Activation Energy For Electron Transport Research Articles

Effect of Backbone Sequence of a Naphthalene Diimide-Based Copolymer on Performance in n-Type Organic Thin-Film Transistors.

We report two newly synthesized naphthalene diimide (NDI)-based conjugated polymers, poly[(E)-2,7-bis(2-decyltetradecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone-vinylene-thiophene-vinylene] (PNDI-VTV) and poly[(E)-2,7-bis(2-decyltetradecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone-vinylene-selenophene-vinylene] (PNDI-VSV) with different donor units as electron-transporting organic semiconductors for organic field-effect transistors (OFETs). Furthermore, we study the effect of vinylene position on electron transport in the NDI polymers by using two similar polymers but with thiophene-vinylene-thiophene (PNDI-TVT) instead of vinylene-thiophene-vinylene or selenophene-vinylene-selenophene (PNDI-SVS) instead of vinylene-selenophene-vinylene. By incorporating vinylene between thiophene (or selenophene) units, the resulting NDI-based polymers PNDI-VTV and PNDI-VSV show larger backbone planarity than PNDI-TVT and PNDI-SVS. The polymers with a shorter acceptor monomer unit (PNDI-VTV and PNDI-VSV) show a strong face-on orientation, whereas those with a longer monomer unit (PNDI-TVT and SVS) exhibit a mixed face-on and edge-on orientation by two-dimensional grazing incidence X-ray diffraction. Optimized PNDI-VTV and PNDI-VSV OFETs exhibit electron mobilities of 0.043 and 0.7 cm2/(V·s), which is quite lower than those of PNDI-TVT and PNDI-SVS. In addition, the activation energies for electron transport of PNDI-VTV and PNDI-VSV were larger than those of PNDI-TVT and PNDI-SVS. Overall, this research provides the insight that the molecular alignment on the substrate can be controlled by the sequence of rigid acceptor monomer molecules for improving the electron transport of NDI polymers.

Temperature Dependent Electron Transport Properties of Gold Nanoparticles and Composites: Scanning Tunneling Spectroscopy Investigations.

Scanning tunneling spectroscopy (STS) is used for investigating variations in electronic properties of gold nanoparticles (AuNPs) and its composite with urethane-methacrylate comb polymer (UMCP) as function of temperature. Films are prepared by drop casting AuNPs and UMCP in desired manner on silicon substrates. Samples are further analyzed for morphology under scanning electron microscopy (SEM) and atomic force microscopy (AFM). STS measurements performed in temperature range of 33 °C to 142 °C show systematic variation in current versus voltage (I-V) curves, exhibiting semiconducting to metallic transition/Schottky behavior for different samples, depending upon preparation method and as function of temperature. During current versus time (I-t) measurement for AuNPs, random telegraphic noise is observed at room temperature. Random switching of tunneling current between two discrete levels is observed for this sample. Power spectra derived from I-t show 1/f2 dependence. Statistical analysis of fluctuations shows exponential behavior with time width τ ≈ 7 ms. Local density of states (LDOS) plots derived from I-V curves of each sample show systematic shift in valance/conduction band edge towards/away from Fermi level, with respect to increase in temperature. Schottky emission is best fitted electron emission mechanism for all samples over certain range of bias voltage. Schottky plots are used to calculate barrier heights and temperature dependent measurements helped in measuring activation energies for electron transport in all samples.

Morphology-dependent space charge polarization and dielectric relaxation of CdO nanomorphotypes

A versatile approach signifying the morphology-dependent dielectric polarization and relaxation mechanisms of cadmium oxide (CdO) nanosphere, nanoflakes and nanoparallelepiped morphotypes as a function of frequency and temperature is presented. Variation of dielectric property is observed due to the changes of space charge/interfacial polarization resulting from the variations of surface to volume ratio of nanomorphology. Accordingly, colossal dielectric constant value has been observed in CdO nanosphere having larger surface to volume ratio. The order of dielectric constant (dc) values observed for the present nanomorphologies is: dc of sphere [Formula: see text] dc of flakes [Formula: see text] dc of parallelepiped resembles the order of surface to volume ratios of the present morphologies respectively. The experimental data of complex impedance values are numerically fitted using theoretical models which provide the information of role of grain resistance on dielectric polarization and Cole–Cole type mechanism of dielectric relaxation process. The activation energies for electron transport are found to be 0.087[Formula: see text]eV for spheres, 0.074[Formula: see text]eV for flakes and 0.067 for parallelepiped nanomorphotypes of CdO. The dielectric and impedance spectroscopic analysis of the present material opens up wide scope for morphology-dependent tuning of nanomaterials for electrical applications.

