A study of structural, optical and magnetic properties of Zn0.97−xCuxCr0.03O diluted magnetic semiconductors

A set of Zn_0.97−xCu_xCr_0.03O (0 ≤ x ≤ 0.03) samples has been synthesized by the sol-gel method. The structural, optical and magnetic properties of the samples were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and vibrating sample magnetometer (VSM). With Cu doping concentration increasing up to 2 at%, the XRD results showed that all diffraction peaks corresponded to wurtzite structure of ZnO, but for Zn_0.94Cu_0.03Cr_0.03O, the secondary phase of Cu emerged. PL measurements showed that Zn_0.97−xCu_xCr_0.03O powders and pure ZnO with the Cu concentration varied from 0.00 to 0.02 exhibited obvious blue shift; the green emission peak could be effectively enhanced with the increase of the Cu concentration. Magnetic measurements indicated that room-temperature ferromagnetism of Zn_0.97−xCu_xCr_0.03O was an intrinsic property when Cu concentration was less than 0.02. The saturation magnetization of Zn_0.97−xCu_xCr_0.03O (x = 0, 0.01, 0.02) increased with the increase of the Cu concentration.     

Structural and optical analysis of ZnBeMgO powder and thin films

We here report the structural and optical studies of Zn_1−x−yBe_xMg_yO (0 ≤ x ≤ 0.15; 0 ≤ y ≤ 0.20) powders and thin films. From the Rietveld refinement of the powder X-ray diffraction (XRD) patterns it was revealed that the value of ‘a’ lattice parameter remains almost unchanged whereas ‘c’ parameter reduces with Be and Mg co-doping in ZnO. The Zn-O bond length also decreases in co-doped samples. Raman studies of the pure ZnO powder showed all the characteristic peaks of the wurtzite hexagonal structure and with (Be, Mg) co-doping new modes appeared which can be attributed to arise as a result of substitution. The XRD of the films prepared from the powders using pulsed laser deposition (PLD) technique exhibited the preferential orientation and with increase in co-doping the (0 0 0 2) peak also shifts to higher 2θ values suggesting the incorporation of Be/Mg at the Zn-site. From the UV-visible optical transmittance measurement it was noticed that the band gap of the pristine ZnO film is 3.3 eV which enhanced up to 4.51 eV for Zn_0.7Be_0.1Mg_0.2O film which lies in the solar blind region and is very useful in the realization of deep UV detectors.     

A novel photocatalyst and improvement of photocatalytic properties by Cu doping

A new series of photocatalysts, Bi₂Zn_2/3−xCu_xTa_4/3O₇ (Cu-β-BZT) crystals with pyrochlore structure were synthesized by the method of solid-state reaction (SSR). With small amount of Cu doped (0.01 ≤ x ≤ 0.04), the phase structure was kept to be monoclinic pyrochlore as pure β-BZT. The diffuse reflectance spectrum of Cu-β-BZT samples showed a red shift. The method of Cu doping enhanced the photocatalytic activity, and when the value of x is 0.03, the sample showed the highest activity, which is about 10 times higher than that of pure β-BZT under UV light. Especially, the samples of Cu-β-BZT showed photocatalytic activities under visible light irradiation (λ > 400 nm). Effects of the Cu doped on the photocatalytic activities of the catalysts were also discussed.     

Tunable light emission and similarities with garnet structure of Ce-doped LSCAS glass for white-light devices

In this paper, we report results concerning tunable light emission and color temperature in cerium-doped low-silica-calcium-alumino-silicate (LSCAS) glass for smart white-light devices. Spectroscopic results, analyzed using the CIE 1931 x-y chromatic diagram, show that this glass presents two broad emission bands centered at 475 and 540 nm, whose intensities can be tuned by the excitation wavelength. Moreover, the same emission can be achieved from a color temperature range from 3200 to 10,000 K, with a color-rendering index (CRI) of around 75% obtained by changing the optical path length of the sample. Our new phosphor LSCAS glass, which is a unique system that exhibits tunable yellow emission, combines all qualities for white-light devices.     

Effect of Cu-doping on the electronic structure and optical properties of LaNi5

Optical properties of intermetallic isostructural compounds LaNi_5−xCu_x (x = 0, 0.6, 1, 1.2) have been studied in the spectral range from 0.22 to 15 μm using the ellipsometry method. It was found that the substitution of copper for nickel leads to local changes in the optical conductivity spectra. Theoretical calculations of the electronic structure and interband optical conductivity of LaNi_5−xCu_x compounds with x = 0, 1, 2, 3 were performed in the generalized gradient approximation within the pseudopotential plane-wave method PWSCF. Both the optical spectroscopic measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated with the Cu 3d → Ni 3d electron transitions and increasing with the growth of copper content.     

