Christopher M Rouleau

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (91)363.33 Total impact

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    ABSTRACT: We describe a two-step approach for suppressing nucleation of graphene on Cu using chemical vapor deposition. In the first step, as received Cu foils are oxidized in air at temperatures up to 500°C to remove surface impurities and to induce the regrowth of Cu grains during subsequent annealing in H2 flow at 1040°C prior to graphene growth. In the second step, transient reactant cooling is performed by using a brief Ar pulse at the onset of growth to induce collisional deactivation of the carbon growth species. The combination of these two steps results in a three orders of magnitude reduction in the graphene nucleation density, enabling the growth of millimeter-size single crystal graphene grains. A kinetic model shows that suppressing nucleation promotes a cooperative island growth mode that favors the formation of large area single crystal graphene, and it is accompanied by a roughly 3 orders of magnitude increase in the reactive sticking probability of methane compared to that in random nucleation growth.
    ACS Nano 05/2014; · 12.06 Impact Factor
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    ABSTRACT: Simple kinetic models of carbon nanotube growth have been able to successfully link together many experimental parameters involved in the growth of carbon nanotubes for practical applications including the prediction of growth rates, terminal lengths, number of walls, activation energies, and their dependences on the growth environment. The implications of recent experiments utilizing in situ monitoring of carbon nanotube growth on our past kinetic model are first reviewed. Then, sub-second pulsed feedstock gas introduction is discussed to explore the nucleation and initial growth of carbon nanotubes in the context of the kinetic model. Moreover, kinetic effects in "pulsed CVD" - using repeated pulsed gas introduction to stop and restart nanotube growth - are explored to understand renucleation, the origin of alignment in nanotube arrays, and incremental growth. Time-resolved reflectivity of the surface is used to remotely understand the kinetics of nucleation and the coordinated growth of arrays. This approach demonstrates that continuous vertically aligned single wall carbon nanotubes can be grown incrementally by pulsed CVD, and that the first exposure of fresh catalyst to feedstock gas is critical to nanotubes site density required for coordinated growth. Aligned nanotube arrays (as short as 60 nm) are shown to nucleate and grow within single, sub-second gas pulses. The multiple-pulse growth experiments (> 100 pulses) show that a high fraction of nanotubes renucleate on subsequent gas pulses.
    02/2014;
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    ABSTRACT: The synthesis of metal nanoparticles by femtosecond laser vaporization of nm-thickness metal films is explored with the goal of comparing the salient features of femtosecond-based through thin film laser ablation (TTFA) to that of ns TTFA, and testing the feasibility of direct synthesis of clean nanoparticle alloys to explore the synthesis of carbon nanotubes by chemical vapor deposition. It is demonstrated that evaporated metal films are cleanly removed from quartz substrates using the technique, producing a highly forward-directed plume of nanoparticles (angle of divergence of ~2.5°) which were cleanly deposited onto different supports for analysis. TEM showed the nanoparticles were spherical with diameters that ranged from a few nm to hundreds of nm in a bimodal fashion. Unlike ns-TTFA, it was found that raising the pressure had no effect on the intensity of the smaller mode within the distribution, suggesting that nanoparticle formation by gas phase condensation was not at play under the present conditions. Close examination of size distributions from a 20 and 10nm Pt film revealed an 80nm downshift in the position of the large mode within the distribution, suggesting film thickness may provide a route to controlling the modal distribution of nanoparticles produced by this method. Lastly, particles sourced by a Fe/Mo bilayer film were found to be effective in growing single wall carbon nanotubes by atmospheric chemical vapor deposition, indicating sufficiently small and catalytically active particles were produced.
