12 research outputs found

    Understanding Capacitance Variation in Sub-nanometer Pores by <i>in Situ</i> Tuning of Interlayer Constrictions

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    The contribution of subnanometer pores in carbon electrodes to the charge-storage mechanism in supercapacitors has been the subject of intense debate for over a decade. Here, we provide a model system based on graphene oxide, which employs interlayer constrictions as a model for pore sizes that can be both controllably tuned and studied <i>in situ</i> during supercapacitor device use. Correlating electrochemical performance and <i>in situ</i> tuning of interlayer constrictions, we observe a peak in specific capacitance when interlayer constriction size reaches the diameters of unsolvated ions, supporting the hypothesized link between loss of ion solvation shell and anomalous capacitance increase for subnanometer pores

    Effect of Catalyst Pretreatment on Chirality-Selective Growth of Single-Walled Carbon Nanotubes

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    We show that catalyst pretreatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotube chemical vapor deposition. Using a SiO<sub>2</sub>-supported cobalt model catalyst and pretreatment in NH<sub>3</sub>, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of carbon nanotube nucleation is of fundamental importance for chiral control, thus identifying the pretreatment atmosphere as a key parameter for control of diameter and chirality distributions

    Introducing Carbon Diffusion Barriers for Uniform, High-Quality Graphene Growth from Solid Sources

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    Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al<sub>2</sub>O<sub>3</sub> barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 ┬░C with domain sizes exceeding 50 ╬╝m, and an average Raman D/G ratio of <0.07. A detailed growth rationale is established via in situ measurements, relevant to solid-state growth of a wide range of layered materials, as well as layer-by-layer control in these systems

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

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    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth

    No full text
    We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For ╬│-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For ╬▒-rich catalyst mixtures, Fe<sub>3</sub>C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk ironÔÇôcarbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes

    The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth

    No full text
    We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For ╬│-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For ╬▒-rich catalyst mixtures, Fe<sub>3</sub>C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk ironÔÇôcarbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes

    Co-catalytic Absorption Layers for Controlled Laser-Induced Chemical Vapor Deposition of Carbon Nanotubes

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    The concept of co-catalytic layer structures for controlled laser-induced chemical vapor deposition of carbon nanotubes is established, in which a thin Ta support layer chemically aids the initial Fe catalyst reduction. This enables a significant reduction in laser power, preventing detrimental positive optical feedback and allowing improved growth control. Systematic study of experimental parameters combined with simple thermostatic modeling establishes general guidelines for the effective design of such catalyst/absorption layer combinations. Local growth of vertically aligned carbon nanotube forests directly on flexible polyimide substrates is demonstrated, opening up new routes for nanodevice design and fabrication

    The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth

    No full text
    We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For ╬│-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For ╬▒-rich catalyst mixtures, Fe<sub>3</sub>C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk ironÔÇôcarbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe<sub>3</sub>C. Our results indicate that kinetic effects dominate the complex catalyst phase evolution during realistic CNT growth recipes
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