Molecular beam epitaxy of GaAsxP1-xusing lowenergy P+ Ion Beam

We describe the epitaxial growth of GaAs_xP_1-x (0 <x < 0.7) on GaAs(00l) substrates using mass-separated low-energy P⁺ ions, and Ga and As₄ molecular beams. Epilayers have been obtained at growth temperatures(Tg) ranging from 300 to 650° C at P⁺ ion energies(E _p+ ) between 50 and 300 eV. We have investigated the growth rate as a function ofE _p+ , and the film composition as a function of the flux ratio of As₄ to P⁺,T _g andE _p+ . The sticking coefficient of phosphorus is markedly enhanced by using P⁺ ion, compared with that of As₄. As the flux ratio of As₄ to P⁺ is increased from 0 to 8, the composition ratiox of GaAs_xP_1-xfilms varies from 0 to 0.5. The composition ratiox decreases slightly with increasingT _g from 400 to 650° C, and increases with increasingE _p+ . Film surfaces are smooth atE _p+ below 100 eV, and their morphology is degraded with increasing energy.     

Achieving Large Second Harmonic Generation Effects via Optimal Planar Alignment of Triangular Units

Trigonal planar units with large polarizability anisotropy and high physicochemical stability are ideal structural units for exploring nonlinear optical (NLO) materials. Integrating the merits of two types of triangular-like moieties, a family of second-order NLO-active hybrid halides, MATX (X = Cl (1), Br ( 2 ), and I ( 3 )), are achieved. MATX crystallizes in a nonpolar space group of $P\overline{6}$2c but exhibits the optimal spatial arrangement and superior NLO performance. The low coordination planar trigonal AgX₃ units enable segregation in layers of the three-winged propeller-like Me₃TPA units. All of the layers are packed in a perfect parallel fashion, making the functional materials exhibit superior NLO performances, including the phase matchable behavior with strong SHG responses (6.2/ 1 , 6.5/ 2, and 7.6/ 3 times that of potassium dihydrogen phosphate), large birefringence (0.232/ 1 , 0.252/ 2 and 0.260/ 3 at 1064 nm), high laser damage threshold, wide transparent window, and easiness of crystal growth. The first-principles calculations reveal that the coexistence of strong linear and nonlinear optical properties are ascribed to the synergistic effect of the trigonal moieties. This study points out a useful path for the rational design of excellent NLO materials.     

Optical absorption in PbGa2Se4 single crystals

Optical measurements are performed in a PbGa2Se4 single crystal. The nature of the optical transitions is determined in the interval of photon energies 2.24–2.46 eV in the temperature range 77–300 K. It is shown that indirect and direct optical transitions take place in the energy intervals 2.28–2.35 eV and 2.35–2.46 eV, corresponding to E _gi =2.228 eV and E _gd =2.35 eV, respectively, at 300 K. The temperature coefficients of E _gi and E _gd are equal to −0.6×10⁻⁴ eV/K and −4.75×10⁻⁴ eV/K, respectively. Fiz. Tekh. Poluprovodn. 33, 39–41 (January 1999)     

Organic soluble indigoids derived from 3-hydroxybenzaldehyde for N-type organic field-effect transistor (OFET) applications

Two new series of organic soluble indigoids 7-7′-dialkoxyindigoids (2a, 2b) and 4,4′-dibromo-7,7′-dialkoxyindigoids (3a, 3b) (alkoxy = n-butoxy and n-octyloxy) were synthesized starting from the inexpensive 3-hydroxybenzaldehyde. The indigoids were soluble in common organic solvents including chloroform, dichloromethane, toluene, ethyl acetate and ethers. The enhanced solubility was suggested to be a lack of intermolecular hydrogen-bonds as confirmed by single crystal X-ray diffraction analyses. It was found that intramolecular hydrogen-bonds in indigoids are crucial to the exhibition of field-effect in OFETs, while intermolecular hydrogen-bonds only caused insolubility of the indigoids. Compared to the pristine insoluble indigo (LUMO = −3.55 eV and E_g = 1.91 eV), the soluble indigoids containing electron donating alkoxy side chains at the indigoid 7 and 7′ positions were shown to have their LUMO decreased by −0.13 to −0.26 eV. Among the indigoid studied, the soluble indigoid 3a containing electron donating alkoxy side chains at the indigoid 7 and 7′ positions and bromine groups at the indigoid 4 and 4′ positions exihibited a narrowest bandgap energy with E_g = 1.66 eV. Employing the same fabrication technique and a bottom-gate-top-contact OFET configuration, the soluble indigoids were found to have electron mobility similar to and within an order of magnitude of the pristine indigo.     

