Crystalline Oxide Layer Research Articles

Spin-coated BiFeO3 films on Si wafers: Low processing temperature but prominent piezoelectricity

The direct integration of crystalline oxide layers with industrial Si substrate, specifically compatible with CMOS technology, requires the development of relatively simple, low-temperature processing routes below 450 °C. Here, a novel nonstoichiometric approach is proposed to achieve fabrication of BiFeO3 films at 450 °C. Of particular importance is that, a saturation and remnant polarization of ∼80 μC/cm2 and ∼60 μC/cm2 and a strain as large as 1% are obtained. This strain stands as one of the most impressive values reported for thin films, comparable to the most superior strain obtained in ferroelectric films fabricated at temperatures exceeding 700 °C. The current work provides a new paradigm with significant simplicity and novel efficacy in reducing processing temperatures, as well offers a promising material for memory and piezo-driven actuating applications, especially meeting the increasing demand for precision position control systems at the nanometer scale.

Electrodeposition of Zn and Cu Nanoparticles into TiO2 Nanotubes on Ti6Al4V: Antimicrobial Effect against S. Epidermidis and Cytotoxicity Assessment

Surface modification of the Ti6Al4V alloy (ASTM grade 5), with the fabrication of vertically oriented TiO2 nanotubes, has been receiving increasing attention both as a way to provide advanced bioactive features and the ability to act as reservoirs for a localized, controlled drug release. In this work, TiO2 nanotubes were grown on the surface of a Ti6Al4V alloy through electrochemical anodization. An ethylene glycol-based electrolyte containing 0.5 wt.% NH4F and 2.5% (v/v) H2O was used. Post-anodizing heat treatments at 500 °C in air atmosphere were performed to achieve a crystalline oxide layer with a higher mechanical stability. Following these treatments, Zn or Cu nanoparticles were incorporated into the nanotubular structures through electrodeposition processes. Then, the antimicrobial performance of the obtained surfaces was assessed against Staphylococcus epidermidis, a Gram-positive bacterium common in implant-related infections. Lastly, the cytotoxicity of the produced surface was evaluated against MC3T3-E1 mouse pre-osteoblast cells. In general, Cu-doped TiO2 nanotubes presented an almost total antimicrobial action, while Zn doped samples had a lower, but still significant antibacterial effect. However, a highly cytotoxic effect against MC3T3-E1 cells was observed on all anodized samples due to the release of vanadium from the alloy. In spite of this, the surface modification reported in this work can be a valid solution for existing commercially available orthopedic implants, considering that similar solutions were already studied in in vivo assays.

Dual‐Mode Operation of Epitaxial Hf0.5Zr0.5O2: Ferroelectric and Filamentary‐Type Resistive Switching

Since the discovery of its ferroelectricity, hafnium oxide is widely used for applications in ferroelectric field‐effect transistors and ferroelectric tunnel junctions. is especially favored for its robust ferroelectricity and high remanent polarization at low thicknesses. In addition, is well established as amorphous or crystalline oxide layer in resistive switching devices. Herein, ferroelectric switching is found coexisting with high on/off ratio resistive switching in sub‐10 nm epitaxial . The resistive switching shows typical characteristics of a filamentary‐type valence change memory (VCM), clearly contradicting the polarization charges as the origin of different resistance states. In contrast to previous observations, no electroforming step is required to initiate VCM switching. The bottom electrode enables a RESET to the virgin state, allowing subsequent ferroelectric hysteresis measurements. It is possible to change between both switching schemes repeatedly without impacting the ferroelectric performance. This indicates that ferroelectric switching and oxygen vacancy movement do not interfere with each other, and both switching phenomena can exist independently. This finding opens up ways to unite the different strengths of both switching mechanisms in the same stack. It becomes possible to assign the two operating principles to artificial neural network training and inference according to their respective advantages.

