Charge Transport Diffusion Research Articles

Diffusive charge transport with strongly renormalized carrier mass in hole-doped Mott insulators (Y1−xCdx)2Mo2O7with frustrated pyrochlore lattice

Diffusive charge transport with strongly renormalized carrier mass in hole-doped Mott insulators (Y<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>Cd<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mi>x</mml:mi></mml:msub></mml:math>)<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>Mo<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>7</mml:mn></mml:msub></mml:math>with frustrated pyrochlore lattice

Inorganic nanowires: a perspective about their role in energy conversion and storage applications

There has been tremendous interest and progress with synthesis of inorganic nanowires (NWs). However, much of the progress only resulted in NWs with diameters much greater than their respective quantum confinement scales, i.e. 10–100 nm. Even at this scale, NW-based materials offer enhanced charge transport and smaller diffusion length scales for improved performance with various electrochemical and photoelectrochemical energy conversion and storage applications. In this paper, these improvements are illustrated with specific results on enhanced charge transport with tin oxide NWs in dye sensitized solar cells, higher capacity retention with molybdenum oxide (MoO3) NW arrays and enhanced photoactivity with hematite NW arrays compared with their nanoparticle (NP) or thin film format counterparts. In addition, the NWs or one-dimensional crystalline materials with diameters less than 100 nm provide a useful platform for creating new materials either as substrates for heteroepitaxy or through the phase transformation with reaction. Specific results with single crystal phase transformation of hematite (a-Fe2O3) to pyrite (FeS2) NWs and heteroepitaxy of indium-rich InGaN alloy over GaN NW substrates are presented to illustrate the viability of using NWs for creating new materials. In terms of energy applications, it is essential to have a method for continuous manufacturing of vertical NW arrays over large areas. In this regard, a simple plasma-based technique is discussed that potentially could be scaled up for roll-to-roll processing of NW arrays.

Charged basal stacking fault scattering in nitride semiconductors

A theory of charge transport in semiconductors showing built-in polarization (polar) is developed in the presence of basal stacking faults. The theory is based on quantum tunneling in conjunction with the semiclassical description of diffusive charge transport. It is shown that the presence of basal stacking faults leads to anisotropy in carrier transport. The theory is applied to carrier transport in gallium nitride films consisting of a large number basal stacking faults, and the result is compared with experimental data.

Aspects of the theory of graphene

Following a brief review of the device-friendly features of graphene, recent work on its Green's functions with and without a normal magnetic field are discussed, for an infinite graphene sheet and also for a quantum dot, with analyses of the Landau-quantized energy spectra of the sheet and dot. The random phase approximation dielectric response of graphene is reviewed and discussed in connection with the van der Waals interactions of a graphene sheet with atoms/molecules and with a second graphene sheet in a double layer. Energy-loss spectroscopy for a graphene sheet subject to both parallel and perpendicular particle probes of its dynamic, non-local response properties are also treated. Furthermore, we discuss recent work on the coupling of a graphene plasmon and a surface plasmon, yielding a collective plasma mode that is linear in wavenumber. Finally, we discuss the unusual aspects of graphene conduction and recent work on diffusive charge transport in graphene, in both the DC and AC regimes.

Width Dependence of the Nonadiabatic Spin-Transfer Torque in Narrow Domain Walls

We analytically determine the spatially varying spin-transfer torque within a domain wall. In the case of ballistic spin and diffusive charge transport, the spin-transfer torque as well as the local degree of nonadiabaticity oscillate within a domain wall. In narrow domain walls, the degree of nonadiabaticity ceases to be a constant material parameter but depends on the domain-wall width including a possible sign change, which is crucial for experiments and the technological utilization in spin-transfer-torque-based storage devices.

