Pulsed Laser Deposition Films Research Articles

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2−xO3 phosphor films

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

Atomic Structure and Dynamics of Unusual and Wide‐Gap Phase‐Change Chalcogenides: A GeTe2 Case

Brain‐inspired computing, reconfigurable optical metamaterials, photonic tensor cores, and many other advanced applications require next‐generation phase change materials (PCMs) with better energy efficiency and a wider thermal and spectral range for reliable operations. Germanium ditelluride (GeTe2), with higher thermal stability and a larger bandgap compared to current benchmark PCMs, appears promising for THz metasurfaces and the controlled crystallization of atomically thin 2D materials. Using high‐energy X‐ray diffraction supported by first‐principles simulation, we investigate the atomic structure in semiconducting pulsed laser deposition films and metallic high‐temperature liquids. The results suggest that the structural and chemical metastability of GeTe2, leading to disproportionation into GeTe and Te, is related to high internal pressure during a semiconductor‐metal transition, presumably occurring in the supercooled melt. Similar phenomena are expected for canonical GeS2 and GeSe2 under high temperatures and pressures.This article is protected by copyright. All rights reserved.

Vertically Aligned Nanocomposite Thin Films Incorporating 3D-Architectures for Micro-Battery Applications

Vertically aligned nanocomposite (VAN) thin films have shown strong potential in oxide nanoionics but are yet to be explored in detail in solid-state battery systems [1,2]. VANs, typically grown by pulsed laser deposition (PLD), are chessboard-like structures comprised of pillars of one phase embedded in a matrix the second phase. They form by solid-state thermodynamic self-assembly, allowing the spontaneous formation of interdigitated battery components in a single deposition without additional processing steps such as etching. Their 3D architectures are attractive because they may allow enhancements in capacity, current and power densities [3]. Furthermore, VAN films grow with a preferential orientation, allowing for selective growth of the optimum crystallographic orientation for high performance by altering the substrate phase and/or orientation. Also, owing to their large interfacial surface areas, VAN could serve as models to study interfaces and solid-electrolyte interphase formation during cycling due to their high interfacial surface areas.In this poster, first we present highly crystalline and epitaxial vertically aligned nanocomposite PLD films comprised of a LixLa0.32±0.05(Nb0.7±0.1Ti0.32±0.05)O3±δ-Ti0.8±0.1Nb0.17±0.03O2±δ-anatase (LL(Nb,Ti)O-(Ti,Nb)O2) electrolyte/anode system, the first anode VAN battery system reported (Figure 1) [4]. This system exhibits an order of magnitude increased Li+ ionic conductivity over that in bulk Li3xLa1/3-xNbO3 (LLNO) and is comparable with the best available Li3xLa2/3-xTiO3 (LLTO) pulsed laser deposition films. Then, we will highlight the considerations and challenges that VANs pose, and how to overcome them [5]. Finally, we will also present our recently submitted work on VAN based cathodes [6]. Our carefully selected materials combination exhibits remarkable high-rate performance, achieving discharge capacities > 70 mAh g-1 up to ~200 C with clear redox behavior of the cathode-of-choice. The enhanced electrochemical performance is enabled by the 3D VAN architecture. Interestingly, we elucidate the relationship between pillar morphology and electrochemical performance, namely there is an optimum size and orientation of cathodic nanopillars such that high discharge capacity under high-rate regimes is possible. These works open up the possibility of incorporating VAN films into an all solid-state battery, either as electrodes or electrolytes, by the pairing of suitable materials.

Decoding the Atomic Structure of Ga2Te5 Pulsed Laser Deposition Films for Memory Applications Using Diffraction and First-Principles Simulations.

