Charge Transport Phenomena Research Articles

Electron transport properties of electrically doped guanine nano-sheet based bio-Zener diode: a first principle paradigm

The quantum ballistic transmission properties of an electrically-doped guanine-nanosheet-based bio-Zener diode are investigated using density functional theory and nonequilibrium Green’s function-based first-principles calculations. The bio-Zener diode is gate-bias modulated, and its various quantum-electronic properties, for example, the V–I characteristic, the transmission spectra, and the device density of states, depend on both the electrical doping concentration and on the applied gate bias voltage. The junctionless highly doped bio-Zener diode shows high levels of reverse-bias current which is dominated by majority charge carriers. It is also found that, due to the presence of a wide bandgap and the backscattering effect, the forward-bias current is highly suppressed. The quantum simulation results confirm a strong reverse gate-bias-modulated biased current–voltage response as well as a charge transport phenomenon through the effective device region. The bio-Zener diode exhibits a specific reverse breakdown that can be varied from −0.78 to −3.2 V without affecting the forward current–voltage characteristic. The current findings are obtained by including the coherent tunneling and incoherent hopping processes with a minimal Hamiltonian model approach.

Interface and doping in carbon dots influence charge transfer and transport

Nanoscale carbon-based materials have the potential for large-scale optoelectronic and biomedical applications due to their tunable photo-physics. Here, we explore the impact of doping and the role of intervening media on both electron transfer and transport from carbon dots (CD) to an acceptor menadione (MD). In a binary mixture compared to undoped-CD (UCD), the amine-based nitrogen-doped-CD (ACD) reflects significant changes in its photo-physics like excitation-independent fluorescence, enhanced lifetime, along with a distinct drop in electron transfer rate (kET) to MD. With micro-emulsion, kET from UCD to MD is enhanced while a substantial reduction is observed for ACDs, emphasizing the critical role of doping, surface states, encapsulation, and interfaces of CTAB micelles. The impact of doping and Coulombic interaction at interfaces on the charge transport phenomena with redox-active MD is further investigated by current-sensing atomic force microscopy. For similar bias-voltage, ACD reflects a higher conductance than UCD, and the conductance is enhanced in the presence of CTAB micelles. Moreover, at a low voltage regime, the current-voltage curve shows a linear growth, while at higher voltage, Fowler-Nordheim (F–N) tunneling is observed. The presence of MD, however, leads to an abrupt rise in conductivity resulting in a transition from F–N to direct tunneling.

Growth, morphology and molecular orientation of controlled Indigo thin films on silica surfaces

Thin films of the natural semiconductor indigo have been grown on Si/SiO2 substrates by physical vapor deposition under ultra-high vacuum conditions, monitoring the influence of the deposition rate on structure, morphology and thickness. This was done to allow well-defined films to be made in a controlled way, suitable for the manufacture of organic devices and the investigation of charge transport phenomena. The films have been characterized by X-ray diffraction, atomic force microscopy, microRaman spectroscopy and transmission electron microscopy, finding that the growth at the solid interface of polymorph B, the only phase present in the films, is governed by the formation of strong multiple hydrogen-bonds between neighbouring molecules, which play a predominant role in the interface organization of the molecular assemblies.

Charge Transport Phenomena in Heterojunction Photocatalysts: The WO3/TiO2 System as an Archetypical Model.

Recent studies have demonstrated the high efficiency through which nanostructured core-shell WO3/TiO2 (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO3 and TiO2 photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO2 and WO3 sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO3 to TiO2 via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO2 to WO3 via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs-ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO3 contribute to the depopulation of W5+ polaronic states, whereas the remaining accumulated holes in WO3 are separated from adjacent electrons in TiO2 up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and experimental approach delivers a robust understanding of the direction of charge separation along with critical time-resolved insights into the evolution of charge transport phenomena in this model heterojunction photocatalyst.

