Showing papers on "Variable-range hopping published in 2016"
TL;DR: Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.
Abstract: We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.
171 citations
TL;DR: Measurements of electrical conductance and thermopower in 10-30 nm thick BP devices indicate that two-dimensional (2D) Mott's variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in the authors' examined thin BP devices.
Abstract: The moderate band gap of black phosphorus (BP) in the range of 03–2 eV, along a high mobility of a few hundred cm2 V–1 s–1 provides a bridge between the gapless graphene and relatively low-mobility transition metal dichalcogenides Here, we study the mechanism of electrical and thermoelectric transport in 10–30 nm thick BP devices by measurements of electrical conductance and thermopower (S) with various temperatures (T) and gate-electric fields The T dependences of S and the sheet conductance (σ□) of the BP devices show behaviors of T1/3 and exp[−(1/T)1/3], respectively, where S reaches ∼04 mV/K near room T This result indicates that two-dimensional (2D) Mott’s variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in our examined thin BP devices We consider the origin of the 2D Mott’s VRH transport in our BPs as trapped charges at the surface of the underlying SiO2 based on the analysis with observed multiple quantum dots
69 citations
TL;DR: In this article, the authors investigated a cooperative shielding model for quantum transport with both nearest-neighbor and infinite-range hopping coupling, where the dynamics within the excited-state subspace are shielded from long range hopping.
Abstract: We investigate a paradigmatic model for quantum transport with both nearest-neighbor and infinite-range hopping coupling (independent of the position). Due to long-range homogeneous hopping, a gap between the ground state and the excited states can be induced, which is mathematically equivalent to the superconducting gap. In the gapped regime, the dynamics within the excited-state subspace is shielded from long-range hopping, namely it occurs as if long-range hopping would be absent. This is a cooperative phenomenon since shielding is effective over a time scale that diverges with the system size. We named this effect cooperative shielding. We also discuss the consequences of our findings on Anderson localization. Long-range hopping is usually thought to destroy localization due to the fact that it induces an infinite number of resonances. Contrary to this common lore we show that the excited states display strong localized features when shielding is effective even in the regime of strong long-range coupling. A brief discussion on the extension of our results to generic power-law decaying long-range hopping is also given. Our preliminary results confirm that the effects found for the infinite-range case are generic.
61 citations
TL;DR: The surface hopping approach, the methods for computation of excited states based on DFT, the connection between these methodologies, and their diverse implementations are reviewed and the shortcomings of the methods are critically addressed.
Abstract: Nonadiabatic dynamics simulation of electronically-excited states has been a research area of fundamental importance, providing support for spectroscopy, explaining photoinduced processes, and predicting new phenomena in a variety of specialties, from basic physical-chemistry, through molecular biology, to materials engineering. The demands in the field, however, are quickly growing, and the development of surface hopping based on density functional theory (SH/DFT) has been a major advance in the field. In this contribution, the surface hopping approach, the methods for computation of excited states based on DFT, the connection between these methodologies, and their diverse implementations are reviewed. The shortcomings of the methods are critically addressed and a number of case studies from diverse fields are surveyed.
58 citations
TL;DR: In this paper, the electrical properties of chalcogenide bulk CdS50S50−−−xSex glassy systems were studied and a two point probe technique was used to measure the electrical resistance and dc-electrical conductivity.
Abstract: This research article was devoted to study some electrical characteristics of chalcogenide bulk Cd50S50 − xSex (30% ≤ x ≤ 50%) glassy systems. CdSSe Bulk glasses were previously prepared by the mechanical milling technique. The products of the grinding process were pressed as disc-shaped pellets of diameter 12 mm and thickness 1.5 mm by using a compressor of pressure about 6.5 MPa. Two point probe technique was used to measure the electrical resistance and dc-electrical conductivity in the temperature range 293 K–435 K. Conduction mechanism studies reveal that, the transition temperature was detected at around 370 K. Below this temperature, the conduction mechanism was via the variable range hopping conduction near Fermi level according to Mott's model. Above 370 K, the mechanism was hopping conduction via localized states by the activated thermionic emission. The activation energies of bulk CdSSe glasses were found to be decreased by adding more Se. The pre-exponential factors, Mott parameters, and the density of localized states near Fermi level, trapping state energy, the potential barrier energy, were evaluated and studied. The relation between cohesive and activation energies was discussed. All obtained data were strongly dependent upon Se-content in the bulk Cd50S50 − xSex glassy compositions.
