One-dimensional Variable Range Hopping Research Articles

Direct current conductance and 1/f-noise in cellulose nanofiber–multi-walled carbon nanotube composites for applications in flexible electronic devices

We report on the studies of conduction mechanism, direct current conductance, and 1f-noise of cellulose nanofiber (CNF) and multiwalled carbon nanotube (MWCNT) composites. The composites were characterized by x-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy. The temperature- and voltage-dependence of the dc conductance Σ were, respectively, probed to investigate the charge transport mechanism and the electrical response of the composite. At room temperature, the increase in Σ with wt. % of MWCNT ϕ showed typical percolation behavior. The Σ−T behavior was fitted to the combination of one-dimensional variable range hopping and the fluctuation-induced tunneling, which were attributed to hopping of charge carriers through 1D MWCNTs and the tunneling of charge carriers between the bundles of MWCNTs, respectively. The non-Ohmic electrical conduction was characterized by the onset voltage V0(T) which scaled with Ohmic conductance Σ0 as V0(T)∼Σ0(T)xT, with xT being the onset exponent increased with ϕ. A scaling description based on the data collapse method was adopted to find the parameters V0(T) and xT. The noise power spectrum SV(f) followed the relation SV(f)∼Vβ with two different power-laws: β1 in the Ohmic and β2 in the non-Ohmic region (β1>β2). Interestingly, this change in power-laws occurs at the same V0(T) obtained from Σ−V curves. A simple model was proposed to explain the noise behavior after V0(T). It is expected that such electrical characterization of CNF-MWCNT nanopaper composite would open up their possibility of application in flexible electronic devices, intelligent networks, sensors, and actuators.

Low-temperature electrical and magnetic properties of La0.6Sr0.4Co0.2Fe0.8O3-δ nanofibers prepared by electrospinning

The microstructural, magnetic, and electrical properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCFO) nanofibers prepared by electrospinning and calcination were systematically investigated. It was found that the as-prepared nanofibers manifested ferromagnetism at low temperatures. Moreover, the resistance and magnetoresistance (MR) data conformed to the one-dimensional variable range hopping (1D VRH) model and revealed that LSCFO nanofibers had different electronic conduction behavior at different temperatures and magnetic fields. LSCFO nanofibers had small negative MR at low temperatures and large positive MR at high temperatures, and the transition temperature ranged between 240 and 260 K. The negative MR and the positive MR of LSCFO nanofibers manifested a fit to the 1D weak localization model and the 1D VRH model, respectively. Therefore, the proposed work can provide an important reference to the design of solid oxide fuel cells (SOFCs) based on the LSCFO cathode.

Polyaniline inclusion into ordered mesoporous silica matrices: Synthesis, characterization and electrical transport mechanism

Abstract Composites of polyaniline (PANI) with MCM-41 and SBA-15 ordered mesoporous silicas were prepared by liquid and vapor monomer adsorption methods. Both methods resulted in polymer encapsulation inside the mesopores, but a higher amount was obtained using the vapor adsorption method. Composites presented a semiconductor behavior and the charge transport mechanism was consistent with a hopping process between localized states. Based on the results of one-dimensional variable range hopping (1D-VRH) conduction parameters, the SBA-15 with polyaniline incorporated by vapor adsorption was the more conductive and ordered material.

Preparation and low-temperature electrical and magnetic properties of La0.33Pr0.34Ca0.33MnO3 nanofibers via electrospinning

We systematically studied the structure, magnetism, resistance, magnetoresistance (MR) and nonlinear current-voltage (I-V) curves of La0.33Pr0.34Ca0.33MnO3 (LPCMO) nanofibers via electrospinning and followed calcination. Magnetic study shows that this nanosystem can form long-range order and generate spin-glass states at low temperatures. The resistance and MR studies indicate the presence of metal insulator transitions and colossal MR effects in the system, respectively. One-dimensional variable range hopping (1D VRH) model is used to explain the high-temperature segment behavior of resistance and MR. The behavior of the nonlinear I-V curves indicates the exist of threshold voltage and zero bias conductance at low temperatures in the LPCMO nanowires, which represents the existence of Coulomb gap.

