Luminescence Studies of the trans-[Cr–Cyclam(CN)2]ClO4(Cyclam = 1,4,7,11-Tetraazacyclotetradecane) Complex: Its Luminescence Spectra in Several Solvents and the Concentration Dependence of Its Luminescence Lifetime

The luminescence lifetime (_lum) and the luminescence spectra of the title complex (I) were determined in eight solvents. We found that in spite of former assumptions, the _lum of I depends on the concentration of the complex. We determined the self-quenching (k _Q) and Stern–Volmer (K _SV) constants, and the relative values of luminescence quantum yields in eight solvents. There was no measurable self-quenching in 1,2-ethanediol and N,N-dimethylformamide; dynamic self-quenching was found (the concentration dependence of _lum and _lum was the same) in N,N-dimethylacetamide, dimethylsulfoxide, water, propylene–carbonate, and pyridine. In acetonitrile we found both dynamic and static self-quenching based on the different concentration dependence of _lum and _lum of I; K _SV and the association constant of I in acetonitrile were computed     

Luminescence spectra and crystal structure of high-temperature superconductors

A study is reported of the effect of temperature (4.2 K≤T≤150 K), the content of weakly bonded oxygen [6.1≤(7?δ)≤6.9], illumination [E _exc=3.4 eV, (Φt)_max～1.5 ×10²⁰ photon cm^?2, and substitution (Cu → Ag, Cr, Fe, Mn) on the luminescence spectra of the high-T _c superconductor YBa₂Cu₃O_7?δ. Only two bands with E _lum～2.4 and ～2.8 eV were observed in the luminescence spectra in all these cases. A clearly pronounced correlation between the electronic and structural changes in YBa₂Cu₃O_7?δ, caused by the influence of temperature, illumination, doping with oxygen or metal ions, and the spectral parameters (peak position E _lum, width Δλ _lum, and intensity I _lum of the luminescence bands), has been established. It is shown that luminescence spectra of HTSCs can be employed as a fairly reliable optical probe to study the electronic processes occurring in these substances, in particular, the electron (hole) transfer between the CuO₂ plane and the CuO_1?δ chain plane serving as a charge reservoir.     

Deactivation pathways of the electronic excitation of ions of lanthanide complexes in polymers with functional groups

Complexes Eu(TTA)₃phen and Eu(MBTA)₃phen, as well as complexes Tb(MBTA)₃phen and Tb(TTA)₃phen, which do not luminesce in solutions, are shown to luminesce in polymer films (TTA is thenoyltrifluoroacetone, MBTA is n-methoxybenzoyltrifluoroacetone, and phen is o-phenanthroline). Luminescence of complexes of Eu and Tb in films of a polymer, poly(methylene-bis-anthranilamide) 1,6-hexamethylenedicarboxylic acid (PAA-5), having a high concentration of functional anthranilate groups, is studied. From the behavior of the luminescence intensity (I _lum), the luminescence decay time, and the luminescence spectra of complexes of these lanthanides in polymer films, the following regular features were revealed. (i) During the film preparation at 90°C, Ln complexes are attached to PAA-5 via anthranilate groups. (ii) Irradiation of these films in the range of the absorption band of ligands (TTA or MBTA) leads to deactivation of the electronic excitation of ions according to the diketone detachment mechanism and to further binding of complexes to polymers. In this case, I _lum(Eu(III)) decreases because the introduction of anthranilate groups of the polymer into the first coordination sphere of Eu(III) complexes enhances the nonradiative deactivation of these ions, whereas I _lum(Tb(III)) increases since the introduction of these groups suppresses the nonradiative deactivation of Tb complexes through triplet states of ligands (TTA and MBTA). (iii) Upon storage of films in the dark (20°C), complexes detach themselves from the polymer and return to their initial structure. In PAA-5 films into which Eu and Tb complexes were simultaneously introduced, the color of the emission from the irradiation spot changes from red to green.     

Spin density functional theory of the temperature-dependent spin susceptibility: Pd and Pt

The finite temperature spin density functional (SDF) formalism is used to derive a variational expression for the temperature-dependent spin susceptibility χ_p(T) of an inhomogeneous electron gas. The use of a simple trial function in the variational expression results in a Stoner form for χ_p(T), i.e., χ_p(T) ≥ χ_s(T)/[1 - I(T) χ_s (T)] where χ_s(T) is the single-particle spin susceptibility including exchange-correlation (XC) effects on the band structure within the framework of the density functional formalism and I(T) is the SDF analog of the Stoner parameter. It should be emphasized that this form for χ_p(T) is derived for a general XC free energy functional F_xc[n, m; T]. χ_s(T) has been calculated self-consistently, including relativi stic effects, using the local approximation for F_xc for Pd and Pt. These results have been used to investigate the temperature dependence of I(T) required to explain their experimental susceptibilities. It is found that when the spin-orbit interaction is included in the calculation of χ_s(T), the temperature variation of I(T) is stronger than that of χ_s(T).     

