Positive Charge Carriers Research Articles

Atmospheric Storm Anomalies Prior to Major Earthquakes in the Japan Region

Connections between atmospheric perturbations, e.g., thunderstorm activity, and major earthquakes are investigated along with the lithosphere–atmosphere coupling mechanism, concerning the earthquake prediction models. The present research attempts to recognize a possible link between atmospheric processes (rainfall, storms) and subsequent earthquakes (M > 6) across a wide area around Japan. Earthquake data and upper-atmosphere sounding data related to the Severe Weather Threat (SWEAT) index and Skew-T plots were obtained from two Japanese radiosonde stations, Hachijojima and Kagoshima. Using the cross-correlation function (CCF) method, it is shown that SWEAT conditions existed within 30 days before six major earthquakes in 2017 in the Japan region. The Seismo-Climatic Index (SCI) reached a mean of 2.00, 7–8, and 13–14 days before these earthquakes, indicating thunderstorms and extreme weather conditions, further supported by Skew-T plots. Low-pressure systems, deviating from the mean by as much as −50 to −250 m, and hot spots of increased precipitation ranging from ~80 to ~140 mm rainfall within 24 h were observed to be geographically associated with these earthquake events. The anomalous atmospheric conditions can be understood based on increased air ionization at the ground-to-air interface due to the influx of positive-hole charge carriers that are stress-activated deep in the lithosphere and spread through the rock column. When the positive electronic charge carriers are accumulated at the lithosphere, preferentially at topographic highs, some steep electric fields are observed capable of field-ionizing the air. The airborne ions then act as condensation nuclei for atmospheric moisture, thermal updrafts, cloud formation, and a statistically significant precipitation increase. This research was conducted based on some experimental indicators in a very important seismological region to examine the successfulness of the proposed mechanism and the given indicators as the possible proxies of pre-earthquake precursors. Hence, the main practical implication of the research can highlight a sustainable way for improving the managerial tools in the field of earthquake prediction.

Charge. transport, conductivity and Seebeck coefficient in pristine and TCNQ loaded preferentially grown metal-organic framework films

This investigation on metal-organic framework (MOF) HUKUST-1 films focuses on comparing the undoped pristine state and with the case of doping by TCNQ infiltration of the MOF pore structure. We have determined the temperature dependent charge transport and p-type conductivity for HKUST-1 films. Furthermore, the electrical conductivity and the current–voltage characteristics have been characterized in detail. Because the most common forms of MOFs, bulk MOF powders, do not lend themselves easily to electrical characterization investigations, here in this study the electrical measurements were performed on dense, compact surface-anchored metal-organic framework (SURMOF) films. These monolithic, well-defined, and (001) preferentially oriented MOF thin films are grown using quasi-liquid phase epitaxy (LPE) on specially functionalized silicon or borosilicate glass substrates. In addition to the pristine SURMOF films also the effect of loading these porous thin films with TCNQ has been investigated. Positive charge carrier conduction and a strong anisotropy in electrical conduction was observed for highly oriented SURMOF films and corroborated with Seebeck coefficient measurements. Van der Pauw four-point Hall sample measurements provide important insight into the electrical behavior of such porous and hybrid organic–inorganic crystalline materials, which renders them attractive for potential use in microelectronic and optoelectronic devices and thermoelectric applications.

Can Organic Solar Cells Beat the Near-Equilibrium Thermodynamic Limit?

Despite an impressive increase over the past decade, experimentally determined power conversion efficiencies of organic photovoltaic cells still fall considerably below the theoretical upper bound for near-equilibrium solar cells. Even in otherwise optimized devices, a prominent yet incompletely understood loss channel is the thermalization of photogenerated charge carriers in the density of states that is broadened by energetic disorder. Here, we demonstrate by extensive numerical modeling how this loss channel can be mitigated in carefully designed morphologies. Specifically, we show how funnel-shaped donor- and acceptor-rich domains in the phase-separated morphology that are characteristic of organic bulk heterojunction solar cells can promote directed transport of positive and negative charge carriers toward the anode and cathode, respectively. We demonstrate that in optimized funnel morphologies this kinetic, nonequilibrium effect, which is boosted by the slow thermalization of photogenerated charges, allows one to surpass the near-equilibrium limit for the same material in the absence of gradients.

