Charge Transport

Modern optoelectronic devices, such as light-emitting diodes, fieldeffect transistors and organic solar cells require well controlled motion of charges for their efficient operation. The understanding of the processes that determine charge transport is therefore of paramount importance for designing materials with improved structure-property relationships. Before discussing different regimes of charge transport in organic semiconductors, we present a brief introduction into the conceptual framework in which we interpret the relevant photophysical processes. That is, we compare a molecular picture of electronic excitations against the Su-Schrieffer-Heeger semiconductor band model. After a brief description of experimental techniques needed to measure charge mobilities, we then elaborate on the parameters controlling charge transport in technologically relevant materials. Thus, we consider the influences of electronic coupling between molecular units, disorder, polaronic effects and space charge. A particular focus is given to the recent progress made in understanding charge transport on short time scales and short length scales. The mechanism for charge injection is briefly addressed towards the end of this chapter. Keywords Charge carrier mobility Á Charge transport Á Organic semiconductors Á Molecular model Á Gaussian disorder model Á SSH model Á Organic optoelectronics Contents

We report on terrylene-3,4:11,12-bis(dicarboximide) (TDI) as electron acceptor for bulk-heterojunction solar cells using poly(3hexyl thiophene) (P3HT) as complementary donor component. Enhanced absorption was observed in the blend compared to pure P3HT. As shown by the very efficient photoluminescence (PL) quenching, the generated excitons are collected at the interface between the donor and acceptor, where they separate into charges which we detect by photoinduced absorption and electron-spin resonance (ESR). Time-of-flight (TOF) photoconductivity measurements reveal a good electron mobility of 10 −3 cm 2 V −1 s −1 in the blend. Nevertheless, the photocurrent in solar cells was found to be surprisingly low. Supported by the external quantum efficiency (EQE) spectrum as well as morphological studies by way of X-ray diffraction and atomic force microscopy, we explain our observation by the formation of a TDI hole blocking layer at the anode interface which prevents the efficiently generated charges to be extracted.

By measuring the drift velocity of electrons in diamond as a function of applied electric field, we
demonstrate that ultra-pure diamond exhibits negative differential electron mobility in the [100] direction
below 140 K. Negative electron mobility is normally associated with III–V or II–VI semiconductors with
an energy difference between different conduction band valleys. The observation of negative mobility in
diamond, an elemental group IV semiconductor, is explained in terms of repopulation effects between
different equivalent conduction band valleys using a model based on the Boltzmann equation.

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

The phase transition of MgzGeO4 from the olivine structure (a-phase) to the spinel structure (Vphase) was determined by complex electrical resistivity measurements in the frequency range 10 Hz up to 100 kHz. The stability fields of the ~-and the y-phase were confirmed up to 2.05 GPa in the temperature range 845 ~ C-1400 ~ C. Based on volume resistivity data, a decrease of about a factor of 5 was found at the e-7 phase transition. Activation energies of electrical conduction E, at 1.05 GPa and 2.05 GPa were calculated using the volume resistivities (p) and the relaxation times (z). The values range from 1.98 eV up to 2.78 eV. The relative dielectric permeability increases with increasing temperature. This is is due to crystal defects and charge transport mechanisms.

Two series of the Li x Mn 2 O 4 spinel samples were studied at low temperatures (200 -300 K) on electrical, thermal (DSC) and structural (X-ray diffraction (XRD)) properties for different lithium contents. Results obtained for deintercalated spinel samples with x f 1 revealed the existence of a broad (100 K) phase transition that can be attributed to the molecular polaron condensation, leading to the orthorhombic distortion of the initial cubic form. The differential scanning calorimetry (DSC) measurement results enable us to regard the phase transition as a form of order -disorder one. Corresponding thermoelectric power (TEP) and electrical conductivity measurements fall within such description, moreover, indicating clear inconsistency between the measured regular DC conductivity of the spinel sample and that observed for the cathode in the working lithium cell. This discrepancy points to an alternative charge transport mechanism existing in the manganese spinel cathode, and it seems to be essential for the lithium cell performance. D

and where his research ranges from the investigation of fundamental physical processes (nonlinear optical activity, charge transport, light harvesting, and emission) in organic nanostructured thin films to the design, fabrication, and testing of lightweight flexible optoelectronic devices based on organic-based materials.

