Thermal conduction across the one-dimensional interface between a MoS<Subscript>2</Subscript> monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.     

Combustion synthesis of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Nanophase of Ga₂O₃ has potentially important applications in photocatalysis. We report the synthesis of nanophase of the metastable γ- and stable β-Ga₂O₃ and demonstrate that it is possible to prepare a continuously varying mixture starting from the pure metastable γ- to the pure β-phase. This is achieved by employing a facile and reliable combustion route, using urea as a fuel. Typical grain sizes, as estimated from XRD studies, are about 3 nm. Given the importance of surface chemistry for potential applications, thermogravimetric coupled with mass spectrometry is used in conjunction with FTIR to elucidate the chemistry of the adsorbed surface layer. Studies on the γ-Ga₂O₃ phase indicate the occurrence of weight loss of 8.1% in multiple steps. Evolved gas analysis and FTIR studies show presence of physisorbed H₂O molecules and chemisorbed –(OH) ions bonded to active surface states and accounts predominantly for the observed weight loss.     

YN<Subscript>2</Subscript> monolayer: Novel p-state Dirac half metal for high-speed spintronics

In spintronics,it is highly desirable to find new materials that can simultaneously possess complete spin-polarization,high-speed conduction electrons,large Curie temperature,and robust ferromagnetic ground states.Using first-principles calculations,we demonstrate that the stable YN2 monolayer with octahedral coordination is a novel p-state Dirac half metal (DHM),which not only has a fully spin-polarized Dirac state,but also the highest Fermi velocity (3.74 x 105 m/s) of the DHMs reported to date.In addition,its half-metallic gap of 1.53 eV is large enough to prevent the spin-flip transition.Because of the strong nonlocal p orbitals of N atoms (N-p) direct exchange interaction,the Curie temperature reaches over 332 K.Moreover,its ferromagnetic ground state can be well preserved under carrier doping or external strain.Therefore,the YN2 monolayer is a promising DHM for high-speed spintronic devices and would lead to new opportunities in designing other p-state DHMs.     

Photoluminescence from chemically exfoliated MoS2

A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.     

Valley polarization in stacked MoS<Subscript>2</Subscript> induced by circularly polarized light

Manipulation of valley pseudospins is crucial for future valleytronics.The emerging transition metal dichalcogenides (TMDs) provide new possibilities for exploring the interplay among the quantum degrees of freedom,including real spin,valley pseudospin,and layer pseudospin.For example,spin-valley coupling results in valley-dependent circular dichroism in which electrons with particular spin (up or down) can be selectively excited by chiral optical pumping in monolayer TMDs,whereas in few-layer TMDs,the interlayer hopping further affects the spin-valley coupling.In addition to valley and layer pseudospins,here we propose a new degree of freedom—stacking pseudospin—and demonstrate new phenomena correlated to this new stacking freedom that otherwise require the application of external electrical or magnetic field.We investigated all possible stacking configurations of chemical-vapor-deposition-grown trilayer MoS2 (AAA,ABB,AAB,ABA,and 3R).Although the AAA,ABA,3R stackings possess a sole peak with lower degree of valley polarization than that in monolayer samples,the AAB (ABB) stackings exhibit two distinct peaks,one similar to that observed in monolayer MoS2 and an additional unpolarized peak at lower energy.Our findings provide a more complete understanding of valley quantum control for future valleytronics.     

Defect-rich MoS<Subscript>2</Subscript> nanowall catalyst for efficient hydrogen evolution reaction

Designing efficient electrocatalysts for the hydrogen evolution reaction (HER) has attracted substantial attention owing to the urgent demand for clean energy to face the energy crisis and subsequent environmental issues in the near future.Among the large variety of HER catalysts,molybdenum disulfide (MoS2) has been regarded as the most famous catalyst owing to its abundance,low price,high efficiency,and definite catalytic mechanism.In this study,defect-engineered MoS2 nanowall (NW) catalysts with controllable thickness were fabricated and exhibited a significantly enhanced HER performance.Benefiting from the highly exposed active edge sites and the rough surface accompanied by the robust NW structure,the defect-rich MoS2 NW catalyst with an optimized thickness showed an ultralow onset overpotential of 85 mV,a high current density of 310.6 mA·cm-2 at η =300 mV,and a low potential of 95 mV to drive a 10 mA.cm-2 cathodic current.Additionally,excellent electrochemical stability was realized,making this freestanding NW catalyst a promising candidate for practical water splitting and hydrogen production.     

