Charge Relaxation Processes Research Articles

Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

Modeling charge relaxation in graphene quantum dots induced by electron-phonon interaction

We study and compare two analytic models of graphene quantum dots for calculating charge relaxation times due to electron-phonon interaction. Recently, charge relaxation processes in graphene quantum dots have been probed experimentally and here we provide a theoretical estimate of relaxations times. By comparing a model with pure edge confinement to a model with electrostatic confinement, we find that the latter features much larger relaxation times. Interestingly, relaxation times in electrostatically defined quantum dots are predicted to exceed the experimentally observed lower bound of ~100 ns.

Effect of arsenic doping on charge relaxation process in silicon nitride film for capacitive RF MEMS switch application

In this work, effects of arsenic doping on dielectric charging/discharging of silicon nitride (SiN) films prepared by low-pressure chemical vapor deposition (LPCVD) have been investigated for the application of capacitive RF MEMS switches. The dielectric charging/discharging processes have been evaluated in an analogous metal–insulator–semiconductor (MIS) structure by capacitance-voltage (C-V) characterization. Arsenic doping was applied to modify the distribution of energy states in the band gap of thin film SiN. The dielectric discharging behaviors were studied with different discharge time under a dc bias. The C-V curves show that low dielectric charging has been observed in the SiN films after arsenic doping. The generation and recombination centers were introduced into the band gap of thin films SiN, and the dielectric charging is modified and further controlled by the doping technology. The arsenic-doped thin films SiN exhibit excellent dielectric properties to meet the application requirement of capacitive RF MEMS switches.

Charge and Energy Transfer Between CdSe Quantum Dots and Polyaniline.

CdSe quantum dots (QDs) and polyaniline (PAni) were mixed to prepare CdSe QDs/PAni complex. PAni can quench the fluorescence of CdSe QDs. Fluorescence intensity of CdSe QDs/PAni complex is related to the size of CdSe QDs and the concentration of PAni. UV-Vis absorption spectra, fluorescence spectra, time-resolved fluorescence spectroscopy were used to analys the quenching phenomenon. The mechanism of fluorescence quenching is dependent on two factors: on one hand, the Förster resonance energy transfer from CdSe to PAni; on the other hand, PAni can intercept the charge relaxation process of CdSe and lead to the interruption of radiative recombination.

Optimization of electret film forming method

The aim of this investigation was to develop a method of fabrication of electrets made from PTFE foil of a 0.5 mm thickness, from Ensinger GmbH. Parameters allowing assessment of the electrets in terms of technical applications are: charge lifetime τ and equivalent voltage Uz. Measurements of electrets equivalent voltage Uz has been carried out using compensation method. Assessment of lifetime required employing a method utilizing thermal stimulation of charge relaxation process. In the investigation, a continuous measurement of equivalent voltage in the condition of linear increase in temperature - TSUz was realized. It was stated that electret properties of a foil depend on the preparation conditions of the material and the forming temperature. The employed method of lifetime measurement enables verification of the applied procedure.

Behavior of charges in the active zone of composite OFET under the source-drain electric field

Charge accumulation and relaxation processes in the OFET active zone are studied. It is found that positive and negative charges injection from metal electrodes under drift field occurs. Characteristic time of these processes is found to be about 30 sec. Behaviour of charges under the combination of source-drain and gate potentials is also investigated.

Thermal Effects and Structural Changes of Photosynthetic Reaction Centers Characterized by Wide Frequency Band Hydrophone: Effects of Carotenoids and Terbutryn.

Photothermal characteristics and light-induced structural (volume) changes of carotenoid-containing and noncontaining photosynthetic reaction centers (RCs) were investigated by wide frequency band hydrophone. We found that the presence of carotenoid either does not play considerable role in the light-induced conformational movements, or these rearrangements are too slow for inducing a photoacoustic (PA) signal. The kinetic component with a few tens of microseconds, exhibited by the carotenoid-less RCs, appears to be similar to that of triplet state lifetimes, identified by other methods. The binding of terbutryn to the acceptor side is shown to affect the dynamics of the RC. Our results do not confirm large displacements or volume changes induced by the charge movements and by the charge relaxation processes in the RCs in few hundreds of microseconds time scale that accompanies the electron transfer between the primary and secondary electron acceptor quinones.

