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Now showing items 1 - 16 of 803

  • Classification of Tea Aromas Using Multi-Nanoparticle Based Chemiresistor Arrays

    Gao, Tuo   Wang, Yongchen   Zhang, Chengwu   Pittman, Zachariah A.   Oliveira, Alexandra M.   Fu, Kan   Zhao, Jing   Srivastava, Ranjan   Willis, Brian G.  

    Nanoparticle based chemical sensor arrays with four types of organo-functionalized gold nanoparticles (AuNPs) were introduced to classify 35 different teas, including black teas, green teas, and herbal teas. Integrated sensor arrays were made using microfabrication methods including photolithography and lift-off processing. Different types of nanoparticle solutions were drop-cast on separate active regions of each sensor chip. Sensor responses, expressed as the ratio of resistance change to baseline resistance (R/R-0), were used as input data to discriminate different aromas by statistical analysis using multivariate techniques and machine learning algorithms. With five-fold cross validation, linear discriminant analysis (LDA) gave 99% accuracy for classification of all 35 teas, and 98% and 100% accuracy for separate datasets of herbal teas, and black and green teas, respectively. We find that classification accuracy improves significantly by using multiple types of nanoparticles compared to single type nanoparticle arrays. The results suggest a promising approach to monitor the freshness and quality of tea products.
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  • Atomic layer deposition synthesis and evaluation of core-shell Pt-WC electrocatalysts

    Hsu, Irene J.   Chen, Jingguang G.   Jiang, Xiaoqiang   Willis, Brian G.  

    Pt-WC core shell particles were produced using atomic layer deposition (ALD) to deposit Pt layers onto WC particle substrates. A range of Pt depositions were used to determine the growth mechanism for the Pt-WC powder system. TEM imaging and Cu stripping voltammetry found that Pt ALD growth on WC powder substrates was similar to that on WC thin films. However, excess free carbon was found to affect Pt ALD by blocking adsorption sites on WC. The Pt-WC samples were evaluated for the oxygen reduction reaction using a rotating disk electrode to obtain quantitative activity information. The mass and specific activities for the 30 and 50 ALD cycle samples were found to be comparable to a 10 wt.% Pt/C catalyst. However, higher overpotentials and lower limiting currents were observed with ALD Pt-WC compared to Pt/C catalysts, indicating that the oxygen reduction mechanism is not as efficient on Pt-WC as on bulk Pt. Additionally, these Pt-WC catalysts were used to demonstrate hydrogen evolution reaction activity and were found to perform as well as bulk Pt catalyst but with a fraction of the Pt loading, in agreement with the previous work on Pt-WC thin film catalysts. (C) 2014 American Vacuum Society.
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  • The role of dangling bondsin H2O-induced oxidation of Si(100)-2 x 1

    Skliar, Dimitri B.   Willis, Brian G.  

    The stability of the water-terminated Si(100)-2 x 1 surface in the 300-550 K temperature range is investigated with ultrahigh vacuum scanning tunneling microscopy experiments and density functional theory calculations. For temperatures below 450 K the surface is found to be stable from hydroxyl decomposition and surface oxidation. In the range of 450-550 K, new surface features associated with oxygen insertion into the silicon dimer bond are observed. It is found that the rate of hydroxyl decomposition and oxygen insertion does not follow simple first order kinetics with respect to the surface hydroxyl groups. Density functional theory calculations of oxygen insertion pathways point toward a catalytic effect of the dangling bonds and suggest that in the 450-550 K range the insertion events should predominantly occur next to unoccupied surface sites. A model is proposed where the dangling bonds diffuse along the dimer rows and promote hydroxyl decomposition. Kinetic Monte-Carlo simulations are used to compare the model with both experiments and density functional theory calculations, and an insertion activation barrier of 1.8 eV is found to give a good fit to the experimental data. On the basis of the findings, a strategy to increase hydroxyl group stability is demonstrated using water termination at cryogenic temperatures.
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  • Binding of styrene on silicon (111)-7 x 7 surfaces as a model molecular electronics system

    Weiland, Conan R.   Yang, Liu   Doren, Douglas J.   Menning, Carl A.   Skliar, Dimitri   Willis, Brian G.   Chen, Jingguang G.   Opila, Robert L.  

