One-pot Microwave Assisted Synthesis of Homogeneously Alloyed CdSexTe1-x Nanocrystals with Tunable Photoluminescence

Aqueous soluble homogeneously alloyed CdSe_xTe_1-x (0 ≤ x ≤ 1) semiconductor nanocrystals (NCs) were synthesized by co-reduction of sodium selenite (Na₂SeO₃) and sodium tellurite (Na₂TeO₃) at 120 °C under microwave irradiation. The optical modulation of the photoluminescence (PL) between 600 and 710 nm could be facilely demonstrated through changing molar ratios of Se to Te. The PL quantum yields (QYs) of CdSe_xTe_1-x NCs were lower than that of CdTe NCs. On the other hand, it could be higher than that of CdSe NCs.     

Microstructure and electrical properties of Ba0.7Sr0.3(Ti1 − xZrx)O3 thin films prepared on copper foils with sol-gel method

Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0, 0.1, 0.2) (BSZT) thin films have been prepared on copper foils using sol-gel method. The films were annealed in an atmosphere with low oxygen pressure so that the substrate oxidation was avoided and the formation of the perovskite phase was allowed. The X-ray diffraction results show a stable polycrystalline perovskite phase, with the diffraction peaks of the BSZT films shifting toward the smaller 2θ with increasing Zr content. Scanning electron microscopy images show that the grain size of the BSZT thin films decreases with increasing Zr content. High resolution transmission electron microscopy shows the clear lattice and domain structure in the film. The dielectric peaks of the BSZT thin films broaden with increasing Zr content. Leakage current density of Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0.1) thin film is the lowest over the whole applied voltage.     

Structural and optical properties of polycrystalline CdSexTe1−x (0 ≤ x ≤ 0.4) thin films

CdSe_xTe_1−x (0 ≤ x ≤ 0.4) ternary thin films have been deposited on quartz substrates at room temperature by a single source thermal evaporation. X-ray diffraction patterns and transmission electron microscope micrographs of these films showed that the films were of polycrystalline texture over the whole range studied and exhibit predominant cubic (zinc blende) structure with strong preferential orientation of the crystallites along (1 1 1) direction. Linear variation of the lattice constant with mole fraction x is observed obeying Vegard's law. The dependence of the optical constants, the refractive index n and extinction coefficient k, of the films on the mole fraction x was studied in the spectral range of 400-2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple-DiDomenco single-oscillator model. CdSe_xTe_1−x thin films of different composition have two direct and indirect transitions corresponding to energy gaps and . The variation in either or with x indicates that this system belongs to the amalgamation type. The variation follows a subquadratic dependence and the bowing parameters were found to be 0.36 and 0.48 eV for the direct, and indirect energy gaps, respectively. Direct linear variation of the ratio N/m^* with x is observed.     

Structure, magnetic and magnetostrictive properties of Sm0.7Pr0.3Fex alloys

The structure, magnetic and magnetostrictive properties of Sm_0.7Pr_0.3Fe_x (1.45 ≤ x ≤ 1.95) alloys is investigated. X-ray diffraction analysis confirms the presence of single cubic Laves phase in Sm_0.7Pr_0.3Fe_x alloys with 1.45 ≤ x ≤ 1.85. The saturation magnetization of the alloys tends to increase with increasing Fe content. Sm_0.7Pr_0.3Fe_1.55 has the highest magnetostriction among all the Sm_0.7Pr_0.3Fe_x alloys at low fields and shows a large magnetostriction λ_// − λ_⊥ = −1008 ppm at a magnetic field of 12 kOe.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Study on the optical property and crystallinity of Ge90Te10 films deposited by electron beam evaporation

Optical property and crystallinity of Ge₉₀Te₁₀ films prepared by electron beam evaporation have been studied. The films grown at different substrate temperatures (Ts) and deposition rates (R) have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and spectroscopic ellipsometry. The polycrystalline film was obtained at Ts = 300 °C, while the amorphous film was obtained when Ts ≦ 200 °C. However, the film showed the columnar structure when Ts ≦ 100 °C. It was found that Ts had the stronger effect on the crystallinity of the film rather than R. The optical constant in the infrared region was determined. All the film exhibited no absorption, but the refractive index was varied with the change of Ts and R. The relationship between optical constant, the film structure and the deposition parameters were also discussed. In addition, the optimum deposition condition of Ge₉₀Te₁₀ film was found.     

