Layer Dependence and Light Tuning Surface Potential of 2D MoS2 on Various Substrates

Here surface potential of chemical vapor deposition (CVD) grown 2D MoS₂ with various layers is reported, and the effect of adherent substrate and light illumination on surface potential of monolayer MoS₂ are investigated. The surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.84 eV with the increase in the number of layer from 1 to 4 or more. Especially, the surface potentials of monolayer MoS₂ are strongly dependent on its adherent substrate, which are determined to be 4.55, 4.88, 4.93, 5.10, and 5.50 eV on Ag, graphene, Si/SiO₂, Au, and Pt substrates, respectively. Light irradiation is introduced to tuning the surface potential of monolayer MoS₂, with the increase in light intensity, the surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.74 eV, while increases from 5.50 to 5.56 eV on Pt substrate. The I–V curves on vertical of monolayer MoS₂/Pt heterojunction show the decrease in current with the increase of light intensity, and Schottky barrier height at MoS₂/Pt junctions increases from 0.302 to 0.342 eV. The changed surface potential can be explained by trapped charges on surface, photoinduced carriers, charge transfer, and local electric field.     

Creation of Ferromagnetic Properties in Ni-Doped Na<Subscript>2</Subscript>WO<Subscript>4</Subscript>—Effect of Hydrogenation Post-treatment

Pure and Ni-doped sodium tungstate (Na₂WO₄) powders were synthesised by simultaneous crystallisation method. The effects of Ni doping on the structural, optical, and magnetic properties of the host Na₂WO₄ powder were studied. The study of X-ray diffraction shows that the incorporated Ni ions occupy locations in interstitial positions and substitution for W ion in the Na₂WO₃ lattice. A monophase cubic structure was obtained when the as-crystallised Na₂WO₄ powder was doped with Ni ions or annealed in hydrogen gas atmosphere (hydrogenation). The optical properties were studied by diffuse reflectance spectroscopy (DRS) technique. It was established that the direct bandgap of Na₂WO₄ exhibits red shift from 4.6 to 3.50 eV with Ni doping and blue shift to 5.13 eV with hydrogenation. The purpose of the present study is to study conditions necessary to prepare powders having room-temperature ferromagnetic (RT-FM) properties. Therefore, the Na₂WO₄ nano-powder was doped with Ni ions. RT-FM properties were obtained with Ni-doped Na₂WO₄ that was strongly enhanced by hydrogenation so that the energy product (EP) was increased by 213 %. This enhancement was attributed to the enhancement of the magnetic medium for the spin-spin (S-S) interaction inside the crystalline medium. In general, an experimental relationship was established between O vacancies, optical absorption, and magnetic properties of the studied crystal. Thus, it was proved, for the first time, the possibility of producing Na₂WO₄ having RT-FM, where magnetic characteristics can be tailored by doping and post-treatment under H₂ atmosphere, thus a new potential candidate to be used in magnetic applications of ferroelectric crystals.     

Effect of annealing on optical properties of thin films with β-FeSi2 quantum dots

Reactive deposition epitaxy was used to prepare β-FeSi₂ nanodots on a Si(001) surface. The influence of annealing on optical properties has been studied. Annealing increases the particle size and decreases absorption. The indirect optical transition at 0.84-0.89 eV dominates the absorption spectra below 1.3-1.5 eV. The film with smaller nanoparticles exhibits higher absorption with direct optical transition energies in the 1.3-1.5 eV range. It suggests that direct transition is dominant only in small (around 3 nm diameter) β-FeSi₂ nanodots. Larger particles are dominated by the indirect optical transition.     

The Electronic Structures and Magnetic Properties of Un-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript>: the First-Principle Calculation

The electronic structures and magnetic properties of native defects in cubic In₂O₃ are investigated systematically by the LDA + U first-principle calculations based on the density functional theory. It is found that the In₂O₃ system is a strongly correlated electron system; therefore, the coulomb potential of In-4d and O-2p should be considered. In this paper, the coulomb potential corrections are U _In?4d=3 eV and U _O?2p=5 eV. The magnetic moments of O interstitial, In vacancy, and In interstitial are 2 u _B , 3 u _B , and 1 u _B , respectively, which are consistent with the analysis of group theory and molecular orbital theory. Moreover, the distributions of these magnetic moments are both local and extended. The magnetic couplings of O interstitials, In vacancies, and In interstitials are ferromagnetic, anti-ferromagnetic, and paramagnetic respectively, which are determined by electronic structures of defects. The formation energy of O interstitial is high, while that of the O Frenkel defect (that is the O interstitial-vacancy) is ?4.98 eV. These results could provide the practical understanding to the room-temperature ferromagnetism in un-doped In₂O₃ system.     

