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Find video protocols related to scientific articles indexed in Pubmed.
Direct in situ observations of single Fe atom catalytic processes and anomalous diffusion at graphene edges.
Proc. Natl. Acad. Sci. U.S.A.
PUBLISHED: 10-20-2014
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Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp(2) carbon, the implementation of a single transition metal atom for growth can provide crucial insight into the formation mechanisms of graphene and carbon nanotubes. This knowledge is particularly important if we are to overcome fabrication difficulties in these materials and fully take advantage of their distinct band structures and physical properties. In this work, we present atomically resolved transmission EM in situ investigations of single Fe atoms at graphene edges. Our in situ observations show individual iron atoms diffusing along an edge either removing or adding carbon atoms (viz., catalytic action). The experimental observations of the catalytic behavior of a single Fe atom are in excellent agreement with supporting theoretical studies. In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.
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Silicon carbide embedded in carbon nanofibres: structure and band gap determination.
Phys Chem Chem Phys
PUBLISHED: 10-14-2014
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Materials drastically alter their electronic properties when being reduced to the nanoscale due to quantum effects. Concerning semiconductors, the band gap is expected to broaden as a result of the quantum confinement. In this study we report on the successful synthesis of wide bandgap SiC nanowires (with great potential for applications) and the local determination of their band gap. Their value was found to be higher with respect to bulk SiC. The nanowires are grown as a heterostructure, i.e. encapsulated in carbon nanofibres via dc hot-filament Plasma-Enhanced Chemical Vapour Deposition on the Si/SiO2 substrate. The structure of the as-produced carbon nanofibres was characterized by means of aberration-corrected high-resolution transmission electron microscopy. Two different pure SiC polytypes, namely the 3C (cubic) and the 6H (hexagonal) as well as distorted structures are observed. The SiC nanowires have diameters in the range of 10-15 nm and lengths of several hundred nanometers. The formation of the SiC is a result of the substrate etching during the growth of the CNFs and a subsequent simultaneous diffusion of not only carbon, but also silicon through the catalyst particle.
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CO2 enhanced chemical vapor deposition growth of few-layer graphene over NiO(x).
ACS Nano
PUBLISHED: 09-03-2014
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The use of mild oxidants in chemical vapor deposition (CVD) reactions has proven enormously useful. This was also true for the CVD growth of carbon nanotubes. As yet though, the use of mild oxidants in the CVD of graphene has remained unexplored. Here we explore the use of CO2 as a mild oxidant during the growth of graphene over Ni with CH4 as the feedstock. Both our experimental and theoretical findings provide in-depth insight into the growth mechanisms and point to the mild oxidants playing multiple roles. Mild oxidants lead to the formation of a suboxide in the Ni, which suppresses the bulk diffusion of C species suggesting a surface growth mechanism. Moreover, the formation of a suboxide leads to enhanced catalytic activity at the substrate surface, which allows reduced synthesis temperatures, even as low as 700 °C. Even at these low temperatures, the quality of the graphene is exceedingly high as indicated by a negligible D mode in the Raman spectra. These findings suggest the use of mild oxidants in the CVD fabrication as a whole could have a positive impact.
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Low Voltage Transmission Electron Microscopy of Graphene.
Small
PUBLISHED: 06-20-2014
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The initial isolation of graphene in 2004 spawned massive interest in this two-dimensional pure sp(2) carbon structure due to its incredible electrical, optical, mechanical, and thermal effects. This in turn led to the rapid development of various characterization tools for graphene. Examples include Raman spectroscopy and scanning tunneling microscopy. However, the one tool with the greatest prowess for characterizing and studying graphene is the transmission electron microscope. State-of-the-art (scanning) transmission electron microscopes enable one to image graphene with atomic resolution, and also to conduct various other characterizations simultaneously. The advent of aberration correctors was timely in that it allowed transmission electron microscopes to operate with reduced acceleration voltages, so that damage to graphene is avoided while still providing atomic resolution. In this comprehensive review, a brief introduction is provided to the technical aspects of transmission electron microscopes relevant to graphene. The reader is then introduced to different specimen preparation techniques for graphene. The different characterization approaches in both transmission electron microscopy and scanning transmission electron microscopy are then discussed, along with the different aspects of electron diffraction and electron energy loss spectroscopy. The use of graphene for other electron microscopy approaches such as in-situ investigations is also presented.
