Nonstoichiometric State Research Articles

2D-Mo3S4 phase as promising contact for MoS2

The search for electrode materials for two-dimensional structures, particularly for semiconducting H-MoS2, is a significant problem for contemporary materials science. This stimulates interest in metastable structures, which motivates us to look at non-stoichiometric metastable states known as Berthollides. An unbiased evolutionary search for new two-dimensional structures in Mo-S system showed the presence of Mo5S4, Mo3S4, and MoS metastable structures (located near the decomposition line) in the low-sulfur area of phase diagram. The predicted novel Cm-Mo3S4 phase exhibits metallic properties and dynamic stability confirmed by the calculation of phonon dispersion spectra. Its lattice parameters are similar to the semiconducting H-MoS2 phase which makes it possible to use Cm-Mo3S4 as a conducting contact in nanodevices based on MoS2. A study of proposed H-MoS2/Cm-Mo3S4 lateral heterostructures showed that in the contact region a transition to the conducting states of the H-MoS2 edge atoms is observed, as well as the preservation of Cm-Mo3S4 conducting properties. The main feature of Cm-Mo3S4 is that it can be structurally obtained by a direct transition of H-MoS2 monolayer with sulfur vacancies. As a result, the predicted Cm-Mo3S4 phase is a promising candidate for conducting contacts in nanoelectronic device components.

The higher-k phase formation in amorphous HfO2 films by swift heavy ion irradiation

The formation of higher-k phase HfO2 is challenging and attractive. In this study, the phase transition of pure HfO2 thin films was induced by swift heavy ions (SHIs) irradiation. The amorphous HfO2 films crystallized after SHIs irradiation and undergone a transformation of monoclinic to cubic/tetragonal phase with the increase of the ion fluences. The as-deposited HfO2 films take on nonstoichiometric state, and became close to the stoichiometric one under SHIs irradiation. This work provides an approach to stabilize the higher-k phase HfO2 and optimize the interface between HfO2 and Si by SHIs irradiation.

Crystallization and bandgap variation of non-stoichiometric amorphous Ga2O3-x thin films during post-annealing process

The evolution of the structural and chemical properties of non-stoichiometric amorphous Ga2O3-x thin films during post-annealing in an atmospheric environment was investigated using scanning electron microscopy, X-ray diffraction, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and ultraviolet–visible light spectroscopy. The amorphous thin films were crystallized into a mixture of α and β phases that were epitaxial to the sapphire (0001) substrate. The AP-XPS results indicate that the non-stoichiometric state of the Ga2O3-x thin films is composed of metallic Ga, Ga2O, and Ga2O3. Ga and Ga2O are oxidized into Ga2O3, achieving a stoichiometric state during the post-annealing process. This gradually increases the optical bandgap from 4.16 eV for the as-grown sample to 4.81 eV for the sample annealed at 600 ℃. Our results support the relationship between the chemical composition and bandgap of non-stoichiometric Ga2O3-x films deposited by radio-frequency powder sputtering.

Исследование структуры бивня мамонта методом ИК спектроскопии

The structure of mammoth tusk was investigated by infrared spectroscopy, including after heat treatment. The whole complex of functional groups of the tusk components - hydroxyapatite, collagen and water - was revealed. It was found that collagen in the IR spectrum is represented mainly by characteristic absorption bands of amide and aliphatic groups. After heat treatment at 600 °C, the organic part is completely removed from the sample. It was found that hydroxyapatite in mammoth tusk is presented in a carbonate-substituted form, however, heat treatment at 900 °C leads to the removal of carbonate anion and water from the sample, which is accompanied by the transition of hydroxyapatite from the nonstoichiometric state to the stoichiometric state.

