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An atomic force microcopy study of the mechanical and electricalproperties of monolayer films of molecules with aromatic end groups

Description: The effect of intermolecular {pi}-{pi} stacking on the electrical and mechanical properties of monolayer films molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA) were used as model systems with different {pi}-{pi} stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than one order of magnitude, MPAA being the most conductive and MPPA the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films distinct molecular conductivity changes are observed upon mechanical perturbation. This effect however was not observed on the MPPA film, where intermolecular {pi}-{pi} interactions are likely weaker.
Date: September 6, 2007
Creator: Fang, Liang; Park, J.Y.; Ma, H.; Jen, A.K.-Y. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Failure analysis of rutile sleeves in MC3080 lightning arrestor connectors.

Description: The purpose of this SAND Report is to document efforts in the extraction and failure analyses of sleeve-style Lightning Arrestor Connectors (LACs). Several MC3080 and MC3079 LACs were recovered from the field and tested as part of the Enhanced Surveillance Campaign. A portion of these LACs failed retesting. Terry Ernest (01733), the LAC Component Engineer, provided eleven MC3080 LACs for evaluation where four of the LACs failed IR/DCW and one failed FRB requirements. The extraction of rutile sleeves from failed LACs was required to determine the source of failure. Rutile sleeves associated with connector function failures were examined for cracks, debris as well as any other anomalies which could have caused the LAC to not function properly. Sleeves that failed FRB or that experienced high FRB exhibited high symmetry, smooth surface, long-flow amicon, and slightly over-sized inside diameter. LACs that failed DCW or IR requirements had rutile sleeves that exhibited breakdown tracks.
Date: February 1, 2006
Creator: Kilgo, Alice C.; Monroe, Saundra L.; Watson, Chad Samuel & Ernest, Terry L.
Partner: UNT Libraries Government Documents Department

Highly Mismatched Alloys for Intermediate Band Solar Cells

Description: It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a ''stepping stone'' for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.
Date: March 21, 2005
Creator: Walukiewicz, W.; Yu, K.M.; Wu, J.; Ager III, J.W.; Shan, W.; Scrapulla, M.A. et al.
Partner: UNT Libraries Government Documents Department

Mechanical and Electrical Properties of CdTe Tetrapods Studied byAtomic Force Microscopy

Description: The mechanical and electrical properties of CdTe tetrapod-shaped nanocrystals have been studied with atomic force microscopy. Tapping mode images of tetrapods deposited on silicon wafers revealed that they contact the surface with the ends of three arms. The length of these arms was found to be 130 {+-} 10 nm. A large fraction of the tetrapods had a shortened vertical arm as a result of fracture during sample preparation. Fracture also occurs when the applied load is a few nanonewtons. Compression experiments with the AFM tip indicate that tetrapods with the shortened vertical arm deform elastically when the applied force was less than 50 nN. Above 90 nN additional fracture events occurred that further shorted the vertical arm. Loads above 130 nN produced irreversible damage to the other arms as well. Current-voltage characteristics of tetrapods deposited on gold indicated semiconducting behavior with a current gap of {approx}2 eV at low loads (<50 nN) and a narrowing to about 1 eV at loads between 60 and 110 nN. Atomic calculation of the deformation suggests that the ends of the tetrapod arms are stuck during compression so that the deformations are due to bending modes. The reduction of the current gap is due to electrostatic effects, rather than strain deformation effects inside the tetrapod.
Date: August 30, 2007
Creator: Fang, Liang; Park, Jeong Young; Cui, Yi; Alivisatos, Paul; Shcrier, Joshua; Lee, Byounghak et al.
Partner: UNT Libraries Government Documents Department

Anomalously high photocurrents in nanostructured electrodes : a new local microchip power source.

