Substoichiometric Oxide Research Articles

Size dependent light absorption modulation and enhanced carrier transport in germanium quantum dots devices

Quantum confinement in closely packed arrays of Ge quantum dots (QDs) was studied for energy applications. In this work, we report an efficient tuning mechanism of the light harvesting and detection of Ge QDs. Thin films of SiGeO alloys, produced by rf-magnetron sputtering, were annealed at 600°C in N2 to induce precipitation of small amorphous Ge QDs into the oxide matrix. Varying the Ge content, the QD size was tailored between 2 and 4nm, as measured by high resolution transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of pure SiO2, as well as the presence of a sub-stoichiometric Ge oxide shell at the QD interface. Light absorption spectroscopy shows a clear size-dependent shift of the QD optical bandgap (Eg), between 1.4 and 2.1eV, and was modeled using the standard effective mass approximation (EMA) or a spatially dependent effective mass approximation (SPDEM) model. The reported quantum confinement effect was exploited to enhance light harvesting capability in Ge QDs-based devices. Metal–insulator–semiconductor devices with Ge QDs in the insulating layer exhibit a significant photo-response under reverse bias. Whereby, we demonstrate a large photoconductive gain (up to 1500%) that is tunable with QD size and is based on a preferential trapping of photo-generated holes by QDs, which enhances the charge carrier collection. Our results provide a new route for the application of Ge QDs in light harvesting devices and solar cells.

A substoichiometric tungsten oxide catalyst provides a sustainable and efficient counter electrode for dye-sensitized solar cells

Development of Pt-free catalyst materials for the counter electrode (CE) in dye-sensitized solar cells (DSSCs) has been regarded as one of the crucial steps to improving energy conversion efficiency and cost effectiveness of DSSCs. In this work, low cost tungsten oxide (WO3-x) counter electrodes, prepared by annealing tungsten metal sheets under an Ar and low O2 atmosphere, exhibited high catalytic activity and energy conversion efficiency. The highest efficiency achieved here for DSSCs with WO3-x counter electrodes, was 5.25%, obtained from a 500°C annealed tungsten sheet. TEM and XPS analysis suggested the formation of sub-stoichiometric tungsten oxide layer (∼WO2.6) with the presence of W6+, W5+ and W4+ oxidation states at the tungsten metal surface after the 500°C annealing. Only W6+ and W5+ oxidation states were detected after a 600°C annealing indicating the formation of a more stoichiometric tungsten oxide layer (∼WO2.8) and resulting in a drop in efficiency of the DSSC. We suggest that mixed valence tungsten states account for the excellent catalytic activity and good electrical conductivity as evidenced by the highest cyclic voltammetry response of 0.76mA/cm2 and the lowest impedance value of 44.33Ω, respectively.

Visible Photoluminescence of Gas Phase Synthesized Well-Defined Germanium Oxide Nanoparticles

Substoichiometric germanium oxide nanoparticles were synthesized through gas aggregation process with a careful control on the size, composition and crystallinity of the nanoparticles. The nanoparticles show broad room temperature photoluminescence (PL) in the 470nm to 600nm wavelength region. The microstructure and optical properties of the nanoparticles were investigated. We found that the photoluminescence behavior of the nanoparticles is critically influenced by their compositions. Through temperature-dependent photoluminescence and time-resolved photoluminescence spectroscopy, we concluded that the broad PL band originated from the defects in the GeOx shell layers, rather than the quantum-confinement effect.

