Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2012"

Binary copper oxide semiconductors: From materials towards devices

Abstract: Copper-oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar-cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin-film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin-film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar-cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p-Cu2O/n-AlGaN is much more promising for the application as efficient solar cells than that of p-Cu2O/n-ZnO heterojunction devices that have been favored up to now.

Molecular chirality at surfaces

Abstract: With the adsorption of larger molecules being increasingly tackled by surface scientists, the aspect of chirality often plays a role. This paper gives a topical review of molecular chirality at surfaces and gives a phenomenological overview of different aspects of adsorption and self-assembly of chiral and prochiral molecules and the principles of mirror-symmetry breaking at a surface. After a brief introduction into the history of molecular chirality and the important role it played for understanding the spatial structure of molecules, definitions of chirality are presented. Topics treated here are principle ways to create single chiral adsorbates, chiral ensembles, and monolayers by achiral molecules, adsorption of intrinsically chiral molecules at achiral and chiral surfaces, long-range symmetry breaking in two-dimensional (2D) crystals due to additional chiral bias, chiral restructuring of solid surfaces under the influence of chiral molecules, switching the handedness of adsorbates, and chirality at the liquid/air interface. An outlook onto further potential research directions and recommendations for further reading, including nonsurface-related sources of chiral topics completes this paper.

Auxetic warp knit textile structures

Abstract: The design, manufacturing and characterization of warp knit textile structures with enhanced drapeability and energy absorption is reported in this paper Four textile structures were produced, all based on a triangular or double arrowhead structure, which is known to lead to a negative Poisson's ratio ν Mechanical testing has confirmed that textile structures can be produced which are auxetic at ± 45° to the warp direction, with ν of up to −022 ± 003

Charge Transport Physics of High‐Mobility Molecular Semiconductors

Abstract: This review is focused on understanding of the charge-transport physics of high-mobility organic semiconductors at a molecular level We review recent high-mobility small-molecule and conjugated polymer materials with a focus on crystalline materials that have been able to exceed mobilities of 05–1 cm2/V s We discuss some of the main, competing factors that govern charge transport in these materials and present theoretical approaches that have been developed to describe systems in which moderately strong intermolecular electronic interactions and strong electron–phonon interactions are present Finally, we review recent experimental results that have aimed to address the important question of whether at room-temperature charge carriers in these high-mobility organic semiconductors are in fact simply extended Bloch electrons that undergo occasional scattering processes or are localized on individual molecules and move by hopping

Instability of amorphous oxide semiconductors via carrier-mediated structural transition between disorder and peroxide state

Abstract: The excited holes occupying the valence band tail (VBT) states in amorphous oxide semiconductors (AOS) are found to induce formation of meta-stable peroxide defects. The VBT states are at least partly characterized by the O–O ppσ* molecular orbital, and the localized-hole-mediated lattice instability results in the formation of the peroxide defects. Along with the O–O bond formation, the ppσ* state is heightened up into the conduction bands, and two electrons are accordingly doped in the electronic ground state. The energy barrier from the peroxide state to the normal disorder state is found to be 0.97 eV in hybrid density functional theory. The hole-mediated formation of the meta-stable peroxide defects and their meta-stability is suggested as an origin of the negative bias and/or illumination stress instability in AOS.

Hybrid photovoltaic cells based on ZnO/Sb2S3/P3HT heterojunctions

Abstract: 1 Introduction Increasing demand for energy andenvironmental crisis drive research activities to developunconventional renewable energy resources as alternativesto the energy from fossil sources. One of the ultimatesolutions is solar energy, because of its permanent avail-abilityandenvironmentalcompatibility.Solarenergycanbeconverted into electricity using photovoltaic (PV) cells.Although silicon PV cells [1], especially those based onsingle crystal, show relatively high power conversionefficiency (PCE), their fabrication is hampered because ofthe high production cost. Therefore, development of highlyefficient and cost effective PV devices is the worldwidefundamental objective to satisfy the overall demands forenergy production [2].Organic/inorganic hybrid PV cells have emerged aspotential devices that may satisfy primary demands for theproduction of electricity [3–5]. Organic semiconductorshave advantage in their availability in abundantquantities and simple processing, while the inorganicsemiconductors usually exhibit high charge carriermobility m. The hybrid systems are expected to combinethe merits of both organic and inorganic semiconductors andthus become low-cost alternatives to current PV cells. It hasbeen shown that zinc oxide (ZnO) is a suitable material forconstruction of hybrid solar cells [4, 6, 7]. Particularadvantage of ZnO is its environmental compatibility, highmobility of charge carriers (m¼ 200cm