(Invited) Simulating Dynamic Excitons Via Quantum Molecular Dynamics: A Case Study in Lead Halide Perovskites

The ultrafast electronic and structural dynamics invoked by photoexcitation, i.e., “dynamic exciton” phenomena, not only are important in the context of practical applications such as solar cells, but also raise many questions from the viewpoint of fundamental science. Experimental measurement, computational simulations, and theoretical interpretation will be the three pillars for deciphering the dynamic exciton phenomena.From the viewpoint of computational simulations, molecular dynamics (MD) techniques combined with quantum chemical calculations, i.e., quantum molecular dynamics (QMD), has been the popular tool to simulate the dynamic exciton phenomena. The quantum chemical calculations, which are typically conducted on the basis of the density-functional theory (DFT), require the large computational resources and time. These have been the limiting factors for the accessible spatial and time scales by the QMD simulations.To extend the coverage of simulations to more complex, large-scale systems, we have developed efficient excited-state QMD methods that can include nonadiabatic effects. Our method combines the density-functional tight binding (DFTB) method, which is an approximate DFT, and the surface hopping method, which is a theoretical framework to incorporate the nonadiabatic effects into the QMD simulations. The method was further improved to be suitable for condensed-phase simulations explicitly including the environment, i.e., solvent, by using a “divide-and-conquer” style quantum chemical calculation technique. These theoretical framework enables us to simulate the coupled electronic–structural dynamics in excited states of systems consisting of 102–103 atoms[1,2,3].In addition, using the developed method, we conducted the real-time simulations of the ultrafast processes invoked by photoexcitation of lead iodide perovskites, which are known as the key materials for perovskite solar cells. The dissociation of the exciton into the positive and negative charge carriers was observed. Moreover, the hot carrier cooling, where the charge carriers dissipate excess energy via the electron–phonon coupling and relax to the band edges, was also tracked. Finally, the direct evidence of the polaron formation, where the structural deformation is induced by the presence of charge carriers, was observed. These results highlight the importance of the coupling between electronic and structural degrees of freedom.In the talk, recent improvements in the methodology and future perspectives will also be presented[5].

Computational study of phase stability, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa

Exploring new materials with interesting physical properties plays a vital role in the development of modern technologies. Herein, using density functional theory based first-principles calculations, we present a study on the structural, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa. The structure stability determined in terms of cohesive energy, formation energy and phonons dispersion curves confirms that alloy is stable and can be considered in experiments. The structural optimization in non-magnetic, ferromagnetic and antiferromagnetic state proves that alloy is ferromagnetic in the ground state with optimized lattice constant of 5.93 Å. The electronic band structure and density of states show that alloy is nearly half-metal (NHF) with 96% spin-polarization at the Fermi level. The effect of pressure on the electronic and magnetic properties investigated in the range 0–25 GPa, reveals that complete half-metallic character at the Fermi level can be obtained for pressure ≥10 GPa with a half-metallic gap of about 1 eV in the minority spin channel and a magnetic moment (Mt) of 3.0 μB/f.u., in agreement with Slater-Pauling rule (Mt = Zt-24). In addition, the electronic structures determined using DFT+U approach exhibit nearly half-metallic property such that amount of spin-polarization increases with increasing Hubbard parameter U. Finally, thermoelectric properties are examined by computing Seebeck coefficient (S), electrical conductivity (σ/τ), electronic thermal conductivity (κ) and power factor (PF) up to 800 K temperature. The positive value of S conveys that heat transport is carried by positive charge carriers. The room temperature values of S, σ/τ, κe/τ and PF are found to be about 5.9 μV/K, 3.06 × 1020 Ω−1m−1s−1, 2.27 × 1015 WK−1m−1s−1 and 1 WK−2m−1s−1, respectively. Collectively, these findings suggest that CrCoIrGa could play a useful role for the development of spintronics and energy production applications.

Highly Efficient Piezoelectrets through Ultra-Soft Elastomeric Spacers.

Piezoelectrets are artificial ferroelectrics that are produced from non-polar air-filled porous polymers by symmetry breaking through high-voltage-induced Paschen breakdown in air. A new strategy for three-layer polymer sandwiches is introduced by separating the electrical from the mechanical response. A 3D-printed grid of periodically spaced thermoplastic polyurethane (TPU) spacers and air channels was sandwiched between two thin fluoroethylene propylene (FEP) films. After corona charging, the air-filled sections acted as electroactive elements, while the ultra-soft TPU sections determined the mechanical stiffness. Due to the ultra-soft TPU sections, very high quasi-static (22,000 pC N−1) and dynamic (7500 pC N−1) coefficients were achieved. The isothermal stability of the coefficients showed a strong dependence on poling temperature. Furthermore, the thermally stimulated discharge currents revealed well-known instability of positive charge carriers in FEP, thereby offering the possibility of stabilization by high-temperature poling. The dependences of the dynamic coefficient on seismic mass and acceleration showed high coefficients, even at accelerations approaching that of gravity. An advanced analytical model rationalizes the magnitude of the obtained quasi-static coefficients of the suggested structure indicating a potential for further optimization.