Host−Guest Directed Assembly of Gold Nanoparticle Arrays

The formation of a three-dimensional assembly of gold nanoparticles driven by host-guest interactions is described. Assembly is achieved via host-guest interactions between cyclodextrin-modified gold nanoparticles which associate with the adamantane unit of a redox active metal complex [Os(CAIPA)(3)](ClO(4))(2), where CAIPA is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]-phenanthroline-1-adamantylamine. The electrochemical properties of thin films formed on glassy carbon electrodes have been probed using cyclic voltammetry. In aqueous LiClO(4), the homogeneous charge transport diffusion coefficient, D(CT), is (8.4 +/- 0.4) x 10(-9) cm(2) s(-1) for both oxidation and reduction of the osmium complexes. Significantly, this charge transport rate is significantly larger than that obtained for a solid deposit of [Os(CAIPA)(3)](ClO(4))(2) alone where D(CT) is 2.3 x 10(-10) cm(2) s(-1). The higher D(CT) value observed for the nanoparticle arrays suggests that the incorporated nanoparticles facilitate electron transfer between the bound osmium centers.

Enhanced Electrochemiluminescence and Charge Transport Through Films of Metallopolymer-Gold Nanoparticle Composites

Water-soluble 4-(dimethylamino) pyridine (DMAP) stabilized gold nanoparticles (DMAP-AuNP) were synthesized by ligand exchange and phase transfer (toluene/water). The DMAP-AuNPs are positively charged with the core diameter of 4 +/- 1 nm. Metallopolymer-gold nanocomposites were prepared by mixing gold nanoparticles and [Ru(bpy)(2)PVP(10)](ClO(4))(2), in water at different mole ratios; bpy is 2,2'-bipyridyl and PVP is poly (4-vinylpyridine). The photoluminescence emission intensity of the metallopolymer decreases with increasing AuNP loading and approximately 57% of the emission intensity is quenched when the Au NP:Ru mole ratio is 14.8 x 10(-2). The rate of homogeneous charge transfer through thin layers of the nanocomposite deposited on glassy carbon electrodes increases with increasing nanoparticle loading. The homogeneous charge transport diffusion coefficient, D(CT), for the composite (AuNP:Ru mole ratio 13.2 x 10(-2)) is (2.8 +/- 0.8) x 10(-11) cm(2) s(-1) and is approximately 3-fold higher than that found for the pure metallopolymer. Significantly, despite the ability of the metal nanoparticles to quench the ruthenium-based emission, the electrochemiluminescence of the nanocomposite with a AuNP:Ru mole ratio of 4.95 x 10(-2) is approximately three times more intense than the parent metallopolymer. This enhancement arises from the increased rate of charge transport that leads to a greater number of excited states per unit time while minimizing the quenching effects. The implications of these findings for the design of electrochemiluminescent sensors are discussed.

Diffusive charge transport in graphene on SiO 2

We review our recent work on the physical mechanisms limiting the mobility of graphene on SiO 2. We have used intentional addition of charged scattering impurities and systematic variation of the dielectric environment to differentiate the effects of charged impurities and short-range scatterers. The results show that charged impurities indeed lead to a conductivity linear in density ( σ ( n ) ∝ n ) in graphene, with a scattering magnitude that agrees quantitatively with theoretical estimates; increased dielectric screening reduces the scattering from charged impurities, but increases the scattering from short-range scatterers. We evaluate the effects of the corrugations (ripples) of graphene on SiO 2 on transport by measuring the height–height correlation function. The results show that the corrugations cannot mimic long-range (charged impurity) scattering effects, and have too small an amplitude-to-wavelength ratio to significantly affect the observed mobility via short-range scattering. Temperature-dependent measurements show that longitudinal acoustic phonons in graphene produce a resistivity that is linear in temperature and independent of carrier density; at higher temperatures, polar optical phonons of the SiO 2 substrate give rise to an activated, carrier density-dependent resistivity. Together the results paint a complete picture of charge carrier transport in graphene on SiO 2 in the diffusive regime.