Mechanical characterisation of a periodic auxetic structure produced by SEBM

Abstract: We present a thorough investigation of the mechanical behaviour of a non-stochastic cellular auxetic structure. A combination of experimental and numerical methods is used to gain a deeper understanding of the mechanical behaviour and its dependence on the geometric properties of the cellular structure. The experimental samples are built from Ti-6Al-4V using selective electron beam melting, an additive manufacturing process giving the possibility to vary the geometry of the structure in a highly controlled manner. The use of finite element simulations and mathematical homogenisation allows us also to investigate off-axis properties of the cellular material. This leads to a more comprehensive understanding of the mechanical behaviour of the auxetics. Ultimately, the gained knowledge can be used to tailor auxetic materials to specific applications.

A three-dimensional rotating rigid units network exhibiting negative Poisson's ratios

Abstract: Materials exhibiting auxetic behaviour get fatter when stretched (i.e. possess a negative Poisson's ratio). This property has been closely related to particular geometrical features of a system and how it deforms. One of the mechanisms which is known to have a potential to generate such behaviour is that of rotating rigid units. Several models based on this concept have been developed, including two-dimensional as well as three-dimensional (3D) models. In this work, we propose a new 3D structure constructed from rigid cuboids which also deform through relative rotation of the units. In particular, analytical models for the mechanical properties, namely the Poisson's ratio and the Young's moduli, are derived and it is shown that for loading on-axis, these systems have the potential to exhibit negative values for all the six on-axis Poisson's ratios.

Origin, secret, and application of the ideal phase‐change material GeSbTe

Abstract: Discovery of the GeSbTe phase-change alloy in particular along the GeTe–Sb2Te3 tie-line took place in the mid-1980s. The amorphous alloys showed ideal properties, for example, high thermal stability at r.t. and laser-induced rapid crystallization with large optical changes. Thereafter, GeSbTe was successively applied to various optical disks such as DVDs and BDs. Through DSC and XRD analyses, the appearance of the metastable phase having a NaCl-type structure was observed over a wide compositional region. This was the “key” to realizing the ideal phase-change properties. During this year, the role of the constituent elements of Ge and Sb became clear by RMC modeling using AXS data at SPring-8, where the “nucleation dominant crystallization process” was well explained. The aspect of the latest Blu-ray Disc (BD) product of Panasonic: GeSbTe phase-change films are utilized in every recording layer. It is seen that the front-side recording layers, L1 and L2, are highly transparent.

Ab initio derived force-field parameters for molecular dynamics simulations of deprotonated amorphous-SiO2/water interfaces

Abstract: We present a set of Coulomb point charges and van der Waals parameters for molecular dynamics simulations of interfaces between natively deprotonated amorphous SiO2 surfaces and liquid water, to be used in combination with standard biomolecular force fields. We pay particular attention to the extent of negative charge delocalisation in the solid that follows the deprotonation of terminal silanol groups, as revealed by extensive Bader analysis of electronic densities computed by density functional theory (DFT). The absolute charge values in our force field are determined from best-fitting to the electrostatic potential computed ab initio (ESP charges). Our proposed parameter set is found to reproduce the energy landscape of single water molecules over neutral and deprotonated amorphous SiO2 surfaces and, after a minor adjustment, over thin oxide films on Si. Our analysis reveals a certain degree of arbitrariness in the choice of the DFT scheme used as the reference for the force-field optimisation procedure, highlighting its intrinsic limits. Interaction between a water molecule and an oxidised Si surface calculated with several DFT and force-field schemes, and delocalisation of the negative charge upon deprotonation of an amorphous SiO2 surface.

