Polaron Relaxation Research Articles

Photoexcited Polaron Relaxation as a Structurally Sensitive Reporter of Charge Trapping in a Conducting Polymer

AbstractConjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity.

Magnetic field modulation of polaron relaxation coherence effects in III-V semiconductor Gaussian quantum wells

We have theoretically investigated the effects of the cyclotron frequency of magnetic field, the initial depth and the range of the asymmetrical Gaussian confinement potential on the vibrational frequency, ground state energy, excited state energy and coherence time of polaron in III-V semiconductors by using a linear combination operator and unitary transformations methods. Through numerical calculations, we find some interesting phenomena that the polaron decreases the energy level and releases the energy of a phonon to jump. From the point of view of the energy level of the polaron, which is αℏωLO energy lower than that of the electron, and the transition of the polaron to the ground state will have a change in lattice position, and the lattice relaxation phenomenon will appear. The theoretical study of the quantum coherence effect of polaron energy levels will provide guidance for the development of optoelectronic devices and the transmission of quantum information.

Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence.

We find evidence for the formation and relaxation of large exciton polarons in 2D organic-inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3-10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds.

Colossal dielectric behavior in Cex(In0.5+Nb0.5)1-xO2 ceramics

(In + Nb) co-doped Cex (In0.5+Nb0.5)1-xO2 (with x = 0, 0.005, 0.05, and 0.1) ceramics were prepared by conventional solid-state method. Dielectric properties of the ceramics were examined between 3 and 475 K and 102 and 106 Hz. All of the doped samples exhibit colossal permittivity (CP) behavior. Among them, the CP behavior of the sample with x = 0.005 possesses the best frequency stability. Two thermally activated relaxations contribute to CP behavior. The low-temperature relaxation (R1) is a polaron relaxation caused by electron hopping between Ce3+ and Ce4+ ions. The high-temperature relaxation (R2) is a dipole relaxation caused by oxygen-vacancy-dopant defect pair.

Small Polarons in Two-Dimensional Pnictogens: A First-Principles Study.

We report the first-principles study of small polarons in the most stable two-dimensional pnictogen allotropes: blue and black phosphorene and arsenene. While both cations and anions of small hydrogen-passivated clusters show charge localization and local lattice distortions, only the hole polaron in the blue allotrope is stable in the infinite size cluster limit. The adiabatic polaron relaxation energy is found to be 0.1 eV for phosphorene and 0.15 eV for arsenene. The polaron is localized on lone-pair orbitals with half of the extra charge distributed among 13 atoms. In the blue phosphorene, these orbitals form the valence band's top with a relatively flat band dispersion. However, in the black phosphorene, lone-pair orbitals hybridize with bonding orbitals, which explains the difference in hole localization strength between the two topologically equivalent allotropes. The polaron's adiabatic barriers for motion are small compared to the most strongly coupled phonon frequency, implying the polaron barrierless motion.

Effect of defect dipoles on the colossal dielectric behaviors of TiO2-x ceramics

Considerable efforts have been made to explore high-dielectric constant ceramics for their promising applications as high-energy density capacitors and fine metal-oxide-semiconductor field effect transistors. Here, we investigated the colossal dielectric behaviors of TiO2-x ceramics influenced by defect dipoles introduced by oxygen vacancies and Ti3+. As temperature decreases from room temperature, three thermally activated relaxations were observed. A room temperature relaxation (R3) was attributed to Maxwell-Wagner relaxation due to the electrode/sample contact. A low-temperature Maxwell-Wagner relaxation (R1) featuring nearly constant loss behavior was associated with frozen carriers. The middle-temperature relaxation (R2) was verified to be the polaron relaxation composed of two segment contributions: polaron hopping and polaron reorienting. The defect dipoles gradually changed from a disordered state into an ordered state in external electric field when temperature falls and reaches a frozen-in state below 217 K. The ordered phase will turn into disordered again when temperature increases to 217 K due to thermal perturbation corresponding to a phase transition. These results indicate that the colossal dielectric constant properties of TiO2 system are attributed to defect dipole clusters. This work exhibits a way to identify the dipole effect. Moreover, we found that the dielectric properties of TiO2-x ceramics hardly depend on temperature and frequency. This implies that TiO2-x ceramics is a kind of promising colossal dielectric materials which can work at various temperatures.

