Stark spectroscopy of excited-state transitions in a conjugated polymer

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.     

Influence of an electric field on photostimulated states in NH4BPh4 films

The influence of an electric field on stable photostimulated triplet states of NH₄BPh₄ at a temperature of 77 K have been studied by EPR spectroscopy. It has been established that, on exposure to UV radiation, electron capture by traps in the band gaps takes place with formation of triplet state. After application of an electric field, triplet states are destructed because, with an increase in the applied voltage, a gradual inclination of energy bands takes place and electrons found in traps on different energy levels are released. The assumption that captured electrons are found in traps on different energy levels is confirmed by earlier studies of thermoluminescence spectra.     

Effect of uniaxial compression on traps of excitons and charge carriers in poly(9-vinylcarbazole) films

The effect of uniaxial pressure (1 × 10⁸ Pa) on the photoluminescence spectra and thermally stimulated luminescence curves of poly(9-vinylcarbazole) has been investigated in the temperature range of 5–295 K. The thermally stimulated luminescence curve of crystalline carbazole has been measured for comparison. The high-temperature wings of the thermally stimulated luminescence curves are approximated by a Gaussian function, the half-width of which characterizes the disorder of energy states of deep structural traps. It is concluded that the pressure inhibits conformational changes of polymer chains of poly(9-vinylcarbazole), which leads to the formation of sandwich-like excimers as well as to an ordering of the spatial arrangement of the side carbazolyl groups. As a result, the concentration of “excimer-forming” centers increases, whereas the degree of disorder of energy states of deep structural traps of charge carriers is reduced by almost half and remains unchanged after the depressurization.     

Multiphoton polymerization with the holographic control of femtosecond and continuous laser radiation

A unit in which two lasers, one with femtosecond and the other continuous radiation, were used was designed and manufactured. Both lasers worked under holographic control conditions provided by two spatial optic modulators. Experiments with creating several polymer structures by several femtosecond optical “traps” simultaneously were performed. The possibility of the production of one polymer article by simultaneous displacement of several femtosecond optical traps was demonstrated. A polymer article was also prepared by several femtosecond traps and manipulated with the use of several continuous traps. The possibility of focusing femtosecond radiation onto an extended object (“thread” 13 μm long) and the preparation of a polymer object by controlling a femtosecond optical trap in three-dimensional space was demonstrated.     

Thermoactivated spectroscopy of heterovalent impurity traps in CdWO4

Abstract

Recombination luminescence emission spectra, TSL and trap spectra estimated by fractional glow technique (FGT), in nominally pure and Li-, Bi- and Ho-doped CdWO₄, crystals are reported. According to the investigations by FGT heterovalent impurities Li, Bi and Ho causes localized electronic states which act as traps for charge carriers. It is shown that TSL results in emission of known blue-green luminescence band by emptying of the Li⁺-related traps in CdWO₄-Li and yellow luminescence band by emptying of the Bi³⁺-related traps in CdWO₄-Bi. It is proposed that blue-green and yellow luminescence occur by recombination correspondingly of free holes and free electrons at different intrinsic tungstate group related luminescence centers.     

Phase transition in a harmonic oscillator with dynamical traps

We consider the characteristic features of a new class of nonequilibrium phase transitions, namely, transitions due to dynamical traps. We studied an individual oscillator with dynamical traps located in a small neighborhood of the x axis of the phase plane {x, v = dx/dt}. The dynamics of this system is analyzed numerically. The mechanism and conditions of the occurrence of various dynamic states is established. We demonstrate that the dynamics of such an oscillator can be represented by a number of random jump-like transitions between long-lived states. For these states to occur, random forces are necessary with intensity lying within a certain interval. If the intensity of the random forces is high or low, phase transitions due to dynamical traps will not occur.     

Evaluation of the energy distribution of trap states in polymer films

This paper reports on the results of an investigation of polymer thin films by the methods of thermally stimulated currents and current-voltage characteristics. It has been found using these methods that, in the band gap of the poly(diphenylene phthalide) films, there are three groups of electron traps with maxima in the density of states at energies of 0.50, 1.06, and 2.40 eV with respect to the bottom of the conduction band. These results correlate well with the data obtained in our investigation of the fluorescence excitation spectra.     