Effect of charge carrier transport on sulfur dioxide monitoring performance of highly porous polyaniline nanofibres

Polyaniline nanofibres (NFs) with controlled diameters were synthesized using a template-free method, with ammonium peroxidisulfate (APS) or ferric chloride (FeCl3) as oxidants. Porosity studies reveal that NFs prepared with FeCl3 possess higher effective surface area and larger pore volume compared to NFs prepared with APS. The FeCl3-assisted NFs show around twofold enhanced sensing response (ca 4.5%) towards 5 ppm of SO2 at room temperature compared to APS-assisted NFs (ca 2%). The enhancement can be attributed to the lower diameter, higher effective surface area and larger porosity of FeCl3-assisted NFs. To further explain this enhanced sensing response, the conduction mechanism was studied. NFs possessing a smaller diameter (ca 10 nm) are found to follow the one-dimensional variable range hopping (VRH) model, whereas NFs with larger diameter (ca 100 nm) follow the conventional three-dimensional VRH model. This can be due to the restriction of charge carrier transport into only one direction due to quantum size effects. Furthermore, the calculated Mott parameters suggest that the NFs prepared using FeCl3 provide a better pathway for charge transport of charge carriers as compared to NFs prepared using APS in terms of shorter hopping distance, lower activation energy and lower hopping energy, and weaker localization of charge carriers.

Distinct electrical effects of multi-walled carbon nanotubes in two composites

The temperature dependent conductivity of multi-walled carbon nanotube film (MWNT) is reported and the different electrical properties of nanotubes in two composites are compared. Due to the disordered structures, our carbon nanotube film displays variable range hopping behavior. While the geometric distributions of carbon nanotubes in the conducting polyaniline (PANI) and insulating polyamide (PA66) are similar, charge carriers transport distinctly. The conductive PANI, following one-dimensional variable range hopping, dominates the electrical properties of MWNT/PANI composites. The effect of MWNTs becomes prominent only at low temperature range. However, the contact junctions composed by adjacent carbon nanotubes, instead of nanotubes themselves or the polymer matrix, determine the electrical properties of MWNT/PA66 composites, showing the fluctuation induced tunneling characteristic.

Dielectric Constant and Transport Mechanism of Percolated Polyaniline Nanoclay Composites.

We report the dielectric constant and transport mechanism of intercalated nanoclay–polyaniline composite, an industrially ready to use novel nanocomposite, which is prepared by a simple mechanochemical method. The effects of clay concentration on structure and structure variations on properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and impedance spectroscopy (20 Hz–1 MHz and temperature range from 300–380 K). The phenomenon of percolation was observed in these composites. The values of Mott’s temperature, density of states at the Fermi level, hopping distance, and barrier height for polyaniline–nanoclay (PANC) composites were calculated. By applying Mott’s theory, it is found that the PANC composites obey the one-dimensional variable range hopping mechanism. This type of percolated sample can be used as a nanocapacitor in many devices because of its enhanced transport properties.

Solution-processed poly(3,4-ethylenedioxythiophene) thin films as transparent conductors: effect of p-toluenesulfonic acid in dimethyl sulfoxide.

Conductivity enhancement of thin transparent films based on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) by a solution-processed route involving mixture of an organic acid and organic solvent is reported. The combined effect of p-toluenesulfonic acid and dimethyl sulfoxide on spin-coated films of PEDOT-PSS on glass substrates, prepared from its commercially available aqueous dispersion, was found to increase the conductivity of the PEDOT-PSS film to ∼3500 S·cm(-1) with a high transparency of at least 94%. Apart from conductivity and transparency measurements, the films were characterized by Raman, infrared, and X-ray photoelectron spectroscopy along with atomic force microscopy and secondary ion mass spectrometry. Combined results showed that the conductivity enhancement was due to doping, rearrangement of PEDOT particles owing to phase separation, and removal of PSS matrix throughout the depth of the film. The temperature dependence of the resistance for the treated films was found to be in accordance with one-dimensional variable range hopping, showing that treatment is effective in reducing energy barrier for interchain and interdomain charge hopping. Moreover, the treatment was found to be compatible with flexible poly(ethylene terephthalate) (PET) substrates as well. Apart from being potential candidates to replace inorganic transparent conducting oxide materials, the films exhibited stand-alone catalytic activity toward I(-)/I3(-) redox couple as well and successfully replaced platinum and fluorinated tin oxide as counter electrode in dye-sensitized solar cells.