PEFC Cathode Catalysts Using Doped and Non-Doped Titanium Oxide Support

Stability of catalyst support materials is one of important issue for improving the durability of PEFC cathode. Due to vulnerability of carbon black to oxidation at high potentials during start-up/shut-down cycles, electro-conductive oxides, nitrides, and carbides are recently attracting much attentions as promising alternative support materials [1]. Among them, sub-stoichiometric (oxygen-deficient) titanium oxide, which is known as Magnéli phase (TinO2n-1), is attractive candidate because of its superior corrosion-resistant and electrically conductive properties. As reported previously, TiOx-supported Pt catalyst shows improved ORR activity with much higher stability against high potentials under RDE and practical MEA evaluation conditions [2]. In addition to introducing oxygen-deficiency to the TiO2 lattice, n-type doping with higher valent cation such as Nb5+ is commonly used technique to obtain conductive titanium oxide. Doped Nb5+ cations introduce carrier electron into Ti-3d band, which leads to the increase of conductivity of the oxide. In this work, titanium oxide supports with different dopant concentration and reduced level were prepared, and its activity for ORR and stability against potential cycles were examined.Nb-doped TiO2 (Nb: 1-10 at.%) nanoparticles were prepared by flame spray method using diluted organometallic compounds as metal sources. The obtained Nb-TiO2 powder is a mixture of rutile and anatase structures and consists of 20-50 nm particles with BET area of 60-80 m2/g. Reducing treatments of the oxide was carried out by irradiating with Nd:YAG laser operated at λ = 355 nm in acetonitrile [3]. Pt nano-particles were deposited on the oxide support (10-20 wt.%Pt) by impregnation-reduction method. The electrochemical measurements was conducted by rotating disk electrode (in 0.1M HClO4) and MEA (at 80°C, 100%RH). Durability of the catalyst was examined according to the FCCJ-2011 protocols (1.0 V-1.5 V vs RHE for catalyst support durability, 0.6 V-1.0 V vs RHE for Pt NPs durability) [4]. Figure 1 shows specific ORR activity (at 0.9 V) of Pt/TiOx and Pt/Nb-TiOx along with that of a conventional Pt/C in O2-saturated solution at 25°C. As niobium content of the oxides increases, the ORR activities decrease. From XRD patterns of the catalysts, formation of ordered Pt-Ti alloy phase was confirmed for Pt/1%Nb-TiOx, whereas no ordered alloy phase was observed for Pt/10%Nb-TiOx. This fact indicates that higher Nb-content suppress the Pt-Ti alloy formation, which leads to lower ORR activity at Pt/Nb-TiOx catalysts. In addition, without reducing treatment with laser irradiation, Pt/Nb-TiOx shows further lower ORR activity. Alloy formation seems to be almost the same with the reduced catalysts so that this activity loss would be partly due to resistivity difference of the Nb-TiOx catalyst supports. To examine the catalyst stability at high potentials, potential cycling test between 1.0 V and 1.5 V was conducted in 0.1 M HClO4 at 60°C. CVs of Pt/1%Nb-TiOx with reducing treatment are fairly stable even after 5,000 cycles, which was similar stability with non-doped Pt/Ti4O7 catalyst [2]. On the other hands, the catalysts with higher Nb content showed much inferior CV stability. Additional data including MEA performance/stability and further discussions will be presented at the meeting.

Electrochemical Characterization of Sb-Doped SnO2 as Electrocatalyst Support for Electrochemical Energy Conversion Systems

Electrochemical energy conversion systems, mainly Unitized Regenerative Fuel Cells (URFC) and Solid Polymer Electrolyzer (SPE), are considered to have a promising future due to the generation of hydrogen in a clean and sustainable way. However, the design of oxygen electrodes is the main limiting factor for an efficient performance. In this matter, most of the studies have been focused in the development of efficient electrocatalyst for the oxygen reduction and water electrolysis reactions, and the use of electrocatalysts supports different from conventional carbon-based materials, which is susceptible of corrosion at the URFC and SPE operation conditions. Corrosion resistance supports, with improved surface area and conductivity, would reduce the electrocatalyst loading,the membrane electrode assembly degradation rate, and the global construction cost of the systems. The present work deals with the synthesis and electrochemical characterization of a Sb – doped SnO2 (ATO) for their potential use as electrocatalyst support in the oxygen electrode in electrochemical energy conversion systems. The synthesis of the support was performed by the sol –gel technique with a surface area modification with dodecylamine as surfactant followed by a thermal treatment. Electrochemical stability was evaluated by cyclic voltammetry (CV) with 250, 400 and 500 cycles in a wide range (0 – 2.0 V vs ENH), while the chronoamperometry (CA) was evaluated at high anodic potential (1.8 V vs ENH) in a continuous period of time of 5 h and 8 h. The results revealed that the Sb doped SnO2 support, present good electrical properties and electrochemical stability at a higher potential of 1.8 V vs. NHE in the presence of O2, as compared to other carbon-based and commercial sub-stoichiometric titanium oxide (Ebonex ®) supports.