Spectral Clustering to Analyze the Hidden Events in Single-Molecule Break Junctions

The single-molecule break junction technique provides a high-throughput method to explore the charge transport phenomena through a molecular junction at the ultimate scale of a single molecule. The...

Charge transport in polythiophene molecular device: DFT analysis.

We report the charge transport phenomenon in polythiophene molecular device and the ways of controlling the nature of charge transport through the device. By using density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalisms, two ways of controlling the nature of charge transport have successfully been demonstrated by introducing conformational changes in the channel and applying external gate potential. Functional groups with negative mesomeric effect such as nitrous and carboxyl and positive mesomeric effect such as amino have been used as substituents as part of introducing conformational changes in the channel. The results indicate that the nature of charge transport in polythiophene molecular device can be changed from hole dominant to electron dominant and vice versa just by introducing minor conformational changes in the channel and by changing the polarity of external gate potential. Moreover, the negative differential resistance (NDR) behavior has been observed in amino-substituted thiophene device. These findings will be very useful in understanding the design of both p and n-type transistors out of same molecule for the next-generation molecular electronics.

Gas sensing properties of palladium-modified zinc oxide nanofilms: A DFT study

The improved gas-detecting ability of ZnO nanostructures has generated considerable attention for further development based on a unique and practical approach. However, reduced gas sensitivity and selectivity, as well as the sophisticated synthesis requiring high-cost technology restricts its efficient use. In this regard, the enhancement of gas sensing properties can be brought about by incorporating the noble metal palladium (Pd) to modify the ZnO surface. This work applied density functional theory (DFT) calculations to study such sensing materials to the target gases (i.e., hydrogen (H2) and ethanol (C2H5OH)), which are extensively applied in energy production, thereby requiring rapid and accurate detection. The modified ZnO thin film-based gas sensor was modeled into Pd-substitutional doped and Pd-decorated ZnO surfaces. The charge transport phenomenon at the atomic scale, binding strength, and other electronic and structural properties were elucidated as the varied number of Pd. Pd-decorated ZnO surfaces enhance the sensitivity of both molecules. The calculation results could provide an atomistic insight and a better understanding of altered electrical and sensing properties of ZnO surface modification by Pd atoms.

Investigation of conduction kinetics in Al/CuInSe2 Schottky device utilizing impedance spectroscopy (IS) measurement and study of its photosensing behaviour

Copper Indium Selenide (CuInSe2) has been synthesized by solvothermal synthesis method. The Schottky diode (SD) has been fabricated by using the material and the interface characteristics of Al/CuInSe2/ITO have been investigated with the help of ac impedance spectroscopy (IS) analysis (under dark condition) and dc current-voltage (I–V) measurements (under both dark and photo condition). IS is a very important and powerful technique to investigate and analyze the impedance at the boundary regions of SDs. Ac impedance spectra of Al/CuInSe2 SD have been recorded in the wide range of frequency from 40 Hz to 20 MHz during dc bias scanning from −0.5 V to 0.5 V under dark condition. From forward I–V characteristics, important parameters such as ideality factor (η), photosensitivity, barrier height (Φb), series resistance (RS) of SD were obtained under dark and photo condition. The photosensitivity of the Al/CuInSe2 SD was found to be 3.36. For better realization of charge transport phenomena through the MS junction, space charge limited current (SCLC) theory has been employed. The effective mobility of the carrier is evaluated in dark and photo condition as 0.42 × 10−3 m2V−1s−1 and 2.11 × 10−3 m2V−1s−1 respectively. It has been observed that the mobility is improved 5 times under illumination compared to the dark condition.

