Hole Hopping Research Articles

Mechanistic insights of enhanced spin polaron conduction in CuO through atomic doping

The formation of a “spin polaron” stems from strong spin-charge-lattice interactions in magnetic oxides, which leads to a localization of carriers accompanied by local magnetic polarization and lattice distortion. For example, cupric oxide (CuO), which is a promising photocathode material and shares important similarities with high Tc superconductors, conducts holes through spin polaron hopping with flipped spins at Cu atoms where a spin polaron has formed. The formation of these spin polarons results in an activated hopping conduction process where the carriers must not only overcome strong electron−phonon coupling but also strong magnetic coupling. Collectively, these effects cause low carrier conduction in CuO and hinder its applications. To overcome this fundamental limitation, we demonstrate from first-principles calculations how doping can improve hopping conduction through simultaneous improvement of hole concentration and hopping mobility in magnetic oxides such as CuO. Specifically, using Li doping as an example, we show that Li has a low ionization energy that improves hole concentration, and lowers the hopping barrier through both the electron−phonon and magnetic couplings' reduction that improves hopping mobility. Finally, this improved conduction predicted by theory is validated through the synthesis of Li-doped CuO electrodes which show enhanced photocurrent compared to pristine CuO electrodes. We conclude that doping with nonmagnetic shallow impurities is an effective strategy to improve hopping conductivities in magnetic oxides.

Impurity band conduction in group-IV ferromagnetic semiconductor Ge1-xFex with nanoscale fluctuations in Fe concentration

We study the carrier transport and magnetic properties of group-IV-based ferromagnetic semiconductor Ge1-xFex thin films (Fe concentration x = 2.3%–14%) with and without boron (B) doping, by measuring their transport characteristics: the temperature dependence of resistivity, hole concentration, mobility, and the relation between the anomalous Hall conductivity versus conductivity. At relatively low x (=2.3%), the transport in the undoped Ge1-xFex film is dominated by hole hopping between Fe-rich hopping sites in the Fe impurity levels, whereas that in the B-doped Ge1-xFex film is dominated by the holes in the valence band in the degenerated Fe-poor regions. As x increases (x = 2.3%–14%), the transport in the both undoped and B-doped Ge1-xFex films is dominated by hole hopping between the Fe-rich hopping sites of the impurity band. The magnetic properties of the Ge1-xFex films are studied by various methods including magnetic circular dichroism, magnetization, and anomalous Hall resistance and are not influenced by B-doping. We show band profile models of both undoped and B-doped Ge1-xFex films, which can explain the transport and the magnetic properties of the Ge1-xFex films.

Theory of electronic magnetoelectric coupling in d5 Mott insulators

Motivated by recent terahertz (THz) spectroscopy measurements in $\alpha$-RuCl$_3$, we develop a theory for magnetoelectric (ME) effects in Mott insulators of $d^5$ transition metal ions in an octahedral crystal field. For $4d$ and $5d$ compounds, the relatively wide-spread orbitals favor charge fluctuations of localized electrons to neighboring ions and a significant ME effect from electronic mechanisms is expected. From a three-orbital Hubbard model with strong spin-orbit coupling, we derive the mechanisms for the electric polarization originating from virtual hopping of the localized holes carrying the spins. We consider the electric polarization generated by pairs of spin operators on nearest neighbor bonds with either an edge-sharing geometry (i.e., two ligands are shared) or a corner-sharing geometry (i.e., one ligand is shared). The allowed couplings are first derived using a symmetry approach. Then, we explicitly calculate the coupling constants and evaluate the effective polarization operator in the ground state manifold using perturbation theory and exact diagonalization. The results are relevant when considering the THz optical conductivity of magnetic systems such as some perovskite iridates or Kitaev materials. In particular, they help explain the recent THz optical measurements of $\alpha$-RuCl$_{3}$ for which the electric-dipole-induced contribution has been shown to be strong.