Metal-organic frameworks induced robust layered Co(OH)2 nanostructures for ultra-high stability hybrid supercapacitor electrodes in aqueous electrolyte

Comparing with intensively studied β-Co(OH)2 with applying on hybrid supercapacitors, α-Co(OH)2 is believed to possess higher electrochemical energy storage performances because of its larger interlayer spacing. However, α-phase is always transforming to β-phase when subject to successive charge-discharge processes with resulting in fast degradation in electrochemical performance. Here, by controlling ZIF-67 hydrolysis with appropriate pH regulating additives, the robust α-Co(OH)2-A nanostructures are obtained with excellent electrochemical energy storing performances, which present higher specific capacity of 87.1 mAh g−1 at 1 A g−1, excellent rate capability of 77% capacity retention at 20 A g−1 and ultra-high cycle stability of over 100% capacity retention over 200, 000 charge-discharge cycles. Through detailed characterizations, such great enhancement is mainly due to the synergistically achieving interlayer crystal water and non-stoichiometric valence states with stable larger interlayer spacing on the robust layered nanostructures. Moreover, the present study also confirms that, instead of most understanding of H+ (de)intercalation, OH− ions significant contributing to pseudocapacitive storage through inserting and reacting with H+ of crystal water and α-Co(OH)2. Thus, the present simple strategy with clear understanding to the energy storage mechanism is beneficial for designing and fabricating mass producible electrode materials of hybrid supercapacitors.

Magnetic and interface properties of the core-shell Fe3O4/Au nanocomposites

Core-shell Fe3O4/Au nanostructures were obtained with an advanced method of two step synthesis and several complementary methodics were applied for investigation structural and magnetic properties of the samples. Along with X-ray diffraction and transmission electron microscopy, electron diffraction, optical, Raman and Mössbauer spectroscopy were used for nanoparticle characterization. It was established that the physical and structural properties Fe3O4/Au nanocomposites are specific of intrinsic properties of gold and magnetite. Mössbauer and Raman spectroscopy data indicated that magnetite was in a nonstoichiometric state with an excess of trivalent iron both in the initial Fe3O4 nanoparticles and in the Fe3O4/Au nanocomposites. As follows from the Mössbauer data, magnetic properties of iron ions in the internal area (in core) and in the surface layer of magnetite nanoparticles are different due to the rupture of exchange bonds at the particles surface. This leads to decrease in an effective magnetic moment at the surface. Gold atoms at the interface of the composites interact with dangling bonds of magnetite and stabilize the magnetic properties of the surface layers of magnetite.

Hydrogen sulphide detection by CMOS integrated tungsten oxide nanowire networks

Metal oxide nanostructures like tungsten oxide nanowires are intensively studied materials for sensor applications. In this work we report on tungsten oxide gas sensors integrated on a CMOS fabricated microhotplate chip. Tungsten oxide gas sensors were prepared by drop coating of a nanowire network suspension onto interdigitated electrodes prefabricated on the CMOS microhotplate chip. The tungsten oxide nanowire network was characterised by TEM and Raman spectroscopy, confirming their non-stoichiometric state. Using tungsten oxide nanowire networks as gas sensing material we observed high sensitivity to hydrogen sulphide: concentrations of 1 ppm have been detected with a sensor response up to 55%.

Dielectric anomaly in La1―xSrxFeO3±δ perovskite nanoparticles

ABSTRACTLa1―xSrxFeO3±δ nanoparticles (where x = 0.05 and 0.10) were synthesized by co-precipitation method. The nanoparticle samples have orthorhombic (Pbnm) phase with small impurities as confirmed from X-ray diffraction (XRD) with Rietveld analysis and transmission electron microscopic (TEM). Nonstoichiometric and valence states of iron ions (Fe3+, Fe4+ and Fe5+) were established by Mössbauer spectroscopy at room temperature (RT). The dielectric constant as a function of frequency (100 Hz to 1 MHz) and temperature (293–503 K) exhibits dielectric anomaly which is an evident the ferroelectric-paraelectric phase transition. Relaxor ferroelectrics were established by differential scanning calorimetric (DSC) which is confirmed by dielectric study. The magnetic properties of the samples exhibit the weak ferromagnetism at RT.

