Free Charge Concentration Research Articles

Impact of film thickness on the structural and linear/nonlinear optical properties in highly oriented Cs3Bi2I9 perovskite films

This study presents the successful preparation of highly oriented Cs3Bi2I9 films with different thicknesses using sol–gel spin-coating method. X-ray diffraction analysis confirmed a single hexagonal crystal phase with a high orientation along the <0 0 l> direction. The crystallite size in the range of 27 and 41 nm increase with increasing film thickness. The band gap energy ranged from 1.91 to 1.97 eV was found to decrease with increasing film thickness which was associated to the film relaxation with increasing film thickness. The refractive index, absorption index, complex dielectric coefficients, complex optical conductivity coefficients, and the ratio of free charge concentration also were found to increase with film thickness. The first-order linear optical susceptibility ranged from 0.22 to 0.38, the non-linear optical susceptibility χ(3) and the nonlinear refractive index parameters n(2) were respectively within the range of 0.4–3.77 × 10−12esu 0.96–7.97 × 10−11esu, the highest value of all the parameters was found in the visible region at 500 nm. These findings position Cs3Bi2I9 as highly promising material for optoelectronics.

Achieving highly tunable negative permittivity in titanium nitride/polyimide nanocomposites via controlled DC bias

Metacomposites with negative permittivity that derived from random structures have drawn considerable attention. Nowadays, negative permittivity was commonly realized in percolative composites via controlling the compositions and microstructures. Herein, highly tunable plasma-like negative permittivities were successfully achieved in titanium nitride/polyimide nanocomposites via applying external DC bias. It is indicated that the trapped charges at the interfaces between titanium nitride and polyimide can escape from the interfaces after absorbing energy from DC bias, resulting in elevated concentration of free charges and enhanced plasma oscillations, hence the enhanced plasma frequencies and negative permittivities. This work demonstrated that applying DC bias is an effective way to achieve highly tailorable negative permittivity, which is of great significance to the exploration of high-performance negative-k metacomposites.

Doping effects on the thermoelectric properties of pristine poly(3,4-ethylenedioxythiophene)

Electronic and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) depend strongly on its geometric structure and the free charge concentration in the PEDOT crystals.

NO2 sensing mechanism of ZnO–Eu2O3 binary oxide under humid air conditions

In order to understand the role of ambient humidity on NO2 detection, electrical resistance and work function changes were recorded simultaneously on ZnO–Eu2O3 material. Thus, surface band bending and electron affinity could be evaluated, according to the Schottky model. The lack of electron affinity changes during NO2 exposure in dry air conditions, reveals that only the free charge concentration is affected. Exposure to NO2 in humid background leads to an increase in sensor signals and a significant change in the electron affinity as well. This effect suggests an interaction between NO2 and surface hydroxyl species; thereby additional charge carriers are involved in the overall sensing process. High resolution XPS analysis proves the presence of Eu solely in the 3+ oxidation state before and after NO2 exposure. A gas-sensing interaction mechanism has been proposed.

Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis

Considerable research effort has gone into improving the performance of traditional thermoelectric materials such as Bi2−x Sb x Te3 through a variety of nanostructuring approaches. Bottom-up, chemical approaches have the potential to produce very small nanoparticles (≪100 nm) with narrow size distribution and controlled shape. For this study, nanocrystalline powder of Bi0.5Sb1.5Te3 was synthesized using a ligand-assisted chemical method, and consolidated into pellets with cold pressing followed by sintering in Ar atmosphere. The thermoelectric transport properties were measured from 7 K to 300 K as a function of sintering temperature. Sintering is found to increase ZT and to move the maximum in ZT to lower temperatures due to a reduction in the free charge concentration. Hall mobility studies indicate that sintering increases the electron mean free path more than it increases the phonon mean free path up to sintering temperature of 598 K. A maximum ZT of 0.42 was measured at temperature of 275 K.

Experimental Characterization of Photonic Fiber-Integrated Modulator

In this paper we present experimental characterization of a fiber-integrated, all-optical silicon based modulator. The operation principle of the proposed device is based upon plasma dispersion effect in silicon, which enables controlling the optical properties of silicon at wavelength of around 1550nm by modifying the concentration of free charges either by applying external electrical field or by using illumination in spectral range that is being absorbed by this semiconductor. The concentration of the generated free charge carriers affects the real and the imaginary part of the refractive index of the silicon semi-conductor i.e. yields controllable absorption and refraction effects over the information carrier wavelength around 1550nm. In this paper silicon slices at varied thicknesses are integrated between an input and an output fiber and the performance of the integrated all-optical modulator are characterized versus various thicknesses of the silicon layer and the wavelength of the control signal when incident on the silicon layer from both sides or from either side of it.

