Thin Surface Oxide Layer Research Articles

Environmental Implications of Nanoparticle Aging in the Processing and Fate of Copper-Based Nanomaterials

Copper nanomaterials are being used in a large number of commercial products because these materials exhibit unique optical, magnetic, and electronic properties. Metallic copper nanoparticles, which often have a thin surface oxide layer, can age in the ambient environment and become even more oxidized over time. These aged nanoparticles will then have different properties compared to the original nanoparticles. In this study, we have characterized three different types of copper-based nanoparticle (NP) samples designated as Cu(new) NPs, Cu(aged) NPs, and CuO NPs that differ in the level of oxidation. The solution phase behavior of these three copper-based nanoparticle samples is investigated as a function of pH and in the presence and absence of two common, complexing organic acids, citric and oxalic acid. The behavior of these three copper-based NP types shows interesting differences. In particular, Cu(aged) NPs exhibit unique chemistry including oxide phases that form and surface adsorption properties. Overall, the current study provides some insights into the impacts of nanoparticle aging and how the physicochemical characteristics and reactivity of nanomaterials can change upon aging.

Structure, Chemical Composition, And Reactivity Correlations during the In Situ Oxidation of 2-Propanol

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.

Stable Aqueous Based Cu Nanoparticle Ink for Printing Well-Defined Highly Conductive Features on a Plastic Substrate

With the aim of inkjet printing highly conductive and well-defined Cu features on plastic substrates, aqueous based Cu ink is prepared for the first time using water-soluble Cu nanoparticles with a very thin surface oxide layer. Owing to the specific properties, high surface tension and low boiling point, of water, the aqueous based Cu ink endows a variety of advantages over conventional Cu inks based on organic solvents in printing narrow conductive patterns without irregular morphologies. It is demonstrated how the design of aqueous based ink affects the basic properties of printed conductive features such as surface morphology, microstructure, conductivity, and line width. The long-term stability of aqueous based Cu ink against oxidation is analyzed through an X-ray photoelectron spectroscopy (XPS) based investigation on the evolution of the surface oxide layer in the aqueous based ink.

Improvement of the wettability of SiMn IF-HSS by liquid zinc by controlling the dew point of the annealing gas atmosphere

The wettability of low-carbon, 0.3 wt%Si–0.4 wt%Mn interstitial-free steel by liquid zinc at 450 °C was investigated using the dispensed sessile drop method. Before the wetting tests, the steel samples were annealed in a 15%H2–Ar gas atmosphere at three different dew points, namely −60, −40, and 0 °C. It was found that as the dew point was increased from −60 to −40 °C, the wettability became poorer. However, as the dew point was increased further to 0 °C, the wettability was dramatically improved and was better than that of −60 °C. In order to understand the dramatic change in wettability, the surfaces of the steel samples after annealing were analyzed with SEM and TEM. It was found that the surface oxide changed from randomly distributed hemisphere particles of 20–30-nm high on a very thin oxide film to a film-like layer ~15-nm thick as the dew point was increased from −60 to −40 °C, and at the dew point of 0 °C, internal oxidation was so pronounced that a very thin surface oxide layer 1–2-nm thick was formed. It was believed that the improvement of the wettability at the dew point of 0 °C was caused by the short diffusion distance in the surface oxide layer.

Fabrication and photoluminescence properties of silicon nanowires with thin surface oxide layers

Si nanowires have been synthesized by a novel oxide-assistant growth mechanism using boron powder and silicon oxide as reactants. Their structures and morphologies have been investigated using X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The obtained nanowire consists of a single-crystalline Si core and a very thin amorphous boron oxide layer. Photoluminescence investigations reveal that the Si nanowires possess a broad red emission band. The outside amorphous oxide layer plays an important role for the luminescence. The study suggests that the Si nanowires can find potential applications in nanoscale electric and optoelectronic devices.

Evaluation of oxide layers formed during the decarburisation of grain-oriented electrical steel using a Fourier transform infrared (FTIR) technique

Electrical steels are highly specialised, magnetically soft materials, used to form the cores that carry the magnetic flux in electrical machines such as motors, generators and transformers. During the production of GO electrical steel, the strip passes through a decarburisation furnace, which promotes the formation of a thin surface oxide layer consisting of predominantly fayalite (Fe 2SiO 4) and silica (SiO 2). During a subsequent high temperature anneal, this layer reacts with magnesia (MgO) to form a forsterite ‘glass film’ layer, which applies a tensile stress to the steel. This reduces the magnetic losses of the material on which the final product is routinely graded. Due to the effect that the oxide layer has on the quality of the final material, it would be beneficial to possess a technique that can rapidly assess its composition and/or morphology. This paper details the assessment of Fourier transform infrared (FTIR) and electrochemical potential (ECP) analysis, and a technique of combining the two. FTIR analysis of the decarburisation oxide layer exhibited evidence of just fayalite, with silica only being observed on the spectra following brief acid etching. To refine the etching process, samples were removed from the acid at various intervals based on the output of the ECP technique. It was established that there was a clear link between the position reached on the ECP profile and absorption bands observed on the corresponding FTIR spectra.

