Activation Energy For Hopping Research Articles

Effect of Phytic Acid, Tin Oxide and Graphite Incorporation in Polyaniline Semiconductive Ink

Enhanced electrical conductivity and pressure sensing functionalities of phytic acid-doped polyaniline ink (PPhyA) incorporated with SnO2 (PPhyAS) and graphite flakes (PPhyAG) as reported in our previous article, encouraged us to explore the electrical conduction mechanism and associated optical properties. In order to realize the enhanced electrical conduction in the reported inks, detailed studies were carried out in our research described here in comparison with normal HCl-doped polyaniline (PANI). This can be attributed to the formation of a highly ordered molecular arrangement with high crystallinity, a change in the hopping pathway between two adjacent molecules and the hopping activation energy of the PANI chain in the phytic acid-doped ink systems. Optical parameters, such as absorption edge, Urbach energy, refractive index, extinction coefficient, optical conductivity and optical dielectric constants, were estimated from the result of UV-Vis spectroscopy. The results of these studies showed enhancement of the optical properties of the phytic acid-doped ink systems (PPhyA, PPhyAS and PPhyAG) in the low photon energy range as compared to HCl-doped PANI. Phytic acid-doped ink systems are paintable on a variety of flexible surfaces, such as paper and plastics. Hence, we suggest that the phytic acid-doped ink systems have a great potential for developing flexible and foldable circuitry for infrared optics and remote sensing.

Marcus Theory and Long-Range Activationless Transport in Molecular Junctions.

Intrachain transport in molecular junctions (MJs) longer than 5 nm has been modeled within the theoretical framework of Marcus theory. We show that in oligo(bisthienylbenzene)-based MJs, electronic transport involves polarons, localized on three monomers that are close to 4 nm in length. They hop and tunnel between adjacent localized sites with reorganization energies λ close to 400-600 meV and electronic coupling parameters Hab close to λ/2. As a consequence, the activation energy for intrachain transport, given by the equation ΔG* = (λ/4)(1 - 2Hab/λ)2, is close to zero, and transport along the chain is activationless, in agreement with experimental observation. On the contrary, similar calculations on conjugated oligonaphthalenefluoreneimine wires show that Hab is much less than λ/2 and predict that the activation energies for intrachain hopping between adjacent sites, close to λ/4, are ∼115 meV. This work proposes a new perspective for understanding long-range activationless transport in MJs beyond the tunneling regime.

Unveiling high resistivity mechanism in (0.8-x)BaTiO3-0.2BiScO3-x(Bi0.5Li0.5)TiO3 ceramics with good dielectric temperature-stability

Focusing on the high resistivity together with low intrinsic conduction in BaTiO3–BiMeO3 materials, it can contribute to the development of reliable high-temperature capacitor materials. In this work, x(Bi0·5Li0.5)TiO3-(0.8-x)BaTiO3-0.2BiScO3 (where x ranges from 0 to 0.2) ceramics were fabricated by a solid reaction method. The temperature stability of dielectric permittivity and resistance are improved significantly by the introduce of (Bi0·5Li0.5)TiO3 (referred to as BL) compared to BaTiO3–BiMeO3. The composition discrepancy due to chemical inhomogeneity can be determined by SEM and EDS mapping although XRD exhibit a single perovskite phase. The characteristics of distinct conductivity behaviors in micro-regions was measured by kelvin probe force microscopy (KPFM). The non-stoichiometry gives rise to two kinds of defects for charge compensation: an ionic compensation and oxygen-vacancy compensation in different grains. The increase of the hopping activation energy of charged carries enhances the high-temperature resistance of the system. The sample of x = 0.15 shows very good temperature stability of dielectric permittivity in the temperature range from 200 °C to 400 °C, and that of x = 0.1 exhibits the highest resistivity of 3.6 MΩ cm at 570 °C. The proposed method gives an efficient strategy for improving the dielectric temperature stability and insulation by the special defect compensation in the perovskite oxides.

