Polaron Transport Research Articles

Spin-polarized tunneling and polaronic transport properties of polycrystalline (Sm1−y Gd y )0.55Sr0.45MnO3 (y = 0.5 and 0.7) compounds

The detail investigations on the magneto-transport properties of the polycrystalline (Sm0.3Gd0.7)0.55Sr0.45MnO3 (SGSMO-1) and (Sm0.5Gd0.5)0.55Sr0.45MnO3 (SGSMO-2) compounds, having a glassy-like and ferromagnetic ground states respectively have been carried out in details. Due to the existence of two different magnetic ground states in the above mentioned systems, the magneto-transport properties are markedly differed from each other, specially at the low temperature region. The highly semi-conducting nature of the SGSMO-1 compound is suppressed with the application of magnetic field, whereas the SGSMO-2 compound exhibits a metal–insulator transition in its pristine state. The high-temperature semiconducting state of both the systems can be well-explained with the polaronic transport mechanisms via small-polaron hopping and variable-range-hopping models. The low-temperature metallic states for both the systems are explored by considering the various contributions arise from the grain boundary effect, electron–electron, electron–phonon, electron-magnon etc scattering processes. The spin-polarized tunneling transport mechanism at the grain boundaries plays a crucial role in the enhancement of low-field magnetoresistance in the studied systems.

Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory.

Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO2-B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO2-B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO2 in photocatalysis and photoelectrochemistry, which we discuss.

2D electrons floating on a suspended atomically thin dielectric

The 2D electrons trapped in vacuum near the atomically thin dielectric (ATD, mono- or N-layer film of h-BN or transition metal dichalcogenide) are considered. ATD is suspended above the back gate and forms the capacitor, which is controlled by the biased voltage determining 2D concentration, n2D. It is found that the leakage current through ATD is negligible, and the effect of the polarizability of ATD is weak if N≤5. At temperatures T=0.1–15 K and n2D=5×108–1010cm−2, one deals with the Boltzmann liquid having a macroscopic thickness of ∼100 A. Due to the bending of ATD, the quadratic dispersion law of the flexural vibrations is transformed into the linear one at small wave vectors. The scattering processes of the electrons caused by these phonons or the monolayer islands on ATD are examined and the momentum and energy relaxation rates are analyzed based on the corresponding balance equations. The momentum relaxation times vary over orders of magnitude in the above region (T, n2D) and N. The response may change from the polaron transport, for a perfect single-layer ATD at low T and high n2D, to the high-mobility (≥107cm2/Vs) regime at high T and low n2D. The quasi-elastic energy relaxation due to phonon-induced scattering is considered, and the conditions for the heating of electrons by a weak in-plane electric field are found.

Formation of a Small Electron Polaron in Tantalum Oxynitride: Origin of Low Mobility

Tantalum oxynitride (I²-TaON) is a potential photoanode because of its suitable band gap and band-edge positions for water-splitting. However, low carrier mobility restricts the solar-to-hydrogen conversion efficiency from the theoretical limit. Here, using the DFT + U formalism, we find that the excess electron tends to form a localized small polaron at the Ta-site (Ta+4 species) over delocalized electrons. The polarization potential created by lattice distortion around Ta+5(d0) generates a driving force to construct Ta+4(d1) by electron capture. The donated electron from n-type single donor defects becomes self-trapped and forms a weakly bound state with the defect. The thermally activated polaronic charge transfer via nearest-neighbor hopping is non-adiabatic using the DFT + U method. However, O substitution at bridging the N site increases the Ta-Ta hopping distance and changes the polaron hopping toward an adiabatic regime. The calculated polaron mobility because of the high migration barrier for both in pristine (0.31 eV) and in the presence of the ON defect (0.36 eV) supports the experimentally observed low mobility and high carrier lifetime in a I²-TaON photoanode. This study provides a mechanistic understanding of the factors controlling the formation and transport of electron polarons, which can guide in designing a I²-TaON photoanode with better efficiency. © copy; 2021 American Chemical Society.