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A growth mechanism for free-standing vertical graphene.
Nano Lett.
PUBLISHED: 05-06-2014
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We propose a detailed mechanism for the growth of vertical graphene by plasma-enhanced vapor deposition. Different steps during growth including nucleation, growth, and completion of the free-standing two-dimensional structures are characterized and analyzed by transmission electron microscopy. The nucleation of vertical graphene growth is either from the buffer layer or from the surface of carbon onions. A continuum model based on the surface diffusion and moving boundary (mass flow) is developed to describe the intermediate states of the steps and the edges of graphene. The experimentally observed convergence tendency of the steps near the top edge can be explained by this model. We also observed the closure of the top edges that can possibly stop the growth. This two-dimensional vertical growth follows a self-nucleated, step-flow mode, explained for the first time.
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Thermal conductivity of mechanically joined semiconducting/metal nanomembrane superlattices.
Nano Lett.
PUBLISHED: 04-22-2014
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The decrease of thermal conductivity is crucial for the development of efficient thermal energy converters. Systems composed of a periodic set of very thin layers show among the smallest thermal conductivities reported to-date. Here, we fabricate in an unconventional but straightforward way hybrid superlattices consisting of a large number of nanomembranes mechanically stacked on top of each other. The superlattices can consist of an arbitrary composition of n- or p-type doped single-crystalline semiconductors and a polycrystalline metal layer. These hybrid multilayered systems are fabricated by taking advantage of the self-rolling technique. First, differentially strained nanomembranes are rolled into three-dimensional microtubes with multiple windings. By applying vertical pressure, the tubes are then compressed and converted into a planar hybrid superlattice. The thermal measurements show a substantial reduction of the cross-sectional heat transport through the nanomembrane superlattice compared to a single nanomembrane layer. Time-domain thermoreflectance measurements yield thermal conductivity values below 2 W m(-1) K(-1). Compared to bulk values, this represents a reduction of 2 orders of magnitude by the incorporation of the mechanically joined interfaces. The scanning thermal atomic force microscopy measurements support the observation of reduced thermal transport on top of the superlattices. In addition, small defects with a spatial resolution of ?100 nm can be resolved in the thermal maps. The low thermal conductivity reveals the potential of this approach to fabricate miniaturized on-chip solutions for energy harvesters in, e.g., microautonomous systems.
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Nanosized carbon black combined with Ni2O3 as "universal" catalysts for synergistically catalyzing carbonization of polyolefin wastes to synthesize carbon nanotubes and application for supercapacitors.
Environ. Sci. Technol.
PUBLISHED: 03-18-2014
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The catalytic carbonization of polyolefin materials to synthesize carbon nanotubes (CNTs) is a promising strategy for the processing and recycling of plastic wastes, but this approach is generally limited due to the selectivity of catalysts and the difficulties in separating the polyolefin mixture. In this study, the influence of nanosized carbon black (CB) and Ni2O3 as a novel combined catalyst system on catalyzing carbonization of polypropylene (PP), polyethylene (PE), polystyrene (PS) and their blends was investigated. We showed that this combination was efficient to promote the carbonization of these polymers to produce CNTs with high yields and of good quality. Catalytic pyrolysis and model carbonization experiments indicated that the carbonization mechanism was attributed to the synergistic effect of the combined catalysts rendered by CB and Ni2O3: CB catalyzed the degradation of PP, PE, and PS to selectively produce more aromatic compounds, which were subsequently dehydrogenated and reassembled into CNTs via the catalytic action of CB together with Ni particles. Moreover, the performance of the synthesized CNTs as the electrode of supercapacitor was investigated. The supercapacitor displayed a high specific capacitance as compared to supercapacitors using commercial CNTs and CB. This difference was attributed to the relatively larger specific surface areas of our synthetic CNTs and their more oxygen-containing groups.
