分数维结构有机物体系中的发光

本文介绍了分数维结构和激子渗透概念,概述了分数维结构有机物体系中能量传递和发光动力学研究的基本现状。     

Master equation techniques for exciton motion,relaxation, capture,and annihilation

Techniques developed recently for the study of exciton dynamics in molecular solids are discussed. They include master equation methods for the analysis of prerelaxation energy transfer, the generalized master equation approach to coherence in exciton motion, and the defect technique as applied to exciton trapping and annihilation.     

Trapping without traps by correlated random walks

Correlated random walk of particles in the infinite cluster of percolating lattices in two dimensions is investigated. For infinitely strong forward correlations (no change of direction except at the boundaries) trapping of the particles in small regions of the infinite cluster is observed.     

The XXZ Heisenberg model on random surfaces

We consider integrable models, or in general any model defined by an R-matrix, on random surfaces, which are discretized using random Manhattan lattices. The set of random Manhattan lattices is defined as the set dual to the lattice random surfaces embedded on a regular d-dimensional lattice. They can also be associated with the random graphs of multiparticle scattering nodes. As an example we formulate a random matrix model where the partition function reproduces the annealed average of the XXZ Heisenberg model over all random Manhattan lattices. A technique is presented which reduces the random matrix integration in partition function to an integration over their eigenvalues.     

Studies on stability of bi-functional P3HT:PCBM:rubrene optoelectronic devices

Investigations are carried out for stability in photovoltaic response of bifunctional electroluminescent and photovoltaic devices, based on ternary blend of poly(3-hexylthiophene) (P3HT), phenyl [6,6′]C₆₁ butyric acid methyl ester (PCBM), and 5,6,11,12-tetraphenylnaphthacene (rubrene). P3HT and PCBM are important and the most frequent materials used for photovoltaic applications, therefore, for relative comparison, photovoltaic cells were also prepared using a binary mixture of P3HT and PCBM. Devices based on the ternary blend exhibited better stability in all photovoltaic parameters and the lifetime was almost doubled, but their photovoltaic efficiency was lower than that of those based on the binary blend. Longer lifetime of ternary blend devices is because of a relatively better thermal, electrochemical, and morphological stabilities of the ternary blend system. However, the lower efficiencies are because of the reduced photo-current and low fill factor (FF) due to an increased recombination and introduction of defects/trapping sites by rubrene molecules.     

Asymptotic properties of multistate random walks. I. Theory

A calculation is presented of the long-time behavior of various random walk properties (moments, probability of return to the origin, expected number of distinct sites visited) formultistate random walks on periodic lattices. In particular, we consider inhomogeneous periodic lattices, consisting of a periodically repeated unit cell which contains a finite number of internal states (sites). The results are identical to those for perfect lattices except for a renormalization of coefficients. For walks without drift, it is found that all the asymptotic random walk properties are determined by the diffusion coefficients for the multistate random walk. The diffusion coefficients can be obtained by a simple matrix algorithm presented here. Both discrete and continuous time random walks are considered. The results are not restricted to nearest-neighbor random walks but apply as long as the single-step probability distributions associated with each of the internal states have finite means and variances.     

Energy transfer as a random walk with long-range steps

We consider the incoherent energy transport in molecular crystals, where the transfer rates stem from Coulombic and exchange interactions. For substitutionally disordered lattices we present in a first passage model the excitation decay due to trapping by randomly distributed traps; the decay is related to the distribution of the number of distinct sites visited during the timet and is expressible through the cumulants of this distribution. The validity domains of approximate decay laws based on the first few cumulants are also discussed. We exemplify the findings for dipolar transfer rates between randomly distributed molecules on a square lattice, by comparing the random walk on the random system to its CTRW (continuous time random walk) counterpart.     

Exciton absorption of light in a smooth random field

A study is made of interband absorption of light in a smooth random field that varies slowly in space. When the exciton energy in the random field is much less than the exciton binding energy, the influence of the random field on the discrete spectrum of the exciton is expressed through the appearance of an exponential “tail” to every exciton line. But if the exciton energy in the random field is much greater than the exciton binding energy, the exciton effects are unimportant because the random field breaks up the excitons.     