Luminescence properties and electronic structure of Sm<Superscript>3+</Superscript>-doped YAl<Subscript>3</Subscript>B<Subscript>4</Subscript>O<Subscript>12</Subscript>

The luminescence properties of Sm³⁺ ions in YAl₃B₄O₁₂ were studied upon synchrotron excitation in the 3.8–11 eV region. In addition to the 4f → 4f excitation bands, the excitation spectra of the Sm³⁺ emission contain broad bands at 6.1 and ~7.0 eV. These bands are attributed to charge transfer transition in Sm³⁺–O²⁻ complexes and 4f → 5d transition of Sm³⁺ ions, respectively. The optical absorption edge of YAl₃B₄O₁₂ was determined at 7.3 eV. A comparison with the results of electronic structure calculations on YAl₃B₄O₁₂ is also made.     

Structural,electronic, and optical properties of orthorhombic and triclinic BiNbO4 determined via DFT calculations

We performed ab initio calculations using the FPLAW method with the local density approximation (LDA) implemented in the WIEN2 k code for the orthorhombic (α) and triclinic (β) phases of BiNbO₄. The modified Becke–Johnson exchange potential (mBJ)-LDA approach was also used to improve the electronic properties. The lattice constants calculated for both structures using the LDA are in good agreement with the experimental values. For the band structure calculations, the mBJ-LDA approach provides reasonable agreement for the band gap value compared with the LDA. The estimated (mBJ)-LDA band gap values are 2.89 eV (3.73 eV) and 2.62 eV (3.15 eV) for the α and β phases of BiNbO₄, respectively. Significant optical anisotropy is clearly observed in the visible-light region. We also calculated and evaluated the electron energy loss spectrum for BiNbO₄. This work provides the first quantitative theoretical prediction of optical properties and electron energy loss spectra for both the orthorhombic and triclinic phases of BiNbO₄.     

Optical properties of translucent CaGa2S4:Ce films prepared by pulsed laser deposition

Translucent CaGa₂S₄:Ce thin films were prepared by pulsed laser deposition. Optical absorption associated with the 4f₁ (²F_5/2) to 5d₂ second-highest excited state transition of the Ce³⁺ ion was clearly observed at 3.60 eV as well as the 4f₁ (²F_5/2) to 5d₁ lowest excited state transition at 2.93 eV. Energy splitting of the 5d state by the crystal field is discussed in terms of a model of a square antiprism structure with D_4d symmetry.     

多相ZrO2几何结构及电子结构第一性原理研究

采用基于第一性原理的密度泛函理论投影缀加平面波,使用广义梯度近似处理交换关联势能,深入研究了弛豫多相 ZrO2几何结构特征及电子结构。研究发现,单斜、四方和立方 ZrO2能带间隙分别约为3.47 eV、3.96 eV 和3.36 eV。近费米能级态密度分析结果表明,多相 ZrO2的基本性质均由 O 2p 态电子和 Zr 4d 态电子决定。     

Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Optical nonlinearity in glasses: the origin and photo-excitation effects

Linear and nonlinear optical properties in oxide and chalcogenide glasses have been studied comparatively. Applying a semiconductor concept to these glasses, we show that maximal nonlinear refractive-index at optical communication wavelengths is ～10^?4 cm²/GW, which can be obtained in materials with bandgap energy of ～1.6 eV. It is also shown for SiO₂ and As₂S₃ that linear and nonlinear optical excitations induce different photostructural changes, which are attributable to different photo-electronic transition probabilities.     

Honeycomb RhI3 Flakes with High Environmental Stability for Optoelectronics

The emerging 2D layered transition metal trihalides (MX₃) have attracted extremely high interest given their exceptional structural and physical properties. Continuing to extend the library of 2D MX₃ is essential for exploring new physical phenomena and enabling new functionality. Herein, the optical and electrical properties and the photodetection behavior of atomically thin RhI₃ flakes exfoliated from bulk crystals are reported. This compound exhibits superior air and thermal stability, as well as thickness-dependent bandgap from 1.1 (18L) to 1.4 eV (2L). Field-effect transistors based on the few-layer RhI₃ flakes display n-type semiconducting behavior with competitive mobility of 2.5 cm² V⁻¹ s⁻¹ and ON/OFF current ratio of 4 × 10⁴. Importantly, the outstanding responsivity of 11.5 A W⁻¹ and high specific detectivity of 2 × 10¹⁰ Jones are recorded from the RhI₃ photodetectors under 980 nm illumination at room temperature in air. These findings indicate a variety of potential applications of atomically thin RhI₃ flakes in future 2D-material-based electronic and optoelectronic devices.     