Charge carrier transport and low electrical percolation threshold in multiwalled carbon nanotube polymer nanocomposites

Electrical transport properties of multiwalled carbon nanotube (MWCNT) polymer nanocomposites based on epoxy and high density polyethylene (HDPE) as matrices have been studied. The composites have been prepared from masterbatches by dilution. Low electrical percolation thresholds of 0.05 and 0.4vol.% of carbon nanotubes in epoxy and HDPE matrices were found respectively. The analysis of the temperature dependence of dc-conductivity shows that the charge transport is fully described as the combination in series of one-dimensional (1-D) and three-dimensional (3-D) variable range hopping (VRH) regimes. The transport is limited by the 1-D VRH regime in the low temperatures limit and by the 3-D VRH regime at room temperature. The analysis of the VRH parameters when the overall volume fraction of MWCNT decreases reveals two distinct situations. In the epoxy the local characteristics of the masterbatch remain practically unchanged and a percolating network is maintained even at very low volume fractions; in the HDPE matrix, the density of the MWCNT bundles packing decreases as revealed by the strong variation of the Mott temperature parameter. Our results indicate that amorphous polymers matrices give lower percolation thresholds than semicrystalline ones when a masterbatch dilution process is used.

Structural and electrical characterizations of nanocrystalline Zn1−xCdxS (0≤x≤0.9) prepared by low cost dip coating

Thin films of nanocrystalline Zn1−xCdxS (0≤x≤0.9) were deposited by a dip coating method on glass substrates from aqueous solution containing cadmium acetate, zinc acetate and thiourea at 200°C. The morphological, structural and electrical properties of the deposited Zn1−xCdxS thin films were studied by atomic force microscopy (AFM), X-ray diffractometer (XRD), selected area electron diffraction (SAED) and photoluminescence (PL). To understand the predominant conduction mechanism of the nanocrystalline Zn1−xCdxS (0≤x≤0.9) thin films, DC electrical conductivity was measured in the temperature range of 300–420K. These measurements revealed that the DC behavior of the films can be described by a one-dimensional variable range hopping (VRH) model in the entire temperature range instead of a three-dimensional variable range hopping (VRH) model. The current density–voltage (J–V) characteristics of Zn1−xCdxS (0≤x≤0.9) thin films shows that the current conduction is ohmic type at the low-voltage region while the charge transport phenomenon appears to be space charge limited current (SCLC) at the higher-voltage regions. The latter conduction is attributed to the presence of a discrete trapping level. Various electrical parameters were determined and studied as a function of Cd-content such as electron mobility (μ0), density of states in conduction band (Nc), Fermi energy (EF), trap energy (Et) and trap electron density (Nt).

PAni-SnO<sub>2 </sub>Nanocomposite: Irradiation Induced Charge Transport Processes

The films of polyaniline–tin oxide (PAni–SnO2) nanocomposites were synthesized by chemical oxidative polymerization technique. These films were irradiated with 90 MeV O7+ ions at the fluences of 5×1010, 1×1011, 5×1011, and 1×1012 ions/cm2. X-ray diffraction studies show that microstrain and domain crystallite size of SnO2 nanoparticles in PAni matrix increase with the increase of ion fluence, resulting in highly ordered PAni–SnO2 nanocomposites. DC electrical conductivity is found to increase with the increase of fluence and conduction mechanism follows a quasi one-dimensional variable-range hopping model. AC electrical conductivity also increases with the increase of ion fluence and obeys correlated barrier-hopping model.

Electrical and optical studies in polyaniline nanofibre–SnO2 nanocomposites

Polyaniline nanofibre–tin oxide (PAni-SnO2) nanocomposites are synthesized and mixed with polyvinyl alcohol (PVA) as stabilizer to cast free-standing films. Composite films are characterized by X-ray diffraction studies (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence spectroscopy (PL) and UV-visible spectroscopy. XRD confirms the formation of PAni nanofibre–SnO2 nanocomposite. From TEM images, diameter of the polyaniline nanofibre and SnO2 nanoparticles in the PAni-SnO2 nanocomposite are found to be 20–60 nm. SEM results show fibrous morphology of the PAni nanofibre and spherical morphology of polyaniline-SnO2 composites. The nanocomposites exhibit high relative photoluminescence intensity in violet as well as green–yellow region of visible spectrum. From electrical conductivity measurement, it is confirmed that PAni nanofibre–SnO2 nanocomposite follows Mott’s one-dimensional variable range hopping (VRH) model.