    02/2014;
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    ABSTRACT: The attractive optoelectronic properties of conducting polymers depend sensitively upon intra- and inter-polymer chain interactions, and therefore new methods to manipulate these interactions are continually being pursued. Here, we report a study of the isotopic effects of deuterium substitution on the structure, morphology and optoelectronic properties of regioregular poly(3-hexylthiophene)s with an approach that combines the synthesis of deuterated materials, optoelectronic properties measurements, theoretical simulation and neutron scattering. Selective substitutions of deuterium on the backbone or side-chains of poly(3-hexylthiophene)s result in distinct optoelectronic responses in poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) photovoltaics. Specifically, the weak non-covalent intermolecular interactions induced by the main-chain deuteration are shown to change the film crystallinity and morphology of the active layer, consequently reducing the short-circuit current. However, side-chain deuteration does not significantly modify the film morphology but causes a decreased electronic coupling, the formation of a charge transfer state, and increased electron-phonon coupling, leading to a remarkable reduction in the open circuit voltage.
    Nature Communications 01/2014; 5:3180. · 10.02 Impact Factor
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    ABSTRACT: The synthesis of metal nanoparticles by ultrafast laser ablation of nanometers-thick metal films has been studied experimentally and computationally. Near-threshold backside laser ablation of 2–20 nm-thick Pt films deposited on fused silica substrates was found to produce nanoparticles with size distributions that were bimodal for the thicker films, but collapsed into a single mode distribution for the thinnest film. Time-resolved imaging of blackbody emission from the Pt nanoparticles was used to reveal the nanoparticle propagation dynamics and estimate their temperatures. The observed nanoparticle plume was compact and highly forward-directed with a well-defined collective velocity that permitted multiple rebounds with substrates to be revealed. Large-scale molecular dynamics simulations were used to understand the evolution of compressive and tensile stresses in the thicker melted liquid films that lead to their breakup and ejection of two groups of nanoparticles with different velocity and size distributions. Ultrafast laser irradiation of ultrathin (few nm) metal films avoids the splitting of the film and appears to be a method well-suited to cleanly synthesize and deposit nanoparticles from semitransparent thin film targets in highly directed beams.
    Applied Physics Letters 01/2014; 104(19):193106-193106-5. · 3.79 Impact Factor
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    ABSTRACT: We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of carbon nanohorns (CNHs) from common solvents such as acetone and water, onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams; in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on timescales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, such as PTFE tape and dielectrics. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications among others.
    ACS Applied Materials & Interfaces 12/2013; · 5.01 Impact Factor
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    ABSTRACT: The kinetics and mechanisms of graphene growth on Ni films at 720-880 °C have been measured using fast pulses of acetylene and real-time optical diagnostics. In situ UV-Raman spectroscopy was used to unambiguously detect isothermal graphene growth at high temperatures, measure the growth kinetics with ∼1 s temporal resolution, and estimate the fractional precipitation upon cooldown. Optical reflectivity and videography provided much faster temporal resolution. Both the growth kinetics and the fractional isothermal precipitation were found to be governed by the C2H2 partial pressure in the CVD pulse for a given film thickness and temperature, with up to ∼94% of graphene growth occurring isothermally within 1 second at 800 °C at high partial pressures. At lower partial pressures, isothermal graphene growth is shown to continue 10 seconds after the gas pulse. These flux-dependent growth kinetics are described in the context of a dissolution/precipitation model, where carbon rapidly dissolves into the Ni film and later precipitates driven by gradients in the chemical potential. The combination of pulsed-CVD and real-time optical diagnostics opens new opportunities to understand and control the fast, sub-second growth of graphene on various substrates at high temperatures.
