Re-distribution of oxygen at the interface between γ-Al2O3 and TiN

Fabrication of anodic and atomic layer deposition-alumina coated titanium implants for effective osteointegration applications

Biomimicking the chemical, mechanical, and topographical properties of bone on an implant model is crucial to obtain rapid and effective osteointegration, especially for the large-area fractures of the skeletal system. Titanium-based biomaterials are more frequently preferred in clinical use in such cases and coating these materials with oxide layers having chemical/nanotopographic properties to enhance osteointegration and implantation success rates has been studied for a long time. The objective of this study is to examine the high and rapid mineralization potential of anodized aluminum oxide (AAO) coated and atomic layer deposition (ALD)-alumina coated titanium substrates on large deformation areas with difficult spontaneous healing. AAO-coated titanium (AAO@Ti) substrates were fabricated via anodization technique in different electrolytes and their osteogenic potential was analyzed by comparing them to the bare titanium surface as a control. In order to investigate the effect of the ionic characters gained by the surfaces through anodization, the oxidized nanotopographic substrates were additionally coated with an ultrathin alumina layer via ALD (ALD@AAO@Ti), which is a sensitive and conformal coating vapor deposition technique. Besides, a bare titanium sample was also coated with pure alumina by ALD (ALD@Ti) to investigate the effect of nanoscale surface morphology. XPS analysis after ALD coating showed that the ionic character of each surface fabricated by anodization was successfully suppressed. In vitro studies demonstrated that, among the substrates investigated, the mineralization capacity of MG-63 osteosarcoma cells were highest when incubated on ALD-treated and bare AAO@Ti samples that were anodized in phosphoric acid (H3PO4_AAO@Ti and ALD@H3PO4_AAO@Ti). Mineralization on these substrates also increased consistently beginning from day 2 to day 21. Moreover, immunocytochemistry for osteopontin (OPN) demonstrated the highest expression for ALD@H3PO4_AAO@Ti, followed by the H3PO4_AAO@Ti sample. Consequently, it was observed that, although ALD treatment improves cellular characteristics on all samples, effective mineralization requires more than a simple ALD coating or the presence of a nanostructured topography. Overall, ALD@H3PO4_AAO@Ti substrates can be considered as an implant alternative with its enhanced osteogenic differentiation potential and rapid mineralization capacity.

Laser-synthesized plasmonic HfN-based nanoparticles as a novel multifunctional agent for photothermal therapy

Hafnium nitride nanoparticles (HfN NPs) can offer appealing plasmonic properties at the nanoscale, but the fabrication of stable water-dispersible solutions of non-toxic HfN NPs exhibiting plasmonic features in the window of relative biological transparency presents a great challenge. Here, we demonstrate a solution to this problem by employing ultrashort (femtosecond) laser ablation from a HfN target in organic solutions, followed by a coating of the formed NPs with polyethylene glycol (PEG) and subsequent dispersion in water. We show that the fabricated NPs exhibit plasmonic absorption bands with maxima around 590 nm, 620 nm, and 650 nm, depending on the synthesis environment (ethanol, acetone, and acetonitrile, respectively), which are largely red-shifted compared to what is expected from pure HfN NPs. The observed shift is explained by including nitrogen-deficient hafnium nitride and hafnium oxynitride phases inside the core and oxynitride coating of NPs, as follows from a series of structural characterization studies. We then show that the NPs can provide a strong photothermal effect under 808 nm excitation with a photothermal conversion coefficient of about 62%, which is comparable to the best values reported for plasmonic NPs. MTT and clonogenic assays evidenced very low cytotoxicity of PEG-coated HfN NPs to cancer cells from different tissues up to 100 μg mL-1 concentrations. We finally report a strong photothermal therapeutic effect of HfN NPs, as shown by 100% cell death under 808 nm light irradiation at NP concentrations lower than 25 μg mL-1. Combined with additional X-ray theranostic functionalities (CT scan and photon capture therapy) profiting from the high atomic number (Z = 72) of Hf, plasmonic HfN NPs promise the development of synergetically enhanced modalities for cancer treatment.

