Browsing by Subject "Electron transport"
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Item Computational studies of electron transport and reaction rate models for argon plasma(2010-08) Min, Timothy T.; Raja, Laxminarayan L.; Hallock, GaryA validation study was performed on a capacitively coupled argon discharge to determine the most suitable models for chemistry and electron transport. Chemical reaction rate and electron transport models choices include equilibrium or non-equilibrium electron EDFs. Experimental studies performed by our collaborative partners in the Colorado School of Mines. Conditions for the studies are 138, 315, and 618 mTorr where the cycle averaged power varied at 20, 50, and 80 Watts in which the voltage supply was driven at 13.56 MHz. Simulations were performed using pressures and voltage used in experiments. The most accurate case was for 138 mTorr at 50 Watts using a non-Maxwellian EDF based chemistry (called Bolsig+ chemistry) and a constant electron momentum transfer cross section of 20 Angstroms which was computed from Boeuf’s paper; this model accurately modeled power deposition to within 2.6%. Furthermore, species number densities, electron temperature, and sheath thicknesses are obtained. Using Bolsig+ chemistry resulted in 20,000K higher electron temperatures than using Arrhenius chemistry rates. Results indicate that power deposition occurs due to electrons gaining energy from the sheath which in turn bombard neutral species producing metastable argon.Item Item Electron transport in nanoparticle single-electron transistors(2007-12) Luo, Kang, 1976-; Yao, Zhen, Ph. D.Electron transport in nanoparticle single-electron transistors (SETs) is a fruitful method to explore a wide range of physical phenomena at the nanometer scale. In this thesis, we investigate electron transport in SETs incorporating various nanoparticles, including gold nanoparticles in both classical and quantum regimes and Pb nanoparticle in both superconducting and normal states. SETs have been successfully fabricated by incorporating individual gold nanoparticles into the gaps between two electrodes. Although single-electron tunneling behavior is prominent, quantized energy levels cannot be resolved in these SETs due to their relatively large particle sizes. A novel method has been developed to achieve SETs incorporating gold nanoparticles whose sizes are small enough to resolve discrete quantum energy levels. The devices consist of spontaneously-formed ultrasmall gold nanoparticles linked by alkanedithiols to gold electrodes. The devices reproducibly exhibit addition energies of a few hundred meV, which enables the observation of single electron tunneling at room temperature. At low temperatures, resonant tunneling through discrete energy levels in the Au nanoparticles is observed, which is accompanied by the excitations of molecular vibrations at large bias voltage. Having explored the SETs in normal state, we have extended the experiments to superconducting single-electron transistors (SSETs). We first fabricated and characterized Pb superconducting electrodes with nanometer-sized separation. Our observation clearly shows that conventional Barden-Cooper-Schrieffer theory remains valid to interpret the tunneling behavior between two nanometer-spaced Pb electrodes. Furthermore, by incorporating Pb nanoparticles between the two Pb electrodes, we have fabricated SSETs and investigated the transport properties of these devices. In the superconducting state, the conductance is suppressed by a combination of the single electron tunneling effect and the absence of density of states within the superconducting gap. In the suppression regime, the tunneling spectroscopy shows current features that arise from quasiparticle tunneling caused by singularity matching. At low temperature, the features can only be observed for odd charge states in SSETs. At high temperature, the odd-even parity effect is smeared out. Upon application of a magnetic field, the superconducting state is suppressed and single-electron tunneling behavior for normal metallic nanoparticles is recovered.Item Electron transport in single-molecule transistors(2006) Chae, Dong-hun; Yao, Zhen, Ph. D.Item Electronic transport under strong optical radiation and quantum chaos in semiconductor nanostructures(2003) Li, Wenjun; Reichl, L. E.Item Experimental study of fast electrons from the interaction of ultra intense laser and solid density plasmas(2008-08) Cho, Byoung-ick, 1976-; Ditmire, Todd R.A series of experiments have been performed to understand fast electron generation from ultra intense laser-solid interaction, and their transports through a cold material. Using Micro-Electro-Mechanical Systems (MEMS), we contrived various shape of cone and wedge targets. The first set of experiment was for investigating hot electron generations by measuring x-ray production in different energy ranges. K[alpha] and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though K[alpha] is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell (PIC) simulations utilizing PICLS and OOPIC codes. We also investigate hot electron transport in foil, wedge, and cone targets by observing the transition radiation emitted from the targets rear side along with bremsstrahlung x-ray measurement. Twodimensional images and spectra of 800 nm coherent transition radiation (CTR) along with ballistic electron transport analysis have revealed the spatial, temporal, and temperature characteristics of hot electron micro-pulses. Various patterns from different target-laser configurations suggest that hot electrons were guided by the strong static electromagnetic fields at the target boundary. Evidence about fast electron guiding in the cone is also observed. CTR at 400 nm showed that two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and j x B heating, create two populations of electrons at the targets front side and drive them to different directions, with distinct temperatures and temporal characteristics. This interpretation is consistent with the results from 3D-PIC code Virtual Laser Plasma Laboratory (VLPL).Item Fabrication of silicon-based nano-structures and their scaling effects on mechanical and electrical properties(2007-12) Li, Bin, 1974 May 21-; Ho, P. S.Silicon-based nanostructures are essential building blocks for nanoelectronic devices and nano-electromechanical systems (NEMS), and their mechanical and electrical properties play an important role in controlling the functionality and reliability of the nano-devices. The objective of this dissertation is twofold: The first is to investigate the mechanical properties of silicon nanolines (SiNLs) with feature size scaled into the tens of nanometer level. And the second is to study the electron transport in nickel silicide formed on the SiNLs. For the first study, a fabrication process was developed to form nanoscale Si lines using an anisotropic wet etching technique. The SiNLs possessed straight and nearly atomically flat sidewalls, almost perfectly rectangular cross sections and highly uniform linewidth at the nanometer scale. To characterize mechanical properties, an atomic force microscope (AFM) based nanoindentation system was employed to investigate three sets of silicon nanolines. The SiNLs had the linewidth ranging from 24 nm to 90 nm, and the aspect ratio (Height/linewidth) from 7 to 18. During indentation, a buckling instability was observed at a critical load, followed by a displacement burst without a load increase, then a fully recoverable deformation upon unloading. For experiments with larger indentation displacements, irrecoverable indentation displacements were observed due to fracture of Si nanolines, with the strain to failure estimated to be from 3.8% to 9.7%. These observations indicated that the buckling behavior of SiNLs depended on the combined effects of load, line geometry, and the friction at contact. This study demonstrated a valuable approach to fabrication of well-defined Si nanoline structures and the application of the nanoindentation method for investigation of their mechanical properties at the nanoscale. For the study of electron transport, a set of nickel monosilicde (NiSi) nanolines with feature size down to 15 nm was fabricated. The linewidth effect on nickel silicide formation has been studied using high-resolution transmission electron microscopy (HRTEM) for microstructural analysis. Four point probe electrical measurements showed that the residual resistivity of the NiSi lines at cryogenic temperature increased with decreasing line width, indicating effect of increased electron sidewall scattering with decreased line width. A mean free path for electron transport at room temperature of 5 nm was deduced, which suggests that nickel silicide can be used without degradation of device performance in nanoscale electronics.Item Item Kinetics and equilibrium studies of the reduction of copper(II) with a tetradentate bis(pyridyl)-dithioether ligand(Texas Tech University, 1986-12) Castillo, Maria C. G.The reduction of [Cu(pmas)]+2 by hydroxyethylferrocene [HEF] has been studied in the presence of azide, imidazolate and pyridine. Both reactants exhibit a first order dependence at 25.0°C, pH 6.0, I = O.IM (NaNO^) with second order rate constant of 1.3 x 10 M~ s~ . +2/+ The apparent Cu(pmas) self-exchange electron transfer rate constant calculated on the basis of relative Marcus theory (k,, = 4.7 x 10 M s ) agrees well with previous findings on ferrocyto--4 +2 chrome £, Fe(CN)g and Ru (NH^)^ py reactions with +2 [Cu(pmas)] . The copper complex reduction has been studied as a function of temperature, and the enthalpy and entropy of activation are 10.1 Kcal/mol and -6 e.u., respectively. The formation constants of the 1:1 and 2:1 complexes of Cu(pmas)+ 2 with azide were measured spectrophotometrically. The values (K^, = 3.3 x 10 M ) and 5 -1 +2( K ^ ^ = 3 . 5 x l O M ) were compared with Cu(tmpa) [tris(2- yridylmethyl)amine] copper(II) binding constants. The latter 2-1 only presented one value (K^ = 6.6 x 10 M ), up to 0.15 M N^ (25.0°C, pH 6.0, I = 0.2 M), which demonstrates that a thioether sulfur atom is displaced in the uptake of a second N^ nit by Cu(pmas)(N^) . The effect of thioether sulfur displacement by azide ion on the redox reaction rate may be understood entirely through the tendency of N^ to shift the position of the redox equilibrium towards reactant side. Cyclic voltammetry measurements showed that the eduction potentials of Cu(pmas) +2 in the presence of 5 and 150 mM N^ are 307 and 254 mV respectively, as compared with that of free complex, 398 mV. The rate constant is decreased when azide is present (k (25.0°C) = 3 x IO^M'-'-S"-'- pH 6.0, I = 0.2 M). This demonstrates the importance of bond breaking in the activated complex for reduction of five-coordinate Cu(II) to four-coordinate (Cu(I) because the aqueous form has a weakly-held coordinated water molecule, but the azide form has a strongly-retained nitrogen-donor ligand. Also the dependence of the reaction rate on acetonitrile concentration will be described.Item Photosynthetic and respiratory electron transport chains in purple and green phototrophic bacteria(Texas Tech University, 1986-05) Wynn, Richard MaxNot availableItem Protein:protein complexes in plant and bacterial electron transfer systems(Texas Tech University, 1988-05) Gray, Kevin AnthonyThe following study is composed of two main parts. Both are concerned with soluble electron transfer proteins but from two diverse systems. The first part involves the elucidation of the initial steps of methylamine metabolism in the Gram-negative bacterium Paracoccus denitrificans. Oxidation-reduction properties and protein-protein interactions for the soluble constituents are reported. The second part concerns several ferredoxindependent enzymes from spinach. The interaction between ferredoxin and ferredoxin-NADP+ oxidoreductase (FNR); ferredoxin-hioredoxin oxidoreductase (FTR) and ferredoxin-nitrite oxidoreductase (nitrite reductase) is studied.Item Stability of the diamond difference approximation in energy to the Spencer-Lewis equation of electron transport(Texas Tech University, 1987-12) Seth, Daniel L.Consider the Spencer-Lewis equation (S-L) of electron transport in an azimuthally symmetric slab geometry setting with energy restricted to a finite interval. Further, S-L is subject to boundary conditions in the form of known incident particle fluxes at the slab faces. The one-dimensional diamond difference approximation is applied in the energy variable to the continuous slowing down term (i.e., the energy derivative) of S-L, This results in a semi-discrete system of integro-differential equations in the spatial and angular variables (D.E.S-L). The numerical stability of D,E.S-L as an approximation to S-L is demonstrated for solutions of D.E,S-L that belong to an L2 function space. The system of integro-differential equations may be rewritten as a system of integral equations. Under certain reasonable conditions on the data, the existence-uniqueness of solutions of the integral equations in a Banach space of square integrable functions with weighted norms is established. This implies the existence of L2 solutions of the integral equations. Under further assumptions on the data, the solutions of the integral equations are shown to be the required solutions of the integro-differential equations as well. Moreover, if the data are all bounded, the solutions are bounded.Item Thermal and thermoelectric transport measurements of one-dimensional nanostructures(2005) Zhou, Jianhua; Shi, Li, Ph. D.This dissertation presents thermal and thermoelectric transport measurements of onedimensional nanostructures including bismuth telluride (BixTe1-x) nanowires and singlewalled carbon nanotubes (SWCNT). Theoretical calculations have predicted that BixTe1-x nanowires may have enhanced thermoelectric figure of merit defined as ZT = (S2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Our measurements showed that the σ of BixTe1-x nanowires was very close to, and the κ was largely reduced compared to the bulk values at 300 K. For a BixTe1-x nanowire with x ≈ 0.46, the room temperature S of 260 µV/K was 60% higher than that of its bulk counterpart, while small negative S was measured for four nanowires with x ≈ 0.54. High ZT can be expected for BixTe1-x nanowires with optimized x. The unique electron transport and heat dissipation mechanisms in current – carrying metallic and semiconducting SWCNTs were studied with the use of Scanning Probe Microscopy (SPM) methods including Electrostatic Force Microscopy (EFM), Scanning Gate Microscopy (SGM) and Scanning Thermal Microscopy (SThM). For several metallic SWCNTs with low-bias resistance above 40 x 103 Ω, the electrical potential profile along the SWCNTs was linear at a voltage bias of 0.1 V, suggesting that the electron mean free path was shorter than the length of the nanotube at the low bias. Heat dissipation along these metallic SWCNTs was uniform at voltage biases above 0.22 V. For several semiconducting SWCNTs with low-bias resistance as low as 20 x 103 Ω, large conduction barriers were induced by SGM probes at locations where defects existed, and the heat dissipation was uniform at voltage biases above 0.12 V. This observation suggests diffusive and dissipative heat dissipation in semiconducting SWCNTs. A large tip-sample thermal contact resistance has made it challenging to obtain the actual temperature rise of the sample surface using the SThM method. We have developed a nanocontact thermometry technique that can potentially be employed for quantitatively mapping surface temperature profiles of nanoelectronics with spatial resolution below 20 nm. This method was tested with metal interconnect structures.Item Turbulent electron thermal transport in fusion plasmas(2008-05) Kim, Juhyung, 1978-; Horton, C. W. (Claude Wendell), 1942-Electron heat transport at the scale of electron gyroradius are investigated via numerical simulation of a fluid model and a role of E x B shear flow with intermediate E x B shearing rate is explored in Euler's equation. The anomalous transport, enhanced transport due to turbulent electro-magnetic fields caused by plasma instabilities, has been a focus of the inter-national fusion research communities. Among the instabilities, the drift-type instabilities from the pressure-gradient universal in magnetic fusion devicesare considered responsible for the anomalous transport. In the current status of wide use of wave heating on electrons and subsequent high core electron temperature, the turbulent heat loss through electrons has one of the most important science element in preventing the large fusion tokamaks from reaching breakeven in the past decade. The Electron Temperature Gradient fluid model consists of electrostatic potential, toroidal magnetic flux function and electron temperature (or pressure) describing electron drift waves. The fluid model proves to be highly useful to electron heat flux analysis in fusion machines. We analyze the discharges in National Spherical Tokamak eXperiment(NSTX) and Tokamak Configuration Variable (TCV) and found that the electron thermal diffusivities can be explained in terms of the mixing length argument based on electron gyroradius, linear theory and our nonlinear fluid simulation. The nonlinear fluid model predicts reasonable heat flux observed in the experiments. Based on the analysis, we investigate the dependences of the dynamics on the ratio of electron and ion temperature T[subscript e]/T[subscript i] and plasma beta [beta subscript e-]. The nonlinear dynamics such as saturation mechanism of the ETG turbulence and the electromagnetic dynamics in terms of micro-tearing at the scale of electron gyroradius are discussed briegly. In most of plasma confinement devices, the equilibrium radial electric field exists and the turbulence-generated electric field is observed. The coherent structure, called as zonal flow, has been know to be effective to suppress the micro-turbulence. But at intermediate E x B shear, where the vortex eddy turn-over time is comparable to E x B shearing rate, the suppression is weak and the flow shear can leads to vortex amplification through interaction of nonlinear dynamics and shear flow.