Breakdown of Stokes–Einstein relation in the supercooled liquid state of phase change materials

Abstract: The application of amorphous chalcogenide alloys as data-storage media relies on their ability to undergo an extremely fast (10–100 ns) crystallization once heated at sufficiently high temperature. However, the peculiar features that make these materials so attractive for memory devices still lack a comprehensive microscopic understanding. By means of large scale molecular dynamics simulations, we demonstrate that the supercooled liquid of the prototypical compound GeTe shows a very high atomic mobility (D ∼10−6 cm2 s−1) down to temperatures close to the glass transition temperatures. This behavior leads to a breakdown of the Stokes–Einstein relation between the self-diffusion coefficient and the viscosity in the supercooled liquid. The results suggest that the fragility of the supercooled liquid is the key to understand the fast crystallization process in this class of materials.

Analyses on multiple resonance behaviors and microwave reflection loss in magnetic Co microflowers

Abstract: Triple dielectric and magnetic resonance behaviors have been observed and studied in Co microflowers within a frequency range of 0.1–18.0 GHz. To characterize such triple-resonance behaviors, the dielectric resonance was resolved by fitting the Lorentzian dispersion law, magnetic resonance spectra was fitted by the Landau–Lifshitz–Gilbert equation. It is demonstrated that our fitted and calculated resonance frequencies agree well with the experimental data. Dielectric resonance in permittivity was due to the dipole polarization. Magnetic resonance in permeability was due to the natural/exchange resonance. Both the permittivity and permeability determined and contributed to the microwave reflection loss.

Prevalence of first-order kinetics in thermoluminescence materials: An explanation based on multiple competition processes

Abstract: Typical materials used in thermoluminescence (TL) dosimetry exhibit the following common characteristics: (i) the temperature of glow peak maximum of individual glow peaks remains practically constant over a wide dose range, (ii) there are no systematic changes in the glow curve shapes with the irradiation dose, and (iii) higher order kinetics is rarely seen in dosimetric materials, while first-order kinetics is a common occurrence in experimental TL work. Theoretical explanation of these experimental characteristics is an open topic of TL research. In the present work these three characteristics are studied by using several models of increasing complexity. The simplest model studied is based on the empirical analytical general order (GO) expressions, followed by two commonly used models, the well-known one trap one recombination center models (OTOR) and the interactive multiple trap system (IMTS). Previous researchers have studied the behavior of these models using arbitrary values of the kinetic parameters in the models, and by varying these parameters within limited physically reasonable ranges. In this paper, a new method of analyzing the results from such models is presented, in which the average behavior of real dosimetric materials is simulated by allowing simultaneous random variations of the kinetic parameters, within several orders of magnitude. The simulation results lead to the conclusion that the presence of many competitive processes during the heating stage of TL, may be correlated to the remarkable stability of the glow curve shapes exhibited by most materials, and to the prevalence of first-order kinetics. This correlation is demonstrated further by a series of simulations in which the number of competitor traps is increased systematically, by adding up to 12 competitor traps in the IMTS model. As the number of competitor traps increases, the average behavior of the TL glow curves tends progressively toward first-order kinetics, and this in turn results in very small average variations in the shape of the TL glow peak. The simulation results in this paper provide a convincing demonstration and explanation of the stability of the shape of TL glow curves in dosimetric materials, and for the prevalence of first-order kinetics in TL.

Range separated functionals in the density functional based tight-binding method: Formalism†

Abstract: A generalization of the density-functional based tightbinding method (DFTB) for the use with range-separated exchange-correlation functionals is presented. It is base d on the Generalized Kohn-Sham (GKS) formalism and employs the density matrix as basic variable in the expansion of the energy functional, in contrast to the traditional DFTB scheme. The GKS-TB equations are derived and appropriate integral approximations are discussed in detail. Implementation issues and numerical aspects of the new scheme are also covered.