    Nanoscale 06/2013; · 6.23 Impact Factor
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    ABSTRACT: Synthesis of a non-ionic, water-soluble poly(thiophene) (PT) derivative, poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3TEGT) with a hydrophilic tri-ethylene glycol side group, is reported and thin films of the polymer suitable for organic field-effect transistors (OFETs) are characterized by combining analysis techniques that include UV-Vis absorption and fluorescence spectroscopy, X-ray diffraction, and atomic force microscopy. After thermal annealing, P3TEGT films exhibit a well-organized nanofibrillar lamellar nanostructure that originates from the strong π–π stacking of the thiophene backbones. P-type organic field-effect transistors (OFETs) with hole mobilities of 10−5 cm2 V−1 s−1 were fabricated from this water-soluble poly(thiophene) derivative, demonstrating the possibility that environmentally friendly solvents may be promising alternatives for the low-cost, green solvent processed organic electronic device manufacturing of OFETs, organic photovoltaics (OPVs), and biosensors.
    Polym. Chem. 01/2013;
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    ABSTRACT: Ordered mesoporous carbon-graphitic carbon composites prepared by the "brick and mortar" method were fluorinated using F2 and investigated as cathodes for primary lithium batteries. The resulting materials have a rich array of C-F species, as measured by XPS, which influence conduction and voltage profiles.
    Journal of Materials Chemistry 01/2013; · 5.97 Impact Factor
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    ABSTRACT: Pulsed laser deposition (PLD) is a widely used growth technique for epitaxy of a wide range of oxide thin films and heterostructures. While the highly energetic PLD process is beneficial for epitaxy, it can deteriorate the interface's structure and chemical abruptness when grown under high vacuum. On page 6423, Ho Nyung Lee and co-workers provide a route to a chemically sharp oxide hetero-interface by atomic layer growth control during pulsed laser epitaxy.
    Advanced Materials 12/2012; 24(48):6422. · 14.83 Impact Factor
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    ABSTRACT: Atomic layer engineering enables fabrication of a chemically sharp oxide heterointerface. The interface formation and strain evolution during the initial growth of LaAlO(3) /SrTiO(3) heterostructures by pulsed laser deposition are investigated in search of a means for controlling the atomic-sharpness of the interface. This study shows that inserting a monolayer of LaAlO(3) grown at high oxygen pressure dramatically enhances interface abruptness.
    Advanced Materials 10/2012; · 14.83 Impact Factor
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    ABSTRACT: Aim: Nanoparticle-enhanced photothermal therapy is a promising alternative to tumor resection. However, quantitative measurements of cellular response to these treatments are limited. This article introduces a Bimodal Enhanced Analysis of Spatiotemporal Temperature (BEAST) algorithm to rapidly determine the viability of cancer cells in vitro following photothermal therapy alone or in combination with nanoparticles. Materials & methods: To illustrate the capability of the BEAST viability algorithm, single wall carbon nanohorns were added to renal cancer (RENCA) cells in vitro and time-dependent spatial temperature maps measured with an infrared camera during laser therapy were correlated with post-treatment cell viability distribution maps obtained by cell-staining fluorescent microscopy. Conclusion: The BEAST viability algorithm accurately and rapidly determined the cell viability as a function of time, space and temperature. Original submitted 13 July 2011; Revised submitted 12 March 2012.
    Nanomedicine 07/2012; · 5.26 Impact Factor
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    ABSTRACT: Very short arrays of continuous single-wall carbon nanotubes (SWNTs) are grown incrementally in steps as small as 25 nm using pulsed chemical vapor deposition (CVD). In-situ optical extinction measurements indicate that over 98% of the nanotubes reinitiate growth on successive gas pulses, and high-resolution transmission electron microscopy (HR-TEM) images show that the SWNTs do not exhibit segments, caps, or noticeable sidewall defects resulting from repeatedly stopping and restarting growth. Time-resolved laser reflectivity (3-ms temporal resolution) is used to record the nucleation and growth kinetics for each fast (0.2 s) gas pulse and to measure the height increase of the array in situ, providing a method to incrementally grow short nanotube arrays to precise heights. Derivatives of the optical reflectivity signal reveal distinct temporal signatures for both nucleation and growth kinetics, with their amplitude ratio on the first gas pulse serving as a good predictor for the evolution of the growth of the nanotube ensemble into a coordinated array. Incremental growth by pulsed CVD is interpreted in the context of autocatalytic kinetic models as a special processing window in which a sufficiently high flux of feedstock gas drives the nucleation and rapid growth phases of a catalyst nanoparticle ensemble to occur within the temporal period of the gas pulse, but without inducing growth termination.