Transient electrophoretic current in a nonpolar solvent

The transient electric current of surfactants dissolved in a nonpolar solvent is investigated both experimentally and theoretically in the parallel-plate geometry. Due to a low concentration of free charges the cell can be completely polarized by an external voltage of several volts. In this state, all the charged micelles are compacted against the electrodes. After the voltage is set to zero the reverse current features a sharp discharge spike and a broad peak. This shape and its variation with the compacting voltage are reproduced in a one-dimensional drift-diffusion model. The model reveals that the broad peak is formed by a competition between an increasing number of charges drifting back to the middle of the cell and a decreasing electric field that drives the motion. After complete polarization is achieved, the shape of the peak stops evolving with the further increase of the compacting voltage. The spike-peak separation time grows logarithmically with the charge content in the bulk. The time peak is a useful measure of the micelle mobility. Time integration of the peak yields the total charge in the system. By measuring its variation with temperature, the activation energy of bulk charge generation has been found to be 0.126 eV.

Phase diagrams of a model diluted fcc magnet with arbitrary spin and modified RKKY interaction: Influence of external magnetic field and structural short-range order

A diluted FCC magnet with modified long-range RKKY interaction and arbitrary Ising spin S is considered within two-sublattice model. In the molecular field approximation the Gibbs free-energy is derived, from which all magnetic thermodynamic properties can be self-consistently obtained. In particular, the phase diagrams are studied for different magnetic ion and free-charge concentration, the atomic short-range-order (Warren-Cowley) parameter being taken into account.

Space Charge Limited Current in Porous Silicon with traces of Nitrogen Dioxide

AbstractThe discovery of the steep increase of electrical conductivity in Porous Silicon (PS) samples in contact with Nitrogen Dioxide, NO2, opens the possibility to study the electrical conduction processes in nanostructured systems from a completely new point of view. The system undergoes a change of conductivity of 5 orders of magnitude when exposed to few ppm of NO2, whereas the free charge concentration increases by only one order of magnitude in the same pressure range. Upon NO2 exposure the system transition from an “insulator-like” to a “semiconductor-like” behaviour can be studied by means of several techniques. In this paper, the results from a Space Charge Limited Current (SCLC) study unambiguously demonstrate the change of the charging status of defects upon NO2exposure, and explain some “exotic” effects recently reported.

DRIFT studies of thick film un-doped and Pd-doped SnO 2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapour

Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO 2 sensors at different temperatures between room temperature and 300 °C. The results show the presence of different surface OH groups as well as coordinated water on the SnO 2 sensor surface. Their intensity changes with temperature. During the temperature cycles the bands’ peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 °C at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO 2 surfaces, the appearance of gaseous CO 2 and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements.

Low-energy particle treatment of GaAs surface

Modification of high-doped GaAs surface by the interaction with RF plasma and a monoenergetic beam of similar chemical composition (Ar with a low content of H 2) was investigated via the evolution of optical, electrical and structural properties. A reduction in the free charge concentration in the GaAs surface region of ∼100 nm in thickness is concluded from Raman spectroscopy. In the processed samples, the surface electric field strength induced by a laser beam (photoreflectance technique) continually decreases with both the energy and fluence of impinging particles. The same results were obtained by evaluation of quasistatic C– V curves of corresponding MOS structures. They confirmed that longer exposure caused the formation of ∼100-nm-thick modified near-surface region with a decreased donor concentration. After the plasma irradiation, an ∼10-nm-thick outermost insulating layer was created in situ. The formation of textured polycrystalline grains with (100) orientation in the surface region was observed by X-ray diffraction.

Scattering of centimeter radiowaves in a gas ionized by radioactive radiation: Cluster plasma formation