Molecular ordering dependent charge transport in [formula omitted]-stacked triphenylene based discotic liquid crystals and its correlation with dielectric properties

We investigate the charge transport in triphenylene core based homologous series of discotic mesogens (HATn) by the time-of-flight method. HAT5, HAT6 and HAT8 exhibit Colh phase whereas HAT4 shows Colhp phase. For the investigated homologous series, we have observed a single transit in the transient photocurrent curves corresponding to positive charge carriers having the mobility in order of 10-2 to 10-4 cm2/vs. The hole mobility decreases with increasing the number of carbon atoms (n) in the alkyl chain attached to the triphenylene core, which follows the empirical exponential relationship: μ≈a.exp(-b/n). Furthermore, the dielectric measurement has been done on these compounds to bring a correlation between the molecular ordering by varying the alkyl chain of the HATn compounds. In the light of structure-property correlation, we have noticed that the hole mobility and permittivity vary according to the length of alkyl chain attached to the triphenylene core. The molecular ordering is also discussed via phase behaviour of compounds and confirmed by polarized optical textures. The change in entropy also decreases with increasing alkyl chain length of the HATn compounds rendering an agreement with that of the mobility and dielectric measurements. The HATn compounds show their potential applications in opto-electronics and could serve as organic semiconductors; however, the poor conductivity remains as a futuristic challenge.

Influence of substrate temperature on delafossite CuFeO2 films synthesized by reactive magnetron sputtering

Delafossite CuFeO2 films have been synthesized by reactive magnetron sputtering at different substrate temperatures from 380 up to 550 °C. The films adopt the rhombohedral structure from 380 to 550 °C. At the highest temperature, the delafossite phase partially dissociates in FCC Cu and Fe3O4 magnetite phases. In addition to the average crystallite size increase, a change of the preferential orientation in the out-of-plane direction from the [012] to [006] directions occurs between 460 and 510 °C. Modelling of the optical properties shows the presence of 2 interband transitions in the visible range. The 1st main transition at 1.5 eV is ascribed to the absorption component parallel to the c-axis whereas the 2nd transition (2.10 – 2.26 eV) is related to the absorption component in the (a, b) plane. Both evolutions with temperature of the refractive index (~ 2.5) and absorption coefficient (~ 10-3 cm-1) in the infrared suggest the formation of secondary phases. The electronic conductivity, dominated by positive charge carrier, varied from 0.01 to 10 S m-1 according to the preferential orientation and to the presence of secondary phases. A very small amount of a short-range ferromagnetic component (magnetization ~ 10 kA m-1 at 1.5 T) is clearly observed at room temperature thanks to magnetometry confirming the formation of secondary phases undetected by X-ray diffraction. In addition to depend on the film orientation, the presence of secondary phases in weak proportion alters the optical and electrical behaviours such as the transmittance in the visible range.

Oxygenation of conducting polymers facilitated by structure‐breaking anions

AbstractConducting polymers are an interesting class of materials that can be tuned to have a range of properties through counterion doping. For most conducting polymers, the insertion of anions (the doping process) leads to the formation of carbocations (positive charge carriers) along the conjugated polymer backbone. In this research, we report on a scenario that arises where certain (commonly used) anions in water induce oxygenation of the conducting polymers heteroatom. This is in contrast to the widely reported doping process, and the recently reported hydrolysis of conducting polymers. We observe that the transition between these different conducting polymer‐interactions/reactions is well described by the concept of structure‐making and structure‐breaking anions. Poly(3,4‐propylenedioxy thiophene dimethyl) (PProDOT‐Me2), polypyrrole (PPy), and poly(3,4‐ethylenedioxy thiophene) (PEDOT) thin films are exposed to a range of anions in water. Both PProDOT‐Me2 and PPy are susceptible to oxygenation, while in contrast PEDOT is doped, when exposed to structure‐breaking anions. All the polymers show hydrolysis for structure‐making anions. The knowledge of the interaction and/or reaction of conducting polymers with anions in water is not only critical to their application in devices for aqueous environments (i.e., sensing), but also for their processing and fabrication using water.

