3D Variable Range Hopping Research Articles

Influence of grain size on the electrical properties of the double-layered LaSr2Mn2O7 manganite

The double-layered LaSr2Mn2O7 manganite was synthesized by the sol–gel process. Two samples with the average grain size from ∼150nm to ∼1μm were prepared by controlling the sintering temperature. Both samples had a tetragonal structure, with a small fraction of impurity phase in the S-1250 sample. In order to investigate the probable influence of grain size on the conduction mechanism, resistivity of the samples was measured as a function of temperature, and the data obtained was analyzed by different conduction mechanisms. It was found that with increase in the grain size, resistivity decreased at all temperature ranges. The results show that the adiabatic small polaron hopping (SPH) model is probably responsible for conduction at high temperature range, and that the 3D variable range hopping (VRH) model shows a better correlation with the experimental data for low temperature range. These analyses indicate the influence of grain size on the parameters obtained from fitting the data by both models.

The effect of tube filling on the electronic properties of Fe filled carbon nanotubes

Carbon nanotubes filled with Fe nanostructures (Fe-CNTs) were synthesized using an injection method in a 1-stage horizontal CVD furnace and a bubbling method in a 2-stage horizontal CVD reactor. Fe-CNTs were obtained through the pyrolysis of a mixture of dichlorobenzene and ferrocene in 5%H2/Ar. Metal impurities from the Fe-CNTs were removed using 1M HCl solution. CNTs filled with crystalline Fe nanoparticles, nanorods and nanowires were obtained using these procedures. An intimate interaction between the Fe and the CNT was established by HRTEM studies. The α-Fe phase was observed to be the most dominant fraction found in the synthesized Fe-CNTs. The Fe2O3 residue obtained from the TGA analysis revealed the amount of Fe filled inside the CNTs and this ranged between 3 and 31% by mass after purification. The temperature dependence of the conductivity in the temperature range between 2.5 and 100K for an entangled network of Fe-CNTs was measured. An increase in conductivity due to the increased Fe filling inside the CNTs with increased temperature was observed. The observed temperature dependence was explained in terms of variable range hopping (VRH) conduction mechanisms. A transition from Efros–Shklovskii behavior at low % Fe filling of the CNTs to Mott 3D VRH behavior at high % Fe filling of the CNTs was observed due to screening of the coulomb potential by electrons as the Fe % increases.

An investigation of the electrical transport properties of graphene-oxide thin films

The electrical transport properties of graphene-oxide (GO) thin films were investigated. The GO was synthesized by a modified Hummers method and was characterized by X-ray diffraction and UV–visible spectroscopy. The thin film of GO was made on a Si/SiO 2 substrate by drop-casting. The surface morphology of the GO film was analyzed by using scanning electron microscopy and atomic force microscopy techniques. Temperature dependent resistance and current–voltage measurements were studied using four-terminal method at various temperatures (120, 150, 175, 200, 250 and 300 K) and their charge transport followed the 3D variable range hopping mechanism which was well supported by Raman spectra analysis. The presence of various functional groups in GO were identified by using high resolution X-ray photo electron (XPS) and Fourier transform infra red (FT-IR) spectroscopic techniques. Graphene-oxide thin film field effect transistor devices show p-type semiconducting behavior with a hole mobility of 0.25 cm 2 V −1 s −1 and 0.59 cm 2 V −1 s −1 when measured in air and vacuum respectively.

Synthesis of nanocrystalline molybdenum disulfide films and studies of their structure, spectral and photoelectrical properties

An approach for preparing MoS2 films in mild conditions is described, which is based on producing the suspensions of nanocrystalline MoS2 particles in organic solvent via exfoliation of MoS2 crystals, followed by the deposition of these suspensions onto substrate. According to X-Ray diffraction data, acetonitrile suspensions yield highly oriented films with the basal planes of MoS2 crystallites being mainly oriented in the plane parallel to the substrate. Atomic force microscopy examination revealed changes in the film surface topography under variation of the film thickness, which varied in the range of 0.03–2.2μm. The optical absorption spectra of the obtained MoS2 films were found to resemble those of thin natural MoS2 single crystals. Dark conductivity of the films with the thickness of d=0.12–2.2μm was determined to be ~10−3S∙cm−1 at 300K. Experimental data on the conductivity in the temperature range of 10–300K were fitted to Mott function for 3D variable-range hopping. The synthesized MoS2 films were found to be photosensitive in the range of 300–800nm. They provided the value of reduced photocurrent density of about 7∙10−4 (A∙cm)/(WV) under photoexcitation at 575nm.