Electric field induced fluorescence hysteresis of single molecules in poly(methyl methacrylate)

Single molecule (SM) chips could serve as the fundamental devices in quantum information processing. In this context, a chip with the non-polar SMs of squaraine-derived rotaxanes embedded in a polar poly(methyl methacrylate) matrix was realized and the SM fluorescence hysteresis induced by the electric field was observed at room temperature. Here, we presented a model considering both of the electron transfer and space charge relaxation processes to explain the fluorescence hysteresis effect, and the model-based simulations agreed reasonably well with the experimental results.

Study of Space Charge in SC Shield / XLPE Interface and Mid-Voltage Cable Perfomance

Three experimental mid-voltage cables C2, C3 and C4 with cross-linked polyethylene (XLPE) isolation, rated respectively as Medium, High and Low in perforation tests over 150KV, are studied. All these cables have been systematically measured by Electroacoustic Pulse (PEA) and Thermally Stimulated with Depolarization Currents (TSDC), using 120 KV of polarization voltage in samples as received, annealed at 90 kV and 120°Cup to 672 hours. Measured internal charge of cable C4 at least doubles that of cables C2 and C3. The interfacial phenomenon has been studied by Infrared spectroscopy measurements Attenuated total reflectance (ATR) that show chemical components that migrate, spread, and transport from the external semiconductor layer to the cross-linked polyethene isolation during the thermic treatment of the cable. The PEA measurements using 120 kV/mm electric field showed the formation and propagation of space charge packet from SC to isolation. These results are coherent with TSDC measurements that also show differences in the areas under the curve of current as a function of temperature for each cable type C2, C3and C4. This is a result of the amount of charge accumulated. To sum up, combination of PEA, TSDC and ATR measurements is a useful tool in understanding charge relaxation processes and XLPE cable performance.

Investigation of Charge Relaxation in Silicon Nitride for the Reliability of Electrostatically Driven Capacitive MEMS Devices

Dielectric charging/discharging in the real-life electrostatic microelectromechanical system device is proposed to be evaluated by the capacitance-voltage response for an analogous metal-insulator-semiconductor (MIS) structure. An analytical model based on this approach has been established. In the experiment, the relaxation behaviors of trapped charges in silicon nitride of MIS structure have been systematically investigated. For both positive and negative dc bias polarities, fast and slow discharge stages were clearly observed in the early and late charge relaxation processes, respectively. The discharge ratio (DR) was found to depend on both the bias polarity and magnitude. It was shown that negative dc bias would cause a high hole injection level but low DR, whereas positive dc bias would lead to a low electron injection level but high DR. Moreover, the DR will increase with the dc bias voltage. It was further found that the hole relaxation reaches the steady state faster than the electron relaxation. To explain the experimental results, we pointed out that the traps close to the interface play an important role in dielectric charging and discharging process.

Injected charge as a cause of the slow dielectric relaxation in thin film Pt/(Ba,Sr)TiO3/Cu structures

Experimental studies of the response of Pt/Ba0.3Sr0.7TiO3/Cu capacitors to switching-off the bias voltage are reported. The remnant capacitance as a function of the bias voltage and the variation of this capacitance with time have been measured and analyzed in terms of the injection model of the space charge origin with consideration for the linear dielectric layer at the Pt/BSTO interface. The energy depths, 0.51 eV, and 0.65 eV, of two trap states controlling the slow (τ ≈ 100 s and τ ≈ 5 × 103 s) space charge relaxation process have been extracted from experiments.

Slow Relaxation Processes in CCTO Detected by Capacitance Versus Voltage Curves

The temperature and frequency dependence of capacitance versus voltage (C–V) curves was investigated for ceramic CaCu3Ti4O12 samples sintered in atmospheres of vacuum, air and oxygen, respectively. A drastic capacitance rise at higher dc voltages and low ac frequencies was observed for all these CaCu3Ti4O12CCTO samples, and it is especially strong for the sample sintered in vacuum, which has not been found previously. The shape as well as the asymmetry of the C–V curves suggest that the dielectric behavior is related to charged trap states. It also indicates that a slow relaxation process, which is strongly correlated with the re‐positioning of oxygen vacancies related trap charges, takes place in CCTO. The main trap charge relaxation process is thought to occur between the boundaries perpendicular and parallel to the external electric field, and the anisotropy of the defect dipoles provide the possibilities for trap charge repositioning which maybe the reason of the huge dielectric constant.