Boehmite (γ‐AlOOH) nanoparticles: Hydrothermal synthesis, characterization, pH‐controlled morphologies, optical properties, and DFT calculations

Abstract: Boehmite nanoparticles (γ-AlOOH) have been successfully synthesized by the hydrothermal method at 180 °C using aluminum nitrate, Al(NO3)3·9H2O, as the aluminum source and sodium metaborate, NaBO2·4H2O, as controlling agent. The size and morphology of boehmite nanoparticles could be controlled by adjusting the pH value of the reaction mixture. The resulting products were characterized by X-ray diffraction (XRD), Fourier transform-infrared spectra (FT-IR), scanning electron microscopy (SEM), UV–Vis spectra, and photoluminescence spectra. The electronic band structure along with density of states (DOS), obtained at the density functional theory (DFT) level indicates that γ-AlOOH has a direct energy bandgap of 4.51 eV. The optical properties, including the dielectric, absorption, reflectivity, and energy-loss spectra of the compound are calculated by the DFT method and analyzed based on the electronic structures.

Manufacture and characterisation of thin flat and curved auxetic foam sheets

Abstract: A new method for the production of thin flat and curved auxetic foam sheets by uniaxial compression is reportted in this paper. Detailed optical microscopy and Poisson's ratio measurement by videoextensometry has revealed that the auxetic behaviour is due to a crumpling of the pores through the foam thickness, caused by a uniaxial compression of 40–60% in combination with shear for the curved foams. Poisson's ratio values of up to −3 have been recorded for the curved foams and −0.3 for the flat sheets.

A comparative study of density functional and density functional tight binding calculations of defects in graphene

Abstract: The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies, and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localized Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone-Wales defects in graphene, as well as various unfunctionalized and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone-Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene.

Effective mechanical properties of concentric cylindrical composites with auxetic phase

Abstract: Materials with unusual mechanical properties can be potentially used as matrices to create high-performance lightweight composites. The appearance of materials with negative Poisson's ratio (auxetics), has led to the evaluation of auxetic composites for possible engineering applications. Because the experimental evaluation of composites with specific properties is expensive and time consuming, computational modelling and simulation provide efficient alternatives to predict the parameters of the composites. In this paper a finite element method was used to find the engineering constants (Young's modulus and Poisson's ratio) of auxetic composites consisting of concentric cylindrical inclusions made of combinations of auxetic and classic (non-auxetic) materials. It has been observed that not only the mechanical properties of the different composite phases influence the effective mechanical properties of the whole composite, but also the location of the same material phases do matter.

Strain-induced ripples in graphene nanoribbons with clamped edges

Abstract: , Phone: þ73 47 2236407, Fax: þ28 23 759Molecular dynamics is employed to study the mechanicalbehavior of graphene nanoribbons with clamped edges underin-plane strain. Buckling of nanoribbons results in theappearance of periodic ripples whose orientation, wavelength,and amplitude can be controlled by varying strain componentsand nanoribbon width. This study shows a way of controllingphysical properties of nanoribbons by introducing strain-induced ripples.

Low temperature diamond growth by linear antenna plasma CVD over large area

Abstract: Recently, there is a great effort to increase the deposition area and decrease the process temperature for diamond growth which will enlarge its applications including use of temperature sensitive substrates. In this work, we report on the large area (20 × 30 cm2) and low temperature (250 °C) polycrystalline diamond growth by pulsed linear antenna microwave plasma system. The influence of substrate temperature varied from 250 to 680 °C, as controlled by the table heater and/or by microwave power, is studied. It was found that the growth rate, film morphology and diamond to non-diamond phases (sp3/sp2 carbon bonds) are influenced by the growth temperature, as confirmed by SEM and Raman measurements. The surface chemistry and growth processes were studied in terms of activation energies (Ea) calculated from Arrhenius plots. The activation energies of growth processes were very low (1.7 and 7.8 kcal mol−1) indicating an energetically favourable growth process from the CO2CH4H2 gas mixture. In addition, from activation energies two different growth regimes were observed at low and high temperatures, indicating different growth mechanism.

