Mott-Hubbard insulating state for the layered van der Waals [Formula: see text] (X: S, Se) as revealed by NEXAFS and resonant photoelectron spectroscopy

A broad family of the nowadays studied low-dimensional systems, including 2D materials, demonstrate many fascinating properties, which however depend on the atomic composition as well as on the system dimensionality. Therefore, the studies of the electronic correlation effects in the new 2D materials is of paramount importance for the understanding of their transport, optical and catalytic properties. Here, by means of electron spectroscopy methods in combination with density functional theory calculations we investigate the electronic structure of a new layered van der Waals [Formula: see text] (X: S, Se) materials. Using systematic resonant photoelectron spectroscopy studies we observed strong resonant behavior for the peaks associated with the [Formula: see text] final state at low binding energies for these materials. Such observations clearly assign [Formula: see text] to the class of Mott-Hubbard type insulators for which the top of the valence band is formed by the hybrid Fe-S/Se electronic states. These observations are important for the deep understanding of this new class of materials and draw perspectives for their further applications in different application areas, like (opto)spintronics and catalysis.

Regulation of Morphology and Electrochemical Properties of Ni0.85Se via Fe Doping for Overall Water Splitting and Supercapacitor

The Fe-doped Ni0.85Se nanosheets array on Ni foam was synthesized successfully through one-step solvothermal method as effective binder-free multifunctional catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall...

Experimental investigation of performance tailoring of the multifunctional sensor using transition metal (Fe) doped ZnO nanorods synthesized via a facile solution-based method

A systematic interpretation of the undoped and Fe doped ZnO based multifunctional sensor developed employing economic and facile low-temperature hydrothermal method is reported. The tailoring of the performance improvement of the sensor was deliberately carried out using varied concentration (1, 3 and 5 Wt%) of Fe dopant in ZnO nanorods. The structural and morphological analysis reveal the undisturbed ZnO hexagonal wurtzite structure formation and 1D morphology grown even when the dopant is added. The optical property study evidences a decreased bandgap (3.10 eV) and decreased defects of 5 Wt% of Fe dopant in ZnO nanorods based sensor compared to the undoped one. The electrical process transpiring in the tailored multifunctional sensor is investigated using photoconductivity and impedance analysis elucidates proper construction of p-n junction between the piezoelectric n-type active layer (undoped and Fe doped ZnO nanorods) and p-type PEDOT:PSS ((poly(3,4-ethylene dioxythiophene) polystyrene sulfonate)) and reduced internal resistance of 5 Wt% of Fe dopant in ZnO nanorods based sensor (131.97 Ω) respectively. The investigation on the experimental piezoelectric acceleration and gas sensing validation and the performance measurement were interpreted using test systems. A revamped output voltage of 3.71 V for 1 g input acceleration and a comprehensive sensitivity of 7.17 V g^-1was achieved for the 5 Wt% of Fe dopant in ZnO nanorods based sensor sensor. Similarly, an upgraded sensitivity of 2.04 and 6.75 for 5 Wt% of Fe dopant in ZnO nanorods based sensor was obtained when exposed to 10 ppm of target gases namely CO and CH₄respectively at room temperature. Appending to this, acceptable stability of the sensor for both the sensing (acceleration and gas) was also attained manifesting its prospective application in multifunctional based systems like sewage systems.

The effect of iron doping on the structural, optical, surface morphological, and temperature-dependent magnetic properties of ZnO nanoparticles

This study reports the role of temperature on the magnetic properties of the Fe-doped (0, 1, 3, and 5 wt%) ZnO nanoparticles (NPs) synthesized using the facile co-precipitation procedure. Powder x-ray diffraction analysis revealed the crystallinity deterioration of the ZnO matrix on trivalent cationic doping and the shifting of peak position due to the mismatch in ionic radius between the Zn²⁺ and Fe³⁺. A clear redshift in the bandgap of the iron-doped ZnO samples is observed from the UV-vis diffused reflectance spectroscopic studies. The existence of lattice defects including the zinc interstitials, zinc vacancies, and oxygen vacancies are confirmed by the room temperature photoluminescence analysis. Scanning electron microscopic investigations showed the synthesized NPs possesses agglomerated spherical morphology. The role of temperature on the magnetization of the iron-doped ZnO nanoparticles has been examined at 300 and 100 K. A 3-fold enhancement of magnetization value perceived for the 5% iron-doped ZnO nanoparticles at 100 K compared to the magnetization value of such sample at 300 K.