Description: An increase in photocurrent has been observed at silicon electrodes coated with nanostructured porous silica films as compared to bare, unmodified silicon. Ultimately, to utilize this effect in devices such as sensors or microchip power supplies, the physical phenomena behind this observation need to be well characterized. To this end, Electrochemical Impedance Spectroscopy (EIS) was used to characterize the effect of surfactant-templated mesoporous silica films deposited onto silicon electrodes on the electrical properties of the electrode space-charge region in an aqueous electrolyte solution, as the electrical properties of this space-charge region are responsible for the photobehavior of semiconductor devices. A significant shift in apparent flat-band potential was observed for electrodes modified with the silica film when compared to bare electrodes; the reliability of this data is suspect, however, due to contributions from surface states to the overall capacitance of the system. To assist in the interpretation of this EIS data, a series of measurements at Pt electrodes was performed with the hope of decoupling electrode and film contributions from the EIS spectra. Surprisingly, the frequency-dependent impedance data for Pt electrodes coated with a surfactant-templated film was nearly identical to that observed for bare Pt electrodes, indicating that the mesoporous film had little effect on the transport of small electrolyte ions to the electrode surface. Pore-blocking agents (tetraalkylammonium salts) were not observed to inhibit this transport process. However, untemplated (non-porous) silica films dramatically increased film resistance, indicating that our EIS data for the Pt electrodes is reliable. Overall, our preliminary conclusion is that a shift in electrical properties in the space-charge region induced by the presence of a porous silica film is responsible for the increase in observed photocurrent.
Date: February 1, 2004
Creator: Hughes, Robert Clark; Dunphy, Darren Robert; Brinker, C. Jeffrey & Brozik, Susan Marie
Partner: UNT Libraries Government Documents Department

Novel Approaches to High-Efficiency III-V Nitride Heterostructure Emitters for Next-Generation Lighting Applications

Description: We report research activities and technical progress on the development of high-efficiency long wavelength ({lambda} {approx} 540nm) green light emitting diodes which covers the second year of the three-year program ''Novel approaches to high-efficiency III-V nitride heterostructure emitters for next-generation lighting applications''. The second year activities were focused on the development of p-type layer that has less/no detrimental thermal annealing effect on green LED active region as well as excellent structural and electrical properties and the development of green LED active region that has superior luminescence quality for {lambda} {approx}540nm green LEDs. We have also studied the thermal annealing effect on blue and green LED active region during the p-type layer growth. As a progress highlight, we obtained green-LED-active-region-friendly In{sub 0.04}Ga{sub 0.96}N:Mg exhibiting low resistivity with higher hole concentration (p=2.0 x 10{sup 18} cm{sup -3} and a low resistivity of 0.5 {Omega}-cm) and improved optical quality green LED active region emitting at {lambda} {approx}540nm by electroluminescence. The active region of the green LEDs was found to be much more sensitive to the thermal annealing effect during the p-type layer growth than that of the blue LEDs. We have designed grown, fabricated green LED structures for both 520 nm and 540 nm for the evaluation of second year green LED development.
Date: January 1, 2006
Creator: Dupuis, Russell D.
Partner: UNT Libraries Government Documents Department


Description: We have developed a novel insulator concept that involves the use of alternating layers of conductors and insulators with periods less than 1 mm. We have demonstrated that these structures perform 2 to 5 times better than conventional insulators in long pulse, short pulse, and alternating polarity applications. We present new testing results showing exceptional behavior at DC, with gradients in excess of 110kV/cm in vacuum.
Date: May 26, 2005
Creator: Watson, J A; Caporaso, G J; Sampayan, S E; Sanders, D M & Krogh, M L
Partner: UNT Libraries Government Documents Department

Interpretation Of Multifrequency Crosswell Electromagnetic Data With Frequency Dependent Core Data

