Energy Of Charge Carriers Research Articles

Synthesis of V2O5 nanostructures and electrical transport properties of V2O5 nanoparticle

In this study, Vanadium pentoxide (V2O5) nanostructures have been synthesized by a simple hydrothermal method at different synthesis temperatures (150, 170, 190 °C). The synthesized V2O5 nanostructures were characterized by X-ray diffraction, Raman spectroscopy, Scanning and High resolution transmission electron microscopy and physical property measurement system for investigation of the structural, vibrational, morphological, and electrical transport properties of nanostructures respectively. X-ray diffraction analysis stands for the growth of nanostructure is in pure orthorhombic phase, and other sub-oxide phases of the metal oxide are absent. Raman spectra define the layered structure of V2O5. The morphological study of V2O5 with synthesis temperature 190 °C shows the uniform formation of nanoparticles with dimensions ∼100 nm. The transport measurements confirm the semiconductor nature of nanoparticles and follow Arrhenius type conduction mechanism, which gives the value of the activation energy of charge carriers for V2O5 nanoparticles to be 100 meV.

Facile synthesis of a versatile Ti/Ti-W@PANI nanocomposite for sustainable hydrogen production under solar irradiation

TiO2/TiO2-WO3 (Ti/Ti-W) reinforced PANI hybrid composite synthesis and investigation of its structural and photocatalytic hydrogen evolution activity are discussed in the present work. The efficiency of the synthesized core/shell structured Ti/Ti-W in re-enforcing polyaniline, a conductive polymer with low band gap and enhanced visible light response for hydrogen evolution reaction is studied. Polyaniline supported mixed oxide composite is successfully fabricated by an in-situ polymerization reaction in presence of the as prepared composite. The interaction at the interface of the polymer and the oxide nanofiller in the resultant composite makes it a better visible light harvestor with significantly enhanced charge separation. The modified PANI/Ti/Ti-W composite yields 20.1 mL of hydrogen from 20 mL water containing 0. 1 g of diethyl amine hydrogen chloride with a quantum efficiency of 20.7%. The improved hydrogen delivering capacity offered by the polymer composite could be better explained in terms of effective utilization of solar energy, surface porosity and efficient separation of charge carriers. The mesoporous catalyst facilitated photocatalytic reaction by rapid diffusion, adsorption of reactants and by fast desorption of product molecules. The composite takes the advantage of both the organic and the inorganic components to achieve active hydrogen generation.

Sensitivity of Thermoelectric Properties from the EPA Method and Its Variants on Variations in Phonon Frequencies

The electron–phonon coupling phenomenon is an important scattering source of charge carriers in thermoelectric materials. As the result, consideration of the scattering mechanism due to electron–phonon coupling is an essential part of the first-principles prediction methods for thermoelectric properties of materials, i.e., the Seebeck coefficient and electrical conductivity. However, a direct, high-resolution treatment of electron–phonon coupling in a realistic situation is too expensive for practical purposes, and approximation methods have been developed for the phenomena at a reasonable level of computational cost. The electron–phonon averaged (EPA) method and its variants have been recently introduced as such approximation methods. These methods comprise two major approximations. First, the electron–phonon coupling matrix elements, which are explicitly dependent on the momentum of charge carriers and that of scattering phonons, are approximated as a function of two energies only, i.e., the energy of the incoming charge carrier before scattering and that of the outgoing charge carrier after scattering. Second, the phonon frequencies in the formula of scattering rate are replaced with their averages in the Brillouin zone. Although these methods have achieved remarkable successes in predicting thermoelectric properties of materials, uncertainty introduced by these approximations and sensitivity of the results to variations of input parameters in these approximations have not been assessed completely. In this study, the uncertainty and sensitivity of thermoelectric properties, predicted with the EPA method and its variants, on variations in phonon frequencies are investigated. A thermoelectric p-type half-Heusler compound, HfCoSb, is used as an example. An empirical bootstrap method is employed to assess the impact of sampled phonon frequencies during the averaging process, whose results show that the impact of variations in phonon frequencies induces a minor but detectable change in predicted thermoelectric properties.

