Transient behaviour of the mechanoluminescence induced by impulsive deformation of fluorescent and phosphorescent crystals

When a crystal is fractured impulsively by the impact of a moving piston, then initially the mechanoluminescence (ML) intensity increases quadratically with time, attains a peak value and later on it decreases with time. Considering that the solid state ML and gas discharge ML are excited due to the charging and subsequent production of electric field near the tip of moving cracks, expressions are derived for the transient ML intensity I, time t_m and intensity I_m corresponding to the peak of ML intensity versus time curve, respectively, the total ML intensity I_T, and for fast and slow decays of the ML intensity. It is shown that the decay time for the fast decrease of the ML intensity after t_m, is related to the decay time of the strain rate of crystals, and the decay time of slow decay of ML, only observed in phosphorescent crystals, is equal to the decay time of phosphorescence. The value of t_m decreases with the increasing impact velocity, I_m increases with the increasing impact velocity, and I_T initially increases and then it tends to attain a saturation value for higher values of the impact velocity. The values of t_m, I_m and I_T increase linearly with the thickness, area of cross-section and volume of the crystals, respectively. So far as the rise, attainment of ML peak, and fast decay of ML are concerned, there is no any significant difference in the time-evolution of solid state ML, gas discharge ML, and the ML emission consisting of both the solid state ML and gas discharge ML. From the time-dependence of ML, the values of the time-constant for decrease of the surface area created by the movement of a single crack, the time-constant for the decrease of strain rate of crystals, and the decay time of phosphorescence of crystals can be determined. A good agreement is found between the theoretical and experimental results. The importance of fracto ML induced by impulsive deformation of crystals is discussed.     

Thermal quenching of electroluminescence assisted by an electric field in thin films of tris-(8-hydroxyquinolinato) aluminum (III)

Temperature dependence of the electroluminescence (EL)-current efficiency of tris-(8-hydroxyquinolinato) aluminum (III) (Alq₃)-based organic light-emitting diodes (OLEDs) operated at a constant current density was investigated. The effects of temperature and electric field on photoluminescence (PL) efficiency of Alq₃ thin layers were also investigated. On the basis of these results, it was found that the EL efficiency decreases more markedly with increasing temperature than does PL efficiency. The temperature dependence of the EL efficiency can be interpreted in terms of the thermal dissociation of excitons that is assisted by the electric field.     

Equilibrium spin current induced by spin-orbital interaction in a quantum dot system

We report a theoretical study of the equilibrium spin current flowing in a quantum dot system. Two electrodes are the two-dimensional electron gas with Rashba or Dresselhaus spin-orbital interaction. By using the Keldysh Green's function technique, we demonstrated that a nonzero spin current can flow in the system without bias. At the weak coupling between electrodes and the quantum dot, the spin current is approximately proportional to the cross product of two average pseudo-magnetizations in two electrodes, which agrees with the result of the linear response theory; whereas at the opposite case, the strong coupling between the quantum dot and electrodes can lead to a non-sinusoidal behavior of the equilibrium spin current. These behaviors of the equilibrium spin current are similar to the Josephson current.     

Optical Defect in GaN-Based Laser Diodes Detected by Cathodoluminescence

GaN-based laser diodes （LDs） with 399 nm wavelength are grown on sapphire substrates by metal organic chemical vapour deposition （MOCVD）. Electroluminescence spectra of the fabricated LDs show that the LDs from some grown wafers failed to emit laser. The SEM and XRD results show the similar surface morphology and interface qualities of multi quantum wells （MQWs） and super-lattices between LDs that succeed and fail to emit laser. However, the cathodoluminescence （CL） measurements reveal a kind of optical defect rather than structural defect in un-emitted LDs. Further depth-dependent CL imaging observation indicates that such optical defects originate from the MQWs to the surface of LDs as a non-irradiative recombination centre that should cause the failure of laser emitting of LDs.     

Thermoluminescence in ZrO2 with impurity of ZnO induced by UV radiation

ZrO₂ pellets doped by ZnO after 302 nm UV irradiation have been studied for ThermoLuminescent (TL) glow. The TL peak at 90°C for the 1100°C sintered ZrO₂ pellet shifted to 85°C with intensity increased three times for the 1% ZnO doped ZrO₂ sintered at 1100°C. The peak intensity at 210°C for the doped one is only one tenth of the undoped one. The emission spectra of thermoluminescence for undoped and ZnO-doped ZrO₂ revealed that the effect of ZnO doping is to increase the number of luminescent centers. The trapping center associated to the 90°C TL peak is explained by the similar model as that of Kirsh et al. For the case of 210°C TL peak, we have proposed two different models of trapping centers; one is the Zr⁴⁺ in an asymmetrical oxygen arrangement, and the other is the defect complex formed from an oxygen vacancy and an anion.     

