Showing papers on "Charge density published in 2004"

Control of carrier density by self-assembled monolayers in organic field-effect transistors

Abstract: Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO2 gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO2 gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.

Ab initio structure solution by charge flipping.

Abstract: In this paper, an extremely simple structure solution method termed charge flipping is presented. It works ab initio on high-resolution X-ray diffraction data in the manner of Fourier recycling. The real-space modification simply changes the sign of charge density below a threshold, while in reciprocal space the moduli Fobs are retained resulting in an Fobs map without weighting. The algorithm is tested using synthetic data for a wide range of structures, the solution statistics are analysed and the quality of reconstruction is checked. Finally, mathematical aspects of the algorithm are considered in detail, and these show that in this chaotic iteration process the solution is a limit cycle and not a fixed point.

Organic single-crystal field-effect transistors

Abstract: Organic electronics constitute an innovative field, with interesting applications complementary to the silicon semiconductor technology. From a scientific perspective, there is large interest in the fundamental understanding of electrical transport in organic semiconductors. However, a well-developed microscopic description is still lacking, due to the complicated character of the many-body polaronic-type of charge carriers in molecular compounds. In this Thesis, we have experimentally studied the intrinsic charge transport properties of organic semiconductors by using organic single-crystal field-effect transistors. The electric field-effect has been frequently used to investigate thin films of organic compounds. Unfortunately, thin-film transistors are not suitable for the study of intrinsic electronic properties of organic conductors, because their characteristics are often strongly affected by imperfections of the film structure and by insufficient purity of organic materials. Thus, for a higher degree of molecular ordering and an improved quality of the FET, we fabricate devices on the surface of a free-standing single crystal of organic molecules. In short, in this work we have achieved successful fabrication of high-quality single-crystal FETs, exhibiting high mobilities and signs of intrinsic transport. Herewith, we have identified new aspects that influence charge transport in organic semiconductor FETs, and we have performed exploratory measurements in the charge density regime approaching one carrier per molecule.

Strong spin-orbit splitting on bi surfaces.

Abstract: Using first-principles calculations and angle-resolved photoemission, we show that the spin-orbit interaction leads to a strong splitting of the surface-state bands on low-index surfaces of Bi. The dispersion of the states and the corresponding Fermi surfaces are profoundly modified in the whole surface Brillouin zone. We discuss the implications of these findings with respect to a proposed surface charge density wave on Bi(111) as well as to the surface screening, surface spin-density waves, electron (hole) dynamics in surface states, and to possible applications to the spintronics.

Ion Transport in Nanofluidic Channels

Abstract: Theoretical modeling of ionic distribution and transport in silica nanotubes, 30 nm in diameter and 5 μm long, suggest that when the diameter is smaller than the Debye length, a unipolar solution of counterions is created within the nanotube and the colons are electrostatically repelled By locally modifying the surface charge density through a gate electrode, the ion concentration can be depleted under the gate and the ionic current can be significantly suppressed It is proposed that this could form the basis of a unipolar ionic field-effect transistor

All-electron self-consistent GW approximation: application to Si, MnO, and NiO.

Abstract: We present a new kind of self-consistent $GW$ approximation based on the all-electron, full-potential linear muffin-tin orbital method. By iterating the eigenfunctions of the $GW$ Hamiltonian, self-consistency in both the charge density and the quasiparticle spectrum is achieved. We explain why this form of self-consistency should be preferred to the conventional one. Some results for Si (a representative semiconductor) are presented. Finally we consider many details in the electronic structure of the antiferromagnetic insulators MnO and NiO. Excellent agreement with experiment is shown for many properties, suggesting that a Landau quasiparticle (energy band) picture provides a reasonable description of electronic structure even in these correlated materials.

Electrochemomechanical Energy Conversion in Nanofluidic Channels

Abstract: When the Debye length is on the order of or larger than the height of a nanofluidic channel containing surface charge, a unipolar solution of counterions is generated to maintain electrical neutrality A pressure-gradient-driven flow under such conditions can be used for ion separation, which forms the basis for electrochemomechanical energy conversion The current−potential (I−φ) characteristics of such a battery were calculated using continuum dynamics When the bulk concentration is large and the channel does not become a unipolar solution of counterions, both the current and potential become small On the other hand, when bulk concentration is so much smaller, the mass diffusion becomes the rate-controlling step and the potential drops rapidly in the high current density region When the Debye length of the solution is about half of the channel height, the efficiency is maximized

Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations

Abstract: We propose a model to determine the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex permittivity is used as a simplified model to describe a biological cell. The measurement takes place between a pair of facing microelectrodes in a microchannel filled with a saline solution. The model incorporates various cell parameters, such as dielectric properties, size and position in the channel. A 3D finite element model is used to evaluate the magnitude of the electric field in the channel and the resultant changes in charge densities at the measurement electrode boundaries as a cell flows past. The charge density is integrated on the electrode surface to determine the displacement current and the channel impedance for the computed frequency range. The complete impedance model combines the finite element model, the electrode-electrolyte interface impedance and stray impedance, which are measured from a real device. The modeled dielectric complex spectra for various cell parameters are discussed and a measurement strategy for cell discrimination with such a system is proposed. We finally discuss the amount of noise and measurement fluctuations of the sensor.