Electron transport properties of indium oxide – indium nitride metal‐oxide‐semiconductor heterostructures

AbstractThe structural, chemical and electron transport properties of In2O3/InN heterostructures and oxidized InN epilayers are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer. The epitaxial In2O3/InN heterojunctions show an increase in the electron concentration due to the increasing band banding at the heterointerface. The oxidation of InN results in improved transport properties and in a reduction of the sheet carrier concentration of the InN epilayer very likely caused by a passivation of surface donors. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth control of GaAs nanowires using pulsed laser deposition with arsenicover-pressure

Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowireshave been systematically investigated. The single-crystal structure and geometry of thenanowires have been characterized for various growth conditions. Arsenic over-pressure withAs2 molecules was introduced into the system by thermal decomposition of polycrystallineGaAs to control the stoichiometry and shape of the nanowires during growth. GaAsnanowires exhibit a variety of geometries under varying arsenic over-pressures, which canbe understood by different growth processes via a vapor–liquid–solid mechanism. WithoutAs2 over-pressure, branched growth of GaAs with uncontrollable size and geometry wasobserved due to the decomposition of GaAs nanowires, producing metallic Ga which servesas catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimalAs2 over-pressure, single-crystal GaAs nanowires with uniform diameter, small diameter distribution, lengthover 20 µm and thin surface oxide layer were obtained and used forI–V characterization.

ASR evaluation of different kinds of coatings on a ferritic stainless steel as SOFC interconnects

The demanding technical requirements for solid oxide fuel cell (SOFC) interconnect materials pose development challenges for commercially-viable systems. SOFC interconnects physically separate and manifold fuel (reducing) and oxidant (oxidizing) gases, while electrically connecting adjacent cell electrodes (anode to cathode) into series, and also permitting SOFC stacking and sealing. Due to the challenging multifunctional requirements, SOFC interconnect material investigations are intensifying to enable deployment of highly-efficient and inexpensive SOFC systems. Commercial ferritic stainless steels are used as interconnects in SOFC operating at ∼ 800 °C. They contain elements such as Rare Earths, Mn, Al, and Ti as microalloying elements, designed to produce a thin surface oxide layer with specific characteristics: high mechanical strength; excellent adhesion to the metallic substrate; low growth rate (interconnect durability must be around 40,000 h); high chemical resistance (to avoid Cr depletion and electrode poisoning) and, low area specific resistance (ASR < 10 − 1 Ω·cm 2). However, long-term performance of commercial and even speciality ferritic stainless steels remains insufficient, and therefore surface treatments and/or coatings are required. These treatments have several important tasks, with the most important being to stabilize condensed-phase Cr(III) compounds in order to limit formation of volatile Cr(VI) compounds, which are responsible of SOFC cathode poisoning. This paper deals with the efficacy of two different kind of promising coatings: reactive element oxide nanolayers, produced by metal–organic chemical vapour deposition (MOCVD); and, dual segment coatings (CrAlYO + CoMnO), obtained by filtered arc deposition for the CrAlYO bottom segment and filtered arc-assisted electron beam physical vapour deposition for the CoMnO top segment. The coatings were applied on Crofer 22 APU (a commercial ferritic stainless steel from Thyssenn Krupp VDM) and tested under conditions simulating the aggressive service environment of a SOFC interconnect.

Effect of surface oxidation on electron transport in InN thin films

The chemical and electron transport properties of oxidized indium nitride epilayers and indium oxide/indium nitride heterostructures are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer using an ozone-assisted oxidation processing. It results in improved transport properties and in a reduction of the electron sheet concentration of the InN epilayer caused by a passivation of the surface donors and a shift of the electron density distribution peak from the surface toward the bulk InN. Using the ensemble Monte Carlo simulation method, the electron mobility for different dislocation densities and surface band bending values has been calculated. The theoretical results correlate well with our experimental data. In opposition to the ozone treatment, in epitaxial oxide/nitride heterojunctions the electron sheet concentration of InN raises due to the increasing band bending at the heterointerface affecting adversely the electron transport properties.