Extraction of interface states at emitter–base heterojunctions in AlGaAs/GaAs heterostructure bipolar transistors using sub-bandgap photonic excitation

Distribution of interface states at the emitter–base heterojunctions in heterostructure bipolar transistors (HBTs) is characterized by using current–voltage characteristics using sub-bandgap photonic excitation. Sub-bandgap photonic source with a photon energy E_ph which is less than the energy bandgap E_g (E_g,GaAs = 1.42, E_g,AlGaAs = 1.76 eV) of emitter, base, and collector of HBTs, is employed for exclusive excitation of carriers only from the interface states in the photo-responsive energy range at emitter–base heterointerface. The proposed method is applied to an Al_0.3Ga_0.7As/GaAs HBT (A_E = W_E × L_E = 250 × 100 μm²) with E_ph = 0.943 eV and P_opt = 3 mW. Extracted interface trap density D_it was observed to be D_it,max  4.2 × 10¹² eV⁻¹ cm⁻² at emitter–base heterointerface.     

Mg-Doped Na4Fe3(PO4)2(P2O7)/C Composite with Enhanced Intercalation Pseudocapacitance for Ultra-Stable and High-Rate Sodium-Ion Storage

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is considered as a promising cathode material for sodium-ion batteries (SIBs) due to its low cost, non-toxicity, and high structural stability, but its electrochemical performance is limited by the poor electronic conductivity. In this study, Mg-doped NFPP/C composites are presented as cathode materials for SIBs. Benefiting from the enhanced electrochemical kinetics and intercalation pseudocapacitance resulted from the Mg doping, the optimal Mg-doped NFPP/C composite (NFPP-Mg5%) delivers high rate performance (capacity of ≈40 mAh g⁻¹ at 20 A g⁻¹) and ultra-long cycling life (14 000 cycles at 5 A g⁻¹ with capacity retention of 80.8%). Moreover, the in situ X-ray diffraction and other characterizations reveal that the sodium storage process of NFPP-Mg5% is dominated by the intercalation pseudocapacitive mechanism. In addition, the full SIB based on NFPP-Mg5% cathode and hard carbon anode exhibits the discharge capacity of ≈50 mAh g⁻¹ after 200 cycles at 500 mA g⁻¹. This study demonstrates the feasibility of improving the electrochemical performance of NFPP by doping strategy and presents a low-cost, ultra-stable, and high-rate cathode material for SIBs.     

Oxygen Defect-Rich WO3−x–W3N4 Mott–Schottky Heterojunctions Enabling Bidirectional Catalysis for Sulfur Cathode

The serious shuttle effect and intrinsically sluggish oxidation–reduction reaction kinetics of polysulfides severely hinder the practical commercialization of lithium–sulfur (Li–S) batteries. Herein, oxygen-defect-rich WO_3−x–W₃N₄ Mott–Schottky heterojunctions are designed as efficient catalysts. Based on theoretical calculations and comprehensive experimental characterization, WO_3−x–W₃N₄ exhibits a lower free energy change (1.03 eV) and Li₂S decomposition energy barrier (0.92 eV) than WO₃ and W₃N₄, which significantly enhances the sulfur reduction reaction (SRR) activity. Furthermore, a relationship between the catalytic activity and the energy gaps in the d and p bands centers (Δd–p) is also established, with the low Δd–p of the heterojunction leading to a lower antibonding state energy, which promotes electron transfer and interfacial redox kinetics. Oxygen vacancies can improve the catalytic effect without affecting adsorption. Hence, the Li–S battery using WO_3−x–W₃N₄@CC/S exhibited outstanding rate and duration performance (913.9 mAh g^–1 at 2 C, stable 400 cycles at 1 C). Impressively, the battery achieves a high areal capacity of 5.0 mAh cm⁻² under a high sulfur loading of 4.98 mg cm⁻².     