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Free-standing single-atom-thick iron membranes suspended in graphene pores.
Science
PUBLISHED: 03-15-2014
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The excess of surface dangling bonds makes the formation of free-standing two-dimensional (2D) metals unstable and hence difficult to achieve. To date, only a few reports have demonstrated 2D metal formation over substrates. Here, we show a free-standing crystalline single-atom-thick layer of iron (Fe) using in situ low-voltage aberration-corrected transmission electron microscopy and supporting image simulations. First-principles calculations confirm enhanced magnetic properties for single-atom-thick 2D Fe membranes. This work could pave the way for new 2D structures to be formed in graphene membranes.
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Room temperature in situ growth of B/BOx nanowires and BOx nanotubes.
Nano Lett.
PUBLISHED: 01-29-2014
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Despite significant advances in the synthesis of nanostructures, our understanding of the growth mechanisms of nanowires and nanotubes grown from catalyst particles remains limited. In this study we demonstrate a straightforward route to grow coaxial amorphous B/BOx nanowires and BOx nanotubes using gold catalyst particles inside a transmission electron microscope at room temperature without the need of any specialized or expensive accessories. Exceedingly high growth rates (over 7 ?m/min) are found for the coaxial nanowires, and this is attributed to the highly efficient diffusion of B species along the surface of a nanowire by electrostatic repulsion. On the other hand the O species are shown to be relevant to activate the gold catalysts, and this can occur through volatile O species. The technique could be further developed to study the growth of other nanostructures and holds promise for the room temperature growth of nanostructures as a whole.
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A cheap and quickly adaptable in situ electrical contacting TEM sample holder design.
Ultramicroscopy
PUBLISHED: 01-09-2014
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In situ electrical characterization of nanostructures inside a transmission electron microscope provides crucial insight into the mechanisms of functioning micro- and nano-electronic devices. For such in situ investigations specialized sample holders are necessary. A simple and affordable but flexible design is important, especially, when sample geometries change, a holder should be adaptable with minimum effort. Atomic resolution imaging is standard nowadays, so a sample holder must ensure this capability. A sample holder design for on-chip samples is presented that fulfils these requisites. On-chip sample devices have the advantage that they can be manufactured via standard fabrication routes.
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Few-layer graphene shells and nonmagnetic encapsulates: a versatile and nontoxic carbon nanomaterial.
ACS Nano
PUBLISHED: 11-12-2013
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In this work a simple and scalable approach to coat nonmagnetic nanoparticles with few-layer graphene is presented. In addition, the easy processing of such nanoparticles to remove their core, leaving only the 3D graphene nanoshell, is demonstrated. The samples are comprehensively characterized, as are their versatility in terms of functionalization and as a material for electrochemical storage. Indeed, these 3D graphene nanostructures are easily functionalized much as is found with carbon nanotubes and planar graphene. Electrochemical investigations indicate these nanostructures are promising for stable long-life battery applications. Finally, initial toxicological investigations suggest no acute health risk from these 3D graphene nanostructures.
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Confined crystals of the smallest phase-change material.
Nano Lett.
PUBLISHED: 09-03-2013
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The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.
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High-mobility graphene on liquid p-block elements by ultra-low-loss CVD growth.