56 citations
TL;DR: From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions.
Abstract: We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS2) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.
56 citations
TL;DR: Quantitative analyses of carrier concentration, carrier mobility, and simulated density of states (DOS) fully support that LixFe7Se8 is an Anderson insulator and provide a unified DOS picture to explain all the experimental results, and a schematic diagram for finding other potential Anderson insulators.
Abstract: Anderson (disorder-induced) localization, proposed more than half a century ago, has inspired numerous efforts to explore the absence of wave diffusions in disordered media. However, the proposed disorder-induced metal-insulator transition (MIT), associated with the nonpropagative electron waves, has hardly been observed in three-dimensional (3D) crystalline materials, let alone single crystals. We report the observation of an MIT in centimeter-size single crystals of Li x Fe7Se8 induced by lattice disorder. Both specific heat and infrared reflectance measurements reveal the presence of considerable electronic states in the vicinity of the Fermi level when the MIT occurs, suggesting that the transition is not due to Coulomb repulsion mechanism. The 3D variable range hopping regime evidenced by electrical transport measurements at low temperatures indicates the localized nature of the electronic states on the Fermi level. Quantitative analyses of carrier concentration, carrier mobility, and simulated density of states (DOS) fully support that Li x Fe7Se8 is an Anderson insulator. On the basis of these results, we provide a unified DOS picture to explain all the experimental results, and a schematic diagram for finding other potential Anderson insulators. This material will thus serve as a rich playground for both theoretical and experimental investigations on MITs and disorder-induced phenomena.
53 citations
TL;DR: In this paper, the authors investigated the electrical conductivity of polycrystalline-amorphous carbon nanocoils (CNCs) over a wide temperature range from 4 to 300 K by employing the four-probe method.
47 citations
TL;DR: In this paper, BaxSr2−xTiFeO6 complex double perovskites have been evaluated for applications as high temperature thermoelectric materials with properties converging to the "phonon-glass electron-crystal" model.
Abstract: In the present work, the prospects of environmental friendly BaxSr2−xTiFeO6 complex double perovskites have been evaluated for applications as high temperature thermoelectric materials with properties converging to the ‘phonon-glass electron-crystal’ model. BaxSr2−xTiFeO6 compositions with 0.0 ≤ x ≤ 0.25 were synthesized by a solid-state reaction method. The oxide samples were then investigated for their crystal structure (single phase) and morphology by XRD and SEM, respectively. Thermo-power or Seebeck coefficient (S) and the electrical conductivity (σ) of these oxide samples were simultaneously measured to calculate the thermoelectric power factor (S2σ). All the BaxSr2−xTiFeO6 compositions showed p-type non-degenerate semiconductor behavior before semiconductor to metal transition occurred as evident from the temperature dependent Seebeck coefficient of these double perovskites. Conduction mechanisms of these oxides were analyzed using variable range hopping and small polaron hopping conduction models. All these double perovskites exhibited more than 100 μV K−1 thermo-power in the wide range of temperature from 300 K to 1223 K. A very high thermo-power (S) value (∼800 μV K−1) was obtained for BaxSr2−xFeTiO6 with x = 0.25 at 1123 K.
44 citations
TL;DR: In this article, the effect of Ti doping on the magnetic, magnetocaloric and transport properties of La0.7Sr0.1 polycrystalline compounds was studied using phenomenological method and the results showed that by increasing Ti doping level, the metal-insulator transition temperature decreases and the system with x ≥ 0.2 becomes an insulator.
43 citations
TL;DR: In this article, low temperature resistivity and magnetotransport measurements were conducted on pristine and electron doped nickelates (SNO) and they showed that the low temperature transport in both pristine and ionized SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples.
