Optical properties of tungsten disulfide single crystals doped with gold

Single crystals of WS₂ doped with gold have been grown by the chemical vapour transport method using iodine as a transporting agent. X-ray diffraction (XRD) pattern analysis revealed presence of mixed three-layer rhombohedral (3R) and two-layer hexagonal (2H) polytypes for the doped crystals while the undoped one shows only 2H form. Hall measurements indicate that the samples are p-type in nature. The doping effects of the materials are characterized by surface photovoltage (SPV), photoconductivity (PC) and piezoreflectance (PzR) measurements. Room temperature SPV and PC spectra reveal a feature located at 60 meV below the A exciton and has been tentatively assigned to be an impurity level caused by Au dopant. Excitonic transition energies of the A, B, d and C excitons detected in PzR spectra show red shift due to the presence of a small amount of Au and the broadening parameters of the excitonic transition features increase due to impurity scattering. The values of the parameters that describe the electron (exciton)–phonon interaction of excitonic transitions of A–B are about two times larger than that of d–C excitonic pairs. The possible assignments of the different origins of A–B and d–C excitonic pairs have been discussed.     

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er³⁺:LiLuF₄. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974 nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552 nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552 nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774 mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er³⁺:LiLuF₄ crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974 nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10 mW of output power at 553 nm.     

Growth and spectroscopic investigation of new promising laser crystal chromium (IV) doped germanoeucryptite Cr:LiAlGeO4

Single crystals of germanoeucryptite doped by chromium (IV) have been grown for the first time. Spectroscopic properties of grown crystals have been investigated. The intensive broadband fluorescence of Cr⁴⁺-ions in the range between 1070 and 1600 nm was observed with lifetime 10 μs. Preliminary investigations show that this material is promising for use as an active medium for new tunable solid-state laser of near IR-region.     

Optical gain calculation of mid-infrared InAsN/GaSb quantum-well laser for tunable absorption spectroscopy applications

We report on optical gain calculations of a dilute-nitride mid-infrared laser structure designed to be grown on InAs substrate. The active region is composed of several strain-compensated type-II “W”-like InAsN/GaSb/InAsN quantum wells adapted to operate near 3.3 μm at room temperature. For typical injected carrier density σ = 1.1012 cm^− 2, the theoretical laser structure performances reveal a gain value at around 1000 cm^− 1 at 300 K, inducing a modal gain value equal to 50 cm^− 1. Low radiative current densities lower than 100 A/cm² are predicted, indicating that this dilute-nitride structure could operate at 300 K with small threshold current density.     

Fluorescence spectroscopy of electrochemically self-assembled ZnSe and Mn:ZnSe nanowires

We report room temperature fluorescence spectroscopy (FL) studies of ZnSe and Mn-doped ZnSe nanowires of different diameters (10, 25, 50?nm) produced by an electrochemical self-assembly technique. All samples exhibit increasing blue-shift in the band edge fluorescence with decreasing wire diameter because of quantum confinement. The 10?nm ZnSe nanowires show four distinct emission peaks due to band-to-band recombination, exciton recombination, recombination via surface states and via band gap (trap) states. The exciton binding energy in these nanowires exhibits a giant increase (～10-fold) over the bulk value due to quantum confinement, since the effective wire radius (taking into account side depletion) is smaller than the exciton Bohr radius in bulk ZnSe. The 25 and 50?nm diameter wires show only a single FL peak due to band-to-band electron-hole recombination. In the case of Mn-doped ZnSe nanowires, the band edge luminescence in 10?nm samples is significantly quenched by Mn doping but not the exciton luminescence, which remains relatively unaffected. We observe additional features due to Mn(2+) ions. The spectra also reveal that the emission from Mn(2+) states increases in intensity and is progressively red-shifted with increasing Mn concentration.     

Optical spectroscopy of La3Ga5SiO14 disordered crystals doped with Fe3+ ions

The circular dichroism, absorption and luminescence spectra of langasite (La₃Ga₅SiO₁₄) doped with iron ions (Fe) are investigated at temperatures T of 300 and 8 K. Analysis of the data obtained reveals the Fe ions are trivalent. The Fe³⁺ impurity ions substitute Ga³⁺ ions and occupy 3f tetrahedral sites. Luminescence from the ⁴T₁ excited state of the dopant was detected at 8 K.     