MgSiAs: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Study of electrically active defects in n-GaN layer

Deep level defects in both p⁺/n junctions and n-type Schottky GaN diodes are studied using the Fourier transform deep level transient spectroscopy. An electron trap level was detected in the range of energies at E_c−E_t=0.23–0.27 eV with a capture cross-section of the order of 10⁻¹⁹–10⁻¹⁶ cm² for both the p⁺/n and n-type Schottky GaN diodes. For one set of p⁺/n diodes with a structure of Au/Pt/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au and the n-type Schottky diodes, two other common electron traps are found at energy positions, E_c−E_t=0.53–0.56 eV and 0.79–0.82 eV. In addition, an electron trap level with energy position at E_c−E_t=1.07 eV and a capture cross-section of σ_n=1.6×10⁻¹³ cm² are detected for the n-type Schottky diodes. This trap level has not been previously reported in the literature. For the other set of p⁺/n diodes with a structure of Au/Ni/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au, a prominent minority carrier (hole) trap level was also identified with an energy position at E_t−E_v=0.85 eV and a capture cross-section of σ_n=8.1×10⁻¹⁴ cm². The 0.56 eV electron trap level observed in n-type Schottky diode and the 0.23 eV electron trap level detected in the p⁺/n diode with Ni/Au contact are attributed to the extended defects based on the observation of logarithmic capture kinetics.     

Structural and optical properties of Cu2FeSnS4 thin film synthesized via a simple chemical method

Cu₂FeSnS₄ thin film, with potential as an effective photovoltaic absorber, was prepared by sulfurizing a (Cu,Sn)S/FeS-structured precursor prepared via successive ionic layer absorption and reaction combined with chemical bath deposition. X-Ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV-vis-NIR absorbance measurements showed that the Cu₂FeSnS₄ thin film exhibits large agglomeration of rod-shaped grains, a bandgap of E_g=1.22 eV, and a high optical absorption coefficient (>10⁴ cm⁻¹).     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Stabilization of the Polar Structure and Giant Second-Order Nonlinear Response of Single Crystal γ-NaAs0.95Sb0.05Se2

The dearth of suitable materials significantly restricts the practical development of infrared (IR) laser systems with highly efficient and broadband tuning. Recently, γ-NaAsSe₂ is reported, and it exhibits a large nonlinear second-harmonic generation (SHG) coefficient of 590 pm V⁻¹ at 2 µm. However, the crystal growth of γ-NaAsSe₂ is challenging because it undergoes a phase transition to centrosymmetric δ-NaAsSe₂. Herein, the stabilization of non-centrosymmetric γ-NaAsSe₂ by doping the As site with Sb, which results in γ-NaAs_0.95Sb_0.05Se₂ is reported. The congruent melting behavior is confirmed by differential thermal analysis with a melting temperature of 450 °C and crystallization temperature of 415 °C. Single crystals with dimensions of 3 mm × 2 mm are successfully obtained via zone refining and the Bridgman method. The purification of the material plays a significant role in crystal growth and results in a bandgap of 1.78 eV and thermal conductivity of 0.79 Wm⁻¹ K⁻¹. The single-crystal SHG coefficient of γ-NaAs_0.95Sb_0.05Se₂ exhibits an enormous value of |d₁₁| = 648 ± 74 pm V⁻¹, which is comparable to that of γ-NaAsSe₂ and ≈20× larger than that of AgGaSe₂. The bandgap of γ-NaAs_0.95Sb_0.05Se₂ (1.78 eV) is similar to that of AgGaSe₂, thus rendering it highly attractive as a high-performing nonlinear optical material.     

The First Improper Ferroelectric of 2D Multilayered Hybrid Perovskite Enabling Strong Tunable Polarization-Directed Second Harmonic Generation Effect

2D organometallic halide perovskites are recently emerging as a robust family of ferroelectrics, of which their inherent spontaneous polarization (P_s) endows fascinating quadratic nonlinear optical properties. However, up to date, few studies are reported to tune and control the second harmonic generation (SHG) effect in this ferroelectric branch. Herein, the first improper ferroelectric of 2D multilayered hybrid perovskites, (IA)₂(EA)₂Pb₃Br₁₀ ( 1 , where IA is isoamylammonium and EA is ethylammonium), which exhibits a high Curie temperature ( ≈ 371 K) and biaxial ferroelectricity with P_s of 2.2  µ C cm⁻² is reported. Strikingly, its unique in-plane ferroelectricity allows strong tunable SHG properties under the polarized-light. That is, the maximum SHG signals are observed with polarized-light parallel to P_s, while the minimum SHG appears along the vertical direction. This SHG anisotropy creates an extremely large dichroism ratio of ≈ 12, as visualized by 2D color mapping, which is the record-high merit for this type of SHG systems. To the best knowledge, this is the first time to achieve tunable SHG effects through ferroelectric polarization. As a pioneering study, the coupling between the SHG effect and ferroelectricity paves a new direction of 2D hybrid perovskite ferroelectrics toward smart optical device applications.     