Neuromorphic computing, reconfigurable optical metamaterials that are operational over a wide spectral range, holographic and nonvolatile displays of extremely high resolution, integrated smart photonics, and many other applications need next-generation phase-change materials (PCMs) with better energy efficiency and wider temperature and spectral ranges to increase reliability compared to current flagship PCMs, such as Ge2Sb2Te5 or doped Sb2Te. Gallium tellurides are favorable compounds to achieve the necessary requirements because of their higher melting and crystallization temperatures, combined with low switching power and fast switching rate. Ga2Te3 and non-stoichiometric alloys appear to be atypical PCMs; they are characterized by regular tetrahedral structures and the absence of metavalent bonding. The sp3 gallium hybridization in cubic and amorphous Ga2Te3 is also different from conventional p-bonding in flagship PCMs, raising questions about its phase-change mechanism. Furthermore, gallium tellurides exhibit a number of unexpected and highly unusual phenomena, such as nanotectonic compression and viscosity anomalies just above their melting points. Using high-energy X-ray diffraction, supported by first-principles simulations, we will elucidate the atomic structure of amorphous Ga2Te5 PLD films, compare it with the crystal structure of tetragonal gallium pentatelluride, and investigate the electrical, optical, and thermal properties of these two materials to assess their potential for memory applications, among others.

Hydroxyapatite Thin Films of Marine Origin as Sustainable Candidates for Dental Implants.

Novel biomaterials with promising bone regeneration potential, derived from rich, renewable, and cheap sources, are reported. Thus, thin films were synthesized from marine-derived (i.e., from fish bones and seashells) hydroxyapatite (MdHA) by pulsed laser deposition (PLD) technique. Besides the physical-chemical and mechanical investigations, the deposited thin films were also evaluated in vitro using dedicated cytocompatibility and antimicrobial assays. The morphological examination of MdHA films revealed the fabrication of rough surfaces, which were shown to favor good cell adhesion, and furthermore could foster the in-situ anchorage of implants. The strong hydrophilic behavior of the thin films was evidenced by contact angle (CA) measurements, with values in the range of 15-18°. The inferred bonding strength adherence values were superior (i.e., ~49 MPa) to the threshold established by ISO regulation for high-load implant coatings. After immersion in biological fluids, the growth of an apatite-based layer was noted, which indicated the good mineralization capacity of the MdHA films. All PLD films exhibited low cytotoxicity on osteoblast, fibroblast, and epithelial cells. Moreover, a persistent protective effect against bacterial and fungal colonization (i.e., 1- to 3-log reduction of E. coli, E. faecalis, and C. albicans growth) was demonstrated after 48 h of incubation, with respect to the Ti control. The good cytocompatibility and effective antimicrobial activity, along with the reduced fabrication costs from sustainable sources (available in large quantities), should, therefore, recommend the MdHA materials proposed herein as innovative and viable solutions for the development of novel coatings for metallic dental implants.

GeTe2 Phase Change Material for Terahertz Devices with Reconfigurable Functionalities Using Optical Activation.

The phenomenon of phase change transition has been a fascinating research subject over decades due to a possibility of dynamically controlled materials properties, allowing the creation of optical devices with unique features. The present paper unravels the optical characteristics and terahertz (THz) dielectric permittivity of a novel phase change material (PCM), GeTe2, prepared by pulsed laser deposition (PLD) and their remarkable contrast in crystalline and amorphous states, in particular, a difference of 7 orders of magnitude in conductivity. The THz spectra were analyzed using the harmonic oscillator and Drude term. Using GeTe2 PLD films, we designed and prepared a THz metasurface in the form of periodic structure and revealed a possibility of tuning the THz resonance either by a thermal control or light-induced crystallization response, thus achieving the dynamic and tunable functionality of the metastructure. We propose controlling the state of metasurface by observing the intensity characteristics of the Raman peak of 155 cm-1. Density functional theory (DFT) modeling demonstrates that in the process of crystallization the mode intensity of 155 cm-1 assigned to Te-Te stretching in amorphous chain fragments decreases and disappears at full crystallization.