Nanocrystalline particles of ZnO and TiO 2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO 2 . Apparent activation energies for electron transport in nanostructured ZnO of e0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO 2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was about 0.23 V lower than the opencircuit potential at the same light intensity. Results may be explained using a multiple trapping model, but as electrons are usually only shallowly trapped in ZnO, an alternative view is presented. If there is significant doping of the ZnO, resulting band bending in the nanocrystals will form energy barriers for electron transport and recombination that can explain the observed properties.

The electrical conductivity of monocrystalline triphylite, Li(Fe2+,Mn2+)PO4, with the orthorhombic olivine-type structure was measured parallel (∥) to the [010] direction and ∥ [001] (space group Pnma), between ∼400 and ∼700 K. Electrical measurements on triphylite are of technological interest because LiFePO4 is a promising electrode material for rechargeable Li batteries. Triphylite was examined by electron microprobe, ICP atomic emission spectroscopy, X-ray diffraction, Mössbauer spectroscopy and microscopic analysis. The DC conductivity σDC was determined from AC impedance data (20 Hz–1 MHz) extrapolating to zero frequency. Triphylite shows σDC with activated behavior measured ∥ [010] between ∼500 and ∼700 K during the first heating up, with activation energy of E A = 1.52 eV; on cooling E A = 0.61 eV was found down to ∼400 K and extrapolated σDC (295 K) ∼10−9 Ω−1cm−1; ∥ [001] E A = 0.65 eV and extrapolated σDC(295 K) ∼10−9 to 10−10 Ω−1cm−1, measured during the second heating cycle. The enhanced AC conductivity relative to σDC at lower temperatures indicates a hopping-type charge transport between localized levels. Conduction during the first heating up is ascribed to ionic Li+ hopping. DC polarization experiments showed conduction after the first heating up to be electronic related to lowered activation energy. Electronic conduction appears to be coupled with the presence of Li+ vacancies and Fe3+, formed by triphylite alteration. For comparison, σDC was measured on the synthetic compound LiMgPO4 with olivine-type structure, where also an activated behavior of σDC with E A ∼1.45 eV was observed during heating and cooling due to ionic Li+ conduction; here no oxidation can occur associated with formation of trivalent cations.

An analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained. This electrolyte contains more than one electroactive species and any number of non-reacting species. The mass and charge transport equations give rise to an implicit form of a set of non-linear algebraic equations which must be solved numerically. The solution is generally applicable and can deal with any kind of overpotential relation at the electrode. The analytical solution is used to determine the mass and charge transport parameters for the Fe(CN)63-/4-redox couple in a KCI solution for two different electrolyte concentrations (0.03 M Fe(CN)63-/4-+ 1.0M KCI and 0.005 M Fe(CN) 3-/4-+ 0.2M KCi). The agreement between the experimental and analytical current densities is perfect for both electrolyte solutions under investigation. It is shown that, although an excess of supporting electrolyte is added, neglecting migration results in an under/overestimation of the diffusion coefficient of ferri-/ferrocyanide of 3 to 5% for the Fe(CN) 3-/4-/KC! ratios investigated. Furthermore, a mathematical background is given for the wide range of values found in the literature for the charge transfer coefficient and the rate constants of the ferri-/ferrocyanide redox system. The same approach can also be useful for other systems, as to parameter identification procedures.

In conventional planar detection structures, photon absorption efficiency is limited by the thickness of the detector, which is itself limited by charge transport properties in the chosen material. Therefore a trade-off must be found between photons absorption efficiency and charge collection efficiency. To overcome this compromise, alternative detector architecture has been proposed [1], where charge collection is realized perpendicularly to the photon absorption plane. The volume of the detector is micro-structured with an array of columnar electrodes (alternatively anodes and cathodes) which penetrate through the whole bulk. Thanks to this new 3D architecture of the detectors, and as long as electrodes are sufficiently close, various semiconductor materials can be considered for room temperature use.

