the -particles as they traverse the layer. Information about the stoichiometry is obtained through the comparison of the energy loss of -particles of different initial energies. Experimental examples for the utilization of this method are presented, in which Sb was grown on Si substrates, GaAs, InAs and AlAs on GaAs and YBCO on YSZ. The experimental precision which can be expected using the method is discussed, together with specific scenarios in which it could be advantageously applied.

The Formation of Cu3Si from Cu/a-Si Multilayers

ABSTRACTThe kinetics of the formation of Cu3Si in Cu/a-Si diffusion couples have been investigated by means of differential scanning calorimetry and x-ray diffraction. Multilayered composites of average stoichiometry Cu3Si were prepared by sputter deposition with individual layer thicknesses varying in different samples between 2 and 100 nm. We observed diffusion limited growth of Cu3 Si upon annealing these diffusion couples below 500 K. Reaction constants were measured for a temperature range of 455 to 495 K for thicknesses of growing Cu3Si between 2.6 and 80 nm. The temperature dependence of the reaction constant, k2, was characterized as k2 = k0 exp(− Ea/kbT) with activation energy, Ea = 1.0 eV/atom and pre-factor, k0 = 1.9×10−3 cm2/s.

Heavy ion time-of-flight ERDA of high dose metal implanted germanium

Heavy ion time-of-flight ERDA and conventional RBS have been used to study the implantation profiles and oxygen uptake of Ge(100) implanted with high dose Ti and Cu ions from a MEVVA ion source. Cu implantations were made at an incident ion dose of 3 × 10 17 atoms/cm 2, with Ge substrate temperatures at ambient, 200°C and 300°C. A significant change in the Cu implantation profile was observed at 300°C, consistent with that seen in thermal annealing studies. Ti implantations were made at 5 × 10 17 atoms/cm 2, with substrate temperatures of 200°C, 300°C and 370°C. Implantation of Ti gave rise to a surface layer of average stoichiometry GeTi 0.8, which grew in thickness with implantation temperature. No significant reaction with air or moisture was seen in the damaged Ge(100) implantation surface region.

Ion beam analysis of ultrathin dielectric films

Nuclear reaction analysis and nuclear resonance profiling methods are currently used to study silicon-based ultrathin (thickness < 10 nm) dielectric films, like silicon oxide, silicon nitride and silicon oxinitrides. The nuclear reactions are used to measure the total amounts of hydrogen, nitrogen and oxygen isotopes that take part in the films, with sensitivities as low as 10 12 atoms cm −2. Nuclear resonance profiling and also step-by-step chemical etching associated with nuclear reaction analyses are used to measure the concentration versus depth profiles of hydrogen, nitrogen, oxygen and silicon isotopes with depth resolutions of about 1 nm. Furthermore, channeling of alpha-particles combined with detection at very grazing angles is used to measure the average stoichiometry of the ultrathin films.

Redox reactions of hydrosulphide ions on the platinum electrode—II. An impedance spectroscopy study and identification of the polysulphide intermediates

Impedance spectroscopy was used to confirm the presence of a poorly conductive sulphur layer on a platinum electrode during the electrolysis of HS −. The double layer capacitance was found to be close to 40μF cm −2 at open circuit potential. When electrolysis commenced the double layer capacitance was greatly reduced (< 5μF cm −2). The charge transfer resistance increased to few hundred ohms cm 2 during electrolysis. Double potential chronocoulometric experiments were also performed. The chronocoulometric analysis showed that the average stoichiometry of the intermediate polysulphide ions in the electrolysis reaction was S 2.5 2− in the anodic step and S 4.3 2− in the cathodic step.