Rietveld refinement of the semiconducting system Bi2−xFexTe3 from X-ray powder diffraction

The semiconducting system Bi_2−xFe_xTe₃ (x = 0.0, 0.02, 0.04 and 0.08) was synthesized at 1000 °C for 30 h. The scanning electron microscope (SEM) image reveals the tendency of the Bi_2−xFe_xTe₃ system to form a sheet structure with more pronounced alignment and to enhance the formation of some microstructure tubes. The structure of the system under study was refined on the basis of X-ray powder diffraction data using the Rietveld method. The analysis revealed the complete miscibility of Fe in the Bi₂Te₃ matrix and hence the formation of single phase. The system crystallizes in the space group R-3m [1 6 6]. The lattice parameters and the unit cell size slightly change by the incorporation of Fe. The refinement of instrumental and structural parameters led to reliable values for the R_B, R_F and Chi².     

Dielectric, ferroelectric and piezoelectric properties of Bi0.5Na0.5TiO3-(Ba0.7Ca0.3)TiO3 ceramics at morphotropic phase boundary composition

(1 − x)Bi_0.5Na_0.5TiO₃-x(Ba_0.7Ca_0.3)TiO₃ (BNT-xBCT, 0 ≤ x ≤ 0.15) solid solutions have been synthesized by a conventional solid state sintering method for obtaining a morphotropic phase boundary (MPB) with good piezoelectric properties. X-ray diffraction patterns reveal that a MPB of rhombohedral and tetragonal phases is formed at compositions 0.09 ≤ x ≤ 0.12. Addition of BCT into BNT greatly lowered coercive field E_c without degrading remanent polarization P_r. The specimen with x = 0.09 has the good piezoelectric properties: d₃₃ = 125 pC/N and k_p = 0.33. A modified Curie-Weiss law was used to fit the dielectric constant of BNT-xBCT ceramics, and a frequency dispersion was observed during the phase transitions from antiferroelectric to paraelectric in specimens with x exceeding 0.06.     

Structural characterization and novel optical properties of defect chalcopyrite ZnGa2Te4 thin films

Stoichiometric thin film samples of the ternary ZnGa₂Te₄ defect chalcopyrite compound were prepared and characterized by X-ray diffraction technique. The elemental chemical composition of the prepared bulk material as well as of the as-deposited film was determined by energy-dispersive X-ray spectrometry. ZnGa₂Te₄ thin films were deposited, by conventional thermal evaporation technique onto highly cleaned glass substrates. The X-ray and electron diffraction studies revealed that the as-deposited and the annealed ZnGa₂Te₄ films at annealing temperature t_a ≤ 548 K are amorphous, while those annealed at t_a ≥ 573 K (for 1 h), are polycrystalline. The optical properties of the as-deposited films have been investigated for the first time at normal incidence in the spectral range from 500 to 2500 nm. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single oscillator model, whereby, the values of the oscillator parameters have been calculated. The analysis of the optical absorption coefficient revealed an in-direct optical transition with energy of 1.33 eV for the as-deposited sample. This work suggested that ZnGa₂Te₄ is a good candidate in solar cell devices as an absorbing layer.     

Microstructure and thermoelectric properties of Sn-doped Bi2Te2.7Se0.3 thin films deposited by flash evaporation method

(Bi_1 − xSn_x)₂Te_2.7Se_0.3 thermoelectric thin films with thickness of 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. The structures, morphology of the thin films were analyzed by X-ray diffraction and field emission scanning electron microscopy respectively. Effects of Sn-doping concentration on thermoelectric properties of the annealed thin films were investigated by room-temperature measurement of Seebeck coefficient and electrical resistivity. The thermoelectric power factor was enhanced to 12.8 μW/cmK² (x = 0.003). From x = 0.004 to 0.01 Sn doping concentration, the Seebeck coefficients are positive and show p-type conduction. The Seebeck coefficient and electrical resistivity gradually decrease with increasing Sn doping concentration.     

