Single RC Circuit Research Articles

Molecular p-doping induced dielectric constant increase of polythiophene films determined by impedance spectroscopy

The dielectric constant (εr) is a fundamental material parameter that governs charge transfer processes in organic semiconductors, yet its value is often assumed rather than measured. Here, we use impedance spectroscopy to determine εr in regioregular poly(3-hexylthiophen-2,5-diyl) (P3HT) thin films p-doped with the molecular dopants hexafluoro-tetracyanonaphthoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We fit the impedance spectra using a single RC circuit model to determine the frequency-dependent capacitance and extract εr. The value of the dielectric constant increases by around two-thirds from 2.9 ± 0.1 (undoped polymer) to 4.9 ± 0.6 on the addition of one F4TCNQ molecule per 500 P3HT monomer units. In contrast, the addition of the weak dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ), which does not undergo ground state charge transfer with P3HT, has no effect on the dielectric constant. Our results support the hypothesis that molecular doping has a considerable impact on the materials dielectric constant via polarizable host-dopant complexes.

Fine-grained magnetoelectric Sr0.5Ba0.5Nb2O6–CoFe2O4 composites synthesized by a straightforward one-pot method

Magnetoelectric (Sr0.5Ba0.5Nb2O6)1−x−(CoFe2O4)x (x = 0.2–0.6) composites were prepared by a one-pot soft-chemistry synthesis using PEG400. Calcining at 700 °C resulted in nanocrystalline composite powders (dcryst. = 24–30 nm) which were sintered between 1050 and 1200 °C to ceramic bodies with relative densities up to 98%. SEM investigations confirm the formation of composite ceramics with a 0–3 connectivity and variable grain sizes from 0.2 to 3.6 μm for sintering up to 1150 °C, while sintering at 1200 °C leads both to a change in the microstructure and a considerable grain growth. Magnetic measurements at 300 K reveal ferrimagnetic behaviour with saturation magnetization values smaller than bulk CoFe2O4 and coercivities between 790 and 160 Oe. Temperature-dependent impedance spectroscopy showed that the relative permittivities decrease both with rising frequency and CoFe2O4 fraction. The frequency dependence of the impedance can be well described using a single RC circuit. Magnetoelectric measurements show the presence of pronounced field hystereses. The maximum magnetoelectric coefficient (αME) depends both on the CoFe2O4 fraction (x) and sintering temperature. The composite with x = 0.3 exhibits the largest αME value of 37 μV Oe−1 cm−1 (@ 900 Hz). With rising frequency of the AC driving field αME increases up to 300–400 Hz and is nearly constant until 1 kHz.

Диэлектрическая спектроскопия как метод исследования тонких пленок диоксида ванадия

The dielectric spectra of thin (1200 Å) vanadium dioxide films, a material with strong electron-electron correlations, were studied. Both the undoped and germanium-doped VO2: Ge films were studied. For the VO2: Ge, an additional maximum has been detected in the frequency of the dielectric loss spectrum. The fine structure of the spectra was investigated and the physical interpretation of two maxima in the frequency dependence of the tangent of the dielectric loss angle and of two semi-circles on the Cole-Cole diagram was performed. The results were analyzed on the basis of the equivalent electrical circuits of the samples: a single RC-circuit for the case of an undoped VO2 film and a double-circuit  for VO2: Ge. The numerical values of the parameters of the model schemes are determined. It has been established that the complicating of equivalent circuit for VO2: Ge compared to undoped VO2 is due to the complication of the electrical response of the doped film to the influence of an electromagnetic field. It is shown that this complication is associated with the acceptor properties of a Ge impurity, which is manifested when the strongly correlated material is doped with an isoelectronic impurity.