Colossal permittivity in (Li + Nb) co-doped Fe2O3 ceramics

(Li + Nb) co-doped (Li + Nb)xFe2-xO3 (with x = 0.0005, 0.005, 0.05, and 0.1) ceramics were prepared by sol-gel method. Their structural, dielectric, humidity, and magnetic properties were investigated. Colossal permittivity (~104) was approached or achieved in all doped samples even with a very small doping level of x = 0.0005. The colossal permittivity behavior is composed of two dielectric relaxations with the low-temperature one being a polaron relaxation due to electrons hopping between Fe3+ and Fe4+ ions and the high-temperature one being a Maxwell-Wagner relaxation caused by humidity-sensing properties.

Small polaron hopping-assisted electrical conduction and relaxation in BCT and Mn-doped BCT samples

ABSTRACTPolycrystalline samples of Ba0.96Ca0.04TiO3 (BCT) and Ba0.96Ca0.04Ti0.96Mn0.04O3 (BCMT) were synthesized using the solid-state reaction route. XRD analysis confirmed the presence of the tetragonal phase in both as well as an additional hexagonal phase in the BCMT samples. The wt % of the segregated phase was found to be ≈8.96% using Rietveld refinement. SEM micrographs showed an increase in the density and a decrease in the grain size of the BCMT sample. P-E measurements confirmed higher remnant polarization in the BCT samples. A single depressed semicircle was observed in the Nyquist plots indicating non-Debye-type behavior and overlapping contributions of the grains and grain boundaries. The increase in the value of the conductivity exponent “n” (and n < 1) with temperature suggested that the electrical conduction mechanism was non-overlapping small polaron tunneling. The electron acceptor nature of the Mn ions and lower grain size of the BCMT samples resulted in higher activation energy as compared to the BCT samples.

Colossal dielectric behavior in BaFeO3-δ ceramics

Abstract Non-stoichiometric, single-phased barium iron oxide (BaFeO3-δ) has been prepared by solid-state reaction. The low-temperature (5–310 K) dielectric properties of the sample were systematically investigated in terms of X-ray diffraction, scanning electron microscope, X-ray photon spectroscopy, and humidity response measurements. Colossal dielectric behavior similar to that of CaCu3Ti4O12 was found. This behavior was argued to be caused by polaron relaxation due to electrons hopping between Fe3+ and Fe4+ ions. Around room temperature range, the sample shows good humidity response, which leads to surface-layer effect giving rise to a Maxwell-Wagner relaxation. Our results indicate that the sample is a promising humidity sensing material.

Dielectric relaxations in fine-grained SrTiO3 ceramics with Cu and Nb co-doping

SrTi1−3x(CuxNb2x)O3 (x = 0.05, 0.1, 0.15 and 0.2) ceramics were synthesized using a solid-state reaction method at 1400 °C in air. Their structures, valence states, conduction mechanisms and dielectric properties were investigated in detail. Fine grains and vibration modes of the samples related to the doping effects were observed. A distorted pseudo-cubic structure was confirmed by XRD and Raman spectroscopy. All ceramics exhibited a colossal dielectric constant and dielectric relaxations. The dielectric constant in the low- and high-frequency ranges for x = 0.05 and 0.1 was ascribed to the contribution of the grain boundary and grain, respectively, and the electrode polarization was significant at x = 0.15 and 0.2. The dielectric relaxation peaks obeyed the T−1/4 law at low temperature for all samples, confirming the polaron relaxation process. The electric modulus analysis confirmed that low-temperature dielectric relaxation was related to the grain response, and the electric conduction exhibited the same behavior with the dielectric relaxation. The variable-range-hopping conduction indicated a highly distorted structure and the localization of carriers in SrTi1−3x(CuxNb2x)O3, which was consistent with the XRD and Raman results. The mixed-valence structure of Cu was identified by XPS, and the polaron hopping between the mixed-valence Cu ions was supposed to be responsible for the dielectric relaxation and electric conduction.

