Photophysical parameters and laser performance of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP): A new laser dye

The spectral properties such as singlet absorption, molar absorptivity, emission spectra, fluorescence quantum yield and excited state lifetime of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP) have been determined in different solvents. DMAFP dye exhibits a large red shift in both electronic absorption and emission spectra as the solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation. A crystalline solid of DMAFP gives an excimer like emission at 566 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. The ground and excited state protonation constants of DMAFP are calculated and amounted to 1.71 and 8.3, respectively. DMAFP acts as a good laser dye upon pumping with nitrogen laser (λ_ex=337.1 nm) in chloroform, methylene chloride and dioxane and gives laser emission in the range 460–590 nm. The laser parameters such as the tuning range, gain coefficient (α), emission cross section (σ_e) and half-life energy (E_1/2) are calculated. The photoreactivity and net photochemical quantum yield of DMAFP in chloromethane solvents are also studied.     

Luminescence characteristics of cobalt doped TiO2 nanoparticles

TiO₂ nanoparticles doped with two different concentrations of Cobalt, 0.02 and 0.04 mol, are prepared by sol–gel method. The crystalline phase of the doped and undoped nanoparticles and particle sizes are observed with X-ray diffraction and transmission electron microscope. FTIR confirms the bonding interaction of Co²⁺ in TiO₂ lattice framework. The UV absorption spectra of the doped material shows two absorption peaks in the visible region related to d–d electronic transitions of Co²⁺ in TiO₂ lattice. Compared to undoped TiO₂ nanoparticles, the cobalt doped samples show a red shift in the band gap. Steady state photoluminescence spectra give emission peaks related to oxygen defects. The decrease in the intensity ratio of UV/visible emission peaks confirms distortion of structural regularity and formation of defects after doping. The intensity ratio of different visible emission peaks is nearly same for undoped and 0.02 Co²⁺. However, this ratio decreases profoundly at 0.04 Co²⁺, due to concentration quenching effect. Photoluminescence excitation spectra, recorded at 598 nm emission wavelength, give different excitation peaks associated with oxygen vacancies and Co²⁺. Time resolved photoluminescence spectra give longer decay time for doped samples, indicating longer relaxation of conduction band electrons on the defect and on dopant sites.     

Low-temperature synthesis of T′-La2CuO4 using CaH2 as reductant

We have successfully synthesized polycrystalline bulk samples of La₂CuO₄ with the Nd₂CuO₄-type (T′) structure at low temperatures. First of all, La₂CuO₄ with the K₂NiF₄-type (T) structure was prepared by the conventional solid-state reaction. Secondly, La₂CuO_3.5+δ with the Sr₂CuO₃-type structure was obtained by heating mixed powder of T-La₂CuO₄ and CaH₂ at 175 °C in an evacuated Pyrex tube. Thirdly, T′-La₂CuO_4+δ was prepared by heating La₂CuO_3.5+δ at 250 °C in air. Finally, T′-La₂CuO_4+δ was reduced using CaH₂ to remove excess oxygen. The powder X-ray diffraction pattern has revealed that the product is of the single phase of T′-La₂CuO₄, but no superconductivity has been observed at T > 2 K in the magnetic susceptibility measurement. The residual excess oxygen may suppress the appearance of superconductivity.     

The influence of the matrix element on angle-resolved photoemission spectra of insulating cuprates

Microscopic model calculations of the matrix element of a dipole moment are carried out in terms of the cluster model. This matrix element determines the transition probability of electron photoemission from a one-electron orbital with symmetry γμ to a free state. The effect of the matrix element on the angular and polarization dependences in the angle-resolved photoemission spectra of insulating cuprates, such as Sr₂CuO₂Cl₂ and Ca₂CuO₂Cl₂, is analyzed under the assumption of a well-isolated ground state of the two-hole CuO ₄ ^5? cluster, namely, the Zhang-Rice singlet. The angular k dependence of the matrix element gives rise to effects, such as the residual Fermi surface, which are typical of metallic systems. An analysis of the experimental data reveals the presence of another electronic state (with different symmetry) in the vicinity of the Zhang-Rice singlet.     

