Publications (38)104.8 Total impact
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Article: Chemical activation of carbon nano-onions for high-rate supercapacitor electrodes
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ABSTRACT: Recent studies have demonstrated that carbon nano-onion (CNO) is a promising candidate for high-power supercapacitors due to the nonporous outer shell, which is easily accessible to electrolyte ions. However, the nonporous ion-accessible outer shells also limit the energy density of the CNOs, which requires large specific surface area. Introducing porosity to the outer shells of CNOs can effectively improve the specific surface area by exposing the inner shells to electrolytes. In this study, the electrochemical performance of supercapacitor electrodes based on CNOs is improved through the controlled introduction of porosity on the outer shells of CNOs by chemical activation. The capacitance of the activated CNOs is five times larger than the pristine ones with a measured power density of 153 kW/kg and an energy density of 8.5 Wh/kg in a 2 mol/l potassium nitrate electrolyte. The capacitance retention ratio of activated CNOs decreases slightly as the current density increases from 0.75 to 25 A/g. About 71% of initial capacitance (at 0.75 A/g) is preserved for activated CNOs at current densities up to 25 A/g. (C) 2012 Elsevier Ltd. All rights reserved.Carbon. 01/2013; 51:52-58. -
Article: Plasmonic-enhanced carbon nanotube infrared bolometers.
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ABSTRACT: Plasmonic nanoantennas show significant potential in photodetection applications, but the extent to which their full potential can be realized is dictated by the volume and location of the active materials within the plasmonic structure. Carbon nanotubes (CNTs) have been used as a novel material in photodetection application due to their excellent electronic and optoelectronic properties. However, difficulties in the integration of CNTs in the gaps of nanoantennas have limited the investigation of antenna-coupled CNT detectors. Here, we demonstrate a unique plasmonic approach for selectively growing CNTs in the gap of nanoantenna arrays for fabrication of plasmonic infrared bolometers operating at room temperature. Strong concentration of light at the tips of nanoantennas was utilized for localized heating and growth of CNTs. Moreover, interaction of this strong optical field with the small volume of CNTs enhanced the photoresponse of the bolometers. Consequently, a high responsivity of about 800 V W(-1) was achieved at room temperature.Nanotechnology 12/2012; 24(3):035502. · 3.98 Impact Factor -
Article: Coherent anti-Stokes Raman scattering and spontaneous Raman spectroscopy and microscopy of microalgae with nitrogen depletion.
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ABSTRACT: Microalgae are extensively researched as potential feedstocks for biofuel production. Energy-rich compounds in microalgae, such as lipids, require efficient characterization techniques to investigate the metabolic pathways and the environmental factors influencing their accumulation. The model green alga Coccomyxa accumulates significant amounts of triacylglycerols (TAGs) under nitrogen depletion (N-depletion). To monitor the growth of TAGs (lipid) in microalgal cells, a study of microalgal cells (Coccomyxa sp. C169) using both spontaneous Raman and coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy were carried out. Spontaneous Raman spectroscopy was conducted to analyze the components in the algal cells, while CARS was carried out to monitor the distribution of lipid droplets in the cells. Raman signals of carotenoid are greater in control microalgae compared to N-depleted cells. Raman signals of lipid droplets appear after N-depletion and its distribution can be clearly observed in the CARS microscopy. Both spontaneous Raman spectroscopy and CARS microscopy were found to be suitable analysis tools for microalgae.Biomedical Optics Express 11/2012; 3(11):2896-906. · 2.33 Impact Factor -
Article: Surface-enhanced Raman spectroscopy using gold-coated horizontally aligned carbon nanotubes.