Abstract: We present low temperature resistivity and magnetotransport measurements conducted on pristine and electron doped ${\mathrm{SmNiO}}_{3}$ (SNO). The low temperature transport in both pristine and electron-doped SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples. Undoped SNO films show a negative magnetoresistance (MR) at all temperatures characterized by spin fluctuations with the evolution of a positive cusp at low temperatures. In striking contrast, upon electron doping of the films via hydrogenation, we observe a crossover to a linear nonsaturating positive $\mathrm{MR}\ensuremath{\sim}0.2%$ at 50 K. The results signify the role of localization phenomena in tuning the magnetotransport response in doped nickelates. Ionic doping is therefore a promising approach to tune magnetotransport in correlated perovskites.
TL;DR: This work shows a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation and demonstrates that variable range hopping controls the electrical properties in as-prepared and thermally treated films.
Abstract: Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp2 content, localization length, and disorder. The measured hopping energies demonstrat...
TL;DR: In this paper, the complex electric modulus and the ac conductivity of carbon nano-onion/polyaniline composites were studied from 1 mHz to 1 MHz at isothermal conditions ranging from 15 K to room temperature.
Abstract: The complex electric modulus and the ac conductivity of carbon nano-onion/polyaniline composites were studied from 1 mHz to 1 MHz at isothermal conditions ranging from 15 K to room temperature. The temperature dependence of the electric modulus and the dc conductivity analyses indicate a couple of hopping mechanisms. The distinction between thermally activated processes and the determination of cross-over temperature were achieved by exploring the temperature dependence of the fractional exponent of the dispersive ac conductivity and the bifurcation of the scaled ac conductivity isotherms. The results are analyzed by combining the granular metal model (inter-grain charge tunneling of extended electron states located within mesoscopic highly conducting polyaniline grains) and a 3D Mott variable range hopping model (phonon assisted tunneling within the carbon nano-onions and clusters).
TL;DR: The results suggest that the existence of the oxygen vacancies in thinner films suppresses the double-exchange mechanism and contributes to the enhancement of disorder, leading to a decrease of the Curie temperature and the low temperature insulating phase in the ultrathin films.
Abstract: Metal-insulator transition is observed in the La0.8Sr0.2MnO3 thin films with thickness larger than 5 unit cells. Insulating phase at lower temperature appeared in the ultrathin films with thickness ranging from 6 unit cells to 10 unit cells and it is found that the Mott variable range hopping conduction dominates in this insulating phase at low temperature with a decrease of localization length in thinner films. A deficiency of oxygen content and a resulting decrease of the Mn valence have been observed in the ultrathin films with thickness smaller than or equal to 10 unit cells by studying the aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy of the films. These results suggest that the existence of the oxygen vacancies in thinner films suppresses the double-exchange mechanism and contributes to the enhancement of disorder, leading to a decrease of the Curie temperature and the low temperature insulating phase in the ultrathin films. In addition, the suppression of the magnetic properties in thinner films indicates stronger disorder of magnetic moments, which is considered to be the reason for this decrease of the localization length.
TL;DR: In this article, the authors investigated the origin of unusual electrical behaviors as negative temperature coefficient (NTC) thermistors in cobalt manganese spinel oxides with high cobalt concentration (CoxMn3-xO4, 2.3 ≤ x ≤ 2.7) and showed that the transition in the hopping motion occurs from variable range hopping (VRH) to nearest neighboring hopping (NNH) motion, which can be responsible for a huge increase of the resistance accompanied by decrease of the factor of thermal sensitivity (B value) in MCO compounds
Abstract: Hopping motions in cobalt manganese spinel oxides with high cobalt concentration (CoxMn3–xO4, 2.3 ≤ x ≤ 2.7) are investigated in order to clarify the origin of unusual electrical behaviors as negative temperature coefficient (NTC) thermistors. Based on the resistance versus temperature (R–T) characteristics, hopping conduction mechanisms in MCO compounds (x = 2.3 and 2.5) are attributed to variable range hopping (VRH) motion with a parabolic distribution of the density of states (DOS) near the Fermi level. However, when Co content increases up to 2.7, transition in the hopping motion occurs from VRH to the nearest neighboring hopping (NNH) motion, which can be responsible for a huge increase of the resistance accompanied by decrease of the factor of thermal sensitivity (B value) in MCO compounds (x = 2.7). Also, hopping distance and activation energies for MCO (x = 2.3 and 2.5) compounds following VRH conduction are calculated as a function of temperature, indicating that higher B value observed in MCO (x...