Samarium doped cerium oxide thin films deposited by pulsed laser deposition

Samarium doped ceria thin films are an important material for the electrochemical devices. Fluorite structured Samarium Doped Cerium Oxide (SDC) thin films were deposited on Si (100) at 500 °C and 600 °C substrate temperatures by PLD (Pulsed Laser Deposition) from Sm0,24 Ce0,13 O0.63 target. X-ray Diffraction (XRD) proved the existence of SDC crystalline compound in both samples, deposited at 500 and 600 °C but not in the target. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) investigations revealed a high quality of the films, with lower roughness, of under 5 nm in the case of the sample deposited at 600 °C. The beginning of sintering process was evidenced by SEM in the case of samples deposited at 600 °C. X-ray Photoelectron Spectroscopy (XPS) indicated the increasing proportion of Ce3+ for PLD films comparatively to the target, in both cases, of substrate temperatures of 500 and of 600 °C, higher in the first case. Spectroscopic Ellipsometry (SE) investigation permitted the calculation of a refractive index under 2.2 in the visible (Vis)-near infrared (near-IR) domain for both samples. Nanoindentation tribometric measurements concluded that both elasticity modulus and hardness decreases with substrate temperature increasing. The obtained films are candidates for potential uses as electrolyte material for a new generation of electrochemical devices.

Improved methods for design of PLD and combinatorial PLD films

Pulsed laser deposition (PLD) is a powerful technique for prototyping thin film materials, both single component (single composition) films and films with a varying composition (e.g., lateral continuous compositional spread, CCS). In this work, we improve one of the simulation methods used to design the deposition of PLD films: We extend the mathematical model for the material spread on the substrate, T1(x,y), for each laser pulse hitting the target, and we use a more accurate method to determine T1(x,y) experimentally. The deposition of the material on the substrate is simulated by repetitively adding T1(x,y), from one or more targets, at the selected location on the substrate. Using the new model, a high agreement between the simulated and grown films’ thickness and composition across the substrate was obtained. The basis for the high agreement is the use of variable angle spectroscopic ellipsometry to carefully determine T1(x,y) by measuring at 794 locations on the 50.8 mm (2 in.) diameter substrates. Factors, such as variation in optical properties and porosity across the plume/calibration films, were considered in the determination of the thicknesses. As test cases, we simulated and deposited (single component) TiO2 thin films and (CCS) TiO2 films doped with Cr and N, deposited on 50.8 mm diameter Si wafers. The modeling and simulations are implemented in an open-source Python library, pyPLD.

Lithium-based vertically aligned nancomposite films incorporating LixLa0.32(Nb0.7Ti0.32)O3 electrolyte with high Li+ ion conductivity

Vertically aligned nanocomposite (VAN) thin films have shown strong potential in oxide nanoionics but are yet to be explored in detail in solid-state battery systems. Their 3D architectures are attractive because they may allow enhancements in capacity, current, and power densities. In addition, owing to their large interfacial surface areas, the VAN could serve as models to study interfaces and solid-electrolyte interphase formation. Here, we have deposited highly crystalline and epitaxial vertically aligned nanocomposite films composed of a LixLa0.32±0.05(Nb0.7±0.1Ti0.32±0.05)O3±δ-Ti0.8±0.1Nb0.17±0.03O2±δ-anatase [herein referred to as LL(Nb, Ti)O-(Ti, Nb)O2] electrolyte/anode system, the first anode VAN battery system reported. This system has an order of magnitude increased Li+ ionic conductivity over that in bulk Li3xLa1/3−xNbO3 and is comparable with the best available Li3xLa2/3−xTiO3 pulsed laser deposition films. Furthermore, the ionic conducting/electrically insulating LL(Nb, Ti)O and electrically conducting (Ti, Nb)O2 phases are a prerequisite for an interdigitated electrolyte/anode system. This work opens up the possibility of incorporating VAN films into an all solid-state battery, either as electrodes or electrolytes, by the pairing of suitable materials.