The use of some sulfonic acids for improving aluminum current collector resistance

The corrosion behavior of aluminum current collectors in the standby and high-load modes was investigated by simulating via immersion and anodization experiments in electrolyte environments. The immersion experiments were carried out using aluminum alloy (AA5754) in camphorsulfonic (CSA), p-toluenesulfonic (PTSA), trifluoromethanesulfonic (TFMSA), and fluorosulfonic (FSA) acid solutions, and the results were compared with those of H 2 SO 4 and methanesulfonic acid (MSA). Corrosion resistance increased in the order of FSA, MSA, TFMSA, PTSA, H 2 SO 4 , and CSA. Thin-film XRD and XPS results revealed sulfonic acid adsorption to Al 2 O 3 . From AFM, the least rough and highest crystalline oxide layer was obtained with CSA, while FSA yielded the opposite. In addition, AA5754 was anodized in 0.1 M CSA, PTSA, and H 2 SO 4 solutions, and its corrosion behavior was examined. R ct values were about one hundred times higher than in the immersion experiments; besides, less rough and thicker layers were obtained on the surface owing to the controlled dissolution. The mixture of CSA:H 2 SO 4 (1:1) was used as an anodization solution, and the maximum resistance (5652 Ω.cm 2 ) was obtained at 20 V, which was a better corrosion behavior according to individual electrolytes. This could be due to the optimum combination of the high acidity of H 2 SO 4 and the ability of CSA to block the surface sites thanks to its larger cyclic structure. Finally, a symmetric supercapacitor cell prepared using a carbon-coated Al current collector in PVA/H 2 SO 4 :CSA gel electrolyte was tested by cyclic voltammetry and GCD. The cell provided 26 Wh.kg -1 and 10.2 kW.kg -1 with 89% efficiency and 64% retention after 5000 cycles over a wide potential range of 1.6 V, exhibiting better capacitive performance than the gel electrolyte without CSA. This increase could be because CSA contributes to the growth of the protective aluminum oxide layer as an anionic surfactant. • Sulfonic acids are adsorbed on aliminum alloy surface during immersion and anodization • Al 2 O 3 has the least rough and highest crystalline in the presence of camphorsulfonic and p-toluenesulfonic acids • The corrosion resistance of AA5754 significantly improves by anodization in the mixtures of sulfonic acids with H 2 SO 4 • Supercapacitor with Al current collector operates in a wide potential range of 1.6 V in PVA/H 2 SO 4 :CSA • The cell provides 26 Wh.kg -1 and 10.2 kW.kg -1 with 89% efficiency

Temperature-dependent oxidation behavior of arc evaporated Al–Ti–B–N coatings

Arc evaporated Al–Ti–B–N coatings were investigated concerning the oxidation behavior. The addition of B promotes the formation of an amorphous scale with mixed oxides of Al, Ti, and B, thus resulting in a higher oxidation rate than the Al–Ti–N coating at 800 °C. At higher temperatures (≥900 °C), B-containing coatings exhibit a better oxidation resistance. The oxidation at the interface between the oxide and nitride led to the continuous formation of the amorphous inner layer with excellent adhesion. Simultaneously, the crystalline outer oxide layer at elevated temperatures inhibited the inward diffusion of oxygen.

A Sustainable Self‐Induced Solution Seeding Approach for Multipurpose BiFeO3 Active Layers in Flexible Electronic Devices

AbstractThe direct integration of crystalline oxide layers into flexible electronic systems requires the development of relatively simple, low‐temperature processing routes. Seeding represents a powerful strategy to reach this objective by the generation of preferential sites for the nucleation of crystalline phases with a reduced energy barrier. Here, a novel approach is reported where nanoseeds are generated in situ from a precursor solution using a solvent‐engineering strategy (solvent‐antisolvent). The controlled addition of an antisolvent (1,3‐propanediol) to a solution of metal salts (Bi/Fe) dissolved in acetic acid results in the formation of nanocrystals (seeds) by supersaturation. The presence of such nanoseeds is also confirmed in the deposited layers, improving significantly the crystallinity of the respective BiFeO3 thin films as complementary, deduced by piezoresponse force microscopy. Using this low‐temperature strategy, crystalline films are directly grown on flexible polymeric substrates at only 350 °C showing a remanent polarization of 10.5 µC cm−2 and a clear photovoltaic effect (11.7 µW cm−2) of interest in computer memories and energy harvesters. The flexibility of the BiFeO3 thin films may enlarge the number of applications of this multifunctional, lead‐free material in next‐generation digital and sustainable electronic devices based on a facile, low‐cost fabrication method with reduced energy consumption.

