Scientific Publications 2004
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
C
2004. "Growth of Oxide Nanorod Arrays through Sol Electrophoretic Deposition." Journal of Physical Chemistry B 108:19921-19931. Abstract This article introduces a process for the growth of oxide nanorod arrays that combines sol preparation and template-based electrophoretic deposition. First, fundamentals and practical approaches in sol-gel processing for synthesis of inorganic or inorganic/organic hybrid materials are briefly described. Second, electrophoresis in colloidal dispersions and electrophoretic deposition technique are discussed. Particular attention is devoted to the electrophoretic deposition for the growth of oxide nanorod arrays from sols. Finally, techniques similar to or derived from sol electrophoretic deposition for the growth of single crystalline oxide nanorod arrays are presented with vanadium pentoxide as a model system. Further examples are shown that the sol electrophoretic deposition is an effective method for the formation of conformal coating of thin films of oxides on metal nanorods to produce metal-oxide core-shell nanocable arrays. Relations between processing conditions, growth parameters, morphologies, microstructures, and properties of nanorod arrays are reviewed.
2004. "Accurate Valence Band Maximum Determination for SrTiO₃(001) ." Surface Science 554(2-3):81-89. Abstract We reexamine a well-established method for determining valence band maxima in semiconductors based on fitting photoemission spectra to theoretical densities of states. This technique is inaccurate for certain oxides because electronic structure methods predict too sharp a leading edge rise, which in turn appears to stem from an underestimation of the extent of metal-oxygen hybridization at the top of the valence band. In contrast, extrapolating the x-ray excited leading edge to the energy axis, in combination with the energy at which the UV-excited leading edge intensity goes to zero, yields consistent and reliable results that are useful for accurate band offset determinations.
2004. "Experimental Determination of Valence Band Maxima for SrTiO3, TiO2, and SrO and the Associated Valence Band Offsets with Si(001)." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 22(4):2205-2215. Abstract Abstract We address the issue of accurate determination of the energy at the top of the valence band for SrTiO₃(001) single crystals, as well as TiO₂(001) anatase and SrO epitaxial films. These measurements are of critical importance in determining valence band offsets for oxides in heterojunctions involving these materials. Three different methods are presented and compared: (1) fitting Gaussian broadened theoretical densities of states to x-ray excited valence band spectra, (2) finding the intersection of a regression line that spans the linear portion of the x-ray excited valence band leading edge with the background between the valence band maximum and the Fermi level, and, (3) determining the energy at which high-resolution ultraviolet photoemission intensity of the leading edge goes to zero. We find that method 1 is not reliable due to limitations in the accuracy of density functional theory when applied to these oxides. In contrast, methods 2 and 3 give physically reasonable results that are in good mutual agreement. The difference in VBM between method 1 and methods 2 & 3 is 0.4 – 0.6 eV, depending on the oxide. The true valence band maximum can be directly and accurately measured using methods 2 & 3 provided the experiment is carried out with adequate energy resolution. PACS numbers: 79.60.Jv a)Electronic mail: sa.chambers@pnl.gov
2004. "Control of Carbon Nanotube Morphology by Change of Applied Bias Field During Growth." Applied Physics Letters 85(22):5373-5375. Abstract Carbon nanotube morphology has been engineered via simple control of applied voltage during dc plasma chemical vapor deposition growth. Below a critical applied voltage, a nanotube configuration of vertically aligned tubes with a constant diameter is obtained. Above the critical voltage, a nanocone-type configuration is obtained. The strongly field-dependent transition in morphology is attributed primarily to the plasma etching and decrease in the size of nanotube-nucleating catalyst particles. A two-step control of applied voltage allows a creation of dual-structured nanotube morphology consisting of a broad base nanocone (~200 nm dia.) with a small diameter nanotube (~7 nm) vertically emanating from the apex of the nanocone, which may be useful for atomic force microscopy.
2004. "Upconversion luminescence of Eu3+ and Mn2+ in ZnS:Mn2+, Eu3+ codoped nanoparticles." Journal of Applied Physics 95(2):667-672. Abstract Strong upconversion luminescence of both Mn2+ and Eu3+ is observed in ZnS:Mn2+, Eu3+ codoped nanoparticles. Laser power dependencies and spectroscopic data show that the upconversion emission is due to two-photon excitation of each specific dopant ion. The relative differences in two-photon excitation cross section result in different relative intensities for the Eu3+ and Mn2+ upconversion at different wavelengths. Spectroscopic data and luminescence lifetime data indicate no evidence of energy transfer between the Mn2+ and Eu3+ ions.