Insights on hexagonal TbMnO3 for optoelectronic applications: From powders to thin films

In this work, we first report on the fabrication of TbMnO3 (TMO) powders and doping with In, Lu, Gd, Sm, Nd, Y. In and Nd atoms were found to significantly reduce the cell size while preserving an orthorhombic structure. Conversely, we show that hexagonal h-TMO, indium doped TMO (h-In-TMO), yttrium doped TMO (h-Y-TMO), and YMnO3 (h-YMO) can be obtained epitaxially using pulsed laser deposition (PLD) on Pt (111)//Al2O3 (0001). The evolution of the optical properties was measured for the PLD films and a direct bandgap of 1.5–1.6 eV was found for h-TMO and doped h-TMO films. The h-TMO, h-In-TMO and h-Y-TMO showed no sign of ferroelectricity at room temperature as opposed to h-YMO. No short-circuit current in solar cell configuration was found under 1 sun illumination. However a photovoltaic behavior was observed by surface photovoltage for the h-Y-TMO and h-YMO, indicating that Y incorporation in h-TMO is helpful for the photovoltaic properties.

Effect of Target Sintering Temperature on the Morphological and Optical Properties of Pulsed Laser Deposited TiO2 Thin Films

In this paper, we report on the effect of titanium dioxide (TiO2) target sintering temperature on the morphological and optical properties of amorphous titanium dioxide thin films synthesized by pulsed laser deposition (PLD) on indium tin oxide (ITO) glass substrate and subsequently heat-treated in air at low temperature (150 °C). Three types of targets were used, unsintered (pressed at room temperature), sintered at 500 °C and sintered at 1000 °C. The surface morphology of the samples was investigated by scanning electron microscopy (SEM), and profilometry was used for thickness measurements. The structural properties of the films were examined by X-ray diffraction (XRD), while their optical properties were studied by UV‒vis spectroscopy. The obtained TiO2 thin films have an amorphous nature, as shown by XRD analysis. Profilometer showed that sintered target samples have more reliable thicknesses than unsintered ones. The SEM studies revealed the sufficient structural homogeneity of sintered target nanosized TiO2 films and agglomerates in the case of unsintered target film. The UV‒vis transmittance spectra showed high transparency in the visible range of PLD films, proportional to the target sintering temperature. The optical band gaps of the films deposited using the 500 °C and 1000 °C sintered targets are closer to those of anatase and rutile phases, respectively, which provides a promising approach to the challenges of amorphous TiO2-based nanostructures.

Substoichiometric Tuning of the Electronic Properties of Titania

Reduced titania phases such as Ti2O3, Ti3O5, and TixO2x-1 (the magńeli phase, 4 ≤ x ≤10) are collectively a promising class of materials due to the stoichiometric tunability of their electronic structure. This flexibility, combined with their relatively high electrical conductivity, high corrosion resistance, and photostability makes reduced titania a candidate for many applications. In this work, stable reduced titania films were synthesized on silicon substrates via pulsed laser deposition (PLD) under a forming gas environment at elevated temperatures. The atomic/electronic structure and the electrical properties of the PLD reduced titania films were investigated using four-point probe, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS). The PLD films demonstrated a notable decrease in resistivity compared to those of stoichiometric TiO2 films reported in the literature. Raman spectroscopy and XAS revealed that the observed resistivities in these films are correlated to substoichiometric titania phases (Ti2O3 between 400-600 °C and the Ti3O5-like at 700 °C) present in them. UPS measurements on the sub-stoichiometric titania indicated a decrease in the work functions with a decrease in the titanium oxidation state. The ability to tune such electrical properties with careful control of preparation is key to using reduced titania as a flexible platform for material design.