Low Power Consumption Nanofilamentary ECM and VCM Cells in a Single Sidewall of High-Density VRRAM Arrays

The technologies of 3D vertical architecture have made a major breakthrough in establishing high-density memory structures. Combined with an array structure, a 3D high-density vertical resistive random access memory (VRRAM) cross-point array is demonstrated to efficiently increase the device density. Though electrochemical migration (ECM) resistive random access (RRAM) has the advantage of low power consumption, the stability of the operating voltage requires further improvements due to filament expansions and deterioration. In this work, 3D-VRRAM arrays are designed. Two-layered RRAM cells, with one inert and one active sidewall electrode stacked at a cross-point, are constructed, where the thin film sidewall electrode in the VRRAM structure is beneficial for confining the expansions of the conducting filaments. Thus, the top cell (Pt/ZnO/Pt) and the bottom cell (Ag/ZnO/Pt) in the VRRAM structure, which are switched by different mechanisms, can be analyzed at the same time. The oxygen vacancy filaments in the Pt/ZnO/Pt cell and Ag filaments in the Ag/ZnO/Pt cell are verified. The 40 nm thickness sidewall electrode restricts the filament size to nanoscale, which demonstrates the stability of the operating voltages. Additionally, the 0.3 V operating voltage of Ag/ZnO/Pt ECM VRRAM demonstrates the potential of low power consumption of VRRAM arrays in future applications.

Ag2O and Fe2O3 modified oxides on the photocatalytic treatment of pulp and paper wastewater

The effects of doping of ZnO and Nb₂O₅ solids with Fe₂O₃ (1.4 wt%) and Ag₂O (1.4 wt%) on its surface and catalytic properties were investigated, as well as its photocatalytic performance on the degradation of a pulp and paper wastewater (PPW). The catalysts were characterized by XRD, SEM, photoacoustic spectroscopy (PAS), NH₃-TPD and textural analysis. The results obtained revealed that Fe₂O₃ doping of Nb₂O₅ conducted at 500 °C resulted in an increase of about 116% for S_BET while Ag₂O treatment exerted a decrease of 33% for S_BET of the doped adsorbents. Doping ZnO with Fe₂O₃ or Ag₂O led to an increase of 80% for S_BET. Iron and silver doping also led to a decrease in band gap energy of at least 6%. The addition of 1.4 wt% Ag₂O on ZnO followed by calcination at 500 °C resulted in an increase of 11% in the value of the reaction rate constant (k_ap) for COD reduction under UV radiation. The treatment of Nb₂O₅ with 1.4 wt% Ag₂O increased by a factor of 2.04 the value of k_ap for the reaction taking place under VIS radiation. The catalysts partially reduced the organic load and the real colour of the wastewater, allowing the achievement of the specifications for release into rivers, so photocatalysis could be an alternative for pulp and paper wastewater final polishing.

The structural and magnetic properties of dual phase cobalt ferrite

The bismuth (Bi³⁺)-doped cobalt ferrite nanostructures with dual phase, i.e. cubic spinel with space group Fd3m and perovskite with space group R3c, have been successfully engineered via self-ignited sol-gel combustion route. To obtain information about the phase analysis and structural parameters, like lattice constant, Rietveld refinement process is applied. The replacement of divalent Co²⁺ by trivalent Bi³⁺ cations have been confirmed from energy dispersive analysis of the ferrite samples. The micro-structural evolution of cobalt ferrite powders at room temperature under various Bi³⁺ doping levels have been identified from the digital photoimages recorded using scanning electron microscopy. The hyperfine interactions, like isomer shift, quadrupole splitting and magnetic hyperfine fields, and cation distribution are confirmed from the Mossbauer spectra. Saturation magnetization is increased with Bi³⁺-addition up to x = 0.15 and then is decreased when x = 0.2. The coercivity is increased from 1457 to 2277 G with increasing Bi³⁺-doping level. The saturation magnetization, coercivity and remanent ratio for x = 0.15 sample is found to be the highest, indicating the potential of Bi³⁺-doping in enhancing the magnetic properties of cobalt ferrite.