Al deposition on Fe: Formation of an iron aluminide surface alloy

An investigation has been made of the formation of a surface iron-aluminum alloy through aluminum adsorption (and subsequent reaction) on Fe(100) single-crystal and polycrystalline Fe surfaces. On the Fe(100) surface, Auger electron spectroscopy and low-energy ion scattering spectroscopy (LEISS) studies indicate that at low Al coverages (&amp;lt;1 ML), and a very low Al deposition rate (&amp;lt;0.03 ML/min), a surface alloy grows uniformly at 25 °C to yield an aluminide with an approximate average stoichiometry of FeAl3. At higher Al exposures the surface becomes more aluminum rich. This Al enrichment is due to a kinetic limitation in the formation of the surface alloy. Heating the surface (300 °C during or after Al deposition) partially overcomes this kinetic limitation, and the topmost surface layer changes to a stable stoichiometry with some reduction in the relative aluminum concentration. Low-energy electron diffraction observations made during the Al deposition (or subsequent heating experiments) gave no indication of long-range order, suggesting the formation of a disordered surface alloy. LEISS experiments show that at 25 °C, the final stoichiometric configuration of the surface is highly dependent on the average arrival rate of Al at the surface. A higher deposition rate ultimately leads to a more aluminum-rich surface than for an equivalent dose at a lower rate. This may be rationalized by noting that at higher deposition rates the Al atoms are more likely to encounter other Al atoms at the surface and may proceed to form two-dimensional microclusters of Al instead of reacting to form a surface or subsurface aluminide alloy. The thermodynamic implications of these observations in the context of the formation of intermetallic alloy phases seen here will be discussed.

Fluorinated fullerenes: Synthesis and characterization

AbstractThe existence of the fully fluorinated fullerene, C60F60, is still subject to controversy. Mass spectrometric evidence shows that it exists, albeit in trace amounts five orders of magnitude in concentration below the most abundant C60F40‐42. Generally, mixtures of C60Fn are obtained where n ranges from 30 to 52, depending on fluorinating conditions. The species with n = 36 is particularly stable. Attempts to increase n by use of the strongly fluorinating halogen fluorides, ClF3 or BrF5, have led to products C60FXXyOz (X = Br or Cl), where the origin of the oxygen is probably hydrolysis during analysis. Fluorinated C60 crystallizes as a mixture of hexagonal close packed (40%) and face‐centered cubic (60%) phases. X‐ray analysis yields an average C‐F bond length of 1.49A. Fluorination of C70 leads to mixtures with maximum average stoichiometries of C70F52.

Local stoichiometry measurement of Y-Ba-Cu-O thin layers grown by metal organic chemical vapor deposition

Y-Ba-Cu-O thin films with different stoichiometry were grown by metal organic chemical vapor deposition on MgO and SrTiO3. The samples showed superconducting behavior in a wide range of average stoichiometry. However, energy dispersive x-ray spectroscopy (EDX) measurements show that best results can be obtained only in a small stoichiometry range. Critical temperatures of Tc≳89 K and critical current densities of jc (77 K)≳2×106 A/cm2 could be obtained with optimized stoichiometry. Spatially resolved EDX measurements show that copper tends to form precipitates on the layers if its average content exceeds 50 mol %, while yttrium is incorporated into the matrix if offered in excess.

Plane-specific oxide stoichiometries on Fe single-crystal surfaces

A high-energy ion scattering study reveals that oxide layers on Fe(100) and Fe(110) have different average stoichiometries if the oxides are grown at room temperature. On Fe(110), Fe 2O 3 is observed, in contrast with Fe(100) where Fe 0.95O has been observed. This difference persists if the surfaces are precovered with one monolayer platinum. On the basis of the stoichiometries the geometrical thicknesses at which the oxidation process saturates at room temperature is found to be equal for both surfaces, despite the larger oxygen uptake by the Fe(110) surface in saturation. Equal thicknesses are in agreement with the Fromhold-Cook theory of metal oxidation.