Oxygen vacancy migration and impact on high voltage DC polarization in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3

AbstractElectrical polarization and defect transport are examined in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3, an attractive capacitor material for high power electronics. Oxygen vacancies are suggested to be the majority charge carrier at or below 250°C with a grain conduction hopping activation energy of 0.97 eV and 0.92 eV for thermally stimulated depolarization current (TSDC) and impedance spectroscopy measurements, respectively. At higher temperature, thermally generated electronic conduction with an activation energy of 1.6 eV is dominant. Significant oxygen vacancy concentration is indicated (up to ∼1%) due to cation vacancy formation (i.e., acceptor defects) from observed Bi (and likely Zn) volatility. Oxygen vacancy diffusivity is estimated to be 10−12.8 cm2/s at 250°C. Low diffusivity and high activation energies are indicative of significant defect interactions. Dipolar oxygen vacancy defects are also indicated, with an activation energy of 0.59 eV from TSDC measurements. The large oxygen vacancy content leads to a short lifetime during high voltage (30 kV/cm), high temperature (250°C) direct current (DC) electrical measurements.

Enhanced high‐temperature electrical properties and charge dynamics of inorganic/organic silicone elastomer nanocomposites via nanostructure grafting and molecular trap construction

AbstractThis study introduces a novel strategy to enhance the high‐temperature electrical performance of the silicone elastomer (SE), while ensuring control over its thermal and mechanical properties for SiC device packaging insulation application. For the same, the ultra‐low‐content inorganic/organic SE nanocomposites are prepared and tested (at room temperature and 150°C) by grafting polyhedral oligomeric silsesquioxane (POSS) nanofillers and doping organic semiconductors. It is found that grafting POSS nanofillers enhances the SE's thermal and mechanical properties. Moreover, doping the grafted SE with different organic semiconductors (ITIC, PCBM, and NTCDA) further improves its electrical properties and charge dynamics at 150°C. The optimal doping content for ITIC, PCBM, and NTCDA is found to be 0.05, 0.10, and 0.15 wt%, respectively. Among these, NTCDA exhibits superior electrical performance at 150°C. Particularly, compared to pure SE, NTCDA doping and POSS grafting reduce the electrical conductivity by an order of magnitude and increase the breakdown strength, charge hopping activation energy, and trap energy level by 65.5%, 0.20 eV and 0.73 eV, respectively. This study finds that organic semiconductors outperform nanostructures in inhibiting electron injection and immobilizing free electrons at high temperatures.Highlights Nano‐POSS grafting enhances material properties by reinforced molecular chains. Charge dynamics is studied by space charge and current integrated charge at HT. Organic semiconductors improve electrical properties and charge dynamics at HT. Organic semiconductors outperform nanofillers in immobilizing electrons at HT. The ideal organic semiconductor doping level correlates with its energy gap.

Dominant mechanisms of thermo-mechanical properties of weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates toward multifunctional thermal/environmental barrier coating applications

Weberite-type RE3TaO7 (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates have been studied as multifunctional thermal/environmental barrier coating materials with working temperatures above 1500 K. This study proposes the dominant mechanisms of their thermo-mechanical properties. The relative ionicity of the RE-O bonds increased with increasing RE+3 ionic radius, which weakened the bond strength and led to decreases in Young's modulus and hardness. A model was proposed to derive the high-temperature thermal conductivity based on the relationship between the phonon thermal conductivity and temperature, and the thermal radiative conductivity was estimated. RE3TaO7 exhibited a lower thermal conductivity than other fluorite-related oxides because its weak bond strength could slow down the phonon speed and enhance the an-harmonic vibrations of the lattices. At low temperatures, the RE atomic weight was a better descriptor of the thermal conductivity than the RE+3 ionic radius, and which could aid in further regulating the thermal conductivity. The thermal expansion coefficients of the different axes were affected by both RE-O and Ta-O bonds when weak RE-O bonds dominated the linear thermal expansion. The low oxygen ion conductivity derived from the strong covalent Ta-O bonds, which increased the activation energy for oxygen hopping in lattices of RE3TaO7. This study elucidates the dominant mechanisms of properties from the characteristics of crystals and chemical bonds, which are significant for high-temperature applications of tantalates.