Optical properties of some laser crystals

 In the following work the results of temperature dependence refractive indices measurements, thermooptical coefficients and dispersion in a wide range of temperature are presented. For measurements of refractive indices the least deviation method was employed. The measurements were carried out in the temperature region of 20–800° C for BBO crystals and 20–600° C for LBO crystals. It is revealed that the character of the refractive indices change in these crystals is different although the refractive indices of both crystals decrease while temperature increases. Whereas the temperature dependence of refractive indices in Ba B₂O₄ is practically linear, the same temperature dependence in LiB₃O₅ has more complicated character. It contains some particular features of refractive indices dispersion and thermooptical coefficients. Received: 11 January 1999/Reviewed and accepted: 15 January 1999     

Development of new NLO borate crystals

Recently new borate crystals, CsLiB₆O₁₀ (CLBO), YCa₄O(BO₃)₃ (YCOB) and Gd _x Y_1−x Ca₄O(BO₃)₃ (Gd _x Y_1−x COB) have been developed by the present authors. Here, the growth and nonlinear optical properties of CLBO, YCOB and Gd _x Y_1−x COB crystals are reviewed and their properties are discussed in relation to those of other nonlinear optical crystals, such asβ-BaB₂O₄ (BBO), and LiB₃O₅ (LBO).     

不同前驱体有机改性溶胶-凝胶膜对氨响应的研究

比较了四甲基硅氧烷(TMOS)分别与甲基三甲氧基硅烷(TMMS)、二甲基二甲氧基硅烷(DiMe-DMOS)和二苯基二甲氧基硅烷(DiPh-DMOS)按不同比例混和作为前驱体所形成的有机改性溶胶-凝胶氨传感敏感膜对氨的响应情况,发现3:1(v/v)TMMS/TMOS膜在0.1～10mg/ml NH3浓度范围内有较好的线性关系、检测限达0.01mg/ml、响应时间为2min左右. 对影响水体中氨光化学传感检测的因素如共存物质、测定温度、稳定性等也作了考察,该膜受测定参数影响较小,有望用于水体微量氨的测定.     

半导体纳米材料Zn(en)3 Se,ZnSe及其光学性能研究

以有机溶剂热技术制备片状Zn（en）3Se（en为乙二胺）纳米材料,DTG、IR、XRD分析结果表明该化合物中乙二胺与中心离子Zn^2＋通过配位键相结合;以制得的纳米Zn（en）3Se为母体,在氮气氛中煅烧至980℃,热分解制得棒状纳米ZnSe;以TEM、ED初步研究了两者的形貌、结构;以提拉法分别将Zn（en）3Se、ZnSe纳米材料涂布在ITO导电玻璃上,制得纳米颗粒／ITO复合膜,并研究其光学特性。结果表明,Zn（en）3Se属立方晶系,呈片状结构;ZnSe属六方纤锌矿型,棒直径在40nm左右;可能的生长机理是乙二胺作为模板分子,首先嵌入到ZnSe无机结构框架中,接着受热分解逃逸出无机框架形成一维纳米棒。PL分析表明Zn（en）3Se的荧光红移至448nm处。     

EPR and optical absorption studies of Cr3+ doped lithium potassium sulphate single crystals

X-band electron paramagnetic resonance (EPR) studies of Cr³⁺ doped lithium potassium sulphate single crystals have been done at room temperature. The Cr³⁺ crystal field and spin Hamiltonian parameters have been evaluated by employing resonance line positions observed in the EPR spectra for different orientations of external magnetic field. The evaluated g, D and E values are: g_x = 2.0763 ± 0.0002, g_y = 1.9878 ± 0.0002, g_z = 1.8685 ± 0.0002 and D = 549 ± 2 × 10^?4 cm^?1, E = 183 ± 2 × 10^?4 cm^?1. Using EPR data the site symmetry of Cr³⁺ ion in the crystal is discussed. Cr³⁺ ion enters the lattice substitutionally replacing K⁺ site. The optical absorption study of the single crystal is also done in 195–925 nm wavelength range at room temperature. By correlating optical and EPR data the nature of bonding in the crystal is discussed. The calculated values of Racah parameters (B and C), crystal field parameter (D_q) and nephelauxetic parameters (h and k) are obtained as: B = 697, C = 3247, D_q = 2050 cm^?1, h = 1.146 and k = 0.21.     

On the oxidation of VS2 2D platelets using tip-enhanced Raman spectroscopy

2D materials are enabling disruptive advancements in electronic and photonic devices yielding to the development of sensing and wearable materials and in the field of energy production and storage as key components of photovoltaic technology and batteries. Nevertheless, little attention has been paid to TMDs and oxides that contain vanadium, as it is the case of vanadium disulfide (VS₂) and vanadium dioxide (VO₂). In this study we review the synthesis and characterization using Raman spectroscopy of VS₂ and its oxidized states. Laser-induced oxidation occurring during the Raman experiments in ambient conditions is described and plateau values of laser power levels to induce oxidation are provided. Furthermore, tip-enhanced Raman spectroscopy (TERS) spectra and maps are conducted to reveal at the single flake level the onset of oxidation mechanisms at the surface of the 2D platelets.     

Optical and Electrical Properties of ZnSe:Cu Ceramics

ZnSe:Cu ceramics were prepared by hot-pressing powder mixtures of ZnSe and Cu at 900–1200°C under a pressure of 30 MPa. The luminescence spectra of the ceramics were found to be dominated by a green emission centered at 540 nm, whose intensity can be controlled by varying the Cu concentration and hot-pressing temperature. The nature of the green-emission centers is discussed. The electrical properties of the ZnSe:Cu ceramics studied can be understood in terms of percolation theory.     