Suppressing Deep Traps in Self-Organized TiO2 Nanotubes by Nb Doping and Optimized Water Content

This manuscript reports on the photoelectrochemical properties of anodized 0.1 and 0.15 at% Nb-doped NTs prepared using electrolytes containing different amount of water (2 to 12 wt%). It is shown that Nb-doped NTs perform better than undoped NTs, achieving a photocurrent density up to 1.35 mA/cm2 at 1 V vs RHE when the water content is sufficiently high (12 wt%). The IPCE also confirms the superiority of Nb-doped NTs, again the larger values (55% IPCE at 300 nm) are obtained when using 12 wt% H2O in the electrolyte. EIS analysis and capacitance measurements highlight the peculiarity of the Nb-doped TiO2 NTs which behave differently than undoped NTs. The electron transport resistance in Nb-doped NTs is similar (103 Ω cm2) to the one of undoped NTs. However, Nb-doped NTs exhibit larger electron mobility and require lower activation energy for electron transport (i.e. a less negative bias) than undoped NTs. Furthermore, Nb-doping, combined with an optimal water content of 12 wt%, is found to be an effective strategy to fully suppress the formation of deep intra band-gap energy states. The presence of Nb in the TiO2 lattice dictates the fate and the energy of defects introduced during anodization and annealing. These defects are shallower than those observed in undoped NTs.

Conductance and activation energy for electron transport in series and parallel intramolecular circuits.

We investigate electron transport through series and parallel intramolecular circuits in the framework of the multi-level Redfield theory. Based on the assumption of weak monomer-bath couplings, the simulations depict the length and temperature dependence in six types of intramolecular circuits. In the tunneling regime, we find that the intramolecular circuit rule is only valid in the weak monomer coupling limit. In the thermally activated hopping regime, for circuits based on two different molecular units Ma and Mb with distinct activation energies Eact,a > Eact,b, the activation energies of Ma and Mb in series are nearly the same as Eact,a while those in parallel are nearly the same as Eact,b. This study gives a comprehensive description of electron transport through intramolecular circuits from tunneling to thermally activated hopping. We hope that this work can motivate additional studies to design intramolecular circuits based on different types of building blocks, and to explore the corresponding circuit laws and the length and temperature dependence of conductance.

Characterization and temperature-dependent conductivity of polyaniline nanocomposites encapsulating gold nanoparticles on the surface of carboxymethyl cellulose

Polyaniline nanocomposites encapsulating gold nanoparticles on carboxymethyl cellulose surface were prepared via the polymerization of aniline hydrochloride with different carboxymethyl cellulose (CMC) concentrations (wt.%) using HAuCl4 as oxidant. The synthesized composites were characterized by Fourier transform infrared (FTIR) spectroscopy. Surface morphology was studied by electron diffraction scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The embedded crystallinity of the composites was investigated by X-ray diffraction (XRD) analyses. The electrical property of the composites was examined by temperature-dependent DC conductivity in the range of 300–500 K. The composites exhibited higher electrical conductivities with increased CMC concentration under equivalent conditions. Activation energy for electron transport was also calculated based on the conductivity data.

Comparative study of bulk and interface transport in disordered fullerene films

The characterization of the charge carrier transport in disordered fullerene films, grown by physical vapor deposition, is important for organic electronics in order to improve carrier mobility and understand transport processes. In this contribution, the electron mobility in the bulk of the fullerene film and at the interface with dielectrics are compared. The bulk mobility is measured in diode structures using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique, which allows a simultaneous study of the electric field, concentration and temperature dependence. The interface mobility is determined using organic field effect transistor (OFET) geometry. The electron mobility values are lower and the dependence on carrier density, field and temperature is stronger in diodes compared to OFETs. In both structures different temperature dependence of the mobility on the carrier concentration and on the electric field is obtained. The dependence shows Meyer–Neldel rule (MN-rule) behavior with similar MN temperatures and MN energies. Activation energy for electron transport plotted as a function of the square root of electric field is linear (Gill's law behavior), in accordance with Poole–Frenkel-type charge carrier transport.