Investigating structural, optical, and electron-transport properties of lithium intercalated few-layer MoS2 films: Unraveling the influence of disorder

Molybdenum disulfide is a promising candidate for various applications in electronics, optoelectronics, or alkali-ion batteries. The natural presence of the van der Waals gap allows intercalating alkali ions, such as lithium, into MoS2 films. Intercalation can modify the electronic structure as well as the electrical and optical properties. Here, we present a structural, optical, and electrical characterization of Li-intercalated few-layer MoS2 films. The intercalation was carried out by annealing MoS2 film in the presence of Li2S powder, serving as a lithium source. The initial MoS2 layers were prepared by pulsed laser deposition (PLD) and by sulfurization of 1 nm thick Mo film (TAC). The presence of lithium was confirmed by synchrotron-based x-ray Photoelectron Spectroscopy. The Raman spectroscopy, x-ray diffraction, and optical absorption measurements confirmed semiconducting behavior for all samples. All samples exhibited the thermally activated dependence of the electrical resistance, R, typical for the Efros–Shklovskii variable range hopping in a disordered semiconductor, ln R(T) ∝ (TES/T)1/2, where kBTES is the hopping activation energy. The PLD-grown MoS2 samples exhibited a relatively mild initial disorder primarily caused by grain boundaries. Lithium intercalation led to an increase in disorder, evident in the increase in kBTES and a substantial rise in electrical resistance. The TAC-grown undoped MoS2 sample already exhibited significant resistance, and the impact of Li intercalation on resistance was minimal. This observation was attributed to the fact that the TAC-grown MoS2 samples exhibit a perturbed stoichiometry (the S:Mo ratio ∼ 2.20), causing strong disorder even before Li intercalation. The electron doping caused by lithium, if any, was completely obscured by the effect of disorder.

Hopping nature of the Hall effect in a samarium nickelate film

We analyzed the temperature dependencies of the DC resistivity and Hall coefficient in thin films of samarium nickelate SmNiO3. A dominating hopping conductivity was revealed in the insulating phase below 400 K, which defines an exponential drop in the Hall coefficient with temperature. The estimated dependencies of the hopping activation energy, Hall mobility, and charge carrier density suggest that small polarons are responsible for hopping. The hopping transport explains the observed sign anomaly of the Hall coefficient in rare-earth nickelates.

Enhanced bulk and interfacial charge transfer in Fe:VOPO4 modified Mo:BiVO4 photoanodes for photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising photoanode material for photoelectrochemical (PEC) water oxidation. However, its performance is greatly hindered by poor bulk and interfacial charge transfer. Herein, to address this issue, iron doped vanadyl phosphate (Fe:VOPO4) was grafted on molybdenum doped BiVO4 (Mo:BiVO4) for significantly enhancing charge transfer and oxygen evolution kinetics simultaneously. Consequently, the resultant Fe:VOPO4/Mo:BVO4 photoanode exhibits a remarkable photocurrent density of 6.59 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, over approximately 5.5 times as high as that of pristine BiVO4. Systematic studies have demonstrated that the hopping activation energy of small polarons is significantly reduced due to the Mo doping, resulting in accelerated bulk charge transfer. More importantly, the deposition of Fe:VOPO4 promotes the interfacial charge transfer between Mo:BiVO4 and Fe:VOPO4 via the construction of V−O−V and P−O bonds, in addition to facilitating water splitting kinetics. This work provides a general strategy for optimizing charge transfer process, especially at the interface between photoanodes and cocatalysts.

Diffusion of hydrogen in the Volterra elastic fields around dislocations in α-Ti: Coupling DFT calculations and elasticity method