A theoretical explanation is proposed for an anomalously high reflectivity of air masses exposed to radioactive radiation relative to electromagnetic waves from the rf range. The mechanism of formation of the reflected signal is connected with a change in the electric parameters of the ionized gas. The concentration of free charges under the typical conditions of radioactive contamination is ten orders of magnitude lower than that required for the formation of an experimentally detectable reflected signal. The discrepancy between the values of reflectivity observed under the real conditions of radar probing and predicted theoretically on the basis of the elementary theory of a weakly ionized gas amounts to 20 orders of magnitude. It is shown that the inclusion of the variation of the mass and the critical capture radius of ions due to their hydration changes the difference between the theoretical predictions and the experimental observations insignificantly. The discrepancy becomes smaller (but only by 1.5 orders of magnitude) when the scattering of radiowaves from turbulent vortices is taken into account. The mechanism of the formation of the high reflectivity is associated with slowing down the recombination and with the accumulation of a profuse population of unrecombined ionic pairs stabilized in the clusters of water molecules. The steady-state concentration of such electrically neutral clusters is several orders of magnitude higher than the concentration of free hydrated ions. A variation of the intensity of ionizing radiation is accompanied by proportional variations of both components. The recombination barrier is formed as a result of drawing dipole molecules into the gap between ions at the final stage of motion of counterions towards one another before their recombination. The accumulation of ionic pairs ensures the multiple enhancement of the sensitivity of the electric properties of cold plasma to the effect of ionizing radiation. A quantitative kinetic theory of the effect is constructed. The numerical calculations of the parameters of the pre-recombination states of ions against the background of the molecular component are made using computer simulation at the microscopic level. The steady-state recombination rate is an exponential function of the pre-recombination barrier height and decreases rapidly even upon an insignificant change in the number of molecules involved in an ion recombination act. The obtained theoretical conclusions are confirmed by the independent results of observations of the strong absorption band in the atmosphere in the middle part of the IR spectrum, which is attributed to the anomalously high concentration of electrically neutral water clusters.

The effect of precipitation on the optical properties of Al-Zn thin films

The optical constants n and k of Al-40 wt% Zn alloy thin films deposited at room temperature were calculated using a transmission spectrum in the wavelength range from 2.5 to 20 μm. The thickness of the films ranged from 10 to 85 nm. The investigated samples were aged at 373 K and 473 K for 1 h and some micro-characteristics of Al-Zn alloy films such as the free charge concentration N, the relaxation time τ, the static conductivity σ, the electron velocity at Fermi surface V f and Fermi energy were determined. The effective absorption assumed the value 63.7% and the absorption due to bound electrons is therefore 36.3%. The role of the solute atoms (Zn) was considered to explain the results.

Study of some optical properties of evaporated TbCu 0.6Al 0.4 thin films

Evaporated TbCu 0.6Al 0.4 thin films with different thicknesses, ranging from 26 to 100 nm, were prepared on a glass substrate. The complex refractive index of thin films has been determined for the first time from the measurements of reflectance, R, and transmittance, T, at almost normal incidence in the spectral range of 400–2200 nm. Some micro-characteristics of TbCu 0.6Al 0.4 alloy including free charge concentration, n c , the relaxation time, τ, the static conductivity, σ, and the electron velocity at the Fermi surface, V F, have been evaluated using the simplified Drude model.

The optical properties of manganese thin films

The optical constants n and k of thin manganese films of different thicknesses ranging from 20 up to 50 nm were determined in the spectral range 3.5–20 μm. The results were compared with the predictions of the Drude free electron theory and anomalous skin effect theory. The previous theories were used to evaluate some micro characteristics of Mn thin films such as the free charge concentration N, the relaxation time τ, the static conductivity σ s, and the electron velocity at Fermi surface V F and the value of the effective area of Fermi surface A F. Using the optical constants, the optical conductivity ( σ = σ 1 + i σ 2) was also estimated. It was found that the contribution of the effective absorption A eff equals 69%, i.e. the absorption due to bound electrons may be 31%.

Optical properties of vanadium thin films

The optical constants of vanadium thin films of different thicknesses were determined in the spectral range of 2.5 to 8.5 μm. These optical constants were used to evaluate some microcharacteristics of vanadium thin films such as the free charge concentration, the relaxation time, the static conductivity, the electron velocity at the Fermi surface, the mean free path and the specularity parameter.

Model for the above-threshold characteristics and threshold voltage in polycrystalline silicon transistors

A new theory of polycrystalline silicon thin-film transistors is proposed, based on a continuous trap state density. Three different regimes are predicted: subthreshold, transitional, and crystallinelike. Two characteristic voltages are identified: the threshold voltage, corresponding to the condition of equal trapped and free charge concentration at the oxide/semiconductor interface and the on-voltage, corresponding to the condition of equal trapped and free charge in the whole space-charge region. In the case of an exponential distribution of gap states, approximated analytical expressions can be deduced and a simple accurate fitting procedure is presented. A very good agreement with the experiment is obtained, confirming the importance of taking into account the detailed energy dependence of the trap distribution.