Polaron Defects Research Articles

Small polaron hopping and tunneling transport in Maxwell–Wagner relaxation dominated Al2O3/TiO2 subnanometric laminates

Maxwell–Wagner relaxation dominated Al2O3/TiO2 nanolaminates (ATA NLs) have recently demonstrated their potential for high-density energy storage applications. In this report, we have unraveled the defect-mediated transport mechanisms prevailing in Al2O3/TiO2 sub-nanometric laminates. Temperature-dependent ac conductivity measurements revealed the signature of small polaron hopping in TiO2 active layers and trap-assisted tunneling transport through Al2O3 barrier layers, which was corroborated by resonant photoelectron spectroscopy and temperature-dependent current–voltage measurement. The polaronic defect states, found ∼1 eV below the Fermi level, served as the hopping centers and leakage paths for current. The signature of quantum tunneling transport and the negative differential conductance observed toward higher electric field was attributed to the splitting of delocalized minibands. These transport properties of Al2O3/TiO2 nanolaminates will help in tailoring these materials for next-generation storage capacitors.

Synthesis, optical characterization and electrical behavior of chitosan/MgO/Cr2O3 nanocomposite for electrical applications

Magnesium oxide nanoparticles (MgO NPs) are straightforward, cheap, biocompatible, and biodegradable, making them one of the most popular inexpensive and non-toxic nanoparticles. In addition, Nanosized chromic oxide (Cr2O3) is a significant transition metal oxide due to its numerous exceptional attributes, it finds application in several domains. It has good electrical conductivity, excellent thermal and chemical stability. It is utilized in several industries for its ability to resist oxidation at high temperatures, as well as its application in refractory materials and catalysts. In this study, we utilized a laser ablation approach to synthesize a ternary nanocomposite composed of MgO and Cr2O3 nanoparticles as it is considered a clean method for the preparation of nanoparticles. Subsequently, these nanoparticles were integrated into Chitosan (CS) solution to create polymer nanocomposite via casting route. XRD confirmed the formation of Cr2O3 nanocrystals with size of 61 nm dispersed in chitosan/MgO matrix. FTIR verified the interactions between chitosan, MgO, and Cr2O3 nanoparticles. The broadening and shifting of the OH and NH bands with increasing Cr2O3 content confirms the interaction and complexation between Cr2O3 and Chitosan/MgO. UV–Vis analysis revealed that incorporating 7 wt% Cr2O3 reduced optical band gap from 5.59 eV for chitosan to 3.91 eV. The band gap values decreased with increasing Cr2O3 content due to polaronic defects. TGA showed improved thermal stability, with the 7 wt% Cr2O3 nanocomposite exhibiting a residual mass of 47 % at 700 °C compared to 28 % for chitosan. Dielectric studies demonstrated high dielectric constant and conductivity (10−5 S/cm at 10 Hz) values for the 7 wt% Cr2O3 nanocomposite, attributed to interfacial polarization effects and nanocapacitor formation by the conductive nanofillers. The ternary chitosan/MgO/Cr2O3 nanocomposites with tunable optical, thermal and electrical properties show promising potential for electronic applications.

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.

Interplay of dopant and polarons in trifunctional semimagnetic semiconductor for supercapacitor applications: Local structure and electronic structure investigations

Diluted magnetic semiconductors (DMS), fractionally doped by magnetic ions, are peculiar, and a single semiconductor effectively contributes to charge and spin properties for various applications, including data and energy storage. Here, a low temperature assisted co-precipitation method produced a series of Co doped ZnO nanoparticles. After structural characterizations, zinc vacancy defects were realized via the hyperfine signal with g ∼ 1.96 (spin resonance) and the emissions (photoluminescence) at 410 and 465 nm. The synergistic effect among the Co dopant, native, and polaron defects makes the system exhibit enhanced polarization and electrochemical reactions. Magnetization hysteresis curves demonstrate the intrinsic magnetic nature of the materials that are essential for realizing useful polaronic states. As an anode material, 20 at. % Co doped electrode has a higher specific capacitance and superior cycling stability with charge retention above 98 %, which is much higher than the other ZnO-based electrodes and is associated with the polaron-assisted rapid redox reactions. These polarons also stimulate a ferroelectric polarization of 0.28 μC cm−2 via local lattice distortion. All these findings confirm that the Co doped semi-magnetic ZnO system with multifunctionality is adequate for employment in device fabrication as a low-cost, highly stable material.