Electric field dependent activation energy of electron transport in fullerene diodes and field effect transistors: Gill’s law

The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene (C60) diodes and at the interface with dielectric of organic field effect transistors (OFETs). Electron mobility values follow a Poole–Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill’s law. The same Gill’s energy EGill=34 meV is measured in diodes and OFETs, which corresponds well to Meyer–Neldel energy (EMN=35 meV). It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.

Meyer–Neldel rule for charge carrier transport in fullerene devices: A comparative study

Charge transport is comparatively studied in the bulk and at the interface of disordered fullerene films fabricated using physical vapour deposition. Charge carrier concentration and temperature dependent electron mobilities are comparatively studied using charge extraction by linearly increasing voltage (CELIV) technique and organic field-effect transistors (OFET) measurements. Electron mobility is at least two orders of magnitude higher than hole mobility in the fullerene films. Lower mobility values and stronger concentration dependence in diodes is observed. Carrier concentration dependent activation energy is experimentally measured in both types of devices. Larger activation energy for electron transport is required at lower carrier concentrations. Meyer–Neldel rule (MNR) for electron mobility is observed in both the bulk of the fullerene films and in the transistor channel at the interface. Meyer–Neldel energy ( E MN = 35 meV), which is interpreted as disorder parameter, is the same in both device geometries, which suggest that the level of disorder and energetical landscape for charge transport is similar in the bulk of fullerene films and at the interface with the insulator. Disorder formalism is used to qualitatively explain that either carrier concentration or the nature of non-equilibrium charge carrier transport in CELIV compared to steady-state OFET measurements is responsible for observed transport properties.

Metal-insulator transition characteristics of VO2 thin films grown on Ge(100) single crystals

Phase transitions exhibited by correlated oxides could be of potential relevance to the emerging field of oxide electronics. We report on the synthesis of high-quality VO2 thin films grown on single crystal Ge(100) substrates by physical vapor deposition and their metal-insulator transition (MIT) properties. Thermally triggered MIT is demonstrated with nearly three orders of magnitude resistance change across the MIT with transition temperatures of 67 °C (heating) and 61 °C (cooling). Voltage-triggered hysteretic MIT is observed at room temperature at threshold voltage of ∼2.1 V for ∼100 nm thickness VO2 films. Activation energies for electron transport in the insulating and conducting states are obtained from variable temperature resistance measurements. We further compare the properties of VO2 thin films grown under identical conditions on Si(100) single crystals. The VO2 thin films grown on Ge substrate show higher degree of crystallinity, slightly reduced compressive strain, larger resistance change across MIT compared to those grown on Si. Depth-dependent x-ray photoelectron spectroscopy measurements were performed to provide information on compositional variation trends in the two cases. These results suggest Ge could be a suitable substrate for further explorations of switching phenomena and devices for thin film functional oxides.

ELECTRONIC PROPERTIES OF THIN FILMS SUBLIMED FROMLa@C82ANDLi@C60

La @ C82and Li @ C60thin films obtained by sublimation in vacuum are studied using four-probe current–voltage measurements and atomic force microscopy. In situ electrical measurements show semiconducting behavior of both films with room-temperature resistivity of 21 ± 8 and 1230 ± 50 Ω · cm for the La @ C82and Li @ C60, respectively. A variable range hopping mechanism of conductance is suggested from the temperature dependences of resistance. The activation energies for electron transport are calculated for both metallofullerenes. Irreversible changes to the Li @ C60film structure increasing the film resistivity to values typical for C60are found at elevated temperatures. The effect of exposure to ambient atmosphere on the conductance of the films is discussed.

Liquid-crystalline dendrimer Cu(II) complexes and Cu(0) nanoclusters based on the Cu(II) complexes: An electron paramagnetic resonance investigation