Probing the intrinsic optical quality of CVD grown MoS<Subscript>2</Subscript>

Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance.The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is,therefore,highly desirable.Here,we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2).We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2.Low-temperature PL measurements are also used to evaluate the structural defects in MoS2,via defect-associated bound exciton emission,which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures.This work therefore provides a sensitive,noninvasive method to characterize the optical properties of TMDs,allowing the tuning of the growth parameters for the development of optoelectronic devices.     

Size-controlled MoS<Subscript>2</Subscript> nanodots supported on reduced graphene oxide for hydrogen evolution reaction and sodium-ion batteries

Transition metal dichalcogenide nanodots (NDs) have received considerable interest.We report a facile bottom-up synthetic route for MoS2 NDs by using molybdenum pentachloride and L-cysteine as precursors in oleylamine.The synthesis of NDs with a narrow size distribution ranging from 2.2 to 5.3 nm,was tailored by controlling the reaction time.Because of its coating characteristics,oleyalmine leads to uniformity and monodispersity of the NDs.Moreover,the NDs synthesized have large specific surface areas providing active sites.Graphene possesses outstanding conductivity.Combining the advantages of the two materials,the 0D/2D material exhibits superior electrochemical performance because of the 2D permeable channels for ion adsorption,energy storage,and conversion.The as-prepared MoS2/rGO (～2.2 nm) showed a stable capacity of 220 mAh·g-1 after 10,000 cycles at the current density of 20 A·g-1.Furthermore,a reversible capacity ～140 mAh·g-1 was obtained at a much higher current density of 40 A·g-1.Additionally,this composite exhibited superior catalytic performance evidenced by a small overpotential (222 mV) to afford 10 mA·cm-2,and a small Tafel slope (59.8 mV·decade-1) with good acid-stability.The facile approach may pave the way for the preparation of NDs with these nanostructures for numerous applications.     

A microwave-assisted route for the solid-state synthesis of lead pyrophosphate,Pb<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

In this study, we have developed a new method based on microwave heating for the preparation of lead pyrophosphate compound, Pb₂P₂O₇. The favoring of microwave heating as synthesis technique is based on the fact being much faster, cleaner and economical than conventional techniques. By following this new route, the lead pyrophosphate, Pb₂P₂O₇, was obtained as a pure phase. The products were characterized by X-ray powder diffraction (XRD) and Fourier Transform IR (FTIR) spectroscopic techniques. Comparing the experimental XRD and FTIR data of the synthesized products with the reported literature data has revealed that there was mutually an excellent agreement.     

Hierarchical MoS<Subscript>2</Subscript> intercalated clay hybrid nanosheets with enhanced catalytic activity

Emerging hierarchical MoSR/pillared-montmorillonite (MoS2/PMMT) hybrid nanosheets were successfully prepared through facile in-situ hydrothermal synthesis of MoS2 within the interlayer of cetyltrimethylammonium bromide PMMT,and their catalytic performance was evaluated by the reduction reaction of 4-nitrophenol (4-NP) using NaBH4 as a reductant.Microstructure and morphology characterization indicated that MoS2/PMMT exhibited hybrid-stacked layered structures with an interlayer spacing of 1.29 nm,and the MoS2 nanosheets were intercalated within the montmorillonite (MMT) layers,with most of the edges exposed to the outside.The catalytic activity and stability of MoS2/PMMT were both enhanced by the MMT.With the MoS2/PMMT as the catalyst,the apparent reaction rate constant of the 4-NP reduction was 0.723 min-1 and was maintained at ～0.679 min-1 after five reaction cycles.The structural evolution of MoS2/PMMT and the possible catalysis mechanism for the reduction reaction of 4-NP were investigated.The as-prepared MoS2/PMMT hybrid nanosheets are promising candidates for catalytic application in the water-treatment and biomedical fields.The strategy developed in this study can provide insights for designing hybrid nanosheets with diverse heterogeneous two-dimensional (2D) nanomaterials.     