TL;DR: In this paper, gate current transient measurements were performed on n-channel MOSFETs operated in a gate-controlled-diode configuration, and the measurements revealed an anomalous non-steady conduction under negative bias through the SiO2/4H-SiC interface.
Abstract: This letter reports on the impact of gate oxide trapping states on the conduction mechanisms in SiO2/4H-SiC metal-oxide-semiconductor field effect transistors (MOSFETs). The phenomena were studied by gate current transient measurements, performed on n-channel MOSFETs operated in “gate-controlled-diode” configuration. The measurements revealed an anomalous non-steady conduction under negative bias (VG > |20 V|) through the SiO2/4H-SiC interface. The phenomenon was explained by the coexistence of a electron variable range hopping and a hole Fowler-Nordheim (FN) tunnelling. A semi-empirical modified FN model with a time-depended electric field is used to estimate the near interface traps in the gate oxide (Ntrap ∼ 2 × 1011 cm−2).
TL;DR: This investigation demonstrates the great potential of the OGNP-BPA for technological applications, given that their structural and electrical behavior is similar to the highly reduced rGO sheets obtained by more sophisticated conventional synthesis methods.
Abstract: The large-scale production of graphene and reduced-graphene oxide (rGO) requires low-cost and eco-friendly synthesis methods. We employed a new, simple, cost-effective pyrolytic method to synthetize oxidized-graphenic nanoplatelets (OGNP) using bamboo pyroligneous acid (BPA) as a source. Thorough analyses via high-resolution transmission electron microscopy and electron energy-loss spectroscopy provides a complete structural and chemical description at the local scale of these samples. In particular, we found that at the highest carbonization temperature the OGNP-BPA are mainly in a sp(2) bonding configuration (sp(2) fraction of 87%). To determine the electrical properties of single nanoplatelets, these were contacted by Pt nanowires deposited through focused-ion-beam-induced deposition techniques. Increased conductivity by two orders of magnitude is observed as oxygen content decreases from 17% to 5%, reaching a value of 2.3 × 10(3) S m(-1) at the lowest oxygen content. Temperature-dependent conductivity reveals a semiconductor transport behavior, described by the Mott three-dimensional variable range hopping mechanism. From the localization length, we estimate a band-gap value of 0.22(2) eV for an oxygen content of 5%. This investigation demonstrates the great potential of the OGNP-BPA for technological applications, given that their structural and electrical behavior is similar to the highly reduced rGO sheets obtained by more sophisticated conventional synthesis methods.
TL;DR: In this paper, the effects of long-range hopping and interparticle interactions on the quantum walk of hard core bosons in ideal and disordered one-dimensional lattices were studied.
Abstract: We study the effects of long-range hopping and long-range interparticle interactions on the quantum walk of hard-core bosons in ideal and disordered one-dimensional lattices. We find that the range of hopping has a much more significant effect on the particle correlation dynamics than the range of interactions. We illustrate that long-range hopping makes the correlation diagrams asymmetric with respect to the sign of the interaction. We examine the relative role of repulsive and attractive interactions on the dynamics of scattering by isolated impurities and Anderson localization in disordered lattices. We show that weakly repulsive interactions increase the probability of tunneling through isolated impurities and decrease the localization.
TL;DR: The Drude model modified by Debye relaxation time was introduced to determine the plasma frequency (ωp) in the surface initiated polymerization (SIP) synthesized β-silicon carbide/polyaniline metacomposites, which demonstrated high positive magnetoresistance (MR) values of up to 57.48% and high MR sensitivity at low magnetic field regimes.
Abstract: The Drude model modified by Debye relaxation time was introduced to determine the plasma frequency (ωp) in the surface initiated polymerization (SIP) synthesized β-silicon carbide (β-SiC)/polyaniline (PANI) metacomposites. The calculated plasma frequency for these metacomposites with different loadings of β-SiC nanoparticles was ranging from 6.11 × 104 to 1.53 × 105 rad s−1. The relationship between the negative permittivity and plasma frequency indicates the existence of switching frequency, at which the permittivity was changed from negative to positive. More interestingly, the synthesized non-magnetic metacomposites, observed to follow the 3-dimensional (3-D) Mott variable range hopping (VRH) electrical conduction mechanism, demonstrated high positive magnetoresistance (MR) values of up to 57.48% and high MR sensitivity at low magnetic field regimes.
