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The Electron Emission Characteristics of Aluminum, Molybdenum and Carbon Nanotubes Studied by Field Emission and Photoemission.

Description: The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.
Date: December 2002
Creator: Sosa, Edward Delarosa
Partner: UNT Libraries

Effects of covalency on the p-shell photoemission of transition metals: MnO

Description: This article discusses the effects of covalency on the p-shell photoemission of transition metals. The treatment of covalency has not been included previously in ab initio theoretical studies of the 2p-shell XPS of transition-metal complexes. In this work, covalent interactions between the metal and ligands are treated on an equal footing with spin-orbit splittings.
Date: April 13, 2006
Creator: Bagus, Paul S. & Ilton, Eugene S.
Partner: UNT College of Arts and Sciences

Structural and Photoelectron Emission Properties of Chemical Vapor Deposition Grown Diamond Films

Description: The effects of methane (CH4), diborone (B2H6) and nitrogen (N2) concentrations on the structure and photoelectron emission properties of chemical vapor deposition (CVD) polycrystalline diamond films were studied. The diamond films were grown on single-crystal Si substrates using the hot-tungsten filament CVD technique. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to characterize the different forms of carbon in the films, and the fraction of sp3 carbon to sp3 plus sp2 carbon at the surface of the films, respectively. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the films. The photoelectron emission properties were determined by measuring the energy distributions of photoemitted electrons using ultraviolet photoelectron spectroscopy (UPS), and by measuring the photoelectric current as a function of incident photon energy.
Date: August 1998
Creator: Akwani, Ikerionwu Asiegbu
Partner: UNT Libraries

Angle-resolved photoemission extended fine structure: Multiple layers of emitters and multiple initial states

Description: Recently, angle-resolved photoemission extended fine structure (ARPEFS) has been applied to experimental systems involving multiple layers of emitters and non-s core-level photoemission in an effort to broaden the utility of the technique. Most of the previous systems have been comprised of atomic or molecular overlayers adsorbed onto a single-crystal, metal surface and the photoemission data were taken from an s atomic core-level in the overlayer. For such a system, the acquired ARPEFS data is dominated by the p{sub o} final state wave backscattering from the substrate atoms and is well understood. In this study, we investigate ARPEFS as a surface-region structure determination technique when applied to experimental systems comprised of multiple layers of photoemitters and arbitrary initial state core-level photoemission. Understanding the data acquired from multiple layers of photoemitters is useful for studying multilayer interfaces, ''buried'' surfaces, and clean crystals in ultra- high vacuum. The ability to apply ARPEFS to arbitrary initial state core-level photoemission obviously opens up many systems to analysis. Efforts have been ongoing to understand such data in depth. We present clean Cu(111) 3s, 3p, and 3d core-level, normal photoemission data taken on a high resolution soft x-ray beamline 9.3.2 at the Advanced Light Source in Berkeley, California and clean Ni(111) 3p normal photoemission data taken at the National Synchrotron Light Source in Upton, New York, USA.
Date: August 1, 1995
Creator: Huff, W.R.A.; Kellar, S.A.; Moler, E.J.; Chen, Y.; Wu, H.; Shirley, D.A. et al.
Partner: UNT Libraries Government Documents Department

Photocharge transport and recombination measurements in amorphous silicon films and solar cells by photoconductive frequency mixing. Annual subcontract report, May 13, 1994--May 12, 1995

Description: The continuous decay of electron drift mobility in intrinsic a-Si:H and a-SiC:H upon light soaking was investigated by the photomixing technique. The photoconductivity, lifetime and drift mobility in intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon carbide (a-SiC:H) while light-soaking were determined using a photomixing technique. In addition to the decay of the photoconductivity and electron lifetime, continuous decay of the electron drift mobility was found during the light soaking process, which reveals a new phenomenon associated with the Staebler-Wronski effect. The drift mobility decreased by a factor of 2 for 20 hour light soaking at 2.5 sun intensity. Experimental data were fitted to a stretched exponential law. Different stretched-exponential parameters for photoconductivity, lifetime and drift mobility were obtained, which indicates the production of defects with different generation kinetics upon light soaking.
Date: October 1, 1995
Creator: Braunstein, R.; Yang, Y. & Dong, S.
Partner: UNT Libraries Government Documents Department

