Conduction Of Charge Carriers Research Articles

Perylenediimide-Graphene Donor-Acceptor Nanoensembles: Synthesis, Characterization and Photophysics

Graphene is an outstanding material, consisting of a two-dimensional (2D) single layer of sp2-hybridized carbon atoms bonded together in a hexagonal “honeycomb” lattice and presents remarkable electronic and mechanical properties, like quantum hall effect at room temperature, an ambipolar effect along with ballistic conduction of charge carriers, tunable bandgap and high elasticity.1 These exceptional properties of graphene make it an ideal candidate in construction of transparent conducting electrodes that could be used in energy conversion and storage systems. However, the preparation of high quality few-layered graphene materials is still an issue that must be circumvented and solvent assisted exfoliation using flat aromatic organic molecules as exfoliating assistants reveals as an extraordinary tool for that purpose.2 Herein, we present the liquid exfoliation of graphite to few layered graphene sheets together with the non-covalent supramolecular functionalization of exfoliated graphene sheets by the custom synthesized 1-N-methylpiperazinyl-N,N’-di(2-ethylhexyl)-perylene-3,4,9,10-tetracarboxydiimide (Pip-PDI).3 The planar aromatic Pip-PDI has the ability to non-covalently interact with exfoliated graphene sheets, stabilizing them and preventing their reassembly. On the other hand, the presence of the piperazine moiety on the bay position of the PDI core in the Pip-PDI makes it an ideal electron donor, nicely coupled with electron accepting exfoliated graphene, hence forming a novel donor-acceptor nanoensemble. The new PDI-graphene ensemble was characterized by complementary spectroscopic and microscopy techniques. Furthermore, electronic communication between the counterparts of the PDI-graphene nanoensemble was identified and probed by photophysical and redox assays, while prototype photoelectrochemical cells were constructed and evaluated.Financial support from GSRT/NSRF 2007-2013 through action “ARISTEIA ΙΙ” project FUNGRAPH (3150) and CTQ2011-26455 and Prometeo 2012/010 is acknowledged.ReferencesA. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183.S. P. Economopoulos, N. Tagmatarchis, Chem. Eur. J. 2013, 19, 12930.N. Karousis, L. Martin-Gomis, K. Ohkubo, T. Hasobe, A. Sastre-Santos, S. Fukuzumi, N. Tagmatarchis, Manuscript in preparation, 2014.

Effect of electron irradiation on morphological, compositional and electrical properties of nanocluster carbon thin films grown using room temperature based cathodic arc process for large area microelectronics

The influence of 8MeV electron beam bombardment on room temperature grown nanocluster carbon using cathodic arc process has been studied here. Atomic force microscopy (AFM) study shows that surface roughness varies with varying electron doses. High doses of electrons could causes thermal induce graphitization and morphological changes in the films. Raman spectroscopy analysis reveals that G-peak vary from 1555cm−1 to 1570cm−1 and D-peak varying from 1361cm−1 to 1365cm−1 indicating the disorderness and presence of both graphitic and diamond-like phases. Room temperature conductivity changes by two to three orders in magnitude. The conductivity in the films could be due to conduction of charge carriers through neighboring islands of conductive chains. Defect states calculated using the differential technique varies from 8×1017cm−3eV−1 to 1.5×1019cm−3eV−1. Irradiation of nanocluster carbon thin films could be helpful to tune the electrical properties and defect densities of the nanocluster carbon films for various large area, flexible electronic and nano electronic applications.

Terahertz conductivity and ultrafast dynamics of photoinduced charge carriers in intrinsic 3C and 6H silicon carbide

The terahertz (THz) conductivity of photoinduced charge carriers in two common polytypes of silicon carbide, 3C-SiC and 6H-SiC, is studied on picosecond time scales using an optical-pump THz-probe technique. We find that the conductivity, measured from 0.7 to 3 THz, is well described by the Drude model, and obtain a velocity relaxation time of 75 fs, independent of sample and charge-carrier density. In contrast, the carrier relaxation rates in the two polytypes differ by orders of magnitude: in 6H- and 3C-SiC, recombination proceeds on a time scale of few picoseconds and beyond nanoseconds, respectively.

Defect chemistry and transport properties of Nd-doped Pb(ZrxTi1−x)O3

The oxygen partial pressure (pO2) dependent conductivity of Nd3+ doped PZT was investigated at different temperatures in order to get information on the nature of the conducting charge carriers and on the chemical oxygen diffusion coefficient. From a positive slope close to 0.25 in log (conductivity) vs. log (pO2) plots it is concluded that PZT with donor-type Nd3+ shows predominant hole conduction already at moderately high oxygen partial pressures, while under reducing conditions electron conduction prevails. This unexpected occurrence of hole conduction in a nominally donor doped system is interpreted in terms of the only partially controllable loss of lead oxide during preparation, which results in the formation of lead vacancies. Those act as acceptors at the temperatures employed in this study (560–615 °C) and turn the nominally donor doped material into a slightly acceptor doped one. From the conductivity relaxation after partial pressure changes, chemical oxygen diffusion coefficients could be extracted and values of about 6.5·10−9 cm2/s at 560 °C and 2·10−8 cm2/s at 611 °C are found in air.

