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Correlation Between the Atomic and Bulk Chemical Potentials of Low work Function Metals

Description: An attempt is made to identify preferred values for the work functions of the rare earth elements by correlating the atomic chemical potential with the work function of the bulk elements. Trends in the alkali and alkali earth metal are evaluated in the same context. Strong linear correlation between the two quantities is observed within the IA, 11A, and IIIB (Se, Y, La) groups. Within the lanthanide series the nature of the correlation between the metallic radius and the work function suggests a dependence on the total angular momentum.
Date: December 22, 1998
Creator: Drummond, T.J.
Partner: UNT Libraries Government Documents Department

Fabrication and characterization of submicron polymer waveguides by micro-transfer molding

Description: Various methods exist for fabrication of micron and submicron sized waveguide structures. However, most of them include expensive and time consuming semiconductor fabrication techniques. An economical method for fabricating waveguide structures is introduced and demonstrated in this thesis. This method is established based on previously well-developed photonic crystal fabrication method called two-polymer microtransfer molding. The waveguide in this work functions by a coupler structure that diffracts the incident light into submicron polymer rods. The light is then guided through the rods. Characterization is done by collecting the light that has been guided through the waveguide and exits the end of these submicron polymer bars. The coupling and waveguiding capabilities are demonstrated using two light sources, a laser and white light.
Date: December 15, 2009
Creator: Wu, Te-Wei
Partner: UNT Libraries Government Documents Department

The sputter generation of negative ion beams

Description: A brief review is given of recent progress toward a quantitative understanding of negative ion formation by sputtering from surfaces covered with fractional layers of highly electropositive adsorbates. Practical models developed for estimating changes in work functions {Delta}{phi} by electropositive adsorbates are described. The secondary negative ion generation process is examined through the use of composite energy/velocity dependent analytical models. These models are used to illustrate the effect of work function on the energy distributions of negative ions sputter ejected from a polycrystalline molybdenum surface covered with fractional layers of cesium. Predictions are also made of the functional dependence of the probability for negative ion formation on cesium coverage. The models predict energy distributions which are in basic disagreement with experimental observations, implying their inappropriateness for describing the sputter negative ion generation process. We have also developed a model for calculating sputter ratios based on the use of simple scaling procedures to bring Sigmund theory into close agreement with experimental observation accounting for the threshold effect. Scaling factors for projectile energies E > 1000 eV are found to be independent of energy while those for projectile energies E{sub th} < E < 1000 eV were found to be energy dependent. In this study, the model and scaling techniques utilized to bring Sigmund theory into agreement with experiment are discussed in detail and several examples provided which illustrate the versatility, accuracy and utility of the model. In the present report, we describe the model and apply it to the case of sputtering a selected number of metals with energetic cesium ions. In particular, we present sputter ratio information for a number of Cs-projectile/metal-target combinations; the targets are bombarded at normal incidence to the surface.
Date: December 31, 1996
Creator: Alton, G.D.
Partner: UNT Libraries Government Documents Department

Growth of silicon carbide on silicon via reaction of sublimed fullerenes and silicon

Description: Epitaxial silicon carbide films are grown on Si(100) substrates at a surface temperature of 1,200 K via fullerene precursors. Films have been grown up to a thickness of 2,500 {angstrom}. The growth rate of the SiC film is not limited by the surface reaction rate of fullerene with silicon at these temperatures, rather by the arrival rate of the reactants Si (by diffusion from substrate or from gas phase) or fullerene. This results in rapid film growth. Films have been characterized by low energy electron diffraction, ultraviolet photoelectron spectroscopy and Auger electron spectroscopy. Stoichiometric, epitaxial SiC films are grown. Supply of silicon to the growing SiC surface via sublimation greatly reduces the tendency for silicon diffusion to form voids at the Si/SiC interface.
Date: February 1, 1996
Creator: Hamza, A.V. & Balooch, M.
Partner: UNT Libraries Government Documents Department