Selectivity Patterns in Alkane Oxidation over Mg3(VO4)2-MgO, Mg2V2O7, and (VO)2P2O7

The catalytic oxidation of ethane, propane, 2-methylpropane, butane, pentane, and cyclohexane has been studied over Mg3(VO4)2-MgO, Mg2V2O7, and (VO)2P2O7. These reactions produced a wide range of products ranging from alkenes, dienes, anhydrides, acids, and carbon oxides. Broadly speaking, the major products were dehydrogenation products over Mg3(VO4)2, combustion products over Mg2V2O7, and oxygen-containing organic products over (VO)2P2O7, although there are exceptions to this broad generalization. From the product selectivities, the moles of oxygen consumed per mole of alkane reacted could be calculated. These were the average oxygen stoichiometries for the reactions. They had values of about unity for ethane oxidation over VPO, about two when oxidative dehydrogenation was the predominant reaction, and about four for the other reactions studied. The selectivity patterns in terms of the formation of dehydrogenation products versus oxygen-containing products including carbon oxides were explained by assuming that there existed a selectivity-determining step, which could be the reaction of the surface alkyl species or the adsorbed alkene before its desorption. The formation of oxygen-containing products would be facilitated if the surface alkyl or adsorbed alkene could interact with the vanadium ions in two adjacent VOx units such that it could easily react with the reactive oxygen in the V-O-V bridge. The results of ethane oxidation, however, could not be explained simply by this model, and other factors needed to be considered.

Decomposition of Molybdenum Hexacarbonyl Deposited on Dehydroxylated Alumina

Deposition and decomposition of Mo(CO)6 on dehydroxylated alumina was studied by XAFS. The rate of deposition and the structure of deposited species depended on deposition temperature. In the deposition at 320 K, the rate was slow and the species on the support was found to be Mo(CO)5. The species formed at 360 K had an average stoichiometry of Mo(CO)2.5 without well-defined structure. Heating of the deposited subcarbonyls in flowing H2 caused first simple decarbonylation and then methanation above 450 K. Around 930 K, highly carbon deficient Mo2C was formed which was converted to metallic Mo upon further heating to 970 K.

Auger electron spectroscopy and secondary-ion mass spectroscopy study of interdiffusion in gold-bismuth oxide and aluminium-bismuth oxide thin films

Thin-film bismuth oxide systems were prepared by depositing gold or aluminum on glass substrates, followed by deposition of bismuth over the gold or aluminium metal layer, after which thermal oxidation was carried out to yield a Bi2O3 layer. Auger electron spectroscopy (AES) and secondary-ion mass spectroscopy (SIMS) were performed on these thin-film systems. AES results show that the average stoichiometry of bismuth oxide films is represented by Bi2O3. SIMS analysis suggests that the oxidation process results in extensive interdiffusion for the BiOx-Al-glass system. The extent of interdiffusion in the BiOx-Au-glass system is certainly far less extensive than in the BiOx-Al-glass system.

Combined X-ray and neutron powder diffraction study of the structure of Ba 4CaCu 3O x: Oxygen stoichiometry and cation order

Cation ordering and oxygen stoichiometry of the nonsuperconducting compound, Ba 4CaCu 3O x , have been studied by a joint X-ray and neutron powder diffraction experiment. In contrast to the related phase Ba 4YCu 3O 8.5+δ, which was refined in the space group Pm3, we found that the higher symmetry space group Im3 m provides a better fit to the diffraction data for Ba 4CaCu 3O x . The cubic unit cell of Ba 4CaCu 3O x has a perovskite-like configuration, but possesses lattice lengths double those of a usual perovskite cell. X-ray scattering data clearly show substantial exchange between the Ca and Ba sites. Refinements of neutron data indicate two oxygen sites: an ( x, 0, 0) site that is essentially fully occupied and a ( 1 4 , 0, 1 2 ) site that is approximately 47% occupied. The joint X-ray and neutron refinement yields an average stoichiometry of Ba 3.92Ca 1.08Cu 3O 8.85 for the compound ( Z = 2) and an effective CuO hole concentration of +0.29. Ba 3.92Ca 1.08Cu 3O 8.85 is a dielectric; the absence of superconductivity in this material is discussed in terms of its structure and oxygen stoichiometry.