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Interactions of structurally modified surfactants with reservoir minerals: Calorimetric, spectroscopic and electrokinetic study

Description: The objective of this project is to elucidate mechanisms of adsorption of structurally modified surfactants on reservoir minerals and to develop a full understanding of the effect of the surfactant structure on the nature of the adsorbed layers at the molecular level. An additional aim is to study the adsorption of surfactant mixtures on simple well-characterized minerals and on complex minerals representing real conditions. The practical goal of these studies is the identification of the optimum surfactant structures and their combinations for micellar flooding. In this work, the experiments on adsorption were focussed on the position of sulfonate and methyl groups on the aromatic ring of alkyl xylene sulfonates. A multi-pronged approach consisting of calorimetry, electrokinetics, wettability and spectroscopy is planned to elucidate the adsorption mechanism of surfactants and their mixtures on minerals such as alumina and kaolinite. 32 refs., 15 figs., 7 tabs.
Date: March 1, 1991
Creator: Somasundaran, P.; Sivakumar, A. & Xu, Q.
Partner: UNT Libraries Government Documents Department

MEASUREMENTS OF LAMBA AND ANTI-LAMBDA POLARIZATION IN LONGITUDINALLY POLARIZED PROTON-PROTON COLLISIONS AT SQRT S(NN) = 200 GEV AT STAR.

Description: Preliminary results for the longitudinal polarization of A and hyperons in longitudinally polarized proton-proton collisions at {radical}s = 200 GeV are presented. The {Lambda}({bar {Lambda}}) candidates are reconstructed at mid-rapidity (|{eta}| < 1) with the time projection chamber of the STAR experiment at RHIC, using 0.5 pb{sup -1} collected in 2003 and 2004 with beam polarizations of up to 45%. Their mean longitudinal momentum fraction x{sub F} is about 8 x 10{sup -3} and their mean transverse momentum p{sub T} is about 1.5 GeV. The analysis uses asymmetries of counts for different spin states of the colliding proton beams and does not require detailed knowledge of the detector acceptance. The preliminary {Lambda}({Lambda}) polarization values are consistent with zero within their statistical uncertainties of 0.05.
Date: October 24, 2005
Creator: XU, Q. (FOR THE STAR COLLABORATION)
Partner: UNT Libraries Government Documents Department

Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacterial System

Description: The objective of this report is to develop an O{sub 2}-tolerant cyanobacterial system for continuous light-driven H{sub 2} production from water. The overall goal is to produce a cyanobacterial recombinant to produce H{sub 2} continuously.
Date: May 1, 2007
Creator: Xu, Q.; Smith, H. O. & Maness, P.-C.
Partner: UNT Libraries Government Documents Department

Stability and Electronic Structures of CuxS Solar Cell Absorbers: Preprint

Description: Cu{sub x}S is one of the most promising solar cell absorber materials that has the potential to replace the leading thin-film solar cell material Cu(In,Ga)Se{sub 2} for high efficiency and low cost. In the past, solar cells based on Cu{sub x}S have reached efficiency as high as 10%, but it also suffers serious stability issues. To further improve its efficiency and especially the stability, it is important to understand the stability and electronic structure of Cu{sub x}S. However, due to the complexity of their crystal structures, no systematic theoretical studies have been carried out to understand the stability and electronic structure of the Cu{sub x}S systems. In this work, using first-principles method, we have systematically studied the crystal and electronic band structures of Cu{sub x}S (1.25 < x {le} 2). For Cu{sub 2}S, we find that all the three chalcocite phases, i.e., the low-chalcocite, the high-chalcocite, and the cubic-chalcocite phases, have direct bandgaps around 1.3-1.5 eV, with the low-chalcocite being the most stable one. However, Cu vacancies can form spontaneously in these compounds, causing instability of Cu{sub 2}S. We find that under Cu-rich condition, the anilite Cu{sub 1.75}S is the most stable structure. It has a predicted bandgap of 1.4 eV and could be a promising solar cell absorber.
Date: July 1, 2012
Creator: Wei, S. H.; Xu, Q.; Huang, B.; Zhao, Y.; Yan, Y. & Noufi, R.
Partner: UNT Libraries Government Documents Department

Insulator to correlated metal transition in V_1-xMo_xO_2

Description: The change from metallic to insulating states is one of the most dramatic transitions that solids undergo on cooling or chemical doping. Many materials display this transition, but only a handful have the right combination of crystal structure and physical properties to serve as model systems. VO{sub 2} is one of those materials. Using Mo as a chemical dopant in VO{sub 2}, we find unanticipated phenomenology for both the electronic and structural characteristics of the resulting insulator to metal transition. The results support a complex, previously proposed scenario involving the coexistence of both electron repulsion and electron pairing for yielding an insulator in VO{sub 2}, but not simply; many issues are raised about local versus itinerant behavior and structure-property correlations in this most iconic ofdoped correlated electron systems.
Date: January 1, 2009
Creator: Klimczuk, Tomasz; Ronning, Filip; Holman, Katherine; Mcqueen, Tyrel M; Williams, Anthony J; Stephens, P W et al.
Partner: UNT Libraries Government Documents Department