A positive (negative) surface ionization source concept for RIB generation

Description: A novel, versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer ({phi}{approximately} = 5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered to {phi} {approximately} = 1.43 eV by continually feeding cesium vapor through the ionizer matrix. The use of Cs to effect low work function surfaces for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB{sub 6} while enhancing the probability for negative ion formation of atomic and molecular species with low to intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for RIB applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in use at the HRIBF The design features and operational principles of the source will be described in this report.
Date: December 31, 1995
Creator: Alton, G.D. & Mills, G.D.
Partner: UNT Libraries Government Documents Department

Thickness dependencies in the calculated properties of metallic ultra-thin films

Description: Ultra-thin film (UTF) electronic structure calculations are a common tool for investigating surface properties. For this approximation to be useful, the UTF must be thick enough that the surfaces are decoupled and the interior is bulk-like, yet thin enough that a high precision electronic structure calculation is affordable. These conditions can only be satisfied simultaneously if the properties of interest converge rapidly as the UTF thickness is increased. In this work, electronic structure calculations for Al(111) films ranging from one to twelve atoms thick are used to illustrate some of the difficulties that can arise when one attempts to determine surface properties of metals with UTF calculations.
Date: December 1, 1997
Creator: Boettger, J.C.
Partner: UNT Libraries Government Documents Department

Work Functions of the transition Metals and Metal Silicides

Description: The work functions of polycrystalline metals are often used to systematize Schottky barrier height data for rectifying contacts to semiconductors. Rectifying contacts to silicon devices are predominantly formed using conductive metal silicides with work functions which are not as well characterized as metal work functions. The present work has two objectives. First, it classifies the transition metals using correlations between the metal work function and the atomic chemical potential. Second, the available data for metal silicides is collected and interpreted using an average charge transfer (ACT) model. The ACT model accounts for the electronic hardness of the component elements in addition to their chemical potentials. New trends in the behavior of silicide work functions are identified.
Date: February 15, 1999
Creator: Drummond, T.J.
Partner: UNT Libraries Government Documents Department


Description: It is the objective of the project to further develop the triboelectrostatic separation (TES) process developed at the Federal Energy Technology Center (FETC) and to test the process at a proof-of-concept (POC) scale. This process has a distinct advantage over other coal cleaning processes in that it does not entail costly steps of dewatering. The POC-scale unit is to be developed based on (i) the charging characteristics of coal and mineral matter that can be determined using the novel on-line tribocharge measuring device developed at Virginia Tech and (ii) the results obtained from bench-scale TES tests conducted on three different coals. During the past quarter, most of the personnel assigned to this project have been performing work elements associated with the engineering design (Task 3) of the TES process. This activity has been subdivided into three subtasks, i.e., Charger Tests (Subtask 3.1), Separator Tests (Subtask 3.2), and Final POC Design (Subtask 3.3). In Subtask 3.1, several different tribocharging devices have been constructed using materials of various work functions. They are currently being tested to establish the best materials to be used for designing and manufacturing the optimum tribochargers that can maximum charge differences between coal and mineral matter. In Subtask 3.2, bench-scale cleaning tests have been conducted to study the effects of the various operating and design parameters on the performance of the electrostatic separator. Two different TES units have been tested to date. One uses drum-type electrodes to separate charged particles, while the other uses plate-type electrodes for the separation. The test results showed that a major improvement in separation efficiency can be achieved by recycling the middlings back to the feed stream. It has also been established that the major source of inefficiency arises from the difficulty in separating ultrafine particles. Understanding the behavior of the ultrafine particles ...
Date: March 10, 1999
Creator: Yan, E.S.; Luttrell, G.H.; Adel, G.T. & Yoon, R.-H.
Partner: UNT Libraries Government Documents Department