Ferroelectricity in Ultrathin HfO2-Based Films by Nanosecond Laser Annealing

Nonvolatile memory devices based on ferroelectric HfxZr1-xO2 (HZO) show great promise for back-end integrable storage and for neuromorphic accelerators, but their adoption is held back by the inability to scale down the HZO thickness without violating the strict thermal restrictions of the Si CMOS back end of line. In this work, we overcome this challenge and demonstrate the use of nanosecond pulsed laser annealing (NLA) to locally crystallize areas of an ultrathin (3.6 nm) HZO film into the ferroelectric orthorhombic phase. Meanwhile, the heat induced by the pulsed laser is confined to the layers above the Si, allowing for back-end compatible integration. We use a combination of electrical characterization, nanofocused scanning X-ray diffraction (nano-XRD), and synchrotron X-ray photoelectron spectroscopy (SXPS) to gain a comprehensive view of the change in material and interface properties by systematically varying both laser energy and the number of laser pulses on the same sample. We find that NLA can provide remanent polarization up to 2Pr= 11.6 μC/cm2 in 3.6 nm HZO, albeit with a significant wake-up effect. The improved TiN/HZO interface observed by XPS explains why device endurance goes beyond 107 cycles, whereas an identical film processed by rapid thermal processing (RTP) breaks already after 106 cycles. All in all, NLA provides a promising approach to scale down the ferroelectric oxide thickness for emerging HZO ferroelectric devices, which is key for their integration in scaled process nodes.

On the Structure of Oxygen Deficient Amorphous Oxide Films

Understanding defects in amorphous oxide films and heterostructures is vital to improving performance of microelectronic devices, thin-film transistors, and electrocatalysis. However, to what extent the structure and properties of point defects in amorphous solids are similar to those in the crystalline phase are still debated. The validity of this analogy and the experimental and theoretical evidence of the effects of oxygen deficiency in amorphous oxide films are critically discussed. The authors start with the meaning and significance of defect models, such as "oxygen vacancy" in crystalline oxides, and then introduce experimental and computational methods used to study intrinsic defects in amorphous oxides and discuss their limitations and challenges. To test the validity of existing defect models, ab initio molecular dynamics is used with a non-local density functional to model the structure and electronic properties of oxygen-deficient amorphous alumina. Unlike some previous studies, the formation of deep defect states in the bandgap caused by the oxygen deficiency is found. Apart from atomistic structures analogous to crystal vacancies, the formation of more stable defect states characterized by the bond formation between under-coordinated Al ions is shown. The limitations of such defect models and how they may be overcome in simulations are discussed.

Charge transfer and X-ray absorption investigations in aluminium and copper co-doped zinc oxide nanostructure for perovskite solar cell electrodes

This study explores influence of charge transfer and X-ray absorption characteristics in aluminum (Al) and copper (Cu) co-doped zinc oxide (ZnO) nanostructures for perovskite solar cell electrodes. Sol-gel technique was employed to synthesize the nanostructures, and their optical and morphological properties were investigated. X-ray diffraction (XRD) analysis confirmed high crystallinity and also single-phase composition of all the samples, particularly up to 5% Al co-doping. Field emission scanning electron microscopy (FESEM) exhibited the formation of pseudo-hexagonal wurtzite nanostructure and the transition to nanorods at 5% Al co-doping. Diffuse reflectance spectroscopy indicated a reduction in the optical band gap of co-doped zinc oxide from 3.11 to 2.9 eV with increasing Al doping. Photoluminescence spectra (PL) exhibited a decrease in peak intensity, suggesting enhanced conductivity in ZnO, also confirmed from I-V measurements. Near-edge X-ray absorption fine structure (NEXAFS) analysis depicts that charge transfer from Al to oxygen (O) species enhanced the photosensing properties of the nanostructure, which was supported by FESEM micrographs and PL spectra. Furthermore, the study discovered that 5% Al co-doping significantly reduced the density of emission defects (deep-level) in Cu-ZnO nanostructure. These findings highlight the potential of Cu and Al co-doped ZnO materials for perovskite solar cell electrodes, as their improved optical and morphological properties resulting from charge transfer could enhance device performance. The investigation of charge transfer and X-ray absorption characteristics provides valuable insights into the underlying mechanisms and behaviors of the co-doped ZnO nanostructures. However, further research is required to delve into the intricate hybridization resulting from charge transfer and explore the broader impact of co-doping on other properties of the nanostructures, enabling a comprehensive understanding of their potential applications in perovskite solar cells.