Modelling the influence of soil properties on performance and bioremediation ability of a pile of soil microbial fuel cells

Worldwide, intense industrial and agricultural activities pose serious issues of land contamination. Soil microbial fuel cells (SMFCs) have great potential as a low-cost, and self-powered solution to soil bioremediation, compatible with operations in remote areas. In this study, we propose a novel tubular SMFC design, in which a ceramic tube acts as the separator between the air-cathode and the anode, while providing structural support. No oxygen reduction reaction catalyst is used, and to reach depth, several SMFC units are piled together.To assess the effect of both the system design and soil properties on performance, a mathematical model, calibrated with experimental data, is proposed, which accounts for chemical and (bio)electrochemical reactions, as well as for charge conservation and transport phenomena. The information generated provides useful indications on optimal design and operational conditions for SMFCs and a guide to effective scale-up strategies for their use in bioremediation.

Influence of dielectric layer thickness on charge injection, accumulation and transport phenomena in thin silicon oxynitride layers: a nanoscale study

Charge injection and retention in thin dielectric layers remain critical issues due to the great number of failure mechanisms they inflict. Achieving a better understanding and control of charge injection, trapping and transport phenomena in thin dielectric films is of high priority aiming at increasing lifetime and improving reliability of dielectric parts in electronic and electrical devices. Thermal silica is an excellent dielectric but for many of the current technological developments more flexible processes are required for synthesizing high quality dielectric materials such as amorphous silicon oxynitride layers using plasma methods. In this article, the studied dielectric layers are plasma deposited SiO x N y . Independently on the layer thickness, they are structurally identical: optically transparent, having the same refractive index, equal to the one of thermal silica. Influence of the dielectric film thickness on charging phenomena in such layers is investigated at nanoscale using Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy. The main effect of the dielectric film thickness variation concerns the charge flow in the layer during the charge injection step. According to the SiO x N y layer thickness two distinct trends of the measured surface potential and current are found, thus defining ultrathin (up to 15 nm thickness) and thin (15–150 nm thickness) layers. Nevertheless, analyses of KPFM surface potential measurements associated with results from finite element modeling of the structures show that the dielectric layer thickness has weak influence on the amount of injected charge and on the decay dynamics, meaning that pretty homogeneous layers can be processed. The charge penetration depth in such dielectric layers is evaluated to 10 nm regardless the dielectric thickness.

Investigation of electrical conduction in Ca6-xNa2Y2(SiO4)6F2:xEu3+ ceramic by complex impedance and electric modulus spectroscopy

The electrical conduction mechanism of Ca6-xNa2Y2(SiO4)6F2:xEu3+ fluorapatite ceramic with (x = 0–0.05 mol%) synthesized by solution combustion method has been investigated by dielectric, complex impedance and electric modulus spectroscopy. Microstructural analysis was carried out by scanning electron microscopy (SEM) which shows that the spheroidal and ellipsoidal grains were distributed uniformly in the prepared ceramic with average grain size of the order of 3 μm. The electrical and dielectric studies of the ceramic were done in the frequency range of 100 Hz–100 KHz and temperature range of 50 °C–500 °C. The temperature and frequency dependent dielectric loss and dielectric constant decreases with an increase in frequency due to the ceasing effect of polarization. Dielectric relaxation behaviour of the prepared ceramics were studied using Complex impedance spectroscopy (CIS). Nyquist plots show that with increase in Eu3+ ion concentration the resistance of grains and grain boundaries decreases for the prepared ceramics. The investigation of charge transport phenomena was done by electric modulus spectroscopy. The drastic increase in a. c conductivity with increase in temperature shows that the ceramic follow the Jonscher’s power law and appear Debye in nature.

Transition of charge transport phenomena from 3D to 1D hopping at low temperatures in polyaniline/graphene composites