Hydrogen around dislocation has a great influence on the mechanical properties of Titanium (Ti) alloys. In this study, the diffusion of hydrogen around dislocations in α-Ti alloys is investigated by coupling density functional theory (DFT) calculations and the elasticity method. Initially, the attempt frequency and hopping activation energy of hydrogen atom in the α-Ti lattice are calculated using the DFT method under various simple strain fields, including volume strain, biaxial strain, and shear strains, within a strain range of −2%–2 %. It is observed that both volume strain and biaxial strain have the most pronounced effects on the hopping activation energy of hydrogen atom, while the attempt frequency remains relatively unaffected across different strain conditions. Subsequently, the hopping activation energy in the elastic strain field has been parameterized using an elasticity method based on elastic dipole and diaelastic polarizability. Using the elasticity method, an analysis is conducted on the effective activation energy of hydrogen atoms and their hopping tendencies in the Volterra elastic strain fields around the perfect edge and screw dislocations. The elastic strain field around screw dislocation enhances the diffusion of hydrogen atoms and simultaneously promotes their migration toward the dislocation core. In the vicinity of edge dislocation, the tensile elastic strain field enhances the diffusion of hydrogen atoms and facilitates their migration toward the dislocation core. Conversely, the compressive strain field retards the diffusion of hydrogen atoms and promotes their migration away from the dislocation core or towards regions with tensile strain.

Electrical Effects of Ionic Deficient Cubic Ferrite Spinels: A Case Study on MgFe2O4

AbstractElectron transport via indirect exchange interactions between octahedral sites in spinels is affected by cationic or anionic vacancies. Thus, the deviation of the electron hopping mechanism is investigated, from a constant energy barrier toward a variable range energy barrier in iron‐deficient and oxygen‐deficient cubic magnesium ferrite spinels. Variable range electron hopping activation energy at high temperatures is consistent with the modified cation–anion interactions in the distorted structure of an ionic deficient cubic magnesium ferrite spinel. For these observations, an instrumental method is developed to evaluate the electron exchange interactions and general redox kinetics on surfaces, managed by exposure to the flow of a wide variety of desired gases. The applications of this instrument are far beyond the example studies reported here.

Small-polaron transport in perovskite nickelates

Knowledge of the explicit mechanisms of charge transport is preeminent for a fundamental understanding of the metal-to-insulator transition in ABO3-type perovskite rare-earth nickelates and for potential applications of these technologically promising materials. Here we suggest that owing to intrinsic Jahn–Teller-driven carrier localization, small-polaron transport is innate in nickelates. We demonstrate experimental evidence for such transport by investigating AC conductivity over a broad range of temperatures and frequencies in epitaxial SmNiO3 films. We reveal the hopping mechanism of conductivity, Holstein-type activation energy for hopping, nonclassical relaxation behavior, and nonclassical consistency between activation and relaxation. By analyzing these observations, we validate small-polaron transport. We anticipate that our findings can lead to precise tailoring of the DC and AC conductivity in nickelates as requested for fruitful employment of these materials. We also believe that further investigations of self-trapped small polarons are essential for a comprehensive understanding of nickelates.

Structure and electrical conductivity of nanocomposite SiOxNy(Si) and SiAlzOxNy(Si) films

In this study, the peculiarities of the structure and electrical conductivity of nanocomposite SiOxNy(Si) and SiAlzOxNy(Si) films in the temperature range of 95 – 340 K are determined. The composite films were obtained by ion-plasma sputtering of Si and co-sputtering of Si and Al targets in oxygen and nitrogen containing atmosphere (Ar+O2 +N2) followed by high temperature annealing. During the annealing of the deposited Si rich oxynitride films, Si nanoinclusions were formed in an amorphous dielectric matrix due to phase separation. Al atoms are found to be in the bound state (no Al metal nanoparticles present). The films have low absorbance and considerable transparency in the ultraviolet – visible – near infrared spectral region. Reduction of some part of Si-O bonds by Al during annealing the Al-containing samples leads to the separation of higher Si concentration as compared to the Al-free samples. The electrical conductivity of the films at low electric fields is found to be by variable-range electron hopping through the traps near the Fermi level. The characteristics of the traps and transport process such as the density of electron traps near the Fermi level, the activation energy of hopping and the hopping length are determined. It is found that the electrical conductivity of the SiAlzOxNy(Si) films annealed in argon is higher as compared to the films annealed in nitrogen and the Al-free films due to higher concentration of electron traps. The region of the conductivity decrease with the increase of electric field is revealed on the current-voltage characteristics of both the SiAlzOxNy(Si) films annealed both in nitrogen and argon and the SiOxNy(Si) films annealed in nitrogen. This effect is explained based on the peculiarities of electric field polarization in the films. The important role is played by the interface traps at the Si nanoparticle / amorphous matrix interfaces that capture electrons. At high electric fields, the film conductivity corresponds to the space charge limited current mechanism with the exponential distribution of the traps in the energy gap.