Optical and Electrical Properties of Polycrystalline ZnSe:Li

ZnSe:Li ceramics were prepared by hot-pressing powder mixtures of ZnSe and LiOH at 1050°C under a pressure of 30 MPa. The luminescence spectra of the ceramics are dominated by a red emission centered at 630 nm. With increasing Li concentration, the intensity and width of the red band increase, and the band shifts to shorter wavelengths. The nature of the emission centers and the effect of the doping level on their parameters are discussed. The electrical properties of the ZnSe:Li ceramics can be understood in terms of percolation in heterogeneous systems.     

(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B:A novel high-entropy monoboride with good electromagnetic interference shielding performance in K-band

A novel equimolar high-entropy(HE)transition metal monoboride,(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B,was designed and prepared in powder and bulk form by high temperature elemental reac-tion method and spark plasma sintering(SPS)method,respectively.XRD analysis shows that HE(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B possesses orthorhombic structure with Pnma space group.Through Rietveld refinement,the lattice parameters of HE(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B are a=5.6675,b=2.9714,c=4.2209 and the theoretical density is 6.95 g/cm3.The Vickers hardness and electrical conductivity of HE(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B bulk with relative density of 90％is 12.3±0.5 GPa and 0.49±0.04×106 S/m,respectively.Due to high electrical conductivity,HE(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B bulk with 3.0 mm thickness displays superior EMI shielding performance in 18.0-26.5 GHz(K-band),and the average values of SET,SER,and SEA are 23.3 dB,13.9 dB,and 9.4 dB,respectively.The EMI shielding mechanism of HE(Cr0.2Mn0.2Fe0.2Co0.2Mo0.2)B mainly results from reflection.     

Optical properties and characterization of zinc nitride nanoneedles prepared from ball-milled Zn powders

Zinc nitride nanoneedles (ZNNs) with diameters at stem and tip parts as 200-300 nm and 30-70 nm respectively have been prepared by the nitridation of ball-milled zinc powders at 600 °C for 120 min under NH₃ gas environment. The structural, compositional and morphological characterizations of the product were conducted by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. From transmission spectrum data, an indirect band gap of 2.72 eV has been calculated for ZNNs whereas photoluminescence studies exhibited a strong UV excitonic mission band at 395 nm as well as two weak defect related blue emissions at 453 and 465 nm. A vapor-solid (VS) process based growth mechanism for the formation of ZNNs has also been discussed briefly.     

Auger electron spectroscopy analysis of interface roughness of Fe/Cr bilayers

In this work we investigate the surface and interfacial properties of Fe/Cr and Cr/Fe bilayers before and after annealing using Auger electron spectroscopy (AES). The roughness of the interface is also determined with the X-ray reflection method. The fitted values of inelastic mean free path λ_{Cr in Fe} reproduce the calculated value for Cr in Fe well, whereas the values of λ_{Fe in Cr} are significantly larger than the calculated ones, suggesting mutual segregation of atoms during growth. The low-energy range Auger spectra demonstrated that the MNN lines of Cr covered with Fe and Fe covered with Cr disappear after the deposition of 1 nm overlayer, this being an indication of continuous deposited film, but not excluding mixing at interfaces. The results of X-ray reflectometry measurements, which give the values of Fe/Cr and Cr/Fe roughness, are in accordance with this observation. The LMM Auger spectra of annealed samples showed that at the largest applied temperature, Cr diffuses into Fe, but the reverse effect of Fe diffusion into Cr is not observed.     

The growth and thermodynamical feasibility of tungsten diselenide single crystals using chemical vapour transport technique

Thermodynamical feasibility study and the growth of layer structured transition metal dichalcogenide single crystals of WSe₂, using iodine as transporting agent, has been reported in this paper. The characterization of the grown samples have been done by X-ray analysis.     

First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co, Ni, Mo and W) compounds

First principles calculations are performed to study the stability, electronic and structural properties of X₂B (X = Cr, Mn, Fe, Co, Ni, Mo and W). The calculated cohesive energy and formation enthalpy of these compounds both have negative values, which indicate that they are thermodynamically stable structures. The ground states of Cr₂B and Mn₂B are anti-ferromagnetic; Fe₂B and Co₂B are ferromagnetic; Ni₂B, Mo₂B and W₂B are paramagnetic. The calculated local magnetic of Fe₂B is 1.962μ_B/Fe, and for Co₂B is 1.182μ_B/Co. They are comparable to the values of Fe₃B (1.97μ_B/Fe) and Co₃B (1.18μ_B/Co), but smaller than pure Fe and Co. The observed magnetic behaviors of X₂B compounds can be explained by Stoner’s model. Two main peaks are observed in the calculated PDOS (partial density of states) of these compounds (P1 and P2). P1 is caused by strong covalent X–B bonds and P2 is attributed to metallic X–X bonds.