Polaron transport in porous graphene nanoribbons

Porous graphene (PG) forms a class of graphene-related materials with nanoporous architectures. Their unique atomic arrangements present interconnected networks with high surface area and high pore volume. Some remarkable PG properties, such as high mechanical strength and good thermal stability, have been widely studied. However, their electrical conductivity, and most importantly, their charge transport mechanism are still not fully understood. Herein, we employed a numerical approach based on a 2D tight-binding model Hamiltonian to first reveal the nature of the charge transport mechanism in PG nanoribbons. Results showed that the charge transport in these materials is mediated by polarons. These carrier species are dynamically stable and present very shallow lattice distortions. The porosity allows for polaron-like charge carriers, and it can preserve the PG semiconducting character even in broader nanoribbons. The polarons move in PG within the optical regime with terminal velocities varying from 0.50 up to 1.15 Å/fs. These velocities are lower than those for polarons in conventional graphene nanoribbons (2.2–5.1 Å/fs).

Mo-doped ZnV2O6/reduced graphene oxide photoanodes for solar hydrogen production

We report the fabrication and characterization of molybdenum (Mo)-doped ZnV2O6/reduced graphene oxide (rGO) composite and its use as photoanode for photoelectrochemical (PEC) hydrogen production. Compared to pure ZnV2O6, Mo ions act as electron donor in the ZnV2O6:Mo lattice increasing charge carrier concentration and subsequently mobility in the bulk by the polaron transport. We measured the hole transfer efficiency for the pure and Mo-doped ZnV2O6 electrodes and revealing a substantial increase from 16 to 25%. The mechanism of enhanced photoactivity of Mo-doped ZnV2O6 was studied by density functional theory calculations. Moreover, electrochemical impedance spectroscopy measurements show that graphene modification improves carrier separation and transfer across the electrode/electrolyte interface. Therefore, the combination of the two strategies triggers a synergistic enhancement in PEC performance in terms of incident photon-to-current efficiency, which is 17% at 370 nm, being 4.5- and 3.6-times greater than those of pristine ZnV2O6 and ZnV2O6:Mo photoanodes, respectively. With photocurrent onset potentials of 0.6 V and photocurrent densities of 2.07 mA/cm2 at 1.23 V vs. RHE, ZnV2O6:Mo/rGO photoanodes are of interest for the design of high performance PEC visible-light-induced water-splitting devices.

Atypical transport for GdTe1.8 upon substitution with Se: Strong electron-phonon coupling in polaronic conduction

The chemistry of new materials and the effects of chemical substitutions on physical properties continues to be of intense interest as it directly affects many technological fields. The synthesis and temperature dependent transport properties of GdTe1.62Se0.18 were investigated revealing polaronic transport upon simple alloying with selenium. Temperature-dependent resistivity and thermopower measurements indicate strong electron-phonon coupling, as compared to the few other chalcogenides that display such transport. In addition, the low thermal conductivity is intrinsic for this material, and a result of the stacking arrangement in the crystal structure as well as of a statistical disordered Te22– anion.

Chiral-induced spin selectivity: A polaron transport model

Weak hyperfine interactions and spin-orbit coupling (SOC) in organic materials result in long spin lifetimes, which is very promising for spintronics. On the other hand, they also make it challenging to achieve spin polarization, which is of crucial importance for spintronics devices. To overcome this obstacle, we have proposed a physical model for spin-polarized electron transport through a chiral molecule based on the chiral-induced spin selectivity. Because the transport in the chiral molecule is not an isolated one, but rather an electron coupled to its surrounding lattice distortions, namely, a spatial localized polaron, an indispensable polaron effect is incorporated in our model. We show that the polaron transport through the chiral molecule exhibits a spin-momentum-locked feature. Interestingly, no matter what their initial spin state is, all of the polarons could transmit through the molecule with their spins being aligned to the same orientation due to the effective ``inverse Faraday effect.'' The coexistence of the electron-lattice coupling and SOC results in the spin and lattice being coupled, which leads to a strongly enhanced spin coherence and then a very high spin polarization of $70%$. In addition, the effects of the helix pitch, polaron size, and drift velocity on spin polarization are also discussed. Our results open the possibility of using chiral molecules in spintronics applications and offer a paradigm for information processing and transmission.