    Styrene on Si(111)-7 x 7 surfaces was investigated as a model molecular electronics system to study the effects of conjugation to the semiconductor surface. Scanning tunneling microscopy imaging, combined with theoretical images show that styrene molecules bind in a [4 + 2] cycloaddition mode, binding at the terminal C of the vinyl substituent and a C on the phenyl ring. These single molecule results are consistent with those of ensembles of molecules as obtained by high resolution electron energy loss spectroscopy. For styrene on Si(111)-7 x 7, the aromaticity of the phenyl ring is broken and the conjugated pi-system does not extend to the silicon surface, potentially hindering conduction between molecule and surface. However, styrene does retain a conjugated pi-system after binding, as is evident in the pi-pi* shakeup transition in the C 1s x-ray photoelectron spectra as well as UV photoemission spectra. This suggests that styrene-type molecules, when bonded to Si surfaces, do not maintain ready conduction between molecule and surface. (C) 2012 American Vacuum Society. []
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  • Atomic Layer Deposition of Pt on Tungsten Monocarbide (WC) for the Oxygen Reduction Reaction

    Hsu, Irene J.   Hansgen, Danielle A.   McCandless, Brian E.   Willis, Brian G.   Chen, Jingguang G.  

    Atomic layer deposition (ALD) was utilized as a synthesis method to deposit monolayers of Pt onto WC substrates for applications as oxygen reduction reaction electrocatalysts. Samples utilizing various Pt ALD cycles were characterized using surface analytical methods and scanning electron microscopy, whereas cyclic voltammetry was used to determine whether the oxygen reduction reaction takes place on the catalyst surface. ALD Pt was found to deposit onto WC substrates following an island growth mechanism. When few Pt ALD cycles are used, discrete Pt particles first formed and dispersed over the WC substrate, but at least 100 ALD cycles is required for the WC substrate to be covered with Pt. Whereas Pt monolayers are not obtained, ALD Pt on WC still shows activity for the oxygen reduction reaction. Cyclic voltammetry conducted in an O(2)-saturated 0.5 MH(2)SO(4) electrolyte indicate that as few as 20 Pt ALD cycles on WC is needed to produce oxygen reduction reaction activity that is comparable to Pt bulk. Efforts to make Pt films that are even thinner and more monolayer-like are desired, and potential approaches are discussed.
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  • Chemical vapor deposition of phase-rich WC thin films on silicon and carbon substrates

    Beadle, Kendra A.   Gupta, Rahul   Mathew, Anoop   Chen, Jingguang G.   Willis, Brian G.  

    Chemical vapor deposition was used to deposit tungsten carbide from a mixture Of WCl6, H-2 and C3H8 at 750-1050 degrees C on silicon and carbon substrates. The phase composition of the films was correlated with substrate temperature, substrate position in the reactor, and total flow rates. X-ray diffraction and X-ray photoelectron spectroscopy were employed to investigate the surface and bulk properties of the thin films. Thick, adherent films of phase-rich hexagonal WC were deposited using 1.3 x 10(3) Pa total pressure, 1050 degrees C substrate temperature, and reactant flow rates of H-2/C3H8/Ar/WCl6 = 1.8 x 10(-2)/3.6 x 10(-3)/8.9 x 10(-4)/1.8 x 10(-4) mol/min, where Ar is the carrier gas. The surface composition was oxygen and carbon rich as compared with the bulk. (c) 2007 Elsevier B.V. All rights reserved.
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  • beta-Diketones As a Model for the Adsorption of Multifunctional Molecules on Si(100)-2 x 1

    Skliar, Dimitri B.   Willis, Brian G.  