Application of electrical discharges at the surface of a ferroelectric material for exciting the thermoluminescence of crystals

A plasma generated at the surface of a ferroelectric ceramic has been applied for exciting the thermoluminescence of the KCl, Gd₃Ga₅O₁₂ (GGG), Al₂O₃, Al₂O₃Cr, and Y₃Al₅O₁₂ (YAG) crystals. It has been found that this excitation technique is very useful in studying the trap centers.     

The origin of luminescence accompanying electrochemical reduction or chemical decomposition of peroxydisulfates

The spectral features of the electrochemiluminescence occurring during the reduction of peroxydisulfate anions at magnesium, silver or platinum electrodes (ecl), the luminescence following their decomposition on a magnesium surface (mcl), and the chemiluminescence accompanying the thermal decomposition of peroxydisulfates in acidic media (tcl), were thoroughly examined in order to discover the origin of the light emission. The intensity of emission followed the order ecl>mcl?tcl and depended on the method of its generation and other experimental conditions. Probable pathways of the reactions leading to the formation of light-emitting species were examined at the density functional theory level. The theoretical studies and experimental findings seem to indicate that the luminescence originates from ¹O₂, ¹(O₂)₂ and ³(O₂)₂, the precursors of which are SO₄^•−, HO^•, HOO^•, HOOH and O₃ formed in the primary and secondary processes following electrochemical reduction or thermal decomposition of peroxydisulfates. Supplementary experiments demonstrated the participation of HOOH in the generation of light emitting entities.     

Discrete-charge quantum circuits and electrical resistance

From the theory of quantum LC circuits with discrete charge, and semiclassical considerations, we obtain approximate energy eigenvalues, depending on the parameter . Next, we include electrical resistance for the quantum RLC circuit, obtaining a relation that strongly reminds us of the Landauer formula.     

Long-term accelerated current operation of white light-emitting diodes

Long-term degradation tests regarding white light-emitting diodes based on InGaN were performed under accelerated current conditions, and the half-life of the light's output was estimated. An estimated mean half-life of 1.5×10⁴ h was obtained under the recommended 20-mA operating condition. The change in the emission spectrum was found to be slight, and the color quality was considered generally satisfactory over the long term.     

Structural change of radiation defects in graphite crystals induced by STM probing

It was found that STM (scanning tunneling microscopy) images of defects in highly oriented pyrolytic graphite introduced by bombardment of 400 eV Ar⁺ ions in ultra-high vacuum exhibit substantial changes in the course of STM probing. Detailed examination of abrupt changes in the tunneling current measured at defect sites during voltage scans shows that the primary cause of the defect-image change was found to be neither the injected current nor the injected power but the absolute value of the voltage applied between the probe tip and the sample. We propose that an electric polarization induced force attracting the sample surface toward the probe tip widens the layer spacing of the graphite surface, leading to an acceleration of the lateral diffusion of interstitial atoms introduced by the ion irradiation, which results in a change in the defect structures and the accompanying electronic structures sensible in the STMimaging. Received: 14 June 2001 / Accepted: 7 September 2001 / Published online: 20 December 2001     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

Tunneling recombination luminescence under excitation of PbWO4:Mo crystals in the defect-related absorption region

Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO₄:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed.     

Thermoluminescence glow curves of CaF2: Dy crystals irradiated by soft X-rays

Applying a deconvolution of the thermoluminescence glow curves, parameters of single glow peaks of CaF₂: Dy TLD 200 dosemeters irradiated by soft X-rays were determined. A dependence of the height ratio of low temperature (T393, 413 and 473 K) single peaks on energy of absorbed photons was measured in a region of 1–22.2 keV. Standard radionuclides¹⁰⁹Cd,²³⁸Pu,⁵⁵Fe and iodine laser produced aluminium plasma (T _e 500 eV) were used as soft X-ray sources. The ratios of the heights of different single peaks are discussed with respect to high local doses. The decreasing ratio of the heights of the first and third and/or second and third peak with increasing photon energy allows to determine reversely a mean photon energy of absorbed soft X-ray radiation.     

Strong mechanoluminescence induced by elastic deformation of rare-earth-doped strontium aluminate phosphors