Metastable surface modification processes for corrosion and oxidation control

This technical note discusses recent advances in the field of high energy surface alloying with particular reference to their non-stoichiometric solid state interaction and their corrosion resistance behaviour by electrochemical techniques. High energy treated surfaces undergo solid state interaction like diffusion, unconventional miscibility, solid solubility and non-stoichiometric compositions which impose a certain degree of metastability on the surface, which alters the electrode surfaces significantly. By introducing a high degree of surface metastability, it is possible to create scope for the physical and chemical properties of the electrodes. This transition of the interface from the metastable state to a stable state is normally accompanied by the formation of a stress free stable thin adherent and protective surface oxide layer. All these aspects are discussed in the present paper in the context of their corrosion resistance industrial usage, in particular to chloroalkali industrial applications. A generalised mechanism of protection of these high energy treated surfaces, is proposed based on surface metastability induced on the surface, as a result of the non-stoichiometry of solid state processing and its electrochemical interactions.

Atomic-scale simulations of nanoindentation-induced plasticity in copper crystals with nanometer-sized nickel coatings

The mechanical behaviour of very small volumes differs from what is typically observed on the macrolevel. In particular, thin metal films with nanometer-sized coatings possess interesting mechanical properties. Here, we use molecular dynamics simulations to elucidate details of plastic deformation and the underlying deformation mechanisms during nanoindentation of thin copper films with epitaxial nickel coatings. Several deformation mechanisms, such as dislocation pile-up on the interface, dislocation cross-slip and movement of misfit dislocations, are observed. Most interestingly, copper films are significantly strengthened by thin nickel coatings. This effect is especially pronounced at a very low temperature. For nickel coatings as thin as 1.6 nm (at low temperature) or 2.5 nm (at room temperature), the coated copper adopts the hardness of bulk nickel, even though the operating deformation mechanism is unique in each case considered and the hardness of bulk copper is significantly lower. The critical thickness at which a coated copper film achieves hardness of bulk nickel depends not only on the temperature, but also on the radius of the indenter tip. Sharper indenters yield higher hardness. The obtained results suggest that thin surface coatings or oxide layers at nanoscale contacts may have a pronounced effect on the perceived hardness of the surface.

AC-Electrochemical Cleaning Versus AC- Anodising of Aluminium

SUMMARYAC-electrochemical cleaning and AC-anodising of rolled AA1050 in phosphoric acid have been investigated under constant AC-current conditions. Inductively coupled plasma optical emission spectrometry (ICP-OES) and potential response measurements demonstrate the transition from AC- anodising to AC-cleaning for different electrolyte concentrations and current densities. The mechanisms of the anodic and cathodic reactions reveal themselves in the respective potential responses. With TEM a thin non-porous oxide film of less than 10 nm is observed on the AC-cleaned substrate. AES depth profiling reveals the presence of remnant phosphorus from the electrolyte in the thin surface oxide layer.

A relation between surface oxide and oxygen-defect complexes in solid-phase epitaxial Si regrown from ion-beam-amorphized Si layers

We present a direct evidence that ion implantation through thin (⩽5 nm) surface oxide layers is a source of O impurities, which form O-defect complexes during thermal treatment. The impurity-defect complexes are identified by correlating the results from positron annihilation spectroscopy, secondary-ion mass spectroscopy, and Monte Carlo simulations. The O atoms are introduced in the bulk by multiple recoil implantation by the primary ions. The signatures of large VmOn formations are observed at 800 °C, which implies the existence of smaller species at lower temperatures.

Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies

ABSTRACTThe effects of Chem-Mechanical Polishing (CMP) and Rapid Thermal Annealing (RTA) on n-type 4H:SiC samples doped with nitrogen were investigated using Raman scattering and X-ray Phtoelectron Spectroscopy (XPS a.k.a. ESCA) measurements. A comparison of the Raman spectra from Mechanically Polished (MP) SiC annealed at 600°C and 800°C displays a frequency shift in the coupled plasmon LO-phonon mode. Since the coupled mode frequency is a direct measure of the free carrier concentration, this observation may suggest the removal of polishing induced carrier traps with increasing annealing temperature. The CMP samples did not show this frequency shift, thereby indicating that such polishing traps were not created in that process. The Si-peak observed in the XPS spectra of the unannealed CMP sample indicates primarily a Si-C bonding, while that for the MP sample is more complex, indicating other bonds beside Si-C. Drastic changes in O, C, Si surface content were observed for annealing between 1000°C and 1100°C. The peaks in the XPS spectra associated with the chemical environment for C, O, and N are complex and may be explained as silicon oxycarbide type structures near the surface or possibly around the interface of the SiC substrate with a thin surface oxide layer.