Magnetic and transport properties of layered LixCo0.5RhO3

Band insulator Li1.5Co0.5RhO3 has been synthesized, in which Co and Rh atoms are partially disordered in the two-dimensional Co1/3Rh2/3O2 layer. Measurements of magnetic susceptibility indicate that Co(III) and Co(III) are both diamagnetic with low-spin configurations. The electrical resistivity and thermopower reveal one-dimensional variable-range hopping below T1=320 K and above T2=630 K. Charges introduced by Li extraction down to x = 0.75 in LixCo0.5RhO3 yield the systematic decreases in resistivity and T1, but the magnetic ordering is observed at TN=30 K, which can be accounted for by spin-orbital coupling with antiferromagnetic interaction.

Localization and electrical transport in onion-like carbon based composites

Electrical transport properties of onion-like carbon (OLC) based composites over a wide (20 Hz–4 THz) frequency range are reported. The dependencies of dc conductivity on temperature can be approximated by the Mott law for one-dimensional variable range hopping below 130 K and by the typical law for fluctuation induced tunneling within the range of 130–300 K. The critical frequency at low temperatures also decreases according to the Mott law for one-dimensional variable range hopping. It was demonstrated that OLC annealing temperature plays a high role on the dielectric and electrical properties of composites at low temperatures. In the terahertz frequency range, the main contribution to the complex electrical conductivity is due to the phonon contribution while the contribution from hopping conduction vanishes at these frequencies.

Electrical conductivity, dielectric properties and optical absorption of organic based nanocrystalline sodium copper chlorophyllin for photodiode application

Sodium copper chlorophyllin (SCC) thin films were successfully prepared, using dip coating technique. Thermal gravimetric analysis (TGA) was performed for studying the thermal stability of SCC film. The surface morphology of thin films was studied by using scanning electron microscopy (SEM). The crystalline structural characteristics were undertaken with the aim of determining the lattice parameters together with a complete list of the Miller indices and interplanar spacing for SCC. The molecular structure and electronic transitions of SCC were investigated by Fourier-transform infrared (FTIR) and absorption spectrum, respectively. Temperature dependence of the DC electrical conductivity, ( σ DC) was investigated in the temperature range 289–373 K. Measurements revealed that the σ DC behavior of the films can be described by Mott's one-dimensional variable range hopping (VRH) model in the entire temperature range. The AC conductivity, ( σ AC( ω)) results were discussed in terms of the correlated barrier hopping (CBH) mechanism for charge carrier transport. The maximum barrier height and the hopping length were estimated. The temperature dependence of the σ AC( ω) shows Arrhenius type with one thermal activation energy for each frequency. The behavior of the real and imaginary parts of the dielectric constant as a function of both temperature and frequency were discussed. The energy band model was applied and the type of the optical transitions responsible for optical absorption was found to be direct allowed transition. Position dependent for SCC thin film photo-detector was studied by using laser diode source.

Correlated conformation and charge transport in multiwall carbon nanotube-conductingpolymer nanocomposites

The strikingly different charge transport behaviours in nanocomposites of multiwall carbonnanotubes (MWNTs) and conducting polymer polyethylenedioxythiophene–polystyrene-sulfonic-acid(PEDOT–PSS) at low temperatures are explained by probing their conformationalproperties using small-angle x-ray scattering (SAXS). The SAXS studies indicate theassembly of elongated PEDOT–PSS globules on the walls of nanotubes, coatingthem partially, thereby limiting the interaction between the nanotubes in thepolymer matrix. This results in a charge transport governed mainly by smallpolarons in the conducting polymer despite the presence of metallic MWNTs. AtT > 4 K, hopping of the charge carriers following one-dimensional variable range hopping is evidentwhich also gives rise to a positive magnetoresistance (MR) with an enhanced localization length (∼5 nm) due to the presence of MWNTs. However, atT < 4 K, the observation of an unconventional positive temperature coefficient of resistivity isattributed to small polaron tunnelling. The exceptionally large negative MR observed inthis temperature regime is conjectured to be due to the presence of quasi-1D MWNTs thatcan aid in lowering the tunnelling barrier across the nanotube–polymer boundary resultingin large delocalization.