Hubbard U through polaronic defect states

Since the preliminary work of Anisimov and co-workers, the Hubbard corrected DFT+U functional has been used for predicting properties of correlated materials by applying on-site effective Coulomb interactions to specific orbitals. However, the determination of the Hubbard U parameter has remained under intense discussion despite the multitude of approaches proposed. Here, we define a selection criterion based on the use of polaronic defect states for the enforcement of the piecewise linearity of the total energy upon electron occupation. A good agreement with results from piecewise linear hybrid functionals is found for the electronic and structural properties of polarons, including the formation energies. The values of U determined in this way are found to give a robust description of the polaron energetics upon variation of the considered state. In particular, we also address a polaron hopping pathway, finding that the determined value of U leads to accurate energetics without requiring a configurational-dependent U. It is emphasized that the selection of U should be based on physical properties directly associated with the orbitals to which U is applied, rather than on more global properties such as band gaps and band widths. For comparison, we also determine U through a well-established linear-response scheme finding noticeably different values of U and consequently different formation energies. Possible origins of these discrepancies are discussed. As case studies, we consider the self-trapped electron in BiVO4, the self-trapped hole in MgO, the Li-trapped hole in MgO, and the Al-trapped hole in α-SiO2.

Polaronic defects in monolayer CeO2: Quantum confinement effect and strain engineering.

We uncover the structure, stability, and electronic properties of polaronic defects in monolayer (ML) CeO2 by means of first-principles calculations, with special attention paid to the quantum confinement effect induced by dimensionality reduction. Results show that the polaron can be more stabilized in ML CeO2 than in the bulk, while formation of oxygen vacancy (Vo2+) and polaron-vacancy complexes [(Vo2+-1polaron)1+, (Vo2+-2polaron)0] tends to be more difficult. The polaronic defect states sit deeper in energy within the bandgap of ML CeO2 compared to the bulk case. We further demonstrate that the epitaxial strain in ceria film, as normally exists when grown on metal substrate, plays a crucial role in regulating the defect energetics and electronic structures. In particular, the formation energies of polarons, Vo2+, (Vo2+-1polaron)1+, and (Vo2+-2polaron)0, generally decrease with tensile strain, leading to controllable defect concentration with strain and temperature. This study not only provides physical insights into the polaronic defects in ultrathin oxide films, but also sheds light on their potential technological applications in nanoelectronics, fuel cells, and catalysts.

Detection and Correction of Delocalization Errors for Electron and Hole Polarons Using Density-Corrected DFT.

Modeling polaron defects is an important aspect of computational materials science, but the description of unpaired spins in density functional theory (DFT) often suffers from delocalization error. To diagnose and correct the overdelocalization of spin defects, we report an implementation of density-corrected (DC-)DFT and its analytic energy gradient. In DC-DFT, an exchange-correlation functional is evaluated using a Hartree-Fock density, thus incorporating electron correlation while avoiding self-interaction error. Results for an electron polaron in models of titania and a hole polaron in Al-doped silica demonstrate that geometry optimization with semilocal functionals drives significant structural distortion, including the elongation of several bonds, such that subsequent single-point calculations with hybrid functionals fail to afford a localized defect even in cases where geometry optimization with the hybrid functional does localize the polaron. This has significant implications for traditional workflows in computational materials science, where semilocal functionals are often used for structure relaxation. DC-DFT calculations provide a mechanism to detect situations where delocalization error is likely to affect the results.

Machine learning for exploring small polaron configurational space

Polaron defects are ubiquitous in materials and play an important role in many processes involving carrier mobility, charge transfer and surface reactivity. Determining small polarons’ spatial distributions is essential to understand materials properties and functionalities. However, the required exploration of the configurational space is computationally demanding when using first principles methods. Here, we propose a machine-learning (ML) accelerated search that determines the ground state polaronic configuration. The ML model is trained on databases of polaron configurations generated by density functional theory (DFT) via molecular dynamics or random sampling. To establish a mapping between configurations and their stability, we designed descriptors modelling the interactions among polarons and charged point defects. We used the DFT+ML protocol to explore the polaron configurational space for two surface-systems, reduced rutile TiO2(110) and Nb-doped SrTiO3(001). The ML-aided search proposes additional polaronic configurations and can be utilized to determine optimal polaron distributions at any charge concentration.