Catalytic Performance and Near-Surface X-ray Characterization of Titanium Hydride Electrodes for the Electrochemical Nitrate Reduction Reaction

The electrochemical nitrate reduction reaction (NO₃RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiH_x, 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH₂ enrichment with increasing NO₃RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH₂/Ti electrodes for use in NO₃RR, thereby decoupling hydride formation from NO₃RR performance. A wide range of NO₃RR activity and selectivity on TiH₂/Ti electrodes between -0.4 and -1.0 V_RHE was observed and analyzed with density functional theory (DFT) calculations on TiH₂(111). This work underscores the importance of relating NO₃RR performance with near-surface electrode structure to advance catalyst design and operation.

Nanocrystalline rhenium-doped TiO2: an efficient catalyst in the one-pot conversion of carbohydrates into levulinic acid. The synergistic effect between Brønsted and Lewis acid sites

A new approach of inserting rhenium into a TiO2 structure generates Brønsted acid sites which are essential for conversion of carbohydrates into levulinic acid.

Effect of surface modification and laser repetition rate on growth, structural, electronic and optical properties of GaN nanorods on flexible Ti metal foil

The effect of flexible Ti metal foil surface modification and laser repetition rate in laser molecular beam epitaxy growth process on the evolution of GaN nanorods and their structural, electronic and optical properties has been investigated. The GaN nanostructures were grown on bare- and pre-nitridated Ti foil substrates at 700 °C for different laser repetition rates (10-30 Hz). It is found that the low repetition rate (10 Hz) promotes sparse growth of three-dimensional inverted-cone like GaN nanostructures on pre-nitridated Ti surface whereas the entire Ti foil substrate is nearly covered with film-like GaN consisting of large-sized grains for 30 Hz growth. In case of the GaN growth at 20 Hz, uniformly-aligned, dense (∼8 × 109 cm-2) GaN nanorods are successfully grown on pre-nitridated Ti foil whereas sparse vertical GaN nanorods have been obtained on bare Ti foil under similar growth conditions for both 20 and 30 Hz. X-ray photoemission spectroscopy (XPS) has been utilized to elucidate the electronic structure of GaN nanorods grown under various experimental conditions on Ti foil. It confirms Ga-N bonding in the grown structures, and the calculated chemical composition turns out to be Ga rich for the GaN nanorods grown on pre-nitridated Ti foil. For bare Ti substrates, a preferred reaction between Ti and N is noticed as compared to Ga and N leading to sparse growth of GaN nanorods. Hence, the nitridation of Ti foil is a prerequisite to achieve the growth of dense and aligned GaN nanorod arrays. The X-ray diffraction, high resolution transmission electron microscopy and Raman studies revealed the c-axis growth of wurtzite GaN nanorods on Ti metal foil with good crystallinity and structural quality. The photoluminescence spectroscopy showed that the dense GaN nanorod possesses a near band edge emission at 3.42 eV with a full width at half maximum of 98 meV at room temperature. The density-controlled growth of GaN nanorods on a flexible substrate with high structural and optical quality holds promise for potential applications in futuristic flexible GaN based optoelectronics and sensor devices.