Charge inversion and flow reversal in a nanochannel electro-osmotic flow.

Abstract: Ion distribution and velocity profiles for electro-osmotic flow in a 3.49 nm wide slit channel with a surface charge density of -0.285 C/m(2) are studied using molecular dynamics simulations. Simulation results indicate that the concentration of the co-ion exceeds that of the counterion in the region 0.53 nm away from the channel wall, and the electro-osmotic flow is in the opposite direction to that predicted by the classical continuum theory. The charge inversion is mainly caused by the molecular nature of water and ions. The flow reversal is caused by the immobilization of counterions adsorbed on the channel wall and due to the charge inversion phenomena.

Density-functional theory of spherical electric double layers and ζ potentials of colloidal particles in restricted-primitive-model electrolyte solutions

Abstract: A density-functional theory is proposed to describe the density profiles of small ions around an isolated colloidal particle in the framework of the restricted primitive model where the small ions have uniform size and the solvent is represented by a dielectric continuum. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-sphere repulsion and a quadratic functional Taylor expansion for the electrostatic interactions. The theoretical predictions are in good agreement with the results from Monte Carlo simulations and from previous investigations using integral-equation theory for the ionic density profiles and the ζ potentials of spherical particles at a variety of solution conditions. Like the integral-equation approaches, the density-functional theory is able to capture the oscillatory density profiles of small ions and the charge inversion (overcharging) phenomena for particles with elevated charge density. In particular, our density-functional theory predicts the formation of a second counterion layer near the surface of highly charged spherical particle. Conversely, the nonlinear Poisson–Boltzmann theory and its variations are unable to represent the oscillatory behavior of small ion distributions and charge inversion. Finally, our density-functional theory predicts charge inversion even in a 1:1 electrolyte solution as long as the salt concentration is sufficiently high.

Ab initio treatment of noncollinear magnets with the full-potential linearized augmented plane wave method

Abstract: The massively parallelized full-potential linearized augmented plane-wave bulk and film program FLEUR for first-principles calculations in the context of density functional theory was adapted to allow calculations of materials with complex magnetic structures---i.e., with noncollinear spin arrangements and incommensurate spin spirals. The method developed makes no shape approximation to the charge density and works with the continuous vector magnetization density in the interstitial and vacuum region and a collinear magnetization density in the spheres. We give an account of the implementation. Important technical aspects, such as the formulation of a constrained local moment method in a full-potential method that works with a vector magnetization density to deal with specific preselected nonstationary-state spin configurations, the inclusion of the generalized gradient approximation in a noncollinear framework, and the spin-relaxation method are discussed. The significance and validity of different approximations are investigated. We present examples to the various strategies to explore the magnetic ground state, metastable states, and magnetic phase diagrams by relaxation of spin arrangements or by performing calculations for constraint spin configurations to invest the functional dependence of the total energy and magnetic moment with respect to external parameters.

Description of bipolar charge transport in polyethylene using a fluid model with a constant mobility: model prediction

Abstract: We present a conduction model aimed at describing bipolar transport and space charge phenomena in low density polyethylene under dc stress. In the first part we recall the basic requirements for the description of charge transport and charge storage in disordered media with emphasis on the case of polyethylene. A quick review of available conduction models is presented and our approach is compared with these models. Then, the bases of the model are described and related assumptions are discussed. Finally, results on external current, trapped and free space charge distributions, field distribution and recombination rate are presented and discussed, considering a constant dc voltage, a step-increase of the voltage, and a polarization–depolarization protocol for the applied voltage. It is shown that the model is able to describe the general features reported for external current, electroluminescence and charge distribution in polyethylene.

Differential charge sensing and charge delocalization in a tunable double quantum dot.

Abstract: We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors allows the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, demonstrating that local charge sensing can be used to accurately determine the interdot coupling in the absence of transport.