Solid polymer electrolytes based on poly (vinyl alcohol) (PVA) complexed with sodium fluoride (NaF) at different weight percent ratios were prepared using solution cast technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD data revealed that the amorphous domains of PVA polymer matrix increased with increase of NaF salt concentration. The complexation of the salt with the polymer was confirmed by FT-IR studies. Electrical conductivity was measured in the temperature range of 303-373 K and the conductivity was found to increase with the increase of dopant concentration as well as temperature. The dielectric constant (e 0 ) increased with the increase in temperature and decreased with the increase in frequency. A loss peak was identified at 365 K in the dielectric loss spectra and is attributed to the orientation of polar groups. Measurement of transference number was carried out to investigate the nature of charge transport in these polymer electrolyte films using Wagner's polarization technique and Watanabe technique. Transport number data showed that the charge transport in these polymer electrolyte systems was predominantly due to ions and in particular due to anions. Using these polymer electrolytes, solid state electrochemical cells were fabricated. Various cell parameters like open circuit voltage (OCV), short circuit current (SCC), power density and energy density were determined.

We have investigated charge transport in ZnTPPdT-Pyr molecular junctions using the lithographic MCBJ technique at room temperature and cryogenic temperature (6K). We combined low-bias statistical measurements with spectroscopy of the molecular levels using I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, which is observed in trace histograms and in current-voltage (I(V)) measurements. Both methods show that multiple stable single-molecule junction configurations can be obtained by modulating the inter-electrode distance. In addition we demonstrate that different ZnTPPdT-Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.

The authors report the investigation of the charge transport behaviors in mixed thin films of N , NЈ-diphenyl-N , NЈ-bis͑1-napthyl͒-1,1Ј-biphenyl-4 , 4Ј-diamine and tris͑8-hydroxyquinoline͒ aluminum. The extracted electron and hole drift mobility were found to be sensitive to the compositional fraction and interpreted by energy levels, charge mobilities of neat compounds, and microscopic networks within the mixed systems. The carrier conduction characteristics, therefore, were used to illustrate the electrical and optical properties of the organic light emitting devices with a mixed layer and present direct evidences on the role of the mixed layer in these devices.

Nanotubes and nanowires have sparked considerable interest in biosensing applications due to their exceptional charge transport properties and size compatibility with biomolecules. Among the various biosensing methodologies incorporating these nanostructured materials in their sensing platforms, liquid-gated field-effect transistors (LGFETs)-based device configurations outperform the conventional electrochemical measurements by their ability in providing label free, direct electronic read-out, and real-time detection. Together with integration of a microfluidic channel into the device architecture, nanotube- or nanowires-based LGFET biosensor have demonstrated promising potential toward the realization of truly field-deployable self-contained lab-on-chip devices, which aim to complement the existing lab-based methodologies. This review addresses the recent advances in microfluidic-integrated carbon nanotubes and inorganic nanowires-based LGFET biosensors inclusive of nanomaterials growth, device fabrication, sensing mechanisms, and interaction of biomolecules with nanotubes and nanowires. Design considerations, factors affecting sensing performance and sensitivity, amplification and multiplexing strategies are also detailed to provide a comprehensive understanding of present biosensors and future sensor systems development.

We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd 1Àx Zn x Te) with zinc content x 0X1±0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. #

We present a comprehensive study of the effective Conformal Field Theory (CFT) describing the low energy excitations of a gas of spinless interacting fermions on a circle in the gapless regime (Luttinger liquid). Functional techniques and modular transformation properties are used to compute all correlation functions in a finite size and at finite temperature. Forward scattering disorder is treated exactly. Laughlin experiments on charge transport in a Quantum Hall Fluid on a cylinder are reviewed within this CFT framework. Edge excitations above a given bulk excitation are described by a twisted version of the Luttinger effective theory. Luttinger CFTs corresponding to the ν = 1/(2p + 1) filling fractions appear to be rational CFTs (RCFT). Generators of the extended symmetry algebra are identified as edge fermions creators and annihilators, thus giving a physical meaning to the RCFT point of view on edge excitations of these sequences.