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ABSTRACT: Gold-coated horizontally aligned carbon nanotube (Au-HA-CNT) substrates were fabricated for surface-enhanced Raman spectroscopy (SERS). The Au-HA-CNT substrates, which are granular in nature, are easy-to-prepare with large SERS-active area. Enhancement factors (EFs) of ∼10(7) were achieved using the Au-HA-CNTs as substrates for rhodamine 6G (R6G) molecules. Maximum enhancement was found when the polarization direction (E-field) of the incident laser beam was parallel to the aligned direction of the HA-CNTs. Simulations using the finite-difference time-domain (FDTD) method were carried out for the granular Au-HA-CNT samples. Enhancement mechanisms and determination of EFs were analyzed. Biological samples, including (13)C- and deuterium (D)-labeled fatty acids and Coccomyxa sp. c-169 microalgae cells, were also measured using this SERS substrate. The limits of detection (LODs) of D- and (13)C-labeled fatty acids on the SERS substrate were measured to be around 10 nM and 20 nM, respectively. Significantly enhanced Raman signals from the microalgae cells were acquired using the SERS substrate.Nanotechnology 05/2012; 23(20):205702. · 3.98 Impact Factor -
Article: 4. T. Guillemet, P.M. Geffroy, J.M. Heintz, N. Chandra,, Y.F. Lu, and J. F. Silvain, An innovative process to fabricate copper/diamond composite films for thermal management applications, Composites, Part A, 42, 1746-1753, (2012)
Composites Part A Applied Science and Manufacturing 01/2012; 42:1746-1753. · 2.69 Impact Factor -
Article: Stress and phase purity analyses of diamond films deposited through laser-assisted combustion synthesis.
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ABSTRACT: Diamond films were deposited on silicon and tungsten carbide substrates in open air through laser-assisted combustion synthesis. Laser-induced resonant excitation of ethylene molecules was achieved in the combustion process to promote diamond growth rate. In addition to microstructure study by scanning electron microscopy, Raman spectroscopy was used to analyze the phase purity and residual stress of the diamond films. High-purity diamond films were obtained through laser-assisted combustion synthesis. The levels of residual stress were in agreement with corresponding thermal expansion coefficients of diamond, silicon, and tungsten carbide. Diamond-film purity increases while residual stress decreases with an increasing film thickness. Diamond films deposited on silicon substrates exhibit higher purity and lower residual stress than those deposited on tungsten carbide substrates.ACS Applied Materials & Interfaces 09/2011; 3(10):4120-5. · 4.53 Impact Factor -
Article: Enhancement of optical emission from laser-induced plasmas by combined spatial and magnetic confinement.
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ABSTRACT: To enhance optical emission in laser-induced breakdown spectroscopy, both a pair of permanent magnets and an aluminum hemispherical cavity (diameter: 11.1 mm) were used simultaneously to magnetically and spatially confine plasmas produced by a KrF excimer laser in air from pure metal and alloyed samples. High enhancement factors of about 22 and 24 in the emission intensity of Co and Cr lines were acquired at a laser fluence of 6.2 J/cm2 using the combined confinement, while enhancement factors of only about 11 and 12 were obtained just with a cavity. The mechanism of enhanced optical emission by combined confinement, including shock wave in the presence of a magnetic field, is discussed. The Si plasmas, however, were not influenced by the presence of magnets as Si is hard to ablate and ionize and hence has less free electrons and positive ions. Images of the laser-induced Cr and Si plasmas show the difference between pure metallic and semiconductor materials in the presence of both a cavity and magnets.Optics Express 07/2011; 19(15):14067-75. · 3.59 Impact Factor -
Article: Fast growth of branched nickel monosilicide nanowires by laser-assisted chemical vapor deposition.
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ABSTRACT: Branched nickel monosilicide (NiSi) nanowires (NWs), for the first time, have been synthesized on Ni foams by laser-assisted chemical vapor deposition using disilane precursor molecules. Studies indicate that 600 °C is the threshold temperature for the growth of a large number of branched NiSi NWs with 100-500 nm long branches extending from the main stems. Below the threshold temperature, unbranched NiSi NWs were obtained. The density of the branched NiSi NWs is relatively higher in comparison to that of the unbranched ones. The growth rate of the branched NiSi NWs at 700 °C is estimated up to 10 µm min(-1). High-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy of the branched NiSi NWs suggest that the formation of these branched nanostructures is ascribed to the Ni-dominant diffusion process. These NiSi NWs with branched nanostructures could bring them new opportunities in nanodevices.Nanotechnology 06/2011; 22(23):235602. · 3.98 Impact Factor -
Article: Generation of high-temperature and low-density plasmas for improved spectral resolutions in laser-induced breakdown spectroscopy.
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ABSTRACT: Improved spectral resolutions were achieved in laser-induced breakdown spectroscopy (LIBS) through generation of high-temperature and low-density plasmas. A first pulse from a KrF excimer laser was used to produce particles by perpendicularly irradiating targets in air. A second pulse from a 532 nm Nd:YAG laser was introduced parallel to the sample surface to reablate the particles. Optical scattering from the first-pulse plasmas was imaged to elucidate particle formation in the plasmas. Narrower line widths (full width at half maximums: FWHMs) and weaker self-absorption were observed from time-integrated LIBS spectra. Estimation of plasma temperatures and densities indicates that high temperature and low density can be achieved simultaneously in plasmas to improve LIBS resolutions.Optics Express 05/2011; 19(11):10997-1006. · 3.59 Impact Factor -
Article: Resonant excitation of precursor molecules in improving the particle crystallinity, growth rate and optical limiting performance of carbon nano-onions.