Effect of the biphase TiO2 nanoparticles on the dielectric and polaronic transport properties of PVA nanocomposite: Structure analysis and conduction mechanism

Polymeric composite films with tunable electrical and dielectric performance have several uses in electronics based on organic materials. This paper reports the fabrication and the role of TiO2 NPS (nanoparticles) on the dielectric and nanocomposites' conduction of pristine polyvinyl alcohol (PVA). The nanopowder of biphase TiO2 and the nanocomposites of different NPS weight percentages have been fabricated via the sol-gel and casting methods. Structure of the synthesized NPS and nanocomposites were analyzed via PXRD (Powder X-ray diffraction), HR-TEM (High-resolution microscopy of transmission electron), and FTIR (Fourier transform-IR). The average size of the biphase TiO2NPS crystallite is around 52 nm, as calculated from Scherer law. Frequency and temperature dependence of dielectric properties have been studied in a wide range. The dielectric constant of the host matrix has been enhanced by adding TiO2NPS. AC conductivity increases with the further addition of TiO2NPS contents, and its frequency dependence follows the universal power law. The nanocomposite with 6.6 wt% of TiO2NPS shows higher conductivity as compared to other concentrations. The exponent power (s) has values in the range of 0.2–0.9, indicating that the conduction mechanism is correlated with barrier hopping (CBH). The dependence of the DC conductivity on absolute temperature has been studied and obeys Arrhenius relation. Three discrete sections were recognized from the AC conductivity spectra. In light of polaronic hopping conduction models, the electrical conductivity data were analyzed. The analysis shows that the high-temperature conductivity is well explained by the polaronic model, while at an intermediate temperature, the Greaves VRH (variable-range hopping) model is appropriate. Also, the non-adiabatic conduction was established in the nanocomposite system via the application of different suggestions. The values of the decay constant and the density of states confirmed the presence of the localized states. Finally, a small polaron coupling constant (γp) is more significant than four, which indicates a strong electron-phonon interaction.

Polaron transport mechanism in maricite NaFePO4: A combined experimental and simulation study

We report, for the first time, systematic investigations on electronic properties of maricite NaFePO4 with different crystallite sizes by a combined experimental and theoretical approach. Ac impedance spectroscopy has been used to study the polaron transport behaviour in maricite NaFePO4 structure with different crystallite sizes over a wide range of temperatures. With the decrease in crystallite size, we observe a polaronic conductivity enhancement of approximately an order of magnitude at the nanoscale level as compared with its bulk counterpart. The temperature dependent dc conductivity has been analysed within the framework of the Mott model of polaron hopping and various physical parameters relevant for the polaron hopping process were extracted. Additionally, by introducing an approximated Mott model with calculated hole polaron migration barrier from density functional theory, we evaluated the polaronic conductivity as function of crystallite size in fair agreement with experimental data. The enhanced polaronic conductivity with crystallite size reduction is found to be due to the combined effect of increased polaron concentration, reduced hopping length, and lowered migration barrier.

Probing Coulomb Interactions on Charge Transport in Few‐Layer Organic Crystalline Semiconductors by the Gated van der Pauw Method

AbstractA general understanding of the nature of the charge motions at the organic/dielectric interface requires an accurate examination on the influence from the Coulomb interactions among carriers. However, difficulties on this substantial issue are due to complicated interpretation and extraction of the intrinsic electrical parameters for the organic films. Here, the carrier transport in highly ordered, few‐layer organic crystalline semiconductors is studied by a geometry‐independent gated van der Pauw method, confining the charge carrier distribution at the precision of molecular layers and minimizing the effects of extrinsic factors. The characteristics of carrier transport are varied in different film layers and show nonideal features. The Fröhlich polaron model well explains the features and suggests that the Coulomb interactions among carriers should be considered at a high carrier density, which is manifested by the characteristic parameter 〈ξ 〉/2 measuring the extra potential for polaron transport. This understanding of the physical process between Coulomb interactions and polaron transport in organic crystalline semiconductors can enable promising strategies for achieving more ideal transistor performance and new device physics.