Molecular Design of Mesoporous NiCo2O4 and NiCo2S4 with Sub‐Micrometer‐Polyhedron Architectures for Efficient Pseudocapacitive Energy Storage

Spinel‐type NiCo₂O₄ (NCO) and NiCo₂S₄ (NCS) polyhedron architectures with sizes of 500–600 nm and rich mesopores with diameters of 1–2 nm are prepared facilely by the molecular design of Ni and Co into polyhedron‐shaped zeolitic imidazolate frameworks as solid precursors. Both as‐prepared NCO and NCS nanostructures exhibit excellent pseudocapacitance and stability as electrodes in supercapacitors. In particular, the exchange of O^2? in the lattice of NCO with S^2? obviously improves the electrochemical performance. NCS shows a highly attractive capacitance of 1296 F g^?1 at a current density of 1 A g^?1, ultrahigh rate capability with 93.2% capacitance retention at 10 A g^?1, and excellent cycling stability with a capacitance retention of 94.5% after cycling at 1 A g^?1 for 6000 times. The asymmetric supercapacitor with an NCS negative electrode and an active carbon positive electrode delivers a very attractive energy density of 44.8 Wh kg^?1 at power density 794.5 W kg^?1, and a favorable energy density of 37.7 Wh kg^?1 is still achieved at a high power density of 7981.1 W kg^?1. The specific mesoporous polyhedron architecture contributes significantly to the outstanding electrochemical performances of both NCO and NCS for capacitive energy storage.     

Structure of 2-(iminocoumar-3-yl)-4-(coumar-3-yl)-thiazole,C2lHl4N2O3S

We have determined the structure of 2-(iminocouma-3-yl)-4-(coumar-3-yl)-thiazole, C₂₁H₁₄N₂O₃S, M_r=372.4, λ = 0.71073 Å, monoclinic, space group P2₁/n, a = 9.472(2) Å, b = 11.284(2) Å, c = 15.821(3) Å, β = 86.50(2)°, V = 1687.9(6) ų, Z = 4, D_x = 1.465 Mg m^?3, μ(Mo Kα) = 0.207 mm^?1, F(000) = 768, S = 1.91, R = 3.82%, wR = 3.26% for 1251 observed reflections (F > 6_σF). In the observed conformation of the molecule the imino group forms an intramolecular H…S bond of length 2.26 Å and the carbonyl oxygen makes a short contact with the hydrogen atom of the thiazolyl cycle. The molecule is planar. In the crystal structure a particular disorder of about 20% of molecules is observed with respect to the pseudo-mirror plane which is perpendicular to the molecular plane and passes through the thiazolyl N atom and the middle of the S? CH bond. The disagreement found between the observed conformation and that predicted by molecular mechanics calculations is explained by the tendency of the molecule to have the lowest value of dipole moment due to minimisation of the electrostatic contribution to the energy.     

A Study of Deep Defect Levels in Semi-Insulating SiC Using Optical Admittance Spectroscopy

Optical admittance spectroscopy (OAS), supported by electron paramagnetic resonance (EPR) measurements, is used to identify the controversial vanadium acceptor levels in vanadium-doped semi-insulating (SI) 4H-SiC and 6H-SiC. The V^3+/4+ levels for the cubic site are likely located at E _c − 0.67 ± 0.02 eV and E _c − 0.70 ± 0.02 eV in 6H-SiC and E _c − 0.75 ± 0.02 eV in 4H-SiC. A peak at 0.87 ± 0.02 eV in the 6H-SiC is tentatively assigned to the same transition at the hexagonal site and the associated transition in 4H-SiC is thought to occur near 0.94 eV. All assignments are supported by the observation of V³⁺ in the EPR spectrum.     