Effect of oxygen deficiency on electronic properties and local structure of amorphous tantalum oxide thin films

The dependence of electronic properties and local structure of tantalum oxide thin film on oxygen deficiency have been investigated by means of X-ray photoelectron spectroscopy (XPS), Reflection Electron Energy Loss Spectroscopy (REELS), and X-ray absorption spectroscopy (XAS). The XPS results showed that the oxygen flow rate change results in the appearance of features in the Ta 4f at the binding energies of 23.2eV, 24.4eV, 25.8, and 27.3eV whose peaks are attributed to Ta1+, Ta2+, Ta3+/Ta4+, and Ta5+, respectively. The presence of nonstoichiometric state from tantalum oxide (TaOx) thin films could be generated by the oxygen vacancies. In addition, XAS spectra manifested both the increase of coordination number of the first Ta-O shell and a considerable reduction of the Ta-O bond distance with the decrease of oxygen deficiency.

CMOS Integrated Tungsten Oxide Nanowire Networks for ppb-level H2S Sensing

Tungsten oxide is an intensively studied semiconductor with the relevant application as H2S gas sensor. In this work we report on H2S gas sensor devices based on tungsten oxide nanowire networks, which are integrated on a CMOS fabricated microhotplate chip. Tungsten oxide gas sensors were prepared by the deposition of nanowire networks onto interdigitated electrodes on CMOS. The material properties of the tungsten oxide nanowires were investigated by TEM and Raman spectroscopy, confirming their non-stoichiometric state. Utilising WO3-x nanowire networks as gas sensing material we obtained extraordinary sensitivity to H2S: concentrations down to 10 ppb have been detected, even in the presence of 50% relative humidity.

In Situ High Resolution Synchrotron X-Ray Powder Diffraction Studies of Lithium Batteries

Lithium ion battery technology is the heart in operating modern technology devices such as mobile phones and laptops. However, as our society is moving towards the utilization of sustainable energy sources, batteries can be foreseen to become an even more important part of the energy infrastructure. They will be used not only for transportation, but also for medium and short term storage as well as for frequency stabilization in intermittent grid scale energy sources such as solar and wind. Thus, the development of new cheaper and safer battery materials with high energy and power density is very important for a successful worldwide energy transition. The understanding of structural and compositional changes of bulk electrodes in batteries is undoubtedly important. However, it is often transport of electrons and ions across and through interfaces [1] (e.g., between lithiated and delithiated domains) which limits the obtainable power density and battery life time. A challenging and important task is to obtain in situ information about the formation and evolution of interfaces in an operating battery system. This work addresses these challenges and for this purpose we have developed a special microcapillary battery cell allowing diffraction information to be obtained from only the active material during battery operation [2]. High resolution synchrotron x-ray powder diffraction technique has been undertaken to obtain detailed structural and compositional information during lithiation/delithiation of commercial LiFePO4 materials [3]. We report results from the first in situ time resolved high resolution powder diffraction experiments at beamline ID22/31 at the European Synchrotron Radiation Facility, ESRF. We follow the structural changes during charge of commercial LiFePO4 based battery materials using the Rietveld method. Conscientious Rietveld analysis shows slight but continuous deviation of lattice parameters from those of the fully stoichiometric end members LiFePO4 and FePO4 indicating a subsequent variation of stoichiometry during cathode delithiation. The application of an intermittent current pulses during charge using GITT technique shows an oscillation of lattice constants that correlates with the applied current and electrochemical relaxation sequence and may indicate the existence of metastable non-stoichiometric states.

HR-MAS NMR reveals a pH-dependent LPS alteration by de-O-acetylation at abequose in the O-antigen of Salmonella enterica serovar Typhimurium

NMR spectroscopy can detect biomolecules like lipopolysaccharide directly on the surface of the cell, thus avoiding isolation and purification, and providing a more realistic description than the one derived from in vitro studies. Here we present a high-resolution magic-angle spinning NMR study of the O-antigen of Salmonella enterica serovar Typhimurium (S. Typhimurium) performed directly on the cells showing the alteration of its acetylation state over time. The O-antigen region of S. Typhimurium consists of the repeating unit [→2)-α-d-Manp-(1→4)-α-l-Rhap-(1→3)-α-d-Galp-(1→] where Man stands for mannose, Rha for rhamnose, and Gal for galactose. Man is substituted with abequose (Abe) O-acetylated at carbon 2. Our studies revealed that the appearance of de-O-acetylated O-antigen in the stationary growth phase is due to the de-O-acetylation of already synthesized O-acetylated O-antigen and that this reaction is caused by the metabolism-induced basic pH of the growth medium. The labile O-acetylation of the O-antigen we observed in S. Typhimurium generates non-stoichiometric O-acetylation states and therefore changes the nature of an immunogenic epitope.

