Christopher M Rouleau

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

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Publications (100)401.44 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560 °C < T < 840 °C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1 s) is revealed at T = 680 °C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T < 680 °C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.
    Carbon 11/2014; 79:256–264. · 6.16 Impact Factor
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    ABSTRACT: Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe2) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe2 are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD-VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications.
    ACS Nano 10/2014; · 12.03 Impact Factor
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    ABSTRACT: We report sputtering measurements of anorthite-like material, taken to be representative of soils found in the lunar highlands, impacted by singly and multicharged ions representative of the solar wind. The ions investigated include protons, as well as singly and multicharged Ar ions (as proxies for the non-reactive heavy solar wind constituents), in the charge state range +1 to +9, at fixed solar-wind-relevant impact velocities of 165 and 310 km/s (0.25 keV/amu and 0.5 keV/amu). A quartz microbalance approach (QCM) for determination of total sputtering yields was used. The goal of the measurements was to determine the sputtering contribution of the heavy, multicharged minority solar wind constituents in comparison to that due to the dominant H+ fraction. The QCM results show a yield increase of a factor of about 80 for Ar+ vs H+ sputtering and an enhancement by a factor of 1.67 between Ar9+ and Ar+, which is a clear indication of a potential sputtering effect.
    Journal of Geophysical Research: Space Physics 09/2014; · 3.44 Impact Factor
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    ABSTRACT: Synthesis of functional metal chalcogenide (GaSe) nanosheet networks by stoichiometric transfer of laser-vaporized material from bulk GaSe targets is presented. Uniform coverage of interconnected, crystalline, and photoresponsive GaSe nanosheets in both in-plane and out-of-plane orientations are achieved under different ablation conditions. The propagation of the laser-vaporized material is characterized by in situ ICCD-imaging. High (1 Torr) Ar background gas pressure is found to be crucial for the stoichiometric growth of GaSe nanosheet networks. Individual 1–3 layer GaSe triangular nanosheets of ≈200 nm domain size are formed within 30 laser pulses, coalescing to form nanosheet networks in as few as 100 laser pulses. The thickness of the deposited networks increases linearly with pulse number, adding layers in a two-dimensional (2D) growth mode. GaSe nanosheet networks show p-type semiconducting characteristics with mobilities reaching as high as 0.1 cm2V−1s−1. Spectrally-resolved photoresponsivities and external quantum efficiencies range from 0.4 AW−1 and 100% at 700 nm, to 1.4 AW−1 and 600% at 240 nm, respectively. Pulsed laser deposition under these conditions appears to provide a versatile and rapid approach to stoichiometrically transfer and deposit functional networks of 2D nanosheets with digital thickness control and uniformity for a variety of applications.
    Advanced Functional Materials 08/2014; · 10.44 Impact Factor
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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; · 12.03 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: We report total and mass resolved sputtering for H^+ and Ar^+q (q=1-9) ions incident on anorthite at 311 km/s, with enhanced O sputtering for Ar^+9 compared to Ar^+.
    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. · 10.74 Impact Factor
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    ABSTRACT: Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthesis of 2D GaSe crystals on SiO2/Si substrates using a vapor phase deposition method. For the first time, uniform, large (up to ~60 μm in lateral size), single-crystalline, triangular monolayer GaSe crystals were obtained and their structure and orientation were characterized from atomic scale to micrometer scale. The size, density, shape, thickness, and uniformity of the 2D GaSe crystals were shown to be controllable by growth duration, growth region, growth temperature, and argon carrier gas flow rate. The theoretical modeling of the electronic structure and Raman spectroscopy demonstrate a direct-to-indirect bandgap transition and progressive confinement-induced bandgap shifts for 2D GaSe crystals. The 2D GaSe crystals show p-type semiconductor characteristics and high photoresponsivity (~1.7 A/W under white light illumination) comparable to exfoliated GaSe nanosheets. These 2D GaSe crystals are potentially useful for next-generation electronic and optoelectronic devices such as photodetectors and field-effect transistors.
    Scientific reports. 01/2014; 4:5497.
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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. · 3.52 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; · 5.90 Impact Factor
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    ABSTRACT: Co3O4 exhibits intriguing physical, chemical and catalytic properties and has demonstrated great potential for next-generation renewable energy applications. These interesting properties and promising applications are underpinned by its electronic structure and optical properties, which are unfortunately poorly understood and the subject of considerable debate over many years. Here, we unveil a consistent electronic structural description of Co3O4 by synergetic infrared optical and in situ photoemission spectroscopy as well as standard density functional theory calculations. In contrast to previous assumptions, we demonstrate a much smaller fundamental band gap, which is directly related to its efficient electro-/photo-activity. The present results may help to advance the fundamental understanding and provide guidance for the use of oxide materials in photocatalysis and solar applications.
    J. Mater. Chem. C. 07/2013; 1(31).
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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; · 6.73 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

Publication Stats

859 Citations
401.44 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