Description: Interpretation of cross-borehole electromagnetic (EM) images acquired at enhanced oil recovery (EOR) sites has proven to be difficult due to the typically complex subsurface geology. Significant problems in image interpretation include correlation of specific electrical conductivity values with oil saturations, the time-dependent electrical variation of the subsurface during EOR, and the non-unique electrical conductivity relationship with subsurface conditions. In this study we perform laboratory electrical properties measurements of core samples from the EOR site to develop an interpretation approach that combines field images and petrophysical results. Cross-borehole EM images from the field indicate resistivity increases in EOR areas--behavior contrary to the intended waterflooding design. Laboratory measurements clearly show a decrease in resistivity with increasing effective pressure and are attributed to increased grain-to-grain contact enhancing a strong surface conductance. We also observe a resistivity increase for some samples during brine injection. These observations possibly explain the contrary behavior observed in the field images. Possible mechanisms for increasing the resistivity in the region include (1) increased oil content as injectate sweeps oil toward the plane of the observation wells; (2) lower conductance pore fluid displacing the high-conductivity brine; (3) degradation of grain-to-grain contacts of the initially conductive matrix; and (4) artifacts of the complicated resistivity/time history similar to that observed in the laboratory experiments.
Date: June 7, 2005
Creator: Kirkendall, B & Roberts, J
Partner: UNT Libraries Government Documents Department

Synthesis and Magnetic, Thermal, and Electrical Measurements on Complex non-Cuprate Superconductors

Description: The project investigated superconductivity in non-cuprate materials with critical temperatures, T{sub c}, in excess of 20 K in order to understand the thermodynamics of several of these materials. The project is a cooperative effort between investigators at Southern University (SU), Louisiana State University (LSU), and Los Alamos National Laboratory (LANL). It involved synthesis of high quality samples, and subsequent detailed magnetic, thermal and electrical measurements on them. The project provided a PhD Thesis research experience and training for a graduate student, Ms. Robin Macaluso. High quality, single crystal samples were synthesized by Ms. Macaluso under the direction of one of the CO-PIS, John Sarao, during the summer while she was a visitor at LANL being supported by this grant. On these samples magnetic measurements were performed at SU, thermal and electrical measurements were made in the LSU Physics and Astronomy Department. The crystallographic properties were determined in the LSU Chemistry Department by Ms. Macaluso under the direction of her dissertation advisor, Dr. Julia Chan. Additional high field magnetic measurements on other samples were performed at the National High Magnetic Field Laboratory (NHMFL) both in Tallahassee and at LANL. These measurements involved another graduate student, Umit Alver, who used some of the measurements as part of his PhD dissertation in Physics at LSU.
Date: February 27, 2006
Creator: Henry, Laurence L
Partner: UNT Libraries Government Documents Department

Comparison of Wide-Bandgap Semiconductors for Power Electronics Applications

Description: Recent developmental advances have allowed silicon (Si) semiconductor technology to approach the theoretical limits of the Si material; however, power device requirements for many applications are at a point that the present Si-based power devices cannot handle. The requirements include higher blocking voltages, switching frequencies, efficiency, and reliability. To overcome these limitations, new semiconductor materials for power device applications are needed. For high power requirements, wide-bandgap semiconductors like silicon carbide (SiC), gallium nitride (GaN), and diamond, with their superior electrical properties, are likely candidates to replace Si in the near future. This report compares wide-bandgap semiconductors with respect to their promise and applicability for power applications and predicts the future of power device semiconductor materials.
Date: January 2, 2004
Creator: Ozpineci, B.
Partner: UNT Libraries Government Documents Department

Neutron and gamma irradiation damage to organic materials.

Description: This document discusses open literature reports which investigate the damage effects of neutron and gamma irradiation on polymers and/or epoxies - damage refers to reduced physical chemical, and electrical properties. Based on the literature, correlations are made for an SNL developed epoxy (Epon 828-1031/DDS) with an expected total fast-neutron fluence of {approx}10{sup 12} n/cm{sup 2} and a {gamma} dosage of {approx}500 Gy received over {approx}30 years at < 200 C. In short, there are no gamma and neutron irradiation concerns for Epon 828-1031/DDS. To enhance the fidelity of our hypotheses, in regards to radiation damage, we propose future work consisting of simultaneous thermal/irradiation (neutron and gamma) experiments that will help elucidate any damage concerns at these specified environmental conditions.
Date: April 1, 2012
Creator: White, Gregory Von, II & Bernstein, Robert
Partner: UNT Libraries Government Documents Department