Fast dynamic waveguides and waveguide arrays in photorefractive Sn_2P_2S_6 induced by visible light

We report on dynamic waveguides and waveguide arrays induced beneath the surface of electro-optic Sn(2)P(2)S(6) crystals by visible light at 514 nm. The waveguide structures are generated by interband photoexcitation and drift or diffusion charge transport mechanism. These structures are probed nondestructively in the transverse direction with a beam at a longer wavelength. We measured the fastest formation of light induced waveguides in the visible up to now. The recording times are below 200 mus for intensities above 0.1 W/cm2. By interfering two light beams, dynamic waveguide arrays are generated with waveguide spacings of 7 microm. If an electric field is applied to the crystal, these arrays can be spatially shifted by 1.5 mum for an applied field of E(0) = 1 kV/cm.

Redox switching and oxygen evolution electrocatalysis in polymeric iron oxyhydroxide films

Outstanding issues regarding the redox switching characteristics and the oxygen evolution reaction (OER) electrocatalytic behaviour of multicycled iron oxyhydroxide films in aqueous alkaline solution have been examined. Charge percolation through the hydrous layer has been quantified, using cyclic voltammetry, in terms of a charge transport diffusion coefficient D(CT) which admits a value of ca. 3 x 10(-10) cm2 s(-1). Steady-state Tafel plot analysis and electrochemical impedance spectroscopy have been used to elucidate the kinetics and mechanism of oxygen evolution. Tafel slope values of ca. 60 mV dec(-1) and ca. 120 mV dec(-1) are found at low and high overpotentials respectively, whereas the reaction order with respect to hydroxide ion activity changes from ca. 3/2 to ca. 1 as the potential is increased. These observations are rationalised in terms of a kinetic scheme involving Temkin adsorption and the rate determining formation of a physisorbed hydrogen peroxide intermediate on the oxide surface. The dual Tafel slope behaviour is ascribed to the potential dependence of the surface coverage of adsorbed intermediates.

Single Ion Diffusive Transport within a Poly(styrene sulfonate) Polymer Brush Matrix Probed by Fluorescence Correlation Spectroscopy

Diffusive transport within complex environments is a critical piece of the chemistry occurring in such diverse membrane systems as proton exchange and bilayer lipid membranes. In the present study, fluorescence correlation spectroscopy was used to evaluate diffusive charge transport within a strong polyelectrolyte polymer brush. The fluorescent cation rhodamine-6G was used as a counterion probe molecule, and the strong polyelectrolyte poly(styrene sulfonate) was the polymer brush. Such strong polyelectrolyte brushes show promise for charge storage applications, and thus it is important to understand and tune their transport efficiencies. The polymer brush demonstrated preferential solvation of the probe counterion as compared to solvation by the aqueous solvent phase. Additionally, diffusion within the polymer brush was strongly inhibited, as evidenced by a decrease in diffusion constant of 4 orders of magnitude. It also proved possible to tune the transport characteristics by controlling the solvent pH, and thus the ionic strength of the solvent. The diffusion characteristics within the charged brush system depend on the brush density as well as the effective interaction potential between the probe ions and the brush. In response to changes in ionic strength of the solution, it was found that these two properties act in opposition to each other within this strong polyelectrolyte polymer brush environment. A stochastic random walk model was developed to simulate interaction of a diffusing charged particle with a periodic potential, to show the response of characteristic diffusion times to electrostatic field strengths. The combined results of the experiments and simulations demonstrate that responsive diffusion characteristics in this brush system are dominated by changes in Coulombic interactions rather than changes in brush density. More generally, these results support the use of FCS to evaluate local charge transport properties within polyelectrolyte brush systems, and demonstrate that the technique shows promise in the development of novel polyelectrolyte films for charge storage/transport materials.

Electrical conductivity and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

Broadband dielectric and terahertz spectroscopy (10(-2)-10(+12) Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce--using the Einstein-Smoluchowski equation--the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel-Fulcher-Tammann and for the latter, an Arrhenius temperature dependence is obtained. There is no indication of a mode arising from the reorientation of stable ion pairs.

