Activation Energy For Hopping Research Articles

Polaron hopping barriers and rates in semiconducting polymers.

Conjugated polymers are potential next-generation materials for organic electronic devices. The ability of these materials to transport charges is a key factor limiting their performance. Charge carriers in conjugated polymers are localized by disorder and polaronic effects. Charge transport in these materials is often described by thermally activated hopping, with a rate given by Marcus theory. The polaron hopping activation energy determines the temperature dependence of the Marcus rate. This energy barrier is dictated by the transition state, in which the charge carrier is equally divided between the initial and final locations. The prefactor for the polaron hopping rate is set by the charge tunneling rate between the initial and final locations. We use a tight-binding polaron model, in which charge carriers are stabilized by both nuclear reorganization and polarization of the surrounding dielectric, to compute the activation energy, charge tunneling rate and overall rate constant for intrachain and interchain charge hopping processes in poly(3-hexylthiophene) (P3HT) crystalline lamellae and amorphous melts. Charge transport in these two environments is limited by interchain hopping processes. Both hopping barriers and rates predicted by the model are in good agreement with experiments on a variety of crystalline and amorphous P3HT materials. Qualitatively, the barriers largely depend on how well the transition state is stabilized by polarization effects, and on the hopping integral between the initial and final locations, both of which penalize hopping over longer distances.

Chemical modulation of valence band and photoelectric properties of epitaxial p-type infrared transparent conducting CuScO2 thin films

Epitaxial p-type infrared transparent conducting oxide (TCO) CuScO2 thin films were grown by polymer-assisted-deposition (PAD) method on a-plane sapphire substrates. Valance band of CuScO2 (largely consisting of localized O-2p and Cu-3d orbitals) was chemically modified by annealing the films under O2 presence with the goal to delocalize carrier holes. Films with c-axis orientation were obtained at 1%–3% O2 pressure. The orientation relationship between CuScO2 and the substrate was [3 R](0001)//a-Al2O3(110). Conduction processes followed the small-polaron hopping mechanism with the hopping activation energies in the 0.032–0.51 eV range (depending on the O2 partial pressure). The films demonstrated p-type conductivity. The conductivity of CuScO2 thin film annealed at 1% O2 partial pressure at room temperature was 22.1 S cm−1. Films prepared with O2 pressure equal to 1 and 3% retained their optical transparency (>90 and >75%, respectively) in the visible and mid-IR regions. To the best of our knowledge, these are the best performing p-type transparent conductive thin films produced by chemical methods. To demonstrate the practical usage of these films, we fabricated p-CuScO2/n-In2O3 heterojunction diodes, which showed ∼2.5 V threshold voltage, which agreed reasonably with CuScO2 bandgap value. High optical transparency and electrical conductivity of these films make them very promising for optoelectronic applications, in which wide wavelength ranges, from visible to IR, are required.

Setting Carriers Free: Healing Faulty Interfaces Promotes Delocalization and Transport in Nanocrystal Solids.

Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC band-edge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.

Morphology Evolution during Lithium-Based Vertically Aligned Nanocomposite Growth.

Ceramic-based nanocomposites are a rapidly evolving research area as they are currently being used in a wide range of applications. Epitaxial vertically aligned nanocomposites (VANs) offer promising advantages over conventional planar multilayers as key functionalities are tailored by the strong coupling at their vertical interfaces. However, limited knowledge exists of which material systems are compatible in composite films and which types of structures are optimal for a given functionality. No lithium-based VANs have yet been explored for energy storage, while 3D solid-state batteries offer great promise for enhanced energy and power densities. Although solid-on-solid kinetic Monte Carlo simulation (KMCS) models of VAN growth have previously been developed, phase separation was forced into the systems by limiting hopping directions and/or tuning the activation energies for hopping. Here, we study the influence of the temperature and deposition rate on the morphology evolution of lithium-based VANs, consisting of a promising LiMn2O4 cathode and a Li0.5La0.5TiO3 electrolyte, by applying a KMCS model with activation energies for hopping obtained experimentally and with minimum restrictions for hopping directions. Although the model considers only the kinetic processes away from thermodynamic equilibrium, which would determine the final shape of the pillars within the matrix, the trends in pillar size and distribution within the simulated VANs are in good agreement with experiments. This provides an elegant tool to predict the growth of VAN materials as the experimental activation energies and higher degrees of freedom for hopping result in a more realistic and low computational cost model to obtain accurate simulations of VAN materials.