Electronic properties of p-GaN(Mg) irradiated with reactor neutrons

The effect of irradiation with full-spectrum reactor neutrons and predominantly fast reactor neutrons (up to a fluence of 8 × 1018 cm−2) on the electrical properties of epitaxial p-GaN(Mg) films at different initial doping levels (in the range of hole concentrations p = 1017–1019 cm−3) is analyzed. It is found that neutron irradiation induces an increase in the resistivity of the initial material to 1010 Ω cm at 300 K. It is shown that, at high neutron fluences, the resistivity of the material decreases because of the hopping conduction of charge carriers over radiation defect states. The study of isochronous annealing at 100–1000°C reveals stages of donor-defect (100–300°C, 500–700°C, 750–850°C) and acceptor-defect (300–500°C, 650–800°C) annealing in the neutron-irradiated p-GaN(Mg) samples.

Dielectric relaxation and ac conductivity behaviour of polyvinyl alcohol–HgSe quantum dot hybrid films

Here we report a comparative study on the dielectric relaxation and ac conductivity behaviour of pure polyvinyl alcohol (PVA) and PVA–mercury selenide (HgSe) quantum dot hybrid films in the temperature range 298 K ⩽ T ⩽ 420 K and in the frequency range 100 Hz ⩽ f ⩽ 1 MHz. The prepared nanocomposite exhibits a larger dielectric constant as compared to the pure PVA. The real and imaginary parts of the dielectric constants were found to fit appreciably with the modified Cole–Cole equation, from which temperature-dependent values of the relaxation times, free charge carrier conductivity and space charge carrier conductivity were calculated. The relaxation time decreases with the quantum dot's inclusion in the PVA matrix and with an increase in temperature, whereas free charge carrier conductivity and space charge carrier conductivity increases with an increase in temperature. An increase in ac conductivity for the nanocomposites has also been observed, while the charge transport mechanism was found to follow the correlated barrier hopping model in both cases. An easy-path model with a suitable electrical equivalent circuit has been employed to analyse the temperature-dependent impedance spectra. The imaginary part of the complex electric modulus spectra exhibit an asymmetric nature and a non-Debye type of behaviour, which has been elucidated considering a generalized susceptibility function. The electric modulus spectra of the nanocomposite demonstrate a smaller amplitude and broader width, as compared to the pure PVA sample.

Optical spectroscopy of erbium-doped SrTiO3 crystals

Results of a detailed study of optical absorption and photoluminescence of slightly erbium-doped (~50ppm) strontium titanate (SrTiO3) single crystals are presented. This study allowed us to specify the structure of energy levels that determine the main internal optical transitions within the 4f11 electron configuration of Er3+ ions in SrTiO3:Er crystals at temperatures below the transition from cubic to tetragonal phase. The numbers of energy levels of the studied multiplets obtained at 2K correspond exactly to the theoretically expected numbers of crystal field levels for multiplets of only one type of Er3+ centers with non-cubic symmetry. Experimental results indicate that Er3+ ions most probably substitute for Sr2+ ions acting in SrTiO3 as ionized donors. Such location of Er3+ ions is consistent with both n-type conductivity and substantial free charge carrier absorption in the infrared spectral region observed in the studied SrTiO3:Er single crystals at room temperature.

Electrical characterization of cobalt phthalocyanine/p-silicon heterojunction

We report the fabrication of organic/inorganic heterojunction of cobalt phthalocyanine (CoPc) with p-type silicon (p-Si) using vacuum thermal evaporation. At ambient conditions, the electrical characteristics of the heterojunction are investigated. The optical band gap of CoPc is calculated from absorption spectrum using Tauc׳s law. The electrical characterization of the heterojunction shows rectifying behavior with a rectification ratio (RR) of 316. Different diode parameters are extracted from the current–voltage (I–V) curves, such as ideality factor n, barrier height ϕ, series resistance Rs and shunt resistance Rsh. These parameters are in good agreement with those calculated from the functions of Cheungs and Norde . The conduction of charge carriers through the interface of p-Si/CoPc is also studied. The fabricated heterojunction could be a promising candidate for its potential use in electronic applications.