TL;DR: In this article, the electrical properties of carbon-nickel films annealed at different temperatures (573, 773, 1073 and 1273 K) in the temperature range 15-300 K were investigated.
Abstract: In this work, the electrical properties of carbon–nickel films annealed at different temperatures (573, 773, 1073 and 1273 K) in the temperature range 15–300 K were investigated. The films were grown by radio frequency magnetron co-sputtering on quartz substrates at room temperature. The multiphonon hopping conduction mechanism is found to dominate the electrical transport in the temperature range 150–300 K. It can be seen that the room-temperature hopping rate (ΓRT) at 773 K has maximum value of 56.8 × 105 s−1. Our results of conductivity measurements at high temperature are in good agreement with strong carrier–lattice coupling model; on the other hand, the conductivity in the range 15–50 K is well described in terms of variable-range hopping (VRH) conduction mechanism. The localized state density around Fermi level N(E
F) and the average hopping energy W
hop at low temperature for the films annealed at 773 K have maximum value of 2.23 × 1023 (cm−3 eV−1) and minimum value of 9.74 × 10−4 eV, respectively.
TL;DR: It is shown that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy, and the system thermalizes through a many-body process involving exchange of a diverging number n∝-logT of phonons in the low temperature limit.
Abstract: We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n∝-logT of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.
TL;DR: In this paper, conductivity measurements were carried out on iron bismuth glasses with the nominal composition (100 −x)Bi 2 O 3 −x Fe 2 O3 (where x=15, 25, 35 and 45 ǫ%) in an attempt to understand the nature of the conduction mechanism governing the DC electrical conductivity and the effect of addition of Fe 2 o 3 on electrical properties in these glasses.
TL;DR: In this paper, the variation of leakage current and polarization hysteresis properties for bulk CH3NH3PbI3 perovskite was studied as a function of temperature.
Abstract: The variation of leakage current and polarization hysteresis properties for bulk CH3NH3PbI3 perovskite was studied as a function of temperature to understand the reported hysteresis in photocurrent and the role of ferroelectricity. The leakage current decreased by two orders of magnitude when the temperature was lowered from 350 K to 100 K. The transitions in leakage current were observed at structural phase transition temperatures. The temperature dependence study allowed the identification of current conduction mechanism based on various models for ferroelectrics and insulating materials. Our results show that the leakage current is governed by the space charge limited conduction mechanism which should be considered in addition to ion conduction and ferroelectricity when analyzing current-voltage hysteresis for thin film and bulk materials. The Mott's variable range hopping model fits well to the experimental data indicating the charge conduction is through hopping mechanism from 300 K to 160 K and poss...
TL;DR: In this article, the authors used the small polaron hopping (SPH) and the variable range hopping (VRH) models to examine conduction in the semiconducting region.
TL;DR: In this article, a structurally induced doping compensation mechanism in doped semiconductor nanowires that results from the reduced size geometry is reported, which can significantly affect the electronic transport properties of the doped wires.
Abstract: We report the observation of a structurally induced doping compensation mechanism in doped semiconductor nanowires that results from the reduced size geometry. This kind of compensation can significantly affect the electronic transport properties of the doped nanowires. We demonstrate that in a crystalline n-type doped Ge wire, compensated by the acceptor-like localized surface states, strong electron-electron interactions occur. Variable range hopping conduction detected in these nanowires is directly generated from strong interactions, exhibiting an unusual large Coulomb gap in the density of states of wires.
TL;DR: In this paper, the electronic properties of the NdNiO 3, prepared at the ambient oxygen pressure condition, were investigated and the metal-insulator transition temperature was observed at 192-K.