Sci Rep
PUBLISHED: 06-07-2013
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The high-quality and low-cost of the graphene preparation method decide whether graphene is put into the applications finally. Enormous efforts have been devoted to understand and optimize the CVD process of graphene over various d-block transition metals (e.g. Cu, Ni and Pt). Here we report the growth of uniform high-quality single-layer, single-crystalline graphene flakes and their continuous films over p-block elements (e.g. Ga) liquid films using ambient-pressure chemical vapor deposition. The graphene shows high crystalline quality with electron mobility reaching levels as high as 7400 cm(2) V(-1) s(-1) under ambient conditions. Our employed growth strategy is ultra-low-loss. Only trace amounts of Ga are consumed in the production and transfer of the graphene and expensive film deposition or vacuum systems are not needed. We believe that our research will open up new territory in the field of graphene growth and thus promote its practical application.
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Spatial recognition of defects and tube type in carbon nanotube field effect transistors using electrostatic force microscopy.
Nanotechnology
PUBLISHED: 05-15-2013
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The exceedingly high current capacity of single wall carbon nanotubes (SWCNTs) and the high currents that SWCNT thin films can sustain are driving significant efforts to fabricate high quality horizontally aligned SWCNTs. Dielectrophoresis is being increasingly used to prepare aligned nanotubes. However, the aligned nanotubes are generally of low quality since the processing involved can introduce defects. In addition, in arrays of tubes, tube-tube contact resistance can be high. Electrostatic force microscopy is a technique similar to atomic force microscopy, but in this case a bias voltage between the conductive tip and sample enables one to probe the long range electrostatic force. In this study we confirm that the technique can be successfully used to spatially determine defects, crossover points between tubes and nanotube electronic type.
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Growth of all-carbon horizontally aligned single-walled carbon nanotubes nucleated from fullerene-based structures.
Nanoscale Res Lett
PUBLISHED: 04-05-2013
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All-carbon single-walled carbon nanotubes (SWCNTs) were successfully synthesized, nucleated using a fullerene derivative. A systematic investigation into the initial preparation of C60 fullerenes as growth nucleators for the SWCNTs was conducted. Enhancement in the yield of the produced SWCNT has been achieved with exploring different dispersing media for the fullerenes, the period, and environment of the initial thermal treatment of the fullerenes in addition to the use of different fullerene-based structures. The systematic studies significantly advance our understanding of the growth of the all-carbon catalyst-free single-walled carbon nanotubes. Field-effect transistors were fabricated using the catalyst-free SWCNT and then electrically characterized, showing current capacity as high as the well-studied catalyst-assisted nanotubes.
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Nanoporous and Highly Active Silicon Carbide Supported CeO2 -Catalysts for the Methane Oxidation Reaction.
Small
PUBLISHED: 03-21-2013
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CeOx @SiO2 nanoparticles are used for the first time for the generation of porous SiC materials with tailored pore diameter in the mesopore range containing encapsulated and catalytically active CeO2 nanoparticles. The nanocasting approach with a preceramic polymer and subsequent pyrolysis is performed at 1300 °C, selective leaching of the siliceous part results in CeOx /SiC catalysts with remarkable characteristics like monodisperse, spherical pores and specific surface areas of up to 438 m(2) ·g(-1) . Porous SiC materials are promising supports for high temperature applications. The catalysts show excellent activities in the oxidation of methane with onset temperatures of the reaction 270 K below the onset of the homogeneous reaction. The synthesis scheme using core-shell particles is suited to functionalize silicon carbide with a high degree of stabilization of the active nanoparticles against sintering in the core of the template even at pyrolysis temperatures of 1300 °C rendering the novel synthesis principle as an attractive approach for a wide range of catalytic reactions.
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Microscopic insight into the bilateral formation of carbon spirals from a symmetric iron core.