Structure-Dependent Spin Polarization in Polymorphic CdS:Y Semiconductor Nanocrystals

Searching for the polymorphic semiconductor nanocrystals would provide precise and insightful structure-spin polarization correlations and meaningful guidance for designing and synthesizing high spin-polarized spintronic materials. Herein, the high spin polarization is achieved in polymorphic CdS:Y semiconductor nanocrystals. The high-pressure polymorph of rock-salt CdS:Y nanocrystals has been recovered at ambient conditions synthesized by the wurtzite CdS:Y nanocrystals as starting material under 5.2 GPa and 300 °C conditions. The rock-salt CdS:Y polymorph displays more robust room-temperature ferromagnetism than wurtzite sample, which can reach the ferromagnetic level of conventional semiconductors doped with magnetic transition-metal ions, mainly due to the significantly enhanced spin configuration and defect states. Therefore, crystal structure directly governs the spin configuration, which determines the degree of spin polarization. This work can provide experimental and theoretical methods for designing the high spin-polarized semiconductor nanocrystals, which is important for applications in semiconductor spintronics.

Correlating the magnetism and gas sensing properties of Mn-doped ZnO films enhanced by UV irradiation

Schematic diagram showing the 2D TOF SIMS overlays of Si⁺, Mn⁺and Zn⁺. The insert corresponds to the correlation between the sensing response and FMR signal as a function of Mn concentration when exposed to various gases.

Defects-assisted ferromagnetism due to bound magnetic polarons in Ce into Fe, Co:ZnO nanoparticles and first-principle calculations

In the DMS systems of Zn_0.94Fe_0.03Ce_0.03O and Zn_0.94Co_0.03Ce_0.03O, the Ce ions attribute lattice defects as well as enhance antiferromagnetic interactions, which have potential in novel spintronic applications.

Realizing ferromagnetic ordering in SnO2 and ZnO nanostructures with Fe, Co, Ce ions

The dopants Co and Ce in SnO₂ nanostructures favour room temperature ferromagnetism, whereas in ZnO, the dopants involve in antiferromagnetic interactions at room temperature.

Understanding the role of iron in the magnetism of Fe doped ZnO nanoparticles

Fe does not contribute directly to ferromagnetic signals, but promotes the formation of defects which are considered as main sources.

Preparation of porous 3D Ce-doped ZnO microflowers with enhanced photocatalytic performance

Porous 3D Ce-doped ZnO microflowers were fabricated using a low temperature hydrothermal method followed by heat treatment for the first time. The influence of the Ce dopant on the structural and photocatalytic properties was investigated.

Synthesis of ZnO nanorods on a flexible Phynox alloy substrate: influence of growth temperature on their properties

A novel flexible alloy substrate (Phynox, 50 μm thick) was used for the synthesis of zinc oxide (ZnO) nanorods via a low-temperature solution growth method.

Controllable morphology and tunable colors of Mg and Eu ion co-doped ZnO by thermal annealing

Controllable morphology and tunable colors of Mg and Eu ion co-doped ZnO by thermal annealing.

Visible light photocatalytic activity of Fe(3+)-doped ZnO nanoparticle prepared via sol-gel technique

The optical properties of a ZnO photocatalyst were enhanced with various dopant concentrations of Fe(3+). Doped ZnO nanoparticles were synthesized via a sol-gel method without the use of capping agents or surfactants and was then characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy. The results showed that ZnO has a wurtzite, hexagonal structure and that the Fe(3+) ions were well incorporated into the ZnO crystal lattice. As the Fe(3+) concentration increased from 0.25 wt.% to 1 wt.%, the crystal size decreased in comparison with the undoped ZnO. The spectral absorption shifts of the visible light region (red shift) and the band gap decreases for each Fe-ZnO sample were investigated. The photocatalytic activities of the ZnO and Fe-ZnO samples were evaluated based on the degradation of 2-chlorophenol in aqueous solution under solar radiation. The samples with a small concentration of Fe(3+) ions showed enhanced photocatalytic activity with an optimal maximum performance at 0.5 wt.%. The results indicated that toxicity removal of 2-chlorophenol at same line of degradation efficiency. Small crystallite size and low band gap were attributed to high activities of Fe-ZnO samples under various concentrations of Fe(3+) ions compared to undoped ZnO.