Adsorption of polymer chains by disordered traps

We study the adsorption cross-over of ideal polymer chains in an environment of disordered traps. Starting from the assumption of an optimal cluster size of traps (optimal fluctuation method) we derive a general scaling form of the free energy function for arbitrary spatial dimensions. For small concentrations of traps we find a cross-over from localized (adsorbed) behavior to delocalized behavior depending on the chain's length and on the depth of the traps; this is connected with the non-monotonic behavior of the chain's extension. In terms of the free energy of the chain this cross-over resembles a first order transition scenario, the chain gets localized at many traps at once. Received 18 November 1998     

Local electrical overstressing in polymer dielectrics

Using a spherical concentric capacitor as a microspike model, the electric field distribution in a polymer dielectric material with shallow and deep electron traps is investigated. It is established that in the course of field emission from microspikes on the electrode and space charge accumulation on the traps, the values of coefficient of electric overstresses depend on temperature, injection barrier height, and trap concentration and depth in the polymer material. The behavior of this coefficient is characterized as a function of the mean field strength. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 2008.     

Anchoring of Polymers by Traps Randomly Placed on a Line

We study the dynamics of a Rouse polymer chain which diffuses in a three-dimensional space under the constraint that one of its ends, referred to as the slip-link, may move only along a one-dimensional line containing randomly placed, immobile, perfect traps. Extensions of this model occur naturally in many fields, ranging from the spreading of polymer liquids on chemically active substrates to the binding of biomolecules by ligands. For our model we succeed in computing exactly the time evolution of the probability P _sl(t) that the chain slip-link will not encounter any of the traps until time t and, consequently, that until this time the chain will remain mobile.     

New oxygen donors in silicon

Oxygen donor traps and oxygen-related precipitates are investigated by deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). The so-called New Donors (ND's) occur after thermal treatments in the temperature range of 650 °–800 ° C. They have a continuous distribution of trap states with respect to energy in the band gap of Si. The concentration of the trap states increases towards the conduction band edge. The precipitates observed are mainly platelets and ribbon-like defects. The formation and annihilation kinetics of ND traps and oxygen-related precipitates are correlated. An SiO _x Interface Model is proposed to explain the origin and the donor-like behavior of the ND traps. The ND trap spectrum is composed of two different types of trap states: interface states at the surface of precipitates and bound states in the Coulombic wells of a fixed positive charge which is located in the SiO _x precipitates.     

Wigner crystallization of electrons and holes in amorphous silicon nitride. Antiferromagnetic ordering of localized electrons and holes as a result of a resonance exchange interaction

The predicted quantitative relation between the density and trapping cross section of traps in Si₃N₄ and the Coulomb repulsion radius in the Wigner crystallization of carriers in localized states is observed experimentally. The absence of ESR for localized electrons and holes in Si₃N₄ is interpreted on the basis of a model of a resonance exchange interaction of electrons on account of tunneling via localized states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 489–494 (10 October 1996)     

Eine Bestimmung der effektiven Zustandsdichte des Bandes für überschüssige Defektelektronen in Anthrazen

Steady-state space-charge-limited current measurements were used to obtain the effective density of states in the band appropriate to excess hole motion in anthracene. Since the energetic trap density decreases exponentially with the energy distance from the band, the traps are filled by the space charge up to a certain quasi Fermi level. The thermal activation energy of the current should be a kind of spatial avarage of this level. This energy was measured and used to calculate the effective density of states. The result, 4·10²¹ cm^?3, is close to the double molecular density which is 8,4·10²¹ cm^?3. Further, a discussion on traps is given in an appendix.     

Influence of surface states on deep level transient spectroscopy in AlGaN/GaN heterostructure

Deep level transient spectroscopy(DLTS) as a method to investigate deep traps in AlGaN/GaN heterostructure or high electron mobility transistors(HEMTs) has been widely utilized.The DLTS measurements under different bias conditions are carried out in this paper.Two hole-like traps with active energies of E_v + 0.47 eV,and E_v + 0.10 eV are observed,which are related to surface states.The electron traps with active energies of E_c-0.56 eV are located in the channel,those with E_c-0.33 eV and E_c-0.88 eV are located in the AlGaN layer.The presence of surface states has a strong influence on the detection of electron traps,especially when the electron traps are low in density.The DLTS signal peak height of the electron trap is reduced and even disappears due to the presence of plentiful surface state.     