Redox induced switching dynamics of a three colour electrochromic metallopolymer film

Thin films of a novel Ru-phenolate based metallopolymer, [Ru(terpy)(box)PVP 20]PF 6, in which one in every twenty of the 4-vinyl pyridine monomer units is labelled with the ruthenium complex have been formed on glassy carbon electrodes, terpy is 2,2′:6′,2″-terpyridine, box is 2-(2-hydroxyphenyl)benzoxazole, and PVP is poly(4-vinylpyridine). Cyclic voltammetry and Raman spectroscopy reveal that the Ru 2+/3+ couple is electrochemically reversible but that the phenolate ligand based oxidation is irreversible. These redox processes are associated with reversible colour changes from wine red (reduced) to red orange (mixed composition) then to light green (oxidized) in the visible region and an irreversible change in the near-IR region, respectively. Scanning electron microscopy reveals that repeated switching in LiClO 4 aqueous solution does not induce any significant structural change within the deposit films. Cyclic voltammetry has been used to determine the electrochromic switching rate under semi-infinite linear diffusion conditions. In aqueous LiClO 4, the homogeneous charge transport diffusion coefficient, D CT, decreases from 3.6 ± 0.3 × 10 −13 to 2.7 ± 0.2 × 10 −13 cm 2 s −1 as the LiClO 4 concentration increases from 0.1 to 1.0 M. This weak dependence of D CT on electrolyte concentration suggests that counterion availability is not rate-determining and that the overall rate of charge transport through the metallopolymer film is limited by the rate of segmental polymer chain motion necessary to bring adjacent centres sufficiently close to allow electron transfer to occur. Also the impact of changing the identity of the charge compensating anion of the redox electrochromic switching rate has been investigated. Finally, the electronic conductivity has been determined using interdigitated array electrodes (IDAs).

Three colour electrochromic metallopolymer based on a ruthenium phenolate complex bound to poly(4-vinyl)pyridine

The photonic and electrochemical properties of a novel Ru–phenolate based metallopolymer are reported. The complex undergoes a ruthenium based reversible oxidation at approximately +0.400 V and irreversible box ligand oxidation at +0.800 V vs. Ag/AgCl. Oxidation of thin films in aqueous electrolyte at +0.500 V reversibly switches the colour from wine red to light green and a red orange colour is observed for mixed redox composition. In contrast, oxidation at potentials more positive than +1.500 V shows no visible colour change but produces a change in the near infra-red region. To determine the electrochromic switching rate and to identify the rate determining step of the, scan rate dependent cyclic voltammetry was performed under semi-infinite linear diffusion conditions in aqueous lithium perchlorate. These data reveal that the homogeneous charge transport diffusion coefficient, D CT, is 3.6 ± 0.2 × 10 −13 cm 2 s −1, i.e., under these conditions it takes approximately 90 s to fully oxidise a 100 nm thick film.

Electric response as a function of applied voltage of Nb-doped Bi 4Ti 3O 12 thin films

We have investigated the electrical properties of sol–gel deposited Nb-doped Bi 4Ti 3O 12 (NBIT) ferroelectric thin films. The obtained values of remanent polarization (2 P r ) and coercive voltage ( V c ) were 7 μC/cm 2 and 2.5 V of NBIT thin film, respectively. From complex dielectric spectra, we observed the dielectric response consisting of two regions for measuring frequency; the low frequency region may be due to diffusion charge transport caused by impurities, while the dielectric relaxation mechanism of high frequency region seems to be the modified Debye type. A model was proposed to account for the observed phenomena, which fits very well to the dielectric dispersion relation: ε ⁎ ⁡ ( ω ) = ε ∞ + ε s − ε ∞ 1 + ( i ω τ ) n + i σ ε 0 ω . The occurrence of an anomaly in n, σ, τ, and ε S − ε ∞ parameters near V c indicates a coupling between the charge carriers and ferroelectricity.