Charge transport mechanism in intercalated polypyrrole aluminum‐pillared montmorillonite clay nanocomposites

AbstractNanocomposites based on intercalated conducting polypyrrole (PPy) into the galleries of inorganic aluminum‐pillared Montmorillonite (Al‐PMMT) clay with varying concentrations of Al‐PMMT were prepared by in situ chemical polymerization. The intercalation was confirmed by X‐ray diffraction pattern. Charge transport mechanism in these composites was investigated by temperature dependent direct current conductivity measurements. An increase in DC conductivity value on addition of (Al‐PMMT) clay in the composites at all temperatures and a transition from three‐dimensional (3D) Mott's variable range hopping (VRH) process in pristine PPy to one‐dimensional (1D) Mott's VRH process in the intercalated polymer composites has been observed. This transition in charge transport mechanism of PPy from 3D VRH to 1D VRH on intercalation has been interpreted in terms of straightening and linearization of polymer chains and decrease in inter‐chain interactions in the intercalated PPy. Enhancement in mechanical properties and increase in thermal stability of the nanocomposites was also observed with the increase in weight percentage of Al‐PMMT in PPy‐Al‐PMMT composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

층상 페로브스카이트 구조인 La0.5Sr1.5Mn0.5Cr0.5-xFexO4(x=0.15, 0.3) 망가나이트의 구조적, 전기적, 자기적 특성의 연구

새로운 층상구조인 페로브스카이트 망가나이트 <TEX>$La_{0.5}Sr_{1.5}Mn_{0.5}Cr_{0.5-x}Fe_xO_4$</TEX> (x=0.15, 0.3) 를 세라믹 방법에 의해 합성하였다. 분말 X-선 측정을 통하여 I4/mmm 의 정방정계 단위세포를 확인하였다. 전기전도도 측정을 통하여 부도체의 특성을 확인하였으며 전기전도도는 3D 홉핑 메커니즘에 의해 나타난다. 자기적 특성은 모두 반강자성의 특성을 보인다. The new layered perovskite manganites <TEX>$La_{0.5}Sr_{1.5}Mn_{0.5}Cr_{0.5-x}Fe_xO_4$</TEX> (x=0.15, 0.3) have been prepared by standard ceramic method. The powder X-ray diffraction studies show that the phases crystallize with tetragonal unit cell in the space group I4/mmm. The electrical transport properties suggest that the phases show insulating behaviour and the electrical conduction in the phases occurs by a 3D variable range hopping mechanism. The magnetic properties suggest that both the phases are antiferromagnetic.

Synthesis and Characterization of Intergrowth Type Perovskite Oxide NdSr2MnCrO7

A new Ruddlesden-Popper phase has been prepared by the standard ceramic method. The powder X-ray diffraction studies suggest that the phase crystallizes with tetragonal unit cell in the space group I4/mmm. The electrical transport properties show that the phase is an electrical insulator and the electrical conduction in the phase occurs by a 3D variable range hopping mechanism. The magnetic studies suggest that the ferromagnetic interactions are dominant.

Synthesis, structure and low temperature study of electric transport and magnetic properties of GdSr2MnCrO7

The layered perovskite oxide, GdSr2MnCrO7, has been prepared by the standard ceramic method. The powder X-ray diffraction studies suggest that the phase crystallizes with tetragonal unit cell in the space group I4/mmm. The electrical transport properties show that the phase is an electrical insulator and the electrical conduction in the phase occurs by a 3D variable range hopping mechanism. The magnetic properties suggest that the ferromagnetic interactions are dominant in the high temperature region.