Monte Carlo study of β-Ga2O3 conductivity

Using semiclassical Monte Carlo simulations a conductivity of β-Ga2O3 is investigated considering a scattering of charge carriers on acoustical phonons, polar optical phonons and charged impurities. Modelling takes into account 12 IR active polar optical phonons modes. Dependence of mean collision frequency on energy of charge carriers is examined. The results are compared with the conclusions of other groups of researchers.

Electron-Phonon Scattering Is Much Weaker in Carbon Nanotubes than in Graphene Nanoribbons.

Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) are lower-dimensional derivatives of graphene. Similar to graphene, they exhibit high charge mobilities; however, in contrast to graphene, they are semiconducting and thus are suitable for electronics, optics, solar energy devices, and other applications. Charge carrier mobilities, energies, and lifetimes are governed by scattering with phonons, and we demonstrate, using ab initio nonadiabatic molecular dynamics, that charge-phonon scattering is much stronger in GNRs. Focusing on a GNR and a CNT of similar size and electronic properties, we show that the difference arises because of the significantly higher stiffness of the CNT. The GNR undergoes large-scale undulating motions at ambient conditions. Such thermal geometry distortions localize wave functions, accelerate both elastic and inelastic charge-phonon scattering, and increase the rates of energy and carrier losses. Even though, formally, both CNTs and GNRs are quantum confined derivatives of graphene, charge-phonon scattering differs significantly between them. Showing good agreement with time-resolved photoconductivity and photoluminescence measurements, the study demonstrates that GNRs are quite similar to molecules, such as conjugated polymers, while CNTs exhibit extended features attributed to bulk materials. The state-of-the-art simulations alter the traditional view of graphene nanostructures and demonstrate that the performance can be tuned not only by size and composition but also by stiffness and response to thermal excitation.

A dual transfer strategy for boosting reactive oxygen species generation in ultrathin Z-scheme heterojunction driven by electronic field

Reactive oxygen species (ROS), which are generated from molecular oxygen activation and oxidization of H2O/OH− in photocatalysis process, are vital for environmental remediation. In this study, a dual strategy for boosting ROS generation is proposed by construction of ultrathin Z-scheme heterojunction of Bi5O7I/g-C3N4. The charge-carrier-quenching effect is inhibited attributed to the prepared ultrathin 2D/2D heterojunction. Profiting from the interfacial charge properties, the boosting molecular oxygen activation and oxidization ability are obtained in the heterojunction. In comparison with pristine catalysts, the yield amount of superoxide (O2−) in Bi5O7I/g-C3N4 case shows enhancement. Furthermore, controllable experiments demonstrate that the generation of hydrogen peroxide (H2O2) is through two-step electron reduction strategy, and it can decompose into hydroxyl radicals (OH) assisted with photoelectrons. The singlet oxygen (1O2) is produced by two transfer pathways including charge transfer and energy transfer. Through DFT calculation, the built-in electronic field (IEF) is formed owing to the electron configuration. The IEF in the heterojunction tunes the charge transfer from type-II to Z-scheme model. Simultaneously, the short charge transfer distance enables more high energy charge carriers to reach the surface of solid catalyst for catalytic reactions. Owing to the boosting ROS generation, the novel composite shows enhanced photocatalytic performance for ciprofloxacin (CIP) degradation. This study gives a proof-of-concept perspective on fabrication of 2D/2D Z-scheme heterojunction with boosting ROS generation for environmental remediation and energy conversion.

Electric field and oxygen concentration-dependent transport properties of nano-graphene oxide

Electrical transport properties of the nano-graphene oxide were investigated by measuring current-voltage characteristics in the wide temperature range of 15 K∼450 K. The n-GO is composed of nanometer-sized intact graphene-like sp2 domains embedded in the sp3 matrix which acts as a charge transport barrier between the highly conductive sp2 domains. The oxygen in the n-GO has the concentration of 4.43 at% in the form of oxygen functional groups. Below the conduction band, four discontinuous localized states with the activation energies of 1.92 meV, 3.27 meV, 5.54 meV, and 6.58 meV were observed. These activation energies decrease with decreasing oxygen concentration and increasing external electric field in the n-GO material. Moreover, we found that the direct tunneling of charge carrier through the sp3 barrier was a dominant transport mechanism for the n-GO material. Also, unlike the activation energy of charge carrier, the transport barrier was independent of both the concentration of the oxygen functional groups and external electric field. The transport barrier was mainly determined by insulation property of the sp3 structure.