Understanding Enzyme Activity Using Single Molecule Tracking (Poster)

Description: This poster describes single-molecule tracking and total internal reflection fluorescence microscopy. It discusses whether the carbohydrate-binding module (CBM) moves on cellulose, how the CBM binds to cellulose, and the mechanism of cellulosome assembly.
Date: June 1, 2009
Creator: Liu, Y.-S.; Zeng, Y.; Luo, Y.; Xu, Q.; Himmel, M.; S., Smith et al.
Partner: UNT Libraries Government Documents Department

Germanium Nanocrystals Embedded in Sapphire

Description: {sup 74}Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by damage. Embedded nanocrystals experience large compressive stress relative to bulk, as embedded in sapphire melt very close to the bulk melting point (Tm = 936 C) whereas experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and measured by Raman spectroscopy of the zone center optical phonon. In contrast, reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding thermal annealing. Transmission electron microscopy (TEM) of as-grown samples those embedded in silica exhibit a significant melting point hysteresis around T{sub m}.
Date: April 15, 2005
Creator: Xu, Q.; Sharp, I. D.; Liao, C. Y.; Yi, D. O.; Ager, J. W., III; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

Photo-oxidation of Ge Nanocrystals: Kinetic Measurements by InSitu Raman Spectroscopy

Description: Ge nanocrystals are formed in silica by ion beam synthesis and are subsequently exposed by selective HF etching of the silica. Under ambient conditions, the exposed nanocrystals are stable after formation of a protective native oxide shell of no more than a few monolayers. However, under visible laser illumination at room temperature and in the presence of O{sub 2}, the nanocrystals rapidly oxidize. The oxidation rate was monitored by measuring the Raman spectra of the Ge nanocrystals in-situ. The intensity ratio of the anti-Stokes to the Stokes line indicated that no significant laser-induced heating of illuminated nanocrystals occurs. Therefore, the oxidation reaction rate enhancement is due to a photo-chemical process. The oxidation rate varies nearly linearly with the logarithm of the laser intensity, and at constant laser intensity the rate increases with increasing photon energy. These kinetic measurements, along with the power dependencies, are described quantitatively by an electron active oxidation mechanism involving tunneling of optically excited electrons through the forming oxide skin and subsequent transport of oxygen ions to the Ge nanocrystal surface.
Date: November 22, 2006
Creator: Sharp, I.D.; Xu, Q.; Yuan, C.W.; Beeman, J.W.; Ager III, J.W.; Chrzan, D.C. et al.
Partner: UNT Libraries Government Documents Department

Ferromagnetism and suppression of metallic clusters in Fe implanted ZnO -- a phenomenon related to defects?

Description: We investigated ZnO(0001) single crystals annealed in high vacuum with respect to their magnetic properties and cluster formation tendency after implant-doping with Fe. While metallic Fe cluster formation is suppressed, no evidence for the relevance of the Fe magnetic moment to the observed ferromagnetism was found. The latter along with the cluster suppression is discussed with respect to defects in the ZnO host matrix, since the crystalline quality of the substrates was lowered due to the preparation as observed by x-ray diffraction.
Date: March 12, 2008
Creator: Arenholz, Elke; Zhou, S.; Potzger, K.; Talut, G.; Reuther, H.; Kuepper, K. et al.
Partner: UNT Libraries Government Documents Department

Mechanism of stress relaxation in Ge nanocrystals embedded in SiO2

Description: Ion-beam-synthesized {sup 74}Ge nanocrystals embedded in an amorphous silica matrix exhibit large compressive stresses in the as-grown state. The compressive stress is determined quantitatively by evaluating the Raman line shift referenced to the line position of free-standing nanocrystals. Post-growth thermal treatments lead to stress reduction. The stress relief process is shown to be governed by the diffusive flux of matrix atoms away from the local nanocrystal growth region. A theoretical model that quantitatively describes this process is presented.
Date: August 30, 2004
Creator: Sharp, I.D.; Yi, D.O.; Xu, Q.; Liao, C.Y.; Beeman, J.W.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department

A Chemical Approach to 3-D Lithographic Patterning of Si and GeNanocrystals

Description: Ion implantation into silica followed by thermal annealingis an established growth method for Si and Ge nanocrystals. Wedemonstrate that growth of Group IV semiconductor nanocrystals can besuppressed by co-implantation of oxygen prior to annealing. For Sinanocrystals, at low Si/O dose ratios, oxygen co-implantation leads to areduction of the average nanocrystal size and a blue-shift of thephotoluminescence emission energy. For both Si and Ge nanocrystals, atlarger Si/O or Ge/O dose ratios, the implanted specie is oxidized andnanocrystals do not form. This chemical deactivation was utilized toachieve patterned growth of Si and Ge nanocrystals. Si was implanted intoa thin SiO2 film on a Si substrate followed by oxygen implantationthrough an electron beam lithographically defined stencil mask. Thermalannealing of the co-implanted structure yields two-dimensionallypatterned growth of Si nanocrystals under the masked regions. We applieda previously developed process to obtain exposed nanocrystals byselective HF etching of the silica matrix to these patterned structures.Atomic force microscopy (AFM) of etched structures revealed that exposednanocrystals are not laterally displaced from their original positionsduring the etching process. Therefore, this process provides a means ofachieving patterned structures of exposed nanocrystals. The possibilitiesfor scaling this chemical-based lithography process to smaller featuresand for extending it to 3-D patterning is discussed.
Date: December 12, 2005
Creator: Sharp, I. D.; Xu, Q.; Yi, D. O.; Liao, C. Y.; Ager, J. W., III; Beeman, J. W. et al.
Partner: UNT Libraries Government Documents Department