Spin-pair correlation driven the colossal magnetoresistance effect in multiferroics CdCr2S4

To understand the anomalous conductivity and colossal magnetoresistance effect of multiferroics CdCr2S4 around magnetic transition temperature TC, we propose the spin-pair correlation dependence of magnetic polarons model. In CdCr2S4, system shows the spontaneous magnetic order at TC and the magnetic order promotes the delocalization of magnetic polarons. According to the proposed model of a dual-conduction behavior, the normal and delocalized magnetic polarons coexist below TC due to the gradual delocalization process of magnetic polarons. Compared to the conductivity of normal magnetic polarons, the conductivity from the delocalized magnetic polarons is dominant. It is suggested that the spin-pair correlation modifies the hopping activation energy of delocalized polarons to realize the anomalous conductivity and colossal magnetoresistance effect. In addition, the applied magnetic field, which promotes the magnetic order and delocalization of magnetic polarons, also leads to the increase of conductivity via spin-pair correlation. It is found that the obtained conductivity and colossal magnetoresistance are in agreement with the experimental results.

A Solid with Liquid-like Diffusion: A Unique Superionic Conductor

Understanding fast ionic diffusion in solids is key to engineering solid electrolytes for energy storage devices such as all-solid-state batteries. In this issue of Chem, Hautier and co-workers conduct atomistic modeling and nuclear magnetic resonance experiments to illustrate how the unique structure of LiTi2(PS4)3 enables unprecedented Li-ion diffusivity.

Multifunctional polyindole/nanometal-oxide composites: Optoelectronic and charge transport properties

This study reports the electronic and optoelectronic properties of polyindole (PIN) based nanometal-oxide composites synthesized by a chemical polymerization method. Optical analyses indicated that molybdenum(VI)-oxide (MoO3) and tungsten(VI)-oxide (WO3) nanoparticles increased both the optical band gap and the fluorescence emission intensity of PIN. In addition, the lower conduction band edge values of MoO3 (−0.24 eV) and WO3 (−0.35 eV) indicated electron migration from these metal oxides to the PIN (0.76 eV). The degree of crystallinity of PIN increased from approximately 25% to 40% with the addition of MoO3 and to 65% for WO3. The interaction between nanoparticles and chain segments increased the onset degradation temperature of PIN by about 45 °C. The charge transport mechanism of PIN and its composites was consistent with the correlated barrier hopping (CBH) model. The required activation energy for hopping of the charge carriers in the PIN matrix increased from 0.17 to 0.19 and 0.23 eV with the addition of the MoO3 and WO3 nanoparticles, respectively, pointing to an increase in the hopping distance of the charge carriers. The experimental analyses indicated that the PIN/MoO3 and PIN/WO3 composites could be used as semiconductor materials in electronics and optoelectronics applications.

Dielectric and electrical study of zinc copper ferrite nanoparticles prepared by exploding wire technique

The experimental investigation of electrical and dielectric characteristics of spinel Zn–Cu ferrite (ZCFO) nanoparticles, synthesized adopting an exploding wire technique (EWT)-based approach, is reported in this paper. The direct current (DC) electrical parameters of prepared nanoparticles were studied by two-probe method in the range of 300 K (RT) to 423 K. The activation energy for hopping of charge carriers, drift mobility and charge concentration was determined through DC analysis. The attributes of dielectric behavior like dielectric constant (e′), loss tangent (tan δ), dielectric loss (e″) and alternating current (AC) conductivity (σAC) were measured at various temperatures in the frequency range of 10 Hz–10 MHz. The increase in AC conductivity with frequency, observed in our study, represented the normal behavior of spinel ferrite. The two-layer model based on space charge polarization could satisfactorily elucidate variation in dielectric constant with frequency. The dielectric parameters at different frequencies were also determined in temperature range of 300–673 K. The decrease in DC resistivity as well as the increase in AC conductivity, with rise in temperature, affirms the semiconducting nature of prepared nanoferrite with the band gap energy of 3.16 eV, as calculated through UV–visible analysis. The capacitance of thin grain boundary region (Cgb), grain boundary resistance (Rgb) and relaxation time (τg) of zinc–copper nanoferrite were estimated through Cole–Cole plot analysis.