Field emission from carbon films deposited by VHF CVD on difference substrates

Description: As previously demonstrated, non-diamond carbon (NDC) films deposited at low temperatures 200-300 C on silicon tips reduced the threshold of field emission. In this paper we will present the results of the study of field emission from flat NDC films prepared by VHF CVD. Emission measurements were performed in a diode configuration at approximately 10{sup {minus}10} Torr. NDC films were deposited on ceramic and on c-Si substrates sputter coated with layers of Ti, Cu, Ni and Pt. The back contact material influences the emission characteristics but not as a direct correlation to work function. A model of field emission from metal-NDC film structures will be discussed.
Date: April 1, 1999
Creator: Abramov, A A; Andronov, A N; Felter, T E; Ioffe, A F; Kosarev, A I; Shotov, M V et al.
Partner: UNT Libraries Government Documents Department

Experimental determination of the temperature dependence of metallic work functions at low temperatures. Progress report, March 1, 1975--February 9, 1976

Description: Progress described includes measurements of the temprature dependence of the work functions for niobium and aluminum between 2 and 20$sup 0$K and the development of a simple theoretical model to explain the effects of adsorbed $sup 4$He gas on the temperature dependence of the work function in niobium. (auth)
Date: January 1, 1976
Creator: Pipes, P.B.
Partner: UNT Libraries Government Documents Department

Surface Dipole Formation and Lowering of the Work Function by Cs Adsorption on InP(100) Surface

Description: The Cs adsorption on InP(100) surface is studied with Synchrotron Radiation Photoelectron Spectroscopy. The charge transfer from Cs to the InP substrate is observed from the Cs induced In4d and P2p components, and this charge transfer results in surface dipole formation and lowering of the work function. The Cs4d intensity saturates at coverage of one monolayer (ML). However, a break point is observed at 0.5 ML, which coincides with the achievement of the minimum work function. This break point is due to the different vertical placement of the first and the second half monolayer of Cs atoms. Based on this information, a simple bi-layer structure for the Cs layer is presented. This bi-layer structure is consistent with the behavior of the charge transfer from the Cs to the InP substrate at different Cs coverages. This, in turn, explains why the work function decreases to a minimum at 0.5 ML of Cs and remains almost constant beyond this coverage. The depolarization of the surface dipoles is attributed to the saturation of charge transfer to the surface In atoms and the polarization of the Cs atoms in the second half monolayer induced by the positively charged Cs atoms in the first half monolayer.
Date: June 8, 2007
Creator: Sun, Y.; Liu, Z. & Pianetta, P.
Partner: UNT Libraries Government Documents Department

Thermotunneling Based Cooling Systems for High Efficiency Buildings

Description: GE Global Research's overall objective was to develop a novel thermotunneling-cooling device. The end use for these devices is the replacement of vapor cycle compression (VCC) units in residential and commercial cooling and refrigeration systems. Thermotunneling devices offer many advantages over vapor cycle compression cooling units. These include quiet, reliable, non-moving parts operation without refrigerant gases. Additionally theoretical calculations suggest that the efficiency of thermotunneling devices can be 1.5-2x that of VCC units. Given these attributes it can be seen that thermotunneling devices have the potential for dramatic energy savings and are environmentally friendly. A thermotunneling device consists of two low work function electrodes separated by a sub 10 nanometer-sized gap. Cooling by thermotunneling refers to the transport of hot electrons across the gap, from the object to be cooled (cathode) to the heat rejection electrode (anode), by an applied potential. GE Global Research's goal was to model, design, fabricate devices and demonstrate cooling base on the thermotunneling technology.
Date: September 30, 2007
Creator: Aimi, Marco; Arik, Mehmet; Bray, James; Gorczyca, Thomas; Michael, Darryl & Weaver, Stan
Partner: UNT Libraries Government Documents Department

Imaging doped silicon test structures using low energy electron microscopy.