We apply to fixed charge network flow (FCNF) problems a general hybrid solution method that combines constraint programming and linear programming. FCNF problems test the hybrid approach on problems that are already rather well suited for a classical 0-1 model. They are solved by means of a global constraint that generates specialized constraint propagation algorithms and a projected relaxation that can be rapidly solved as a minimum cost network flow problem. The hybrid approach ran about twice as fast as a commercial mixed integer programming code on fixed charge transportation problems with its advantage increasing with problem size. For general fixed charge transshipment problems, however, it has no effect because the implemented propagation methods are weak.

A chemical vapor deposition (CVD) procedure has been used for the synthesis of boron-doped carbon microspheres (CSMs) using BF 3 /MeOH as the boron source, and acetylene as the carbon source. The boron-doped carbon microsphere samples were characterized by transmission electron microscopy (TEM), Raman spectroscopy and laser ablation mass spectrometry analysis. The average diameter and the shell thickness of the carbon microspheres are strongly influenced by the boron content. The intensity of the D-band laser excitation line increased after the boron incorporation into the carbon microspheres. Electrical conductivity of the boron-doped carbon microspheres has been measured. The conductivity of the B-doped microsphere sample is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which impedes the charge transport in this material compared to the undoped material.

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Einstein's early thoughts about superconductivity are discussed as a case study of how theoretical physics reacts to experimental findings that are incompatible with established theoretical notions. One such notion that is discussed is the model of electric conductivity implied by Drude's electron theory of metals, and the derivation of the Wiedemann-Franz law within this framework.

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic ...

Recent advances in nanobiotechnology involve the use of biomolecule-nanoparticle (NP) hybrid systems for bioelectronic applications. This is exemplified by the electrical contacting of redox enzymes by means of Au-NPs. The enzymes, glucose oxidase, GOx, and glucose dehydrogenase, GDH, are electrically contacted with the electrodes by the reconstitution of the corresponding apo-proteins on flavin adenine dinucleotide (FAD) or pyrroloquinoline quinone (PQQ)-functionalized Au-NPs (1.4 nm) associated with electrodes, respectively. Similarly, Au-NPs integrated into polyaniline in a micro-rod configuration associated with electrodes provides a high surface area matrix with superior charge transport properties for the effective electrical contacting of GOx with the electrode. A different application of biomolecule-Au-NP hybrids for bioelectronics involves the use of Au-NPs as carriers for a nucleic acid that is composed of hemin/G-quadruplex DNAzyme units and a detecting segment complementary to the analyte DNA. The functionalized Au-NPs are employed for the amplified DNA detection, and for the analysis of telomerase activity in cancer cells, using chemiluminescence as a readout signal.

The charge transport through a single ruthenium atom clamped by two terpyridine hinges is investigated, both experimentally and theoretically. The metal-bis(terpyridyl) core is equipped with rigid, conjugated linkers of para-acetyl-mercapto phenylacetylene to establish electrical contact in a two-terminal configuration using Au electrodes. The structure of the [Ru II (L) 2 ](PF 6 ) 2 molecule is determined using single-crystal X-ray crystallography, which yields good agreement with calculations based on density functional theory (DFT). By means of the mechanically controllable break-junction technique, current-voltage (I-V), characteristics of [Ru II (L) 2 ](PF 6 ) 2 are acquired on a single-molecule level under ultra-high vacuum (UHV) conditions at various temperatures. These results are compared to ab initio transport calculations based on DFT. The simulations show that the cardan-joint structural element of the molecule controls the magnitude of the current. Moreover, the fluctuations in the cardan angle leave the positions of steps in the I-V curve largely invariant. As a consequence, the experimental I-V characteristics exhibit lowest-unoccupied-molecular-orbit-based conductance peaks at particular voltages, which are also found to be temperature independent.