Here, we present the charge transport properties of polyaniline/graphene composites prepared by a chemical oxidation method in the presence of four different loading concentrations (2, 4, 6, and 8 wt. %) of graphene. The synthesized materials are characterized for surface and chemical bonding analyses through field-emission scanning electron microscopy and Fourier transform infrared spectroscopy techniques, respectively. The change in the chemical structure of the prepared composites with graphene loading concentrations revealed the possible increment in electrical conductivity. The room-temperature dc conductivity of the prepared composites was found to increase from ∼22 to 217 S/cm with an increase in the loading concentration of graphene from 2 to 8 wt. %. The temperature-dependent electrical conduction behavior of the prepared samples is investigated under Mott's variable-range hopping conduction mechanism. It is found that all composite samples follow three-dimensional (3D) hopping in the higher temperature region (>44 K), which transforms into one-dimensional (1D) hopping at lower temperatures (<44 K). A decrease in hopping distance (1.07–0.96 nm) and an increase in density of states (3.20 × 1021–4.95 × 1021 cm−3 eV−1) in three dimensions with an increase in the graphene loading concentration from 2 to 8 wt. % suggest the requirement of lower hopping energy (61.3–55.5 meV) for conduction. The estimated hopping parameters also revealed a nonadiabatic small-polaron hopping conduction mechanism that is followed by the charge carriers in the present samples for both one- and three-dimensional variable range hoppings.

Anthracene-Based Fluorophore and Its Re(I) Complexes: Investigation of Electrical Properties and Schottky Diode Behavior.

A novel fluorophore (HL) [1-((E)-(quinolin-8-ylimino)methyl)anthracen-2-ol] using a suitably designed anthrol and quinoline derivative was synthesized and well characterized. Then, two Re(I) complexes with the fac-[Re(CO)3]+ moiety were prepared with the ligand under different reaction conditions. Both the complexes [Re(L)(CO)3] (1) and [Re(HL)(CO)3Cl] (2) absorbed in the visible region. Steady-state fluorescence measurements and time-correlated single-photon count experiments were performed to elucidate the nature of the excited state. The ground- and excited-state geometries were theoretically investigated using density functional theory (DFT) calculations. The electrical properties of the ligand and the complexes have been explored with the help of a sandwich-structured thin-film device of an Al/sample/indium tin oxide (ITO) configuration at room temperature. The thermionic emission (TE) theory was adopted for the extraction of Schottky diode parameters such as ideality factor, barrier height, and series resistance. Further, the space-charge-limited current (SCLC) theory was employed for a better understanding of the charge transport phenomenon.

Measurement of the Protonic and Electronic Conductivities of PEM Water Electrolyzer Electrodes.

Reducing anode catalyst layer proton- and electron-transport resistances in polymer electrolyte membrane water electrolyzers is critical to improving its performance and maximizing catalyst utilization at high current density. A hydrogen pump technique is adapted to measure the protonic conductivity of IrOx-based catalyst layers. The protonic resistance of the catalyst layer is obtained by subtracting the protonic resistance of an assembly of two NRE211 membranes hot-pressed together from an assembly of two NRE211 membranes with an IrOx intermediate layer. The through-plane and in-plane electronic conductivities were also measured using two- and four-probe methods, respectively. Using these techniques, the protonic and electronic conductivities of the IrOx catalyst layers with varying Nafion loading were measured. The results show that the limiting charge-transport phenomena in the IrOx catalyst layer can be either proton or electron transport, depending on the ionomer loading in the catalyst layer. These results are validated by numerical simulation, as well as by comparison to the high-frequency resistance of an electrolyzer with the same layer.

Synthesis, characterization and charge transport properties of Pr–Ni Co-doped SrFe2O4 spinel for high frequency devices applications