Suppressed polaronic conductivity induced sensor response enhancement in Mo doped V2O5 nanowires

In this paper, we show the direct correlation between the suppression of the polaronic oxygen vacancy defect (Vo) density and gas sensor response of 1 at. % Mo-doped V2O5 nanowires (MVONWs). Doping 1 at. % Mo5+ leads to substitution at the V5+ site in V2O5 nanowires (VONWs) and, therefore, reduction in Vo defects. This, in turn, affects the charge carrier hopping sites and, subsequently, enhances the sensor response at lower temperatures (<320 °C). The Mo5+ dopants lead to the lowering of Fermi energy (EF) toward valence band maxima due to the reduced Vo donor density. The polaron suppression is confirmed with the activation energy of polaron hopping, increasing from 195 to 385 meV in VONWs and MVONWs. As a result, the response to ethanol gas enhanced as the depletion width is widened for the given cross section of the nanowires. This may lead to a large depletion controlled cross-sectional area and, therefore, better sensitivity. At about 350 °C, VONWs show a change in the slope of resistance vs temperature (MIT), which is not observed in the case of MVONWs. This is attributed to the presence of the enhanced non-stoichiometry of V ion resulting in metallic behavior and accompanied by a sudden rise in the sensor response at this temperature. Moreover, the absence of MIT may be attributed to the lack of such a sudden rise in the response in MVONWs.

Grain Boundary Engineering with Cl-Terminated Ti2C Quantum Dots for Enhancing Perovskite Solar Cell Performance

The power conversion efficiency and stability of polycrystalline perovskite solar cells are compromised by grain-boundary-dominated ion migration. Herein, the grain boundaries of CH3NH3PbI3 films were engineered using Ti2C quantum dots. Ti2C quantum dots had a strong interaction with Pb2+ and I– ions, which retarded crystal growth, resulting in larger grain size and less grain boundary in perovskite films. Additionally, the Ti2C quantum dots at the grain boundary could anchor ions and passivate defects, increasing activation energy for ion hopping and reducing trap density. In consequence, perovskite films were used to fabricate solar cells with significantly improved efficiency and stability. These findings may allow the development of versatile grain boundary modifiers for perovskite optoelectronic devices that combine stability and efficiency.

Gamma-ray Shielding Characteristics and Electrical Properties of Na2O Doped Zinc-Boro-Vanadate Glasses

A set of glasses of composition (ZnO)0.3-(V2O5)0.6–x-(B2O3)0.1-(Na2O)x with x = 0.1, 0.15, 0.2 and 0.3 were prepared and their non-crystallinity were confirmed. Density and molar volume were found to decrease with increasing Na2O concentration. Using Phy-X/PSD and XCOM softwares, radiation-shielding efficiency was determined by estimating mass attenuation coefficient, linear attenuation coefficient, effective atomic number, half value layer, tenth value layer and mean free path of glasses for gamma energy from 0.005 to 15 MeV. Obtained shielding parameters led to propose the glasses for γ-ray shielding at lower energy regime. Conductivity variation with temperature was analyzed using Mott’s small polaron hopping model and activation energy was determined. It was found to decrease with increasing Na2O content both at high and low temperature. The density of states at Fermi level was determined using Mott’s and Greave’s variable range hopping (VRH) models at low temperature.