Versatile Electronic Devices Based on WSe2/SnSe2 Vertical van der Waals Heterostructures.

Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from those of existing (Si, Ge, and III-V bulk material based) technologies. Here, we report high-performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the heterostructure devices show superb characteristics, with a high current on/off ratio of ∼3 × 108, an average subthreshold slope of 126 mV/dec over 5 dec of current change due to band-to-band tunneling, an ultrahigh rectification ratio of ∼3 × 108, and a current density of more than 104 A/cm2. Furthermore, a small signal half-wave rectifier circuit based on a majority-carrier-transport-dominated diode is successfully demonstrated, showing great potential in future high-speed electronic applications.

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

Abstract The (K0.90Li0.10) (Nb0.80Ta0.20)0.99Mn0.01O3 (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.

Combined influence of Urbach’s tail width energy and mobility of charge carriers on the photovoltaic performance of bulk heterojunction organic solar cells

Bulk heterojunction organic solar cells with four different Urbach’s tail width energies ( $${E}_{U}$$ ) are simulated, and the combined influence of $${E}_{U}$$ and charge carrier mobility on the short circuit current ( $${J}_{sc})$$ , open circuit voltage ( $${V}_{oc})$$ , fill factor (FF), power conversion efficiency (PCE), Langevin and tail state recombination is investigated. The results show that $${J}_{sc}$$ decreases more drastically with increasing $${E}_{U}$$ in the lower mobility range and then asymptotically converges to a single value in the large mobility range. Likewise, with the increase in mobility, $${V}_{oc}$$ decreases nearly linearly with different slopes depending on different $${E}_{U}$$ in the lower mobility range and then different slopes converge to a single slope in the larger mobility range. Thus, $${V}_{oc}$$ becomes $${E}_{U}$$ independent in the higher mobility range. Furthermore, it is found that by increasing $${E}_{U}$$ , the maximum values of FF and PCE shift towards the higher mobility range. It is shown that the influence of combined optimization of $${E}_{U}$$ and mobility enhances both FF and PCE, but its effect on FF is much more significant than that on PCE.

Electronic polarization in dipolar organic molecular semiconductors: The case study of 1,2,3,4-tetrafluoro-6,7-dimethylnaphthalene crystal

Electronic polarization has an important impact on the site energies of charge carriers that play a key role in determining the charge transport in organic semiconductors. Dipolar molecules have strong intermolecular interactions and widespread applications in organic optoelectronics. However, compared with nonpolar organic semiconductors, electronic polarization for dipolar systems has been rarely studied. Here, taking 1,2,3,4-tetrafluoro-6,7-dimethylnaphthalene as representative, we have calculated the electronic polarization energies of dipolar organic molecular crystals by means of a polarizable force-field method. Surprisingly, our results point to that the polarization energies for this dipolar system are similar to those of nonpolar systems. In addition, the π-π stack contributes only about 30%∼40% to the total polarization energy, thus the polarization effects along the three dimensions should be treated equally even for the one-dimensional stacking crystals.

Enhanced plasmonic rectification of terahertz radiation in spatially periodic graphene structures towards the charge neutrality point

The rectification of terahertz radiation in a periodically almost-depleted graphene structures with dual grating gate is studied. Plasmonic electron-hole ratchet based rectified current grows with decreasing the Fermi energy of charge carriers in the gated parts of graphene. It is shown that the charge carriers accumulated at boundaries of depleted parts of graphene enhance the magnitude and inhomogeneity of oscillating electric field of the plasmon, which leads to significant growth in the rectification efficiency. Interaction of plasmon modes in different gated parts of graphene can increase the rectification efficiency due to excitation of the hybrid plasmon modes.