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ABSTRACT: A catalyst-free and highly efficient synthetic method for growing carbon nano-onions (CNOs) in open air has been developed through the laser resonant excitation of a precursor molecule, ethylene, in a combustion process. Highly concentric CNO particles with improved crystallinity were obtained at a laser wavelength of 10.532 µm through the resonant excitation of the CH(2) wagging mode of the ethylene molecules. A higher growth rate up to 2.1 g h( - 1) was obtained, compared with that without a laser (1.3 g h( - 1)). Formation of the CNOs with ordered graphitic shells is ascribed to the decomposition of polycyclic aromatic hydrocarbons (PAHs) into C(2) species. The optical limiting performances of the CNOs grown by the combustion processes were investigated. CNOs grown at 10.532 µm laser excitation demonstrated improved optical limiting properties due to the improved crystallinity.Nanotechnology 04/2011; 22(16):165604. · 3.98 Impact Factor -
Article: Influence of WC-Co substrate pretreatment on diamond film deposition by laser-assisted combustion synthesis.
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ABSTRACT: The quality of diamond films deposited on cemented tungsten carbide substrates (WC-Co) is limited by the presence of the cobalt binder. The cobalt in the WC-Co substrates enhances the formation of nondiamond carbon on the substrate surface, resulting in a poor film adhesion and a low diamond quality. In this study, we investigated pretreatments of WC-Co substrates in three different approaches, namely, chemical etching, laser etching, and laser etching followed by acid treatment. The laser produces a periodic surface pattern, thus increasing the roughness and releasing the stress at the interfaces between the substrate and the grown diamond film. Effects of these pretreatments have been analyzed in terms of microstructure and cobalt content. Raman spectroscopy was conducted to characterize both the diamond quality and compressive residual stress in the films.ACS Applied Materials & Interfaces 04/2011; 3(4):1134-9. · 4.53 Impact Factor -
Article: Plasma confinement by hemispherical cavity in laser-induced breakdown spectroscopy
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ABSTRACT: An aluminum hemispherical cavity (diameter: 11.1 mm) was used to confine plasmas produced by a KrF excimer laser in air from a steel target with a low concentration manganese in laser-induced breakdown spectroscopy. A significant enhancement (factor >12) in the emission intensity of Mn lines was observed at a laser fluence of 7.8 J/cm2 when the plasma was confined by the hemispherical cavity, leading to an increase in plasma temperature about 3600 K. The maximum emission enhancement increased with increasing laser fluence. The spatial confinement mechanism was discussed using shock wave theory.Applied Physics Letters 03/2011; 98(13):131501-131501-3. · 3.84 Impact Factor -
Article: Fast growth of graphene patterns by laser direct writing
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ABSTRACT: Rapid single-step fabrication of graphene patterns was developed using laser-induced chemical vapor deposition (LCVD). A laser beam irradiates a thin nickel foil in a CH4 and H2 environment to induce a local temperature rise, thereby allowing the direct writing of graphene patterns in precisely controlled positions at room temperature. Line patterns can be achieved with a single scan without pre- or postprocesses. Surprisingly, the growth rate is several thousand times faster than that of general CVD methods. The discovery and development of the LCVD growth process provide a route for the rapid fabrication of graphene patterns for various applications.Applied Physics Letters 03/2011; 98(12):123109-123109-3. · 3.84 Impact Factor -
Article: A. Veillere, T. Guillemet, Z.Q. Xie, C.A. Zuhlke, D.R. Alexander, J.F. Silvain, J.M. Heintz, N. Chandra, Y.F. Lu, Influence of WC-Co Substrate Pretreatment on Diamond Film Deposition by Laser Assisted Combustion Synthesis, Applied Materials and Interfaces, 3(4), 1134-1139, (2011
ACS Applied Materials & Interfaces 01/2011; 3(4):1134-1139. · 4.53 Impact Factor -
Article: 13. J.F. Silvain, A. Veillère, J.M Heintz, C. Vincent, T. Guillemet, G. Lacombe, Y.F. Lu, and N. Chandra, The role of controlled interfaces in the thermal management of copper-carbon composites, accepted for publication, Emerging Materials Research, (2011)
emerging materials research. 01/2011; -
Article: , Influence of the interface structure on the thermo-mechanical properties of Cu-X (X=Cr of B)/carbon fiber composites
ACS Applied Materials & Interfaces 01/2011; 3:4120-4125. · 4.53 Impact Factor -
Article: Assembly of carbon nanotube devices by tip-induced optical trapping
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ABSTRACT: Fabrication of nanoscale devices by assembling individual carbon nanotubes (CNTs) remains challenging despite enormous effort made in this field. Fulfilling the promise of CNTs requires more efficient assembly techniques. In this study, we have developed an in-situ assembly method for precise and cost-effective integration of CNTs using a laser-assisted chemical vapor deposition (LCVD) process. Results show that CNTs can be trapped between sharp tip-shaped electrodes due to the optical gradient forces around the tip apexes generated by a CO2 laser irradiation. This method enables the precise assembly of CNT-based field-effect transistors (FETs) and paves the way for the successful implementation of the CNT-based nanoelectronics.MRS Proceedings. 12/2010; 1365. -
Article: Diameter modulation by fast temperature control in laser-assisted chemical vapor deposition of single-walled carbon nanotubes.
