Photoemission studies of oxygen adsorbed on a LiAl(1 1 0) alloy surface: Role of Li segregation

We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlO_x and Li₂O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O₂ exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation.     

Tip-induced local anodic oxidation on p-GaAs surface with non-contact atomic force microscopy

Nano-sized oxide structures resulted from localized electrochemical oxidation induced by a negatively biased atomic force microscopy (AFM) tip operated with the non-contact mode were fabricated on p-GaAs(1 0 0) surface. The geometrical characteristics of the oxide patterns and their dependences on various fabrication parameters, e.g., the anodization time, the biased voltages, the tip scanning rates, as well as the formation mechanism and relevant growth kinetics are investigated. Results indicate that the height of the protruded oxide dots grow exponentially as a function of time in the initial stage of oxidation and soon reaches a maximum height depending linearly with the anodized voltages, in according with the behaviors predicted by space charge limited local oxidation mechanism. In addition, selective micro-Auger analysis of the anodized region reveals the formation of Ga(As)O_x, indicating the prominent role played by the field-induced nanometer-size water meniscus in producing the nanometer-scale oxide dots and bumps on p-GaAs(1 0 0) surface.     

Local chemical distribution and electronic structure of Ge1−xTx DMS single crystals (T=Cr,Mn, Fe)

The spatial concentration distribution and local electronic structure of ferromagnetic Ge_1−xT_x (T=Cr, Mn, Fe) DMS single crystals have been investigated by using scanning photoelectron microscopy (SPEM), X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). It is found that doped T ions in Ge_1−xT_x crystals are chemically phase-separated, suggesting that the observed ferromagnetism arises from the phase-separated T-rich phases in Ge_1−xT_x.     

Features of microarc oxidation coatings formation technology in slurry electrolytes

A group of electrolytes composed of different powders entered into electrolyte presents a separate direction in microarc oxidation (MAO) technology. This group of electrolytes is most complex and however it has not been sufficiently studied yet. This paper shows that addition of powders of different nature into electrolyte influences both on properties of obtained coatings and on speed of their formation. The MAO-coatings formed in similar electrolytes-suspensions or slurry electrolytes (SE) contain in their composition an essential share of powder material entered into electrolyte as corpuscles of disperse phase (DP). Different nonmetallic combinations are used: oxides, carbides, borides, nitrides of metals and others. The particular conditions of microarc discharge regime (high temperature and pressure in a zone of breakdown) cause a thermolysis of electrolyte together with weighed corpuscles of solid matter, the realization of plasma chemical reactions on oxidizing surface and provide the synthesis of chemical combinations in an oxide matrix. A new composition of slurry electrolyte which allows improving the quality of obtained coatings on the basis of held investigations was developed.     

On the growth of conversion chromate coatings on 2024-Al alloy

The initial growth of chromate conversion coatings on aluminium 2024-T3 alloy has been investigated by scanning Auger microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The coating initiation is shown to be influenced by the alloy microstructure. In agreement with previously proposed growth models, Cr(VI) to Cr(III) reduction begins on the Al-Cu-Fe-Mn intermetallic second-phase particles, which act as cathodic sites, and then over the entire Al matrix surface. The less noble Al-Cu-Mg second-phase particles demonstrate dual behaviour during the initial stage of coating; some dealloy, with formation of a Cu-rich sponge-like structure, while others show no evidence for etching during the first few seconds and coating deposits on them similar to the situation for the Al-Cu-Fe-Mn particles. XPS measurements show more Cr(III) at the very initial stage of nucleation and growth, whereas the amount of Cr(VI) in the coating increases with the length of the chromating treatment. This is discussed in relation to Raman spectroscopy measurements made in a separate study.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.     