Crystal fields, disorder, and antiferromagnetic short-range order in (Yb{sub 0.24}Sn{sub 0.76})Ru

Description: We report extensive measurements on a new compound (Yb{sub 0.24}Sn{sub 0.76})Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L{sub 3} x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb{sup 3+} is split by the crystalline electric field (CEF) into a Γ{sub 7} doublet ground state and a Γ{sub 8} quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ{sub 6} excited state resides at 32 meV; however, the Γ{sub 8}/Γ{sub 6} transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ{sub 7} ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.
Date: July 18, 2011
Creator: Klimczuk, T.; Wang, C. H.; Lawrence, J. M.; Xu, Q.; Durakiewicz, T.; Ronning, F. et al.
Partner: UNT Libraries Government Documents Department

Kinetics of visible light photo-oxidation of Ge nanocrystals:Theory and in situ measurement

Description: Photo-oxidation of Ge nanocrystals illuminated with visible laser light under ambient conditions was investigated. The photo-oxidation kinetics were monitored by in situ measurement of the crystalline Ge volume fraction by Raman spectroscopy. The effects of laser power and energy on the extent of oxidation were measured using both in situ and ex situ Raman scattering techniques. A mechanistic model in which the tunneling of photo-excited carriers to the oxide surface for electron activated molecular oxygen dissociation is proposed. This quantitative model successfully describes all experimental photo-oxidation observations using physical parameters.
Date: November 14, 2006
Creator: Sharp, I.D.; Xu, Q.; Yuan, C.W.; Beeman, J.W.; Ager III, J.W.; Chrzan, D.C. et al.
Partner: UNT Libraries Government Documents Department

Superheating and supercooling of Ge nanocrystals embedded inSiO2

Description: Free-standing nanocrystals exhibit a size-dependant thermodynamic melting point reduction relative to the bulk melting point that is governed by the surface free energy. The presence of an encapsulating matrix, however, alters the interface free energy of nanocrystals and their thermodynamic melting point can either increase or decrease relative to bulk. Furthermore, kinetic contributions can significantly alter the melting behaviors of embedded nanoscale materials. To study the effect of an encapsulating matrix on the melting behavior of nanocrystals, we performed in situ electron diffraction measurements on Ge nanocrystals embedded in a silicon dioxide matrix. Ge nanocrystals were formed by multi-energy ion implantation into a 500 nm thick silica thin film on a silicon substrate followed by thermal annealing at 900 C for 1 h. We present results demonstrating that Ge nanocrystals embedded in SiO{sub 2} exhibit a 470 K melting/solidification hysteresis that is approximately symmetric about the bulk melting point. This unique behavior, which is thought to be impossible for bulk materials, is well described using a classical thermodynamic model that predicts both kinetic supercooling and kinetic superheating. The presence of the silica matrix suppresses surface pre-melting of nanocrystals. Therefore, heterogeneous nucleation of both the liquid phase and the solid phase are required during the heating and cooling cycle. The magnitude of melting hysteresis is governed primarily by the value of the liquid Ge/solid Ge interface free energy, whereas the relative values of the solid Ge/matrix and liquid Ge/matrix interface free energies govern the position of the hysteresis loop in absolute temperature.
Date: August 21, 2006
Creator: Xu, Q.; Sharp, I.D.; Yuan, C.W.; Yi, D.O.; Liao, C.Y.; Glaeser,A.M. et al.
Partner: UNT Libraries Government Documents Department

Stable, free-standing Ge nanocrystals

Description: Free-standing Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5 nm are grown in amorphous SiO{sub 2} by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain free-standing nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Free-standing nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For free-standing as opposed to embedded Ge nanocrystals, an additional amorphous-like contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms.
Date: January 28, 2005
Creator: Sharp, I.D.; Xu, Q.; Liao, C.Y.; Yi, D.O.; Beeman, J.W.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department

Large melting point hysteresis of Ge nanocrystals embedded inSiO2

Description: The melting behavior of Ge nanocrystals embedded within SiO{sub 2} is evaluated using in situ transmission electron microscopy. The observed melting point hysteresis is large ({+-} 17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.
Date: May 4, 2006
Creator: Xu, Q.; Sharp, I.D.; Yuan, C.W.; Yi, D.O.; Liao, C.Y.; Glaeser,A.M. et al.
Partner: UNT Libraries Government Documents Department