(Invited) A Practical Model of Oxygen Exchange Reaction on a Mixed Conducting Electrode for SOFC and SOEC

Oxygen exchange on mixed conducting electrodes for SOFC/SOEC have been investigated by many researchers, and the reaction mechanism has been made clearer in these decades. In a practical modeling works, however, a Butler-Volmer equation with unproven parameters is still used to describe current-voltage (I-V) relationships due to the complexity of realistic reaction models. Recently, we have collaborated with Muramatsu and Terada to develop a simulation code “SIMUDEL” that can calculate oxygen potential distribution and the resulting chemical strain in SOFC/SOEC components [1]. In this code, porous electrode is described as a homogeneous medium in which oxygen exchange reaction is given as the source term in the continuity equation. In this code, the surface reaction rate is given as a function of oxygen potentials in the gas phase and at the solid surface. This work aimed to provide a simple but plausible model of oxygen exchange at a mixed conducting electrode surface.The work started with modeling I-V characteristics of a dense film electrode. Steady state dc measurements were made with La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) films deposited by pulsed laser deposition (PLD) on doped ceria substrates. Oxygen vacancy concentration of the film was estimated from the chemical capacitance by ac impedance measurements and was used as a parameter to represent the I-V relationships. The reaction rate on a dense electrode was compared with the volumetric surface reaction rate in a porous electrode assuming one-dimensional transport approximation. Several porous electrodes with micro- and nano- meter-sized particles were fabricated by slurry coating or metal organic decomposition. A plausible reaction equation obtained for a film electrode was applied to estimate I-V curves of a porous electrodes, and the result was compared with the experiments to test the model.The surface reaction resistance on a dense film electrode showed an activation energy in the range of 1.5 to 2.0 eV. It was much larger than that of a porous electrode when it was obtained as the volumetric reaction resistance of the porous medium and normalized to the actual surface area. The magnitude of the reaction resistance on a porous electrode was close to that observed for a sample powder by pulse isotope exchange experiments. No remarkable difference was observed between the micro and nano porous electrodes. The large activation energy on the dense film electrode is partly due to the reduced oxygen vacancy concentration in a PLD film electrode. As many other researchers pointed out, oxygen exchange reaction on a mixed conductor is strongly affected by the concentration and mobility of oxygen vacancy. The apparent activation energy of surface reaction is the sum of activation energies for vacancy formation and reaction. The difference between the dense and porous electrode, however, was not fully explained only with the difference in vacancy concentration. Furthermore, the reaction rate on a dense film electrode was found to be strongly affected by the coexisting chemicals on the surface. As we reported earlier, applying LaSrCoO4 (R-P phase) particles on the surface dramatically enhanced the surface reaction rate. It also decreased the activation energy. Existence of R-P phase was found to clean the Sr enriched segregates on the surface. On the contrary, significant degradation was observed for reaction on a sintered ceramic surface due to the segregation of Sr rich layer. These observations suggested the existence of two series reaction barriers for oxygen exchange; i.e. the surface reaction and sub-surface transport. The I-V curve of a dense film electrode was modelled on the basis of the above consideration. The surface process was described to be proportional to the oxygen vacancy concentration, and the sub-surface process was given to overcome some energy barrier. For a porous electrode, the sub-surface process was assumed to be negligible. The difference in oxygen vacancy concentration and lack of sub-surface process explained fairly well the oxygen partial pressure dependence of I-V curves of the porous electrodes. The obtained model is to be applied to the description of the surface reaction in SIMDEL code. Acknowledgements This work was partly supported by Japan Science and Technology Agency for the financial support during JST-CREST program (JPMJCR11C1, 2011-2016). Development of SIMDEL was made under the support by NEDO.

Comparative Study on Structural, Optical, and Electrical Properties of ZnO Thin Films Prepared by PLD and Sputtering Techniques