LC-MS/MS Proteoform Profiling Exposes Cytochrome c Peroxidase Self-Oxidation in Mitochondria and Functionally Important Hole Hopping from Its Heme.

LC-MS/MS profiling reveals that the proteoforms of cytochrome c peroxidase (Ccp1) isolated from respiring yeast mitochondria are oxidized at numerous Met, Trp, and Tyr residues. In vitro oxidation of recombinant Ccp1 by H2O2 in the absence of its reducing substrate, ferrocytochrome c, gives rise to similar proteoforms, indicating uncoupling of Ccp1 oxidation and reduction in mitochondria. The oxidative modifications found in the Ccp1 proteoforms are consistent with radical transfer (hole hopping) from the heme along several chains of redox-active residues (Trp, Met, Tyr). These modifications delineate likely hole-hopping pathways to novel substrate-binding sites. Moreover, a decrease in recombinant Ccp1 oxidation by H2O2 in vitro in the presence of glutathione supports a protective role for hole hopping to this antioxidant. Isolation and characterization of extramitochondrial Ccp1 proteoforms reveals that hole hopping from the heme in these proteoforms results in selective oxidation of the proximal heme ligand (H175) and heme labilization. Previously, we demonstrated that this labilized heme is recruited for catalase maturation (Kathiresan, M.; Martins, D.; English, A. M. Respiration triggers heme transfer from cytochrome c peroxidase to catalase in yeast mitochondria. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 17468-17473; DOI: 10.1073/pnas.1409692111 ). Following heme release, apoCcp1 exits mitochondria, yielding the extramitochondrial proteoforms that we characterize here. The targeting of Ccp1 for selective H175 oxidation may be linked to the phosphorylation status of Y153 close to the heme since pY153 is abundant in certain proteoforms. In sum, when insufficient electrons from ferrocytochrome c are available to Ccp1 in mitochondria, hole hopping from its heme expands its physiological functions. Specifically, we observe an unprecedented hole-hopping sequence for heme labilization and identify hole-hopping pathways from the heme to novel substrates and to glutathione at Ccp1's surface. Furthermore, our results underscore the power of proteoform profiling by LC-MS/MS in exploring the cellular roles of oxidoreductases.

Proton–Electron Conductivity in Thin Films of a Cobalt–Oxygen Evolving Catalyst

The conductivities of thin film cobalt oxygen evolving catalysts (Co-OECs) are examined for four variants differing in phosphate and borate anion compositions and pH. The effects of these parameters on the Co-OEC activity is explored by (a) examination of the intermediate range structural order within films through high energy X-ray scattering and pair distribution function analysis, and (b) direct measurement of film conductivity, which has been measured with an interdigitated array microelectrode. We find that the nature of the anion is a critical determinant of the size of cobaltate clusters comprising the Co-OEC thin films which in turn is a critical determinant of film conductivity. Improved conductivity of the Co-OEC films with the size of the metalate clusters is consistent with a model for electron conduction that arises from the hopping of hole equivalents delocalized over metalate clusters. The hole hopping is fast relative to the rate-determining step of OER, which may become limited by proton ...

Mixed Ionic-Electronic Conduction in NiFe2 O4 -Ce0.8 Gd0.2 O2-δ Nanocomposite Thin Films for Oxygen Separation.

NiFe2 O4 -Ce0.8 Gd0.2 O2-δ (NFO/CGO) nanocomposite thin films were prepared by simultaneously radio-frequency (RF) magnetron sputtering of both NFO and CGO targets. The aim is the growth of a CO2 -stable composite layer that combines the electronic and ionic conduction of the separate NFO and the CGO phases for oxygen separation. The effect of the deposition temperature on the microstructure of the film was studied to obtain high-quality composite thin films. The ratio of both phases was changed by applying different power to each ceramic target. The amount of each deposited phase as well as the different oxidation states of the nanocomposite constituents were analyzed by means of X-ray photoelectron spectroscopy (XPS). The transport properties were studied by conductivity measurements as a function of temperature and pO2 . These analyses enabled (1) selection of the best deposition temperature (400 °C), (2) correlation of the p-type electronic behavior of the NFO phase with the hole hopping between Ni3+ -Ni2+ , and (3) following the conductivity behavior of the grown composite layer (prevailing ionic or electronic character) attained by varying the amount of each phase. The sputtered layer exhibited high ambipolar conduction and surfaceexchange activity. A 150 nm-thick nanograined thin film was deposited on a 20 μm-thick Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-δ asymmetric membrane, resulting in up to 3.8 mL min-1 cm-2 O2 permeation at 1000 °C under CO2 atmosphere.