New nanostructured materials, namely, the liquid-crystalline copper(II) complexes that contain poly(propylene imine) dendrimer ligands of the first (ligand 1) and second (ligand 2) generations and which have a columnar mesophase and different copper contents (x = Cu/L), are investigated by EPR spectroscopy. The influence of water molecules and nitrate counterions on the magnetic properties of complex 2 (x = 7.3) is studied. It is demonstrated that water molecules can extract some of the copper ions from dendrimer complexes and form hexaaqua copper complexes with free ions. The dimer spectra of fully hydrated complex 2 (x = 7.3) are observed at temperatures T < 10 K. For this complex, the structure is identified and the distance between the copper ions is determined. It is shown that the nitrate counterion plays the role of a bridge between the hexaaqua copper(II) complex and the dendrimer copper(II) complex. The temperature-induced valence tautomerism attended by electron transport is revealed for the first time in blue dendrimer complexes 1 (x = 1.9) with a dimer structure. The activation energy for electron transport is estimated to be 0.35 meV. The coordination of the copper ion site (NO4) and the structural arrangement of green complexes 1 (x = 1.9) in the columnar mesophase are determined. Complexes of this type form linear chains in which nitrate counterions serve as bridges between copper centers. It is revealed that green complexes 1 (x = 1.9) dissolved in isotropic inert solvents can be oriented in the magnetic field (B0 = 8000 G). The degree of orientation of these complexes is rather high (Sz = 0.76) and close to that of systems with a complete ordering (Sz = 1) in the magnetic field. Copper(0) nanoclusters prepared by reduction of complex 2 (x = 7.3) in two reducing agents (NaBH4, N2H4 · H2O) are examined. A model is proposed for a possible location of Cu(0) nanoclusters in a dendrimer matrix.

Spider Silk as an Active Scaffold in the Assembly of Gold Nanoparticles and Application of the Gold–Silk Bioconjugate in Vapor Sensing

Spider silk is being viewed with interest by materials scientists due to its excellent resilience and mechanical properties. In this paper we show that spider silk is an excellent scaffold for the one-step synthesis and assembly of gold nanoparticles. Formation of a gold nanoparticle-spider-silk bioconjugate material is accomplished by simple reaction of the fibers with aqueous chloroauric acid. The gold nanoparticles thus formed are strongly bound to the spider-silk fiber surface enabling study of the electrical properties of the nanobioconjugate. Using the well-known contraction/expansion behavior of the fibers in solvents of varying polarity, we show that exposure of the gold nanoparticle-spider silk bioconjugate to vapors of methanol and chloroform leads to changes in electrical transport through the nanoparticles and thus, the possibility of developing a vapor sensor. The bioconjugate shows excellent response time and cycling efficiency to methanol vapors. The activation energy of electron transport from one gold nanoparticle to another in the nanobiocojugate was determined from temperature-dependent electron-transport measurements to be approximately 1.7 eV.

Comparison of Dye-Sensitized ZnO and TiO2 Solar Cells: Studies of Charge Transport and Carrier Lifetime

Nanocrystalline particles of ZnO and TiO2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO2. Apparent activation energies for electron transport in nanostructured ZnO of ≤0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was abo...

Interpretation of Apparent Activation Energies for Electron Transport in Dye-sensitized Nanocrystalline Solar Cells

Electron transport in dye-sensitized nanocrystalline solar cells appears to be a slow diffusion-controlled process. Values of the apparent electron diffusion coefficient are many orders of magnitude smaller than those reported for bulk anatase. The slow transport of electrons has been attributed to multiple trapping (MT) at energy levels distributed exponentially in the band gap of the nanocrystalline oxide. In the MT model, release of immobile electrons from occupied traps to the conduction band is a thermally activated process, and it might therefore be expected that the apparent electron diffusion coefficient should depend strongly on temperature. In fact, rather small activation energies (0.1-0.25 eV) have been derived from time and frequency resolved measurements of the short circuit photocurrent. It is shown that the MT model can give rise to such anomalously low apparent activation energies as a consequence of the boundary conditions imposed by the short circuit condition and the quasi-static relationship between changes in the densities of free and trapped electrons. This conclusion has been confirmed by exact numerical solutions of the time-dependent generation/collection problem for periodic excitation that provide a good fit to experimental data.

Activation Energy of Electron Transport in Dye-Sensitized TiO2 Solar Cells

Various characteristics of dye-sensitized nanostructured TiO2 solar cells, such as electron transport and electron lifetime, were studied in detail using monochromatic illumination conditions. The electron transport was found to be a thermally activated process with activation energies in the range of 0.10-0.15 eV for light intensities that varied 2 orders of magnitude. Electron lifetimes were determined using a new method and found to be significantly larger (>1 s) than previously determined. An average potential was determined for electrons in the nanostructured TiO2 under illumination in short-circuit conditions. This potential is about 0.2 V lower than the open-circuit potential at the same light intensity. The electron transport time varies exponentially with the internal potential at short-circuit conditions, indicating that the gradient in the electrochemical potential is the driving force for electron transport in the nanostructured TiO2 film. The applicability of the conventionally used trapping/detrapping model is critically analyzed. Although experimental results can be fitted using a trapping/detrapping model with an exponential distribution of traps, the distribution parameters differ significantly between different types of experiment. Furthermore, the experimental activation energies for electron transport are smaller than those expected in a trapping/detrapping model.