Charge transport in two dimensions limited by strong short-range scatterers: Going beyond parabolic dispersion and Born approximation

We investigate the conductivity of charge carriers confined to a two-dimensional system with the non-parabolic dispersion $k^N$ with $N$ being an arbitrary natural number. A delta-shaped scattering potential is assumed as the major source of disorder. We employ the exact solution of the Lippmann-Schwinger equation to derive an analytical Boltzmann conductivity formula valid for an arbitrary scattering potential strength. The range of applicability of our analytical results is assessed by a numerical study based on the finite size Kubo formula. We find that for any $N>1$, the conductivity demonstrates a linear dependence on the carrier concentration in the limit of a strong scattering potential strength. This finding agrees with the conductivity measurements performed recently on chirally stacked multilayer graphene where the lowest two bands are non-parabolic and the adsorbed hydrocarbons might act as strong short-range scatterers.

Frequency Dispersion of Conductivity and Complex Impedance in Li2Ge7O15 Single Crystal

Electric properties of lithium heptagermanate Li2Ge7O15 single crystal have been studied in AC external field (101–105 Hz) in the temperature range 300–700 K. It has been shown that below 500 K conductivity σ is determined by polarization processes whereas above this temperature charge transfer phenomena become dominant. It is assumed that at high temperatures conduction is determined by mobile interstitial lithium ions АLi. The spectra of complex impedance ρ* testify for hopping АLi conduction and accumulation of charge carriers near to blocking Pt electrodes.

Electrical transport properties of polycrystalline CVD graphene on SiO2/Si substrate

Abstract Transport behavior of single layer graphene (SLG) grown by chemical vapor deposition technique on copper foil and transferred to SiO 2 /Si substrate has been studied by measuring the dc conductivity and Hall mobility in the temperature range 2–460 K. The samples of size 1 × 1 cm 2 have been found to be polycrystalline in nature. Raman spectrum has been studied at various locations of the sample and the formation of SLG has been confirmed. From dc conductivity and mobility measurements it has been concluded that the one dimensional grain boundary defects are mainly responsible for the deterioration of mobility and conductivity of charge carriers in the polycrystalline samples.

Investigation of electrical and dielectric properties of NaI doped synthesized systems

A polymeric system has been synthesized using Triethylenetetramine (TETA), 1-3 Propane Sultone and Glutaraldehyde (GA) in the ratio 1:1:1. The system is doped with NaI to make it conducting. Its electrical and dielectric properties are discussed in terms of conductivity, mobility and number of charge carriers as a function of salt concentration and temperature. Maximum conductivity (~10−4Scm−1) is obtained for 50% NaI containing system. Before melting phase transition (Tm), conductivity, number of charge carriers, mobility and diffusion coefficient are all increasing with temperature and follow Arrhenius nature but after Tm mobility follows a decreasing trend. Both in ε″ vs logω and tanδ vs logω curves peaks become sharper after melting phase transition. Distribution of relaxation time, decreases with increasing temperature and salt concentration, indicating that material has a tendency to achieve Debye type behavior.

Thermoelectric properties of layered antiferromagnetic CuCrSe2

Here we study thermoelectric and magnetic properties of CuCrSe2 samples sintered at various temperatures. Structural analysis with XRD shows an order-disorder transition for Cr atoms when the sintering temperature is increased above 1273 K. Metal-like electrical resistivity and anomalously large Seebeck coefficient are found about room temperature. Analysis of electrical conductivity and Seebeck coefficient of the partially-disordered phase suggests hopping conduction of charge carriers. For both the ordered and disordered phases magnetic susceptibility follows Curie–Weiss temperature dependence at high temperatures above 150 K and shows an antiferromagnetic transition around 55 K. For the disordered phase, the effective magnetic moment is determined at 3.62 μB; this low value in comparison to the spin only value for Cr3+ of 3.89 μB indicates spin fluctuations in the paramagnetic state. The thermal conductivity in these phases is low and dominated by the lattice contribution. Values for the thermoelectric figure of merit (ZT) at room temperature are estimated to be 0.17 and 0.05 for the ordered and disordered phases, respectively.

Characterization of intrinsic and induced lateral conduction in space dielectric materials

Characterization of lateral charge carrier conduction in space dielectrics is of high importance for the prediction of charging behavior and electrostatic discharges on satellites. In the present paper, a new experimental approach for the analysis of surface conduction, which is not well understood and characterized in the literature, is established and discussed. Though this method, based on the use of two Kelvin probes, we have been able to discriminate between lateral and bulk charge transports and to reveal the presence of an intrinsic lateral conductivity on Teflon® FEP irradiated with low energy electron beam. We demonstrated that lateral intrinsic conductivity is enhanced when incident current density increases and when approaching the sample surface. The experimental results are analyzed through trapping/detrapping and hopping models. Depending on radiation configuration mode, we have revealed as well the presence of a lateral conductivity that is enhanced by radiation ionization processes, and explained as well with a trapping/recombination model.