Living with Oxygen.

Work on the electronic structures of metal–oxo complexes began in Copenhagen over 50 years ago. This work led to the prediction that tetragonal multiply bonded transition metal–oxos would not be stable beyond the iron–ruthenium–osmium oxo wall in the periodic table and that triply bonded metal–oxos could not be protonated, even in the strongest Brønsted acids. In this theory, only double bonded metal–oxos could attract protons, with basicities being a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent iron–oxos are intermediates in biological reactions critical to life on Earth.In this Account, we focus attention on the oxygenations of inert organic substrates by cytochromes P450, as these reactions involve multiply bonded iron–oxos. We emphasize that P450 iron–oxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high-valent iron–oxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examining many thousands of structures in the Protein Data Bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent iron–oxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by soluble protein or small molecule reductants. As the Earth’s oxygenic atmosphere is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin, O2.

Parton Theory of Magnetic Polarons: Mesonic Resonances and Signatures in Dynamics

When a mobile hole is moving in an anti-ferromagnet it distorts the surrounding Neel order and forms a magnetic polaron. Such interplay between hole motion and anti-ferromagnetism is believed to be at the heart of high-Tc superconductivity in cuprates. We study a single hole described by the t-Jz model with Ising interactions between the spins in 2D. This situation can be experimentally realized in quantum gas microscopes. When the hole hopping is much larger than couplings between the spins, we find strong evidence that magnetic polarons can be understood as bound states of two partons, a spinon and a holon carrying spin and charge quantum numbers respectively. We introduce a microscopic parton description which is benchmarked by comparison with results from advanced numerical simulations. Using this parton theory, we predict a series of excited states that are invisible in the spectral function and correspond to rotational excitations of the spinon-holon pair. This is reminiscent of mesonic resonances observed in high-energy physics, which can be understood as rotating quark antiquark pairs. We also apply the strong coupling parton theory to study far-from equilibrium dynamics of magnetic polarons observable in current experiments with ultracold atoms. Our work supports earlier ideas that partons in a confining phase of matter represent a useful paradigm in condensed-matter physics and in the context of high-Tc superconductivity. While direct observations of spinons and holons in real space are impossible in traditional solid-state experiments, quantum gas microscopes provide a new experimental toolbox. We show that, using this platform, direct observations of partons in and out-of equilibrium are possible. Extensions of our approach to the t-J model are also discussed. Our predictions in this case are relevant to current experiments with quantum gas microscopes for ultracold atoms.

Hole Hopping Rates in Organic Semiconductors: A Second-Order Cumulant Approach.

Second-order cumulant expansion of the time dependent reduced density matrix has been employed to evaluate hole hopping rates in pentacene, tetracene, picene, and rubrene homodimers. The cumulant expansion is a full quantum mechanical approach, which enables the use of the whole set of nuclear coordinates in computations and the inclusion of both the effects of the equilibrium position displacements and of normal mode mixing upon hole transfer. The time dependent populations predicted by cumulant approach are in good agreement with those obtained by numerical solution of time dependent Schrödinger equation, even for ultrafast processes, where the Fermi Golden Rule fails.