The electrical conductivity, current–voltage (I–V) characteristics and optical absorption of pure and AgNO3-doped polyvinyl alcohol (PVA) films have been studied under different conditions. The electrical conductivity increased with increasing dopant concentration up to 0.5 wt.% of the dopant and then showed a decrease beyond this concentration. The increase in conductivity for dopant concentration up to 0.5 wt.% is attributed to formation of charge transfer complexes while the decrease for concentrations above 0.5 wt.% may be due to segregation. The variation of electrical conductivity with temperature showed two regions of activation with activation energies 0.8 and 0.3 eV for undoped films and 1.3 and 0.3 eV for doped films. I–V characteristics indicated that Schottky emission is the dominant charge transport mechanism in both undoped and AgNO3-doped films. Optical absorption studies in the wavelength range 190–600 nm showed additional peaks in the wavelength region 391–417 nm for differently doped films, in addition to the peak at 273 nm for undoped PVA. The optical energy gaps and band edge values shifted to lower energies on doping up to a dopant concentration of 0.5 wt.% but showed an increasing tendency for further increase in dopant concentration.

We consider charge transport properties of 2+1 dimensional conformal field theories at non-zero temperature. For theories with only Abelian U(1) charges, we describe the action of particle-vortex duality on the hydrodynamic-to-collisionless crossover function: this leads to powerful functional constraints for self-dual theories. For N =8 supersymmetric, SU(N ) Yang-Mills theory at the conformal fixed point, exact hydrodynamic-to-collisionless crossover functions of the SO(8) Rcurrents can be obtained in the large N limit by applying the AdS/CFT correspondence to Mtheory. In the gravity theory, fluctuating currents are mapped to fluctuating gauge fields in the background of a black hole in 3+1 dimensional anti-de Sitter space. The electromagnetic self-duality of the 3+1 dimensional theory implies that the correlators of the R-currents obey a functional constraint similar to that found from particle-vortex duality in 2+1 dimensional Abelian theories.

We construct a theory of charge transport by the surface states of topological insulators in three dimensions. The focus is on the experimentally relevant case when the electron doping is such that the Fermi energy $\epsilon_F$ and transport scattering time $\tau$ satisfy $\epsilon_F \tau/\hbar \gg 1$, but sufficiently low that $\epsilon_F$ lies below the bottom of the conduction band. Our theory is based on the spin density matrix and takes the quantum Liouville equation as its starting point. The scattering term is determined accurately to linear order in the impurity density. We consider scattering by charged impurities and short-range scatterers such as surface roughness. We calculate also the polarization function in topological insulators, emphasizing the differences from graphene. We find that the main contribution to the conductivity is $\propto n_i^{-1}$, where $n_i$ is the impurity density, and will have different carrier density dependencies for different forms of scattering. Two different contributions to this conductivity are traced to the scalar and spin-dependent terms in the Hamiltonian and their relative weight depends on the doping density. Our results contain all contributions to the conductivity to orders zero and one in the impurity density. We discuss also a way to determine the dominant scattering angles by studying the ratio of the transport relaxation time to the Bloch lifetime as a function of the Wigner-Seitz radius $r_s$. We also discuss the effect on the surface states of adding metallic contacts.

Complex impedance spectroscopy (CIS) analysis on polycrystalline lanthanum modified lead zirconate titanate (PLZT) has been carried out to investigate its electrical, dielectric and electromechanical properties. CIS has been shown to be an important and very convenient experimental tool to correlate the electrical and dielectric behaviour of ferroelectric ceamics with their microstructure/structure in terms of the relaxations of defect species. The physical structure of the sample pellet has been visualized as comprising of grains separated by grain boundary in accordance with the impedance spectrum observations. The electrical processes in the sample has been modeled in the form of an electrical equivalent circuit made up of a series combination of two parallel RC circuits attributed to grains and grain boundaries. The real and imaginary parts of dielectric constants have been evaluated from impedance spectrum results and expressed in terms of complex permittivity spectrum (Cole-Cole plot). The dielectric phenomenon in PLZT has been observed to be of polydispersive nature with marked deviation from Debye type relaxation process. This result has also been confirmed by complex modulus analysis. Conductivity dispersion has been observed throughout the frequency range in the present investigation up to 400 • C beyond which low frequency dispersion and high frequency plateau was found. It has been attributed to Warburg type diffusion phenomenon arising due to ion migration/charge transport at higher temperature.