Synthesis and conductivity of Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-based cermet material

SHS synthesis of Zn₂SnO₄-based cermet material from Zn + NiO + SnO₂ compacted powder mixtures has been studied. The final product is obtained as a monolithic cylindrical block composed of a ZnO-based outer layer and Zn₂SnO₄-based central part, in which the metal phase is distributed. The phase composition and microstructure of combustion products have been studied by x-ray phase analysis (XPA), electron microscopy, and microprobe techniques. It is established that the structure of the obtained cermet material has a dramatic effect on their conductivity.     

Ultrathin Co(Ni)-doped MoS<Subscript>2</Subscript> nanosheets as catalytic promoters enabling efficient solar hydrogen production

The design of efficient artificial photosynthetic systems that harvest solar energy to drive the hydrogen evolution reaction via water reduction is of great importance from both the theoretical and practical viewpoints.Integrating appropriate co-catalyst promoters with strong light absorbing materials represents an ideal strategy to enhance the conversion efficiency of solar energy in hydrogen production.Herein,we report,for the first time,the synthesis of a class of unique hybrid structures consisting of ultrathin Co(Ni)-doped MoS2 nanosheets (co-catalyst promoter) intimately grown on semiconductor CdS nanorods (light absorber).The as-synthesized one-dimensional CdS@doped-MoS2 heterostructures exhibited very high photocatalytic activity (with a quantum yield of 17.3％) and stability towards H2 evolution from the photoreduction of water.Theoretical calculations revealed that Ni doping can increase the number of uncoordinated atoms at the edge sites of MoS2 nanosheets to promote electron transfer across the CdS/MoS2 interfaces as well as hydrogen reduction,leading to an efficient H2 evolution reaction.     

Real-time electrical detection of epidermal skin MoS<Subscript>2</Subscript> biosensor for point-of-care diagnostics

Various approaches have been proposed for point-of-care diagnostics,and in particular,optical detection is preferred because it is relatively simple and fast.At the same time,field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection.In this work we present highly sensitive,epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors,read-out circuits,and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA).Regardless of the physical forms or mechanical stress conditions,our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg.mL-1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM)and tapping-mode atomic force microscopy (tm-AFM).Furthermore,current modulation induced by the binding process was stably maintained for longer than 2-3 min.The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.     

Homogeneity ranges of Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Cl and Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Br

This paper systematizes phase-diagram data for the Tl-TlCl(Br)-Te systems and presents their 500-K subsolidus phase diagrams. Tl₅Te₂Cl and Tl₅Te₂Br (Tl₅Te₃ structure) are shown to be nonstoichiometric compounds with wide homogeneity ranges, which have been accurately determined using emf measurements, x-ray diffraction, and microhardness tests. Using emf data for reversible concentration cells with a thallium electrode, we have evaluated the partial thermodynamic functions of the thallium in the alloys studied, the standard thermodynamic functions of formation of Tl₅Te₂Cl and Tl₅Te₂Br, and their standard entropies. The crystal chemistry of these phases of variable composition is discussed in relation to the Tl₅Te₃ structure.     

Influence of Uniaxial Strain on the Electronic Properties of (TMTSF)<Subscript>2</Subscript>ReO<Subscript>4</Subscript> Compounds

To study the influence of uniaxial strain on anion-orderings and superconductivity, we measured electrical resistance along the a (R_xx) and c*-axes (R_zz) under uniaxial strain on (TMTSF)₂ReO₄. Compared to the pressure-temperature phase diagrams under hydrostatic pressure, those under uniaxial strain depend on the direction along which the pressure is applied. The difference is particularly large when the uniaxial strain is applied along the c*-axis. The compression along only one direction diminishes (1/2,1/2,1/2) anion ordering transition similar to those under hydrostatic pressure. On the other hand, (0,1/2,1/2) anion ordering transition is completely suppressed and anomalous behavior which enhances non-metallic ground state is observed when the samples are compressed along the c*-axis. The superconducting transition of (TMTSF)₂ReO₄ is suppressed in R_zz under pressure along the c*-axis.     