Anodized titanium with calcium and phosphorus surface enhancements for dental and orthopedic implant applications

Titanium anodization in mixed-acid electrolytes containing phosphoric acid has been shown to result in phosphorus (P)-doped amorphous or crystalline titanium oxide layers. Recently, aqueous hydrothermal soak treatments have been shown to further enhance the bioactivity of titanium and titanium oxide through the addition of calcium (Ca) to the specimen surfaces. For the present study, a combination of mixed-acid electrolyte anodization and subsequent calcium nitrate tetrahydrate (Ca(NO3)2) hydrothermal treatments were used to create optimized titanium specimens with favorable Ca/P surface ratios. Commercially pure titanium specimens were anodized in two mixed-acid electrolytes to final forming voltages of 72 V, 108 V, or 180 V. Three specimens per group were subsequently soaked for 72 h at 80°C in 1.2 M Ca(NO3)2 solution. Energy dispersive spectroscopy revealed the Ca(NO3)2-soaked specimens anodized to 108 V in each electrolyte exhibited favorable surface Ca/P ratios averaging of 2.27 and 1.67. Bioactivity testing revealed increased apatite formation on 108 V Ca(NO3)2-soaked specimens compared to their un-soaked counterparts. Pre-osteoblast cell culture studies showed similar cellular proliferation and differentiation but increased mineralization on Ca(NO3)2-soaked specimens. This two-step combination of anodization and subsequent Ca(NO3)2 hydrothermal treatments shows great promise for future biomaterials applications.

In-situ and ex-situ comparison of oxidation of Inconel 718 manufactured by selective laser melting and conventional methods up to 650 °C

This work reveals the initial stages of Inconel 718 (IN718) oxidation manufactured by selective laser melting (SLM). In-situ and ex-situ investigations were conducted on SLM-IN718 and a commercially obtained Inconel 718 sample (Comm-IN718) to compare their oxidation behavior. In-situ and ex-situ testing was performed using a unique synchronized system integrating precise thermal control with Raman spectroscopy, optical microscopy and scanning probe techniques. X-Ray diffractometry (XRD) and Scanning Electron Microscopy (SEM) were used for microstructure analyses before and after heating. Raman spectroscopy indicated that, in both samples, there is a threshold temperature for oxidation at around 300 °C. The highest rate of oxidation was observed up to 550 °C as different oxides form. After 550 °C, there was no noticeable formation of new compounds up to 650 °C. Extended heating at 650 °C resulted in further changes in the Comm-IN718 sample, however the SLM-IN718 sample was unchanged by longer heating times. XRD of cooled samples did not detect any oxides because of the structurally disordered nature of the oxides. After being cooled, Comm-IN718 showed undesirable oxides like iron oxides in an inhomogeneous crystalline oxide layer. For SLM-IN718 the continuous unbroken oxide layer on the cooled SLM-IN718 sample imparted more effective protection of the alloy.

Suppression of vapor-liquid-solid (VLS) mechanism in the growth of α-Sb2O4 nanobelts by a vapor-deposition approach

In this work we report on the synthesis of novel α-Sb2O4 nanostructures synthesized by a gold catalyzed vapor deposition method in which metallic Sb is used as a precursor. Belts, rods and zigzag morphologies were obtained and characterized by a group of techniques which also provided data on crystalline structure and compositional aspects. Structural characterizations revealed that although the addition of gold nanoparticles plays an important role in the final growth of the nanostructures, no evidence of this element was found in the as-grown samples. In order to clarify the causes of the suppression of the VLS mechanism in these conditions, we carried out experiments in which the syntheses were interrupted in a controlled manner. Our findings revealed that at the early stages of the growth VLS mechanism is suppressed when high levels of Sb supersaturation are reached. A thick crystalline oxide layer rapidly grows at the liquid-gas interface providing preferential sites for the VS growth to take place. Low temperature photoluminescence measurements revealed a strong emission in the visible portion of electromagnetic spectra which can be associated to the presence of oxygen vacancies in these nanostructures.