ZnO thin films were formed on c-plane sapphire and p-GaN substrates by pulsed laser deposition (PLD) and radio frequency (RF) magnetron sputtering techniques. XRD analysis including omega scan depicted the formation of highly textured wurtzite ZnO with c-axis. The texture was primarily introduced by the substrate effects as the planes lying at oblique angles also exhibited six-fold symmetry during phi scan. Atomic force microscopy exhibited the surface roughness of 4.33 and 12.99 nm for PLD and sputtered ZnO films, respectively. In photoluminescence (PL) measurements, a strong UV emission was observed at 3.30 eV for both ZnO films. However, deep-level emission was observed at around 2.61 eV in PLD film, but it had a wide range from 2.61 to 2.29 eV in case of sputter-deposited film. From the transmission spectra, the optical band gap values were found to be 3.29 and 3.28 eV for PLD and sputtered ZnO films, respectively. Hall measurement revealed the resistivity values of 0.0792 and 0.4832 Ω cm and carrier concentrations of 2.28 × 1018 and 1.73 × 1018 cm–3 for respective PLD and sputtered films. I(V) current–voltage curves clearly demonstrated the n-ZnO|p-GaN hetero-junction with turn-on voltage of 3.8 and 5.2 V for PLD and sputtered samples, respectively.

Electrical and optical properties of degenerate and semi-insulating ZnGa2O4: Electron/phonon scattering elucidated by quantum magnetoconductivity

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors.

Vortex pinning properties at dc and microwave frequencies of YBa2Cu3O7-x films with nanorods and nanoparticles

YBa2Cu3O7−x (YBCO) nanocomposites for wire applications need to operate in a broad range of frequencies, ranging from dc in magnets to GHz in cavities and screenings of future particle accelerators. We have investigated the in-field and angular vortex pinning performance in dc and at 50 GHz of two types of nanocomposites, pulsed laser deposition (PLD) YBCO with mixed Ba2YNbO6 + Ba2YTaO6 (BYNTO) nanorods and chemical solution deposited (CSD) YBCO with BaHfO3 (BHO) nanoparticles (NPs), and the pristine counterpart films, grown on top of single-crystalline substrates. Transport measurements performed up to 9 T between 5 and 77 K show that CSD nanocomposites exhibit a smooth field decay and increased single-to-collective crossover field H* compared to pristine samples, associated to the enhanced isotropic pinning contribution induced by the NPs, while PLD films exhibit unchanged H* and superior critical current densities up to higher irreversibility fields, associated to the anisotropic contribution introduced by the rods. Microwave in-field measurements of the pinning constant kp revealed CSD NCs exhibit a qualitatively similar, but smoother kp(H) than pristine samples, whereas for PLD samples, a growing kp(H) dependence is observed as a result of the increased relevance of the stiffness of the fluxons pinned by nanorods.

PLD-fabricated perovskite oxide nanofilm as efficient electrocatalyst with highly enhanced water oxidation performance

The practical application of perovskite oxides as electrocatalysts for oxygen evolution reaction (OER) has been greatly hindered by their limited catalytically active sites and poor electrical conductivity. Herein, we report the use of a perovskite-oxide nanofilm as highly improved electrocatalyst for OER. A facile fabrication method, pulsed laser deposition (PLD), is employed for the first time to fabricate the SrCo0.85Fe0.1P0.05O3−δ nanofilm on nickel foam (SCFP-NF) with abundant active sites and enhanced electrical conductivity. We find that the SCFP-NF fabricated by PLD exhibits excellent OER activity and durability in alkaline solutions. Especially, the PLD method can create a large amount of oxygen vacancies on the surface of the perovskite nanofilm, leading to a high concentration of highly oxidative oxygen O22−/O−. This O22−/O− species is generally recognized to be active for OER. As a result, the SCFP-NF catalyst with low mass loading (≈30 μg cm-2) demonstrates a remarkable OER activity with a low overpotential of 290 mV and a superb stability up to 200 h at 10 mA cm−2 in alkaline media. The catalytic performance of this perovskite nanofilm is superior to most of the reported metal-oxide-based catalysts and benchmark RuO2. Our findings show that the catalytic performance of oxide-based electrocatalysts can be greatly improved by defect-engineering, and the PLD and other film fabrication methods may help us to realize practical application.