Two-dimensional Aluminum Oxide Nanosheets Decorated with Palladium Oxide Nanodots for Highly Stable and Selective Hydrogen Sensing

Hydrogen (H₂) sensing materials such as semiconductor metal oxides may suffer from poor long-term stability against humidity and unsatisfactory selectivity against other interfering gases. To address the above issues, highly stable and selective H₂ sensing built with palladium oxide nanodots decorating aluminum oxide nanosheets (PdO NDs//Al₂O₃ NSs) has been achieved via combined template synthesis, photochemical deposition, and oxidation. Typically, the PdO NDs//Al₂O₃ NSs are observed with thin NSs (≈17 nm thick) decorated with nanodots (≈3.3 nm in diameter). Beneficially, the sensor prototypes built with PdO NDs//Al₂O₃ NSs show excellent long-term stability for 278 days, high selectivity against interfering gases, and outstanding stability against humidity at 300 °C. Remarkably, the sensor prototypes enable detection of a wide-range of 20 ppm – 6 V/V% H₂, and the response and recovery times are ≈5 and 16 s to 1 V/V% H₂, respectively. Theoretically, the heterojunctions of PdO NDs-Al₂O₃ NSs with a large specific surface ratio and Al₂O₃ NSs as the support exhibit excellent stability and selective H₂ sensing. Practically, a sensing device integrated with the PdO NDs//Al₂O₃ NSs sensor prototype is simulated for detecting H₂ with reliable sensing response.     

Long afterglow phosphorescent characteristics of BaAl2O4:Eu,Dy films

BaAl₂O₄:Eu,Dy (BAO) films have been fabricated on Si substrate by laser ablation, and their fundamental optical property and afterglow characteristics are discussed in comparison with the SrAl₂O₄:Eu,Dy (SAO) films. The intense green emission near 500 nm that originates from 5d to 4f transition in Eu²⁺ ions was clearly observed from the BAO films. This photoluminescence peak was at a shorter wavelength than that of the SAO films (λ = 520 nm). The afterglow intensity from the BAO films disappeared within a few minutes whereas that of the SAO films lasts over 20 min. The hole-trap depth (E_t) created by Dy as the auxiliary activators, which strongly affects the afterglow characteristics, was estimated on the basis of the thermally stimulated luminescence (TSL) result. The TSL glow curve for BAO films showed two broad peaks at 320 K and 450 K. The calculated E_t for each peak was 0.2 eV (for the 320 K peak) and 1.2 eV (for the 450 K peak). On the other hand, E_t = 0.5 eV was obtained from the SAO films. The hole-trap depths of the BAO film are either too shallow or too deep to affect the afterglow characteristics at room temperature.     

Thin film solar cells based on the ternary compound Cu2SnS3

Alongside with Cu₂ZnSnS₄ and SnS, the p-type semiconductor Cu₂SnS₃ also consists of only Earth abundant and low-cost elements and shows comparable opto-electronic properties, with respect to Cu₂ZnSnS₄ and SnS, making it a promising candidate for photovoltaic applications of the future. In this work, the ternary compound has been produced via the annealing of an electrodeposited precursor in a sulfur and tin sulfide environment. The obtained absorber layer has been structurally investigated by X-ray diffraction and results indicate the crystal structure to be monoclinic. Its optical properties have been measured via photoluminescence, where an asymmetric peak at 0.95 eV has been found. The evaluation of the photoluminescence spectrum indicates a band gap of 0.93 eV which agrees well with the results from the external quantum efficiency. Furthermore, this semiconductor layer has been processed into a photovoltaic device with a power conversion efficiency of 0.54%, a short circuit current of 17.1 mA/cm², an open circuit voltage of 104 mV hampered by a small shunt resistance, a fill factor of 30.4%, and a maximal external quantum efficiency of just less than 60%. In addition, the potential of this Cu₂SnS₃ absorber layer for photovoltaic applications is discussed.     

Aligned Heterointerface‐Induced 1T‐MoS2 Monolayer with Near‐Ideal Gibbs Free for Stable Hydrogen Evolution Reaction

1T‐phase molybdenum disulfide (1T‐MoS₂) exhibits superior hydrogen evolution reaction (HER) over 2H‐phase MoS₂ (2H‐MoS₂). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T‐MoS₂ monolayer formed at edge‐aligned 2H‐MoS₂ and a reduced graphene oxide heterointerface (EA‐2H/1T/RGO) using a precursor‐in‐solvent synthesis strategy are reported. Theoretical prediction indicates that the edge‐aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS₂ and graphene. As an electrocatalyst for HER, EA‐2H/1T/RGO displays an onset potential of ?103 mV versus RHE, a Tafel slope of 46 mV dec^?1 and 10 h stability in acidic electrolyte. The unexpected activity of EA‐2H/1T/RGO beyond 1T‐MoS₂ is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (ΔG_H*) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H‐MoS₂. The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.     