Studies on gamma silver iodide

γ-AgI with an Ag +-disordered zincblende structure has been synthesized by several methods including a wet route, based on LiI and a mechano-chemical reaction involving Ag powder and iodine crystals, and thoroughly characterized by XRD ( a = 0.6507 nm) and density (5.96 g mL −1) measurements. γ-AgI is found to be thermodynamically stable at room temperature and pressure, up to 413 K and 588 MPa. β-AgI is formed beyond pressures of 883 MPa. AC/DC electrical conduction in γ-AgI is promoted by the migration of cationic Frenkel defects (Ag + interstitials) that are already present below 250 K and by the creation of more such defects above 250 K but below 420 K. The activation energy for Ag + ion migration is 0.39 eV. Electronic conductivity through I–V characteristics, of γ-AgI shows the existence of thermally generated electrons and their migration and establishes γ-AgI as an n-type semiconductor, like β-AgI. The activation energy for electron transport is 0.42 eV. Weaker excitonic effects (relative to β-AgI) are observed in γ-AgI from optical absorption measurements.

Effects of Temperature on Electron Transport in Arum maculatum Mitochondria

The effects of temperature upon the respiratory pathways of Arum maculatum mitochondria have been studied. The alternate oxidase sustained a greater proportion of the total respiration at low temperatures than at higher temperatures. Arrhenius plots of respiratory activities show two discontinuities, one at 14 degrees C and one at 21 degrees C. The lower temperature discontinuity was associated with electron transport from succinate dehydrogenase to the alternative oxidase, enzymes that face the inner side of the membrane while the higher temperature discontinuity was associated with electron transport from the external NADH dehydrogenase to cytochrome c oxidase, which face the outer side of the membrane. Both discontinuities resulted in a decrease in the activation energy for electron transport on one side of the membrane. Arrhenius plots of transmembrane electron transport showed discontinuities at both 14 degrees and 21 degrees C but the upper discontinuity resulted in an increase in the activation energy. Activation energies determined for the respiratory activities show that above 21 degrees C the exogenous NADH-cytochrome pathway and the succinate-alternative oxidase pathway were lower than those for the NADH-alternative pathway or the succinate cytochrome pathway.

An X-Ray Photoelectron Study of Doped and Supported Nickel Oxide

The surfaces of lithium-and chromium-doped nickel oxide and of nickel oxide supported on alumina have been examined by X-ray photoelectron spectroscopy. The concentration of the nickel(III) species increased for the lithium-doped oxide and decreased for the chromium-doped oxide relative to the undoped oxide. The effects of doping were manifested most clearly however by the amount of oxygen-containing species adsorbed on the oxide surface rather than by variations in the nickel(III) peak intensity. Lithium-doped oxides were also shown to reduce more readily than undoped or chromium-doped oxides in the presence of carbon-containing impurities. The rate of reduction is influenced by the activation energy for electron transport which is related to the availability of the nickel(III) species. Supported oxides showed significant surface enhancement of nickel(II) as well as an absence of the nickel(III) species. The nickel(II) species in the supported oxide was not easily reduced to nickel(0).

Site selective laser spectroscopy of the insulator-to-semiconductor transition in CdF2:Er3+

The individual Er3+ sites in CdF2:Er3+ are determined by site selective laser spectroscopy and are monitored as the crystal is changed from the insulating to the semiconducting state. The site distribution is controlled by clustering of the Er3+ dopant ions and their charge compensating fluoride interstitials (F′i). Simple, single pair sites where an Er3+ ion is locally compensated by a Fi are found to have minor importance in contrast with other fluorites. The conversion to a semiconductor state results from oxidation of the F′i that locally compensate the Er3+ clusters. No new sites are observed that arise from local compensation by electrons. Differences in the oxidation potential between the various clusters suggest differences in the numbers of Fi associated with each cluster. It is suggested that the changes in the site distribution that occur for dopants with different ionic radii account for the inability of some dopants like La3+, Ce3+, and Pr3+ to produce the semiconducting state. The observation of the dominance of clustering in CdF2 lends support to previous interpretations of the activation energies for electron transport.