Isotropic magnetoresistance and enhancement of ferromagnetism through repetitious bending moments in flexible perovskite manganite thin film

For achieving more functionalities of perovskite manganese oxides film, an epitaxial La0.8Sr0.2MnO3 (LSMO) film was deposited on a flexible substrate (mica) by pulsed laser deposition. The film shows a paramagnetic-ferromagnetic (PM-FM) phase transition at 134 K together with metal-insulator transition (MIT) at 149 K. The measurement of hysteresis loop indicates that the easy-magnetization axis lied in film plane. The transport behavior can be fitted well with the small polaron model, giving out the activation energy Ehop = 176.9 meV. Different from the common anisotropic magnetoresistance (AMR) in manganite film, an isotropic magnetoresistance (IMR) effect was observed in LSMO film and the relatively small thickness of LSMO film (∼50 nm) is a major reason for it. After the cyclic flexural bending for 1.0×104 times, we found the magnetic properties of LSMO film were improved. Micromagnetic detection shows that the activation energy for polaron hopping (Ea = 144.7 meV) remarkably decreases by the cyclic flexural bending process. We suggested that the inhomogeneous/incompact part located at the zone of neighbouring domains become uniform and tight after circulating bending, which facilitates the rotation of domains and the polarons hopping so that the magnetic and electronic performance are enhanced.

Electron attachment induced ion transport — Part I: Conductivities and activation energies

The charge carrier transport in an ion conducting glass has been investigated by means of the charge attachment induced transport (CAIT) technique. For the first time we describe the direct measurement of the absolute bulk conductivity and the activation energy for ion hopping in a glass by attachment of low-energy electrons to the front side of this glass, which is in contact with a metal electrode at the backside. Both, the conductivity as well as the activation energy agree well with a cation attachment induced transport experiment establishing the universal character of the CAIT technique. The possibility that blocking electrodes and/or depletion zones evolving at the interface between the glass and the metal electrode interfere with the conductivity measurement is discussed but does not constitute a limitation under the conditions of the experiment.

Switching failure behaviors and doping enhanced performances of Ni/Al2O3/p+Si resistive switching devices

Device reliability is of great significance to resistive switching applications, and reset failure dominates the deterioration of cycling endurance. Although it has been found that the excessive aggregation of movable ions could lead to the reset failure, the quantitative studies on the defect movement have seldom been conducted. Hence, the Ni/Al2O3/p+Si sandwiched structure is fabricated by magnetron sputtering, and the reset failure phenomenon is analyzed. The measurements on the resistive switching behaviors demonstrate that the space-charge-limited current mechanism is responsible for the electroforming process, while the current conduction in subsequent switching cycles obeys the hopping mechanism. Temperature-dependent I-V measurements reveal that the resistance states are closely related with both the hopping distance (R) and hopping energy barrier (W) between adjacent localized states. Short hopping distance of 0.66 ± 0.02 nm and low hopping activation energy of 1.72 ± 0.06 meV will lead to the unrecoverable breakdown of Al2O3 dielectric layer, large leakage current, and deteriorative memory window. 1.9 at. % ZnO doped into Al2O3 dielectric layer can lower the switching voltages and the compliance current of the devices, which will alleviate the aggregation of the localized states during the cycling process. As a result, the R and W values in high resistance state are stabilized at 2.24 ± 0.04 nm and 5.76 ± 0.11 meV during 100 direct current switching cycles, and the memory window is significantly improved. A physical model is proposed to understand the reset failure mechanism of Ni/Al2O3/p+Si devices.

Spectroscopic, dielectric properties and phase transition of Bis (diethylammonium) hexachlorido stannate(IV)

The [C4H10NH2]2SnCl6 compound is characterized by using the X-ray powder analysis, thermogravimetric analysis, differential scanning calorimetry analysis, 13C and 119Sn (CP/MAS NMR) analysis, vibrational spectroscopy and complex impedance spectroscopic data. The results show that this compound exhibits a phase transition at 326 K which was characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, Raman spectroscopy and dielectric measurements. The most important changes are observed for four lines at 3207 cm−1, 3056 cm−1, 3000 cm−1 and 1538 cm−1 (at room temperature) issued from asymmetric and symmetric stretching vibrations of νs(NH2), νs(NH2), νas(NH2) and νδ(NH2) band, respectively. The assignment of the observed IR and Raman lines was performed by comparison with the homologous compounds. Dielectric data were analyzed using complex permittivity ε* and complex electrical modulus M* for the sample at various temperatures. The Z′ and Z” versus frequency plots are well fitted to an equivalent circuit model. The analysis of Nyquist plots are well fitted to an equivalent circuit consisting of series of combination of grains and grain boundary elements. The AC conductivity was studied using the following equation: σAC(ω)=σs1+τ2ω2+σ∞τ2ω21+τ2ω2+Aωs. The conductivity follows the Arrhenius relation. Hopping frequency was determined and activation energy of hopping is almost equal to the activation energy of conduction. The temperature dependence of the power law exponent s and a.c conductivity σac is reasonably interpreted by the quantum mechanical tunneling (QMT) and the barrier hopping (CBH) model. The modulus plot can be characterized by full width at half height or in terms of a non-experiential decay function ϕ(t) = exp(−t/τ)β.