Analysis of carbon black networking in elastomers by dielectric spectroscopy

The dielectric response of various series of cross-linked and non-cross-linked solution-styrene-butadiene-rubber and/or carbon black composites is investigated in the frequency range from $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. Two types of rubber, differing in vinyl content, and two carbon black grades, differing in particle size, are considered. The dielectric spectra are analyzed in the framework of percolation theory, describing the effect of structural disorder of the conducting carbon black network on the diffusive charge transport. Various deviations from the predictions of percolation theory are observed, which are related to a superimposed kinetic aggregation or flocculation process. It is demonstrated that due to the heat treatment during vulcanization, a pronounced flocculation of filler particles takes place in the rubber matrix, especially in the case of a small particle size, leading to a drastic modification of the dielectric spectra. In particular, the percolation threshold decreases significantly and a second high-frequency relaxation transition appears, which is traced back to the tunneling of charge carriers over small gaps between adjacent carbon black particles. The gap size is found to be independent of the carbon black concentration in the range of $2--5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Organic solar cells with an ultra thin organized hole transport layer

Discotic liquid crystals (LC) are promising materials to manufacture devices for organic photovoltaic conversion. These molecules possess a mesophase permitting the fabrication of columnar films having particularly desirable properties of charge transport and exciton diffusion along the axes of the columns. After the deposition of an organic layer on the Indium Tin Oxide (ITO) substrate, a thermal annealing, up to the clarification temperature of the LC, is generally necessary to reach a uniform orientation of the columns. It generally leads to a pronounced droplet formation on the substrate if no surface treatment had been performed on the ITO prior to the film deposition. This paper deals with the study of how this partial dewetting can contribute to good solar cell performances with the formation of an ultra thin organized layer.

Simultaneous enhancement of charge transport and exciton diffusion in poly(p-phenylene vinylene) derivatives

Time-resolved luminescence spectroscopy has been used to investigate exciton diffusion in thin films of poly($p$-phenylene vinylene) (PPV)--based derivatives. Due to chemical modifications the PPV derivatives differ by three orders of magnitude in charge carrier mobility as a result of a reduced energetic disorder. From the photoluminescence decay curves of PPV/fullerene heterostructures, the exciton diffusion coefficient was found to increase by one order of magnitude with decreasing disorder. This increase in the diffusion coefficient is compensated by a decrease of the exciton lifetime, leading to an exciton diffusion length of $5--6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the various PPV derivatives.

Correlation of Tomonaga–Luttinger liquid, superconductivity, and spin entanglement in carbon nanotubes

AbstractWe report successful observation of proximity‐induced superconductivity in multi‐walled carbon nanotubes (MWNTs) within a diffusive charge transport regime and also show enhancement of spin entanglement in coherent electron pairs of this MWNT as the 1D anisotropy of MWNTs becomes stronger. By reducing length of this MWNT below mean free path, MWNT can exhibit Tomonaga–Luttinger liquid behaviors. We show a possibility that Cooper pairs, injected from a superconductor electrode, are separated into individual spin by this TLL. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Measurement of Lateral Charge Propagation in [Os(bpy)2(PVP)nCl]Cl Thin Films: A Scanning Electrochemical Microscopy Approach

The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (DCT) in metallopolymers of the type [Os(bpy)2(PVP)nCl]Cl, bpy = 2,2‘-bipyridyl and PVP = poly(4-vinylpyridine), is described. In this approach, a triple potential step technique is employed in which the ultramicroelectrode (UME) tip of the SECM is used to electrogenerate a solution phase oxidant, Ru(CN)63-, in an initial potential step, via the oxidation of Ru(CN)64-. This moiety diffuses from the tip to the underlying polymer film where electron transfer occurs, causing the local oxidation of the polymer-bound complex of OsII to OsIII. The form of the current−time characteristic in this step provides information on the kinetics of the ET process between the solution species and the polymer-bound moiety, as well as the concentration of redox-active species in the polymer film. This process creates lateral concentration gradients of OsII and OsIII along the film. After the first potential step, a waiting period is introduced in which Ru(CN)63- is converted back to Ru(CN)64- at the UME and OsII can recover in concentration by electron self-exchange between OsIII and OsII moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN)63-. The current−time behavior associated with this step is influenced significantly by the extent of lateral electron hopping in the waiting period. It is shown that SECM is capable of measuring DCT values as low as 10-10 cm2 s-1 with good precision. We report experimental measurements on spin-coated films of [Os(bpy)2(PVP)nCl]Cl, where n = 5 or 10, which indicate that DCT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).