Superlubricity of epitaxial monolayer WS<Subscript>2</Subscript> on graphene

We report the superlubric sliding of monolayer tungsten disulfide (WS₂) on epitaxial graphene (EG) grown on silicon carbide (SiC). Single-crystalline WS₂ flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG. Microscopic and diffraction analyses indicate that the WS₂/EG stack is predominantly aligned with zero azimuthal rotation. The present experiments show that, when perturbed by a scanning probe microscopy (SPM) tip, the WS₂ flakes are prone to slide over the graphene surfaces at room temperature. Atomistic force field-based molecular dynamics simulations indicate that, through local physical deformation of the WS₂ flake, the scanning tip releases enough energy to the flake to overcome the motion activation barrier and trigger an ultralow-friction rototranslational displacement, that is superlubric. Experimental observations show that, after sliding, the WS₂ flakes come to rest with a rotation of nπ/3 with respect to graphene. Moreover, atomically resolved measurements show that the interface is atomically sharp and the WS₂ lattice is strain-free. These results help to shed light on nanotribological phenomena in van der Waals (vdW) heterostacks, and suggest that the applicative potential of the WS₂/graphene heterostructure can be extended by novel mechanical prospects.

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Superconductivity at 5.5 K in Nb<Subscript>2</Subscript>PdSe<Subscript>5</Subscript> Compound

We report superconductivity in as-synthesized Nb₂PdSe₅, which is similar to a recently discovered Nb₂PdS₅ compound having very high upper critical field, clearly above the Pauli paramagnetic limit Zhang et al. (Sci. Rep. 3:1446, 2013). A bulk polycrystalline Nb₂PdSe₅ sample is synthesized by a solid-state reaction route in a phase-pure structure. The structural characterization has been done by X-ray diffraction, followed by Rietveld refinements, which revealed that the Nb₂PdSe₅ sample is crystallized in a monoclinic structure within the space group C2/m. Structural analysis revealed the formation of sharing of one-dimensional PdSe₂ chains. Electrical and magnetic measurements confirmed the superconductivity in Nb₂PdSe₅ compound at 5.5 K. Detailed magneto-resistance results exhibited the value of upper critical field to be around 8.2 T. The estimated H _c2(0) is within the Pauli paramagnetic limit, which is unlike the Nb₂PdS₅.     

Crystallographically selective nanopatterning of graphene on SiO<Subscript>2</Subscript>

Graphene has many advantageous properties, but its lack of an electronic band gap makes this two-dimensional material impractical for many nanoelectronic applications, for example, field-effect transistors. This problem can be circumvented by opening up a confinement-induced gap, through the patterning of graphene into ribbons having widths of a few nanometres. The electronic properties of such ribbons depend on both their size and the crystallographic orientation of the ribbon edges. Therefore, etching processes that are able to differentiate between the zigzag and armchair type edge terminations of graphene are highly sought after. In this contribution we show that such an anisotropic, dry etching reaction is possible and we use it to obtain graphene ribbons with zigzag edges. We demonstrate that the starting positions for the carbon removal reaction can be tailored at will with precision.     

Microstructure and properties of epitaxial Sr<Subscript>2</Subscript>FeMoO<Subscript>6</Subscript> films containing SrMoO<Subscript>4</Subscript> precipitates

Thin films of Sr₂FeMoO₆ (SFMO) were grown by pulsed laser deposition in non-optimized argon ambient pressures. The films were found to contain a high number of precipitates of foreign phases. The nature and microstructure of these phases were investigated in detail by high-resolution scanning transmission electron microscopy (STEM) and X-ray diffractometry (XRD). We found out that the dominant foreign phase embedded in the SFMO film matrix was SrMoO₄ (SMO). Through STEM and XRD analysis, we determined that the SMO phase grows epitaxially with respect to the surrounding SFMO matrix and has a fairly good crystallinity. Although the SFMO films include many foreign precipitates, they still exhibit good conducting properties and moderate magnetization values. Tuning the growth of the SMO phase on top of SFMO films to obtain a natural tunnel barrier might pave the way for future applications of SFMO in spintronic devices.     

Effect of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> concentration in Bi-containing high-temperature solutions on the luminescence of epitaxial Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> films

Single-crystal gadolinium gallium garnet films have been grown by liquid-phase epitaxy on (111) Gd₃Ga₅O₁₂ substrates from supercooled Bi₂O₃-B₂O₃ fluxed melts at different Gd₂O₃ concentrations. The luminescence spectra of the films have been measured at 10 and 300 K under unmonochromatized synchrotron X-ray excitation and selective UV synchrotron excitation. The Bi³⁺ luminescence is discussed.     