A theoretical study on the stability and ionic conductivity of the Na11M2PS12 (M = Sn, Ge) superionic conductors

The search for next-generation solid-state superionic conductors has attracted significant attention. Among Na superionic conductors, Na11Sn2PS12 has been reported to have a room temperature ionic conductivity of 1.4 mS/cm. In this study, we employ density functional theory to study the stability of Na11Sn2PS12 and further explore the substitution of Sn with Ge. Our results indicate that Na11Ge2PS12 is more stable than Na11Sn2PS12. Furthermore, substituting Sn with Ge increases the band gap, improves the room temperature ionic conductivity by a factor of 2, and lowers the activation energy of Na hopping. Statistical analysis suggests that Na11Ge2PS12 has a faster diffusion along the ab-plane compared to the c-axis. The Na diffusion in Na11Ge2PS12 appears to occur with two different mechanisms depending on temperature: 1) an ion hopping process at lower temperatures (<800 K); 2) a fluid-like distribution of Na ions at higher temperatures (>1000 K). The computations suggest that Na11Ge2PS12 is a promising candidate as a solid Na electrolyte due to its high room temperature ionic conductivity and phase stability. In light of these simulation results, we expect to stimulate further experimental studies on Na11Ge2PS12.

Complex Cation and Spin Orders in the High-Pressure Ferrite CoFe3O5.

A ferrite in the Sr2Tl2O5-type MFe3O5 family with M = Co has been synthesized at 12 GPa pressure. Neutron diffraction shows the sample to be Co deficient with composition Co0.6Fe3.4O5. The Co/Fe cation distribution is found to be profoundly different from those of MFe3O5 analogs and lies between normal and inverse limits, as Co2+ substitutes across trigonal prismatic and one of the two octahedral sites. CoFe3O5 shows complex magnetic behavior with weak ferromagnetism below TC1 ≈ 300 K and a second transition to ferrimagnetic order at TC2 ≈100 K. Spin scattering of carriers leads a substantial increase in the hopping activation energy below TC1, and a small negative magnetoresistance is observed at low temperatures.

Molecular dynamics simulation of polymer-coupled ion transport in the crystalline polymer electrolyte poly(ethylene oxide)3:NaI

While solid polymer electrolytes (SPEs) possess a combination of the requisite properties to merit their use in all-solid-state rechargeable batteries, their low ionic conductivity (σ) at room temperature limits their widespread use in advanced commercial applications. To overcome this limitation, it is necessary to understand the crucial molecular factors that govern the energetics and transport of ions in SPEs. In this Article, all-atom molecular dynamics (MD) and adaptive biasing force (ABF) simulations are used to characterize the mechanism and energetics of polymer-coupled ion transport and to probe the nature of interactions of polymers and ions in a crystalline SPE (PEO3:NaI). In particular, the free energy profiles associated with the cation transport along the polymer helix, the activation energy for hopping of a cation between adjacent energetically favourable coordination sites, and crucial factors determining the features of the free energy profiles are examined. The calculated free energy profiles exhibit well-defined minima (correspond to favourable cation coordination sites) separated by relatively high energy barriers (12–14 kcal/mol) indicating that ion hopping is the likely mechanism of cation transport in PEO3:NaI crystals. The influence of the conformational flexibility of the polymer on the energetics of cation transport is also examined. The dissection of interaction of the cations provides insights into the interplay of ion-ion, ion-polymer, and polymer-polymer interactions in determining the activation barrier for cation transport in the crystalline PEO3:NaI.

Lithium Phosphidogermanates α- and β-Li8GeP4—A Novel Compound Class with Mixed Li+ Ionic and Electronic Conductivity

To utilize all-solid-state batteries as high power and energy density energy storage devices it is necessary to improve the current electrolyte materials and the cell architecture. In this work, we present α- and β-Li8GeP4 as potential compounds for the use in cathode composites of solid-state batteries because of their ability to conduct both Li+ ion and electrons. Each polymorph was synthesized via mechanical alloying of the elements and subsequent annealing. Structural analysis of α- and β-Li8GeP4 via X-ray diffraction reveals isolated [GeP4]8– tetrahedra. α-Li8GeP4 (Pa3) and β-Li8GeP4 (P43n) are isotypic with Li8SiP4 and Li8SnP4, respectively. The lithium ion mobility indicated by partially filled octahedral voids was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy and reveals low activation energies for lithium hopping in the range from 34 and 42 kJ mol–1 as well as high ionic conductivities of up to 8 × 10–5 S cm–1 and electronic conducti...