TiO2 Polarons in the Time Domain: Implications for Photocatalysis.

Exploiting the availability of solar energy to produce valuable chemicals is imperative in our quest for a sustainable energy cycle. TiO2 has emerged as an efficient photocatalyst, and as such its photochemistry has been studied extensively. It is well-known that polaronic defect states impact the activity of this chemistry. As such, understanding the fundamental excitation mechanisms deserves the attention of the scientific community. However, isolating the contribution of polarons to these processes has required increasingly creative experimental techniques and expensive theory. In this Perspective, we discuss recent advances in this field, with a particular focus on two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT), and discuss the implications for photocatalysis.

Strain‐Engineered Formation, Migration, and Electronic Properties of Polaronic Defects in CeO2

First‐principles investigations on the strain‐engineered formation, electronic structures, and migration properties of polaronic defects in ceria (CeO2), including single polarons, oxygen vacancies (Vo2+), and polaron–vacancy complexes [(Vo2+–1polaron)1+, (Vo2+–2polaron)0], are reported. Results show that the formation energy of oxygen vacancy increases with both tensile and compressive biaxial strain, whereas the formation energies of polarons and polaron–vacancy complexes reduce (increase) with tensile (compressive) strain, so that their defect concentrations behave drastically different with strain and temperature. Interestingly, due to the distinct deformation potentials, the polaronic defect states can shift toward band edges with strain, beneficial for enhanced electrical conductivity. The migration of polarons in CeO2 is further explored, and a tendency of strain‐induced directional migration is revealed. These findings not only shed light on the fundamental properties of polaronic defects in CeO2 but also provide an attractive approach of combining defect engineering with strain engineering for oxide‐based functional microelectronics, batteries, and catalysts.

Evaluation of similarities and differences of LiTaO3 and LiNbO3 based on high-T-conductivity, nonlinear optical fs-spectroscopy and ab initio modeling of polaronic structures

Different aspects of ferroelectric LiTaO3 (LT) such as polaronic defects, optical response and electrical conductivity are investigated by the most recent theoretical and experimental approaches. Comparing the results with the state-of-the-art knowledge of the widely studied LiNbO3 (LN), we evaluate the general assumption that there is little difference between the aforementioned properties of LT and LN. First-principles calculations reveal the existence of point defects in LT qualitatively compatible with the polaronic picture established in LN. Though, peculiar differences with respect to the individual binding energies and polaronic deformation can be revealed. Accordingly, (sub-)picosecond transient absorption measurements show pronounced differences in the kinetics in the sub-ps time domain of small polaron formation and, even more pronounced, in the long-term evolution identified with small polaron hopping. In contrast, (sub-)ps transient luminescence, attributed to the relaxation of self-trapped excitons in LN, shows very similar kinetics. Electrical conductivity measurements are performed in air as function of temperature. Up to about 600 °C they demonstrate similar temperature dependence for the two materials, from which rather comparable activation energies can be extracted. However, in the high-temperature range from about 600 °C to 920 °C both materials show noticeable differences. The results suggest that the fundamental microscopic understanding of LN can be in part transferred to LT. However, due to differences in structure, energetic landscape and temperature behavior, discrepancies between the two materials bear a striking potential for novel applications, even at high temperatures.

A Faster Path to Solar Water Splitting

Finding new wide-bandgap light absorbers that are stable in aqueous solutions is a long-standing challenge in photoelectrochemical water splitting research. Two papers in this issue describe recent advances in high throughput experimentation that may accelerate the discovery of suitable materials.

Temperature-dependent magnetic and electrical properties of Cr-doped AlFeO3 ceramics

ABSTRACT The solid-state reaction method was selected to prepare polycrystalline AlFe1−x Cr x O3 (x = 0, 0.02, 0.05, 0.07 and 0.1) samples. The Samples were found to be pure phases possessing the orthorhombic space group Pna21. The unit cell volume decreased with increases in the Cr content. The magnetic transition temperature and the saturation magnetization of the ceramics decreased due to dilution of the magnetic interaction with Cr content. The Dielectric responses were systematically investigated by temperature-dependent dielectric/impedance spectroscopy from 160 K to 500 K. It was noticed that incorporation of Cr3+ in the structure led to an increase in the dielectric constant up to x = 0.05 before decreasing, whereas the dielectric loss decreased. Two different dielectric relaxation processes that occurred in the grain interior of AlFe1-xCrxO3 polycrystals were clarified. One relaxation process was thought to be associated with localized transfer of electrons between Fe3+ and Fe2+, while the other arising at low temperature was assigned to interplay between lattice and polaronic defects such as oxygen vacancies.