Photo-induced Charge Separation across the Graphene–TiO2 Interface Is Faster than Energy Losses: A Time-Domain ab Initio Analysis

Graphene-TiO(2) composites exhibit excellent potential for photovoltaic applications, provided that efficient photoinduced charge separation can be achieved at the interface. Once charges are separated, TiO(2) acts as an electron carrier, while graphene is an excellent hole conductor. However, charge separation competes with energy losses that can result in rapid electron-hole annihilation inside metallic graphene. Bearing this in mind, we investigate the mechanisms and, crucially, time scales of electron transfer and energy relaxation processes. Using nonadiabatic molecular dynamics formulated within the framework of time-domain density functional theory, we establish that the photoinduced electron transfer occurs several times faster than the electron-phonon energy relaxation (i.e., charge separation is efficient in the presence of electron-phonon relaxation), thereby showing that graphene-TiO(2) interfaces can form the basis for photovoltaic and photocatalytic devices using visible light. We identify the mechanisms for charge separation and energy losses, both of which proceed by rapid, phonon-induced nonadiabatic transitions within the manifold of the electronic states. Electron injection is ultrafast, owing to strong electronic coupling between graphene and TiO(2). Injection is promoted by both out-of-plane graphene motions, which modulate the graphene-TiO(2) distance and interaction, and high-frequency bond stretching and bending vibrations, which generate large nonadiabatic coupling. Both electron injection and energy transfer, injection in particular, accelerate for photoexcited states that are delocalized between the two subsystems. The theoretical results show excellent agreement with the available experimental data [Adv. Funct. Mater. 2009, 19, 3638]. The state-of-the-art simulation generates a detailed time-domain atomistic description of the interfacial charge separation and relaxation processes that are fundamental to a wide variety of applications, including catalysis, electrolysis, and photovoltaics.

Cure and morphology of epoxy/polyester powder coatings containing titanium dioxide: a thermally stimulated discharge study

This article describes the use of thermally stimulated electrical discharge (TSD) measurements in the study of the cure processes occurring for a range of titanium dioxide, silica, and zinc oxide-pigmented epoxy polyester films. The unfilled polyester–epoxy resin system exhibits a simple TSD profile indicative of a combination of space charge and dipole relaxation processes. The addition of the Modaflow, which is used to aid in the dispersion of the pigment in the system, induces phase separation under certain cure conditions. In a number of systems, two lower temperature Tg peaks were initially observed, which collapsed into a single peak on post cure. The principal difference between the two grades of titanium dioxide was the size of the space charge peak, which is a direct indication of the mobile ion content in the material. The sulfate has higher mobile ion content than the chloride route material. Differences were observed between traces of silica and zinc oxide, which reflect the ability of the particles to interact with the resin and form a dense matrix. Silica at low concentrations is able to reduce the space charge peak which implies it can trap ions. The TSD study indicates that the pigments are able to change the morphology of the matrix which is being formed.

Formation of a disorderd hetero‐junction by diffusion of CuI from CuSCN into In2S3 layers: A surface photovoltage study

AbstractCharge‐selective disordered hetero‐junctions were formed in evaporated In2S3 layers by diffusing at 200 °C CuI from a CuSCN source. The thicknesses of In2S3 layers and diffusion times were varied between 5 and 80 nm and between 2 and 19 min, respectively. In some cases CuSCN layers were etched back with pyridine. Spectral and time‐dependent surface photovoltage measurements were carried out in the capacitor arrangement. It was observed that a competing process of charge separation and relaxation was initiated together with the formation of the charge‐selective In2S3/In2S3:Cu hetero‐junction.Modulated SPV amplitude for different annealing times and thicknesses of the evaporated In2S3 layers. magnified image

Effect of recrystallization on the molecular mobility of a copolymer of vinylidene fluoride and hexafluoropropylene