Description: This document is the final SAND Report for the LDRD Project 105877 - 'Novel Diagnostic for Advanced Measurements of Semiconductor Devices Exposed to Adverse Environments' - funded through the Nanoscience to Microsystems investment area. Along with the continuous decrease in the feature size of semiconductor device structures comes a growing need for inspection tools with high spatial resolution and high sample throughput. Ideally, such tools should be able to characterize both the surface morphology and local conductivity associated with the structures. The imaging capabilities and wide availability of scanning electron microscopes (SEMs) make them an obvious choice for imaging device structures. Dopant contrast from pn junctions using secondary electrons in the SEM was first reported in 1967 and more recently starting in the mid-1990s. However, the serial acquisition process associated with scanning techniques places limits on the sample throughput. Significantly improved throughput is possible with the use of a parallel imaging scheme such as that found in photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM). The application of PEEM and LEEM to device structures relies on contrast mechanisms that distinguish differences in dopant type and concentration. Interestingly, one of the first applications of PEEM was a study of the doping of semiconductors, which showed that the PEEM contrast was very sensitive to the doping level and that dopant concentrations as low as 10{sup 16} cm{sup -3} could be detected. More recent PEEM investigations of Schottky contacts were reported in the late 1990s by Giesen et al., followed by a series of papers in the early 2000s addressing doping contrast in PEEM by Ballarotto and co-workers and Frank and co-workers. In contrast to PEEM, comparatively little has been done to identify contrast mechanisms and assess the capabilities of LEEM for imaging semiconductor device strictures. The one exception is the ...
Date: January 1, 2010
Creator: Nakakura, Craig Yoshimi; Anderson, Meredith Lynn & Kellogg, Gary Lee
Partner: UNT Libraries Government Documents Department


Description: Work continued during the past quarter to improve the performance of the POC-scale unit. For the charging system, a more robust ''turbocharger'' has been fabricated and installed. All of the internal components of the charger have been constructed from the same material (i.e., Plexiglas) to prevent particles from contacting surfaces with different work functions. For the electrode system, a new set of vinyl-coated electrodes have been constructed and tested. The coated electrodes (i) allow higher field strengths to be tested without of risk of arcing and (ii) minimize the likelihood of charge reversal caused by particles colliding with the conducting surfaces of the uncoated electrodes. Tests are underway to evaluate these modifications. Several different coal samples were collected for testing during this reporting period. These samples included (i) a ''reject'' material that was collected from the pyrite trap of a pulverizer at a coal-fired power plant, (ii) an ''intermediate'' product that was selectively withdrawn from the grinding chamber of a pulverizer at a power plant, and (iii) a run-of-mine feed coal from an operating coal preparation plant. Tests were conducted with these samples to investigate the effects of several key parameters (e.g., particle size, charger type, sample history, electrode coatings, etc.) on the performance of the bench-scale separator.
Date: October 1, 2000
Creator: Yoon, R.-H.; Luttrell, G.H.; Luvsansambuu, B. & Walters, A.D.
Partner: UNT Libraries Government Documents Department


Description: Our results from optical absorption and surface external reflection FTIR reveal that we can grow self-assembled multilayers consisting of electron acceptors like OHBP and donors like NiPc. In order to investigate the evolution of the surface physical properties of self-assembled multilayers, we applied Kelvin probe techniques and surface photovoltage spectroscopy to monitor the growth of self-assembled OHBP and NiPc systems. Kelvin probe results indicate that the surface electronic potential is very sensitive to the presence of self-assembled molecular layers on surfaces. We find that there is an approximately constant gap of 350 meV between the surface work function of OHBP and NiPc terminated self-assembled multilayers. In other words, the surface work function or surface electronic potentials is a periodic function of the terminating layer, oscillating between {approximately}450 mV for OHBP layers and {approximately}100 mV for NiPc layers. On the other hand, the photo-induced effects can be also correlated to the terminating layer of the OHBP-NiPc system. This shows that the self-assembled layers also control the photo-induced effects to some extent. Over the whole excitation spectrum (300-710 nm), the photo-induced band bending change oscillates between two values mostly governed by the properties of the terminating layer of the self-assembled systems.
Date: May 1, 2001
Creator: LI, L. S. & LI, A. D. Q.
Partner: UNT Libraries Government Documents Department