Fabrication and characterization of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript>/LaNiO<Subscript>3 </Subscript>heterostructures

Epitaxial Ba0.6Sr0.4TiO3 (BST)/LaNiO3 (LNO) heterostructures were fabricated on LAO (100) substrates using pulsed laser deposition (PLD). Their structural properties were investigated by X-ray diffraction (XRD). The θ–2θ scans showed single crystalline BST and LNO layers with a (100) orientations perpendicular to the substrate plane. Phi scans (ϕ) on the (220) plane of BST layer indicated that the films have two in-plane orientations with respect to the substrate. The atomic force microscope (AFM) surface morphologies showed a smooth and crack-free surface with the average grain size of 55 nm and the root-mean-square (RMS) of 4.53 nm for BST films. Capacitance–voltage curves are measured. From the capacitance, a dielectric constant of 762, tunabilty of 82.81% and loss tangent of 0.032 are obtained. The current–voltage curve shows that the leakage current is 2.41 × 10⁻⁷ A/cm² under an applied voltage of 2 V.     

Special features of reconstruction of defect structure of epitaxial films of CeO<Subscript>2</Subscript> and La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> in alternating magnetic field

Results obtained by methods of X-ray spectroscopy and atomic force microscopy (AFM) and special features of reconstruction of the defect structure of epitaxial films of cerium oxide CeO₂ and lanthanum zirconate La₂Zr₂O₇ during their processing in an alternating magnetic field or during their magnetic structural processing (MSP) are reported. The epitaxial films of CeO₂ and La₂Zr₂O₇ are seed and barrier layers of a buffer current-carrying element of a high-temperature superconducting second-generation wire (HTSC-2 wires). Changes in the crystalline structure of epitaxial films of CeO₂ and La₂Zr₂O₇ are reflected in changes in their X-ray diffraction patterns and AFM data. The X-ray spectroscopy indicated a change in the crystal structure of a biaxially textured substrate of Ni–5% W alloy as a result of MSP of sample buffer CeO₂/La₂Zr₂O₇. The obtained data indicate the existence of complex and fast processes in the crystal structure of epitaxial films of CeO₂ and La₂Zr₂O₇ and of Ni–5% W alloy at MSP, which result in irreversible changes in the crystal structure of these materials; in other words, it is found that, in the epitaxial films of CeO₂ and La₂Zr₂O₇ and in the Ni–5% W alloy, a magnetic structural effect is observed in an alternating magnetic field [1–3].     

The effect of growth conditions and N<Subscript>2</Subscript>/O<Subscript>2</Subscript> ambient on LO-phonon replicas during epitaxial growth of ZnO on c-sapphire

High quality heteroepitaxial thin films of ZnO:N were grown by pulsed laser deposition using a two-step growth method and annealed in situ at different temperatures and ambient conditions. Films were analyzed by X-ray diffraction (XRD), electrical measurements, and photoluminescence experiments at low temperatures to investigate the effect of nitrogen doping. The XRD results demonstrate epitaxial growth on the c-sapphire substrates, with average grain size of 57 nm. Photoluminescence spectra reveals a peak at 3.061 eV (405.1 nm) which is part of the longitudinal-optical-phonon replicas of excitons bound to neutral acceptors \textA₁⁰  \textX_\textA {\text{A}}_{1}^{0} \,{\text{X}}_{\text{A}} at 3.348 eV (370.4 nm), attributed in recent investigations to a newly reported donor–acceptor pair. Electrical resistivity and Hall effect measurements were performed using standard four point van der Pauw geometry at room temperature. Fresh films exhibited a resistivity of 3.1 × 10⁻³ Ω cm, a carrier density of 1.3 × 10¹⁹ cm⁻³, and a mobility of 53 cm²/V s. During approximately 2 weeks the as-deposited films presented a p-type behavior, as shown by the positive sign of the Hall constant measured. Thereafter, films reverted to n-type. From electrical measurements and photoluminescence spectra, the acceptor energy was determined to be 150 meV, in close agreement with reported values. These results are consistent with those presented in the literature for high purity crystals or homoepitaxial thin films, even though samples for the present study were processed at lower annealing temperature.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Interfacial capacitance in epitaxial heterostructures La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO

Epitaxial trilayer heterostructures of the type La_0.67Ca_0.33MnO₃/SrTiO₃/La_0.67Ca_0.33MnO₃ were grown by laser ablation on (001)[(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7] substrates. The real part of the dielectric permittivity ε and the loss factor tan δ of a 1100-nm-thick SrTiO₃ interlayer were studied in the temperature interval T=4.2–300 K in a nonbiased state and at a bias voltage of ±2.5 V applied to the manganite electrodes. Using the temperature dependence ε(T) measured for the SrTiO₃ layer grown between the manganite electrodes, we have estimated the capacitance of La_0.67Ca_0.33MnO₃/SrTiO₃ interfaces (C₁≈2 μF/cm²) related to the electric field penetrating from the interlayer into La_0.67Ca_0.33MnO₃.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.     