Magnetic, optical, dielectric, and sintering properties of nano-crystalline BaFe0.5Nb0.5O3 synthesized by a polymerization method

A one-pot polymerization method using citric acid and glucose for the synthesis of nano-crystalline BaFe0.5Nb0.5O3 is described. Phase evolution and the development of the crystallite size during decomposition of the (Ba,Fe,Nb)-gel were examined up to 1100 °C. Calcination at 850 °C of the gel leads to a phase-pure nano-crystalline BaFe0.5Nb0.5O3 powder with a crystallite size of 28 nm. The shrinkage of compacted powders starts at 900 °C. Dense ceramic bodies (relative density ≥ 90%) can be obtained either after conventional sintering above 1250 °C for 1 h or after two-step sintering at 1200 °C. Depending on the sintering regime, the ceramics have average grain sizes between 0.3 and 52 µm. The optical band gap of the nano-sized powder is 2.75(4) eV and decreases to 2.59(2) eV after sintering. Magnetic measurements of ceramics reveal a Neel temperature of about 23 K. A weak spontaneous magnetization might be due to the presence of a secondary phase not detectable by XRD. Dielectric measurements show that the permittivity values increase with decreasing frequency and rising temperature. The highest permittivity values of 10.6 × 104 (RT, 1 kHz) were reached after sintering at 1350 °C for 1 h. Tan δ values of all samples show a maximum at 1–2 MHz at RT. The frequency dependence of the impedance can be well described using a single RC-circuit.

The effect of substrate pore size on the network interconnectivity and electrical properties of dropcasted multiwalled carbon nanotube thin films

Abstract

The illustrated brief application of defect distribution model for heterojunction device by admittance spectroscopy

The dielectric properties of the devices can be studied by using admittance spectroscopy (AS). Majority carrier traps are experimentally analyzed without sophisticated mathematical manipulation. Experimental evidence obtained from AS of the devices may display several features that cannot be explained by the usual single RC circuits representing a depleted junction region and undepleted bulk elements. In this study, ac behavior of typical pn junction diode is simulated using temperature depended AS in a broad frequency range (5Hz to 13MHz). The frequency-dependent admittance data of typical pn junction has been discussed based on a discrete deep or shallow trap model. The analytically obtained frequency depended −dC/dω and tanδ (loss tangent) curves are peaks function. The position of the peak depends on the temperature and dc-bias. This peak position gives us information about deep or shallow trap states. The trapping time and trap energy levels can be deduced from −dC/dω versus ω or tanδ versus ω curves. This investigation shows that suitable complex admittance measurement data obtained during the capacitance–voltage measurement process are used to calculate the trapping time and energetic position of the traps. A theoretical analysis and computer simulation are presented in order to illustrate the nature of the trap and the technique by which accurate trapping time and energy position of the trap state can be obtained.

Impedance Spectroscopy in Monitoring the Maturation of Stem Cell-Derived Retinal Pigment Epithelium

The development and differentiation of stem cell-derived impermeable retinal pigment epithelium (RPE) with tight junctions (TJs) is a gradual process that is, at confluence, controlled by cell-to-cell contact. The objective of this study was to evaluate the use of electric impedance spectroscopy (EIS) to follow the maturation and development of barrier function in human embryonic stem cell-derived RPE (hESC-RPE). Barrier function was assessed using EIS, permeability measurements, and microscopic inspection in intact cells and following calcium sequestration with ethylene glycol tetraacetic acid (EGTA). The results showed that the cultures with the most mature morphology had the highest impedance and the lowest permeability values. The EIS of samples of high integrity fitted well to the equivalent model of a single RC circuit, whereas the semicircular shape of the Nyquist plots was distorted for samples of lower integrity. EGTA treatment resulted in lower impedance values and changes in the shapes of plots. Our results show that EIS-as a measure of overall maturity and integrity of the epithelium-is useful when evaluating the maturity of cell cultures. It is highly warranted in future transplantation therapies and in in vitro cell culture models in drug development.

Simple RC network models for simulating dielectric response with two relaxation loss peaks

In this contribution, we analyze the dielectric response of two simple RC models. Both RC-models are composed of single RC circuits with the exponential response. The relaxation time of single RC relaxor is defined by the product of resistance and capacitance: τ = RC. The symmetric relaxation loss peak is observed when the Gaussian distribution of the relaxation time is assumed. The introduction of coupling among the RC circuits generates a new relaxation mode occurring in low frequency range. Thus, the simple RC network models can be used for simulating dielectric spectra with two relaxation modes. In addition, by changing the value of coupling resistors, R S, it is possible to control the separation between these two relaxation peaks. As the value of coupling resistors is low (high degree of coupling), the dielectric response reveals the so-called ‘excess wing’ phenomenon.