Photoelectron holography applied to surface structural determination

Description: Photoemitted electron waves are used as coherent source waves for angstrom-scale holographic imaging of local atomic geometry at surfaces. Electron angular distribution patterns are collected above a sample surface and serve as a record of the interference between source wave and waves scattered from surrounding ion cores. Using a mathematical imaging integral transformation, the three-dimensional structural information is obtained directly from these collected patterns. Patterns measured with different electron kinetic energies are phase-summed for image improvement. Pt (111) surface is used as a model system. A pattern 9.6{angstrom}{sup {minus}1} (351 eV) is used to generate a full 3-D image of atom locations around an emitter with nearest neighbors within 0.l{angstrom} of the expected bulk positions. Atoms several layers beyond the nearest neighbors are also apparent. Twin-image reduction and artifact suppression is obtained by phase-summing eight patterns measured from 8.8 to 10.2{angstrom}{sup {minus}1} (295 to 396 eV). 32 were measured in 0.2{angstrom}{sup {minus}1} steps from 6.0 to 12.2{angstrom}{sup {minus}1} (137 to 567 eV) are presented here. Simple models of two-slit interference are compared with electron scattering to illuminate understanding of holographic recording of the structural information. This also shows why it sometimes fails due to destructive interferences. Simple theoretical models of electron scattering are compared to experiment to show the origin of the structural information and the differences that result from atomic scattering and from the source wave. Experimental parameters and their relation to imaging is discussed. Comparison is made to the Pt pattern measured at 351 eV using the simple theoretical model. The remaining data set is also modeled, and the eight appropriate theoretical patterns are used to regenerate the multiple-wavenumber experimental result. A clean Cu (001) surface is also measured and imaged.
Date: May 1, 1995
Creator: Petersen, B.L.
Partner: UNT Libraries Government Documents Department

Near-Field Spectroscopy of Selectively Oxidized Vertical Cavity Surface Emitting Lasers

Description: Selectively oxidized vertical cavity surface emitting lasers (VCSELS) have been studied by spectrally resolved near field scanning optical microscopy (NSOM). We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and significantly different from those measured in the far field.
Date: December 9, 1999
Partner: UNT Libraries Government Documents Department

Photoelectron holography of platinum (111)

Description: Platinum atoms near a (111) single-crystal face have been imaged using photoelectron holography. Electron angular intensity patterns were collected at equally spaced wavenumbers from 6 to 12{Angstrom}{sup {minus}1}. Images of atoms near expected atomic positions are obtained from single-wavenumber analyses over the range of the data set. Positions are detected further from the emitter than we have seen previously, and symmetry assumptions are not required. We have also adopted a three dimensional means of representing the data in order to help understand the results. Twin image suppression and artifact reduction in the holographically reconstructed data are set are obtained when images at different wavenumbers are correctly phase-summed. We are assessing the capability of the technique for rendering true three-dimensional structural information for unknown systems.
Date: April 1, 1993
Creator: Petersen, B.L.; Terminello, L.J.; Barton, J.J. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Fermi surfaces, Fermi patches, and Fermi arcs in high T{sub c} superconductors.

Description: A defining property of metals is the existence of a Fermi surface: for two dimensions, a continuous contour in momentum space which separates occupied from unoccupied states. In this paper, I discuss angle resolved photoemission data on the cuprate superconductor BSCCO and argue that it is not best thought of in this conventional picture. Rather, the data are consistent with patches of finite area connected by more conventional arcs. Novel physics is associated with the patches, in that the states contained in a patch are dispersionless and thus interaction dominated. In the pseudogap phase, the patches are gapped out, leaving the Fermi arcs disconnected. This unusual situation may be the key to understanding the microscopic physics of the high temperature superconductors, in that the pairing correlations are strongest in the patches, yet the superfluid density lives only on the arcs.
Date: May 19, 1999
Creator: Norman, M. R.
Partner: UNT Libraries Government Documents Department


Description: Recent instrumentation developments in photoemission are providing new insights into the physics of complex materials. With increased energy and momentum resolution, it has become possible to examine in detail different contributions to the self-energy or inverse lifetime of the photohole created in the photoexcitation process, Employing momentum distribution and energy distribution curves, a detailed study of the optimally doped cuprate, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub g+{delta}}, shows that the material behaves like a non-Fermi liquid with no evidence for the quasi-particles characteristic of a Fermi liquid.
Date: October 13, 1999
Creator: JOHNSON,P.D.
Partner: UNT Libraries Government Documents Department