Luminescent properties of Na3Gd1−xEux(PO4)2 and energy transfer in these phosphors

A series of Eu³⁺ activated Na₃Gd_1−xEu_x(PO₄)₂ (0 ≤ x ≤ 1) phosphors were synthesized by solid-state reaction method. The structures and photo-luminescent properties of these phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the orthorhombic Na₃Gd(PO₄)₂. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The intensities of magnetic dipole transition ⁵D₀ → ⁷F₁ and forced electric dipole transition ⁵D₀ → ⁷F₂ are comparable, and the energy ratio (⁵D₀ → ⁷F₁/⁵D₀ → ⁷F₂) is 1.1. The emission spectra exhibit strong reddish orange performance (CIE chromaticity coordinates: x = 0.62, y = 0.38), which is due to the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions. The correlation between the structure and the photo-luminescent properties of the phosphors was studied. The energy transfer and concentration quenching of the phosphors were discussed. Na₃Gd_1−xEu_x(PO₄)₂ has a potential application for white light-emitting diodes.     

Power factor enhancement in Zn-doped Na0.8CoO2

The thermoelectric properties of Na_0.8Zn_xCo_1−xO₂/(ZnO)_y (x ≤ 0.01, 0 ≤ y ≤ 0.14) have been systematically investigated. The results suggest that doping divalent Zn ions within solubility limit x^* ∼ 0.01 leads to simultaneous reduction in resistivity and enhancement of thermopower. Analysis of the results show that the reduction of resistivity may be attributed to improved mobility of carriers, while the enhancement of thermopower may originate from the geometric relaxation of distorted CoO₆ octahedra caused by partial Zn substitution, leading to a narrower band width in the strongly correlated environment, consequently resulting in a remarkable 20% improvement in power factor.     

Transparent p-type CuxS thin films

The effect of different mild post-annealing treatments in air, at 270 °C, for 4-6 min, on the optical, electrical, structural and chemical properties of copper sulphide (Cu_xS) thin films deposited at room temperature are investigated. Cu_xS films, 70 nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu₂S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (∼10²² cm⁻³), low electrical resistivity (∼10⁻⁴ Ω cm) and inconclusive p-type conduction. After the mild post-annealing, these films display increasing values of resistivity (∼10⁻³ to ∼10⁻² Ω cm) with annealing time and exhibit conclusive p-type conduction. An increase of copper content in Cu_xS phases towards the semiconductive Cu₂S (chalcocite) compound with annealing time is reported, due to re-evaporation of sulphur from the films. However, the latter stoichiometry was not obtained, which indicates the presence of vacancies in the Cu lattice. In the most resistive films a Cu₂O phase is also observed, diminishing the amount of available copper to combine with sulphur, and therefore the highest values of optical transmittance are reached (65%). The appearance on the surface of amorphous sulphates with annealing time increase is also detected as a consequence of sulphur oxidation and replacement of sulphur with oxygen. All annealed films are copper deficient in regards to the stoichiometric Cu₂S and exhibit stable p-type conductivity.     

Effect of Ca-doping on the structural and electrical properties of CuY1−xCaxO2 (0 ≤x ≤0.10) ceramics

Delafossites CuY_1−xCa_xO₂ (0 ≤x ≤0.10) ceramics have been prepared by solid state reaction using Cu₂O, Y₂O₃ and CaCO₃. Liquid phase sintering, which obviously accelerates the reaction speed of Cu₂O-Y₂O₃-CaCO₃ system and promotes the formation of CuYO₂ phase is evidenced for the Ca-doped samples. During the sintering process, CuO can react with CaO to form two intermediate compounds, CaCu₂O₃ and Ca₂CuO₃, which decompose into CaO and liquid phase during 1273-1323 K. In the dopant range of 0 ≤x ≤0.10, both electrical conductivity and density of the samples are increased by Ca-doping. The room temperature conductivity of CuY_0.94Ca_0.06O₂ is more than four orders of magnitude higher than that of CuYO₂.     

Upconversion luminescence in Er/Yb codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y₂CaGe₄O₁₂ doped with Er³⁺ and Er³⁺/Yb³⁺ reveal upconversion emission in visible spectral range under near-infrared excitation, λ_ex = 980 nm. For the solid solution Er_xY_2−xCaGe₄O₁₂ concentration dependencies for the green and red lines of the visible emission around 526 nm (²H_11/2 → ⁴I_15/2), 545 nm (⁴S_3/2 → ⁴I_15/2) and 670 nm (⁴F_9/2 → ⁴I_15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y₂CaGe₄O₁₂:Er³⁺ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y₂CaGe₄O₁₂:Er³⁺ and Y₂CaGe₄O₁₂:Er³⁺, Yb³⁺ has been proposed. Conditions for visible emission occurrence under quasi-resonance λ_ex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition ⁴I_13/2 → ⁴I_15/2 in erbium ions has been detected.     