    Small 03/2012; 8(10):1534-42. · 7.82 Impact Factor
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    ABSTRACT: The catalytic dissociation of hydrogen molecules by metal nanoparticles and spillover of atomic hydrogen onto various supports is a well-established phenomenon in catalysis. However, the mechanisms by which metal catalyst nanoparticles can assist in enhanced hydrogen storage on high-surface area supports are still under debate. Experimental measurements of metal-assisted hydrogen storage have been hampered by inaccurate estimation of atomically stored hydrogen deduced from comparative measurements between metal-decorated and undecorated samples. Here we report a temperature cycling technique combined with inelastic neutron scattering (INS) measurements of quantum rotational transitions of molecular H2 to more accurately quantify adsorbed hydrogen aided by catalytic particles using single samples. Temperature cycling measurements on single-wall carbon nanohorns (SWCNHs) decorated with 2-3 nm Pt nanoparticles showed 0.17 % mass fraction of metal-assisted hydrogen storage (at 0.5 MPa) at room temperature. Temperature cycling of Pt-decorated SWCNHs using a Sievert s apparatus also indicated metal-assisted hydrogen adsorption of 0.08 % mass fraction at 5 MPa at room temperature. No additional metal-assisted hydrogen storage was observed in SWCNH samples without Pt nanoparticles cycled to room temperature, or in Pt-SWCNHs when the temperature was cycled to less than 150K. The possible formation of C-H bonds due to spilled-over atomic hydrogen was also investigated using both INS and density functional theory calculations.
    Carbon 01/2012; 50(13):4953-4964. · 5.87 Impact Factor
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    ABSTRACT: We show that the analysis of single-shot surface x-ray diffraction transients in terms of time-dependent coverages allows quantitative determination of interlayer transport in pulsed-laser deposition of SrTiO3. The fast interlayer transport during and immediately after the arrival of the laser plume and before crystallization represents the dominant mechanism for redistribution of the deposited material that is completed on a μs-range or faster time scale. Following crystallization interlayer transport is more than four orders of magnitude slower because it is driven only by sluggish thermally activated processes, which represent a small fraction of total interlayer transport that decreases with increasing laser repetition rate. The analysis of growth kinetics shows that it is fast interlayer transport driven by hyperthermal energy species and not thermal annealing that governs layer completion that determines the growth mode and the formation of atomically sharp interfaces in pulsed-laser deposition of epitaxial oxide films and similar energy-enhanced growth processes.
    Physical review. B, Condensed matter 11/2011; 84(19). · 3.77 Impact Factor
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    ABSTRACT: A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.
    Advanced Materials 11/2011; 23(46):5529-35. · 14.83 Impact Factor
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    ABSTRACT: The nucleation and growth kinetics of single-wall carbon nanotubes in aligned arrays have been measured using fast pulses of acetylene and in situ optical diagnostics in conjunction with low pressure chemical vapor deposition (CVD). Increasing the acetylene partial pressure is shown to decrease nucleation times by three orders of magnitude, permitting aligned nanotube arrays to nucleate and grow to micrometers lengths within single gas pulses at high (up to 7 μm/s) peak growth rates and short ∼0.5 s times. Low-frequency Raman scattering (>10 cm(-1)) and transmission electron microscopy measurements show that increasing the feedstock flux in both continuous- and pulsed-CVD shifts the product distribution to large single-wall carbon nanotube diameters >2.5 nm. Sufficiently high acetylene partial pressures in pulsed-CVD appear to temporarily terminate the growth of the fastest-growing, small-diameter nanotubes by overcoating the more catalytically active, smaller catalyst nanoparticles within the ensemble with non-nanotube carbon in agreement with a growth model. The results indicate that subsets of catalyst nanoparticle ensembles nucleate, grow, and terminate growth within different flux ranges according to their catalytic activity.