Effect of annealing temperatures on the electrical conductivity and dielectric properties of Ni1.5Fe1.5O4 spinel ferrite prepared by chemical reaction at different pH values

The electrical conductivity and dielectric properties of Ni1.5Fe1.5O4 ferrite has been controlled by varying the annealing temperature of the chemical routed samples. The frequency activated conductivity obeyed Jonscher’s power law and universal scaling suggested semiconductor nature. An unusual metal like state has been revealed in the measurement temperature scale in between two semiconductor states with different activation energy. The metal like state has been affected by thermal annealing of the material. The analysis of electrical impedance and modulus spectra has confirmed non-Debye dielectric relaxation with contributions from grains and grain boundaries. The dielectric relaxation process is thermally activated in terms of measurement temperature and annealing temperature of the samples. The hole hopping process, due to presence of Ni3+ ions in the present Ni rich ferrite, played a significant role in determining the thermal activated conduction mechanism. This work has successfully applied the technique of a combined variation of annealing temperature and pH value during chemical reaction for tuning electrical parameters in a wide range; for example dc limit of conductivity ~10−4–10−12 S cm−1, and unusually high activation energy ~0.17–1.36 eV.

Activation Energies for Electron Transfer from TiO2 to Oxidized Dyes: A Surface Coverage Dependence Correlated with Lateral Hole Hopping

Three ruthenium(II) sensitizers, [Ru(L)2(dcb)]2+, were anchored to mesoporous TiO2 thin films where the ligand L = 4,4′-(CH3)2-bpy (dmb), 4,4′-(C(CH3)3)2-bpy (dtb), and 4,4′-(CF3)2-bpy (bpyCF3) controls the thermodynamics and electronic coupling for self-exchange intermolecular RuIII/II “hole hopping”. Apparent electron difussion coefficients, Dapp, were reported to increase in the order bpyCF3 ≪ dtb < dmb. Nanosecond transient absorption measurements made over an 80° temperature range were conducted to abstract average charge recombination rate constants, kcr, under conditions of sub-percolation and saturated sensitizer surface coverages. For sensitizers [Ru(dmb)2(dcb)]2+ and [Ru(dtb)2(dcb)]2+, the kcr values at saturation coverages were significantly larger than those at low coverages, by a degree that followed the trend in Dapp. The inability of [Ru(bpyCF3)2(dcb)]2+ to introduce hole transport was afirmed by recombination kinetic data that were insensitive to the sensitizer surface converage. An Arrhen...

Effects of temperature on conduction mechanism, ac electrical and dielectric properties of NdFe0.9Ni0.1O3 by employing impedance spectroscopy

In the present work, the effects of temperature and frequency on the ac electrical and dielectric properties of polycrystalline NdFe0.9Ni0.1O3, prepared by solid-state reaction method, are investigated. From the impedance spectroscopic measurements, three relaxation processes are observed, which are related to grains, grain boundaries and electrode-semiconductor contacts in the measured temperature and frequency range. For NdFe0.9Ni0.1O3 sample, the applicable equivalent circuit model is (R g Q g)(R gb Q gb)(R c Q c) in the measured temperature range. Decrease in resistances and relaxation times of the grains and grain boundaries with temperature confirm the involvement of thermally activated conduction mechanisms. With the increase in temperature, the relaxation frequencies increase for all three processes and the conduction mechanism changes from Mott variable range hole hopping to adiabatic small polaronic hole hopping in this material. The permittivity of grains is from 8 to 10, while the high permittivity observed at higher temperatures is extrinsic (due to Maxwell-Wagner type polarizations), which is primarily due to the formation of different Schottky barriers. The grain boundaries, ceramic surfaces and electrode-semiconductor contacts mainly contribute in the colossal value of permittivity of NdFe0.9Ni0.1O3. The ac conductivity plots show the positive values of the ‘dσ/dT’ which indicate that the charge carriers are localized. The results of conductivity signify that the delocalization of charge carriers augment with increase in the temperature in this system.