Magneto-optical and photoemission studies of ultrathin wedges

Description: Magnetic phase transitions of Fe wedges grown epitaxially on Cu(100) are detected via the surface magneto-optical Kerr effect and used to construct a phase diagram for face centered Fe. Also, the confinement of Cu sp- and d-quantum-well states is studied for Cu/Co(wedge)/Cu(100) utilizing undulator-based photoemission experiments.
Date: December 1, 1995
Creator: Bader, S. D. & Li, Dongqi
Partner: UNT Libraries Government Documents Department

Electron transfer and physical and chemical processes at low temperatures

Description: We summarize some phenomena that occur at temperatures of the order of 15K, and are dominated by quantum mechanical tunneling. Although electron tunneling dominates many conduction processes at low temperatures, we discuss evidence that phenomena like oxidation, as well as the solution of alkali metals in ammonia, can also be dominated by electron tunneling. Both phenomena demonstrate that the chemical potential of a metastable system can equilibrate at low temperatures by electron tunneling. The case of alkali metal clusters covered with ammonia is contrasted to covering the clusters with Xe. In this case changes in the activated conduction are observed which are consistent with the dielectric constant of the rare gas.
Date: October 1, 1995
Creator: Strongin, M.; Xia, B. & Jacobsen, F.M.
Partner: UNT Libraries Government Documents Department

Study of an instability of the PEP-II positron beam (Ohmi effect and multipactoring)

Description: The paper is organized in the following way. First, Ohmi effect induced by direct flow of primary photoelectrons is studied for the PEP-II parameters. The production rate and kinematics take into account the antechamber of the LER. We discuss the effect of the secondary emission of electrons in the AL chamber, where the yield is larger than one. Resonance multipactoring is considered, and then the average density of the secondary electrons is estimated taking into account the space-charge effect and the interaction with the beam. We show that in the extreme case there is a self-consistent regime similar to the regime of the space-charge dominated cathode. Finally, the rate of ion production by accumulated electrons and the possibility of the ion induced pressure instability is discussed.
Date: November 1, 1995
Creator: Heifets, S.A.
Partner: UNT Libraries Government Documents Department

BSCCO superconductors : hole-like fermi surface and doping dependence of the gap function.

Description: We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at ({pi},{pi}), Then, the superconducting gap along the FS is precisely determined from ARPES measurements on over-doped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k{sub x})-cos(k{sub y}) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.
Date: August 18, 1999
Creator: Campuzano, J. C.; Ding, H.; Fretwell, H. M.; Kadowaki, K.; Kaminski, A.; Mesot, J. et al.
Partner: UNT Libraries Government Documents Department

Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Emittance measurements for the Illinois/CEBAF polarized electron source

Description: The transverse thermal properties of the electrons photoemitted from GaAs determine the intrinsic beam emittance, an important quantity in applications such as polarized electron sources and high-brightness sources. In this paper, emittance measurements using the Illinois/CEBAF polarized electron source are described. The emittance was measured as a function of both the laser beam spot size and laser wavelength at low currents. The data was used to infer the transverse thermal energy of the electrons photoemitted from GaAs for wavelengths between 514 and 840 nm. Near the bandgap the transverse energy is {approximately}34 meV, a factor of 3 lower than that of the beam from a typical thermionic electron gun. 8 refs., 2 figs.
Date: January 1, 1995
Creator: Dunham, B.M.; Cardman, L.S. & Sinclair, C.K.
Partner: UNT Libraries Government Documents Department

Surface and interface electronic structure: Final report, December 1, 1994--November 30, 1995

Description: A year and a half ago, we terminated our operations at the National Synchrotron Light Source and moved our research program to the Advanced Light Source in Berkeley. This move was motivated primarily by the exciting possibilities and research directions made possible by operation at the newer ALS facility. The ALS facility was commissioned in October 1993. We are members of a `Participating Research Team` (PRT) associated with undulator beamline 7.0 at the ALS. This beamline was commissioned for experiments the following winter and spring of 1994. We have thus had about a year to debug the beamline and to try to get some experiments accomplished. Reportedly due to insufficient operating funds, the facility provided beam to users only nine 8-hour shifts per week. In the past year, our program has been devoted primarily to becoming operational at the ALS and to assembling experiments and experimental equipment associated with our program there.
Date: December 31, 1995
Creator: Kevan, S.D.
Partner: UNT Libraries Government Documents Department

Correlation of magnetic dichroism in x-ray absorption and photoelectron emission using ultrathin magnetic alloy films