Variable-range-hopping conduction and polaron dielectric relaxation in Cu and Nb co-doped BaTiO3

BaTi0.7(Cu0.1Nb0.2)O3 ceramic was prepared using a solid-state method and its structure, valence states, conduction mechanism and dielectric properties were investigated in detail. A fine-grained microstructure and a distorted pseudo-cubic perovskite structure were confirmed by scanning electron microscopy, X-ray diffraction analysis and Raman spectroscopy. X-ray photoelectron spectroscopy analysis suggested that Cu in BaTi0.7(Cu0.1Nb0.2)O3 was polyvalent but the valence states of Ti and Nb were invariable. Mott's variable-range-hopping (VRH) conduction was observed. The two colossal dielectric constant plateaus in low- and high-temperature ranges were ascribed to the electrode and grain boundary responses, respectively. The VRH model described the low-temperature relaxation well, indicating that the dielectric relaxation was a polaron relaxation rather than Maxwell–Wagner type. Both grain and grain boundary resistances were well fitted by the VRH model, suggesting that the VRH mechanism was tenable in both grain and grain boundaries. The electron paramagnetic resonance signal was ascribed to Cu ions, and the linewidth showed a linear relationship with T−1/4, corresponding to the charge transfer between different valence via Cu+-O-Cu2+ and Cu2+-O-Cu3+ paths. The hopping of carriers was also responsible for the conduction and polaron dielectric relaxations.

High permittivity behavior and microstructure in a two-phase barium-silicon titanate

Barium titanate (140 nm) was coated with silicon dioxide using atomic layer deposition after which the powders were sintered at 1050 °C for 5 min at 50 MPa using a direct current sintering furnace. Dielectric measurements showed large permittivity (up to 4 × 105) with X-ray and electron diffraction confirming a cubic titanate and tetragonal silicate-based mixture. Transmission electron microscopy revealed planar faults within the tetragonal phase where such defects could assist in the charge storage along with grain boundaries. Polaron relaxation was also observed in this material.

Polaron relaxation and charge dynamics near the mobility edge of a possible Wigner lattice of polarons

We performed impedance spectroscopy and optical investigations below the reported mobility edge of two models systems (${X}_{n}$/${X}_{1.95\ensuremath{-}\ensuremath{\delta}}{\mathrm{Al}}_{1.95}{\mathrm{Si}}_{2.05}{\mathrm{O}}_{8.00}$; $X=\mathrm{K}$ or Rb, $n$ is the doping density of guest atoms) used to explore the metal-insulator transition in a deformable lattice. The bulk ac conductivity and imaginary dielectric permittivity of insulating samples near the mobility edge in both systems show a power-law behavior and variable-range hopping (VRH) conduction that is indeed related to the proposed polaron relaxation and conduction. We show that polaron relaxation and VRH occur simultaneously above a particular density of an interacting gas of small (bi)polarons, which is commensurate with the electron-lattice coupling strength and the intersite electron transfer energy. Based on our experimental results, we propose a rich phase diagram for the charge dynamics in a deformable lattice from the perspective of a Wigner lattice of small (bi)polarons.

Surface layer and its effect on dielectric properties of SiC ceramics

The dielectric properties of SiC ceramics were investigated in the temperature range from room temperature to 873 K and frequency range from 102 to 106 Hz. Giant dielectric behavior was observed in SiC ceramics in the temperature range above 600 K. This behavior strongly depends on sample thickness. Our results revealed that the surface of the sintered SiC sample was covered by a SiO2 layer. Oxygen vacancies in the surface layer diffuse into the inner part giving rise to two thermally activated dielectric relaxations. The low-temperature relaxation was argued to be a polaron relaxation caused by the hopping motions of oxygen vacancies, whereas the high-temperature one is a Maxwell-Wagner relaxation mainly caused by the surface layer.