Oxidation in wire arc additive manufacturing of aluminium alloys

Abstract Wire Arc Additive Manufacturing is a near-net-shape machining technology that enables low-cost production of large and customised metal parts. In the present work, oxidation effects in Wire Arc Additive Manufacturing of the aluminium alloy AW4043/AlSi5(wt%) were investigated. Two main oxidation effects, the surface oxidation on aluminium parts and the oxidation anomalies in aluminium parts were observed and analysed. The surface oxidation on aluminium parts changed its colour during Wire Arc Additive Manufacturing from transparent to white. In the present work, it was shown by high-speed imaging that this change in the surface oxidation took place in the process zone, which was covered by inert gas. Since the white surface oxidation formed in an inert gas atmosphere, it was found that the arc interacts with the existing amorphous oxide layer of the previously deposited layer and turns it into a white duplex (crystalline and amorphous) oxide layer. In addition to the analysis of the white surface oxidation, oxidation anomalies, which occur at low shielding from the environment, were investigated. It was shown by physical experiments and Computational Fluid Dynamics simulations, that these oxidation anomalies occur at inadequate gas flow rates, too big nozzle-to-work distances, process modes with too high heat input, or too high wire feed rates. Finally, a monitoring method based on light emission spectroscopy was used to detect oxidation anomalies as they create peaks in the spectral emission when they occur.

Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics.

Spin electronic devices based on crystalline oxide layers with nanoscale thicknesses involve complex structural and magnetic phenomena, including magnetic domains and the coupling of the magnetism to elastic and plastic crystallographic distortion. The magnetism of buried nanoscale layers has a substantial impact on spincaloritronic devices incorporating garnets and other oxides exhibiting the spin Seebeck effect (SSE). Synchrotron hard x-ray nanobeam diffraction techniques combine structural, elemental, and magnetic sensitivity and allow the magnetic domain configuration and structural distortion to be probed in buried layers simultaneously. Resonant scattering at the Gd L2 edge of Gd3Fe5O12 layers yields magnetic contrast with both linear and circular incident x-ray polarization. Domain patterns facet to form low-energy domain wall orientations but also are coupled to elastic features linked to epitaxial growth. Nanobeam magnetic diffraction images reveal diverse magnetic microstructure within emerging SSE materials and a strong coupling of the magnetism to crystallographic distortion.

Thin TiO2 Coatings by ALD Enhance the Cell Growth on TiO2 Nanotubular and Flat Substrates.

The present work exploits Ti sheets and TiO2 nanotube (TNT) layers and their surface modifications for the proliferation of different cells. Ti sheets with a native oxide layer, Ti sheets with a crystalline thermal oxide layer, and two kinds of TNT layers (prepared via electrochemical anodization) with a defined inner diameter of 12 and 15 nm were used as substrates. A part of the Ti sheets and the TNT layers was additionally coated by thin TiO2 coatings using atomic layer deposition (ALD). An increase in cell growth of WI-38 fibroblasts (>50%), MG-63 osteoblasts (>30%), and SH-SY5Y neuroblasts (>30%) was observed for all materials coated by five cycles ALD compared to their uncoated counterparts. The additional ALD TiO2 coatings changed the surface composition of all materials but preserved their original structure and protected them from unwanted crystallization and shape changes. The presented approach of mild surface modification by ALD has a significant effect on the materials' biocompatibility and is promising toward application in implant materials.

Synthesis of bioactive glass-based coating by plasma electrolytic oxidation: Untangling a new deposition pathway toward titanium implant surfaces

HypothesisAlthough bioactive glass (BG) particle coatings were previously developed by different methods, poor particle adhesion to surfaces and reduced biological effects because of glass crystallization have limited their biomedical applications. To overcome this problem, we have untangled, for the first time, plasma electrolytic oxidation (PEO) as a new pathway for the synthesis of bioactive glass-based coating (PEO-BG) on titanium (Ti) materials. ExperimentsElectrolyte solution with bioactive elements (Na2SiO3-5H2O, C4H6O4Ca, NaNO3, and C3H7Na2O6P) was used as a precursor source to obtain a 45S5 bioglass-like composition on a Ti surface by PEO. Subsequently, the PEO-BG coating was investigated with respect to its surface, mechanical, tribological, electrochemical, microbiological, and biological properties, compared with those of machined and sandblasted/acid-etched control surfaces. FindingsPEO treatment produced a coating with complex surface topography, Ti crystalline phases, superhydrophilic status, chemical composition, and oxide layer similar to that of 45S5-BG (~45.0Si, 24.5 Ca, 24.5Na, 6.0P w/v%). PEO-BG enhanced Ti mechanical and tribological properties with higher corrosion resistance. Furthermore, PEO-BG had a positive influence in polymicrobial biofilms, by reducing pathogenic bacterial associated with biofilm-related infections. PEO-BG also showed higher adsorption of blood plasma proteins without cytotoxic effects on human cells, and thus may be considered a promising biocompatible approach for biomedical implants.