Role of long-range exact exchange in polaron charge transition levels: The case of MgO

Predicting the degree of localization and calculating the trapping energies of polarons in insulators by density functional theory (DFT) is challenging. Hybrid functionals are often reparametrized to obtain accurate results and the a priori selection of these parameters is still an open question. Here we test the accuracy of several range-separated hybrid functionals, all reparametrized to produce an accurate band gap, by calculating the charge transition levels (CTLs) of experimentally well-studied hole polaron defect centers in MgO. We show that the functional with screened long-range exact exchange is moderately but consistently more accurate than functionals which do not include long-range exact exchange. We provide evidence that the source of the improved accuracy is the eigenvalue associated with the valence band maximum of the bulk material. We discuss the extent to which this accuracy relates to Koopmans' compliance of the defect energy level.

Broad luminescence from donor-complexed LiZn and NaZn acceptors in ZnO

Zn substitutional lithium (${\mathrm{Li}}_{\text{Zn}}$) and sodium (${\mathrm{Na}}_{\text{Zn}}$) acceptors and their complexes with common donor impurities (${\mathrm{Al}}_{\text{Zn}}, {\mathrm{H}}_{i}$, and ${\mathrm{H}}_{\text{O}}$) in ZnO have been studied using hybrid functional calculations. The results show that the complexes are not exclusively charge neutral, but rather exhibit a thermodynamic ($+/0$) transition level close to the valence band maximum. The positive charge states are associated with a polaronic defect state, similar to those of the well-studied charge-neutral isolated acceptors. This incomplete passivation has profound consequences for the optical properties of the complexes. Indeed, electron transitions from the conduction band minimum to the ($+/0$) transition level of the complexes result in broad luminescence bands that are blueshifted with respect to those originating from the isolated acceptors. Such complexes are proposed as a potential defect origin of the green luminescence observed at the high-energy side of the orange luminescence band (caused by ${\mathrm{Li}}_{\text{Zn}}$) in hydrothermally grown ZnO. This prediction is supported by experimental photoluminescence and secondary ion mass spectrometry data on a hydrothermally grown ZnO sample. We have also explored how the parameters controlling the fraction and screening of exchange in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional influence the results by comparing two parametrization approaches: (i) the conventional one where the exchange fraction is adjusted to reproduce the experimental band gap, and (ii) tuning both parameters in order to also comply with the generalized Koopmans theorem (gKT). Interestingly, these approaches were found to yield similar results.

Polyaniline encapsulated graphite: A sensitive system for resistive detection of methanol

Polyaniline in its emeraldine salt form has been prepared on graphite coated cellulose through vapor phase polymerization technique to prepare polyaniline encapsulated graphite (PANI-G) system. The resistive response of polyaniline in alcoholic medium has been reported in this article. The prepared PANI-G system appeared crystalline as revealed from X-ray diffraction measurements. π electrons from graphite facilitates the electronic conduction mechanism of polaron defect states in polyaniline, and are responsible for observed electronic behavior of polyaniline film and its response with the presence of methanol. The interaction of coupled polaron and electrons of PANI-G with alcohol, and thus the variation of resistivity of the PANI-G films has been the basis of this work for preparing an alcohol sensor. The OH− ion of alcohol localizes the coupled polaron-electron in alcoholic medium. Higher the concentration of alcohol higher is the polaron-electron localization and thus greater is the resistivity. Resistive methanol sensitivity of 11.2 has been obtained at pure methanol.