Optical constants of electroplated Bi2Te3 films by Mueller matrix spectroscopic ellipsometry

We synthesized polycrystalline Bi_2 + xTe_3 − x (− 0.2< x <0.2) thin films by electrodeposition in acidic medium. Since Bi₂Te₃-like structure may be uniaxially anisotropic due to its rhombohedral crystallographic system, we investigated their optical behavior using ex and in situ Mueller matrix spectroscopic ellipsometry in the wavelength range of 470 to 830 nm (1.5-2.6 eV). We found that room-temperature electroplated polycrystalline appears optically isotropic and that no depolarization effect occurs from the first steps of growth until several micrometers thick films. Additional ex situ measurements permit to obtain their optical constants from far-ultraviolet to near-infrared (190-2100 nm).     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Characterization of phase transition in silver photo-diffused Ge2Sb2Te5 thin films

Surface activity of thermally evaporated amorphous chalcogenide films of Ge₂Sb₂Te₅ has been investigated. Silver (Ag) is readily deposited on such films from appropriate aqueous ionic solution and Ag diffuses into the films upon irradiation with energetic photons. The composition of Ge₂Sb₂Te₅ thin films and the amount of Ag photo-diffused has been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. The composition of the films was found to be very close to the bulk used to deposit films and the amount of Ag photo-diffused was ∼ 0.38 at. %. X-ray diffraction and temperature dependent sheet resistance studies have been used for the structural analysis of the bulk alloy, as-deposited, Ag photo-diffused and annealed films at different temperatures. The films remain amorphous after Ag photo-diffusion into the amorphous Ge₂Sb₂Te₅ films. The reflectivity, reflectivity contrast and extinction coefficient of the crystalline and amorphous photo-diffused thin films are presented. The optical band gaps of the amorphous and crystalline photo-diffused (Ge₂Sb₂Te₅)_100−xAg_x=0.38 phase change thin films have also been calculated from absorption data using UV-VIS spectroscopy.     

Synthesis and electrical properties of cubic NaxWO3 thin films across the metal-insulator transition

Highly oriented (1 0 0) Na_xWO₃ thin films were fabricated in the composition range 0.1 ≤ x ≤ 0.46 by pulsed laser deposition technique. The films showed transition from metallic to insulating behaviour at a critical composition between x = 0.15 and 0.2. The pseudo-cubic symmetry of Na_xWO₃ thin films across the transition region is desirable for understanding the composition controlled metal-insulator transition in the absence of any structural phase transformation. The electrical transport properties exhibited by these films across the transition regime were investigated. While the resistivity varied as T² at low temperatures in the metallic regime, a variable range hopping conduction was observed for the insulating samples. For metallic compositions, a non-linear dependence of resistivity in temperature was also observed from 300 to 7 K, whose exponent varied with the composition of the film.     

Growth of single-phase CuInGaSe2 photo-absorbing alloy films by the selenization method using diethylselenide as a less-hazardous Se source

Selenization growth of purely single-phase, polycrystalline CuIn_1 − xGa_xSe₂ (0 ≤ x ≤ 1) alloy films was demonstrated using a less-hazardous metalorganic selenide, diethylselenide [(C₂H₅)₂Se: DESe], without additional thermal annealing. Approximately 2.0-μm-thick films of the alloys exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Low temperature photoluminescence spectra of the alloy films were dominated by a couple of characteristic donor-acceptor pair emissions that are particular to the state-of-the-art CuInGaSe₂ photo-absorbing layers.     

Growth of single-phase Cu(In,Al)Se2 photoabsorbing films by selenization using diethylselenide

Selenization growth of purely single-phase, polycrystalline CuIn_1 − xAl_xSe₂ (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C₂H₅)₂Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor-acceptor pair emissions of the state-of-the-art CuIn_1 − xAl_xSe₂ photoabsorbing layers.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

Structural and magnetic properties of La0.85Ca0.15Mn1 − xCrxO3

Chromium (Cr) is doped at the Mn site of La_0.85Ca_0.15MnO₃ system to explore its complex ferromagnetic insulating (FMI) state. The Rietveld refinement of neutron diffraction data indicates that there is no structural change owing to Cr substitution in La_0.85Ca_0.15Mn_1 − xCr_xO₃ (0 ≤ x ≤ 0.1). Nevertheless, it strengthens the magnetic couplings and the system shifts towards enhanced ferromagnetic (FM) ordering. Doping with Cr is found to stabilise the FMI state at low temperatures. The magnetic moment of the parent compound (for x = 0) obtained from neutron diffraction data recorded at low temperature (17 K) is found to be ~ 3.53(5) μ_B and is close to the theoretically estimated value of 3.85 μ_B. This value is higher than previously reported value of 2.90 μ_B.     

Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.