XPS study of residual oxide layers on p-GaAs surfaces

The total thickness and composition of a residual oxide layer after chemical etching of p-GaAs:Zn + In has been studied by X-ray photoelectron spectroscopy (XPS). The variation of the Ga to As oxides ratio along the depth has been determined. A concentration correlation of doping isovalent impurity and the dislocation density with the composition of residual oxides is looked for. The total thickness of the residual oxide layer on p- and semi-insulating GaAs is about 5–6 Å. It is found that the Ga₂O₃ quantity in the oxide bulk is greater than the same value of As₂O₃ in highly In-doped samples. In-doping in concentrations over 1.5 × 10¹⁹ cm⁻³ increases the Ga₂O₃ content and the density of the residual oxide. This influence is determined by reducing the dislocation density and changing the point defect environment. The presence of As-rich precipitates on the dislocations and in the matrix decreases the sputtering time and changes the composition of the residual oxide. The correlation between the type of high temperature dislocations revealed by Abrahams-Buiocchi (AB) etching and the oxide layer composition is shown. The results obtained could be used in the first stages of epitaxial growth, metallization and other technological processes of semiconductor device and ICs fabrication.     

Four cyclo­alkane­spiro‐4′‐imidazolidine‐2′,5′‐dithiones

The crystal structures of four cyclo­alkane­spiro‐4′‐imidazolidine‐2′,5′‐dithiones, namely cyclo­pentane­spiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diaza­spiro­[4.4]­nonane‐2,4‐dithione}, C₇H₁₀N₂S₂, cyclo­hexane­spiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diaza­spiro­[4.5]decane‐2,4‐dithione}, C₈H₁₂N₂S₂, cyclo­heptane­spiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diaza­spiro­[4.6]undecane‐2,4‐dithione}, C₉H₁₄N₂S₂, and cyclo­octane­spiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐di­aza­spiro­[4.7]dodecane‐2,4‐dithione}, C₁₀H₁₆N₂S₂, have been determined. The three‐dimensional packing in all of the structures is based on closely similar chains, in which hydantoin moieties are linked through N—H⋯S hydrogen bonding. The size of the cyclo­alkane moiety influences the degree of its deformation. In the cyclo­octane compound, the cyclo­octane ring assumes both boat–chair and boat–boat conformations.     

Hydrogen isotope exchange in a coverage obtained on Ir(111) by ethylene adsorption

HREELS and SIMS studies of hydrogen isotopc exchange in a coverage obtained on Ir(111) by ethylene adsorption are carried out at 180–450 K and at a hydrogen (deuterium) pressure up to 8×10⁶ Pa. The ethylidyne species have shown a high stability towards hydration and structural changes upon hydrogen (deuterium) exposures. Under these conditions hydrogen exchange in the methyl groups is a slow process. With increasing temperature the hydrogen exchange in the decomposition products of ethylidyne (C₂H species) is quick and depends on the exchanged amount of atomically adsorbed hydrogen.     

Biomonitoring of elemental atmospheric deposition: spatial distributions in the 2015/2016 moss survey in Bulgaria

Journal of Radioanalytical and Nuclear Chemistry - In the most recent 2015/2016 campaign of the European-scale moss survey, Bulgaria joined for the fifth consecutive time. To investigate elemental...     

Comparative study on energetic distortions of SO42− ions matrix-isolated in compounds with kröhnkite-type chains,K2Me(CrO4)2·2H2O and Na2Me(SeO4)2·2H2O (Me = Mg,Co, Ni,Zn, Cd)