A quasi-two-dimensional charge transport model of AlGaN/GaN high electron mobility transistor has been developed that is capable of accurately predicting the drain current as well as small-signal parameters such as drain conductance and device transconductance. This model built up with incorporation of fully and partially occupied subbands in the interface quantum well, combined with a numerically self-consistent solution of the Schro¨dinger and Poisson equations. In addition, nonlinear polarization effects, self-heating, voltage drops in the ungated regions of the device are also taken into account. Also, to develop the model, the accurate two-dimensional electron gas mobility and the electron drift velocity have been used. The calculated model results are in very good agreement with existing experimental data for Al m Ga 1Àm N/GaN HEMT devices with Al mole fraction within the range from 0.15 to 0.50, especially in the linear regime of I-V curve. r

Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numerical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent Manuscript He is currently an Associate Professor of electronics with the University of Brescia and teaches courses on electronic devices and power electronics. He is the author of more than 100 scientific publications, including international journals, conference proceedings, and international patents. His research interests include various aspects of microelectronics, including device physics and characterization, current transport in semiconductor materials and devices, amorphous and organic thin-film transistors, submicrometer metal-oxide-semiconductor field-effect-transistor design, the numerical analysis of semiconductor devices, integrated analog circuit design, electromagnetic interference analysis, low-power portable ultrawideband radars, and integrated voltage converters. Zsolt M. Kovacs-Vajna (M'90-SM'00) received the D.Eng. degree (with honors) and the Ph.D. degree in electronics engineering and computer sciences from

The three-dimensional topological insulator (originally called "topological insulators") is the first example in nature of a topologically ordered electronic phase existing in three dimensions that cannot be reduced to multiple copies of quantum-Hall-like states. Their topological order can be realized at room temperatures without magnetic fields and they can be turned into magnets and exotic superconductors leading to world-wide interest and activity in topological insulators. These experimentally demonstrated unique properties of 3D topological insulators have turned the topic into a field of intense experimental activity. It is also only the third topologically ordered phase in weakly interacting systems to be discovered in nature, with the first two belonging to the quantum Hall-like topological insulator class consisting of the 2D integer quantum Hall state (IQH) and the 2D quantum spin Hall state (QSH). The 2D quantum spin Hall state (QSH) can be thought of as two copies of IQH put together leading to a timereversal invariant version of IQH state. All of the 2D topological insulator examples (IQH, QSH) including the fractional one (FQH) involving Coulomb interaction are understood in the standard picture of quantized electron orbits in a spin-independent or spin-dependent magnetic field, the 3D topological insulator defies such description and is a novel type of topological order which cannot be reduced to multiple copies of quantum-Hall-like states. In fact, the 3D topological insulator exists not only in zero magnetic field, and differs from the 2D variety in three very important aspects: 1) they possess topologically protected 2D metallic surfaces (a new type of 2DEG) rather than the 1D edges, 2) they can work at room temperature (300K and beyond, largegap topological insulators) rather than cryogenic (mK) temperatures required for the QSH effects and 3) they occur in standard bulk semiconductors rather than at buried interfaces of ultraclean semiconductor heterostructures thus tolerate stronger disorder than the IQH-like states. One of the major challenges in going from quantum Hall-like 2D states to 3D topological insulators is to develop new experimental approaches/methods to precisely probe this novel form of topological-order since the standard tools and settings that work for IQH-state also work for QSH states. The method to probe 2D topological-order is exclusively with charge transport (pioneered by Von Klitzing in the 1980s), which either measures quantized transverse conductance plateaus in IQH systems or longitudinal conductance in quantum spin Hall (QSH) systems. In a 3D topological insulator, the boundary itself supports a two dimensional electron gas (2DEG) and transport is not (Z2) topologically quantized hence cannot directly probe the topological invariants νo or the topological quantum numbers analogous to the Chern numbers of the IQH systems. This is unrelated to the fact that the present materials have some extrinsic or residual/impurity conductivity in their naturally grown bulk. In this paper, we review the birth of momentum-and spin-resolved spectroscopy as a new experimental approach and as a directly boundary sensitive method to study and prove topological-order in three-dimensions via the direct measurements of the topological invariants νo that are associated with the Z2 topology of the spin-orbit band structure and opposite parity band inversions, which led to the experimental discovery of the first 3D topological insulator in Bi-Sb semiconductors (KITP Proceeding 2007 which further led to the discovery of the Bi2Se3 class -the most widely researched topological insulator to this date. We discuss the fundamental properties of the novel topologically spin-momentum locked half Dirac metal on the surfaces of the 3D topological insulators and how they emerge from topological phase transitions due to increasing spin-orbit coupling in the bulk. These electronic and spin properties of topological surface states discovered via the methods reviewed here are already guiding the interpretation of surface transport measurements as they are beginning to be possible further advancing the field potentially towards device applications. These methods and their derivatives are also being applied by many others world-wide for further investigations of topological-order and for discovering new topological insulator states as well as exotic topological quantum phenomena (the list is too long to review here). We also review how spectroscopic methods are leading to the identification of spin-orbit superconductors that may work as Majorana platforms and can be used to identify topological superconductors -yet another class of new state of matter.