Description: We have begun a program to characterize magnetic alloy overlays using both magnetic x-ray circular dichroism (MXCD) and magnetic x-ray linear dichroism (MXLD). This will allow a direct comparison of MXCD-absorption and MXLD-photoelectron emission. First results from the Advanced Light Source will be presented.
Date: April 1, 1996
Creator: Tobin, J.G.; Goodman, K.W.; Mankey, G.J.; Willis, R.F.; Denlinger, J.D.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

High T{sub c} superconductors: New insights from angle-resolved photoemission

Description: Recent angle-resolved photoemission (ARPES) studies of the high T{sub c} superconductors by the Argonne group are briefly reviewed. First we discuss sum rules to establish a spectral function interpretation of the data, and the use of ARPES to obtain the momentum distribution. We then apply these ideas to the normal and superconducting state spectra for Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. Among the topics discussed are the Fermi surface, polarization selection rules, bilayer splitting and the superconducting gap.
Date: November 1, 1995
Creator: Randeria, M.
Partner: UNT Libraries Government Documents Department

Bulk and surface electronic structure of actinide, rare earth, and transition metal elements and compounds

Description: This is the final report for a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project sought to study of unusual magnetic and structural properties of rare earth, actinide, and transition metals through high-precision electronic structure calculations. Magnetic moment anisotropies in bulk and surface systems were studied, with emphasis on novel surfaces with unusual magnetic properties with possible applicability in magnetic recording. The structural stability, bonding properties, and elastic response of the actinides, as well as transition and rare earth elements and compounds, were also studied. The project sought to understand the unusual crystallographic and cohesive properties of the actinides and the importance of correlation to structural stability and the nature of the delocalization transition in these elements. Theoretical photoemission spectra, including surface effects, were calculated for rare earths and actinides.
Date: July 1, 1996
Creator: Wills, J.W. & Eriksson, O.
Partner: UNT Libraries Government Documents Department

An investigation of resonant photoemission in Gd with x-ray linear dichroism

Description: The constructive summing of direct and indirect channels above the absorption threshold of a core level can cause a massive increase in the emission cross section, leading to a phenomenon called "resonant photoemission". Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiments and theoretical simulations, we have probed the nature of the resonant photoemission process in Gd metal. It now appears that temporal matching as well as energy matching is a requirement for true resonant photoemission.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Spin polarization of the conduction bands and secondary electrons of Gd(0001)

Description: Angle- and spin-resolved photoemission was utilized to investigate the 5d bulk bands and the surface state of Gd(0001) in the temperature range. of 130 - 350 K The bulk bands at 1-2 eV below the Fermi energy E{sub F} show Stoner-like behavior, while the temperature dependence of the surface state near E{sub F} indicates spin-mixing behavior due to fluctuating local 5d moments. The secondary electron spectra of the Gd surfaces both before and after initial oxygen adsorption show a polarization dip at low kinetic energies due to the extra scattering channel for minority electrons via the unoccupied 4f level. The temperature dependences of the surface and bulk magnetization are separated using the spin polarization of the surface state and the bulk exchange splitting.
Date: December 31, 1995
Creator: Li, Dongqi; Pearson, J. & Bader, S.D.
Partner: UNT Libraries Government Documents Department

Angle-resolved photoemission extended fine structure study of chemisorbed c(2{times}2)P/Fe(100): Comparison with self-consistent- field X{alpha} scattered wave calculations

Description: Angle-resolved photoemission extended fine structure (ARPEFS) was used to determine the structure of c(2x2)P/Fe(100). P 1s core-level photoemission data were collected normal to the (100) surface and 45{degree} off-normal along the [011] direction at room temperature. Close analysis of the auto-regressive linear prediction based Fourier transform and multiple-scattering spherical-wave calculations indicate that the P atoms adsorb in the high-coordination fourfold hollow sites. The P atoms bond 1.02 {angstrom} above the first layer of Fe atoms and the Fe-P-Fe bond angle is 140.6{degree}. It was also determined that there is no expansion of the Fe surface. Self-consistent-field X{alpha} scattered wave calculations were performed for the c(2x2)P/Fe(100) and the c(2x2)S/Fe(100) systems. These independent results are in excellent agreement with this P/Fe structure and the S/Fe structure previously published, confirming the ARPEFS determination that the Fe{sub 1}-Fe{sub 2} interlayer spacing is contracted from the bulk value for S/Fe but not for P/Fe.
Date: February 1, 1996
Creator: Huff, W.R.A.; Chen, Y. & Zhang, X.S.
Partner: UNT Libraries Government Documents Department