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ABSTRACT: Diameter modulation by fast temperature control in laser-assisted chemical vapor deposition (LCVD) was successfully achieved to tune the diameters of single-walled carbon nanotubes (SWNTs) in different segments. Due to the inverse relationship between the SWNT diameter and the growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were integrated in electrodes to form field-effect transistors (FETs) and to investigate their electronic transport properties. The SWNTs in the FET structures have electronic properties similar to Schottky diodes, indicating clear evidence of different bandgap structures at the two ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of the SWNTs.Nanotechnology 10/2010; 21(39):395601. · 3.98 Impact Factor -
Article: Optical and field-emission properties of ZnO nanostructures deposited using high-pressure pulsed laser deposition.
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ABSTRACT: ZnO nanostructures were deposited on GaN (0001), Al2O3 (0001), and Si (100) substrates using a high-pressure pulsed laser deposition (PLD) method. Vertically aligned hexagonal-pyramidal ZnO nanorods were obtained on the Al2O3 and Si substrates whereas interlinked ZnO nanowalls were obtained on the GaN substrates. A growth mechanism has been proposed for the formation of ZnO nanowalls based on different growth rates of ZnO polar and nonpolar planes. Both ZnO nanorods and nanowalls exhibit a strong E2H vibration mode in the micro-Raman spectra. The corresponding fluorescence spectra of ZnO nanorods and nanowalls showed near band emission at 3.28 eV. The ZnO nanorods grown on the Si substrates exhibited better crystalline and optical properties compared with the ZnO structures grown on the GaN and Al2O3 substrates. The high aspect ratio, good vertical alignment, and better crystallinity of the ZnO nanorods with tapered tips exhibited promising field emission performance with a low turn-on field of 2 V/μm, a high current density of 7.7 mA/cm2, and a large field enhancement factor.ACS Applied Materials & Interfaces 09/2010; 2(10):2863-9. · 4.53 Impact Factor -
Article: Towards carbon-nanotube integrated devices: optically controlled parallel integration of single-walled carbon nanotubes.
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ABSTRACT: Where it starts and where it goes? Controlled integration of single-walled carbon nanotubes (SWNTs) into pre-designed nano-architectures is one of the major challenges to be overcome for extensive scientific research and technological applications. Various serial assembly techniques have been proposed and developed. However, they are still a long way from practical applications due to the drawbacks on reliability, yield and cost. Here we demonstrate a laser-based strategy to achieve parallel integration of SWNTs into pre-designed nano-architectures through an optically controlled in situ growth process. Optical driving forces originated from tip-induced optical near-field enhancement and laser beam polarization were applied in this study to realize the controlled placement of SWNTs at designated sites following wanted orientations on the nanometer scale. Parallel integration of SWNT arrays was achieved by adjusting laser beam diameter to cover interested nano-architectures. The laser-based process suggests an efficient and cost-effective approach for fabricating and integrating SWNT-based devices and circuits.Nanotechnology 08/2010; 21(31):315601. · 3.98 Impact Factor
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Institutions
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2006–2012
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University of Nebraska at Lincoln
- Department of Electrical Engineering
Lincoln, NE, USA
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