Infrared spectra of compounds with kröhnkite-type infinite octahedral–tetrahedral chains, K₂Me(CrO₄)₂·2H₂O and Na₂Me(SeO₄)₂·2H₂O (Me = Mg, Co, Ni, Zn, Cd), as well as infrared spectra of the title double salts containing matrix-isolated SO₄^2? guest ions are presented and discussed in the regions of the X–O stretching modes.The SO₄^2? guest ions matrix-isolated in selenate and chromate matrices exhibit four infrared bands corresponding to the four site-group components of the stretching modes in good agreement with the low site symmetry of the host ions (C₁ site symmetry). The values of Δν₃ (site-group splitting) and Δν_max (the difference between the highest and the lowest wavenumbered components of the stretching modes) are used as an adequate measure for the extent of energetic distortion of the matrix-isolated SO₄^2? guest ions.The influence of different crystal-chemical parameters (Me²⁺–OXO₃ bond strengths, sizes of the Me²⁺ and Me⁺ ions, electronic configurations of the Me²⁺ ions, hydrogen bond strengths, and unit-cell volumes of the host compounds) on the extent of energetic distortion of both the host SeO₄^2? and CrO₄^2? ions, and the SO₄^2? guest ions is analyzed. Correlations between the values of Δν₃ and Δν_max of the guest ions and both the degree of covalency of the respective Me²⁺–OXO₃ bonds and the electronic configurations of the Me²⁺ ions have been found and will be discussed. For example, the energetic distortion of SO₄^2? ions included in the chromate lattices decreases in the order Zn > Cd > Mg as a result of the decreasing covalency of the respective Me²⁺–O bonds in the same order (Δν₃ have values of 73, 58 and 36 cm^?1, respectively). Furthermore, the values of Δν₃ and Δν_max are larger when the metal ions have CFSE ≠ 0 (crystal field stabilization energy, Co²⁺, Ni²⁺). These cations are more resistant to angular deformations of the MeO₆ octahedra (i.e. changes in the O–Me–O bond angles), thus facilitating the extent of distortion of the matrix-isolated SO₄^2? ions as compared to those having CFSE = 0 (Mg²⁺, Zn²⁺ and Cd²⁺). For example, Δν₃ and Δν_max of SO₄^2? ions matrix-isolated in K₂Zn(CrO₄)₂·2H₂O have values of 73 and 163 cm^?1, and 116 and 207 cm^?1 in Na₂Zn(SeO₄)₂·2H₂O, whereas in the respective nickel lattices Δν₃ and Δν_max have values of 88 and 173 cm^?1 (K₂Ni(CrO₄)₂·2H₂O) and 127 and 212 cm^?1 (Na₂Ni(SeO₄)₂·2H₂O).The SO₄^2? guest ions included in selenate matrices, Na₂Me(SeO₄)₂·2H₂O, are remarkably much distorted than in chromate ones, K₂Me(CrO₄)₂·2H₂O, as deduced from the values of Δν₃ and Δν_max owing to a stronger static field caused by the smaller Na⁺ ions as compared to that caused by the larger K⁺ ions. The smaller unit-cell volumes of the selenate host compounds, i.e. the higher repulsion potential at the lattice sites at which the guest ions are situated additionally favor the extent of energetic distortion of the sulfate guest ions in the selenate matrices.     

Preparation,crystal structure and infrared spectroscopy of the new compound Rb4Be(SeO4)2(HSeO4)2·4H2O

A new compound, Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O, crystallizes in a comparatively wide concentration range from mixed beryllium rubidium selenate solutions (from solutions containing 29.06 mass% beryllium selenate and 25.75 mass% rubidium selenate up to solutions containing 12.53 mass% beryllium selenate and 55.32 mass% rubidium selenate).Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O crystallizes in the acentric orthorhombic space group Pmn2₁ (a = 32.607(4), b = 10.676(2), c = 6.069(1) Å, V = 2112.8 ų, Z = 4, R1 = 0.047 for 4059 F_o > 4σ(F_o) and 311 variables). The crystal structure is composed of Be(H₂O)₄ tetrahedra arranged in layers at x = 0 and x = ½, alternating with broad layers built up from SeO₄ and HSeO₄ selenate tetrahedra and Rb cations. The beryllium–water layers are linked to the rest of the structure via hydrogen bonds only. The H₂O molecules as well as the OH molecules of the acid HSeO₄ groups form strong to very strong hydrogen bonds with donor–acceptor distances between 2.58 and 2.74 Å.Vibrational spectra (infrared and Raman) of Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O are presented and discussed in the region of the fundamentals of both the selenate and the beryllium tetrahedra (skeleton motions) as well as in the region of the OH vibrations at ambient and liquid nitrogen temperature (LNT). The appearance of four Raman bands corresponding to ν₁ of the selenate ions reflects the existence of four crystallographically different selenate tetrahedra in the structure. The spectroscopic experiments reveal that the ν₁ modes of the selenate ions appear at higher frequencies than some components of ν₃. Bands of an AB doublet structure (2950, 2390 cm^?1) arising from the OH stretching modes of the HSeO₄^- ions are recognized in the infrared spectra. The appearance of two infrared bands (1308, 1250 cm^?1) corresponding to δ(OH) (in-plane bending modes of the OH groups) confirms the structural data regarding the existence of two crystallographically different OH groups. The water librations are also briefly commented. The appearance of a band at a comparatively large wavenumber (1013 cm^?1) corresponding to rocking librations of the water molecules indicate that strong hydrogen bonds are formed in the title compound.     

Interfacial layers from the protein HFBII hydrophobin: dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.     

Acetylene adsorption on a clean iridium surface

The interaction of C₂H₂ with a polycrystalline iridium surface at 140 KT500 K has been studied by AES, XPS and UPS. A model of the bonding between C₂H₂ and Ir by the 1 orbitals of the hydrocarbon is proposed. The more pronounced energy splitting between the orbitals of acetylene during adsorption compared to that in the gas phase is ascribed to deformation of the acetylene molecule.

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C₂H₂ . 140 KT500 K AES, XPS UPS. C₂H₂ Ir 1- . - , , .

     

Real-time holography in ruthenium-doped bismuth sillenite crystals at 1064 nm

Real-time holographic recording and an improvement of the response time in ruthenium (Ru)-doped Bi(12)SiO(20) (BSO) crystal at 1064 nm is obtained. Using green light pre-exposure, a significant operation speed of 60 ms is achieved. In addition, the ability for image reconstruction is demonstrated in Ru-doped BSO, supporting further applications as reversible media for real-time image processing at the near-IR spectral range.