Tuning of optical and electrical properties of polyaniline on flexible cellulose through Eosin Y dye interaction

Polyaniline, which is an attractive macromolecular system for electronic applications, has been chosen in this work in order to bring a tuning and alignment of energy levels for controlling its electrical and optical properties through Eosin Y dye interaction. Polyaniline has been synthesized over flexible cellulose substrate through a very simple oxidative chemical vapor polymerization technique. Prepared polyaniline incorporated cellulose was then allowed to interact with the anionic dye of Eosin Y to control its charge transport and optical properties. The dye interaction leads to better crystallinity of polyaniline over cellulose as revealed from x-ray diffraction pattern. From optical characterization, a remarkable change of band gap varying from 2.629 eV to 2.471 eV has been observed with increasing dye interaction time. The effects of dye interaction on charge transport and carrier dynamics were studied from I-V curve, which shows an excellent improvement of electrical conductivity. The color generating chromophores present in dye are likely to interact with positively charged polaron defect states present in polyaniline. Thus it opens a wider and simpler way of functionalizing π-conjugated polymers on flexible cellulose substrate in exploring their applications as electronic materials in sensor, memory devices and field effect devices.

Ferroelasticity, anelasticity and magnetoelastic relaxation in Co-doped iron pnictide: Ba(Fe0.957Co0.043)2As2

The hypothesis that strain has a permeating influence on ferroelastic, magnetic and superconducting transitions in 122 iron pnictides has been tested by investigating variations of the elastic and anelastic properties of a single crystal of Ba(Fe0.957Co0.043)2As2 by resonant ultrasound spectroscopy as a function of temperature and externally applied magnetic field. Non-linear softening and stiffening of C66 in the stability fields of both the tetragonal and orthorhombic structures has been found to conform quantitatively to the Landau expansion for a pseudoproper ferroelastic transition which is second order in character. The only exception is that the transition occurs at a temperature (TS ≈ 69 K) ~10 K above the temperature at which C66 would extrapolate to zero ( ≈ 59 K). An absence of anomalies associated with antiferromagnetic ordering below TN ≈ 60 K implies that coupling of the magnetic order parameter with shear strain is weak. It is concluded that linear-quadratic coupling between the structural/electronic and antiferromagnetic order parameters is suppressed due to the effects of local heterogeneous strain fields arising from the substitution of Fe by Co. An acoustic loss peak at ~50–55 K is attributed to the influence of mobile ferroelastic twin walls that become pinned by a thermally activated process involving polaronic defects. Softening of C66 by up to ~6% below the normal—superconducting transition at Tc ≈ 13 K demonstrates an effective coupling of the shear strain with the order parameter for the superconducting transition which arises indirectly as a consequence of unfavourable coupling of the superconducting order parameter with the ferroelastic order parameter. Ba(Fe0.957Co0.043)2As2 is representative of 122 pnictides as forming a class of multiferroic superconductors in which elastic strain relaxations underpin almost all aspects of coupling between the structural, magnetic and superconducting order parameters and of dynamic properties of the transformation microstructures they contain.

Polaron localization in polyaniline through methylene blue dye interaction for tuned charge transport and optical properties

An excellent control over the charged transport process and optical properties in polyaniline has been attained through successful localization of polarons formed in the polyaniline chain. The localization of polaron in polyaniline has been achieved through its interaction with methylene blue (MB) dye. Polyaniline has been synthesized on paper substrate from aniline using FeCl3 as polymerizing agent. On optical measurement of prepared sample, a remarkable red shift in the optical band gap has been observed varying from 2.63 to 2.47 eV. The charge transport behavior as reflected from its I-V measurement shows a nice tuning in the logarithmic scale. The chemical interaction between the MB dye and the polaronic charged defect states brings this brilliant tuning in the optical and electrical properties of polyaniline. Predominantly, the intra-chain charge transport of the polyaniline backbone is affected by the MB dye interaction.

Limitation of Fermi level shifts by polaron defect states in hematite photoelectrodes

The optical band gap is a major selection criterion for an absorber in photocatalytic water splitting. Due to its ideal value hematite has been intensively investigated without reaching the expectation, yet. In this work, the Fermi level positions in hematite due to doping and contact formation are investigated. An upper boundary for the Fermi level position at 1.8 eV above the valence band maximum due to the formation of polarons is identified. This results in a different concept of the effective band gap for hematite which we believe is transferable to any material with competing polaron formation after optical excitation: the optical band gap of 2.2 eV deviates from an effective electronic band gap of 1.75 eV. The polaron state acts as a limit in (quasi-)Fermi level shift, restricting the potential of charge transfer reactions. Additionally, it has led to an incorrect determination of the band edge positions of hematite in electrochemical contacts.