Conduction Mechanism in Electrically Conducting Polymers

The conduction mechanism in conducting polymers is reviewed and experimental results of temperature dependence of electrical conductivity of PF6 doped polypyrrole in temperature range of 77 to 300 K are discussed. The room-temperature conductivitywas experimentally determined to be 73 ± 3.4 S/m and temperature dependence follows the Mott’s variable range hopping model. The average hopping distance at 298 K was (6.75 ± 0.97) ×10-8 cm. The coefficient of decay of the localized states, the density states at the Fermi level, and the hopping activation energy were calculated to be (3.5±0.51) ×107 cm-1, (1.92 ± 0.83) ×1022 cm-3 eV-1, and 0.040 ± 0.001 eV respectively. KEYWORDS: Electrically Conducting Polymers; Doped Polypyrrole; Temperature Dependence of Conductivity; Hopping Activation Energy; Density of State at Fermi level

Hole doping effect on structure, transport and magnetic properties of Dy1-xBaxMnO3 (0 ≤x ≤1)

Structural, electrical and magnetic properties of Dy1-xBaxMnO3 compounds synthesized by the standard solid state reaction route are reported in this paper. For x = 0.0 to 0.5, the compounds have orthorhombic structure and for x = 0.8 and 1.0, the structure changed to hexagonal structure corresponding to 2H. Raman spectra of the materials compliment structural phase transition. Phonon lifetimes estimated from the Raman line width were found to be in the order of pico seconds. X-ray photoelectron spectra of Dy1-xBaxMnO3, revealed the presence of Dy, Ba, Mn and O elements with respective binding energies. Mn ions were found to be in the mixed valent states of Mn (III) and Mn (IV) and their ratio decreased with increasing Ba concentration. Temperature behavior of electrical resistivity of all the compounds showed semiconducting behavior that persisted in the entire temperature range with small polaron hopping conduction mechanism. As Ba content increased, activation energies for hopping of polarons decreased in the orthorhombic phase and increased in the hexagonal phase. Magnetic studies as a function of temperature indicated that ordering temperatures of rare earth (Dy) moments are suppressed with Ba substitution. Random occupation of magnetic Dy3+, Mn3+ and Mn4+ ions in Dy1-xBaxMnO3 system resulted in complex magnetic structures. The reduction in the experimental effective magnetic moments determined from the Curie-Weiss fit parameters followed the magnetic dilution. Isothermal magnetization curves confirmed the presence of multiple interactions between the magnetic ions.

Electronic and Electrochemical characterization of DNA - templated CdS nanowires

Cadmium sulfide (CdS) nanowires have been prepared by precipitation of Cd(II) ions with hydrosulfide (HS-) in the presence of DNA templates. CdS nanowires are of interest for chemirestive sensing elements in which a mat of nanowires is drop-cast across metal microelectrodes. We have demonstrated by FTIR, UV-Vis and pXRD measurements that CdS forms on both Lambda (λ) DNA and Calf-thymus (CT) with similar optical and chemical properties, but the structural properties determined by microscopy (AFM, fluorescence) and the electrical behaviour are very different. In particular the greater length of the λ-DNA template results in higher conductivity of the drop-cast nanowire mats. The current-voltage (I-V) characteristics and impedance spectra are consistent with a model of charge transport in the films which involves thermally-activated hopping (activation energy about 1.1 eV) and coupled electron-ion motion similar to that in redox polymers

Electronic transport and Raman spectroscopic properties of Co doped (110) PrBa2Cu3O7 thin film

The authors deposited Co doped (110) PrBa2 Cu3O7 (PBCCO) epitaxial thin film using pulsed laser deposition technique and investigated its electronic transport and Raman spectroscopic properties. The authors observed an increase in electrical resistivity by a factor of 50 at 77 K and the existence of three-dimensional variable range hopping mechanism among the localized electronic states on the (110) PBCCO thin film. The activation energies for hopping of the (110) PBCCO thin film was calculated to be 168 meV at 300 K and 75 meV at 77 K suggesting that the hopping was mainly thermally assisted. The Raman spectroscopy measurement on the (110) PBCCO thin film revealed a Raman mode at ∼620 cm−1 as an evidence of Cu ion replacement with Co ions at the Cu-O chain site of the PBCO. The replacement of the Cu ions with the Co ions induced disorder and localization of the charge carriers at the chain sites, thereby increasing the electrical resistivity of the (110) PBCCO thin film. In view of higher electrical resistivity and structural and chemical compatibility with YBa2Cu3O7 (YBCO), (110) PBCCO thin film could serve as an excellent insulator on YBCO based trilayer Josephson junction.