Depth-resolved compositional analysis of W/B4C multilayers using resonant soft X-ray reflectivity

W/B₄C multilayers (MLs) consisting of ten layer pairs with varying boron carbide layer thicknesses have been investigated. The ML structures were characterized using grazing-incidence hard X-ray reflectivity (GIXR), resonant soft X-ray reflectivity (RSXR), hard X-ray photoelectron spectroscopy (HAXPES) and X-ray absorption near-edge spectroscopy (XANES). Depth-resolved spectroscopic information on the boron carbide layer in W/B₄C MLs was extracted with sub-nanometre resolution using reflectivity performed in the vicinity of the B K-edge. Interestingly, these results show that the composition of boron carbide films is strongly dependent on layer thicknesses. HAXPES measurements suggest that most of the boron is in the chemical state of B₄C in the multilayer structures. XANES measurements suggest an increase in boron content and C-B-C bonding with increase in boron carbide layer thickness.

Control of TiN oxidation upon atomic layer deposition of oxides

The interfaces of a physical-vapor deposited (PVD)-TiN electrode with atomic-layer deposited (ALD) HfO2 layers were studied using photoelectron spectroscopy with high kinetic energies of photoelectrons enabling nondestructive in-depth chemical profiling and phase analysis. Our results reveal the presence of only TiNxOy at the TiN/ALD-HfO2 interface with no measurable traces of the TiO2 phase. By contrast, the interface formed by ALD of HfO2 on top of PVD TiN contains both TiO2 and TiNxOy compounds and may be compared to an HfO2/TiN interface with intentional ALD TiO2 interlayer (IL) formation prior to HfO2 growth. Pre-growth of ALD Al2O3 IL drastically reduces the TiO2 and TiNxOy amounts present at the HfO2/TiN interface, which can be ascribed to oxygen scavenging from the initially oxidized TiN surface by energetically more favorable Al-O bonds. The present study demonstrates that the amount of TiO2 phase can be effectively controlled, i.e., increased or decreased, during the ALD process enabling engineering of vacancy-mediated processes.

Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials

We introduce a novel approach that addresses the probing of interfacial structural phenomena in layered nano-structured films. The approach combines resonant soft x-ray reflection spectroscopy at grazing incidence near the "critical angle" with angular dependent reflection at energies around the respective absorption edges. Dynamic scattering is considered to determine the effective electron density and hence chemically resolved atomic profile across the structure based on simultaneous data analysis. We demonstrate application of the developed technique on the layered model structure C (20 Å)/B (40 Å)/Si (300 Å)/W (10 Å)/substrate. We precisely quantify atomic migration across the interfaces, a few percent of chemical changes of materials and the presence of impurities from top to the buried interfaces. The results obtained reveal the sensitivity of the approach towards resolving the compositional differences up to a few atomic percent. The developed approach enables the reconstruction of a highly spatio-chemically resolved interfacial map of complex nano-scaled interfaces with technical relevance to many emerging applied research fields.

Probing hydrogen bonding interactions and impurity intercalation in gibbsite using experimental and theoretical XANES spectroscopy

The Al and O K-edge XANES spectra of Bayer process gibbsite are examined, both experimentally and with plane wave density functional theory, and transition assignments are made. We confirm that the broad transitions in Al K-edge spectra are fundamentally the same as those in more ordered and well-studied corundum, and typical of octahedrally coordinated aluminium. Analysis of O K-edge spectra reveals that while much of the initial edge transition structure (535-548 eV) is due to O 2p hybridization with Al orbitals, the lowest energy region (∼535-539 eV) is particularly influenced by O-H states which are heavily perturbed by hydrogen bonding interactions. We therefore tentatively suggest that an unexplained structure at 539 eV in O K-edge spectra of corundum may be due to surface hydroxylation. Finally, we examined Na K-edge spectra, as sodium is the major impurity in these materials and implicated in perturbation of the evolution of materials properties with calcination through the transition aluminas. Density functional theory calculations find sodium replacement of in-sheet H-atoms is the most thermodynamically stable site for sodium inclusion. We subsequently confirm this assignment spectroscopically, explicitly demonstrating the precise location of Na+ impurities for the first time.