Sci Rep
PUBLISHED: 02-07-2013
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Mirrored carbon-spirals have been produced from pressured ferrocene via the bilateral extrusion of the spiral pairs from an iron core. A parametric plot of the surface geometry displays the fractal growth of the conical helix made with the logarithmic spiral. Electron microscopy studies show the core is a crystalline cementite which grows and transforms its shape from spherical to biconical as it extrudes two spiralling carbon arms. In a cross section along the arms we observe graphitic flakes arranged in a herringbone structure, normal to which defects propagate. Local-wave-pattern analysis reveals nanoscale defect patterns of two-fold symmetry around the core. The data suggest that the bilateral growth originates from a globular cementite crystal with molten surfaces and the nano-defects shape emerging hexagonal carbon into a fractal structure. Understanding and knowledge obtained provide a basis for the controlled production of advanced carbon materials with designed geometries.
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Synthesis and toxicity characterization of carbon coated iron oxide nanoparticles with highly defined size distributions.
Biochim. Biophys. Acta
PUBLISHED: 01-20-2013
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Iron oxide nanoparticles hold great promise for future biomedical applications. To this end numerous studies on iron oxide nanoparticles have been conducted. One aspect these studies reveal is that nanoparticle size and shape can trigger different cellular responses through endocytic pathways, cell viability and early apoptosis. However, systematic studies investigating the size dependence of iron oxide nanoparticles with highly defined diameters across multiple cells lines are not available yet.
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CVD growth of large area smooth-edged graphene nanomesh by nanosphere lithography.
Sci Rep
PUBLISHED: 01-04-2013
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Current etching routes to process large graphene sheets into nanoscale graphene so as to open up a bandgap tend to produce structures with rough and disordered edges. This leads to detrimental electron scattering and reduces carrier mobility. In this work, we present a novel yet simple direct-growth strategy to yield graphene nanomesh (GNM) on a patterned Cu foil via nanosphere lithography. Raman spectroscopy and TEM characterizations show that the as-grown GNM has significantly smoother edges than post-growth etched GNM. More importantly, the transistors based on as-grown GNM with neck widths of 65-75 nm have a near 3-fold higher mobility than those derived from etched GNM with the similar neck widths.
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Understanding the growth of amorphous SiO2 nanofibers and crystalline binary nanoparticles produced by laser ablation.
Nanotechnology
PUBLISHED: 12-16-2011
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The pulsed-laser evaporation synthesis of silica nanofibers and crystalline binary nanoparticles is investigated in detail. By careful adjustment of the synthesis parameters one can tailor the product to form high yield nanofibers or binary nanoparticles. Some control on their diameters is also possible through the synthesis parameters. Oxidation of the nanofibers occurs upon exposure to air after the reaction.
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Structural distortions in few-layer graphene creases.
ACS Nano
PUBLISHED: 11-28-2011
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Folds and creases are frequently found in graphene grown by chemical vapor deposition (CVD), due to the differing thermal expansion coefficients of graphene from the growth catalyst and the flexibility of the sheet during transfer from the catalyst. The structure of a few-layer graphene (FLG) crease is examined by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). A study of 2D fast Fourier transforms (FFTs) taken about the region of the crease allowed for the crystal stacking structure of the system to be elucidated. It was found that strain-induced stacking faults were created in the AB Bernal-stacked FLG bulk around the region proximal to the crease termination; this is of interest as the stacking order of FLG is known to have an effect on its electronic properties and thus should be considered when transferring CVD-grown FLG to alternate substrates for electronic device fabrication. The FFTs, along with analysis of the real space images, were used to determine the configuration of the layers in the crease itself and were corroborated by multislice atomistic TEM simulations. The termination of the crease part way through the FLG sheet is also examined and is found to show strong out of plane distortions in the area about it.
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Graphene: Piecing it together.