Interlayer Spacing Regulation by Single-Atom Indiumδ+–N4 on Carbon Nitride for Boosting CO2/CO Photo-Conversion

Simultaneous optimization on bulk photogenerated-carrier separation and surface atomic arrangement of catalyst is crucial for reactivity of CO₂ photo-reduction. Rare studies capture the detail that, better than in-plane regulation, interlayer-spacing regulation may significantly influence the carrier transport of the bulk-catalyst thereby affecting its CO₂ photo-reduction in g-C₃N₄. Herein, through a single atom-assisted thermal-polymerization process, single-atom In-bonded N-atom (In^δ+–N₄) in the (002) crystal planes of g-C₃N₄ is originally constructed. This In^δ+–N₄ reduces the (002) interplanar spacing of g-C₃N₄ by electrostatic adsorption, which significantly enhances the separation of bulk carriers and greatly promotes the reactivity of CO₂ photoreduction. The CO₂ photo-conversion performance of this resulted single-atom In modified g-C₃N₄ is significantly superior to other single atom loaded carbon nitride catalysts. Moreover, the In^δ+–N₄ enhances the CO₂ adsorption on g-C₃N₄, reduces the *COOH formation energy, and optimizes the reaction path. It achieves a remarkable 398.87 µmol g⁻¹ h⁻¹ yield rate, 0.21% apparent quantum efficiency, and nearly 100% selectivity for CO without any cocatalyst or sacrificial agent. Through d₍₀₀₂₎ modulation of carbon nitride by single In atom, this study provides a ground-breaking insight for reactivity enhancement from a double-gain view of bulk structural control and surface atomic arrangement for CO₂-reduction photocatalysts.     

The structure and optical properties of nanocrytalline lead sulfide films

The crystal structure of lead sulfide (PbS) films produced by chemical deposition onto a glass substrate is studied by X-ray diffraction analysis. The thickness of the synthesized films is ∼120 nm, the dimensions of the regions of coherent scattering are 70–80 nm, and the magnitude of microstrains is ∼0.20%. It is established that the crystal structure of the synthesized PbS films and of the same films annealed at the temperature 293–423 K is cubic (space group Fm 3m) and corresponds to the D0₃ type that differs from the B1 type typical of coarse-grained PbS. In the cubic structure of the PbS nanofilms, there is a latent nonstoichiometric distribution of S atoms and vacancies among octahedral sites 4(b) and tetrahedral positions 8(c). The optical transmittance of the nanocrystalline PbS films is measured in the wavelength range from 200 to 3270 nm. The most pronounced variations in the transmittance is observed in the wavelength range from 700–800 to 1600–2000 nm (corresponding to the photon energy range from ∼1.8 to ∼0.7 eV). It is established that the band gap E _g is 0.83–0.85 eV, i.e., it is larger than the band gap of single-crystal PbS, 0.41 eV.     

Physical investigations on Sb2S3 sprayed thin film for optoelectronic applications

Sb₂S₃ thin films have been obtained at 250 °C on glass substrates using the spray pyrolysis techniques. The structural study by means of XRD analysis shows that Sb₂S₃ thin film crystallized in the orthorhombic phase. The discussion of some structural constants has been made by means of both XRD and AFM investigations. Moreover, the optical analysis via the transmittance and the reflectance measurements reveals that Sb₂S₃ sprayed thin film has a direct transition with the band gap energy E_g equal to 1.72 eV. The analysis in 300–2500 nm domain of the refractive index data through Wemple–DiDomenico model leads to the single oscillator energy (E₀=2.32 eV), and the dispersion energy (E_d=10.03 eV). The electrical study leads to the dc activation energy is of the order of 0.72 eV and the maximum barrier high is W_M=0.87 eV. From the power exponent variation in terms of the heated temperature, it is found that the mechanism of conduction matches well the correlated barrier hopping CBH model.     

Band gap of CdTe and Cd<Subscript>0.9</Subscript>Zn<Subscript>0.1</Subscript>Te crystals

The band gap E _g of the CdTe and Cd_0.9Zn_0.1Te crystals and its temperature dependence are determined by optical methods. This is motivated by considerable contradictoriness of the published data, which hampers the interpretation and calculation of characteristics of detectors of X-ray and γ radiation based on these materials (E _g = 1.39–1.54 and 1.51–1.6 eV for CdTe and Cd_0.9Zn_0.1Te, respectively). The used procedure of determination of E _g is analyzed from the viewpoint of the influence of the factors leading to inaccuracies in determination of its value. The measurements are performed for well-purified high-quality samples. The acquired data for CdTe (E _g = 1.47–1.48 eV) and Cd_0.9Zn_0.1Te (E _g = 1.52–1.53 eV) at room temperature substantially narrow the range of accurate determination of E _g.