On Arising Nanohydrides in Reduced Alkaline Solution

Studying electron properties of liquid water in the frame of band theory shows that obtaining its non-stoichiometric state is a simple way to vary physical and chemical properties, and changing a Reduction–Oxidation (RedOx) potential of any aqueous solution. In this connection, Fermi level in the band gap, as a measurable characteristic of non-stoichiometric liquid water, is the most convenient energy for monitoring and managing its RedOx potential. The hypo-stoichiometric state, H2O1–z, of liquid water is realized when the position of Fermi level is shifted to the bottom of conduction band. This state can be fixed by micro emulsifying gaseous hydrogen in liquid water or by electro-reducing the alkaline solution (catholyte) with possible forming alkaline (A) nanohydrides (AH?H2O) n. As strong reducers, they can be quasi-stable in the aqueous solution and be an effective means for holding the negative RedOx potential of liquid water.

Aqueous Nanofluid as a Two-Phase Coolant for PWR

Density fluctuations in liquid water consist of two topological kinds of instant molecular clusters. The dense ones have helical hydrogen bonds and the nondense ones are tetrahedral clusters with ice-like hydrogen bonds of water molecules. Helical ordering of protons in the dense water clusters can participate in coherent vibrations. The ramified interface of such incompatible structural elements induces clustering impurities in any aqueous solution. These additives can enhance a heat transfer of water as a two-phase coolant for PWR due to natural forming of nanoparticles with a thermal conductivity higher than water. The aqueous nanofluid as a new condensed matter has a great potential for cooling applications. It is a mixture of liquid water and dispersed phase of extremely fine quasi-solid particles usually less than 50 nm in size with the high thermal conductivity. An alternative approach is the formation of gaseous (oxygen or hydrogen) nanoparticles in density fluctuations of water. It is possible to obtain stable nanobubbles that can considerably exceed the molecular solubility of oxygen (hydrogen) in water. Such a nanofluid can convert the liquid water in the nonstoichiometric state and change its reduction-oxidation (RedOx) potential similarly to adding oxidants (or antioxidants) for applying 2D water chemistry to aqueous coolant.

Effect of Li/Ti ratio on microstructure and thermal diffusivity of lithium titanate for solid breeding material

A non-stoichiometric solid state compounds of lithium titanate of the Li 2SnO 3-type structure with the space group C2/ c (No. 15), Z = 8 (denoted as Li 2TiO 3 ss) were successfully synthesized using the powders of LiOH·H 2O and H 2TiO 3 as the starting materials. The crystal structure of the compounds was analyzed by X-ray diffractometry using Cu Kα radiation. The Li/Ti ratio in the compounds was analyzed by an inductively coupled plasma–optical emission spectroscopy. The microstructure was analyzed by scanning electron microscopy. Thermal diffusivity was measured by the laser flash method. The influences of the Li/Ti ratio on microstructure, Li density, and thermal diffusivity were investigated. Li density of the compounds increased due to the synergy effect of the good sintering behavior and the Li-rich composition. The increase in the Li/Ti ratios of the compounds encouraged the sintering and increased the thermal diffusivity.

Exotic Quantum Materials

Quantum critical transitions (QCT) occur when temperatures approach absolute zero and are caused by quantum, rather than thermal, fluctuations. In the vicinity of these transitions, materials often exhibit exotic transport properties. QCTs are usually accessed by varying an external parameter such as the pressure or the magnetic field or, alternatively, by using chemical doping to fine-tune the material into a nonstoichiometric (and thus disordered) state. Matsumoto et al. (p. [316][1]) now achieve quantum criticality without any such tuning in the compound β-YbAlB4. Because of the mixed-valence nature of this heavy fermion material, it is likely that instead of a conventional quantum critical point, the experiments are accessing an exotic quantum critical phase. [1]: /lookup/doi/10.1126/science.1197531

ChemInform Abstract: Propene Metathesis Over Silica-Supported Tungsten Oxide Catalyst- Catalyst Induction Mechanism.