Single‐walled carbon nanotubes filled with nickel halogenides: Atomic structure and doping effect

Abstract: NiX2@SWCNT (X = Cl, Br) nanostructures were prepared by capillary filling of single-walled carbon nanotube channels with nickel halogenide melts with slow cooling down to room temperature for better crystallization. The HRTEM data indicated formation of well-ordered 1D NiBr2 crystals, with the experimental atomic structure representing a fragment of the bulk structure. The lattice constant coincides with the corresponding distance in bulk lattice. The 1D crystal structure was modeled using DFT within the PW-GGA approach. According to the Raman, X-ray photoelectron, X-ray and optical absorption spectroscopic data and the DFT results obtained within the rigid band model, nickel halogenides display acceptor behavior with the corresponding charge transfer from the single-walled carbon nanotube walls to the NiX2 nanocrystals.

Controlling quantum dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators

Abstract: This paper reviews the recent advances obtained by integrating semiconductor epitaxial films with embedded self-assembled quantum dots (QDs) on top of single-crystal piezoelectric substrates made of lead magnesium niobate–lead titanate (PMN-PT). This combination allows us to study in detail the effects produced by variable strains (up to about ± 0.2%) on the excitonic emission of single QDs and to add a powerful “tuning knob” to QDs. Biaxial stress can be used to reversibly shift the emission wavelength of QDs in a spectral range wider than 10 meV and to modify the relative binding energies of excitonic species. Anisotropic stress has instead a strong influence on the fine structure splitting of neutral excitons. Finally, we present experimental results on the effect of biaxial strain on the optical modes of microring optical resonators and show a simple approach enabling the compensation of piezo-creep via a closed-loop system. Schematic illustration of a QD membrane integrated on top of a PMN-PT substrate. Stress provided by the piezoelectric substrate allows broad range tuning of the emission properties of the overlying dots.

Effect of chemical treatment on the thermoelectric properties of single walled carbon nanotube networks

Abstract: Carbon nanotube networks showing superior electric properties, high chemical stability, strong mechanical properties, and flexibility are also known to exhibit thermoelectric effects. However, the experimental thermoelectric figure of merit, ZT, of pristine carbon nanotubes is typically in the range of 10−3–10−2, which is still not attractive for thermal energy conversion applications. In this work, we show possible ways to improve the thermoelectric properties of single walled carbon nanotubes (SWCNTs) by means of chemical treatments. In this study, we primarily investigated the effect of chemical treatment on the electrical conductivity and thermoelectric power (TEP) of the entangled network of nanotubes, also, known as “buckypaper”. This chemical treatment increased the electrical conductivity due to p-type doping, thereby, showing a decrease in the TEP given by the Seebeck coefficient, whereas the n-type dopants changed the sign and value of the TEP from about 40 to −40 µV K−1. Neutral polymers, in terms of doping, such as PVDF, PMMA, PVA, PS, and PC, were expected to hinder phonon transport through the nanotube network, increasing the Seebeck coefficient up to ca. 50 µV K−1. Our results reveal the importance of chemical doping determining the sign and the magnitude of the TEP, and role of the polymer matrix in the development of more effective thermoelectric composites based on carbon nanotubes.

Structural properties of metal‐organic frameworks within the density‐functional based tight‐binding method

Abstract: Density-functional based tight-binding (DFTB) is a powerful method to describe large molecules and materials. Metal-organic frameworks (MOFs), materials with interesting catalytic properties and with very large surface areas, have been developed and have become commercially available. Unit cells of MOFs typically include hundreds of atoms, which make the application of standard density-functional methods computationally very expensive, sometimes even unfeasible. The aim of this paper is to prepare and to validate the self-consistent charge-DFTB (SCC-DFTB) method for MOFs containing Cu, Zn, and Al metal centers. The method has been validated against full hybrid density-functional calculations for model clusters, against gradient corrected density-functional calculations for supercells, and against experiment. Moreover, the modular concept of MOF chemistry has been discussed on the basis of their electronic properties. We concentrate on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn4O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)), and aluminum oxide AlO4(OH)2 octahedron (MIL-53). We show that SCC-DFTB predicts structural parameters with a very good accuracy (with less than 5% deviation, even for adsorbed CO and H2O on HKUST-1), while adsorption energies differ by 12 kJ mol−1 or less for CO and water compared to DFT benchmark calculations.