Photoferroelectric Thin Films for Flexible Systems by a Three‐in‐One Solution‐Based Approach

AbstractThe effective incorporation of (multi)functional oxides into next‐generation flexible electronics systems requires novel fabrication technologies that enable the direct integration of crystalline oxide layers in them. Unfortunately, this is considerably challenging due to the thermal incompatibility between the crystallization temperatures of metal oxides (>600 °C) and the thermal stability of the flexible polymer substrates conventionally used (<400 °C). Here, it is shown that BiFeO3 thin films can be grown on flexible plastic by solution processing involving three different but complementary strategies to induce the crystallization of the perovskite phase at a lower temperature limit of 325 °C. This “three‐in‐one” approach is based on the synthesis of tailored metal precursors i) with a molecular structure resembling the crystalline structure of the oxide phase, which additionally allows both ii) photochemical and iii) internal combustion reactions taking place in the thin films. The flexible BiFeO3 thin films obtained from a specifically designed molecular complex with N‐methyldiethanolamine yield a large remnant polarization of 17.5 µC cm−2, also showing photovoltaic and photocatalytic effects. This result paves the way for the direct integration of an interesting class of oxides with photoferroelectric properties in flexible devices with multiple applications in information and communication technology, and energy.

Surface oxidation and crystallization of FeNi-Based soft magnetic nanocrystalline and amorphous nanocomposite alloys

In this work, we present characterization of surface oxidation and crystallization behavior of Metal Amorphous Nanocomposite (MANC) (Fe 70 Ni 30 ) 80 Nb 4 Si 2 B 14 alloys via complementary x-ray diffraction, electron microscopy, and x-ray photoelectron spectroscopy techniques. An adherent amorphous oxide of roughly 15–20 nm in thickness is observed on the wheel side of the ribbon, enriched in Fe relative to Ni and with significant content of Si and Nb as would be expected based upon a comparison of the Gibbs free energy of formation of the stable oxide phases. The amorphous character of the oxide is likely to result from the significant Si-content coupled with the amorphous and nanocrystalline character of the underlying metallic layer. Higher density of FeNi-enriched nanocrystals is also observed within ∼100 nm of the wheel-side surface, presumably due to reduction in local glass former concentration. A rough, crystalline copper oxide layer is also present and forms through contact between the copper wheel and the solidifying melt during the rapid solidification process. On the air side of the ribbon, a crystalline oxide layer of ∼15–20 nm is formed as well as intergranular oxidation that penetrates along the grain boundaries between highly textured surface crystallites of ∼50–100 nm in size. The crystalline nature of the oxide on these surfaces is asserted to result from the lack of significant Si-content and the pre-existing surface crystallites that form during the rapid solidification process which promote a crystalline oxide surface layer. These results shed new insights into the details of surface oxidation and can help to inform processing and alloy composition adjustments which seek to tailor surface properties.

Effect of strain on magnetic and orbital ordering of LaSrCrO3/LaSrMnO3 heterostructures

We investigate the effect of strain and film thickness on the orbital and magnetic properties of LaSrCrO$_3$ (LSCO)/LaSrMnO$_3$ (LSMO) heterostructures using bulk magnetometry, soft X-ray magnetic spectroscopy, first-principles density functional theory, high-resolution electron microscopy and X-ray diffraction. We observe an anti-parallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as 2 unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn $x^2-y^2$ (in-plane) d-orbitals for tensile strain and $3z^2-r^2$ (out-of-plane) orbitals under compressive strain leading to competing ferromagnetic and anti-ferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural and spin degree of freedom and their tunability in atomically-thin crystalline complex oxide layers.