Imaging by atomic force microscopy of the properties difference of the layers covering the facets created during SIMS analysis

Abstract Atomic force microscopy (AFM) is used in tapping mode in order to study the roughness created in the crater bottom during secondary ions mass spectrometry (SIMS) analysis in silicon, using O 2 + primary ions without flooding. Previous studies of the chemical composition of the facets created during the analysis have lead to the conclusion that the facets oriented toward the O 2 + beam during the ionic bombardment were close to SiO 2 in composition, while the facets hidden from the beam were covered with a sub-stoichiometric oxide SiO x (with x 2 due to the different properties of the materials (hardness, adhesion, etc.). As a comparison, an observation of a surface covered with SiO 2 and Si (SiO 2 deposed with a 90 nm or 4 nm thickness, and partially removed from a Si surface) shows the same kind of contrast.

Crystalline Mesoporous Films of Titanium Oxides with Various Degree of Substoichiometry for Fuel Cells

The first results of the synthesis of mechanically stable, crystalline mesoporous films of titanium oxides with various stoichiometry, which will be applied either as an anode or a cathode for fuel cell electrodes, will be presented. The composition of TiO2 and substoichiomteric TinO2n-1 Magneli phases which has been obtained in a high temperature and in reductive atmosphere is expected to have a potential to serve as an anode for enhanced oxidation of organic fuel and to be employed as a cathode when modified with the co-catalyst (NiO/Ni) to allow an effective oxygen reduction. These applications require high degrees of conductivity and a large contact area with the reaction media. In addition, highly crystalline oxides are desired in order to optimize the properties such as electroactivity or electronic conductivity. The synthesis approach of substoichiometric TinO2n-1 oxides and the structural characterization in view of their potential application in fuel cell systems will be the objective of the presentation.

Ge interface engineering using ultra-thin La2O3 and Y2O3 films: A study into the effect of deposition temperature

A study into the optimal deposition temperature for ultra-thin La2O3/Ge and Y2O3/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La2O3 and Y2O3) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La2O3 has been found to be more reactive to Ge than Y2O3, forming LaGeOx and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y2O3/Ge deposited at 400 °C allows for an ultra-thin GeO2 layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeOx/Ge interface. The Y2O3/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y2O3/Ge stacks has been estimated to be 5.7 ± 0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y2O3/Ge stack exhibits a higher conduction band offset (>2.3 eV) than the La2O3/Ge (∼2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (∼10−7 A/cm2 at 1 V) five orders of magnitude lower than the respective La2O3/Ge stack. Our study strongly points to the superiority of the Y2O3/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology.

Towards forming-free resistive switching in oxygen engineered HfO2−x

We have investigated the resistive switching behavior in stoichiometric HfO2 and oxygen-deficient HfO2−x thin films grown on TiN electrodes using reactive molecular beam epitaxy. Oxygen defect states were controlled by the flow of oxygen radicals during thin film growth. Hard X-ray photoelectron spectroscopy confirmed the presence of sub-stoichiometric hafnium oxide and defect states near the Fermi level. The oxygen deficient HfO2−x thin films show bipolar switching with an electroforming occurring at low voltages and low operating currents, paving the way for almost forming-free devices for low-power applications.

Silicon suboxide (SiOx): laser processing and applications

Substoichiometric silicon oxide SiOx with x 105 cm−1 in the deep UV below 250 nm, comparable to strongly absorbing polymers such as polyimide. This enables precise ablation with, e.g., excimer lasers at moderate fluences. For example, UV-transparent diffractive elements or phase masks are made by laser patterning of an appropriate SiOx film and subsequent oxidation to SiO2. Modifications of the basic film ablation process lead to novel surface topographies such as blister or cup arrays with potential non-optical applications, e.g., in micro-/nanofluidics.

Enhanced activity, durability and anti-poisoning property of Pt/W18O49 for methanol oxidation with a sub-stoichiometric tungsten oxide W18O49 support

In this work, sub-stoichiometric tungsten oxide W18O49 was first studied as a support for a Pt catalyst.

Substoichiometric two-dimensional molybdenum oxide flakes: a plasmonic gas sensing platform.