Investigation of the thermoelectric response in conducting polymers doped by solid-state diffusion

Thermoelectric effect is a physical phenomenon which intricately relates the thermal energy of charge carriers to their charge transport. Understanding the mechanism of this interaction in different systems lies at the heart of inventing novel materials which can revolutionize thermoelectric power generation technology. Despite the recent surge of interest in organic thermoelectric materials, the community has had difficulties in formulating the charge transport mechanism in the presence of a significant degree of disorder. Here, we analyze the thermoelectric properties of various conducting polymers doped by solid-state diffusion of dopant molecules based on a transport model with a power law energy dependence of transport function. A fine control of the degree of doping via postdoping annealing provides an accurate empirical evidence of a strong energy dependence of the carrier mobility in the conducting polymers. A superior thermoelectric power factor of conducting polymers doped by solid-state diffusion to that of other doping methods can be attributed to a resulting higher intrinsic mobility and higher free carrier concentration.

Effect of Charge Carrier Relaxation on Donor-Acceptor Recombination Spectra Taking into Account Coulomb Correlations

This study presents the results of numerical simulation of donor-acceptor recombination spectra in compensated semiconductors taking into account electrostatic fluctuations associated with the presence of ionised impurities. The presence of Coulomb correlations in a system of partially ionised impurity centers results in distinctive features appearing in recombination spectra, depending on charge-carrier energy relaxation. The authors have considered the following limiting cases: absence of relaxation, partial relaxation of majority and minority charge carriers, as well as complete relaxation of a system of electrons and holes localized on impurities. The results obtained through numerical simulation have been interpreted using a previously developed analytical model, in which contributions to recombination from equilibrium and photoexcited charge carriers are considered separately. It has been shown that, under certain experimental conditions, emission lines corresponding to these contributions can be spectrally resolved.

High efficient nanostructured PbSe0.5Te0.5 exhibiting broad figure-of-merit plateau

To have a very good energy conversion efficiency, thermoelectric (TE) material should exhibit higher figure-of-merit (ZT) for broad range of temperatures. In that direction, n-type PbTe0.5Se0.5 material with enhanced and temperature insensitive figure-of-merit was developed through nanostructured approach. A temperature insensitive ZT of 0.7 was observed from 400 K to 600 K. The enhanced and stable ZT over a wide temperature range was ascribed to the presence of various types of nanostructures that facilitated scattering of mid and long wavelength phonons, keeping the thermal conductivity low in addition to scattering low energy charge carriers elevating the Seebeck coefficient (−380 μVK−1 at 600 K). Most thermoelectric n-type materials such as PbQ (Q = Te, Se) materials exhibits very low ZT at room temperature which hampers the overall conversation efficiency. However, the developed material exhibits highest ZT at room temperature among PbQ (Q = Te,Se) materials that enables this material to be used in wearable thermoelectric applications. In this study, newly introduced technique was adopted to calculate TE conversion efficiency, which addresses overestimations of conventional efficiency calculations. The developed material showed power generation efficiency higher than many state-of-the-art TE n-type materials in 300 K–600 K range making it a competitive material for waste heat recovery applications.

Ballistic 3-port interferometric logic gates in the quantum Hall regime

We show that a 3-port interferometer, defined by quantum point contacts in a two-dimensional electron gas, and working in the quantum Hall regime, can implement different logic gates at each port depending on the energy of charge carriers and the operating conditions. In all cases the input logic states are encoded in the potentials applied on the quantum point contacts, while the output logic states are encoded in the values of the overall transmission coefficients at each port. In addition, the output of such compact configurations can be reprogrammed by combining two 3-port interferometers.

Precise regulation of the emissive layer for ultra-high performance white organic light-emitting diodes in an exciplex forming co-host system

Ultra-high performance WOLEDs were achieved by solving the trade-off between charge carrier trapping and energy transfer in electroluminescence processes.