2004. "Full-Color Emission From In2S3 and In2S3:Eu3+ Nanoparticles." Journal of Physical Chemistry B 108(32):11927-11934. Abstract Blue (425 nm) and green (510 nm) emission is observed from In2S3 nanoparticles which are synthesized using a novel method. Both the blue and green emissions have large Stokes shifts of 62 and 110 nm, respectively. Excitation with longer wavelength photons causes the blue emission to shift to longer wavelength while the green emission wavelength remains unchanged. The lifetimes of both green and blue emissions are similar to reported values for excitonic recombination. When doped with Eu3+, in addition to the broad blue and green emissions, a red emission at 615 nm attributed to Eu3+ is observed. Temperature dependences on nanoparticle thin films indicate that with increasing temperature, the green emission wavelength remains constant, however, the blue emission shifts toward longer wavelengths. Based on these observations, the blue emission is attributed to exciton recombination and the green emission to Indium interstitial defects. These nanoparticles show full-color emission with high efficiency, fast lifetime decays, good stability, and are relatively simple to prepare thus making them a new type of phosphor with potential applications in lighting, flat-panel displays, and communications.
2004. "Optical Properties and Potential Applications of Doped Semiconductor Nanoparticles." Journal of Nanoscience and Nanotechnology 4(8):919-947. Abstract Recent studies on the optical properties, in particular luminescence, of a variety of doped semiconductor nanoparticles are reviewed. The effects of quantum confinement, temperature, and pressure on luminescent properties are discussed. In addition, electroluminescence, cathodoluminescence, magnetoluminescence and related applications involving doped semiconductor nanoparticles are presented. A new phenomenon, upconversion luminescence of doped nanoparticles, is reviewed and its potential applications are discussed. While more research efforts are necessary in order to fully understand the fundamentals and explore the great technological potential behind doped nanoparticles, recent results already show that this is a new and exciting field with applications in many fields. In particular, the emerging field of "spintronics", where spin states are exploited in analogy to conventional electronic states, is discussed and the advantages of using doped semiconductor nanoparticles are elucidated.
2004. "Temperature and Isotope Substitute Effects on the Structure and NMR Properties of the Pertechnetate Ion in Water." Journal of the American Chemical Society 126(37):11583-11588. Abstract The uniquely well-resolved 99Tc NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the 99Tc chemical shift as a function of temperature has been measured over the range 10-45 °C for the three isotopomers Tc(16O)4ˉ, Tc(16O)3(18O)ˉ, and Tc(16O)3(17O)ˉ at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc-O scalar coupling was determined for the Tc(16O)3(17O)ˉ isotopomer. Values for these parameters were computed for three different theoretical models, which include both an unsolvated ion approximation and treatments of the solvated ion based on the COSMO approach. According to these analyses, the temperature- and isotope-dependent shifts in energy observed in the 99Tc NMR spectra can be quantitatively explained by changes in the average Tc-O bond length of the order of 1 x 10ˉ4 Å, which are induced by a redistribution of vibrational state populations and solvent-ion interactions. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental determinations.
2004. "Aqueous and Micelle-bound Structural Characterization of the Epidermal Growth Factor Receptor Juxtamembrane Domain Containing Basolateral Sorting Motifs." Journal of Biomolecular Structure and Dynamics 21(6):813-826. Abstract The EGF receptor is the prototype for four highly related receptors constituting the ErbB family. The EGF receptor is normally targeted to the basolateral membrane in polarized epithelial cells, where it relays information from underlying tissues. Two basolateral sorting signals have been mapped to the cytoplasmic juxtamembrane region of the receptor, a dominant signal comprised of a polyproline core (667-PXXP) and a preceding basic residue (Arg662), and a consensus leucine-based signal (658-LL) responsible for residual sorting when the 667-PXXP signal is absent or defective. The goal of this study was to define the structure of these signals, and gain some insights into how these structures might be regulated by cellular microenvironment. Structural information was acquired for two peptides corresponding to EGF receptor residues Arg645 and Ala674 in aqueous solution or in the presence of membrane-mimicking dodecylphosphocholine micelles, using a variety of NMR and CD spectroscopic methods. Chemical shift data indicate that the 667-PXXP signal does not bind to the micelles and is in random coil state in both aqueous solution and a micellar environment, raising the possibility that 667-PXXP switches to an ordered structure during interaction with the basolateral sorting machinery. In contrast, the adjacent region including 658-LL does bind to micelles mediated by a highly positively charged region located between Arg645 and Arg656. The micelle-bound region also includes Thr654, a known substrate for PKC. This suggests a distinct mode of regulation for this signal involving membrane association and/or phosphorylation.