Thermally activated motion of cold excitons in conjugated polymers

AbstractThermally activated migration of localized cold excitons were investigated in archetypal conjugated polymers; trans‐polyacetylene, polythiophene, and poly(p‐phenylene vinylene). Room temperature thermal energy has been found to provide sufficient energy to avail substantial mobility for excitons. A simple undistorted but displaced harmonic oscillator model has been assumed to predict the activation energy for exciton hopping between identical sites in perfectly planar conjugated chains, promoted by vibrational modes with Bu symmetry. The activation energies of exciton hopping has been found to increase in the order of trans‐polyacetylene (6.4 meV), polythiophene (29.5 meV), and poly(p‐phenylene vinylene) (147.4 meV). A rough Monte‐Carlo simulation based on incoherent hopping mechanism reveals excitons can displace in an average of 2.17 nm in polythiophene whereas those distances are much smaller in poly(p‐phenylene vinylene) (0.24 nm) and trans‐polyacetylene (0.37 nm). The simulation results partially explain why polythiophene based polymers are better suited for organic photovoltaic applications whereas poly(p‐phenylene vinylene) are more appropriate for organic light emitting diode applications.

Small-polaron conductivity in perovskite ferroelectric BaTiO3 films

In ABO3 perovskite oxide ferroelectrics, electrical conductivity ranges from insulator- to superconductor-type and is virtually critical for all applications of these materials. Compared to bulk ceramics and crystals, ferroelectric thin films can enable advanced control of the conductivity. Here, small-polaron hopping conductivity was evidenced and examined in various pulsed-laser-deposited films of ferroelectric BaTiO3 and reference films of SrTiO3. For this, AC conductivity was studied in a broad range of temperatures and frequencies for films sandwiched between the bottom and top electrodes. In the BaTiO3 films, with increasing temperature, a significant increase in activation energy for small-polaron hopping was found and ascribed to strong electron–phonon coupling and complex lattice oscillations therein. Plain relations of the activation energy to microstructure, composition, or phase transitions were lacking, which corroborated the critical role of phonons. Additionally, a phonon-less transport was detected. It was anticipated that owing to strong electron–phonon coupling, rich phonon ensembles, and coexistence of phonon-stimulated and phonon-less processes, the small-polaron conductivity can heavily vary in ferroelectric films that necessitates further studies.

Analysis of Al adatoms random hopping on Si(100)-c(4 [formula omitted] 2) surface observed by STM at low temperature: Determination of the hopping barriers

Mobility of single aluminium atoms adsorbed on Si(100)-c(4×2) surface was studied by means of scanning tunneling microscopy (STM) at a temperature of 115 K. Due to a strongly anisotropic surface migration along silicon dimer rows the hopping in one-dimensional (1-D) bounded areas of various sizes was observed. Distributions of distances traversed in a time between acquisition of two subsequent STM images were obtained. The experimental conditions were interpreted by a model of 1-D random walk of Al adatoms in areas of limited size with reflective boundaries. Kinetic Monte Carlo (KMC) simulations were used for analysing dependence of a calculated activation energy for a single hop on a size of the 1-D bounded area. The developed method allowed restoration of a real value of the activation energy for the Al adatom hopping in a direction parallel to the silicon dimer rows unaffected by the limited size of the bounded area. The activation energy for hops in a direction perpendicular to the dimer rows was estimated from a distribution of time intervals between individual perpendicular hops.

Electron transport through composite SiO2(Si)&FexOy(Fe) thin films containing Si and Fe nanoclusters

In this study, electron transport mechanisms at a direct current in SiO2(Si)&FexOy(Fe) granular composite films containing Si and Fe nanoinclusions in the temperature range of 95–340 K were determined. The composite films were obtained by co-sputtering of Si and Fe targets in oxygen containing atmosphere (Ar+O2) followed by temperature annealing. It was found out that hopping conductivity with a variable-range hopping was realized at low electric fields. In the temperature range of 115<T<180 K, the electron transport could be reasonably described by the Efros-Shklovsky model taking into account the Coulomb interaction. At higher temperatures (180<T<340 K), the Mott model have been shown to be applicable. Using this model, a number of the characteristics of traps and transport process, namely the density of electron traps near the Fermi level, the activation energy of hopping, and the hopping length were determined. In the range of intermediate and high electric fields, the field enhanced thermal activation of electrons in the conduction band (Poole-Frenkel mechanism) was concluded. At this, the temperature independent current was obtained at the highest values of the electric field. This effect was explained by the influence of the energy position of electron traps taking part in the conductivity as well as the temperature dependence of a dielectric constant.