Decisive role of mixed‐valence structure in colossal dielectric constant of LaFeO3

AbstractThe role of mixed‐valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO3 ceramics by tuning the ratio of Fe2+/Fe3+ through substituting Al for Fe. The ratio of Fe2+/Fe3+ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al3+. Two clear‐cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe2+/Fe3+ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to , where is the intrinsic dielectric constant. These findings underscore the role of the mixed‐valence structure in CDC behavior and suggest that adjusting the mixed‐valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition‐metal oxides.

Absorption Tails of Donor:C60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation.

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D-A complexes occur at photon energies below the optical gaps of both the donors and the C60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and Manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy-hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.

Dielectric properties of pure and Mn-doped CaCu3Ti4O12 ceramics over a wide temperature range

The effect of manganese doping on the dielectric properties of CaCu3Ti4-xMnxO12 (x = 0, 0.02, 0.04) were investigated over a broad temperature range (93–723 K) in the frequency range from 100 Hz to 10 MHz. Two dielectric relaxations and two dielectric anomalies were observed. The low-temperature relaxation appearing in the temperature range below 200 K is the characteristic relaxation for CaCu3Ti4O12. This relaxation was attributed to the polaron relaxation due to electron hopping between Ti3+ and Ti4+ states. Due to the negative factors of notable decreases in the Ti3+/Ti4+ and Cu3+/Cu2+ ratios and the concentration of oxygen vacancies as revealed by X-ray photoemission spectroscopy, Mn-doping was found to gradually destroy rather than move this relaxation to a higher temperature. The high-temperature relaxation occurring around room temperature was found to be a Maxwell-Wagner relaxation caused by grain boundaries. Our results confirm that the colossal dielectric behavior in the tested samples results from both polaron and Maxwell-Wagner relaxations, but is predominated by the latter relaxation. The low-temperature anomaly behaves as a phase-transition-like behavior. It was argued to be created by oxygen vacancies transition from static disorder to dynamic disorder. The high-temperature anomaly is an artificial effect caused by negative capacitance.

Dielectric relaxations in multiferroic La2Ti2O7 ceramics

La2Ti2O7 ceramic samples were prepared via conventional solid-state reaction route. The dielectric properties of La2Ti2O7 were investigated as functions of temperature (113–1073 K) and frequency (100 Hz–1 MHz). Our results revealed that La2Ti2O7 ceramics exhibit intrinsic dielectric response with a dielectric constant of ∼57 in the temperature range below 250 K. Three thermally activated dielectric relaxations were observed when the temperature higher than 250 K. The low-temperature relaxations R1 with the activation energy of 0.38 eV is found to be a polaron relaxation results from hopping holes. The high-temperature relaxations R2 and R3 are related to the conduction progress associated with the singly and doubly ionized oxygen vacancies, respectively.

Dielectric Relaxations in Rutile TiO2

Dielectric properties of high‐purity (4N degree) rutile TiO2 ceramics were investigated over a wide temperature (100–1073 K) and frequency (20 Hz–10 MHz) ranges. X‐ray photoemission spectroscopy measurement revealed the sample possesses mixed‐valent states of Ti3+/Ti4+. Four thermally activated relaxations were observed. The lowest temperature relaxation (R1) features two Arrhenius segments with activation energy of 30 and 80 meV for the low‐ and high‐temperature segments, respectively. This relaxation was argued to be a polaron relaxation due to electrons hopping between Ti3+ and Ti4+ ions. The second relaxation (R2) appears around room temperature showing activation energy of 0.68 eV is believed to be a Maxwell‐Wagner relaxation. The high‐temperature relaxations R3 and R4 with activation energy of 0.84 and 1.26 eV were ascribed to the conduction process due to the hopping motions of singly and doubly charged oxygen vacancies, respectively.