Equivalent circuit analysis for ferroelectrics

The purpose of this study is to make up the equivalent circuit for bulk relaxation, in which excluded a grain boundary and electrode relaxation, and to simulate its impedance under various conditions. As ac field applied to dielectric or ferroelectric materials, the dipole displacement and the charge carrier migration occur simultaneously. With constructing the Debye equivalent circuit and a single RC circuit in parallel(briefly DRC), two dispersions in only grain may be interpreted. The parameters in DRC circuit is estimated on the basis of real situation. As a result of simulation, (i) the resistance for DRC is affected by only Rh , not Rd , even if Rd < Rh , and (ii) the relaxation behavior significantly varied according to the ratios, Rh / Rd and Ch /C d2. The most relaxation behaviours of dielectrics and ferroelectrics can be represented with a combination of parameters in DRC, not including the complicate mathematics.

Thermoelectrical analysis of the human skin barrier

The influence of temperature on the resistive and capacitative properties of human stratum corneum in vitro was studied to determine where within substructures of stratum corneum, the electrical resistance R and capacitance C components reside. Heating-cooling cycles were designed in accordance with earlier calorimetric and spectroscopic studies of thermal transitions of human stratum corneum lipids and/or proteins. Two different protocols were used. (A) Heat treatment and electrical analysis were carried out simultaneously in pH 7.4 phosphate buffered saline, starting with prehydrated stratum corneum (70% w/w) of pH 7.4. (B) Heat treatment was performed before electrical analysis, using dried stratum corneum (< 10% w/w), followed by prehydration and measurement of the electrical properties in phosphate buffered saline at 20°C. Square-wave alternating current pulses of 13 μA cm −2 were applied every 60 s. Analysis of the resulting voltage waveform across stratum corneum yielded an equivalent electrical model of stratum corneum composed of a series connection of two RC circuits ( R 1 ∥ C 1 and R 2 ∥ C 2 ). Below 60°C a constant activation energy of 5.4 ± 0.7 kcal mol −1 was measured, which was close to the activation energy of K + diffusion in a fluid aqueous medium. The total resistance of stratum corneum was less than 100 kΩ cm 2 , which is very low compared to the resistance of black lipid membranes (1–10 MΩ cm 2 ). Both the low activation energy and resistance of human stratum corneum suggest the presence of highly conductive pathways through the membrane. Between 60 and 75°C an abrupt decline of the resistances R 1 and R 2 and a rapid rise of the capacitances C 1 and C 2 was observed. This temperature interval corresponded to the temperature interval of the second thermal transition observed in human stratum corneum, which is a lipid phase transition. Beyond 75°C, the resistances were fairly constant, while the capacitances continued to increase. The changes in the resistances and capacitances brought about by heating to 75 and 95°C were completely irreversible. This is in agreement with X-ray diffraction studies, which have shown that the originally predominant lamellar structure with a repeat distance of 6.4 nm does not reappear after recrystallization from 75 and 90°C. The final resistances after heating desiccated stratum corneum (⩽ 10% w/w hydration) to temperatures of 75 and 95°C and subsequent cooling were less different from the original values than those of hydrated samples, suggesting that the impact of heating depends on the level of stratum corneum hydration. After chloroform-methanol lipid extraction, stratum corneum was characterized by a single RC circuit. The resistance of extracted stratum corneum was less than 1% of the original pre-extraction value, which indicates that the resistance of stratum corneum is mainly determined by the intercellular lipids. Taken together, the correlation between the temperature dependence of the resistances and capacitances between 20 and 75°C suggests that the electrical barrier and the charge storage capacity of stratum corneum are determined by the same substructures. Most probably these structures are the intercellular lipid lamellae, because the changes of the electrical properties mainly take place within the interval 60 to 75°C, which corresponds to the phase transition of free intercellular lipids, and also because these changes are irreversible. This conclusion is also confirmed by chloroform-methanol extraction. However, the continued increases of the capacitances between 75 and 95°C, indicates that the charge storage capacity of stratum corneum also depends on lipids attached to protein. The low resistance and activation energy at temperatures between 20 and 60°C indicate that stratum corneum possesses highly conductive pathways.