Computational study on hole conduction in normal alkanes: Anisotropy and effect of dynamic disorder

Despite its importance, carrier conduction in electrical insulators is poorly understood. This work presents a computational study of hole conduction in single crystalline alkanes (n-C18H38 and n-C36H74). Hole mobilities are computed with the combination of molecular dynamics simulation, quantum chemical calculation, and the kinetic Monte Carlo method. The hole hopping rate is computed by the Fermi golden rule rate kernel without high temperature approximation. A strong correlation between the anisotropy of hole mobility and crystalline morphology is found. Hole mobilities in the direction of the c axis are more than an order of magnitude larger than those in the a − b plane. At room temperature, hole mobility is increased by roughly a factor of 10 due to the thermal motion of molecules. Computed anisotropic hole mobilities are in reasonable agreement with experimental values when the effect of dynamic disorder is taken into account. The results strongly indicate that hole transfer in crystalline alkane occurs in the phonon-assisted transport regime.

Metal-semiconductor transition and Seebeck inversion in CoFe2O4 nanoparticles

A Semiconductor to metal transition has been observed in hydrothermally synthesized CoFe2O4 (CFO) nanoparticles. Temperature dependent Seebeck coefficient was measured in order to confirm the type of majority charge carriers and conductivity vs. temperature measurement was performed to get the activation energy. The CFO nanoparticles shows p-type semiconducting behaviour at lower temperature as confirmed by the sign of Seebeck coefficient and at elevated temperatures a sign inversion in Seebeck voltage was observed. The p-type and n-type semiconducting behaviour have been attributed to hopping of holes and electrons between Co3+–Co2+ and Fe2+–Fe3+ respectively dominating at different temperatures.

On the mechanism of highly efficient p-type conduction of Mg-doped ultra-wide-bandgap AlN nanostructures

Free hole concentrations up to ∼6 × 1017 cm−3 were measured in Mg-doped AlN nanowires at room-temperature, which is several orders of magnitude larger than that of previously reported AlN epilayers. Detailed studies suggest that such unusually high carrier concentrations stem from the efficient hole hopping conduction in the Mg impurity band, driven by the significantly enhanced Mg-dopant incorporation in nearly defect-free AlN nanostructures. Distinct signatures of hole hopping conduction in the Mg impurity band are observed experimentally, including a relatively small activation energy for electrical conductivity and an increase in hole mobility with increasing temperature.

LC-MS/MS suggests that hole hopping in cytochrome c peroxidase protects its heme from oxidative modification by excess H2O2.

We recently reported that cytochrome c peroxidase (Ccp1) functions as a H2O2 sensor protein when H2O2 levels rise in respiring yeast. The availability of its reducing substrate, ferrocytochrome c (CycII), determines whether Ccp1 acts as a H2O2 sensor or peroxidase. For H2O2 to serve as a signal it must modify its receptor so we employed high-performance LC-MS/MS to investigate in detail the oxidation of Ccp1 by 1, 5 and 10 M eq. of H2O2 in the absence of CycII to prevent peroxidase activity. We observe strictly heme-mediated oxidation, implicating sequential cycles of binding and reduction of H2O2 at Ccp1's heme. This results in the incorporation of ∼20 oxygen atoms predominantly at methionine and tryptophan residues. Extensive intramolecular dityrosine crosslinking involving neighboring residues was uncovered by LC-MS/MS sequencing of the crosslinked peptides. The proximal heme ligand, H175, is converted to oxo-histidine, which labilizes the heme but irreversible heme oxidation is avoided by hole hopping to the polypeptide until oxidation of the catalytic distal H52 in Ccp1 treated with 10 M eq. of H2O2 shuts down heterolytic cleavage of H2O2 at the heme. Mapping of the 24 oxidized residues in Ccp1 reveals that hole hopping from the heme is directed to three polypeptide zones rich in redox-active residues. This unprecedented analysis unveils the remarkable capacity of a polypeptide to direct hole hopping away from its active site, consistent with heme labilization being a key outcome of Ccp1-mediated H2O2 signaling. LC-MS/MS identification of the oxidized residues also exposes the bias of electron paramagnetic resonance (EPR) detection toward transient radicals with low O2 reactivity.

