Three-state Trapping Model Research Articles

Effect of Gd doping concentration and sintering temperature on structural, optical, dielectric and magnetic properties of hydrothermally synthesized ZnO nanostructure

Abstract Nanoparticles of Gd 3+ ions doped ZnO were prepared by hydrothermal method. To obtain the desired nanocrystalline phase and also, to enhance the oxygen vacancy inside the host ZnO nanoparticles, the as prepared sample is sintered at 400 and 600 °C for 2 h in vacuum atmosphere. X-ray diffractograms (XRD) analysis reveals the absence of any impure phases in the Gd 3+ ions doped ZnO nanostructure. The presence of intrinsic defects/oxygen vacancies were confirmed by the analysis of UV–Visible, PL and Raman spectra of the doped sample. The qualitative and quantitative analysis of intrinsic defects was successfully explained by three-state trapping model using positron annihilation study. Room temperature ferromagnetism was found in the sample of 5% Gd 3+ ions doped ZnO, which has been successfully explained by the vacancy assisted bound magnetic polaron model. Frequencies as well as temperature dependent dielectric constant of the samples were investigated. The dielectric constants were recorded at 40 Hz and 100 kHz frequencies and the values are nearly ∼2.76 х 10 6 and 12782 respectively, in the 5% Gd 3+ ions doped ZnO sample.

Early stages of solute clustering in an Al–Mg–Si alloy

Abstract The processes taking place during natural ageing (NA) of the aluminium alloy Al–0.4Mg–0.4Si (wt.%) are characterised by positron annihilation lifetime spectroscopy carried out both in situ during ageing and ex situ after cooling down samples to low temperatures (−180 °C to −60 °C) between NA steps. We find a pronounced dependence of positron lifetime on temperature, which is the signature of the presence of vacancy-free solute clusters in addition to vacancy-related defects. Such clusters are also found in as-quenched samples, indicating that clustering takes place already during quenching. The decomposition of positron lifetime spectra into various components allows us to apply the three-state trapping model and to determine the trapping rates into vacancy-related defects and solute clusters. By modelling the time dependence of positron data we are able to describe the natural ageing process as a gradual decrease of vacancy density associated with a pronounced increase of the density of solute clusters.

Kinetic analyses and mathematical modeling of primary photochemical and photoelectrochemical processes in plant photosystems

In this paper the model and simulation of primary photochemical and photo-electrochemical reactions in dark-adapted intact plant leaves is presented. A descriptive algorithm has been derived from analyses of variable chlorophyll a fluorescence and P700 oxidation kinetics upon excitation with multi-turnover pulses (MTFs) of variable intensity and duration. These analyses have led to definition and formulation of rate equations that describe the sequence of primary linear electron transfer (LET) steps in photosystem II (PSII) and of cyclic electron transport (CET) in PSI. The model considers heterogeneity in PSII reaction centers (RCs) associated with the S-states of the OEC and incorporates in a dark-adapted state the presence of a 15-35% fraction of Q(B)-nonreducing RCs that probably is identical with the S₀ fraction. The fluorescence induction algorithm (FIA) in the 10 μs-1s excitation time range considers a photochemical O-J-D, a photo-electrochemical J-I and an I-P phase reflecting the response of the variable fluorescence to the electric trans-thylakoid potential generated by the proton pump fuelled by CET in PSI. The photochemical phase incorporates the kinetics associated with the double reduction of the acceptor pair of pheophytin (Phe) and plastoquinone Q(A) [PheQ(A)] in Q(B) nonreducing RCs and the associated doubling of the variable fluorescence, in agreement with the three-state trapping model (TSTM) of PS II. The decline in fluorescence emission during the so called SMT in the 1-100s excitation time range, known as the Kautsky curve, is shown to be associated with a substantial decrease of CET-powered proton efflux from the stroma into the chloroplast lumen through the ATPsynthase of the photosynthetic machinery.

Algorithm for analysis of OJDIP fluorescence induction curves in terms of photo- and electrochemical events in photosystems of plant cells:: Derivation and application

The algorithm for simulation of the OJDIP fluorescence induction curve in chloroplasts under variable conditions is presented. It is derived from analyzes of chlorophyll a fluorescence kinetics upon excitation with single- (STF), twin- (TTF) and repetitive STF excitations, and from the rate equations that describe the sequence of transfer steps associated with the reduction of the primary quinone acceptor Q A and the release of photochemical fluorescence quenching of photosystem II (PSII) in multi-turnover excitation (MTF). The fluorescence induction algorithm (FIA) considers a photochemical O–J–D, a photo-electrochemical J–I and an I–P component (phase) which probably is associated with a photo-electric interaction between PSI and PSII. The photochemical phase incorporates the kinetics associated with the double reduction of the acceptor pair [PheQ A ] in Q B –nonreducing reaction centers (RCs) and the associated doubling of the variable fluorescence, in agreement with the three-state trapping model (TSTM) of PSII. Application of and results with the algorithm are illustrated for MTF-induced OJDIP curves, measured in dark-adapted, in STF pre-excited and in DCMU inhibited thylakoids.