Charge transport in amorphous chalcogenide-GST used for memory devices is modeled using two contributions: hopping of trapped electrons and motion of band electrons in extended states. The type of feedback that produces the snapback phenomenon is described as a filamentation in energy that is controlled by electron-electron interactions between trapped electrons and band electrons. The model thus derived is implemented within a state-of-the-art simulator. An analytical version of the model is also derived and is useful for discussing the snapback behavior and the scaling properties of the device.

We report on both the intrinsic and the extrinsic stability of a formamidinium lead bromide [CH(NH 2) 2 PbBr 3 = FAPbBr 3 ] perovskite solar cell that yields a high photovoltage. The fabrication of FAPbBr 3 devices, displaying an outstanding photo-voltage of 1.53 V and a power conversion efficiency of over 8%, was realized by modifying the mesoporous TiO 2 −FAPbBr 3 interface using lithium treatment. Reasons for improved photovoltaic performance were revealed by a combination of techniques, including photothermal deflection absorption spectroscopy (PDS), transient-photovoltage and charge-extraction analysis, and time-integrated and time-resolved photoluminescence. With lithium-treated TiO 2 films, PDS reveals that the TiO 2 −FAPbBr 3 interface exhibits low energetic disorder, and the emission dynamics showed that electron injection from the conduction band of FAPbBr 3 into that of mesoporous TiO 2 is faster than for the untreated scaffold. Moreover, compared to the device with pristine TiO 2 , the charge carrier recombination rate within a device based on lithium-treated TiO 2 film is 1 order of magnitude lower. Importantly, the operational stability of perovskites solar cells examined at a maximum power point revealed that the FAPbBr 3 material is intrinsically (under nitrogen) as well as extrinsically (in ambient conditions) stable, as the unsealed devices retained over 95% of the initial efficiency under continuous full sun illumination for 150 h in nitrogen and dry air and 80% in 60% relative humidity (T = ∼60 °C). The demonstration of high photovoltage, a record for FAPbBr 3 , together with robust stability renders our work of practical significance.