Two-dimensional (2D) molybdenum oxides at their various stoichiometries are promising candidates for generating plasmon resonances in visible light range. Herein, we demonstrate plasmonic 2D molybdenum oxide flakes for gas sensing applications, in which hydrogen (H2) is selected as a model gas. The 2D molybdenum oxide flakes are obtained using a grinding-assisted liquid exfoliation method and exposed to simulated sunlight to acquire its substoichiometric quasi-metallic form. After the exposure to H2 gas molecules, the quasi-metallic molybdenum oxide flakes are partially transformed into semiconducting states, thus gradually losing their plasmonic properties. The novel 2D plasmonic sensing platform is tested using different concentrations of H2 gas at various operating temperatures to comprehensively assess its sensing performance. The presented 2D plasmonic system offers great opportunities for future sensing and optical applications.

Electrochromic active layers from ultrathin nanowires of tungsten oxide

Electrochromic devices have benefited from the use of nanocrystalline films of particles to improve upon coloration and bleaching times while simultaneously enabling the use of solution based film making processes. In this work, electrochromic properties of ultrathin nanowires of substoichiometric tungsten oxide deposited on ITO via electrophoretic deposition are studied. These films show large coloration changes with very rapid coloration and bleaching times of 4.1 and 2.9 seconds respectively for an optical density change of 0.58 using H+ as the charge-balancing ion. These fast switching times are due to the combination of fast diffusion of ions into the narrow diameter wires and density control of the film through the deposition process. We also show good film stability over 300 colour–bleach cycles. This development not only advances the potential of WOx films in display applications, but can also improve their performance in smart windows.

A Raman spectroscopy study of cerium oxide in a cerium–5 wt.% lanthanum alloy

This paper describes a study of a cerium–5wt.% lanthanum (Ce–5wt.% La) alloy using Raman spectroscopy and X-ray diffraction (XRD). Examination of the alloy microstructure by optical microscopy and Raman spectroscopy revealed the presence of inclusions which were identified as cerium oxide (CeO2). The study also highlighted the need to avoid excessive laser power during acquisition of the Raman spectra as this appeared to cause the oxidation of the region being analysed where previously no cerium oxide peak had been detected. The propensity of cerium to oxidise in air results in the formation of a CeO2 layer on the surface of the alloy. Raman spectroscopy of the oxide layer formed on the alloy after exposure to air for 21 days found that the Raman peak denoting cerium oxide was seen at between 5 and 7cm−1 lower than the value for CeO2 (465cm−1). This is attributed to a combination of a sub-stoichiometric oxide layer and the presence of La in the alloy.

Physicochemical properties and electrochemical behavior of Ebonex/Pt-based materials

Physicochemical properties and electrochemical behavior of Ebonex®/Pt-based electrodes obtained with the use of a combined electrochemical method by electrodeposition of a thin platinum layer on substoichiometric titanium oxides (Ebonex®) followed by heat treatment are studied. Phase composition is found to depend substantially on the temperature of the electrode treatment. At temperatures above 230°C, a titanium-dioxide-hollandite phase is formed and facilitates thermal diffusion of platinum deep into the substrate. A previously unknown titanium-oxygen phase (310°C) that affects the electrochemical behavior of the electrodes is discovered. Ebonex®/Pt-based materials are found to be n-type semiconductors, the flat band potentials and the number of charge carriers of which are determined by the formation conditions.

Activity and Stability of Sub-Stoichiometric Titanium Oxide Supported Pt Nano-Particles

not Available.

Electron microscopy investigations of metal-support interaction effects in M/Y2O3 and M/ZrO2 thin films (M=Cu, Ni)

Model systems of the clean and pure oxides Y2O3 and ZrO2, as well as Cu/Cu2O and Ni/NiO particles embedded in the respective oxides have been used to study the reduction behavior of the oxides and the eventually associated metal-support interaction effects in oxide-supported systems. Particular emphasis has also been given to the influence of the phase transformation in ZrO2-containing systems on metal-support interaction. Whereas Y2O3 has been found to be an outstandingly structurally and thermally stable oxide even upon reduction in hydrogen up to 1073 K, ZrO2 was found to undergo a series of phase transformations from amorphous ZrO2 to polycrystalline tetragonal ZrO2 (∼673 K) and subsequently to monoclinic ZrO2 (above 873 K). Both phase transformations were found to be basically dependent on gas partial pressure and annealing rate. However, substantial reduction of the oxides did not take place during the phase transformations. In turn, both Cu- and Ni-containing systems were not observed to be substantially affected by any (strong) metal-support interaction effects such as encapsulation by sub-stoichiometric oxides or reductive formation of intermetallic phases, at least up to temperatures of 1073 K. Equally, for the ZrO2-containing systems, also the phase transformations occurring at elevated temperatures did not cause structural or thermo-chemical alterations of the Cu or Ni-particles. Differences in the metal-support interaction between Cu- and Ni-particles have only been obtained in the structural “reference” systems, that is, if supported on SiO2. Whereas Cu/Cu2O particles on SiO2 are basically unaffected by the reductive treatment at elevated temperatures, a Ni3Si2 intermetallic phase is formed if SiO2-supported Ni/NiO particles are treated in hydrogen at 673 K and above.