Sputter-Deposited Titanium Oxide Layers as Efficient Electron Selective Contacts in Organic Photovoltaic Devices

Organic photovoltaics (OPVs) has recently reached power conversion efficiencies of 17.3%, making it a green technology that not only offers short energy payback times and diverse photovoltaic integration schemes, but also can deliver competitive power outputs. OPVs typically employs electron selective contact layers made from low work function n-type metal oxide semiconductors, such as titanium oxide (TiO2) or zinc oxide (ZnO), developed from a variety of deposition techniques. However, in the case of TiO2 interlayers, the appearance of unwanted s-shape characteristics has been reported extensively in the literature in the past, for a variety of different deposition methods used. It has been shown that the s-shape arises from negatively charged chemisorbed oxygen, and that it can be deactivated by UV light illumination, which, however, is hardly compatible with real-life OPV application. In this work, we introduce sputtered crystalline titanium oxide layers as efficient s-shape-free electron selective extraction layers in organic solar-cell devices. We demonstrate that the onset of crystallization takes place at substrate growth temperatures of approximately 100 °C for the TiOx thin films, and that the crystallization onset temperature correlates well with a strong increase in device performance and the removal of any s-shape characteristics. Optical, structural, compositional, and electronic energy-level characterizations of the TiOx layers are shown in the present work, and point to the formation of an oxide with a low surface-defect density, developed from the sputter–crystallization process. Importantly, well-functioning s-shape-free PTB7:PC70BM devices are demonstrated for TiOx growth temperatures of 155 °C.

Comparison of Al and Cu masks used for patterning boron-doped diamonds in oxygen plasma

In this paper we investigate the influence of different metallic (or metal-based) masking materials and plasma techniques for etching and patterning polycrystalline boron-doped diamond thin films. Lift-off technique was used to prepare various testing mask patterns with dimensions ranging from 1 μm to 15 μm. The results of plasma etching utilizing 100 nm Al and Cu masks are compared. Radio frequency and inductively coupled pure oxygen plasma techniques were used to obtain the fine etched structures. A simple etching scheme describing the obtained results is presented for each type of plasma technique and mask type. Although the Al mask is widely believed to have outstanding properties over other metallic materials, we found that the Cu mask exhibits lower side edge etching for both types of plasma techniques. A formation of thick and crystalline copper oxide layer in contrast with thin amorphous aluminum oxide is believed to be the reason for this behavior. Consequently, the etched structures also possessed much better side wall angles which is a key parameter for micro-fabrication of boron doped diamond microelectrodes, microfluidics, sensors and microelectromechanical devices in general.

Chemical diversity of iron species and structure evolution during the oxidation of C14 Laves phase Zr(Fe,Nb)2 in subcritical environment

C14 Laves phase Zr(Fe,Nb)2 was fabricated and oxidized in a subcritical environment. Electron energy loss spectroscopy was used to study the migration and chemical state evolution of the elements. The oxidation process was accompanied by the formation of multilayered oxide scale, consisting of a crystalline oxide layer, amorphous oxide layer (Fe-poor) and Zr-Nb-Fe-O amorphous layer. The formation of closely bonded, semi-free and metallic iron species as well as the out diffusion of Fe contribute to the multilayered structure growth. The amorphization of the C14 phase occurred with the incorporation of oxygen, which preferentially bonded with Zr and Nb.

Phase Transformations in the Surface Layer of Compact Zirconium Nitride-Based Ceramics

We have studied structural, morphological, and phase transformations in the surface layer of compact zirconium nitride-based ceramics during high-temperature annealing in vacuum, nitrogen, and air. The formation of oxygen-containing phases during heat treatment of ZrN is controlled by bulk and grain-boundary oxygen diffusion. At the beginning of the process, oxygen is supplied by an amorphous surface layer, which always forms during the ceramic preparation process. In air, the process is driven by oxygen indiffusion from the gas phase across an initially forming porous oxide layer. Grain-boundary oxygen diffusion leads to the formation of a crystalline zirconium oxide layer on grain boundaries and crack nucleation.