Integrated chemiresistor and work function microsensor array with carbon black/polymer composite materials

Description: An array of chemically-sensitive field-effect transistors (CHEMFETs) that measure both work function and bulk resistance changes in thin films was used to detect volatile organic compounds. Carbon black/organic polymer composite films were deposited onto the CHEMFETs using an automated microdispensing method.
Date: May 1, 1998
Creator: Domansky, K.; Zapf, V.S.; Grate, J.W.; Ricco, A.J.; Yelton, W.G. & Janata, J.
Partner: UNT Libraries Government Documents Department

Electron field emission from undoped and doped DLC films

Description: Electron field emission and electrical conductivity of undoped and nitrogen doped DLC films have been investigated. The films were grown by the PE CVD method from CH{sub 4}:H{sub 2} and CH{sub 4}:H{sub 2}:N{sub 2} gas mixtures, respectively. By varying nitrogen content in the gas mixture over the range 0 to 45%, corresponding concentrations of 0 to 8 % (atomic) could be achieved in the films. Three different gas pressures were used in the deposition chamber: 0.2, 0.6 and 0.8 Torr. Emission current measurements were performed at approximately 10{sup -6} Torr using the diode method with emitter-anode spacing set at 20 {micro}m. The current - voltage characteristics of the Si field electron emission arrays covered with DLC films show that threshold voltage (V{sub th}) varies in a complex manner with nitrogen content. As a function of nitrogen content, V{sub th} initially increases rapidly, then decreases and finally increases again for the highest concentration. Corresponding Fowler-Nordheim (F-N) plots follow F-N tunneling over a wide range. The F-N plots were used for determination of the work function, threshold voltage, field enhancement factor and effective emission area. For a qualitative explanation of experimental results, we treat the DLC film as a diamond-like (sp{sup 3} bonded) matrix with graphite-like inclusions.
Date: June 1, 1999
Creator: Chakhovskoi, A G; Evtukh, A A; Felter, T E; Klyui, N I; Kudzinovsky, S Y; Litovchenko, V G et al.
Partner: UNT Libraries Government Documents Department

Simulations of Ion Migration in the LCLS RF Gun and Injector

Description: The motivation for this work was the observed surface contamination of the first LCLS RF gun copper cathode. We will present the results of simulations in regards to ion migration in the LCLS gun. Ions of residual gases will be created by interaction of molecular gas species with the UV drive laser beam and by the electron beam itself. The larger part of those ionized molecules remain in the vicinity of creation, are transported towards beam line walls or away from the cathode. However a small fraction gains enough kinetic energy, focused by RF and magnetic fields and propagates to the cathode, producing an undesirable increase of the cathode's surface work function. Although this fraction is small, during long term operation, this effect may become a significant factor limiting the source performance.
Date: June 25, 2012
Creator: Brachmann, Axel; /SLAC; Dowell, David & /SLAC
Partner: UNT Libraries Government Documents Department