Asymptotic properties of multistate random walks. II. Applications to inhomogeneous periodic and random lattices

The previously developed formalism for the calculation of asymptotic properties of multistate random walks is used to study random walks on several inhomogeneous periodic lattices, where the periodically repeated unit cell contains a number of inequivalent sites, as well as on lattices with a random distribution of inequivalent sites. We concentrate on the question whether the random walk properties depend on the spatial arrangement of the sites in the unit cell, or only on the number density of the different types of sites. Specifically we consider lattices with periodic and random arrangements of columns and lattices with periodic and random arrangements of anisotropic scatterers.     

Positron lifetimes and lineshape factors in deformed copper

Measurements of positron annihilation lifetime and lineshape factors in deformed OFHC copper, for 14 different pairs of specimen, ranging up to 66% deformation, are discussed. The influence of annihilation in the source is carefully examined and the data are consequently corrected. The lifetime varies from (124±2) ps for annealed copper to (179±2) ps for dislocation saturated samples. Trapping cross sections and trapping efficiencies are calculated. The possibility of interference of vacancies formed during the deformation is discussed.     

Ultrafast exciton relaxation in microcrystalline pentacene films

The exciton dynamics in microcrystalline pentacene films is investigated by transient absorption measurements with 30 fs time resolution. It is found that the emission from photoexcited Frenkel excitons decays within 70 fs due to the ultrafast formation of an excitonic species with a strongly reduced transition dipole to the ground state and an absorption dipole in the plane of the film. We propose that an excimer exciton is formed and stabilized by changes of the local crystal structure. The subsequent dynamics is dominated by diffusion controlled annihilation and trapping.     

Computer simulation of irreversible kinetic gelation: Crossover between random and kinetic growth,radical conservation,poisons, and chain-preferred reactions

A Monte-Carlo study is presented for irreversible kinetic gelation in binary polymers in three dimensions. Using a growth scheme where attempts are made to form bonds with the help of radicals with probabilityk/(1+k) and without the help of radicals with probability 1/(1+k), wherek=0,1,2, ..., , we have studied the crossover from one universality class of critical phenomena in kinetic gelation to another universality class of random percolation. In the radical assisted gelation, several features are incorporated to make the model more realistic, for example, the conservation of the number of radicals during the growth (more suited for living anionic polymerization), the introduction of poisons and chain-preferring reactions are considered. Conserving the radicals delays gelation as well as trapping. The trapping can also be delayed by increasing the functionality of the monomers. The effect of the poison is rather severe; even a small concentration of poison (c _p >2%) terminates the reaction well before the gel point in a mixture of binary and ternary monomers, and it seems to affect the critical phenomena in the sol-gel phase transitions. In our growth mechanism, where reactions prefer to growL length steps each in a random direction, the critical phenomena remain unaffected by the length scale, but the flexibility of varying the chain stepL allows us to study the changeover between the phase transition in different gel structures.     

A vectorizable random lattice

We describe a family of random lattices in which the connectivity is determined by the Voronoi construction while the vectorizability is not lost. We can continuously vary the degree of randomness so in a certain limit a regular lattice is recovered. Several statistical properties of the cells and bonds of these lattices are measured. We also study anisotropy effects on the numerical solution of the Laplace equation for varying degrees of randomness.     

Laser time-resolved spectroscopy: Investigations of energy transfer in Eu3+ and Er3+ doped YVO4

Host-sensitized energy transfer in YVO₄ crystals doped with Eu³⁺ and Er³⁺ ions was investigated using laser excited time-resolved spectroscopy techniques. The results are consistent with a model in which the energy is transferred by a single step process from self- trapped excitons to activators at low temperatures, and by thermally activated exciton hopping at high temperatures. The rate of the latter process is found to be strongly affected by the trapping step with activator-induced host traps playing an important part in the trapping process. Time-resolved spectroscopy techniques are shown to be expecially useful in separately determining the characteristics of migration and trapping processes in exciton energy transfer. The parameters describing these processes are determined for the systems investigated using both diffusion and generalized random walk theories.     

Self-avoiding random walk with multiple site weightings and restrictions

We introduce a new class of models for polymer collapse, given by random walks on regular lattices which are weighted according to multiple site visits. A Boltzmann weight omegal is assigned to each (l+1)-fold visited lattice site, and self-avoidance is incorporated by restricting to a maximal number K of visits to any site via setting omegal=0 for l>or=K. In this Letter we study this model on the square and simple cubic lattices for the case K=3. Moreover, we consider a variant of this model, in which we forbid immediate self-reversal of the random walk. We perform simulations for random walks up to n=1024 steps using FlatPERM, a flat histogram stochastic growth algorithm. We find evidence that the existence of a collapse transition depends sensitively on the details of the model and has an unexpected dependence on dimension.     