Directional percolating pathways in demixing blends on a wetting substrate

Understanding the dynamical evolution of a percolating network during liquid–liquid demixing is crucial for many technological applications, including plastic electronics, such as organic photovoltaics, whose performance depends on the efficiency to transport the positive and negative charge carriers to the corresponding electrodes. The transport and collection of the charge carriers require sufficient asymmetry between the donor and acceptor phases by attaining a minimum concentration of the majority fluids in it, called the percolation threshold. We investigate demixing in symmetric binary blends on a substrate preferentially wet by one of the fluids from the perspective of such a percolation threshold to achieve directed and connectivity percolation. We also study the influence of the strength of the substrate interaction and the property of the blend right before the quench with respect to the critical point. It is commonly assumed that the bicontinuous morphology of a symmetric blend guarantees percolation, where the average concentration of the blend distinguishes the two phases. However, if the percolation threshold is larger than the average concentration, we find that percolating pathways grow monotonically and a percolating cluster forms only after a time lag. Furthermore, we find that this time lag is characterized by two universal kinetic regimes that can explain all our observations. The first regime is associated with the percolation threshold itself, which grows exponentially. The second regime displays an algebraic growth with an exponent of 1/3 and we argue that it must be associated with the directional connectivity of the wetting phase to the substrate.

Effect of isopropanol doped Poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) on emission characteristics of organic light emitting diodes

The performance of polymer based organic light-emitting devices (OLEDs) that contained isopropanol (IPA) doped poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) utilized as positive charge carriers (hole) injecting electrode were investigated. In this study, morphological changes in PEDOT:PSS thin films have been analyzed with atomic force microscopy (AFM). The results of device performance with doped PEDOT:PSS were compared with the result of non doped PEDOT:PSS device. Fabricated OLED with the concentration 1:1 (PEDOT:PSS to IPA) shows the best performance among the others with almost 8000 cd/m2 brightness and 1.75 cd/A efficiency. Additionally, effect of IPA doping mechanisms proposed in the literature have been surveyed.

Effective Temperatures of Olivine Dust Impact Plasmas

The effective temperatures of the positive and negative charge carriers in impact plasma are measured experimentally. The measurements are performed using a dust accelerator using polypyrrole (PPy)-coated olivine dust particles impacting onto a tungsten (W) target in four velocity bins: 2–3.6, 5–6.6, 8–12, and 12–18 km/s. The setup measures the retained impact charge as a function of applied bias potential, and the temperatures are calculated by fitting the data. The effective temperatures of the cations are approximately 7 eV and independent of the impact speed. The effective temperature of the negative charge carriers changes from 1 eV for the lowest speed to values comparable to that of the cations at higher speeds. The temperature measurements presented here are significantly different from prior studies using Fe dust particles. The discrepancy is possibly due to a larger fraction of negative ions in the impact plasma that likely originates from the PPy coating.

SEPARAZIONE DI CARICA IN MISCELE DI MATERIALI ORGANICI: VALE LA LEGGE DI COULOMB?

After a brief introduction to organic photovoltaic materials, I discuss the problem of charge separation in these systems. In the best devices, the excitations produced by absorption of a photon can be converted into positive and negative charge carriers (holes and electrons, respectively) with near-100% efficiency. Such efficiencies are apparently at odds with Coulomb’s law, which would lead to change recombination due to their mutual electrostatic attraction. A theoretical model developed within my group overcomes this apparent contradiction, by taking into account quantum mechanical effects such as the delocalization of the charge carriers.

Doping-driven electronic and lattice dynamics in the phase-change material vanadium dioxide

Doping is generally understood as a strategy for including additional positive or negative charge carriers in a semiconductor, thereby tuning the Fermi level and changing its electronic properties in the equilibrium limit. However, because dopants also couple to all of the microscopic degrees of freedom in the host, they may also alter the nonequilibrium dynamical properties of the parent material, especially at large dopant concentrations. Here, we show how substitutional doping by tungsten at the 1 at. % level modifies the complex electronic and lattice dynamics of the phase-change material vanadium dioxide. Using femtosecond broadband spectroscopy, we compare dynamics in epitaxial thin films of pristine and tungsten-doped $\mathrm{V}{\mathrm{O}}_{2}$ over the broadest wavelength and temporal ranges yet reported. We demonstrate that coupling of tungsten atoms to the host lattice modifies the early electron-phonon dynamics on a femtosecond timescale, altering in a counterintuitive way the ps-to-ns optical signatures of the phase transition. Density functional theory correctly captures the enthalpy difference between pristine and W-doped $\mathrm{V}{\mathrm{O}}_{2}$ and shows how the dopant softens critical V-V phonon modes while introducing new phononic modes due to W-V bonds. While substitutional doping provides a powerful method to control the switching threshold and contrast of phase-change materials, determining how the dopant dynamically changes the broadband optical response is equally important for optoelectronics.