Synthesis and characterization of Ruddlesden-Popper (RP) type phase LaSr2MnCrO7

New Ruddlesden-Popper (RP) type phase LaSr2MnCrO7 has been synthesized by ceramic method. Rietveld profile analysis shows that the phase crystallizes with tetragonal unit cell in the space group I4/mmm. The electrical resistivity of the phase has been measured in the temperature range of 10–300 K using Leybold closed cycle helium cryostat. The phase shows insulator-metal (I-M) transition at low temperature, the phenomenon often associated with giant magnetoresistance. 3D variable range hopping governs the electrical conduction in the insulator region above the I-M transition temperature. Magnetic susceptibility of the phase has been measured in the temperature range of 100–300 K. Magnetic studies suggest that the phase is ferromagnetic.

Electrical studies of low energy Ar 9+ irradiated conducting polymer PANI–PVA

Polyaniline (PANI) prepared using water soluble support polymer polyvinyl alcohol (PVA) was subjected to irradiation with 150 keV Argon (Ar 9+) ion at the fluence of 7.68×10 14 ion/cm 2 with beam current of 2 μA (electrical). There has been implantation of Ar atoms. The field dependence of dielectric response has been measured in the frequency range from 100 Hz to 1 MHz and in the temperature range from room temperature to the glass transition temperature. Dielectric constants have been found to be more for the Ar 9+-implanted polymer than for the pristine sample. The frequency dependence of the imaginary complex impedance ( Z″) is found to obey Arrhenius law with an activation energy of 0.29 eV for unirradiated and 0.53 eV for the Ar 9+-implanted polymer. Using the Cole–Cole model, an analysis of the real and imaginary parts of the impedance is performed, which reflects an increase in polydispersive nature after irradiation. Resistivity analysis shows three-dimensional (3D) Variable Range Hopping (VRH) at low temperature in contrast with two-dimensional (2D) VRH. There has been a decrease in the localization length as well as an increase in the energy gap due to the scission of bonds by the energy deposited by Ar 9+ ions.

Optical, mechanical, and electrical studies of polymer composites based on charge transfer complex of phenothiazine–iodine with poly(vinyl chloride)

Charge transfer complex (CTC) of phenothiazine and iodine (1:2 molar ratio) was prepared by solvent evaporation method in diethyl ether and its composite with poly(vinyl chloride) was prepared in benzene by diffusion method. Infra-red spectra showed overlapped peaks for both components and intensity of individual component was proportional to feed ratio. Optical photographs and scanning electron microscopy of composites showed template growth and connectivity in insulator matrix was proportional to wt% of CTC. Mechanical strength of composites was found to increase with wt% of PVC. The current–voltage study showed percolation threshold of 8 wt% of CTC. The temperature dependence of conductivity showed semiconducting nature of the materials. Transport property of charges were explained by regression analysis of σ dc vs. T −1/1+ n data and meets the basis for the Mott's 2D, 3D variable range hopping or thermoionic emission model, depending on temperature and wt% of CTC content. The impedance spectroscopy was performed between 40 Hz–100 kHz range. Circuit elements consists only combination of resistance and capacitance, which showed homogeneous nature of composites. Thermoelectric factor ‘ S’ was also evaluated and has value of <1 at 303 K in all cases.

The large anisotropy of the magnetic and transport properties in the Ba 5Co 5ClO 13 single crystal

In this Letter, the single crystals of Ba 5Co 5ClO 13 were grown by the flux method successfully. Their structure, magnetic and transport properties were studied. A large anisotropy of the magnetic and transport properties has been detected in this compound. Below the T N ∼ 108 K , the magnetic susceptibility exhibits an antiferromagnetic peak in χ c and an upturn transition in χ a b . We suggest that this behavior is consistent with the competition of the ferromagnetic (FM) intra-blocks coupling and antiferromagnetic (AFM) inter-blocks coupling in this compound. The temperature dependence of the resistivity displays a hump in ρ a b and a kink in ρ c around T N , suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the three-dimensional (3D) variable range hopping (VRH) mechanism.