Arrays of ungated GaAs field emitters fabricated by wet or dry etching

Dry and wet etching methods are used to fabricate arrays of ungated GaAs field emitters. Comparisons were made among tips, wedges, pillars and walls. Effects of the etching technique and subsequent deoxidation procedure on the GaAs surface are evaluated in terms of emitter geometry, uniformity, heating and integration. Moreover, the current emission capability of field emitter arrays is investigated. In particular, the influence of a thin surface oxide layer on the emission stability is observed. An extraction of geometrical parameters is attempted on the basis of a statistical examination of the emission characteristics and the validity of this approach is discussed.

Electronic structures of adsorbed buffer layer on metallic electrodes

We present electronic and structural features of adsorbed nitrogen buffer layers on nickel electrodes covered with a thin surface oxide layer. The electrical structures of adsorbed buffer layer were obtained by a new in situ monitoring technique, which is named as angle-resolved transmission current spectroscopy in magnetic field. The probe is electron beam, whose momentum is controlled by external magnetic field. The measure quantity is transmission probability T of the incident electrons into the surface as functions of electron energy and magnetic field. The results are expressed as contour plots of d T/d k ‖ in two-dimensional wave number space k=(k ‖, k ⊥) . The information on atomic arrangement on the surface was derived at grid potential V g=−1 V for controlling the incident electrons. Significant pressure dependence of the surface electronic structures were derived in 10 −7 Torr< P N2<1×10 −3 Torr. At P<10 −7 Torr, the NiO surface showed a superlattice structure with 2D lattice constant, a=1.894 nm, b=0.936 nm which were 6 times and 3 times of a modified α-NiO structure, whose lattice constant is 6% larger than that of the equilibrium phase. Adsorbed nitrogen atoms formed extra spots in contour plots and a phase transition of surface electronic structure occurred at P N2=6×10 −5 Torr. At P N2=10 −3 Torr, the adsorbed nitrogen molecules showed a clear three-dimensional structure which were related to the sites and energies of adsorbates.

Removal of Surface Oxides on Copper by Pulsed Laser Irradiation

The removal of an oxide layer from a copper surface by pulsed laser irradiation was studied to improve the soldering property of Sn–Pb. The copper surface was irradiated by ArF excimer laser or the second harmonic pulsed Nd3+:YAG (532 nm) laser. It was found that the thin surface oxide layer can be removed by laser irradiation even in air without degradation of the copper surface layer. However, highly intense irradiation causes significant changes in the surface morphology, resulting in the reduced wettability for the Sn–Pb solder.

Improvement in high frequency magnetic properties of amorphous cores by thin surface oxide layers on ribbons

A thin oxide layer with the effective thickness of about 150 nm was prepared on a surface of a thin Co-based amorphous ribbon by acid-treatment followed by annealing, and magnetic properties of cores made of it were studied. The oxide layers increased electric resistance between the ribbons and improved high frequency magnetic properties of the cores. For a core made ofa 6.8-μm-thick ribbon, the acid-treatment increased the real part of the complex permeability - , μ', by 60% and decreased tanδ /μ' (δ: the loss angle) by 48% at 10MHz. The magnetic loss at a large excitation level was also reduced by the treatment. The obtained improvement was more remarkable than that obtained previously for the cores made of the ribbon with silicone interlayer insulation.

Chromium oxide gas sensors for the detection of hydrogen, oxygen and nitrogen oxide

The gas sensitivity of chromium oxide, a semiconductor metal oxide, to hydrogen in air and to oxygen in hydrogen is presented and the operation principle is described. It is shown that when this oxide is doped Pd promoter, it has high sensitivity to hydrogen concentrations of a few ppm in air. When this oxide is doped with Pt and In it acquires room-temperature sensitivity. Sensors tested in flowing forming gas (50–100 cm 3 min −1 detect oxygen down to a few ppm concentration. It is shown that Pd-doped sensors have a switching characteristic at hydrogen/oxygen stoichiometric ratios of 1:1, resulting in a very sharp change in their conductance. It is found that the role of In, an n-type promoter, is to transform Cr 2O 3 from a high-resistivity p-type to a low-resistivity n-type oxide. Pt provides room-temperature sensitivity due to its thin surface oxide layer. In flowing forming gas the sensors detect oxygen concentrations of 50 ppm at room temperature, but at about 100 °C the oxygen sensitivity increases and a concentration of a few ppm is detectable.