Spectral characteristics and fluorescence quenching of N,N′-bis(4-pyridyl)-3,4:9,10-perylenebis(dicarboximide) (BPPD)

The photophysical properties such as electronic absorption, molar absorptivity, emission spectra, fluorescence quantum yield and fluorescence lifetime of N,N′-bis(4-pyridyl)-3,4:9,10-perylene bis(dicarboximide) (BPPD) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity, while the fluorescence quantum yield (?_f) is solvent dependent. The ground state geometry has been computed by using density functional theory (DFT), the transition from HOMO to LUMO from perylene core with maximum absorption at 512 nm and HOMO–LUMO energy difference equal 2.53 eV. BPPD dye undergoes molecular aggregation to dimmer or higher aggregates in dimethyl sulfoxide (DMSO). Crystalline solids of BPPD gives excimer-like emission at 676 nm. The fluorescence quenching of BPPD is also studied using hydrated ferric oxide nanoparticle (FeOOH), and the Stern–Volmer rate constants (K_sv) were calculated as 8×10⁶ and 9.2×10⁶ M^?1 in ethanol and ethylene glycol, respectively.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Investigation on Tm3+-doped silicate glass for 1.8 μm emission

A Tm³⁺-doped silicate glass (SiO₂–CaO–Na₂O–K₂O) with good thermal stability is prepared by the melt-quenching method. Intense 1.8 μm emission is obtained when pumped by an 808 nm laser diode. Based on the measured absorption spectra, radiative properties are predicted using Judd–Ofelt theory and Judd–Ofelt parameters Ω_λ (λ=2, 4, 6), as well as absorption and emission cross-sections are calculated and analyzed. The difference between the measured Tm³⁺:³F₄ lifetime and the calculated lifetime is also discussed. The emission property together with good thermal property indicates that Tm³⁺-doped silicate glass is a potential kind of laser glass for efficient 2 μm laser.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Characteristics of nitrogen-incorporated silicon oxycarbide films and plasmas for plasma enhanced chemical vapor deposition with TMOS/N2/NH3

Plasma enhanced chemical vapor deposition of nitrogen-incorporated silicon oxycarbide thin films obtained from the gas mixture of TMOS (tetramethoxysilane), N₂, and NH₃ is studied. The effects of the TMOS to N₂ pressure ratio on the properties of the film and the plasma are investigated. The deposited films are analyzed by in situ ellipsometry, ex situ Fourier transform infrared spectroscopy (FTIR), and by X-ray photoelectron spectroscopy (XPS). The plasma is characterized by using optical emission spectroscopy (OES). The mass spectra of the constituents in the plasma are obtained by quadrupole mass spectroscopy. The correlation between the film properties and the plasma characteristics is explained wherever possible. As the partial pressure of N₂ is decreased, the refractive index begins to decrease, reaches a minimum, and then saturates. The FTIR absorption bands are observed from about 850 to 1000 cm⁻¹ and from 1000 to 1250 cm⁻¹, and can be attributed to the formation of a nitrogen-incorporated silicon oxycarbide thin film. The variation of the refractive index is discussed in relationship with the deposition rate, the OES spectra, the mass spectra of the plasma, the film composition obtained by XPS, and FTIR spectra.     

Hole superconductivity in Nd2 − zCezCuO4

The reclassification of Nd_2 − zCe_zCuO₄from an electron superconductor to a hole superconductor is suggested by simple valence and electrostatics arguments: In Nd_2 − zCe_zCuO₄, isolated Ce substituting for Nd is not an electron donor; the proposed actual dopant is a defect-complex of Ce and interstitial oxygen which provides holes, making Nd_2 − zCe_zCuO₄a hole superconductor.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

The spectroscopic properties of Yb3+ doped α-BBO crystal

2.0 mol% (relative to Ba²⁺) Yb³⁺ doped α-BaB₂O₄ (α-BBO) crystal was obtained by the Czochralski method. The doped crystal structure was determined by means of an X-ray diffraction analysis. The absorption, near-infrared (NIR) luminescence spectra and fluorescence decay curve of Yb³⁺ doped α-BBO crystal were investigated. NIR emission under 940 nm and 980 nm LDs (laser diodes) excitation was observed in the Yb doped α-BBO crystal.     