A note on triplet-triplet fission of singlet excitons in molecular crystals

The singlet exciton lifetime in the reaction of triplet-triplet fission due to purely electronic interaction is estimated in an ideal molecular crystal with favorable positions of singlet and triplet bands, for coherent plane wave form of exciton wave functions. An explicit form of the interaction of molecules in molecular crystal in second quantization reprezentation is presented, including exchange and charge-transfer terms. New commutation relations of molecular and exciton creation and annihilation operators are introduced.Na Slovance 2, Praha 8, Czechoslovakia.     

PAL spectroscopy as experimental probe for extended free-volume defects in inorganic glasses and ceramics

Extended free-volume defects trapping positrons are studied in inorganic glasses and functional ceramics using positron annihilation lifetime spectroscopy. Obtained results are treated in terms of general macroscopic approach grounded on physical–chemical properties of the tested materials which depend on the value of free volume content. At the example of chalcogenide glasses of ternary Ge–Sb–S system and spinel-type mixed transition-metal manganite Cu_0.4Co_0.4Ni_0.4Mn_1.8O₄ and magnesium aluminate MgAl₂O₄ ceramics, it is shown that strict correlation exists between numerical values of free volumes of potential positron traps and lifetime parameters determined within two-state positron trapping model.     

A high accuracy diffusion kinetics formalism for random multicomponent alloys: application to high entropy alloys

In this paper, a new, lighter, version of the highly accurate Moleko, Allnatt and Allnatt formalism for describing both tracer (self) and collective diffusion kinetics in multicomponent random alloys is presented. Verification of the resulting expressions is performed by means of kinetic Monte Carlo simulation. The accuracy of the new formalism is much higher than that of the combined Manning and Holdsworth and Elliott formalism discussed recently. Using this formalism the possible range of the tracer diffusion ratio of the highest to the lowest atomic component is examined for equiatomic (or near equiatomic) binary, ternary, quaternary and quinary alloys. It is shown that in the random alloy model, the correlation effect is the highest with a reduction of the fastest tracer diffusion by 40–55%, when moving from two pure metals to their equiatomic binary alloy. By adding the third component (with an intermediate mobility) this effect can be further increased with a possible total reduction of the fastest tracer diffusion by up to 70% (depending on the combinations of mobilities), while adding the fourth component brings this reduction up to 80% and with a possible maximum of up to 85% reduction for the 5-component alloy (again depending on the combinations of mobilities). But the slowest diffusing components are not affected by this. This suggests that kinetics arguments alone are not enough for explaining the sluggish diffusion observed of all atomic components in (equiatomic) high-entropy alloys.     

The Ising model on random lattices in arbitrary dimensions

We study analytically the Ising model coupled to random lattices in dimension three and higher. The family of random lattices we use is generated by the large N limit of a colored tensor model generalizing the two-matrix model for Ising spins on random surfaces. We show that, in the continuum limit, the spin system does not exhibit a phase transition at finite temperature, in agreement with numerical investigations. Furthermore we outline a general method to study critical behavior in colored tensor models.     

Exciton--polaritons in ternary mixed crystals

In this paper the bulk exciton polaritons in ternary mixed crystals (TMCs) are investigated in the Born--Huang approximation. The numerical results of the polariton frequencies as functions of the wave-vector and the compositions for ternary mixed crystals Al电震荡 三重混合晶体 激子 电磁学polariton, exciton, ternary mixed crystalProject supported partly by PhD Progress Foundation of Higher Education Institutions of China (Grant No~20040126003) and the Natural Science Foundation of Inner Mongol of China (Grant No~200408020101).2006-10-08In this paper the bulk exciton polaritons in ternary mixed crystals （TMCs） are investigated in the Born-Huang approximation. The numerical results of the polariton frequencies as functions of the wave-vector and the compositions for ternary mixed crystals AlxGa1-xAs, CdxZn1-xSe, and AlxGa1-xN are obtained and discussed. The new dispersion characteristics for exciton-polaritons in TMC systems are found in comparison with binary crystals. The splitting of the two branches of exciton-polariton frequencies varies nonlinearly with the composition of TMCs and has a minimum in the long-wavelength range.