Thermal conductivity of Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Gd<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions

We have measured the thermal conductivity of Bi₂Te₃-Sb₂Te₃-Gd₂Te₃ solid solutions at temperatures from ～80 to 300 K and have determined the electronic and lattice components of their total thermal conductivity and the contributions of Sb₂Te₃ and Gd₂Te₃ to their thermal resistance. The results indicate that heat in these materials is transported largely by phonons and that three-phonon processes play a key role in determining the lattice thermal conductivity of the solid solutions.     

Effect of Eu<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on the crystallization behavior of BaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

We gave studied the crystallization behavior of 30BaO · 25Bi₂O₃ · 45B₂O₃ glasses doped with Eu₂O₃ to different levels. At a Eu₂O₃ content of 7 mol % or higher, the glasses undergo volume crystallization. The only precipitating phase is a solid solution between europium and bismuth oxides. With increasing europium concentration in the glass, the structure of the crystallites changes from cubic to rhombohedral. We have investigated the morphology, physicochemical properties, and luminescence spectra of the glasses and glass-ceramics.     

Growth and properties of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript>-NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

Platelike Li_{1 ? x} Na _x Cu₂O₂ single crystals up to 2 × 10 × 10 mm in dimensions have been grown by slowly cooling (1 ? x)Li₂CO₃·xNa₂O₂·4CuO melts in alundum crucibles in air. Li_{1 ? x} Na _x Cu₂O₂ solid solutions in the LiCu₂O₂-NaCu₂O₂ system have been shown to exist in the composition range 0.78 < x < 1. The temperature stability ranges of NaCu₂O₂ and LiCu₂O₂ are 780–930 and 890–1050°C, respectively. The Mössbauer spectra and electrical conductivity of the crystals have been measured.     

CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-B<Subscript>2</Subscript>Al<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-H<Subscript>2</Subscript>O (B = Na,K) zeolites

The structures of gismondine and amicite are analyzed in comparison with one another and with those of the zeolites whose compositions lie in the hypothetical plane “CaAl₂Si₃O₁₀”-Na₂Al₂Si₃O₁₀-H₂O. It is shown that the structures are similar to each other and may undergo mutual transformations.     

Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Construction of point-line-plane (0-1-2 dimensional) Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-SnO<Subscript>2</Subscript>/graphene hybrids as the anodes with excellent lithium storage capability

The assembly of hybrid nanomaterials has opened up a new direction for the construction of high-performance anodes for lithium-ion batteries (LIBs). In this work, we present a straightforward, eco-friendly, one-step hydrothermal protocol for the synthesis of a new type of Fe₂O₃-SnO₂/graphene hybrid, in which zero-dimensional (0D) SnO₂ nanoparticles with an average diameter of 8 nm and one-dimensional (1D) Fe₂O₃ nanorods with a length of ~150 nm are homogeneously attached onto two-dimensional (2D) reduced graphene oxide nanosheets, generating a unique point-line-plane (0D-1D-2D) architecture. The achieved Fe₂O₃-SnO₂/graphene exhibits a well-defined morphology, a uniform size, and good monodispersity. As anode materials for LIBs, the hybrids exhibit a remarkable reversible capacity of 1,530 mA·g^?1 at a current density of 100 mA·g^?1 after 200 cycles, as well as a high rate capability of 615 mAh·g^?1 at 2,000 mA·g^?1. Detailed characterizations reveal that the superior lithium-storage capacity and good cycle stability of the hybrids arise from their peculiar hybrid nanostructure and conductive graphene matrix, as well as the synergistic interaction among the components.

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Synthesis and high-temperature heat capacity of Yb<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Lu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Yb₂Sn₂O₇ and Lu₂Sn₂O₇ have been prepared by solid-state reactions, by firing mixtures of Yb₂O₃ or Lu₂O₃ and SnO₂ at 1473 K, and the molar heat capacity of these compounds (pyrochlore structure) has been determined by differential scanning calorimetry. The C _p (T) data have been used to evaluate the thermodynamic properties of the stannates: enthalpy increment, entropy change, and reduced Gibbs energy.