Ferrites are materials of interest due to their broad applications in high technological devices and a lot of research has been focused to synthesize new ferrites. In this regard, an effort has been devoted to synthesize spinel Pr–Ni co-substituted strontium ferrites with a nominal formula of Sr1-xPrxFe2-yNiyO4 (0.0 ≤ x ≤ 0.1, 0.0 ≤ y ≤ 1.0). The cubic structure of pure and Pr–Ni co-substituted strontium ferrite samples calcinated at 1073 K for 3 h has been confirmed through X-ray diffraction (XRD). Average sizes of crystallites (18–25 nm) have been estimated from XRD analysis and nanometer particle sizes of synthesized ferrites have been further verified by scanning electron microscopy (SEM). SEM results have also shown that particles are mostly agglomerated and all the samples possess porosity. It has been observed that at 298 K, the values of resistivity (ρ) increase, while that of AC conductivity, dielectric loss, and dielectric constants decrease with increasing amounts of Pr3+ and Ni2+ ions. The values of dielectric parameters initially decrease with frequency and later become constant and can be explained on the basis of dielectric polarization. Electrochemical impedance spectroscopy (EIS) studies show that the charge transport phenomenon in ferrite materials is mainly controlled via grain boundaries. Overall, synthesized ferrite materials own enhanced resistivity values in the range of 1.38 × 109–1.94 × 109 Ω cm and minimum dielectric losses, which makes them suitable candidates for high frequency devices applications.

Performance evaluation of poly(aniline-co-pyrrole) wrapped titanium dioxide nanocomposite as an air-cathode catalyst material for microbial fuel cell.

A simple, inexpensive in situ oxidative polymerization of aniline and pyrrole using ammonium persulfate (APS) as oxidant and hydrochloric acid (HCl) as dopant has been used to synthesize a hybrid (PAni-Co-PPy)@TiO2 nanocomposite with titanium oxide (TiO2) nanoparticles (NPs) wrapped into (PAni-Co-PPy) copolymer. The synthesized nanocomposite has been shown with higher oxygen reduction reactions (ORR) as an excellent cathode material for higher performance in the complex of (PAni-Co-PPy)+/TiO2(O-). The charge transport phenomenon between TiO2 and (PAni-Co-PPy)+ were found adequate with subsequent delocalization of electron/s at PAni and PPy. The self-doping nature of TiO2 (O-) played a vital role in oxygen adsorption and desorption process. With higher electrical conductivity and surface area, these were tested in microbial fuel cells (MFCs) for ORRs at cathode. This yielded a relatively higher current and power density output as compared to PAni@TiO2, PPy@TiO2, and commercially available Pt/C cathode catalysts in MFC system. In overall, the prepared (PAni-Co-PPy)@TiO2 nano-hybrid cathode delivered ~2.03 fold higher power density as compared to Pt/C catalyst, i.e. ~987.36±49mW/m2 against ~481.02±24mW/m2. The properties of electro-catalysts established an improved synergetic effect between TiO2 NPs and (PAni-Co-PPy). In effect, the enhanced surface area and electrochemical properties of the prepared (PAni-Co-PPy)@TiO2 nano-hybrid system is depicted here as an effective cathode catalyst in MFCs for improved performance.

Insight into the Specific Adsorption Properties of Pristine Ionic Liquid: Temperature Dependent Flatband Potential Study

Specific adsorption of ions of ionic liquids on an electrode plays an important role in controlling the mass and/or charge transport phenomena in electrochemical techniques. Insufficient experimental evidences put limit on wider acceptance and usage of ionic liquids in the field of energy devices. In the present study, two different photoelectrochemical cells (PECs) having configurations as (i) PbOx/Fe(CN)6 -4/-3/Pt and (ii) PbOx/[Bmim]Tf2N/Pt were utilized to provide the experimental pathway and to unveil the adsorption characteristics of ion-pairs of any ionic liquid. The photoresponse curves of PECs under chopped light illumination conditions were used to extract the flatband potential of PbOx film. The band edge positions and electron affinity of PbOx films were calculated with the help of Mott-Schottky plots. It is well known that the adsorption properties of electrolytes can be understood by calculating the point of zero charge (pHpzc). Hence, this parameter was calculated by analyzing the temperature dependent flatband potential which may further unveil the adsorption characteristics of ionic liquids. This study is an unexplored approach to get closer to the understanding of inherent properties of ionic liquids which can be of utmost interest to either basic or applied field researchers.

Frequency Dependent Charge Transport and Spin State Switching Characteristics of Fe(phen)₂(NCS)₂ in Polymer.