Theory of spin-charge-coupled transport in a two-dimensional electron gas with Rashba spin-orbit interactions

Abstract: We use microscopic linear response theory to derive a set of equations that provide a complete description of coupled spin and charge diffusive transport in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO) interaction. These equations capture a number of interrelated effects including spin accumulation and diffusion, Dyakonov-Perel spin relaxation, magnetoelectric, and spin-galvanic effects. They can be used under very general circumstances to model transport experiments in 2DEG systems that involve either electrical or optical spin injection. We comment on the relationship between these equations and the exact spin and charge density operator equations of motion. As an example of the application of our equations, we consider a simple electrical spin injection experiment and show that a voltage will develop between two ferromagnetic contacts if a spin-polarized current is injected into a 2DEG, that depends on the relative magnetization orientation of the contacts. This voltage is present even when the separation between the contacts is larger than the spin diffusion length.

Effect of polyethylene interface on space charge formation

Abstract: This paper reports on an investigation into the space charge formation and decay at different material interfaces. In particular, the influence of the interface between electrode and polymer or polymer and polymer on the space charge dynamics has been studied. Planar samples were subjected to high DC electric stresses for extended periods of time and space charge measurements taken using the pulsed electroacoustic (PEA) technique. It has been found that the types of interface between electrode and polymer play a significant role in determining the charge distribution in the insulation and that the interface between polymer and polymer acts as a potential barrier to electrons while allowing positive charge carriers through easily.

Quantum-mechanical investigation of field-emission mechanism of a micrometer-long single-walled carbon nanotube.

Abstract: A quantum-mechanical simulation is carried out to investigate the charge distribution and electrostatic potential along a 1 microm long (5,5) single-walled carbon nanotube under realistic field-emission experimental conditions. A single layer of carbon atoms is found sufficient to shield most of the electric field except at the tip where strong field penetration occurs. The penetration leads to a nonlinear decrease of potential barrier for emission, which is equally responsible for the low threshold voltage besides the well-known geometrical field enhancement factor.

Nonuniform total-dose-induced charge distribution in shallow-trench isolation oxides

Abstract: A new approach for modeling the radiation-induced charge distribution in shallow-trench isolation (STI) structures shows that much less charge is trapped near the top of the trench. We found that charges inside the STI oxide are pushed down by the vertical electric field coming from the positive gate bias, leaving much less total-dose-induced charge close to the top of trench. This nonuniformity significantly affects the measured leakage current.

Measurement of polarization charge and conduction-band offset at InxGa1−xN/GaN heterojunction interfaces

Abstract: The spontaneous and piezoelectric polarization fields in group-III nitride semiconductors lead to the presence of large electrostatic sheet charge densities at nitride semiconductor heterojunction interfaces. Precise quantitative knowledge of these polarization-induced charge densities and of the band-edge discontinuities at nitride heterojunction interfaces is therefore essential in nitride semiconductor device design and analysis. We have used capacitance–voltage profiling to measure the conduction-band offset and polarization charge density at InxGa1−xN/GaN heterojunction interfaces with x=0.054 and x=0.09. We obtain conduction-band offsets ΔEC=0.09±0.07 eV for x=0.054 and ΔEC=0.22±0.05 eV for x=0.09, corresponding to an averaged conduction-to-valence-band offset ratio ΔEC:ΔEV of 58:42. Our measurements yield polarization charge densities of (1.80±0.32)×1012 e/cm2 for x=0.054 and (4.38±0.36)×1012 e/cm2 for x=0.09. These values are smaller than those predicted by recent theoretical calculations, but in good agreement with values inferred from a number of optical experiments.

Force microscopy with light-atom probes.

Abstract: The charge distribution in atoms with closed electron shells is spherically symmetric, whereas atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 picometers in the charge density of adatoms on a (001) tungsten surface. We imaged these charge distributions by means of atomic force microscopy with the use of a light-atom probe (a graphite atom), which directly measured high-order force derivatives of its interaction with a tungsten tip. This process revealed features with a lateral distance of only 77 picometers.

Si-E (E = N, O, F) bonding in a hexacoordinated silicon complex: new facts from experimental and theoretical charge density studies.

Abstract: The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si−E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment.

Charge Density Study of Naphthalene Based on X-ray Diffraction Data at Four Different Temperatures and Theoretical Calculations

Abstract: The crystal electron density of naphthalene has been investigated on the basis of highly redundant X-ray diffraction data collected to high resolution at 100, 135, 170, and 205 K and from quantum chemical calculations. An analysis of the X-ray diffraction data showed that for the data collected below 200 K thermal motion can be successfully deconvoluted from the diffraction data. A topological analysis of the resulting static crystal electron-density map revealed that the intramolecular bond critical points have characteristics that are very similar to those of the isolated molecule. There is excellent agreement between the bond critical points corresponding to the intermolecular interactions in the experimental and theoretical crystal electron densities. The strongest intermolecular interaction is a C−H···π interaction that causes a change in the electron distribution of the C−H bond.