Tunable deep-level emission in ZnO nanoparticles via yttrium doping

Yttrium-doped ZnO nanoparticles (Zn_1−xY_xO, x = 0, 0.03, 0.05) were synthesized by sol-gel technique. The effects of yttrium doping concentration on the structures, morphologies and optical properties of as-synthesized Zn_1−xY_xO nanoparticles were investigated in detail. The results from structural characterizations clearly demonstrated that yttrium ions were successfully doped into the crystal lattice of ZnO matrix. Besides a UV emission centered at ∼383 nm, the PL spectra of all the samples exhibited a broad deep-level emission, which can be deconvoluted into two Gauss peaks centered at 539 nm (P1) and 598 nm (P2), respectively. As the concentration of Y doping increased from 0% to 5%, the peak position with maximum intensity in deep-level emission band was gradually tuned from 539 nm to 598 nm and the relative intensity ratio of I_P1/I_P2 also decreased step by step, which revealed a unique optical property of yttrium-doped ZnO nanoparticles.     

Composition dependence of structure and optical properties of Cu2ZnSn(S,Se)4 solid solutions: An experimental study

The evolution of structure and optical properties of Cu₂ZnSn (S_xSe_1−x)₄ (CZTSSe) solid solutions in a wide composition range (0 ≤ x ≤ 1) has not been fully elucidated. We have performed comprehensive characterization on the CZTSSe powders with different S/Se ratios, which were synthesized by the solid state reaction method. X-ray diffraction patterns demonstrate that the lattice parameters a and c of CZTSSe decrease lineally when S replace Se gradually, which obeys the Vegard's rule. The A₁ Raman modes of CZTSSe show a typical two-mode behavior. The absorption spectra reveal that the band gap of CZTSSe can be tuned monotonously between 0.96 and 1.5 eV with almost linearity, and a small band gap bowing constant (b ≈ 0.08 eV) is deduced.     

Structural and magnetic nanoclusters in Cu50Zr50−xGdx (x = 5 at.%) metallic glasses

It is shown that phase-separated metallic glasses on the nanoscale can be prepared by rapid quenching of Cu₅₀Zr_50−xGd_x melts with a low concentration of gadolinium (x = 5 at.%). Gd-enriched clusters of 2 nm size are formed as early stages of decomposition in the deeply undercooled melt. The key physical parameter to obtaining such a nanoclustered microstructure upon quenching is the critical temperature of liquid-liquid phase separation which has to be close to the glass transition temperature. Thus, the thermodynamic properties of the liquid phase even in the metastable deeply undercooled melt essentially determine the structure formation. Analysis of the spatial atomic arrangement by atom probe tomography after annealing in the supercooled liquid state provides direct evidence of the spinodal character of the decomposition by uphill diffusion. The Gd-enriched nanoclusters exhibit ferromagnetic ordering below 50 K and the cluster size regime derived from magnetization measurements is in good agreement with that obtained from atom probe tomography investigations. The first stage of crystallization of Cu₅₀Zr₄₅Gd₅ glass is observed to be Ostwald-type ripening on a nanoscale. The phase-separated glass acts as a precursor for the formation of a metastable nanocrystalline structure.     

Calculation of band offsets in Cd1−xXxTe alloys, X = Zn, Mg, Hg and Mn and magnetic effects in CdMnTe

Due to the large variety of properties offered by the telluride binaries CdTe, ZnTe, MgTe, HgTe and MnTe as well as their mixed ternary alloys, an accurate knowledge of their electronic band parameters is crucial. These materials have been extensively studied but, some points bearing on several properties have never previously reported or are still not clear. In this paper, we report results on the conduction and valence band offsets of the pseudo-morphically strained Cd_1−xX_xTe layer on relaxed Cd_1−yX_yTe substrate, X = Zn, Hg, Mg and Mn. Based on the Van Der Walle model, calculations have been performed for the all range of material and substrate 0 ≤ x,y ≤ 1. These discontinuities have not yet calculated for X = Mg, Mn or Hg in the all range 0 ≤ x,y ≤ 1. For the CdMnTe diluted magnetic semiconductor which we focus more interest due to its considerable current interest for applications, calculations have been done without and with correction taking into account magnetic effect of magnesium ions Mn²⁺. It is found that the introduction of only a few percent of Mn into CdTe provides a unique opportunity to combine two important fields in physics, semiconductivity and magnetism. We can take advantage both of possibility of applications in solid-state lasers and exceptional magnetic properties offered by this magnetic diluted semiconductor.This study presents important quantities that are required to model quantum structures and offers a fast and inexpensive way to check device designs and processes.     