Studies on Fourier transform infrared spectroscopy, scanning electron microscope, and direct current conductivity of polyaniline doped zinc ferrite

AbstractIn situ polymerization of aniline was carried out in the presence of zinc ferrite to synthesize polyaniline/ZnFe2O4 composites (PANI/ZnFe2O4) by chemical oxidation method. The composites have been synthesized with various compositions (10, 20, 30, 40, and 50 wt %) of zinc ferrite in PANI. From the Fourier transform infrared spectroscopy (FTIR) studies on polyaniline/ZnFe2O4 composites, the peak at 1140 cm−1 is considered to be measure of the degree of electron delocalization. The surface morphology of these composites was studied with scanning electron micrograph (SEM). The dc conductivity has been studied in the temperature range from 40–160°C and supports the one‐dimensional variable range hopping (1DVRH) model proposed by Mott. The results obtained for these composites are of scientific and technological interest. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Enhancements in Conductivity and Thermal Stabilities of Polypyrrole/Polyurethane Nanoblends

The current work deals with enhancements in conductivity and thermal stabilities of thermoplastic polyurethane (TPU)/dodecylbenzenesulfonic acid doped polypyrrole (PPy.DBSA) nanoblends prepared by solution intercalation (SB) and in situ (IS) methods. It is observed that the electrical conductivity (σdc) of nanoblends is influenced by interaction between PPy.DBSA and TPU as well as homogeneous nanolevel dispersion and content of PPy.DBSA in TPU matrix. The maximum value of σdc has been found at 30 wt % PPy.DBSA for the SB (0.26 S cm−1) nanoblend due to the presence of the hexagonal network. The percolation threshold is attained at 2.5 wt % of PPy.DBSA for IS and SB nanoblends. The temperature variation of conductivity for different nanoblends follows one-dimensional variable range hopping (VRH) model. A significant improvement (32 °C) in thermal stability has been observed at 50% wt loss for SB nanoblend containing 30 wt % PPy.DBSA. The PL peak of the TPU nanoblend is red-shifted with increasing wt % of PP...

Electrical Conductivity of Polyaniline/NiZnO3 Composites: A Solid State Electrolyte

In situ polymerization of aniline was carried out in the presence of nickel zinc oxide to synthesize polyaniline/NiZnO3 composites (PANI/NiZnO3) by chemical oxidation method. The composites have been synthesized with various compositions (10, 20, 30, 40 and 50 wt%) of nickel zinc oxides in PANI. From the infrared spectroscopy (FTIR) studies on polyaniline/NiZnO3 composites, the peak at 1140 cm−1 is considered to be the measure of the degree of electron delocalization. The surface morphology of these composites was studied with the scanning electron micrograph (SEM). The dc conductivity was studied in the temperature range from 40–160°C and supports the one-dimensional variable range hopping (1DVRH) model proposed by Mott. The results obtained for these composites are of scientific and technological interest.

Structure, DC and AC conductivity of oxazine thin films prepared by thermal evaporation technique

Thin films of oxazine (OXZ) were prepared using thermal evaporation technique under high vacuum. The thermogravimetric analysis (TGA/DSC) was investigated. Thin films of nano-crystalline OXZ were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Temperature dependence of the DC electrical conductivity was investigated in the temperature range 300–425 K. Measurements revealed that the DC behavior of the films can be described by Mott's one-dimensional variable range hopping (VRH) model in the entire temperature range. The AC conductivity of the sample is found to be proportional to ω s . The temperature dependence of the AC conductivity and the frequency exponent, s is reasonably well interpreted in terms of the correlated barrier-hopping CBH model. Frequency dependence of the capacitance in the frequency range 42 Hz–5 MHz indicates that the capacitance is strongly frequency dependent. The capacitance was determined to decrease with increasing frequency at low frequency and less rapidly at higher frequencies. This is quantitatively interpreted using an equivalent circuit model.