Description: The work reported here took place at the University of Minnesota from 07/01/1989 to 06/30/2006. Most of this work focused on computational materials applied to oxides during the first part of this funding period and to nanoscale materials toward the end of the funding period. This funding resulted in three monographs, 36 invited articles or book chapters, 160 articles in refereed journals and 89 invited talks. The funding helped train 13 PhD students and one masters student. The PI received two national research awards for this work. A list of these contributions are presented.
Date: February 27, 2009
Creator: Chelikowsky, James R.
Partner: UNT Libraries Government Documents Department

Liquid Silane Routes to Electronic Materials

Description: New chemistries based upon liquid cyclohexasilane (Si{sub 6}H{sub 12} or CHS) have been used as precursors to silicon-containing electronic materials. Spin-coating of CHS-based inks with subsequent UV light and/or thermal treatment yielded amorphous silicon (a-Si:H) films. While initial ink chemistries gave a-Si:H with high resistivity (i.e., > 10{sup 6} {Omega}.cm), several doping strategies are under development to address this limitation. In this contribution, the current status of solution processed rectifying diodes and field effect transistors fabricated from CHS-based inks will be presented. Additionally, a new printing approach termed collimated aerosol beam direct write (CAB-DW{sup TM}) was developed that allows the deposition of printed Ag lines 5 {mu}m in width. A status update will be given where CHS-based inks have been used to CAB-DW silicon-based features with linewidths <10 {mu}m. Assuming silicon thin film materials with good electrical properties will be developed, there may be significant cost advantages associated with the ability to controllably deposit the semiconductor in a metered fashion.
Date: December 4, 2008
Creator: Schulz, Douglas L.; Dai, Xuliang; Nelson, Kendric J.; Pokhodnya, Konstantin; Hoey, Justin M.; Akhatov, Iskander S. et al.
Partner: UNT Libraries Government Documents Department

Improvements in hot-wire electroexplosive devices

Description: Several possible design improvements in hot-wire electroexplosive devices were investigated. These were: an arc-resistant header, greater bridgewire-post heat dissipation area, reacting metal bridgewires, and a secondary explosive substitute for primary explosive. Tests using an SE-1 test fixture with four header-bridgewire models showed highly promising results for raising the overall hot-wire ignition safety margins in electrostatic and rf environments without large increases in firing energy. Various electrical characteristics of the four models are given, together with electrical characteristics and function times of the devices loaded with the secondary explosive. (auth)
Date: October 1, 1973
Creator: Joppa, R.M.
Partner: UNT Libraries Government Documents Department

Phase-sensitive LVDT transducer testing system

Description: A system has been developed to measure mechanical and electrical parameters of ultralinear STA0.05% (full scale) nonlinearity! displacement transducers of the linear variable differential transformer (LVDT) type. From these measurements, transducer pretravel, over travel, maximum percent nonlinearity, null voltage, and half-range output voltage can be determined. The system measures transducer stylus position with a precision digital micrometer. Corresponding output/input voltage ratio (transfer ratio) data are measured using a phase-sensitive, AC bridge circuit. From these measuremerts the maximum percent nonlinearity of an LVDT transducer is determined with an accuracy of plus or minus 0.024% (full scale) for a 0.200-inch (5.08-mm) transducer. (auth)
Date: April 1, 1974
Creator: Turner, P.C.
Partner: UNT Libraries Government Documents Department

Electrical Contacts to Individual Colloidal Semiconductor Nanorods

Description: We report the results of charge transport studies on single CdTe nanocrystals contacted via evaporated Pd electrodes. Device charging energy, E{sub c}, monitored as a function of electrode separation drops suddenly at separations below {approx}55 nm. This drop can be explained by chemical changes induced by the metal electrodes. This explanation is corroborated by ensemble X-Ray photoelectron spectroscopy (XPS) studies of CdTe films as well as single particle measurements by transmission electron microscopy (TEM) and energy dispersive X-Rays (EDX). Similar to robust optical behavior obtained when Nanocrystals are coated with a protective shell, we find that a protective SiO2 layer deposited between the nanocrystal and the electrode prevents interface reactions and an associated drop in E{sub c,max}. This observation of interface reactivity and its effect on electrical properties has important implications for the integration of nanocrystals into conventional fabrication techniques and may enable novel nano-materials.
Date: April 1, 2008
Creator: Trudeau, Paul-Emile; Sheldon, Matt; Altoe, Virginia & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Non-magnetic compensation in ferromagnetic Ga1-xMnxAs and Ga1-xMnxP synthesized by ion implantation and pulsed-laser melting