Electronic Surface Structures of Coal and Mineral Particles

Description: Surface science studies related to tribocharging and charge separation studies were performed on electrostatic beneficiation of coal. In contrast to other cleaning methods, electrostatic beneficiation is a dry cleaning process requiring no water or subsequent drying. Despite these advantages, there is still uncertainty in implementing large scale commercial electrostatic beneficiation of coal. The electronic surface states of coal macerals and minerals are difficult to describe due to their chemical complexity and variability [1]. The efficiency in separation of mineral particles from organic macerals depends upon these surface states. Therefore, to further understand and determine a reason for the bipolar charging observed in coal separation, surface analysis studies using Ultra-violet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS) were performed on coal samples and several materials that are used or considered for use in tribocharging. Electrostatic charging is a surface phenomenon, so the electronic surface states of the particles, which are influenced by the environmental conditions, determine both polarity and magnitude of tribocharging. UPS was used to measure the work function of the materials as typically used in ambient air. XPS was used to determine the surface chemistry in the form of contamination and degree of oxidation under the same environmental conditions. Mineral bearing coals are those amenable to electrostatic beneficiation. Three types of coal, Illinois No. 6, Pittsburgh No. 8, and Kentucky No. 9 were investigated in this study. Pulverized coal powder was tribocharged against copper. Pyritic and other ashes forming minerals in coal powders should charge with a negative polarity from triboelectrification, and organic macerals should acquire positive charge, according to the relative differences in the surface work functions between the material being charged and the charging medium. Different types of minerals exhibit different magnitudes of negative charge and some may also charge positively against copper [2]. Only the ...
Date: April 1, 2001
Creator: Mazumder, M.K.; Lindquist, D.A.; Tennal, K.B.; Trigwell, Steve; Farmer, Steve; Nutsukpul, Albert et al.
Partner: UNT Libraries Government Documents Department

The work function of sub-monolayer cesium-covered gold: A photoelectronspectroscopy study

Description: Using visible and X-ray photoelectron spectroscopy we measured the work function of a Au(111) surface at a well-defined sub-monolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61 {+-} 0.08 eV consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.
Date: June 13, 2008
Creator: LaRue, J.L.; White, J.D.; Nahler, N.H.; Liu, Z.; Sun, Y.; Pianetta, P.A. et al.
Partner: UNT Libraries Government Documents Department

In-Situ Cleaning of Metal Cathodes using a Hydrogen Ion Beam

Description: Metal photocathodes are commonly used in high-field RF guns because they are robust, straightforward to implement and tolerate relatively poor vacuum compared to semi-conductor cathodes. However these cathodes have low quantum efficiency (QE) even at UV wavelengths, and still require some form of cleaning after installation in the gun. A commonly used process for improving the QE is laser cleaning. In this technique the UV drive laser is focused to a small diameter close to the metal's damage threshold and then moved across the surface to remove contaminants. This method does improve the QE, but can produce non-uniform emission and potentially damage the cathode. Ideally an alternative process which produces an atomically clean, but unaltered surface is needed. In this paper we explore using a hydrogen ion (H-ion) beam to clean a copper cathode. We describe QE measurements over the wavelength range of interest as a function of integrated exposure to an H-ion beam. We also describe the data analysis to obtain the work function and derive a formula of the QE for metal cathodes. Our measured work function for the cleaned sample is in good agreement with published values, and the theoretical QE as a function of photon wavelength is in excellent agreement with the cleaned copper experimental results. Finally, we propose an in-situ installation of an H-ion gun compatible with existing s-band RF guns.
Date: March 29, 2006
Creator: Dowell, D.H.; King, F.K.; Kirby, R.E.; Schmerge, J.F. & /SLAC
Partner: UNT Libraries Government Documents Department

Measuring current emission and work functions of large thermionic cathodes.

Description: As one component of the nations Stockpile Stewardship program, Los Alamos National Laboratory is constructing a 20 MeV, 2 kA (with a 4 kA upgrade capability), 3ps induction linac for doing x-ray radiography of explosive devices. The linac is one leg of a facility called the Dual-Axis Radiography Hydrodynamic Test Facility (DARHT). The electron gun is designed to operate at 3.2 MV. The gun is a Pierce type design and uses a 6.5' cathode for 2 kA operation and an 8' cathode for 4 kA operation. We have constructed a small facility called the Cathode Test Stand (CTS) to investigate engineering and physics issues regarding large thermionic dispenser-cathodes. In particular, we have looked at the issues of temperature uniformity on the cathode surface and cathode quality as measured by its work function. We have done thermal imaging of both 8' and 6.5' cathodes. Here we report on measurements of the cathode work function, both the average value and how it vanes across the face of the cathode.
Date: January 1, 2001
Creator: Fortgang, C. M. (Clifford M.)
Partner: UNT Libraries Government Documents Department