Radiative and non radiative recombinations study in the novel nanocomposites BiVO4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiVO4/3DOM-ZnO: Application to the photocatalysis

A theoretical investigation of optical proprieties of three nanostructures such as BiOV4/3DOM TiO2, ZnO/3DOM TiO2 and BiOV4/3DOM ZnO has been presented and discussed. The study based on a one band effective mass approximation model was developed to calculate the charge carrier energies on various parameters of nanostructures including their size. We computed the optical properties for both spherical BiVO4 and ZnO nanoparticles incorporated in the three dimensionally ordered macroporous inverse opal TiO2 nanocomposites (3DOM-TiO2). At first and by solving a three dimension Schrödinger equation we calculated the dependence of charge carriers energies on the size of nanoparticles. Then we investigated the variation of the excitonic energy of BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO as function of the radius of these nanostructures into taken account the effects of dielectric and coulomb interaction between charge carriers. We showed that the radiative recombination lifetime is enhanced with decreasing BiVO4 and ZnO quantum dots (QDs) size. It was found that as the size of the dot is reduced the confinement of charge carriers and the optical gap are increased. The optical gap of the ZnO/3DOM-TiO2 is larger than both BiOV4/3DOM-TiO2 and BiOV4/3DOM-ZnO especially for small radius. It was also found that radiative life time in the BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO increased by increasing the radius of these nanostructures. In addition, we have also showed that, the radiative lifetime in the ZnO/3DOM-TiO2 nanostructure is less than both BiOV4/3DOM-TiO2 and BiOV4/3DOM-ZnO nanostructures. Finally, we have studied and discussed the Auger process in BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO nanostructures. At first, we have showed that the auger process is important when the radiuses of nanostructures are less than 2 nm. Then we have also showed that this process is more important for ZnO/3DOM-TiO2 compared to the other nanostructures.

Mechanistic study of energy dependent scattering and hole-phonon interaction at hybrid polymer composite interfaces for optimized thermoelectric performance

Low energy barriers for charge transport may improve thermoelectric performance in heterostructured inorganic materials and hybrid polymer composites. In this study, earth abundant Cu12Sb4S13 nanoparticles were synthesized and incorporated into poly (3,4-ethylenedioxythiophene) (PEDOT) matrices. The surface of nanoparticles was modified with ligands that improve carrier transport while maintaining the Seebeck coefficient. We demonstrated optimized thermoelectric figures of merit, ZT, were obtained at around 5 wt.% nanoparticle content, which we attribute to a low hole-phonon interaction, an effective phonon scattering, and mainly to the presence of a proper density of low energy barriers, 0.17 eV, between the energy states of Cu12Sb4S13 nanoparticle and PEDOT nanofiber and which selectively scatter low energy charge carriers. A 2.5-fold increase of ZT over pristine PEDOT nanofiber, ZT = 0.0098, was obtained by polymer composite at the optimal nanoparticle concentration. Remarkably, electrical conductivity and Seebeck coefficient decreased with an increase in nanoparticle content beyond 5 wt.%, which is attributed to the non-energy dependent hole-phonon interaction. These findings are expected to create an economical and new route for enhancing ZT in hybrid thermoelectric polymer composite and devices.

Desorption induced by low energy charge carriers on Si(111)-7 × 7: First principles molecular dynamics for benzene derivates.

We use clusters for the modeling of local ion resonances caused by low energy charge carriers in STM-induced desorption of benzene derivates from Si(111)-7 × 7. We perform Born-Oppenheimer molecular dynamics for the charged systems assuming vertical transitions to the charged states at zero temperature, to rationalize the low temperature activation energies, which are found in experiment for chlorobenzene. Our calculations suggest very similar low temperature activation energies for toluene and benzene. For the cationic resonance transitions to physisorption are found even at 0 K, while the anion remains chemisorbed during the propagations. Further, we also extend our previous static quantum chemical investigations to toluene and benzene. In addition, an in depth analysis of the ionization potentials and electron affinities, which are used to estimate resonance energies, is given. © 2018 Wiley Periodicals, Inc.