Visible photoluminescence in porous GaAs capped by GaAs

A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I₁ and I₂, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I₁ and I₂ PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I₁ PL band exhibits a red shift while the I₂ PL band exhibits a blue shift with increasing injection levels. The I₂ PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I₂ PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states.     

Fast luminescence processes in KI : In caused by excitation in the a absorption band

The temperature dependence of the decay time (τ) of the luminescence pulses from the 440 nm emission band as well as the rise time (τ_R) of luminescence pulses from the 575 nm emission band has been investigated for KI : In by pulsed light excitation in the A absorption band. From the agreement of τ and τ_R independent of the temperature it can be concluded that the X minimum on the ³T₁₄ adiabatic potential energy surface (APES) is mainly populated by a radiationless transition from the tetragonal T minimum of the same APES.Furthermore, a comparison of parameters for the impurity centre luminescence corresponding to the tetragonal emission band in KI : In and KI : Tl has been drawn.     

Mechanisms of direct microwave absorption in Y1Ba2Cu3O7−x

Direct microwave absorption was studied as a function of magnetic field up to 8 kG and in a temperature range between 10 K and 150 K in Y₁Ba₂Cu₃O_7−x. Large microwave absorption was observed below T_c over a wide magnetic field range. The microwave power absorption is approximately described by I=A(T) H+C(T), where A(T) and C(T) are temperature dependent constants. A(T) is proportional to the square root of the microwave power, while C(T) shows a different behaviour. We propose different origins for A(T) and C(T), namely fluxons in grains and weak Josephson links. Moreover, also flux trapping was observed in the superconducting state. Possible mechanisms for the absorption are discussed.     

Creation of excitons and defects in a CsI crystal during pulsed electron irradiation

Data presented on the influence of the temperature in the range 80–650 K on the spectral kinetics of the luminescence and transient absorption of unactivated CsI crystals under irradiation by pulsed electron beams (〈E〉=0.25 MeV, t _1/2=15 ns, j=20 A/cm²). The structure of the short-wavelength part of the transient absorption spectra at T=80–350 K exhibits features, suggesting that the nuclear subsystem of self-trapped excitons (STE’s) transforms repeatedly during their lifetime until their radiative annihilation at T⩾80 K, alternately occupying di-and trihalide ionic configurations. It is established that a temperature-induced increase in the yield of radiation defects, as well as F and H color centers, and quenching of the UV luminescence in CsI occur in the same temperature region (above 350 K) and are characterized by identical thermal activation energies (∼0.22 eV). It is postulated that the STE’s in a CsI crystal can have a trihalide ionic core with either an on-center or off-center configuration; the high-temperature luminescence of CsI crystals is associated with the radiative annihilation of an off-center STE with the structure (I⁻(I⁰I⁻ e ⁻))*. Fiz. Tverd. Tela (St. Petersburg) 40, 640–644 (April 1998)     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0 0 0 1) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800 °C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (0 0 2) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800 °C in N₂. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (V_O) and interstitial oxygen (O_i). The biggest ratio of the PL intensity of UV emission to that of visible emission (I_NBE/I_DLE) is observed from ZnO thin films annealed at 800 °C in N₂. Therefore, we suggest that annealing temperature of 800 °C and annealing atmosphere of N₂ are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.     

Luminescence of fluctuation tails of disordered solid solutions

The form of the stationary luminescence spectra of excitons, localized by composition fluctuations, in disordered solid solutions under weak excitation is calculated. The tail states for which there are no nonradiative transition channels are distinguished by means of continuum percolation theory. Such states are responsible for the “zero-phonon” luminescence band. The shape of the short-wavelength luminescence band edge is determined mainly by the number of isolated localizing clusters and their smallest complexes, which decreases rapidly near the mobility threshold. The real luminescence spectrum is due to the simultaneous emission of phonons. The phonon emission determines the form of the long-wavelength wing of the emission band. The computed shape of the emission spectrum is compared with the experimental luminescence spectra of the solid solution CdS_(1−c)Se_c. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 274–279 (10 February 1997)     

Temperature dependences of the order parameter for sodium nitrite embedded into porous glasses and opals

The temperature dependences of the order parameter η(T) for sodium nitrite NaNO₂ embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO₂ in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.     

Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co by time-resolved spectroscopy

This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl₂O₄:Co²⁺. Besides the well-known fourfold-coordinated Co²⁺(T_d) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr³⁺(O_h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co²⁺(T_d) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for T_d systems is related to the excited-state crossover (⁴T₁↔²E), making the emission band to transform from a narrow-like emission from ²E at low temperature to a broad structureless band from ⁴T₁ at room temperature.     