Polaron Diffusion in Pentathienoacene Crystals

Molecular crystals have been used as prototypes for studying the energetic and dynamic properties of charge carriers in organic electronics. The growing interest in oligoacenes and fused-ring oligothiophenes in the last two decades is due, in particular, to the success achieved in conceiving pentacene-based organic photovoltaic devices. In the present work, a one-dimensional Holstein-Peierls model is designed to study the temperature-dependent polaron transport in pentathienoacene (PTA) lattices. The tight-binding Hamiltonian employed here takes into account intra and intermolecular electron-lattice interactions. Results reveal that polarons in PTAs can be stable structures even at high temperatures, about 400 K. During the dynamical process, these charge carriers present a typical 1D random walk diffusive motion with a low activation energy of 13 meV and a room temperature diffusivity constant of 1.07 × 10−3 cm2 s−1. Importantly, these critical values for the polaron diffusion and activation energy are related to the choice of model parameters, which are adopted to describe pristine lattices.

Generalized input-output method to quantum transport junctions. I. General formulation

In this work, we put forward a generalized input-output method (GIOM) for studying charge transport in molecular junctions accounting for strong electron-vibration interactions and including electronic and phononic environments. The method radically expands the scope of the input-output theory, which was originally proposed to treat quantum optic problems. Based on the GIOM we derive a Langevin-type equation of motion for molecular operators, which posses a great generality and accuracy, and permits the derivation of a stationary charge current expression involving only two types of transfer rates. Furthermore, we devise the so-called "Polaron Transport in Electronic Resonance" (PoTER) approximation, which allows to feasibly simulate electron dynamics in generic tight-binding models with strong electron-vibration interactions. For short chains, the charge current reduces to known limits and reasonably agrees with exact numerical simulations. For extended junctions the current displays a turnover from phonon-assisted to phonon-suppressed transport. Nevertheless, the onset of ohmic behavior requires extensions beyond the PoTER approximation. As an additional application, we consider a cavity-coupled molecule junction. Here we identify a cavity-induced suppression of charge current in the single-site case, and observe signatures of polariton formation in the current-voltage characteristics in the strong light-matter coupling regime. A critical understanding gained from the GIOM-PoTER scheme is that the single-site vibrationally-coupled model is deceptively simpler, and amenable to approximations than multi-site models. Therefore, benchmarking of methods should not be concluded with the single-site case. The work manifests that the input-output framework, which is normally employed in quantum optics, can serve as a powerful and feasible tool in the realm of electron transport junctions.

Polarons in Halide Perovskites: A Perspective.

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron-phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Low potassium mobility in iron pyrophosphate glasses

Structural and electrical properties of glasses of the molar composition xK2O−(40−x/2)Fe2O3−(60−x/2)P2O5, x = 0–36 mol%, are investigated by Raman, Mössbauer and Impedance spectroscopy. All glasses show pyrophosphate structure with a growing tendency for depolymerization with increasing K2O content. The DC conductivity changes in a non-monotonic fashion, exhibiting a broad flat maximum as K2O gradually substitutes Fe2O3 and P2O5. The observed conductivity trend could be related to the changes in the number density of polarons which depends on Fe2O3 and fraction of ferrous ions, indicating dominant polaronic conduction mechanism in all glasses. Interestingly, a relatively small addition of K2O, up to 12 mol%, has a positive effect on the polaronic transport due to slight modifications of glass network. Potassium ions are weakly mobile in iron pyrophosphate glass network and have a negligible contribution to the electrical transport even when their molar fraction is as high as 36 mol%.