Networking hole and electron hopping paths by Y-shaped host molecules: promoting blue phosphorescent organic light emitting diodes

Compared with a V shaped host, a Y shaped host promotes the efficiency of PhOLEDs due to its networking crystal stacking.

Solvent Dependence of Lateral Charge Transfer in a Porphyrin Monolayer

Lateral charge transport in a redox-active monolayer can be utilized for solar energy harvesting. A model porphyrin system was chosen to study the influence of the solvent on lateral hole hopping, which plays a crucial role in the charge-transfer kinetics. We examined the influence of water, acetonitrile, and propylene carbonate as solvents. Hole-hopping lifetimes varied by nearly 3 orders of magnitude among solvents, ranging from 3 ns in water to 2800 ns in propylene carbonate, and increased nonlinearly as a function of added acetonitrile in aqueous solvent mixtures. These results elucidate the important roles of solvation, molecular packing dynamics, and lateral charge-transfer mechanisms that have implications for all dye-sensitized photoelectrochemical device designs.

Variable Doping Induces Mechanism Swapping in Electrogenerated Chemiluminescence of Ru(bpy)32+ Core-Shell Silica Nanoparticles.

The impact of nanotechnology on analytical science is hardly overlooked. In the search for ever-increasing sensitivity in biomedical sensors, nanoparticles have been playing a unique role as, for instance, ultrabright labels, and unravelling the intimate mechanisms which govern their functioning is mandatory for the design of ultrasentitive devices. Herein, we investigated the mechanism of electrogenerated chemiluminescence (ECL) in a family of core-shell silica-PEG nanoparticles (DDSNs), variously doped with a Ru(bpy)32+ triethoxysilane derivative, and displaying homogeneous morphological, hydrodynamic, and photophysical properties. ECL experiments, performed in the presence of 2-(dibutylamino)ethanol (DBAE) as coreactant, showed two parallel mechanisms of ECL generation: one mechanism (I) which involves exclusively the radicals deriving from the coreactant oxidation and a second one (II) involving also the direct anodic oxidation of the Ru(II) moieties. The latter mechanism includes electron (hole) hopping between neighboring redox centers as evidenced in our previous studies and supported by a theoretical model we have recently proposed. Quite unexpectedly, however, we found that the efficiency of the two mechanisms varies in opposite directions within the DDSNs series, with mechanism I or mechanism II prevailing at low and high doping levels, respectively. Since mechanism II has an intrinsically lower efficiency, the ECL emission intensity was also found to grow linearly with doping only at relatively low doping levels while it deviates negatively at higher ones. As the ζ-potential of DDSNs increases with the doping level from negative to slightly positive values, as a likely consequence of the accumulating cationic charge within the silica core, we attributed the observed change in the ECL generation mechanism along the DDSN series to a modulation of the electrostatic and hydrophobic/hydrophilic interactions between the DDSNs and the radical cationic species involved in the ECL generation. The results we report therefore show that the ECL intensity of a nanosized system cannot be merely incremented acting on doping, since other parameters come into play. We think that these results could serve as valuable indications to design more efficient ECL nano- and microsized labels for ultrasensitive bioanalysis.

Enhancement of Carrier Mobility through Deformation Potential in Metal-Containing Insulated Molecular Wires

We used first-principles calculations to investigate the hole mobility of metal-containing poly(phenylene ethynylene) insulated molecular wires. The metal–organic bond effects were considered using ruthenium(II) porphyrin–pyridyl and platinum(II) acetylide as the organometallic moieties. We found that high hole mobility can be achieved even when the metal–organic dπ–pπ interaction is weak. The weak metal–organic interaction reduces the structural deformation that accompanies hole hopping and compensates the reduced conjugation inside the molecular wire. Our results suggest a new principle for the design of functionalized metallopolymers with high carrier mobilities.