Electron Energy Loss Spectroscopy Characterization of TANOS (TaN/Al2O3/Si3N4/SiO2/Si) Stacks

The interfacial layer between the Al₂O₃ layer and the Si₃N₄ layer formed after postdeposition annealing (PDA) of TaN/Al₂O₃/Si₃N₄/SiO₂/Si (TANOS) stacks was investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy, and electron energy loss spectroscopy (EELS). From the result of the TEM analysis, it was found that the 2-nm-thick interface layer between Al₂O₃ and Si₃N₄ layers was amorphous. The high-loss EELS analysis showed that the phases of the interfacial layer weakly bound together instead of the substoichiometric silicon oxide and amorphous Al₂O₃ near the bottom interface of the crystalline Al₂O₃. The low-loss EELS analysis showed that aluminum existed in metallic state at the interface. Therefore, we speculated that SiO(x)N(y) could be formed by oxidation of Si₃N₄ during PDA and that metallic aluminum could be formed by the decomposition of weakly bound amorphous Al₂O₃ during electron irradiation. These complicated reactions near the interface could induce oxygen deficiency in the Al₂O₃ layer and finally degrade the retention properties of TANOS stacks.

Magnetic field strength influence on the reactive magnetron sputter deposition of Ta2O5

Reactive magnetron sputtering enables the deposition of various thin films to be used for protective as well as optical and electronic applications. However, progressing target erosion during sputtering results in increased magnetic field strengths at the target surface. Consequently, the glow discharge, the target poisoning, and hence the morphology, crystal structure and stoichiometry of the prepared thin films are influenced. Therefore, these effects were investigated by varying the cathode current Im between 0.50 and 1.00 A, the magnetic field strength B between 45 and 90 mT, and the O2/(Ar + O2) flow rate ratio Γ between 0% and 100%. With increasing oxygen flow ratio a substoichiometric TaOx oxide forms at the metallic Ta target surface which further transfers to a non-conductive tantalum pentoxide Ta2O5, impeding a stable dc glow discharge. These two transition zones (from Ta to TaOx and from TaOx to Ta2O5) shift to higher oxygen flow rates for increasing target currents. In contrast, increasing the magnetic field strength (e.g., due to sputter erosion) mainly shifts the TaOx to Ta2O5 transition to lower oxygen flow rates while marginally influencing the Ta to TaOx transition. To allow for a stable dc glow discharge (and to suppress the formation of non-conductive Ta2O5 at the target) even at Γ = 100% either a high target current (Im ⩾ 1 A) or a low magnetic field strength (B ⩽ 60 mT) is necessary. These conditions are required to prepare stoichiometric and fully crystalline Ta2O5 films.

Influence of Graphite Source on Chemical Oxidative Reactivity

Although all graphites share the same idealized chemical structure, marked differences in fact exist between their reactivities, such as the propensity for oxidation, that need to be taken into consideration for the development of applications. Here we show that five different commercially sourced natural and synthetic graphites differ significantly in their response to a modified Staudenmaier oxidation that produces substoichiometric graphene oxides (sub-GOx). The dominant oxidation product is hydroxyl groups, which can be dehydrate to epoxy groups under mild heating even below 120 °C. The extent of oxidation correlates broadly with the defect band intensity in the starting graphites as measured by Raman spectroscopy. FTIR shows there is a significant concentration of H defects at the % atom level. The results suggest that defects in the graphite plane are more prevalent than previously thought. Finally, the properties of the thermally reduced sub-GOx are also different. The product from the least defective starting graphite ultimately exhibits the lowest activation energies for both electron and hole transport, of the order of 10 μeV below 25 K, that is characteristic of band-like transport. These results are important because they show that the quality of the starting graphite significantly affects the properties of the derived products.