Characterization of defects in ZnO nanocrystals: Photoluminescence and positron annihilation spectroscopic studies

Defects present in ZnO nanocrystals prepared by a wet chemical method have been characterized by photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques. The as-prepared sample was heat treated at different temperatures to obtain nanocrystals in the size range of 19–39nm. X ray diffractograms confirmed the single-phase wurtzite structure formation. Photoluminescence measurements showed a strong violet band at 434nm, which has been identified as due to electronic transitions from the zinc interstitial defect level to the top of the valence band. A marked decrease in the intensity of the violet emission with increasing heat-treatment temperature has been observed, which is attributed to recombination of zinc interstitials with zinc vacancies. Positron annihilation spectroscopy has been employed to understand the dynamics of the vacancy-type defects and their annealing behavior. The observed variation of the defect related lifetime components with heat-treatment temperature has been successfully explained by using a three-state trapping model. The results of PL and PAS studies in the present case are found to be complementary to each other.

Photoelectric effects on chlorophyll fluorescence of photosystem II in vivo. Kinetics in the absence and presence of valinomycin

Fluorescence induction curves (F(t)) in low intensity 1s light pulses have been measured in leaf discs in the presence and absence of valinomycin (VMC). Addition of VMC causes: (i) no effect on the initial fluorescence level Fo and the initial (O-J) phase of F(t) in the 0.01-1 ms time range. (ii) An approximately 10% decrease in the maximal fluorescence Fm in the light reached at the P level in the O-J-I-P induction curve. (iii) Nearly twofold increase in the rate and extent of the F(t) rise in the J-I phase in the 1-50 ms time range. (iv) A 60-70% decrease in the rise (I-P phase) in the 50-1000 ms time range with no appreciable effect, if at all, on the rate. System analysis of F(t) in terms of rate constants of electron transfer at donor and acceptor sides have been done using the Three State Trapping Model (TSTM). This reveals that VMC causes: (i) no, or very little effect on rate constants of e-transfer reactions powered by PSII. (ii) A manifold lower rate constant of radical pair recombination (k(-1)) in the light as compared to that in the control. The low rate constant of radical pair recombination in the reaction center (RC) in the presence of VMC is reflected by a substantial increase in the nonzero trapping efficiency in RCs in which the primary quinone acceptor (Q(A)) is reduced (semi-open centers). This causes an increase in their rate of closure and in the overall trapping efficiency. Data suggest evidence that membrane chaotropic agents like VMC abolish the stimulation of the rate constant of radical pair recombination by light. This light stimulation that becomes apparent as an increase in Fo has been documented before [Biophys. J. 79 (2000) 26]. It has been ascribed to effects of (changes in) local electric fields in the vicinity of the RC. The decrease of the I-P phase is attributed to a decrease in the photoelectric trans-thylakoid potential in the presence of VMC. Such effects have been hypothesized and illustrated.

Temperature-dependent positron trapping in copper and aluminum tubes after tensile deformation

Doppler-broadening measurements on aluminum and copper polycrystals were performed in situ during tensile tests and temperature-dependent after different degrees of deformation. The sensitivity limit of positrons to defects produced by plastic deformation was found at a resolved shear stress of $\ensuremath{\tau}=(4\ifmmode\pm\else\textpm\fi{}1) \mathrm{MPa}$ in aluminum and $\ensuremath{\tau}=(9\ifmmode\pm\else\textpm\fi{}1) \mathrm{MPa}$ in copper. This corresponds to dislocation densities of $\ensuremath{\varrho}\ensuremath{\approx}2\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{12} {\mathrm{m}}^{\ensuremath{-}2}.$ Temperature-dependent Doppler-broadening measurements revealed distinct differences of the positron trapping at dislocations in copper after weak and strong plastic deformation. The transition of the trapping behavior occurred between $\ensuremath{\tau}=20$ and 30 MPa. In aluminum such a dependence of the positron trapping behavior on the degree of deformation was not observed. The measurements were analyzed in terms of a three-state trapping model which provided numerical values for the trapping rates and the binding energy of the positron to dislocation lines.