Individual charge traps in silicon nanowires

We study anomalies in the Coulomb blockade spectrum of a quantum dot formed in a silicon nanowire. These anomalies are attributed to electrostatic interaction with charge traps in the device. A simple model reproduces these anomalies accurately and we show how the capacitance matrices of the traps can be obtained from the shape of the anomalies. From these capacitance matrices we deduce that the traps are located near or inside the wire. Based on the occurrence of the anomalies in wires with different doping levels we infer that most of the traps are arsenic dopant states. In some cases the anomalies are accompanied by a random telegraph signal which allows time resolved monitoring of the occupation of the trap. The spin of the trap states is determined via the Zeeman shift.     

Current-voltage characteristics of poly(diphenylenephthalide) thin films

Charge transfer near the threshold of polymer film transition (induced by a low uniaxial pressure) to the high-conductivity state is studied in an attempt to tackle the question of how the energy band structure of a wide-gap organic insulator varies near this threshold. The I-V characteristics of poly(diphenylenephthalide) films of various thickness versus uniaxial pressure are analyzed. The results obtained are treated within the model of space-charge-limited injection currents. The parameters of the injection model, such as the equilibrium concentration of electrons, electron mobility, the occupation of traps, etc., are estimated. It is concluded that deep traps due to an excess charge may appear in the energy gap of the polymer near the imref. This probably causes a narrow subband to arise, and charge transfer via this subband increases the charge carrier mobility and, hence, conductivity.     

Electrothermographic characteristics of poly(styrene-allyl alcohol)

The paper describes electrothermographic characteristics of poly(styrene-allyl alcohol) such as charge acceptance, decay of charge at room temperature and under infrared exposure, contrast potential, residual potential and fatigue effect. The polymer layer (65 μm thick) is found to have high charge acceptance: initial surface charge density is 5.72×10⁻⁴ Coulomb/m², long retentivity of charge at room temperature (28% decay in 500s) and maximum contrast potential of 1300V at 70°C. The high charge acceptance of the polymer has been related to the presence of phenyl and hydroxyl groups in the polymer structure which form traps of depth 1.2eV. The decay of surface potential is in the temperature range 50–70°C and is related to the glass-rubber transition of the polymer at 58°C.     

Fluorescence quenching by charge carriers in indolo[3,2-b]carbazole-based polymer

The fluorescence quenching by the applied voltage in a new indolocarbazole-based polymer poly{5,11-di-2,3-epoxypropyl-6-pentyl-5,11-dihydroindolyl[3,2-b]carbazolyl-alt-4,4′-tiobisbenzentiole} sandwiched between ITO and Al electrodes has been investigated. Time-resolved fluorescence measurements have shown that the excitons are quenched during their entire life-time. The fluorescence intensity showed complex dependence on the applied voltage and the excitation light intensity, memory effects and dynamics on time scale of tens of seconds. These effects are explained by the exciton quenching by mobile and trapped charge carriers. Charge carrier density dynamics calculated by using a simplified model evaluating carrier injection, photogeneration, recombination and trapping was used to model variations of the fluorescence intensity induced by the periodically applied voltage. These effects point out the importance of the fluorescence quenching by trapped carriers in organic materials possessing high density of carrier traps or with broad distribution of density of states.     

Effect of deformation of a polymer matrix on the optical properties of CdS nanocrystals incorporated into it

A study is made of the photodegradation kinetics of luminescence and absorption of cadmium sulfide nanocrystals incorporated into a plastic polymer matrix. The reversibility effect of photodegradation under mechanical deformation (stretching) of a polymer film is revealed. It is suggested that the photodegradation reversibility is related to the extraction of electrons, photoexcited in nanocrystals, from deep-lying traps in the surrounding polymer matrix during its deformation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 101–104, January–February, 2000.