    Reaction pathways of the beta-diketones acetylacetone and dipivaloylmethane on Si(100)-2 x 1 are analyzed using density functional theory calculations and compared with experimental data. Of the multiple pathways available to the molecules, hydroxyl dissociation via a 1,7 H-shift mechanism is determined to be the dominant adsorption pathway. Several other pathways including [2 + 4] addition, [2 + 2] C=O intradimer addition, [2 + 2] C=O intradimer addition with OH dissociation on an adjacent dimer, [2 + 2] C=C intradimer addition, and "ene" addition are also found to be barrierless with respect to the entrance channel, and have small barriers relative to an adsorption precursor intermediate. Pathways involving 1,3 and 1,2 intramolecular H-shifts are found to be highly activated and are expected to be inaccessible at room temperature. Several product interconversions are considered as well. The results are compared with experiment and provide insight to the competitive adsorption pathways for multifunctional molecules on silicon.
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  • In-situ spectroscopic ellipsometry study of copper selective-area atomic layer deposition on palladium

    Jiang, Xiaoqiang   Wang, Han   Qi, Jie   Willis, Brian G.  

    Selective area copper atomic layer deposition on palladium seed layers has been investigated with in-situ real-time spectroscopic ellipsometry to probe the adsorption/desorption and reaction characteristics of individual deposition cycles. The reactants are copper bis(2,2,6,6-tetramethyl-3,5-heptanedionate) vapor and hydrogen gas. Self-limiting atomic layer deposition was observed in the temperature range of 135-230 degrees C in a low pressure reactor. Under optimal conditions, growth occurs selectively on palladium and not on silicon dioxide or silicon nitride layers. Based on in-situ ellipsometry data and supporting experiments, a new mechanism for growth is proposed. In the proposed mechanism, precursor adsorption is reversible, and dissociatively adsorbed hydrogen are the stable surface intermediates between growth cycles. The mechanism is enabled by continuous diffusion of palladium from the seed layer into the deposited copper film and strong H* binding to palladium sites. Less intermixing can be obtained at low growth temperatures and short cycle times by minimizing Cu/Pd inter-diffusion. (C) 2014 American Vacuum Society.
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  • Tunable plasmonic response of metallic nanoantennna heterodimer arrays modified by atomic-layer deposition

    Wambold, Raymond A.   Borst, Benjamin D.   Qi, Jie   Weisel, Gary J.   Willis, Brian G.   Zimmerman, Darin T.  

    We present a systematic study of tunable, plasmon extinction characteristics of arrays of nanoscale antennas that have potential use as sensors, energy-harvesting devices, catalytic converters, in near-field optical microscopy, and in surface-enhanced spectroscopy. Each device is composed of a palladium triangular-prism antenna and a flat counter-electrode. Arrays of devices are fabricated on silica using electron-beam lithography, followed by atomic-layer deposition of copper. Optical extinction is measured by employing a broadband light source in a confocal, transmission arrangement. We characterize the plasmon resonance behavior by examining the dependence on device length, the gap spacing between the electrodes, material properties, and the device array density, all of which contribute in varying degrees to the measured response. We employ finite-difference time-domain simulations to demonstrate good qualitative agreement between experimental trends and theory and use scanning electron microscopy to correlate plasmonic extinction characteristics with changes in morphology. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)
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  • Dielectrophoretic Assembly of Gold Nanoparticles in Nanoscale Junctions for Rapid,Miniature Chemiresistor Vapor Sensors

    Fu, Kan   Chen, Shutang   Zhao, Jing   Willis, Brian G.  