Current-voltage characteristics of YBa2Cu3O7−x single crystals and Kosterlitz-Thouless transition

The nonlinear I-V characteristic (V(I)) of YBa₂Cu₃O_7−x single crystal was investigated near the transition from the resistive to the superconducting state in the absence of a magnetic field. A modulation Fourier analysis at temperature T* (the maximum of the amplitudes of the higher (n>1) harmonics of the response voltage) was used to determine an analytic dependence V(I) which accurately describes the experimental results (direct measurements and harmonics) in the range of currents I<30 mA (j<310 A/cm²). It is shown that at T* the power approximation of the I-V characteristic V∼I ³ is only found in the low current density limit (j≪j ₀=140 A/cm²). The results are interpreted in terms of the Kosterlitz-Thouless (KT) transition model. It is established that T* corresponds to the temperature of the KT transition T _KT, which means that T _KT can be determined directly. The deviation of V(I) from a power dependence is caused by the nonlogarithmic variation of the vortex interaction energy as a function of the distance between them. Fiz. Tverd. Tela (St. Petersburg) 40, 202–204 (February 1998)     

Luminescence spectroscopy and electronic structure of ZrP2O7 and KZr2(PO4)3 crystals

The photoluminescence (PL) of ZrP₂O₇ and KZr₂(PO₄)₃ phosphate crystalline micro-powders grown by spontaneous crystallization method is studied under vacuum ultra-violet (VUV) synchrotron radiation excitations (4–20 eV region of excitation photon energies) in 8–300 K temperature region. The electronic structures (partial densities of states) and optical absorbance spectra of the crystals are calculated by the Full-Potential Linear Augmented Plane Wave Method. Both phosphate crystals reveal PL emission band in the UV spectral region peaking near 300 and 295 nm for ZrP₂O₇ and KZr₂(PO₄)₃ respectively. The spectral profile of the band weakly depends on temperature. The excitation spectra of the UV emission in each crystal contain intensive excitation band peaking at 189 and 182 nm for ZrP₂O₇ and KZr₂(PO₄)₃ respectively. The excitation band of the UV emission is related to band-to-band electronic transitions with charge transfer from O 2p to Zr 4d states. The energy band gaps E_g of ZrP₂O₇ and KZr₂(PO₄)₃ are estimated as 6.7 and 6.6 eV respectively.     

Photoluminesence of Iodine‐Doped Crystals of ZnSe

The luminescence spectra and the luminescenceexcitation spectra of ZnSe crystals grown by the Piper–Polish method have been investigated in the temperature interval 81–300 K. An intense edge emission assigned to the presence of iodine has been detected. The analysis of the longwave photoluminescence band by the Alentsev–Fok method has revealed a yellow luminescence band which is attributed to the associative emission centers involving I _Se.     

Steady-state and time-resolved spectroscopy of porous silicon

We present experimental data on steady-state properties, time-resolved properties and on polarization characteristics of porous silicon photoluminescence and models for the decay processes of the red-orange band. The manifold manifestation of inhomogeneous broadening of this band in emission, excitation, polarization, kinetics and degradation supports the model in which porous silicon is treated as a network of crystallites connected via an oxide interface. Spectral inhomogeneties of the red-orange band can be described in terms of varying shape and size of silicon clusters. The polarization of emission is explained by coexistence of dot-like and wire-like entities, i.e. spherical and non-spherical clusters. The relative weight of these species determines the polarization degree, whereas the kinetics are controlled by the transport of excitations among the clusters. The decay is modeled by a modified stretched exponential function with the local lifetime, the migration lifetime, and a scaling factor. The latter is determined by the dimensionality of the space available for migration which was found to be close to but less than unity. On the nanosecond range two distinct bands in the blue-green region are evaluated that need further studies for interpretation. Generally, arguments are proposed in favor of a quantum confinement origin of the red-orange band and a bridge between quantum-wire and quantum-dot models is provided.     

Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l0¹⁹e/cm². In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI ₂ during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section _t(T) T ⁿ withn= –1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.A.B.O.S., on leave from University of Kinshasa, Zaïre     

Nontraditional temperature shift of the fundamental absorption edge for layered substances and the ferroelastic phase transition in Cs3Bi2I9

We have carried out investigations of the exciton reflection spectra of Cs₃Bi₂I₉ layered crystals as a function of temperature. For the first time for the layered substances we have found nontraditional temperature shift of the energy gap E_g(T) described by the Varshni formula. We have registered a transition region in the temperature broadening of the half-width, H(T), of the exciton band with increasing of temperature in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. We have also found a surge in H(T) at the ferroelastic phase transition point