When rare-earth-doped strontium aluminate phosphor mixed in an epoxy resin, is deformed elastically by applying a uniaxial pressure, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. After t_m, initially, the ML intensity decreases exponentially at a fast rate and then it decreases exponentially at a slow rate. The ML appears after a threshold pressure and then, initially at low pressure, the peak intensity I_m of ML increases linearly with the magnitude of applied pressure, and for high pressure, I_m increases exponentially with the magnitude of applied pressure. The value of I_m increases linearly with the density of filled hole traps. The ML emission also takes place during the release of applied pressure. There should be a significant effect of temperature on the ML intensity of rare-earth-doped strontium aluminate phosphors. The ML intensity of rare-earth-doped strontium aluminates decreases with successive number of the applications of pressure and the diminished ML intensity can be recovered with the exposure of the samples to UV-radiation. The ML spectra of rare-earth-doped strontium aluminate phosphors are similar to their photoluminescence spectra. As only the piezoelectric-phase of the strontium aluminate phosphors exhibit ML during their elastic deformation, the ML emission can be attributed to the piezoelectrification of the crystals. Considering that the piezoelectric field causes decrease in the trap-depth of the hole traps and, therefore, the holes transferred from traps to the valence band recombine with (Eu¹⁺)^* ions, whereby the Eu²⁺ ions are excited, expressions are derived for different parameters of ML, which are able to satisfactorily explain the experimental results. It is shown that the lifetimes of holes in the shallow traps in stressed and unstressed materials, and the threshold pressure P_t for the ML emission, and other parameters of the ML, can be determined from the ML measurements. Finally, the criteria for tailoring strong elasico-mechanoluminescent materials are explored.     

Time-resolved spectroscopy and energy transfer in Pr3+-doped Gd2(SO4)3·8H2O

The luminescence of Pr³⁺ in gadolinium sulfate hydrate is reported for 195 nm laser excitation. the Pr³⁺ ion acts as a sensitizer of the host lattice emission. The Pr³⁺Gd³⁺ energy transfer occurs in two different ways. In the octahydrate the transfer occurs from the lowest component of the 4f5d configuration of Pr³⁺, but in the samples with less water there is energy transfer from the¹ S _o level of Pr³⁺ to several Gd³⁺ levels. The Pr³⁺ emission in the two modifications is, of course, also strikingly different.Deceased 24 December 1994     

Heteroepitaxial growth of oxides on sapphire induced by laser radiation in the solid–liquid interface

2 O₃, Fe₂O₃ and MnO₂ on sapphire from an aqueous solutions of either CrO₃, FeCl₃, or KMnO₄, respectively, under laser irradiation of the interface sapphire/liquid. The interface is exposed through the sapphire substrate to the radiation of a copper vapor laser (wavelength of 510 nm). The etching of sapphire is accompanied by the deposition of oxide films, which are shown to grow epitaxially on the sapphire substrate, while the deposition of the polycrystalline oxide film occurs on a glass substrate under the same experimental conditions. Similarly, the epitaxial growth of cubic Fe₂O₃ and orthorhombic MnO₂ is observed, though their crystallographic structure is different from the hexagonal structure of sapphire. Received: 26 June 1997/Accepted: 7 July 1997     

Fabrication of Al nanoparticles and their electrical properties studied by capacitance-voltage measurements

Nanometer-scale Al particles are fabricated and are embedded in a GaAs matrix using molecular beam epitaxial technique. The Al particle is self-assembled on GaAs by supplying an Al molecular beam. The average particle size is found to be 25 nm. The density is 7 × 10¹⁰ cm⁻² when Al of 6.2 × 10¹⁵ atoms/cm² is supplied on (1 0 0)GaAs at a substrate temperature of 300 °C. Clear hysteresis and plateaus in capacitance-voltage (C-V) curves are found in an Al-embedded sample, whereas monotonic increase of capacitance is obtained in a reference sample having an AlAs layer instead of Al. This difference results from trapping of electrons by the Al particles, suggesting that the particles have metallic character.     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.     

A novel method for extracting oscillator strength of select rare-earth ion optical transitions in nanostructured dielectric materials

A new technique to obtain the oscillator strength of select rare-earth optical transitions in nanostructured dielectric materials (nanophosphors) is presented. It is based on the experimentally observed nanophosphor lifetime dependence on the embedding medium. A constant oscillator strength and parity-allowed electric dipole transitions of the RE ion emission are assumed. The oscillator strength is obtained from the slope of the 1/τ_ij vs. n(n²+2)² plot, where τ_ij is the radiative lifetime of transition between states i and j, and n is the index of refraction of the embedding medium. The use of the technique is illustrated for the Y ₂SiO₅:Ce nanophosphor.     

Dependence of the anomalous fading of the TL and blue-OSL of fluorapatite on the occupancy of the tunnelling recombination sites

The anomalous fading (AF) of thermoluminescence (TL) and optically stimulated luminescence (OSL) signals in Durango apatite is attributed to tunnelling effects. Electrons from the TL and OSL traps in this material are transferred, via a tunnelling effect, to the recombination sites. The availability of recombination sites for tunnelled electrons is of major importance for the degree of AF rate observed in this material. It is expected that a variation of the number of the electron recombination sites will be reflected in the experimentally measured AF rate. In the present work an investigation of the recombination sites for the tunnelled electrons is attempted by studying the AF effect using a special technique, in which the anomalously faded TL (OSL) is replaced by an equal amount of TL (OSL) induced by a beta dose.