    A method for fabricating integrated arrays of nanoscale chemiresistor vapor sensors using functionalized gold nanoparticles is presented. Controlled placement of nanoparticles was accomplished with dielectrophoresis, achieving localized nanoparticle assembly between 50-nm-thick, 100-nm-wide nanofabricated electrodes with 50 nm spacing. Each individual sensor comprises an assembly of thiol-functionalized 10-nm-diameter gold nanoparticles, making a total active sensing volume with thickness of 30 to 40 nm and area dimension 50 nm x 50 nm. The small electrode spacing enables contiguous films of just 3 to 4 layers of nanoparticles. Combination of top-down lithographic fabrication and bottom-up directed assembly allows multiple sensors spaced by 200 mu m to be fabricated on a single chip. A second set of chemiresistor sensors with larger 20 mu m electrode spacing and 200-300 nm film thickness were fabricated for comparison. Nanoscale sensors fabricated using 4 different types of thiolated capping ligands exhibited response sensitivity and selectivity similar to the larger chemiresistor sensors, but with a signal-to-noise degradation to 25% of the micron scale devices. The results demonstrate that nanofabricated sensors with dense arrays of many different types of functionalized nanoparticles can be integrated on a single chip, and it should be possible to create integrated, independent nanoscale sensors separated by only hundreds of nanometers.
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  • Atomic layer deposition synthesis of platinum-tungsten carbide core-shell catalysts for the hydrogen evolution reaction

    Hsu, Irene J.   Kimmel, Yannick C.   Jiang, Xiaogiang   Willis, Brian G.   Chen, Jingguang G.  

    Pt was deposited onto tungsten carbide powders using atomic layer deposition to produce core-shell catalysts for the hydrogen evolution reaction (HER). The Pt loading on these catalysts was reduced nearly ten-fold compared to a bulk Pt catalyst while equivalent HER activities were observed.
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  • An Evaluation of Density Functional Theory and ab Initio Predictions for Bridge-Bonded Aluminum Compounds

    Willis, Brian G.   Jensen, Klavs F.  

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  • Molecular level investigation of 2,2,6,6-tetramethyl-3,5-heptanedione on Si(100)-2 x 1: Spectroscopic and computational studies

    Perrine, Kathryn A.   Skliar, Dimitri B.   Willis, Brian G.   Teplyakov, Andrew V.  

    The molecular level chemistry of 2,2,6,6-tetramethyl-3,5-heptanedione (dpmH) has been investigated on a Si(100)-2 x 1 surface. The dpmH compound is a beta-diketone, whose deprotonated form is used as a ligand in chemical precursors for metal-organic chemical vapor deposition (MOCVD). A combination of multiple internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and density functional theory (DFT) were employed to analytically detect and monitor surface species under different thermal conditions. Upon adsorption at cryogenic temperatures dpmH was shown to be present in the enolic form, while primarily OH dissociation and [2 + 2] carbonyl cycloaddition were revealed at room temperature. Upon heating from room temperature to 900 K, isobutene evolution into the gas phase was found to be a minor reaction pathway. The remainder of dpmH decomposes on the surface to release hydrogen into the gas phase. (C) 2008 Elsevier B. V. All rights reserved.
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  • Gas-Phase Reaction Pathways of Aluminum Organometallic Compounds with Dimethylaluminum Hydride and Alane as Model Systems

    Willis, Brian G.   Jensen, Klavs F.  

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  • Reversible Molecular Adsorption and Detection Using Inelastic Electron Tunneling Spectroscopy in Monolithic Nanoscopic Tunnel junctions

    Gupta, Rahul   Appelbaum, Ian   Willis, Brian G.  

    Monolithic nanoscopic tunnel junctions fabricated by atomic layer deposition are used to investigate the reversible adsorption and detection of a vapor phase molecule by inelastic electron tunneling spectroscopy. Molecular adsorption of acetic acid at room temperature is enhanced by a direct current electric field between. the electrodes to attract and trap the molecules. Step-like features in current versus time plots are observed and assigned to discrete adsorption events. Inelastic electron tunneling spectra recorded at cryogenic temperatures match the signatures of acetic acid vibrational modes and indicate that the molecules are generally physisorbed in the junction. Annealing the tunnel junctions above room temperature desorbs molecules from the junction, and the removal of molecular features is verified by the tunneling spectra. Molecules are readsorbed in the same tunnel junctions at room temperature, which demonstrates reversible adsorption.
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  • Introduction of Guest Editor, Dr. Danny G. Willis

    Thomas, Sandra P.  

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