Epitaxial and polycrystalline CuInS2 thin films: A comparison of opto-electronic properties

Epitaxial and polycrystalline thin CuInS₂ (CIS) layers were grown by means of molecular beam epitaxy (MBE) on single crystalline silicon substrates of 4 inch diameter. Photoluminescence (PL) studies were performed to investigate the opto-electronic properties of these layers. For the epitaxial CIS, low-energy-hydrogen implantation leads to the passivation of deep defects and several donor-acceptor (DA) pair recombinations (from 1.034 eV to 1.439 eV) and two free-to-bound (FB) transitions (at 1.436 eV and 1.485 eV) become observable at low temperatures (5 to 100 K). Excitonic luminescence is completely absent for all investigated epitaxial CIS layers. This contrasts sharply with the PL of the polycrystalline films which is dominated by excitonic luminescence (1.527 eV). Also a donor-to-valence band transition at 1.465 eV (BF-1) and one donor-acceptor recombination at 1.435 eV (DA-1) were observed, while luminescence from deep levels is not present at all. Based on these data, a refined defect model for CuInS₂ with two donor and two acceptor states is presented. Under comparable growth conditions, the electronic quality of polycrystalline CIS is superior to epitaxially grown material.     

Impact of Pd nanoparticle loading method on SnO2 surface for natural gas detection in humid atmosphere

To improve the sensor response to low concentrations of methane (CH₄) at low operating temperatures in humid atmospheres, we prepared Pd-loaded SnO₂ (Pd-SnO₂) nanoparticles via two different Pd-loading processes: (i) a general impregnation method and (ii) a new loading method using poly(N-vinyl-2-pyrrolidone) (PVP) as a protective agent for Pd receptor particles. According to the measured electric resistances, the Pd particles limited the hydroxyl-poisoning of the SnO₂ particle surface. Because Pd is oxidized to PdO, a p–n junction is formed at the interface between PdO and SnO₂, and such interface gives the enlargement of the electron depletion layer. Therefore, Pd further improved the resistance against hydroxyl poisoning of the SnO₂ surface in humid air. In addition, although the sensor based on neat SnO₂ did not respond to low-concentration CH₄ at 200–400 °C, both the sensors based on the Pd-loaded SnO₂ samples exhibited high sensor response to 200 ppm CH₄ in a humid atmosphere. The Pd-SnO₂ obtained by the new loading method exhibited a higher response to CH₄ at lower concentrations in the lower operating temperature range (200–250 °C). This improvement in the sensor response is probably due to the catalytic activity of the larger Pd nanoparticles. According to high-resolution transmission electron microscopy–energy-dispersive X-ray spectroscopy images, the new loading method successfully provided Pd-loaded SnO₂ nanoparticles with Pd nanoparticles dispersed uniformly on the SnO₂ particle surface. The average particle size of Pd nanoparticles loaded on the surface of SnO₂ by the new loading method was slightly larger than that of the Pd nanoparticles loaded by the impregnation method. As the Pd particle size increases, it is thought that crystalline PdO particles are formed more easily, thereby improving the combustion activity of CH₄ under humid conditions. These results are of great significance for further decreasing the energy consumption of the CH₄ sensor and increasing its sensor response in humid atmospheres.

     

The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Growth and optical properties of ZnIn2Se4 films

Thin films of ZnIn₂Se₄ were deposited on quartz substrates at 297 K by the conventional thermal evaporation technique. The as-deposited films were amorphous. On annealing at 623 K under vacuum for 3 h, the films crystallized with a preferred (1 1 2) orientation corresponding to the chalcopyrite-type structure. Films deposited on a quartz substrate heated to 573 K were also crystalline. The optical constants were computed from the measured transmittance and reflectance at normal incidence of light in the wavelength range 400 to 2000 nm. The analysis of the data gave a direct gap of 2.2 and 2.06 eV for the amorphous and crystallized films, respectively. The dispersion curve exhibited a peak above the absorption edge. An indirect gap of 1.8 eV for the crystallized films and a direct forbidden gap of 1.75 eV for the amorphous films were also deduced. A direct allowed transition with a gap of 2.065 eV and an indirect transition with a gap of 1.69 eV were deduced for the crystalline films deposited on the heated substrate.     

The First 2D Homochiral Lead Iodide Perovskite Ferroelectrics: [R‐ and S‐1‐(4‐Chlorophenyl)ethylammonium]2PbI4

2D organic–inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics [R‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ and [S‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P2₁/c at room temperature, exhibiting no high‐temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high‐performance 2D lead iodide perovskite ferroelectrics with great application prospects.