    ACS Nano 09/2011; 5(10):8311-21. · 12.06 Impact Factor
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    ABSTRACT: This article reports a direct chemical pathway for antioxidant deactivation on the surfaces of carbon nanomaterials. In the absence of cells, carbon nanotubes are shown to deplete the key physiological antioxidant glutathione (GSH) in a reaction involving dissolved dioxygen that yields the oxidized dimer, GSSG, as the primary product. In both chemical and electrochemical experiments, oxygen is only consumed at a significant steady-state rate in the presence of both nanotubes and GSH. GSH deactivation occurs for single- and multi-walled nanotubes, graphene oxide, nanohorns, and carbon black at varying rates that are characteristic of the material. The GSH depletion rates can be partially unified by surface area normalization, are accelerated by nitrogen doping, and suppressed by defect annealing or addition of proteins or surfactants. It is proposed that dioxygen reacts with active sites on graphenic carbon surfaces to produce surface-bound oxygen intermediates that react heterogeneously with glutathione to restore the carbon surface and complete a catalytic cycle. The direct catalytic reaction between nanomaterial surfaces and antioxidants may contribute to oxidative stress pathways in nanotoxicity, and the dependence on surface area and structural defects suggest strategies for safe material design.
    Small 08/2011; 7(19):2775-85. · 7.82 Impact Factor
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    ABSTRACT: An integrated field-portable surface enhanced Raman scattering (SERS) sensing system has been developed and evaluated for quantitative analysis of energetics such as perchlorate (ClO(4)(-)) and trinitrotoluene (TNT) at environmentally relevant concentrations and conditions. The detection system consists of a portable Raman spectrometer equipped with an optical fiber probe that is coupled with novel elevated gold bowtie nanostructural arrays as a sensitive and reproducible SERS substrate. Using the standard addition technique, we show that ClO(4)(-) and TNT can be quantified at concentrations as low as 0.66 mg L(-1) (or ~6.6 µM) and 0.20 mg L(-1) (~0.9 µM), respectively, in groundwater samples collected from selected military sites. This research represents the first step toward the development of a field SERS sensor which may permit rapid, in situ screening and analysis for various applications including national security, chemical, biological and environmental detection.
    The Analyst 03/2011; 136(8):1697-702. · 4.23 Impact Factor
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    ABSTRACT: Non-equilibrium, pulsed gas delivery and pulsed heating synthesis approaches are used to explore and compare the kinetics and mechanisms of carbon nanotube and graphene growth on metal thin-films. Time-resolved, in situ optical reflectivity of growing nanotubes and graphene reveal the growth kinetics resulting from well-controlled, pulsed fluxes of acetylene by chemical vapor deposition. Alternatively, pulsed laser heating of substrates is used to provide well-defined transient growth temperature profiles for growth by chemical vapor deposition. Pulsed gas fluxes are shown to control the density and diameter of nanotubes in vertically-aligned nanotube arrays with nanoparticles of different size repeatedly nucleating, growing, and terminating growth in accordance with an empirical growth model. The pulsed processing approach is used to grow vertically aligned nanotube arrays with variable density. Research sponsored by the Materials Science and Engineering Division, Basic Energy Sciences, U.S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.
    03/2011;

Publication Stats

832 Citations
363.33 Total Impact Points

Institutions

  • 1994–2014
    • Oak Ridge National Laboratory
      • • Center for Nanophase Materials Sciences
      • • Materials Science and Technology Division
      • • Chemical Sciences Division
      • • Solid State Division
      Oak Ridge, Florida, United States
  • 2011
    • University of Alabama
      • Department of Electrical and Computer Engineering
      Tuscaloosa, AL, United States