AbstractRelaxation processes in copolymers of vinylidene fluoride with hexafluoropropylene (93/7) were studied by means of a dielectric method. The initial extruded film was recrystallized by free heating to temperatures above the melting point and by subsequent cooling. This increased both the perfection of the crystal phase and the degree of crystallinity. The impact of recrystallization on both the relaxation times (τ's) and the activation parameters of the local mobility (β process) and micro‐Brownian cooperative mobility in amorphous phase (αa process) was almost negligible, whereas the τ's of the αc relaxation were an order of magnitude higher after recrystallization. Qualitatively, it was predicted by the soliton model for the αc relaxation. The recrystallization affected characteristics of the transition at the highest temperature (α) registered in the region of the melting of the crystals even more. The process is related to the relaxation of the space charge formed by ionogenic impurities, which migrate through the amorphous phase with high free volume. It was shown that the dynamics in the amorphous phase controlled the drift mobility of free charge carriers and, by that, determined the τ of the space charge relaxation process. The structuring processes during the recrystallization also affected the parameters of the order–disorder transition in the low‐perfect ferroelectric (antiferroelectric) phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Dielectric Relaxation Investigations in Copolyesteramide (Vectra B950) Polymer Liquid Crystal: Thermally Stimulated Polarization Current Technique

Thermally stimulated polarization currents (TSPC) in copolyesteramide (Vectra B950) polymer liquid crystal have been investigated in the temperature region 20°–250°C under different polarizing fields ranging from 0.7 to 7 kV/cm and with different heating rates. The low temperature region of the TSP current spectra shows a pronounced peak (P1) around 30°C. This peak has been attributed to the dipolar relaxation process associated with ester group. The peak (P2) observed around 140°C has been attributed to a combination of two relaxation processes, the dipolar and space charge relaxation processes. A steady rise in the TSP currents by several orders of magnitude is observed above 150°C. Saturation in the TSP current around 220°C reveals the presence of a relaxation process owing to the movement of large segmental groups in LC phase.

Measurement of relaxation of electrical charge distribution on fused silica sample

Fused silica test masses (mirrors) of interferometric gravitational wave detectors may accumulate electrical charges which interact with surroundings. Fluctuations of the charge or of its distribution create fluctuating force which acts on the test mass. To study this effect we have carried out measurements of relaxation of the charge deposited by contact electrification on fused silica sample. In vacuum we have not found relaxation of the charge exceeded measurement errors. At the same time the slow drift of the residual charge distribution (existed on the sample which was not specially electrified) has been observed. Evidently, there exists a variety of the charge relaxation processes on fused silica samples, and charges trapped into traps of different types relax in different ways.

Sub-electron charge relaxation via 2D hopping conductors

We have extended Monte Carlo simulations of hopping transport incompletely disordered two-dimensional (2D) conductors to the processof external charge relaxation. In this situation, a conductor of areaL × W shunts an externalcapacitor C withinitial charge Qi. At low temperatures, the charge relaxation process stops at some ‘residual’charge value corresponding to the effective threshold of the Coulombblockade of hopping. We have calculated the root mean square (rms) valueQR of the residual charge for a statistical ensemble of capacitor-shunting conductorswith random distribution of localized sites in space and energy and randomQi, as a function of macroscopic parameters of the system. Rather unexpectedly,QR has turned out to depend only on some parameter combination: for negligible Coulomb interaction and for substantial interaction. (Hereν0 is the seed density oflocalized states, while κ is the dielectric constant.) For sufficiently large conductors, both functionsQR/e = F(X) follow the powerlaw F(X) = DX−β, but withdifferent exponents: β = 0.41 ± 0.01 for negligible and β = 0.28 ± 0.01 for significant Coulomb interaction. We have been able to derive this law analytically forthe former (most practical) case, and also explain the scaling (but not the exact value ofthe exponent) for the latter case. In conclusion, we discuss possible applications of thesub-electron charge transfer for ‘grounding’ random background charge in single-electrondevices.

Monte Carlo simulation of pneumatic tribocharging in two-phase flow for high-inertia particles

Pneumatic transport, triboelectrostatic experimentation studying the removal of carbon from fly ash has shown the importance of particle charging on process performance. To further elucidate the influence of particle charging, a tribo-electrification model for charging spherical particles in vertical pipes is proposed. Unlike previous numerical approaches, the charging mechanism is studied in the framework of a charge relaxation process in which the equilibrium charge for uniform and localized charge distributions on the particle surfaces are determined. A recently proposed statistical model was also adopted to evaluate particle–wall collisions for wall-bounded flows, taking into account the influence of particle–particle collisions. Effects of particle concentrations, along with particle size and turbulence intensity, are also investigated. Model predictions are compared with experimental results, with good agreement found if the charge is assumed to be distributed on small surface sector of the particles. After defining suitable probability density functions for the variables considered, a Monte Carlo analysis is employed for simulating particle charge distributions and compared with published measurements to identify the important parameters during particle tribocharging.