Interfacial reactions of Ba2YCu3O6+z with coated conductor buffer layer,LaMnO3

Chemical interactions between the Ba₂YCu₃O_6+x superconductor and the LaMnO₃ buffer layers employed in coated conductors have been investigated experimentally by determining the phases formed in the Ba₂YCu₃O_6+x–LaMnO₃ system. The Ba₂YCu₃O_6+x–LaMnO₃ join within the BaO–(Y₂O₃–La₂O₃)–MnO₂–CuO_x multi-component system is non-binary. At 810 °C (p_O2 = 100 Pa) and at 950 °C in purified air, four phases are consistently present along the join, namely, Ba_2?x(La_1+x?yY_y)Cu₃O_6+z, Ba(Y_2?xLa_x)CuO₅, (La_1?xY_x)MnO₃, (La,Y)Mn₂O₅. The crystal chemistry and crystallography of Ba(Y_2?xLa_x)CuO₅ and (La_1?xY_x)Mn₂O₅ were studied using the X-ray Rietveld refinement technique. The Y-rich and La-rich solid solution limits for Ba(Y_2?xLa_x)CuO₅ are Ba(Y_1.8La_0.2)CuO₅ and Ba(Y_0.1La_1.9)CuO₅, respectively. The structure of Ba(Y_1.8La_0.2)CuO₅ is Pnma (No. 62), a = 12.2161(5) Å, b = 5.6690(2) Å, c = 7.1468(3) Å, V = 494.94(4) Å³, and D_x = 6.29 g cm^?3. YMn₂O₅ and LaMn₂O₅ do not form solid solution at 810 °C (p_O2 = 100 Pa) or at 950 °C (in air). The structure of YMn₂O₅ was confirmed to be Pbam (No. 55), a = 7.27832(14) Å, b = 8.46707(14) Å, c = 5.66495(10) Å, and V = 349.108(14) Å³. A reference X-ray pattern was prepared for YMn₂O₅.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Investigation of electronic states of infinite-layer SrFeO2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO₂ epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3d_xy, 3d_xz + 3d_yz, and 3d_x²_–y². In addition, the indirect bandgap value of the SrFeO₂ film was determined to be 1.3 eV by transmission and absorption spectroscopies.     

Synthesis,characterization and magnetic properties of lightly doped La2−xSrxCuO4 (x = 0.04) nanoparticles

Lightly doped La_2−xSr_xCuO₄ (x = 0.04) nanoparticles with different particle sizes have been successfully prepared by a sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared transmission (IR) spectra and superconducting quantum interference device (SQUID) magnetometer. All samples are single phase and have an orthorhombic unit cell. As the particle size reduces, it is found that the IR band at around 685 cm⁻¹ corresponding to the in-plane Cu–O asymmetrical stretching mode shifts to higher frequency and the magnetization exhibits a large enhancement at low temperature. The magnetic susceptibility of all samples follows a modulated Curie law between ∼20 K and ∼100 K and the Curie constant displays a strong dependence on the particle size. It is suggested that as the particle size decreases surface effects should play an important role in the magnetic properties of the nanoparticles.     

Relationship between structural phase transition and carrier concentration in La2−x−ySrxRyCuO4 (R = Ce or Tb)

We have studied the relationship between the crystal structure and the carrier concentration in La_2?x?ySr_xCe_yCuO₄ by low-temperature X-ray diffraction method. The analysis for the [1 1 0]_t peak of the tetragonal index confirms that the high-temperature tetragonal phase changes to the low-temperature orthorhombic one in both La_1.89Sr_0.11CuO₄ and La_1.88Sr_0.11Ce_0.01CuO₄. We have also examined the effects of Tb substitution for La-site on the superconductivity and the structure in La_2?x?ySr_xTb_yCuO₄. A dip of the critical temperature T_c(x) due to the 1/8 anomaly and a maximum of T_c(x) at the optimum carrier concentration do not depend on the Tb concentration. This result suggests the possibility that Tb is introduced as the trivalent ion for x = 0.07–0.18.     

Superconductivity in bulk T′-(La,Sm)2CuO4 prepared via a molten alkaline hydroxide route

We have synthesized La_2?xSm_xCuO₄ (0 ? x ? 2.0) with the Nd₂CuO₄ structure via a molten alkaline hydroxide route at temperatures as low as 400–480 °C. After reduction heat treatment in vacuum at 600–750 °C for removal of excess oxygen atoms at the interstitial apical site, superconductivity with T_c = 20–24 K was observed in the samples with x = 0.05–1.0. The superconducting volume fraction is nearly 100% for x = 0.3–0.7. Our results demonstrate that La_2?xSm_xCuO₄ with no nominal carrier doping is a bulk superconductor.     

Magnetic and dielectric study of Bi2CuO4

Magnetic and dielectric properties have been investigated for Bi₂CuO₄, which has the same chemical formula as that of the parent materials of cuprate superconductors R₂CuO₄ (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu²⁺ spins at ～42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100–500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La₂CuO₄. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions.