This work investigates the electronic properties of very small gold and semiconductor particles using Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Ballistic Electron Emission Spectroscopy (BEES). Complementary theoretical works were also performed. The first theoretical work was to calculate the quantized states in the CdS/HgS/CdS quantum-well-quantum-dot nanocrystals. An eight-band envelope function method was applied to this system. This method treats exactly the coupling between the conduction bands, the light-hole bands, the heavy-hole bands, and the spin-orbit split bands. The contributions of all other bands were taken into account using second order perturbation theory. Gold nanocrystals with diameters of 1.5 nm have discrete energy levels with energy spacings of about 0.2 eV. These values are comparable to the single electron charging energy, which was about 0.5 eV in our experimental configuration. Since bulk gold doesnt have an energy gap, we expect the electron levels both below and above the Fermi level should be involved in the tunneling. Measured spectroscopy data have rich features. In order to understand and relate these features to the electronic properties of the nanocrystals, we developed a tunneling model. This model includes the effect of excited states that have electron-hole pairs. The relaxation between discrete electron energy levels can also be included in this model. We also considered how the nanocrystals affect the BEES current. In this work an ultra-high vacuum and low-temperature STM was re-designed and rebuilt. The BEEM/BEES capabilities were incorporated into the STM. We used this STM to image gold nanocrystals and semiconductor nanocrystals. STS and BEES spectra of gold nanocrystals were collected and compared with calculations.

The current technological age is embodied by a constant push for increased performance and efficiency of electronic devices. This push is particularly observable for technologies that comprise free electron sources, which are used in various technologies including electronic displays, x-ray sources, telecommunication equipment, and spacecraft propulsion. Performance of these systems can be increased by reducing weight and power consumption, but is often limited by a bulky electron source with a high energy demand. Carbon nanotubes (CNTs) show favorable properties for field electron emission (FE) and performance as electron sources. This dissertation details the developments of a uniquely designed Spindt type CNT field emission array (CFEA), from initial concept to working prototype, to specifically prevent electrical shorting of the gate. The CFEA is patent pending in the United States. Process development enabled fabrication of a CFEA with a yield of up to 82%. Furthermore, a novel oxygen plasma etch process was developed to reverse shorting after CNT synthesis. CFEA testing demonstrates FE with a current density of up to 293 μA/cm² at the anode and 1.68 mA/cm² at the gate, with lifetimes in excess of 100 hours. A detailed analysis of eighty tested CFEAs revealed three distinct types of damage. Surprisingly, about half of the damaged chips are not electrically shorted, indicating that the CFEAs are very robust. Potential applications of this technology as cathodes for spacecraft electric propulsion were explored. Exposure to an operating electric propulsion thruster showed no significant effect or damage to the CFEAs, marking the first experimental study of CNT field emitters in an electric propulsion environment. A second effort in spacecraft propulsion is a collaboration with the Air Force Institute of Technology (AFIT). CFEAs are the payload on an AFIT developed Cube Satellite, called ALICE, to test electron emission in the space environment. ALICE has passed flight tests and is awaiting launch scheduled for 5 December 2013.

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No description available.

<p>  </p> <p>Electron emission plays a vital role in many modern technologies, from plasma medicine to heavy ion beams for fusion. An accurate theoretical model based upon the physics involved is critical to efficient operation of devices pushing the boundaries of complexity. The interactions between different electron emission mechanisms can severely alter device performance, especially when operating in extreme conditions. This dissertation studies electron emission from the perspectives of increasing geometric and physical mechanism complexities </p> <p>One half of this dissertation derives new relations for space-charge limited emission (SCLE) in non-planar geometries. SCLE is the maximum stable current that may be produced by electron emission before the electric field of the electrons themselves self-limits further emission. In planar devices, this is modeled by the well-established Child-Langmuir (CL) equation. The Langmuir-Blodgett (LB) equations remain the most commonly accepted theory for SCLE for cylindrical and spherical geometries after nearly a century; however, they suffer from being approximations based on a polynomial series expansion fit to a nonlinear differential equation. I derive exact, fully analytic equations for these geometries by using variational calculus to transform the differential equation into a new form that is fully and exactly solvable. This variational approach may be extended to any geometry and offers a full description of the electric field, velocity, and charge density profiles in the diode. </p> <p>SCLE is also an important mechanism for characterizing the operation of devices with an external magnetic field orthogonal to the electric field. This “crossed-field” problem decreases the limiting current as electrons travel longer, curved paths, effectively storing some charge in the gap (moving parallel to the emitter). At a critical magnetic field called the Hull cutoff, electron paths become so tightly curved that the circuit can no longer be completed, a condition called magnetic insulation. Crossed-field SCLE has been accurately modeled in planar devices by Lau and Christenson. Using the variational approach, I replicate their planar results and extend the calculation to cylindrical geometry, a common choice for magnetron devices. Further, I derive additional equations with simplified assumptions that, for the first time, provide an analytic description of experimental results below the Hull cutoff field. Following this I incorporate a series resistor: device resistance (or impedance) changes non-linearly with current and voltage, so I couple Ohm’s Law (OL) to all the models of crossed-field devices. For devices just below the Hull cutoff, I predict analytically and show in simulation novel bi-modal behavior, oscillating between magnetically insulated and non-insulated modes. With crossed-field device assessment, the variational calculus approach to space-charge may be used for numerous applications, including high power microwave sources, relativistic klystron devices, heavy ion beams, Hall thrusters, and plasma processing. </p> <p>The other half of this dissertation derives analytic theories to solve for emission current with three or more electron emission mechanisms simultaneously. In addition to the CL law, SCLE may also occur in neutral, non-vacuum diodes, modeled by the Mott-Gurney (MG) equation. These are the two limiting mechanisms I study; the other major modality of electron emission is direct electron production, the source of current in the device. Electrons are ejected when impelled by high temperature or electric field at the emission surface. These mechanisms are thermionic (or thermal) emission, modeled by the Richardson-Laue-Dushman (RLD) equation, and field emission, modeled by the Fowler-Nordheim (FN) equation, respectively. Additionally, just as I calculated the impedance of devices operating in a crossed-field configuration, all these models can be similarly coupled to OL. I derive models unifying FN, MG, and CL (with an extension linking OL, mentoring an undergraduate) and RLD, FN, and CL. These models are relevant for modern device design, especially as micro- and nano-scale devices seek to eliminate vacuum requirements and as space and military applications require higher temperature tolerances.</p> <p>While multi-physics models, like the ones described above, are important, the single-physics models (FN, RLD, MG, CL, OL) are still valid (and much easier to use) in their respective asymptotic limits. For example, a circuit behaves purely according to OL for very high resistances, according to MG for very high pressures, and so forth. Importantly, when devices operate in transition regions between these asymptotic limits, <em>none </em>of the asymptotic equations match the predictions of multi-physics models. Yet, intersections between the asymptotic equations are easily found, say for a certain set of voltage, gap distance, and pressure, CL=MG. Since both asymptotic equations give the same prediction, we may conclude that both must be inaccurate for those physical parameters! This gives rise to what I term “nexus theory:” solving two or more asymptotic equations simultaneously to rapidly and accurately predict sets of physical parameters at which multi-physics models (specifically, the physics leading to the “nexus point” parameters, points or curves at which nexus conditions are satisfied) are required for accurate device predictions. In fact, I show that multi-physics models are necessary within roughly one to two orders of magnitude from a nexus. In effect, nexus theory provides a simple, powerful tool to determine how complex a model is necessary for a particular device. Both nexus theory and multi-physics results in this dissertation have been successfully used to design devices to operate in specific transition regimes and identify the resulting device behavior.  </p>

Im Rahmen dieser Dissertation wurden Experimente zur Wechselwirkung von schnellen Atomen mit Oberflächen von Oxidkristallen, Metallkristallen und ultradünnen Oxidfilmen auf Metalloberflächen durchgeführt und modellhaft beschreiben. Die Experimente wurden im Regime der streifenden Streuung für Energien im keV-Bereich durchgeführt. Diese Streugeometrie bietet den Vorteil einer außerordentlich hohen Oberflächensensitivität und somit die Möglichkeit, die kristallographischen Eigenschaften der obersten Atomlage zu untersuchen. Darüber hinaus wurden Experimente zur Bestimmung des Energieverlustes der an den verschiedenen Oberflächen gestreuten Projektile und zur, durch diese Projektile induzierten, Elektronenemission durchgeführt. Die Anregungsenergie für die Elektronenemission und Exzitonen wurde an der Alumina/NiAl(110)- und der SiO2/Mo(112)- Oberfläche für die Streuung von He bestimmt. Durch die Bestimmung der Anzahl von emittierten Elektronen in Abhängigkeit des azimutalen Winkels konnten die Strukturen von obersten Lagen von Adsorbaten mit der Methode der Ionenstrahltriangulation bestimmt werden. / In the framework of the present dissertation the interactions of fast atoms with surfaces of bulk oxides, metals and thin films on metals were studied. The experiments were performed in the regime of grazing incidence of atoms with energies of some keV. The advantage of this scattering geometry is the high surface sensibility and thus the possibility to determine the crystallographic and electronic characteristics of the topmost surface layer. In addition to these experiments, the energy loss and the electron emission induced by scattered projectiles was investigated. The energy for electron emission and exciton excitation on Alumina/NiAl(110) and SiO2/Mo(112) are determined. By detection of the number of projectile induced emitted electrons as function of azimuthal angle for the rotation of the target surface, the geometrical structure of atoms forming the topmost layer of different adsorbate films on metal surfaces where determined via ion beam triangulation.

In dieser Arbeit wird mit Hilfe der koinzidenten Messung des Energieverlusts gestreuter Projektile zusammen mit der Anzahl emittierter Elektronen pro Streuprozess die Wechselwirkung schneller Atome mit Metalloberflächen untersucht. Dabei wird insbesondere die streifende Streuung der Edelgasatome Helium, Neon und Argon an Al(110)-, Al(111)- und Cu(111)-Oberflächen betrachtet. In den ersten Kapiteln wird die Messmethode der streifenden Streuung, der verwendete experimentelle Aufbau sowie der Charakterisierung der untersuchten Metalloberflächen anhand ihrer geometrischen und elektronischen Eigenschaften erläutert. Nach einführenden Betrachtungen zu Energieverlust und Elektronenemission bei der streifenden Streuung von Heliumatomen erfolgt eine detaillierte Diskussion zum Schwellenverhalten der kinetischen Elektronenemission (KE). Dabei wird gezeigt, dass das Schwellenverhalten durch ein Modell basierend auf dem zentralen Stoss zwischen Projektil und Valenzelektronen beschrieben werden kann. Aus den Schwellengeschwindigkeiten werden Informationen über Elektronenimpulse und Elektronenpotentiale vor Metalloberflächen abgeleitet. Ferner wird die Streuung von Neon- und Argonatomen untersucht. Für das System Ne-Al(111) wird ab Senkrechtenergien von ca. 25 eV Elektronenpromotion beobachtet. KE unterhalb der klassischen Schwelle für die Streuung von Neon- und Argonatomen wird im Rahmen von zwei alternativen Modellen interpretiert: der Effekt der Impulsdichteverteilung der Elektronen vor der Oberfläche oder die gleichzeitige Anregung zweier benachbarter Elektronen mit anschließenden Auger-Zerfall. Abschließend die unterschiedlichen KE für axiales und planares Oberflächen-Channeling untersucht und interpretiert. / The kinetic electron emission (KE) in coincidence with the energy loss of noble gas atoms impinging on metal surfaces under grazing angles of incidence is studied. Results for scattering of helium, neon, and argon atoms from Al(111), Al(110), and Cu(111) are presented and discussed. In the first chapters the experimental technique of grazing scattering, the experimental setup, and a geometric and electronic characterization of the surfaces is described. General aspects of projectile energy loss and KE for grazing scattering of helium atoms are discussed. It is shown that the threshold behavior of the KE can be interpreted in terms of a simple model of binary-encounter between projectiles and valence electrons. From threshold velocities for KE electron momenta and potentials in front of the studied metal surfaces are derived. Then special features for the grazing scattering of neon and argon are discussed. For neon impinging on Al(111) with energies normal to the surface above 25 eV electron promotion was found to dominate the electron emission process. Sub-threshold KE for scattering of neon and argon atoms is interpreted by two models. The first one is based on the local electron momentum distribution in front of the surface. The second one involves correlated excitation of two conduction electrons well above the Fermi level with subsequent emission of an electron via Auger deexcitation. Furthermore an explanation for the different KE yields under axial and planar surface channeling conditions is provided.

In the nanoscale metrology industry, there is a need for low-cost instruments, which have the ability to probe the structrure and elemental composition of thin films. This dissertation, describes the research performed to design and simulate a miniature Cylindrical Mirror Analyzer, (CMA), and Auger Electron Spectrometer, (AES). The CMA includes an integrated coaxial thermionic electron source. Electron optics simulations were performed using the Finite Element Method, (FEM), software COMSOL. To address the large Secondary Electron, (SE), noise, inherent in AES spectra, this research also included experiments to create structures in materials, which were intended to suppress SE backgound noise in the CMA. Laser Beam Machining, (LBM), of copper substrates was used to create copper pillars with very high surface areas, which were designed to supress SE’s. The LBM was performed with a Lumera SUPER RAPID‐HE model Neodymium Vanadate laser. The laser has a peak output power of 30 megawatts, has a 5x lens and a spot size of 16 μm. The laser wavelength is in the infrared at 1064 nm, a pulse width of 15 picoseconds, and pulse repetition rate up to 100 kHz. The spectrometer used in this research is intended for use when performing chemical analysis of the surface of bulk materials and thin films. It is applicable for metrology of thin films, as low as 0.4 nm in thickness, without the need to perform destructive sample thinning, which is required in Scanning Tranmission Electron Microscopy, (STEM). The spectrometer design is based on the well known and widely used coaxial cylinder capacitor design known as the Cylindrical Mirror Analyzer, (CMA). The coaxial tube arrangement of the CMA allows for placing an electron source,which is mounted in the center of the inner cylinder of the spectrometer. Simulation of the electron source with an Einzel Lens was also performed. In addtion, experiments with thin film coatings and Laser Beam Machining to supress Secondary Electron emission noise within the Auger electron spectrum were completed. Design geometry for the miniature CMA were modeled using Computer Aided Design, (CAD). Fixed Boundary Conditions, (BC), were applied and the geometry was then meshed for FEM. The electrostatic potential was then solved using the Poisson equation at each point. Having found the solution to the electrostatic potentials, electron flight simulations were performed and compared with the analytical solution. From several commercially available FEM modeling packages, COMSOL Multiphysics was chosen as the research platform for modeling of the spectrometer design. The CMA in this design was reduced in size by a factor of 4 to 5. This enabled mounting the CMA on a 2 ¾ in flange compared to the commercial PHI model 660 CMA which mounts onto a 10 in flange. Results from the Scanning Electron Microscopy measurements of the Secondary Electron emission characteristics of the LBM electron suppressor will also be presented.

In modern low pressure plasma discharges, the electron mean free path often exceeds the device dimensions. Under such conditions the electron velocity distribution function may significantly deviate from Maxwellian, which strongly affects the discharge properties. The description of such plasmas has to be kinetic and often requires the use of numerical methods. This thesis presents the study of kinetic effects in inductively coupled plasmas and Hall thrusters carried out by means of particle-in-cell simulations. The important result and the essential part of the research is the development of particle-in-cell codes. <p>An advective electromagnetic 1d3v particle-in-cell code is developed for modelling the inductively coupled plasmas. An electrostatic direct implicit 1d3v particle-in-cell code EDIPIC is developed for plane geometry simulations of Hall thruster plasmas. The EDIPIC code includes several physical effects important for Hall thrusters: collisions with neutral atoms, turbulence, and secondary electron emission. In addition, the narrow sheath regions crucial for plasma-wall interaction are resolved in simulations. The code is parallelized to achieve fast run times. <p>Inductively coupled plasmas sustained by the external RF electromagnetic field are widely used in material processing reactors and electrodeless lighting sources. In a low pressure inductive discharge, the collisionless electron motion strongly affects the absorption of the external electromagnetic waves and, via the ponderomotive force, the density profile. The linear theory of the anomalous skin effect based on the linear electron trajectories predicts a strong decrease of the ponderomotive force for warm plasmas. Particle-in-cell simulations show that the nonlinear modification of electron trajectories by the RF magnetic field partially compensates the effects of electron thermal motion. As a result, the ponderomotive force in warm collisionless plasmas is stronger than predicted by linear kinetic theory. <p>Hall thrusters, where plasma is maintained by the DC electric field crossed with the stationary magnetic field, are efficient low-thrust devices for spacecraft propulsion. The energy exchange between the plasma and the wall in Hall thrusters is enhanced by the secondary electron emission, which strongly affects electron temperature and, subsequently, thruster operation. Particle-in-cell simulations show that the effect of secondary electron emission on electron cooling in Hall thrusters is quite different from predictions of previous fluid studies. Collisionless electron motion results in a strongly anisotropic, nonmonotonic electron velocity distribution function, which is depleted in the loss cone, subsequently reducing the electron wall losses compared to Maxwellian plasmas. Secondary electrons form two beams propagating between the walls of a thruster channel in opposite radial directions. The secondary electron beams acquire additional energy in the crossed external electric and magnetic fields. The energy increment depends on both the field magnitudes and the electron flight time between the walls. <p>A new model of secondary electron emission in a bounded plasma slab, allowing for emission due to the counter-propagating secondary electron beams, is developed. It is shown that in bounded plasmas the average energy of plasma bulk electrons is far less important for the space charge saturation of the sheath than it is in purely Maxwellian plasmas. A new regime with relaxation oscillations of the sheath has been identified in simulations. Recent experimental studies of Hall thrusters indirectly support the simulation results with respect to the electron temperature saturation and the channel width effect on the thruster discharge.

This thesis concerns multi-ionization coincidence measurements of atoms and small molecules using a magnetic bottle time-of-flight (TOF) spectrometer designed for multi-electron coincidence studies. Also, a time-of-flight mass spectrometer has been used together with the TOF electron  spectrometer for electron-ion coincidence measurements. The multi-ionization processes have been studied by employing a pulsed discharge lamp in the vacuum ultraviolet spectral region and synchrotron radiation in the soft X-ray region. The designs of the spectrometers are described in some detail, and several timing schemes suitable for the light sources mentioned above are presented. Studies have been performed on krypton, molecular oxygen, carbon disulfide and a series of alcohol molecules. For the latter, double ionization spectra have been recorded and new information has been obtained on the dicationic states. A recently found rule-of-thumb  and quantum chemical calculations have been used to quantify the effective distance of the two vacancies in the dications of these molecules. For Kr, O2, and CS2, single-photon core-valence spectra have been obtained at the synchrotron radiation facility BESSY II in Berlin and interpreted on the basis of quantum chemical calculations. These spectra show a remarkable similarity to conventional valence photoelectron spectra. Spectra of triply charged ions were recorded, also at BESSY II, for Kr and CS2 by measuring, in coincidence, all three electrons ejected. The complex transition channels leading to tricationic states were mapped in substantial detail for Kr. It was found that for 3d-ionized krypton, the tricationic states are dominantly populated by cascade Auger decays via distinct intermediate states whose energies have been determined. The triple ionization spectra of CS2 from the direct double Auger effect via S2p, S2s and C1s hole states contain several resolved features and show selectivity based on the initial charge localisation and on the identity of the initial state.