Abstract The propene metathesis reaction was studied from the point of view of elucidating the mechanism of catalyst induction and establishing conditions for maximum activity. Instrumental techniques such as ESR, IR, and TPD were used to study the various aspects. During catalyst induction, trace quantities of acetone and acetaldehyde were detected in the product stream, indicating that lattice oxygen from tungsten oxide might be responsible for these products. Induction appeared to proceed via two steps since pretreatment of the catalyst with nitrogen and hydrogen yielded a decreased amount of acetone in the latter case whereas acetaldehyde remained unaffected. ESR studies indicated some interaction between tungsten oxide and silica at the catalyst preparatory stage as well as stabilisation of reduced tungsta species on the catalyst after its use and regeneration. Catalyst activity appeared to depend on conditions of pretreatment. Change in nitrogen pretreatment temperature from 500 to 600°C resulted in transition from strong to negligible external mass transfer behaviour of the catalyst. TPD studies in this context showed possible loss of lattice oxygen from tungsten oxide under the above-mentioned conditions of catalyst pretreatment. ESR studies indicated the reduction of W03 to a nonstoichiometric oxidation state. Hence catalytic activity appears to be related to the nonstoichiometric state of tungsten oxide, which may be WO 2.9 (as deduced from the blue-violet colour of the used catalyst).

Efficient Organic Photovoltaic Cells through Structural Modification of Chloroaluminum Phthalocyanine/Fullerene Heterojunctions

The structure−function relationship of organic photovoltaic (OPV) cells based on the chloroaluminum phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction are explored. Optimization is achieved with the use of a molybdenum oxide (MoOx) and an underlying 3,4,9,10-perylene tetracarboxylic acid (PTCDA) interlayer at the hole extracting electrode, the latter acting as a structural template for the subsequent growth of the ClAlPc donor layer. OPV cells demonstrate power conversion efficiencies of 3.0% under simulated AM1.5G (air mass 1.5 global) illumination, with the short-circuit current (Jsc) showing an ∼25% improvement relative to a device without a templating layer. Results from X-ray diffraction and electronic absorption spectroscopy suggest an improved packing and crystallinity in the ClAlPc layer when deposited on the PTCDA template, which suggests an enhancement in charge transport through the film. External quantum efficiency measurements confirm an overall improvement in Jsc in the ClAlPc spectral region upon templating. The effect of the MoOx interlayer is to minimize losses in the open-circuit voltage and fill factor caused by significant band bending and pinning of the adjacent organic layer highest occupied molecular orbital levels to nonstoichiometric defect states in the near Fermi level region of MoOx. The results present an improved strategy for the development of higher-performance OPV cells based on small molecule heterojunctions.

Atomic models of non-stoichiometric layered diborides M 1−xB 2 (M = Mg, Al, Zr and Nb) from first principles

Two atomic models of non-stoichiometric metal diborides M 1− x B 2 are now assumed: (i) the presence of cation vacancies and (ii) the presence of “super-stoichiometric” boron which is placed in cation vacancy site. We have performed first principle total energy calculations using the VASP-PAW method with the generalized gradient approximation (GGA) for the exchange-correlation potential in a perspective to reveal the trends of their possible stable atomic configurations depending on the type of M cations (M = Mg, Al, Zr or Nb) and the type of the defects (metal vacancies versus metal vacancies occupied by “super-stoichiometric” boron in forms of single atoms, dimers B 2 or trimers B 3). Besides we have estimated the stability of these non-stoichiometric states (on the example of magnesium–boron system) as depending on the possible synthetic routes, namely via solid-state reaction method, as well as in reactions between solid boron and Mg vapor; and between these reagents in gaseous phase. We demonstrate that the non-stoichiometric states such as B 2 and B 3 placed in metal sites may be stabilized, while the occupation of vacancy sites by single boron atoms is the most unfavorable.

Metastable surface modification processes for corrosion and oxidation control

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