Metastable self-trapping of positrons in MgO

Low-temperature positron annihilation measurements have been performed on MgO single crystals containing either cation or anion vacancies. The temperature dependence of the S parameter is explained in terms of metastable self-trapped positrons which thermally hop through the crystal lattice. The experimental results are analyzed using a three-state trapping model assuming transitions from both delocalized and self-trapped states to deep trapped states at vacancies. The energy level of the self-trapped state was determined to be (62\ifmmode\pm\else\textpm\fi{}5) meV above the delocalized state. The activation enthalpy for the hopping process of self-trapped positrons appears to depend on the kind of defect present in the crystals.

Positron trapping in Au

Trapping of positrons by lattice defects has proved to be a powerful tool in probing both the density and the structure of defects. In this work, we are concerned with a three-state trapping model i.e. free (bulk), monovacancy and divacancy states in the case of Au. An attempt is made to calculate the fractions of positrons annihilating in each state and their dependence on both time and sample temperature. A significant divacancy contribution to the annihilation rate is observed for temperatures near melting.

Evidence for trapping of positrons in thallium

Abstract For the first time the positron lifetimes have been measured in thallium in the temperature range 80 to 570 K. At 507 K the transition from the α- to the β-phase has been detected. By use of a three-state trapping model, including detrapping out of shallow traps a vacancy formation enthalpy of 0.39 eV and a binding energy in the shallow trap of 0.06 eV was found in the α-phase.

Structural implications on positron lifetimes in lamellar polyethylene with chain defects

New positron lifetime data in a series of isothermally crystallized polyethylene pairs with known concentrations of chain defects, having a lamellar structure, have been measured. The materials were characterized by small-angle and wide-angle x-ray scattering techniques. Positron lifetime data were computer analyzed and only three components could be resolved. It is shown that the annihilation mechanisms can be understood in terms of average distance between lamellar crystals (long period), crystal thickness, and chain defect concentration. The present results confirm, in consonance with the conclusions of other authors, the longest component to be due to ortho-positronium pick-off annihilation. The intermediate component may be ascribed to positronium trapped at the crystal-amorphous interface. The application of a three-state trapping model yields a lifetime of about 900 ps for this state. It is suggested that the shortest component may be due to free positron annihilation, para-positronium self-annihilation, and annihilation of positrons from a tightly bound state localized at the chain defects. An approximate calculation provides lifetime values around 250 ps for this short-lived state.

Temperature dependence of positron annihilation parameters in neutron irradiated molybdenum

Positron lifetime measurements have been performed for molybdenum samples containing different densities of voids and dislocation loops. The samples consisted of single crystal molybdenum exposed to 2.7×1018 fast neutrons/cm2 at 60°C, and subsequently annealed at 650°, 725°, 800°, and 875°C in vacuum (p<10−7 Torr). After each annealing, where the densities of voids and loops were changed, positron lifetime measurements were performed in the temperature interval [−194°, 285°C]. In two-term fits of the measured spectra the longer lifetime, τe2-460 ps corresponds to an intensityI e2 increasing with sample temperature. The shorter lifetime τe1 decreases with increasing temperature. A three-state trapping model with and without detrapping is discussed, and appears to be incapable of explaining the observed temperature dependences. A four-state positron trapping model including detrapping is necessary and satisfactory. It describes positron trapping to voids and trapping to dislocation loops, which is followed by a competition between detrapping and positron transition to jogs or other dislocation-bound defects. Mathematical expressions of the four-state trapping model including detrapping are worked out and calculations of the intensityI e2 are compared with the experimental values ofI e2. By use of special models for the temperature dependence of trapping rates, numerical values can be determined for the positron-dislocation-binding energy and for specific positron trapping rates.

Positron annihilation on pure and carbon-dopedα-iron in thermal equilibrium

Positron-annihilation $S$-parameter measurements in thermal equilibrium on pure and carbon-doped (50 and 750 at. ppm) $\ensuremath{\alpha}$-iron are presented. It is shown that trapping of positrons in both monovacancies and carbon-vacancy pairs occurs, even far above the dissociation temperature of the vacancy pairs. Therefore, a three-state trapping model is used in the analysis of the measured $S$ curves. The vacancy-formation enthalpy in both the paramagnetic and ferromagnetic state is deduced: It is found to be 1.79 \ifmmode\pm\else\textpm\fi{} 0.10 eV in the paramagnetic state and 2.0 \ifmmode\pm\else\textpm\fi{} 0.2 eV in the ferromagnetic state. These values are larger than those published so far. The activation enthalpy for vacancy migration obtained by combining the values cited above with recently published self-diffusion enthalpy values confirms the applicability of the one-interstitial model in $\ensuremath{\alpha}$-iron.