Abstract: We report the electronic properties of the NdNiO 3 , prepared at the ambient oxygen pressure condition. The metal–insulator transition temperature is observed at 192 K, but the low temperature state is found to be less insulating compared to the NdNiO 3 prepared at high oxygen pressure. The electric resistivity, Seebeck coefficient and thermal conductivity of the compound show large hysteresis below the metal–insulator transition. The large value of the effective mass ( m*~ 8 m e ) in the metallic state indicates the narrow character of the 3d band. The electric conduction at low temperatures ( T =2–20 K) is governed by the variable range hopping of the charge carriers.
TL;DR: In this article, a huge band gap tuning and low temperature transport phenomenon in highly transparent antimony doped tin oxide thin film (Sb:SnO 2 ) under the influence of swift heavy ions irradiation (SHII) is reported.
TL;DR: In this paper, a graphite oxide reduced by sodium borohydride was characterised and its electrical conduction investigated with impedance spectroscopy, and the thermal dependence of electrical modulus was calculated from complex impedance spectra, an approach that prevents any peak in dielectric loss (imaginary component) from being swarmed by large dc conductivity.
Abstract: Graphite oxide reduced by sodium borohydride was characterised and its electrical conduction investigated with impedance spectroscopy. Thermal dependence of electrical modulus (instead of permittivity, its inverse) was calculated from complex impedance spectra, an approach that prevents any peak in dielectric loss (imaginary component) from being swarmed by large dc conductivity. Two loss peaks appeared at each tested frequency, in a sample of either degree of reduction. The set of weaker peak should arise from the relaxation of some polar bonds, as proposed earlier by us. The stronger loss peaks may correspond to the hopping of conduction electrons; variable range hopping is also consistent with the observed thermal dependence of conductivity. However, nearer ambient temperature there is a change in mechanism, to band transport, with an activation energy of fairly similar values as derived from both loss peaks and conductivity.
TL;DR: In this paper, the effect of Zn amount on the structural, optical, electrical and supercapacitive properties of pure Co3O4 and ZnCo2O4 (Co/Zn: 9/1, 8/2 and 7/3) was characterized.
Abstract: Pure Co3O4 and ZnCo2O4 (Co/Zn: 9/1, 8/2 and 7/3) nanostructured thin films were successfully deposited on the glass and indium tin oxide substrates from acetate based sols, by spin coating technique. The effect of Zn amount on the structural, optical, electrical and supercapacitive properties of ZnCo2O4 nanostructured films were characterized. X-ray diffraction, infrared spectroscopy and field emission scanning electron microscopy results revealed that the increase of Zn content does not lead to a significant change in the structure and morphology, nevertheless it caused drastic effects on the optical, electrical and electrochemical properties of the films. Mono-phase ZnCo2O4 structure even in high amounts of Zn exhibited good solubility of Zn in the Co3O4 structure in this preparation method. The quantity of allowed direct, forbidden direct, allowed indirect and forbidden indirect transitions were calculated from the UV–Visible spectroscopy results, and the nature of allowed direct band gap of Co3O4 in 1.52 eV was discussed as a controversial topic. Resistance measurements as a function of temperature suggested variable range hopping as predominant mechanism for transport in room temperature, which switches to thermionic conduction in higher temperatures. Cyclic voltammetry results revealed that, addition of Zn to the Co3O4 structure results interfacial capacitance of the films to be increased more than 12 times.
TL;DR: It is demonstrated experimentally that the contact radius, between touching heavily doped ZnO nanocrystals, controls the electron transport mechanism.
Abstract: Nanocrystal assemblies are being explored for a number of optoelectronic applications such as transparent conductors, photovoltaic solar cells, and electrochromic windows. Majority carrier transport is important for these applications, yet it remains relatively poorly understood in films comprised of touching nanocrystals. Specifically, the underlying structural parameters expected to determine the transport mechanism have not been fully elucidated. In this report, we demonstrate experimentally that the contact radius, between touching heavily doped ZnO nanocrystals, controls the electron transport mechanism. Spherical nanocrystals are considered, which are connected by a circular area. The radius of this circular area is the contact radius. For nanocrystals that have local majority carrier concentration above the Mott transition, there is a critical contact radius. If the contact radius between nanocrystals is less than the critical value, then the transport mechanism is variable range hopping. If the contact radius is greater than the critical value, the films display behavior consistent with metallic electron transport.