Description: The electronic and magnetic effects of intentional compensation with non-magnetic donors are investigated in the ferromagnetic semiconductors Ga1-xMnxAs and Ga1-xMnxP synthesized using ion implantation and pulsed-laser melting (II-PLM). It is demonstrated that compensation with non-magnetic donors and MnI have similarqualitative effects on materials properties. With compensation TC decreases, resistivity increases, and stronger magnetoresistance and anomalous Hall effect attributed to skew scattering are observed. Ga1-xMnxAs can be controllably compensated with Te through a metal-insulator transition through which the magnetic and electrical properties vary continuously. The resistivity of insulating Ga1-xMnxAs:Te can be described by thermal activation to the mobility edge and simply-activated hopping transport. Ga1-xMnxP doped with S is insulating at all compositions but shows decreasing TC with compensation. The existence of a ferromagnetic insulating state in Ga1-xMnxAs:Te and Ga1-xMnxP:S having TCs of the same order as the uncompensated materials demonstrates that localized holes are effective at mediating ferromagnetism in ferromagnetic semiconductors through the percolation of ferromagnetic 'puddles' which at low temperatures.
Date: February 5, 2008
Creator: Scarpulla, M. A.; Stone, P. R.; Sharp, I. D.; Haller, E. E.; Dubon, O. D.; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

On the Relationship Between Microstructure and Electrical and Hydraulic Properties of Sand-Clay Mixtures

Description: A series of laboratory experiments were performed on saturated sand-clay mixtures, including electrical properties, permeability and porosity. Different mixtures and configurations of quartz sand and 0 to 10% Na-montmorillonite clay saturated with solutions of CaCl{sub 2} and deionized water were investigated. The electrical properties were dependent on clay content, fluid conductivity, and microstructure in a complex fashion. Two main regions of conduction exist: a region dominated by surface conduction and a region where the ionic strength of the saturating fluid controlled conduction. For low fluid conductivities, the sample geometry was found to greatly affect the magnitude of the surface conductance. The influence of the microstructural properties on the electrical properties was quantified by estimating formation factors, {Lambda}-parameters, and surface conductances. We suggest that high and low bounds on the expected surface and bulk conductance of natural systems can be derived from the measurements on these artificial configurations.
Date: October 25, 1999
Creator: Wildenschild, D.; Roberts, J.J. & Carlsberg, E.D.
Partner: UNT Libraries Government Documents Department

Compact X-band high power load using magnetic stainless steel

Description: We present design and experimental results of a high power X-band load. The load is formed as a disk-loaded waveguide structure using lossy, Type 430 stainless steel. The design parameters have been optimized using the recently developed mode-matching code MLEGO. The load has been designed for compactness while maintaining a band width greater than 300 MHz.
Date: May 1, 1995
Creator: Tantawi, S.G. & Vlieks, A.E.
Partner: UNT Libraries Government Documents Department

Structural and electronic properties of clean and defected Si-SiC(001) surfaces

Description: We have studied the reconstructions and electronic properties of both clean and defected Si-terminated (001) surfaces of cubic SiC, by performing -first principles computations within density functional theory. We find that the unstrained bulk exhibits a stable p(2xl) reconstruction, whereas a bulk under tensile stress shows a c(4x2) 1econstruction Furthermore our calculations indicate that ad-dimers are common defects on the Si-terminated SIC(001) surface These results permit the interpretation of recent STM and X-ray- photoemission experimental data.
Date: June 1, 1998
Creator: Galli, G.
Partner: UNT Libraries Government Documents Department