Electrical properties of polyaniline doped with metal ions

Electrical transport properties of undoped polyaniline (Pani) and Pani doped with Fe, Ni and La ions (named Pani–Fe, Pani–Ni and Pani–La) have been investigated using dc and ac conductivity data. It is shown that the Pani–Fe sample follows 3D variable range hopping (VRH) conduction while the Pani, Pani–Ni and Pani–La samples follow 1D VRH conduction. These results are discussed by comparing with conventional HCl-doped Pani. Ac conductivity studies carried out in the frequency range 42 Hz–5 MHz reveal the influence of dopants on inter-chain interactions. The dielectric response exhibits two well resolved relaxation processes, which appear to indicate phase segregation of doped and undoped regions in the samples.

Hopping conduction on PAni/PSS blends

Conducting blends of polyaniline (PAni) and sulfonated polystyrene (PSS) were obtained by in situ polymerization. We prepared PAni/PSS blends with molar ratio of 1:2 and PSS sulfonated at the degrees of 13, 21 and 30 mol%. The blends were investigated by d.c. conductivity and scanning electron microscopy (SEM). Increasing sulfonation degree increases the conductivity and the miscibility of the blends was observed by SEM. The electrical conductivity of blends was evaluated from 100 to 300 K. We show that three-dimensional variable range hopping is observed. From the hopping parameter we obtained the mean hopping distance, the density of active centers and the energy associated with it. The hopping distance was compared to the mean distance of sulfonic groups of the PSS, indicating that interchain 3D variable range hopping is dominant at low temperatures.

A Morphological Model for the Solvent‐Enhanced Conductivity of PEDOT:PSS Thin Films

AbstractThe well‐known enhanced conductivity of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films that is obtained by addition of high‐boiling solvents like sorbitol to the aqueous dispersion used for film deposition is shown to be associated with a rearrangement of PEDOT‐rich clusters into elongated domains, as evidenced from STM and AFM. Consistently, temperature dependent conductivity measurements for sorbitol‐treated films reveal that charge transport occurs via quasi 1D variable range hopping (VRH), in contrast to 3D VRH for untreated PEDOT:PSS films. The typical hopping distance of 60–90 nm, extracted from the conductivity measurements is consistent with hopping between the 30–40 nm sized grains observed with scanning probe microscopy.

Variable-range hopping in Fe70Pt30 catalyzed multi-walled carbon nanotubes film

Using low-pressure chemical vapour deposition (LPCVD), multi-walled carbon nanotubes (MWNTs) are grown on nanocrystalline Fe70Pt30 film. The Fe70Pt30 nanocrystalline film is deposited by vapour condensation technique. The size of the nanoparticles varies from 5–10 nm, as inferred from SEM micrographs of Fe70Pt30 film. SEM and TEM observations of as-grown CNTs film reveal that these are multi-walled and their diameter varies from 30–80 nm and length is of the order of several micrometers respectively. There is a structural change from ordinary geometry of CNTs to bamboo shaped as suggested by TEM image. Raman spectra shows sharp G and D bands with a higher intensity of G band showing the presence of graphitic nature of the nanotubes. An experimental study of the temperature dependence of electrical conductivity of MWNTs film is done over a wide temperature range from (293–4 K). The measured data gives a good fit to variable-range hopping (VRH) and the results are interpreted using Mott's (VRH) model. The conduction mechanism of the MWNTs film shows a crossover from the exp[ -(To/T)1/4] law in the temperature range (293–110 K) to exp[ -(Tm/T)1/3] in the low temperature range (110–4 K). This behaviour is attributed to temperature-induced transition from three-dimension (3D) to two-dimension (2D) VRH. Various Mott's parameters like characteristic temperature (Tm), density of states at Fermi level N(EF), localization length (ξ), hopping distance (R), hopping energy (W) have also been calculated using above-mentioned model.