We report on the bistability in spin states of spin crossover (SCO) compound Fe(phen)₂(NCS)₂ in polymer (polypyrrole) by frequency (1-100 kHz) and temperature dependent (305-457 K) electrical conductivity measurements. The structure and growth of SCO compounds in conducting polymer are obtained by scanning electron microscopy, X-ray diffraction and optical absorption measurements. The thermal dependence of ac conductivity σ(ω) shows the clear formation of a hysteresis loop in its cooling and heating cycle due to the difference in conductivity in high spin and low spin state. The size, shape and width of the hysteresis loops are found to be critically dependent on the applied frequency and/or the ratio between SCO and polymer. The ac conductivity is found to exhibit a dispersive behavior following Jonscher's law: σ(ω) ∝ ωn below a critical frequency ωc, above which it is found to monotonically decrease with increasing frequency. The thermal dependence of the exponent n and ωc is also explored. The charge transport phenomena are explained in the framework of hopping of charge carriers. The data reveals that addition of polymer can play an important role to tune the conductivity of SCO compounds and its spin state dependence characteristics which may be quite helpful for fabricating future spin-based devices. Temperature dependent magnetic susceptibility measurement also confirms the spin transition behavior of the SCO/ppy composite samples. These SCO/ppy composite samples can be taken as the reliable nanomaterials fabricated with the concept of future spin based nanoarchitectonics.

Insight into the Specific Adsorption Properties of Pristine Ionic Liquid: Temperature Dependent Flatband Potential Study

Specific adsorption of ions of ionic liquids on an electrode plays an important role in controlling the mass and/or charge transport phenomena in electrochemical techniques. Insufficient experimental evidences put limit on wider acceptance and usage of ionic liquids in the field of energy devices. In the present study, two different photoelectrochemical cells (PECs) having configurations as (i) PbOx/Fe(CN)6 -4/-3/Pt and (ii) PbOx/[Bmim]Tf2N/Pt were utilized to provide the experimental pathway and to unveil the adsorption characteristics of ion-pairs of any ionic liquid. The photoresponse curves of PECs under chopped light illumination conditions were used to extract the flatband potential of PbOx film. The band edge positions and electron affinity of PbOx films were calculated with the help of Mott-Schottky plots. It is well known that the adsorption properties of electrolytes can be understood by calculating the point of zero charge (pHpzc). Hence, this parameter was calculated by analyzing the temperature dependent flatband potential which further unveiled the adsorption characteristics of ionic liquids. This study is an unexplored approach to get closer to the understanding of inherent properties of ionic liquids which can be of utmost interest to either basic or applied field researchers.

Efficient evaluation of Coulomb interactions in kinetic Monte Carlo simulations of charge transport.

The application of predictive and reliable modeling techniques for the simulation of charge transport in functional materials is an essential step for the development of advanced platforms for electronics, optoelectronics, and photovoltaics. In this context, kinetic Monte Carlo (KMC) methods have emerged as a valuable tool, especially for the simulation of systems where charge transport can be described by the hopping of charge carriers across localized quantum states, as, for example, in organic semiconductor materials. The accuracy, computational efficiency, and reliability of KMC simulations of charge transport, however, crucially depend on the methods and approximations used to evaluate electrostatic interactions arising from the distribution of charges in the system. The long-range nature of Coulomb interactions and the need to simulate large model systems to capture the details of charge transport phenomena in complex devices lead, typically, to a computational bottleneck, which hampers the application of KMC methods. Here, we propose and assess computational schemes for the evaluation of electrostatic interactions in KMC simulations of charge transport based on the locality of the charge redistribution in the hopping regime. The methods outlined in this work provide an overall accuracy that outperforms typical approaches for the evaluation of electrostatic interactions in KMC simulations at a fraction of the computational cost. In addition, the computational schemes proposed allow a spatial decomposition of the evaluation of Coulomb interactions, leading to an essentially linear scaling of the computational load with the size of the system.