Radio-Frequency Characteristics of Ge-Doped Vanadium Dioxide Thin Films with Increased Transition Temperature

This work investigates and reports on the radio-frequency (rf) behavior in the frequency range of 5–35 GHz of germanium-doped vanadium dioxide (Ge-doped VO2) thin films deposited on silicon substrates via sputtering and pulsed laser deposition (PLD) with estimated Ge concentrations of 5 and 5.5%. Both films exhibit critical transition temperatures (Tc) of 76.2 and 72 °C, respectively, which are higher compared to that of the undoped VO2 which undergoes reversible insulator-to-metal phase transition at 68 °C. Both types of Ge-doped films show low hysteresis (<5 °C) in their conductivity versus temperature characteristics and preserve high off-state dc-conductivities (corresponding to the insulating state of the phase change material) of 13 S/m for the sputtered and 55 S/m for the PLD-deposited film, respectively. The dc on-state (corresponding to the conductive state of the phase change material) conductivity reaches 145,000 S/m in the case of the PLD film, which represents a significant increase compared to the state-of-the art values measured for undoped VO2 thin films deposited on identical substrates. In order to further understand the off-state dissimilarities and rf behavior of the deposited Ge-doped VO2 films, we propose an original methodology for the experimental extraction of the dielectric constant (εr) in the GHz range of the films below 60 °C. This is achieved by exploiting the frequency shift of resonant filters. For this purpose, we have fabricated coplanar waveguide structures incorporating ultracompact Peano space-filling curves, each resonating at a different frequency between 5 and 35 GHz on two types of substrates, one with the Ge-doped VO2 thin films and another one using only SiO2 to serve as the reference. The reported results and analysis contribute to the advancement of the field of metal–insulator–transition-material technology with high Tc for rf industrial applications.

Pulsed Laser Deposition Films Based on CdSe-Doped Zinc Aluminophosphate Glass

A composite material with applications in optoelectronics has been investigated. Pulsed laser deposition CdSe-doped glass film was prepared by the combinatorial deposition from two targets, namely pure CdSe and glass belonging to the 20Li2O-10Al2O3-7BaO-2La2O3-2ZnO-59P2O5 system. Exciton peaks in the Vis domain, related to electron–hole pairs transitions from the valence band to the conduction band, were revealed in the optical absorption spectra of the CdSe-doped film. CdSe quantum dots (QDs) band gap energy depends on the CdSe quantum confinement effect. CdSe-doped film photoluminescence exhibits peaks in the red domain assigned to CdSe transitions from the excited state to the ground state. The size of CdSe nanoclusters, determined from x-ray diffraction is correlated with scanning electron microscopy–energy dispersive x-ray spectroscopy and atomic force microscopy results. Vibration modes specific both to CdSe QDs and to the vitreous network have been evidenced by Fourier transform infrared and Raman spectroscopy.

Hole Accumulation at the Grain Boundary Enhances Water Oxidation at α-Fe2O3 Electrodes under a Microwave Electric Field

We studied the effects of surface morphology and carrier distribution of α-Fe2O3 electrodes on the enhancement of water electrolysis under microwave (MW) irradiation. We deposited α-Fe2O3 electrodes with various morphologies on Nb-doped rutile TiO2 (100) substrates. α-Fe2O3 films with rough and flat surfaces were deposited using electrodeposition (ED) and pulsed laser deposition (PLD), respectively. The ED α-Fe2O3 film showed a larger response to the MW electric field applied to the electrodes than did the PLD film. In addition, the response was linearly correlated with the MW electric field intensity. Using scanning MW microscopy, we found that the local MW susceptibility of the α-Fe2O3 electrode was enhanced at the grain boundary of the ED α-Fe2O3 film. Analysis of the surface band structure of both ED and PLD α-Fe2O3 films using electrochemical impedance spectroscopy showed that the ED α-Fe2O3 film had a wider depleted layer, indicating increased accumulation of holes on the surface of the electrode to enhance water oxidation. We concluded that the accumulation of holes at the grain boundary of the ED α-Fe2O3 film determines the enhancement of water oxidation under an MW electric field.