The correlation between the electron work function and yield strength of metals

Abstract: In this study, an effort is made to establish the correlation between electron work function (EWF) and mechanical properties of metals such as yielding strength and hardness. In general, the intrinsic resistance of metals to plastic deformation depends on their atomic bond strength that is essentially governed by the electron behavior. Based on the Peierls–Nabarro model, an intrinsic sixth-power dependence of the yield strength of metals on their EWF is derived. Such a relation can also be extended to hardness. The established correlations are verified by the reported experimental results of transition metals and rare-earth metals.

Extensions of DFTB to investigate molecular complexes and clusters

Abstract: Molecular complexes and clusters provide bridges between molecular and solid states physics. Containing tens to few thousands of atoms, such systems can hardly be approached via traditional ab initio wavefunction based methods at the moment. Density functional theory (DFT) and density functional based tight binding methods (DFTB) have strongly developed with respect to computational efficiency to cover this size range. However both DFT and currently implemented DFTB face difficulties to describe realistically and accurately the typical interactions met in molecular clusters, in particular long range interactions such as Coulomb interactions between distant charge fluctuations, charge resonance in ionic clusters, and van der Waals interactions. The present article aims at providing an overview of how extensions of DFTB can circumvent some of the above deficiencies and turn out to be realistic and efficient tools to investigate the properties of molecular clusters and complexes, focusing on structural, electronic, energetic, and spectroscopic properties.

Coupling of single nitrogen-vacancy defect centers in diamond nanocrystals to optical antennas and photonic crystal cavities

Abstract: We demonstrate the ability to modify the emission properties and enhance the interaction strength of single-photon emitters coupled to nanophotonic structures based on metals and dielectrics. Assembly of individual diamond nanocrystals, metal nanoparticles, and photonic crystal cavities to meta-structures is introduced. Experiments concerning controlled coupling of single defect centers in nanodiamonds to optical nanoantennas made of gold bowtie structures are reviewed. By placing one and the same emitter at various locations with high precision, a map of decay rate enhancements was obtained. Furthermore, we demonstrate the formation of a hybrid cavity quantum electrodynamics system in which a single defect center is coupled to a single mode of a gallium phosphite photonic crystal cavity.

Origin of Bi3+-related luminescence centres in Lu3Al5O12:Bi and Y3Al5O12:Bi single crystalline films and the structure of their relaxed excited states

Abstract: Single crystalline films (SCFs) of Y3Al5O12:Bi and Lu3Al5O12:Bi with different Bi3+ contents are studied at 1.7–300 K by the time-resolved luminescence spectroscopy methods under excitation in the 2.4–20 eV energy range. The ultraviolet (UV) emission of these SCFs is shown to arise from the radiative decay of the metastable and radiative minima of the triplet relaxed excited state (RES) of a single Bi3+ centre, which are related to the 3P0 and 3P1 levels of a free Bi3+ ion, respectively. At T 100 K, both the high-energy shift of the maximum and the shortening of the decay time of the UV emission with increasing temperature are caused by thermally stimulated non-radiative transitions between the metastable and radiative minima of the triplet RES. Their excitation bands located around 4.6, 5.2 and 5.95 eV are assigned to the 1S0 3P1, 1S0 3P2 and 1S0 1P1 transitions, respectively, in free Bi3+ ions. The luminescence of dimer Bi3+–Bi3+ centres is not detected in the SCFs studied. The lower-energy (≈2.6 eV) visible (VIS) emission of these SCFs is due to an exciton, localized near a single Bi3+ ion, while the higher-energy (2.75 eV) VIS emission, an exciton, localized near a dimer Bi3+–Bi3+ centre. The phenomenological models are proposed to describe the excited-state dynamics of all the luminescence centres studied. Application of the two- or three-excited-level models on the temperature evolution of the luminescence decay times has allowed determination of characteristic parameters of the corresponding RES: the energy separations between the excited states and the rates of the radiative and non-radiative transitions from these states.