Nanocrystalline TiO<Subscript>2</Subscript> preparation by microwave route and nature of anatase–rutile phase transition in nano TiO<Subscript>2</Subscript>

Nanopowders of TiO₂ has been prepared using a microwave irradiation-assisted route, starting from a metalorganic precursor, bis(ethyl-3-oxo-butanoato)oxotitanium (IV), [TiO(etob)₂]₂. Polyvinylpyrrolidone (PVP) was used as a capping agent. The as-prepared amorphous powders crystallize into anatase phase, when calcined. At higher calcination temperature, the rutile phase is observed to form in increasing quantities as the calcination temperature is raised. The structural and physicochemical properties were measured using XRD, FT–IR, SEM, TEM and thermal analyses. The mechanisms of formation of nano-TiO₂ from the metal–organic precursor and the irreversible phase transformation of nano TiO₂ from anatase to rutile structure at higher temperatures have been discussed. It is suggested that a unique step of initiation of transformation takes place in Ti_1/2O layers in anatase which propagates. This mechanism rationalizes several key observations associated with the anatase–rutile transformation.     

MoS<Subscript>2</Subscript> as a long-life host material for potassium ion intercalation

Electrochemical potassium ion intercalation into two-dimensional layered MoS2 was studied for the first time for potential applications in the anode in potassium-based batteries.X-ray diffraction analysis indicated that an intercalated potassium compound,hexagonal K0.4MoS2,formed during the intercalation process.Despite the size of K+,MoS2 was a long-life host for repetitive potassium ion intercalation and de-intercalation with a capacity retention of 97.5％ after 200 cycles.The diffusion coefficient of the K+ ions in KxMoS2 was calculated based on the Randles-Sevcik equation.A higher K+ intercalation ratio not only encountered a much slower K+ diffusion rate in MoS2,but also induced MoS2 reduction.This study shows that metal dichalcogenides are promising potassium anode materials for emerging K-ion,K-O2,and K-S batteries.     

Production and characterization of metastable Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> ceramic materials

Producing nanostructured materials through metastable phases is interesting in the field of ceramic materials. Metastable phases can be obtained by the Atmospheric Plasma Spray (APS) technique which, is a well-known technique to produce coatings. The initial powders are melted during the spraying obtaining a homogenized phase due to their solubility in the liquid state. Afterwards, the molten droplets are quenched in a cooled medium, producing the sought metastable phases. Finally, during material consolidation, the metastable structure evolves due to a dual structure. A suppression of the grain growth is produced as a consequence of the immiscibility of both phases in the solid state. Due to their small grain size and uniform structure, these nanostructured materials exhibit very interesting properties such as higher hardness and toughness. The aim of this research has been to produce nanostructured Al₂O₃–TiO₂ ceramic powders through APS + quenching route, starting from commercially available micron-sized powders. A complete characterization of the obtained structures using XRD, SEM, FESEM and EDS has been carried out in the Thermal Spray Center (CPT) of the University of Barcelona.     

A facile two-step hydrothermal route for the synthesis of γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocrystals and their magnetic properties

This paper proposes a facile two-step hydrothermal route for the synthesis of maghemite (γ-Fe₂O₃) nanocrystals. The synthesis route included two steps: (i) hydrothermal synthesis of Fe₃O₄ nanocrystals, and (ii) hydrothermal oxidation of the Fe₃O₄ nanocrystals to their γ-Fe₂O₃ counterpart. Phase transition from γ-Fe₂O₃ to hematite was studied by in situ XRD; the γ-Fe₂O₃ nanocrystals exhibited enhanced phase transition temperature (>600 °C). The magnetization curves revealed that the γ-Fe₂O₃ nanocrystals showed ferromagnetic behavior with high saturation magnetization of 68 emu/g at room temperature.     

Synthesis of Ba<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript>−Sr<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript> solid solution and their dielectric properties

Oxides of the type, Ba_3-xSr_xZnNb₂O₉ (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.