Igli Larashi-Transporti Ajror, çmimet e biletave, kostot e fluturimit dhe kostot e shërbimeve në Shqipëri. All rights reserved 2017.04.27 Transporti Ajror, çmimet e biletave, kostot e fluturimit dhe kostot e shërbimeve në Shqipëri. Eshtë folur shumë nga mediat vendase të cilat kane shkruajtur dhe diskutuar pse në Shqipëri mungojnë kompanitë që ofrojnë çmimet e ulta çdo periudhë të sezonit dhe në çdo destinacion. Ka patur shumë diskutime dhe gishtin mediat gjithmonë e kanë drejtuar nga çmimet aeroportuale të shërbimeve të Aeroportit Nenë Tereza. Por kjo është vetëm njëra nga disa arsye po aq të rëndësishme në mbartjen e situates që kemi sot. Në ditët e sotme, çdo kompani përballet me tre kostot kryesore të cilat janë të detyrueshme të paguhen për të realizuar fluturimet. Njëri është çmimet e korridoreve ajrore, të cilat janë të centralizuara dhe të pa negociushme. Për të lehtësuar këtë barrë kompanitë detyrohen të drejtojnë fluturimet e tyre nga ato rrugë ku këto çmime janë më të lirat. Faktori I dytë është çmimi I karburantit. Ky është I negocushëm në varësi të volumit të fluturimeve. Dmth çdo kompani sa më shumë fluturoin dhe merr karburant nga një ofrues aq më ulje pëson çmimi I karburantit. Dhe së treti, janë çmimet aeropoortuale, të shërbimeve që marrin kompanitë dhe të tarifave të uljeve dhe ngritjeve të avionave. Këto diktojnë pjesën e dukshme të kostove që një kompani ka në mënyrë të detyrueshme të mbulojë për të realizur një fluturim, por jo të gjithë tabelën e kostove të cilat shtohen nga siguria në avion gjatë fluturimit, mirëmbajtja teknike e avionëve, pagesa e stafit, trajnimi I stafit e kështu me

Charge transport mechanism in 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenyl amino)-9,9′-spirobifluorene (spiro-MeOTAD) has been investigated as a function of temperature and organic layer thicknesses. Hole only devices of different thicknesses were fabricated in configuration ITO/spiro-MeOTAD/Au by vacuum evaporation technique. The hole current is space charge limited which provides a direct measurement of the hole mobility μ as a function of electric field and temperature. Gaussian disorder model has been used to explain the temperature and field dependent behavior of mobility. The values of energetic disorder (σ = 0.088 eV), positional disorder (Σ = 3.35) and mobility prefactor (μ0 = 0.0147 cm2/V s) have been evaluated using this model.

Charge transport and shelf-degradation of MEH-PPV thin-films were investigated through stationary (e.g. current versus voltage -JxV) and transient (e.g. Time-of-Flight -ToF, Dark-Injection Space-Charge-Limited Current -DI-SCLC, Charge Extraction by Linearly Increasing Voltage -CELIV) current techniques. Charge carrier mobility in nanometric films was best characterized through JxV and DI-SCLC. It approaches 10 − 6 cm 2 /Vs under a SCLC regime with deep traps for light-emitting diode applications. ToF measurements performed on micrometric layers (i.e.~3 μm) confirmed studies in 100 nm-thick films as deposited in OLEDs. All results were comparable to a similar poly(para-phenylene vinylene) derivative, MDMO-PPV. Electrical properties extracted from thin-film transistors demonstrated mobility dependence on carrier concentration in the channel (~10 − 7 -10 − 4 cm 2 /Vs). At low accumulated charge levels and reduced free carrier concentration, a perfect agreement to the previously cited techniques was observed. Degradation was verified through mobility reduction and changes in trap distribution of states.

Coatings prepared from polyaniline-nano-TiO 2 particles synthesized by in situ polymerization were found to exhibit excellent corrosion resistance much superior to polyaniline (PANI) in aggressive environments. The corrosion studies were carried out on steel plates coated with these formulations containing 10 wt% polyaniline prepared with different concentrations of nano-TiO 2 . The electrochemical impedance spectroscopy was studied at periodic intervals during exposure to hot saline (65 • C) conditions for prolonged durations over a period of 90 h. The open circuit potential (OCP) was found to shift with time from −0.38 V SCE to more anodic side (−0.2 V SCE) much above that of bare steel (−0.5 V SCE). The presence of nano-TiO 2 was found to be vital in the prevention of corrosion and the shift of OCP to anodic side. From these data, one could envisage more than 100 times improvement in the corrosion resistance especially for polyaniline prepared with 4.18 wt% nano-TiO 2 . The exceptional improvement of performance of these coatings has been associated with the increase in barrier to diffusion, prevention of charge transport by the nanosize TiO 2 , redox properties of polyaniline as well as very large surface area available for the liberation of dopant due to nano-size additive.