Microstructural and transport properties of Mg doped CuFeO2 thin films: A promising material for high accuracy miniaturized temperature sensors based on the Seebeck effect

Delafossite type Mg doped CuFeO2 thin films have been deposited on fused silica by radio-frequency magnetron sputtering. As-deposited 300 nm thick films have been obtained and post-annealed between 350 and 750 °C under primary vacuum. The delafossite structure appears for the samples annealed above 550 °C. The microstructural analysis showed the presence of cracks and an inhomogeneous distribution of the dopant in the thickness. Only the sample annealed at 700 °C showed CuFeO2 stable phases, lower impurities amount, a high and constant Seebeck coefficient (+416 ±3 μV K−1) and good electrical conductivity (1.08 S cm−1 at 25 °C). High accuracy temperature sensors based on the Seebeck effect not only need high Seebeck coefficient without any drift with the temperature, but also a sufficient electrical conductivity and high phase stability. Thanks to its properties and also its low thermal conductivity (4.8 ±0.6 W m−1K−1 at 25 °C) due to the thin film configuration and the polaronic transport, the Mg doped CuFeO2 thin film annealed at 700 °C was found to be a very good p-type material for high accuracy miniaturized temperature measurement sensors based on the Seebeck effect in the medium temperature range.

Evaluation of Polaron Transport in Solids from First‐principles

AbstractPolarons are formed in polar or ionic solids, either molecular or crystalline, due to local distortions of the lattice induced by charge carriers. Polaron hopping is the primary mechanism of charge transport in these materials, such as functional ceramic compounds, with applications in photovoltaics, thermoelectrics, two‐dimensional electron gas transistors, magnetic sensors, spin valve devices, and memories. Understanding the fundamental physics of polaron hopping is, therefore, of prime technological importance. This article provides a brief physical background of polarons and their hopping mechanism, focusing on first‐principles calculations of polaron properties. Herein, we review recent selected studies applying the density functional theory (DFT), and describe the merits and challenges in applying DFT for such calculations, highlighting the need to address both electronic and vibrational aspects. The vibrational component of the polaron is evaluated based on structural and total energy calculations, whereas the electronic component is derived from both total energy and electron density calculations. To address the most compelling challenge of calculating polaron properties using DFT, which is the issue of electron localization, we propose to employ calculations of selected vibrational properties, such as the sound velocity, shear modulus, and Grüneisen parameter, to represent the polaron hopping energy; all of which originate from the stiffness of inter‐atomic bonds. Such methodology is expected to be more straightforward than the existing ones, however demands standardization.

Defects and polaronic electron transport in Fe2WO6.

We report the synthesis of phase pure Fe2WO6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200-950 °C) and pO2 (1-10-3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe2WO6 are found to be 113(5) kJ mol-1 and 41(5) J mol-1 K-1, respectively. Electrons residing as Fe2+ in the Fe3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of XFe2+≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u0 = 400(50) cm2 kV-1 s-1. We report thermal conductivity for the first time for Fe2WO6. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe2WO6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.

Improved polaronic transport under a strong Mott–Hubbard interaction in Cu-substituted NiO

We report the origin of improved hole conduction without band-gap collapse in Cu-doped NiO.

Weak-field polaron dynamics in organic ferromagnets.

With a nonadiabatic dynamical method the polaron dynamics in organic ferromagnets with spin radicals is investigated under weak electric fields. The results reveal two novel phenomena different from those in normal polymers due to the existence of spin radicals. One is that the velocity of the polaron is asymmetric upon the reversal of the applied electric field, which is explained from the asymmetric polarity of the polaron charge density in different directions of the field, and hence its effect on the lattice distortion. The other is the 'intermittent rebound' of the polaron, where the polaron intermittently moves against the electric field force during a short interval behaving like a negative current. The details of lattice distortion and charge distribution of the polaron during the process have been revealed. We further found that there exist different critical fields for the above two phenomena. With an increase of the electric field, the 'intermittent rebound' of the polaron vanishes first and subsequently the asymmetric polaron velocity. This work demonstrates the unique properties of polaron transport in organic ferromagnets, and will be helpful in the future design of organic ferromagnetic devices.