Charge and Mass Transport Properties of BaCe0.45Zr0.4Y0.15O3-δ

BaCe0.45Zr0.4Y0.15O3-δ (BCZY4015) was synthesized by a solid state reaction method and its charge and mass transport properties were analyzed. The solutions of analytical defect equations were obtained in terms of various defect concentrations to calculate the partial conductivity of various charge carriers. The calculation of partial conductivities showed that p-type conduction dominated the overall conductivity of BCZY4015 across the entire 700–850°C temperature range. The electrical conductivity was monitored by 4-probe dc-conductivity measurement with respect to the changes in oxygen partial pressure (pO2) and water vapor pressure (pH2O) at different temperatures. The electrical conductivity relaxation during oxidation/reduction in −2.25 ≤ log(pO2/atm) ≤ −0.65 range at a fixed pH2O, and during hydration/dehydration in air in −3.15 ≤ log(pH2O/atm) ≤ −2.35 range was recorded, and the surface exchange coefficients and chemical diffusivities of oxygen and hydrogen were calculated from Fick's second law by nonlinear least squares fitting of the conductivity data, as proposed by Yoo et al. [H. I. Yoo, J. Y. Yoon, J. S. Ha, and C. E. Lee, Phys. Chem. Chem. Phys. 10 (2008) 974–982.].

Graphene: A new beginning for semiconductor devices beyond silicon

Currently, graphene is a topic of very active research fields due to its exceptional electronic properties. For various RF circuit applications, including low-noise amplifiers, the unique ambipolar nature of graphene field-effect-transistors can be utilized for high-performance frequency multipliers, mixers, and highspeed radiometers. Direct current (DC) and radio frequency (RF) measurements were performed on graphene field-effect transistors to find out the DC and RF properties. Two sets of GFETs were measured; first chip was fabricated with SiC process and the second with CVD process. The SiC GFET impedance levels were too high to measure RF properties. RF-measurements were performed on CVD GFETs. The CVD GFET cut-off frequency was found to be approximately 80 MHz, which is in the same range as the calculated cut-off frequency. MOSFET small-signal model was used for GFETs and the model parameters are presented .The results of the DC measurements were analyzed and the data was fitted according to an existing device resistance model. The curve-fit to total device resistance gives estimations on parameters such as contact resistance, residual charge carrier concentration and conductivity mobility.

Nanoscale Materials: A General Approach for Fast Detection of Charge Carrier Type and Conductivity Difference in Nanoscale Materials (Adv. Mater. 48/2013)

Nanoscale Materials: A General Approach for Fast Detection of Charge Carrier Type and Conductivity Difference in Nanoscale Materials (Adv. Mater. 48/2013)

A General Approach for Fast Detection of Charge Carrier Type and Conductivity Difference in Nanoscale Materials

A general method using a biased atomic force microscopy tip that allows a qualitative, fast, and reliable determination of key electronic properties such as metallic, n-, or p-doped characteristics has been reported for the first time. This method eliminates the detrimental effect of contact in the traditional transport measurement and is much simpler than the common-electrostatic force microscopy detection method, thus providing a powerful tool for fast characterizations of nanomaterials.

High sensitivity to humidity of an element based on a carbon-nanotube bundle

The results of development and studies of humidity-sensitive elements based on a carbon-nanotube bundle are presented. It has been demonstrated that arrays of nanotubes grown by the low-temperature plasma-chemical method on planar silicon structures have exceptional sensitivity to humidity. The ratio of the structure resistance in dry and humid states is more than 105-fold. Such a high relative resistance change is caused by the character of changes in conductivity of the charge carriers between individual tubes of the bundle upon adsorption of water molecules.

Numerical Simulation and Modeling of Resistive and Recombination Losses in MWT Solar Cells

This study analyzes the impact of resistive and recombination losses in metal wrap through (MWT) solar cells through technology computer aided design (TCAD) numerical simulations. Two types of MWT architectures are considered in this study: “point busbar,” featuring one circular tabbing contact for each via at the back side, and “continuous busbar,” in which the rear busbar connects all the vias along a line. A comparison with conventional, H-pattern, front contact (FC) solar cells is performed by adopting the surface recombination velocity at the rear-contact isolation region as a parameter representative of possible passivation options. The differences under dark and light conditions are highlighted. Moreover, the following resistive losses in MWT cells are investigated: via resistance, shunting effect, and lateral conduction of charge carriers above rear busbar. An analytical model to account for the lateral conduction of charge carriers is proposed and validated by means of numerical simulations. While the advantage of MWT over FC cells is confirmed by simulation, we quantitatively show how the resistive and recombination losses limit the efficiency of MWT cells.