2004. "Peak Fitting to Resolve CN-Isotope Ratios in Biological and Environmental Samples using TOF-SIMS." Applied Surface Science 231-232(Sp. Iss. SI):912-916. Abstract Our research has focused on developing TOF-SIMS to measure organic 15N in environmental samples [Appl. Environ. Microbiol. 68 (8) (2002) 4067]. Our goal was to develop a peak-fitting algorithm that would successfully remove the isobaric interferences of Al- and 13C14N- from 12C15N- ions under conditions of low mass resolution inherent in environmental samples. We tested a variety of peak-fitting models and found that the EMG + GMG (E+G) model performed better than the standard peak shape shifting method under conditions of high mass resolution, unless Al- was present as an interference. Under conditions of Al- interference and low 15N content, the standard method performed better than the E + G model. As 15N content increased, the E + G model worked comparably or better than the standard method. Limited mass resolution during analysis of organic 15N standards on kaolin clay dictated using the standard method which performed acceptably on standards containing greater than 1 at.% 15N. These data emphasize the potential utility of using analytical models to resolve isobaric interferences in TOF-SIMS.
2004. "The Raised Midpoint Potential of the [2Fe2S] Cluster of Cytochrome bc1 is Mediated by Both the Q(o) Site Occupants and the Head Domain Position of the Fe-S Protein Subunit ." Biochemistry 43(8):2217-2227. Abstract We have previously reported that mutant strains of Rhodobacter capsulatus that have alanine insertions (+nAla mutants) in the hinge region of the iron sulfur (Fe-S) containing subunit of the bc1 complex have increased redox midpoint potentials (Em) for their [2Fe2S] clusters. The alteration of the Em in these strains, that contain mutations far from the metal binding site, implied that the local environment of the metal center is indirectly altered by a change in the interaction of this subunit with the hydroquinone oxidizing (Qo) site (Darrouzet, E., Valkova-Valchanova, M., and Daldal, F. (2002) Journal of Biological Chemistry 277, 3464- 3470). Subsequently, the Em changes have been proposed to be predominantly due to a stronger or more stabilized hydrogen bonding between the reduced [2Fe2S] cluster and the Qo site inhabitant ubiquinone (Q) (Shinkarev, V. P., Kolling, D. R. J., Miller, T. J., and Crofts, A. R. (2002) Biochemistry 41, 14372-14382). To further investigate this issue, Fe-S protein-Q interactions were monitored by EPR spectroscopy and the findings indicated that the wild type and mutant proteins interactions with Q are similar. Moreover, when the Qpool was chemically depleted, the Em of the [2Fe2S] cluster in mutant bc1 complexes remained more positive than a similarly treated native enzyme (e.g., the [2Fe2S] Em of the +2Ala mutant was 55 mV more positive than the wild type). These data suggest that the increased Em of the [2Fe2S] cluster in the +nAla mutants is in part due to the cluster’s interaction with Q, and in part to additional factors that are independent of hydrogen bonding to Q. One such factor, the possibility of a different position of the Fe-S at the Qo site of the mutant proteins versus the native enzyme, was addressed by determining the orientation of the [2Fe2S] cluster in the membrane using EPR spectroscopy. In the case of the +2Ala mutant, the [2Fe2S] cluster orientation in the absence of inhibitor is different than that seen in the native enzyme. However, the +2Ala mutant cluster shared a similar orientation with the native enzyme when both samples were exposed to either stigmatellin or myxothiazol. In addition, Qpool extracted membranes of +2Ala mutant exhibited fewer overall orientations, with the predominant one being more similar to that observed in the non Q-depleted membranes of +2Ala mutant than the Q-depleted membranes of a wild type strain. Therefore, additional component(s) that are independent of Qo site inhabitants and that originate from the newly observed orientations of the [2Fe2S] clusters in the +nAla mutants also contribute to the increased midpoint potentials of their [2Fe2S] clusters. While the molecular basis of these components remains to be determined, salient implications of these findings in terms of Qo site catalysis are discussed.
2004. "Rotational Analysis of Bands in the High-Resolution Infrared Spectra of the Three Species of Butadiene-1,4-d2; Refinement of the Assignments of the Vibrational Fundamentals." Journal of Molecular Spectroscopy 228(2):401-413. Abstract Samples of trans, trans and cis, cis forms of butadiene-1, 4-d2 have been synthesized and found to contain useful amounts of the cis, trans species as a contaminant. Assignments of fundamental frequencies for the three isotopomers of butadiene-1,4-d2 have been extended and improved from investigations of their Raman spectra as well as their IR spectra. High-resolution IR spectra have been recorded for the three isotopomers, and a rotational analysis has been completed for strong bands of each species. Ground state and some upper state rotational constants have been fit. Corresponding ground state moments of inertia compare faborably with equilibrium moments of inertia obtained from B3LYP/6-311++G** theory. In the absence of rotational constants for 13C isotopomers it is premature to attempt an improved structural analysis of butadiene to assess how π-electron delocalization affects structure.