Adsorption of BTSE and γ-GPS organosilanes on different microstructural regions of 7075-T6 aluminum alloy

The adsorption of bis-1,2-(triethoxysilyl)ethane (BTSE) and γ-glycidoxypropyltrimethoxysilane (γ-GPS) on mirror-polished 7075-T6 aluminum alloy was studied with an emphasis on the different microstructural regions of the alloy surface, specifically the alloy matrix and the two main types of second-phase particles, as well as how the adsorption was affected by a heating pre-treatment and by changes in the pH of the γ-GPS solution. Surface characterizations were undertaken with scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). BTSE at its natural pH (4.3) adsorbed at all micro-regions of the air-oxidized surface, while γ-GPS at its natural pH (5.7) was largely ineffective. Adsorption of γ-GPS on all micro-regions was possible after adjusting the solution pH to a lower value (3.2), or by using the solution of natural pH after pre-treating the sample by heating at 200 °C for 15 min. TOF-SIMS measurements indicated that direct metal-O-Si covalent bonding occurred at each silane interface formed to the different micro-regions of the alloy surface, with Al-O-Si bonding being predominant in each case.     

The effect of pH and role of Ni in zinc phosphating of 2024-Al alloy: Part II: Microscopic studies with SEM and SAM

Coatings formed on 2024-T3 aluminum alloy were studied by scanning electron microscopy (SEM) and scanning Auger microscopy (SAM) after dipping in zinc phosphating (ZPO) baths at different acidities, with or without the Ni²⁺ additive. The objective was to learn more about the ZPO coating mechanism on the different microstructural regions of 2024-T3. When the initial coating solution pH is 4 (optimal acidity), a slower etching rate at the Al-Cu-Fe-Mn intermetallic particle causes significant precipitation of ZnO, which differs from the coating on other regions of the surface where phosphate predominates. The larger crystals (∼μm dimension) on the matrix and the Al-Cu-Mg particle contain more phosphate compared to other areas on the surface. When Ni²⁺ is added to the coating solution, the Al-Cu-Mg particle is more thickly coated compared to when the Ni²⁺ is not present. The slower rate of precipitation when Ni²⁺ is present in the coating solution increases the exposure of the alloy substrate to the acidic environment, so allowing more dissolution of Mg and Al from the Al-Cu-Mg particle. This results in the particle becoming more cathodic in nature, and therefore more coating deposits at this location. Evidence from SAM supports the presence of NiAl₂O₄, hypothesized in Part I, forming at coating pores later in the process.     

New TiO2 precursor for TiO2:poly(N-vinylcarbazole) (PVK) thin film: Synthesis and reactivity to hydrolysis of titanium tetrakis (9H-carbazole-9-yl-ethyl-oxy)

The chemical reactivity of the ligand initially coordinated on the TiO₂ precursor plays a decisive role in the morphology of TiO₂:poly(N-vinylcarbazole) (PVK) thin film elaborated by in situ generation of the inorganic phase in the polymer matrix. The final aim of this study is to prepare a new nanocomposite TiO₂:poly(N-vinylcarbazole) (PVK) thin film from hydrolysis-condensation of titanium alkoxide in polymer thin film. In this context, we synthesized a new TiO₂ precursor, the tetrakis (9H-carbazole-9-yl-ethyl-oxy) [Ti(OeCarb)₄], bearing ligands close to the repetitive unit structure of the PVK to improve the interaction between both materials. In this study, the synthesis and reactivity to hydrolysis of Ti(OeCarb)₄ is presented. Ti(OeCarb)₄ was elaborated from alcoholysis reaction between titanium isopropoxide [Ti(ⁱOPr)₄] and 9H-carbazole-9-ethanol (ECOH) and identified by ¹H and ¹³C NMR spectroscopy. The reactivity to hydrolysis-oxolation of Ti(OeCarb)₄ was evaluated first in aqueous media by in situ ¹H NMR spectroscopy analysis. Moreover, reactivity of Ti(OeCarb)₄ to surrounding humidity was evaluated in thin film by X-ray photoelectron spectroscopy (XPS). Results show that steric hindrance of 9H-carbazole-9-yl-ethyl-oxide ligands do not influence the hydrolysis-condensation process in aqueous media in our experimental conditions when compared to [Ti(ⁱOPr)₄] but decrease the reactivity when the precursor is simply exposed to air humidity.