Hopping conduction in double layered La 2−2 xCa 1+2 xMn 2O 7 manganite

Double layered La 2−2 x Ca 1+2 x Mn 2O 7 manganite has been synthesized using solid state reaction method for different dopant concentration x = 0.0–0.5. Their temperature dependence of resistivity ( ρ − T) has been studied in the semiconducting region. The experimental observations were compared with the theoretically simulated temperature dependence of resistivity curves based on nearest neighbour hopping, Efros–Shklovskii variable range hopping, and Mott's 2D and 3D variable range hopping models. From the analysis of these results, Mott's 3D variable range hopping mechanism seemed to be most suitable mechanism describing the semiconducting behaviour of these double layered manganites. Temperature dependent activation energy also supported the Mott's 3D variable range hopping model. The Mott's activation energy was found to vary with the dopant concentration x and it showed a minimum value for x = 0.3.

Electrical transport and morphological study of PLD-grown nanostructured amorphouscarbon thin films

Nanostructured carbon thin films have been actively investigated recently for theirelectroresistance (ER) properties. Furthermore, carbon films with nonlinear current–voltage(I–V) characteristics have potential application in field-emission devices. This hasmotivated us to study the effect of various growth parameters on the physical andmorphological properties of carbon films grown by pulsed laser deposition (PLD). Carbonfilms have been deposited using a graphite target at different partial pressures ofargon. The morphology of film surfaces deposited at various growth conditionswas monitored using an atomic force microscope (AFM). AFM studies showednanostructured grain growth with average grain size of about 80–90 nm. As thedeposition time was decreased down to 1 min, the grain size was also found to decreasecorrespondingly. From Raman spectroscopic measurements an increase in theI(D)/I(G) ratio and a decrease in FWHM (G) clearly revealed the promotion ofsp2 hybridization as the substrate temperature increased. All the films showsemiconducting behaviour with the dominant conduction process being thethree-dimensional (3D) variable range hopping (VRH) mechanism. NonlinearI–V curves were obtained for carbon films deposited on p-type Si indicating diode-likebehaviour. The most significant result of this study was the observation of a largeelectroresistance value.

Structural and electrical studies on tetrameric cobalt phthalocyanine and polyaniline composites

Polyaniline and oligomeric cobalt phthalocyanine are blended in different proportions by chemical methods. These blends are characterised by spectroscopic methods and dielectric measurements. Dielectric studies on the conducting polymer blends are carried out in the frequency range of 100 kHz to 5 MHz from room temperature (300 K) to 373 K. Dielectric permittivity and dielectric loss of these blends are explained on the basis of interfacial polarisation. From the dielectric permittivity studies, ac conductivity of the samples were calculated and the results are correlated. In order to understand the exact conduction mechanism of the samples, dc electrical conductivity of the blends is carried out in the temperature range of 70–300 K. By applying Mott's theory, it is found that the conducting polymer composites obey a 3D variable range hopping mechanism. The values of Mott's temperature ( T 0), density of states at the Fermi energy ( N( E F)), range of hopping ( R) and hopping energy ( W) for the composites are calculated and presented.

Crystal growth and magnetic properties of rare earth borosilicides

Abstract B12 icosahedra cluster-containing rare earth borosilicides REB44Si2 (RE = Tb, Dy, Ho, Er, Tm, Yb) were grown by the floating zone method and their physical properties were investigated. REB44Si2 compounds are isostructural to REB50 and they are unique among the newly discovered boron-rich phases in that it is possible to grow large crystals with dimensions exceeding 10 mm. The rare earth atoms form a coupled chain structure like a ladder along the c-axis which is also the axis along which there is a B12 icosahedral chain. Magnetic properties, resistivity, and specific heat of the compounds were investigated. The conductivity follows the 3D variable range hopping mechanism and it was determined that localization lengths ξ are extremely short, actually close to the distances of the atoms themselves. Magnetic transitions above 4 K are indicated for all the compounds except for TmB44Si2 and it is found that specific heat measurement is the expedient probe of such compounds in which the low temperature susceptibility is influenced by external conditions such as crystal growth rate. Despite dimer-like features to the magnetic transition a spin gap is not observed. The magnetic coupling in these systems is indicated to be different from the conventional mechanisms in f-electron systems of dipole-dipole coupling or RKKY interaction.