Optimization of the electron transport layer in quantum dot light-emitting devices

Quantum dot light-emitting devices have emerged as an important technology for display applications. Their emission is a result of recombination between positive and negative charge carriers that are transported through the hole and electron conductive layers, respectively. The selection of electron or hole transport materials in these devices not only demands the alignment of energy levels between the layers but also balances the flow of electrons and holes toward the recombination sites. In this work, we examine a method for device optimization through control of the charge carrier kinetics. We employ impedance spectroscopy to examine the mobility of charge carriers through each of the layers. The derived mobility values provide a path to estimate the transition time of each charge carrier toward the emitting layer. We suggest that an optimal device structure can be obtained when the transition times of both charge carriers toward the active layer are similar. Finally, we examine our hypothesis by focusing on thickness optimization of the electron transport layer.

Carrier transport in LDPE and its nanocomposites

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

Fabrication of SiNWs-FET Nanostructure Via Atomic Force Microscopy Lithography

The unique electrical properties of silicon nanowires (SiNWs) is one of the reasons it become an attractive transducer for biosensor nowadays. Positive (holes) and negative (electron) charge carriers from SiNWs can simply interact with either positive or negative charge of sensing target. In this paper, we have studied the fabrication of silicon nanowires field effect transistor (SiNWs-FET) nanostructure patterned on 15 Ω resistivity of p-type silicon on insulator (SOI) wafer fabricated via atomic force microscopy lithography technique. To fabricate SiNWs-FET nanostructure, a conductive AFM tip, Cr/Pt cantilever tip, was used then various value of applied voltage, writing speed and relative humidity were studied. Subsequent, followed by wet etching processes, admixture of tetramethylammonium hydroxide (TMAH) and isopropyl alcohol (IPA) were used to remove the undesired of silicon layer and diluted hydrofluoric acid (HF) was used to remove the oxide layer. From the results, it shows that, cantilever tip at 9 V with 0.4 μm/s writing speed and relative humidity between 55% - 60% gives the best formation of silicon oxide to fabricate SiNWs-FET nanostructure.

Flexible PMOS Inverter and NOR Gate Using Inkjet-Printed Dual-Gate Organic Thin Film Transistors

We report the fabrication of low voltage dual-gate organic thin film transistors, and their possible applications for PMOS inverters and NOR logic gates, on $125~\mu \text{m}$ -thick flexible polyethylene naphthalate (PEN) substrates using solution-processed techniques. Source, drain, and gate electrodes are made by inkjet-printing, while active semiconductor is printed by a dispenser system over parylene dielectric. Bottom and top channel interfaces have been characterized by taking long drain current transients. Formation of stable top channel interface, with low trap density, is a reason for higher top channel current than bottom channel. Increase in drain current transients, and positive threshold voltage shift is attributed to the accumulation of extra positive charge carriers induced by slow polarization of the dipoles in the parylene dielectric.

Ionization of large helium nanodroplets

SynopsisWe have produced and mass-selected large, highly charged helium droplets containing up to 1010 He atoms and several tens of charges. They are stable with both multiple positive and negative charge carriers and are stabilized by losing small fragments of excess charge instead of disintegrating (highly asymmetric fission).

Examination of a relationship between atmospheric blocking and seismic events in the Middle East using a new seismo-climatic index

This study statistically examines the role of atmospheric blocking as a precursor of major seismic events. Atmospheric blocking archive and earthquake databases for the Middle East region are compiled for 2000–2013. Correlations between atmospheric blocking events and seismicity are examined based on defined seismo-climatic index (SCI) based on variations of earthquake frequency and magnitude before and after blocking events. Limiting the SCI index to values > 6, 16 out of 26 major earthquakes (M > 6), i.e. 62%, are shown to have occurred within 14 days after blocking events over their respective epicentral regions. The correlation between blocking events and subsequent seismicity falls into a range of 0.694–0.803. Additional blocking-related atmospheric anomalies such as cyclogenesis, cloud coverage, and anomalous rainfall prior to major earthquakes can be understood as processes that take place in the Earth’s crust and at the ground-to-air interface as a result of the stress activation of positive hole charge carriers at depth, in the hypocentral rock volume, and their rapid migration to the Earth’s surface. Hence, atmospheric blocking events in a seismically active region may be categorized as an earthquake precursory phenomenon.