Multiwavelength excited novel red-emitting phosphor Eu-activated Li2Zn2(MoO4)3

Eu³⁺-activated Li₂Zn₂(MoO₄)₃ multiwavelength excited red-emitting phosphors were synthesized via a solid state reaction. The structure and photoluminescence characteristics were investigated by X-ray powder diffraction and fluorescent spectrophotometry, respectively. The excitation spectrum included a strong broadband ranging from 250 to 350 nm and some sharp peaks at 363, 384, 395, 465, and 533 nm, which matchs the radiations of near-UV or blue light-emitting diodes chip well. Upon excitation either of near-UV or blue even green light, the intense red emission with 615 nm peak can be observed, which is ascribed to the ⁵D₀-⁷F₂ transition of Eu³⁺ ions. The chromaticity coordinates (x = 0.65, y = 0.34) of the as-obtained phosphor is very close to the National Television Standard Committee standard values (x = 0.67, y = 0.33). All these characteristics suggest that Eu³⁺-doped Li₂Zn₂(MoO₄)₃ wavelength-conversion material to be suitable candidate red component for phosphor-converted white light-emitting diodes.     

Vacuum ultraviolet spectroscopic properties of KSrPO4:Tb

KSrPO₄:Tb³⁺ phosphors were prepared by a solid-state method and their photoluminescence properties were investigated under vacuum ultraviolet excitation. In the excitation spectrum monitoring at 544 nm, the band in the region of 120-162 nm can be attributed to be the overlap of host absorption and charge transfer transition of O²⁻ → Tb³⁺, and the band ranging from 162 to 300 nm was assigned to the f-d transition of Tb³⁺. The photoluminescence spectrum shows that the phosphors exhibited a strong green emission around 544 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ under the excitation of 147 nm. Optimal emission intensity was obtained when x = 7% in KSr_1-xPO₄:xTb³⁺ and the luminescent chromaticity coordinates were calculated to be (x = 0.317, y = 0.522) for KSr_0.93PO₄:7%Tb³⁺.     

Dual role of the six-coordinated lead and copper ions in structure of the copper-lead-tellurate glasses

Glasses from xCuO·(100 − x)[4TeO₂·PbO₂] system where x = 0-40 mol% were studied by density measurements, FTIR, UV-vis and EPR spectroscopy in order to obtain information about the changes that appear in the structure of matrix glass with the doping of copper ions.The FTIR data analysis shows that the copper ions will be distributed in the six-coordinated interstices ([PbO₆] structural units) and some [TeO₄] structural units will be converted to [TeO₃] structural units because the lead ions have a strong affinity towards these groups containing non-bridging oxygens, with negative charge.From the EPR studies, we can conclude that Cu⁺² ions have an environment elongated along the z-axis and the ground state of the Cu⁺² is dx²−y² orbital (²B_1g state). When the concentration of CuO is increased beyond 5 mol%, the intensity and width of both the parallel and perpendicular hyperfine components are observed to increase.     

Acoustically induced optical effects in Pr and Tm doped calcium gadolinium oxyborate nanocomposites

Acoustically induced two-photon absorption (AITPA) and optical Kerr effect (AIOKE) in Pr and Tm doped calcium gadolinium oxyborate Ca₄GdO(BO₃)₃ (CGOB) nanocrystallites (with sizes below 60 nm) prepared by sol-gel method and embedded into the PMMA matrices were studied. The source of the acoustical field generated the frequency equal to about 1.2 MHz and achieved maximal acoustical power density equal to about 6 W/cm². The AITPA was measured at fundamental Er:glass laser wavelength (1.54 μm) and was observed only during applying of the prolonged acoustical field. The AITPA effect was substantially higher for the Tm³⁺ doped composites than for the Pr³⁺ doped ones. With increasing acoustical power, the AITPA increases achieving its maximal value at acoustical power density equal to about 4.70 W/cm². Temperature dependences have demonstrated the maximal values of the AITPA at temperatures equal to about 310 K corresponding to the AITPA value equal to about 3.4 cm/GW. At the same time at acoustical power density about 3.8 W/cm² the AIOKE achieves its minimum for the Pr doped CGOB nanocomposites. So one can expect that these two effects have different signs of contribution caused by the acoustical field.