Adv. Mater. Weinheim
PUBLISHED: 11-22-2011
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Graphene has a multitude of striking properties that make it an exceedingly attractive material for various applications, many of which will emerge over the next decade. However, one of the most promising applications lie in exploiting its peculiar electronic properties which are governed by its electrons obeying a linear dispersion relation. This leads to the observation of half integer quantum hall effect and the absence of localization. The latter is attractive for graphene-based field effect transistors. However, if graphene is to be the material for future electronics, then significant hurdles need to be surmounted, namely, it needs to be mass produced in an economically viable manner and be of high crystalline quality with no or virtually no defects or grains boundaries. Moreover, it will need to be processable with atomic precision. Hence, the future of graphene as a material for electronic based devices will depend heavily on our ability to piece graphene together as a single crystal and define its edges with atomic precision. In this progress report, the properties of graphene that make it so attractive as a material for electronics is introduced to the reader. The focus then centers on current synthesis strategies for graphene and their weaknesses in terms of electronics applications are highlighted.
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Catalyst poisoning by amorphous carbon during carbon nanotube growth: fact or fiction?
ACS Nano
PUBLISHED: 10-27-2011
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The influence of amorphous carbon on FePt catalyst particles under chemical vapor deposition conditions typically applied for CNT growth is examined through two routes. In the first, FePt catalyst particles supported on alumina are exposed to a well-established cyclohexane thermal CVD reaction at various temperatures. At higher temperatures where self-pyrolysis leads to copious amorphous carbon and carbon tar formation, carbon nanotubes are still able to form. In the second route, an amorphous carbon film is first deposited over the catalyst particles prior to the CVD reaction. Even for reactions where further amorphous carbon is deposited due to self-pyrolysis, graphitization is still demonstrated. Our findings reveal that the presence of amorphous carbon does not prevent catalytic hydrocarbon decomposition and graphitization processes. We also show an additional catalytic reaction to be present, catalytic hydrogenation, a process in which carbon in contact with the catalyst surface reacts with H(2) to form CH(4).
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Metallization and investigation of electrical properties of in vitro recrystallized mSbsC-eGFP assemblies.
Nanotechnology
PUBLISHED: 08-22-2011
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Surface layer (SL) proteins are self-assembling nanosized arrays which can be recrystallized in solution or on surfaces. In this paper, we investigate the metallization, contact potential difference and conductivity of in vitro recrystallized mSbsC-eGFP tube-like assemblies for possible applications in nanobiotechnology. Treatment of mSbsC-eGFP tube-like structures with 150 mM Pt salt solution resulted in the formation of metallized SL assemblies decorated with Pt nanoparticles (? > 3 nm) which were closely packed and aggregated into metal clusters. Kelvin probe force microscopy (KPFM) measurements revealed that metallized and unmetallized SL templates showed different surface potential behaviours, demonstrating that the metal coating changes the electrostatic surface characteristics of SL assemblies. In situ conductivity measurements showed that unmetallized SL assemblies were not conductive. Metallized samples showed linear I-V dependence between - 1 and + 1 V with a conductivity of ? 10(3) S m( - 1).
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Atomic structure of interconnected few-layer graphene domains.
ACS Nano
PUBLISHED: 08-05-2011
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The atomic structure at the boundary interface between interconnected few-layer graphene (FLG) domains, synthesized by atmospheric pressure chemical vapor deposition (AP-CVD), is examined using aberration-corrected high-resolution transmission electron microscopy. Moire? patterns in the HRTEM images reveal the presence of rotational stacking faults in the boundary region that extend over distances of ?100 nm. We show that FLG domains interconnect via two principle processes: graphene sheets from one domain grow over the top of a neighboring domain, while other graphene domains interconnect by direct atomic bonding. Differentiating between these two types of interconnects was found to be possible by examining the HRTEM contrast profiles produced at the interface. Graphene sheets that terminate were found to produce strong edge contrast with increasing defocus values, as well as a broader edge cross section, whereas atomically bonded interfaces were found to not exhibit any contrast, even under large defocus values. These findings are reinforced by correlating with multi-slice TEM image simulations of appropriate structures.
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Carbon nanotube nanoelectronic devices compatible with transmission electron microscopy.
Nanotechnology
PUBLISHED: 04-21-2011
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We report on a novel method to fabricate carbon nanotube (CNT) nanoelectronic devices on silicon nitride membrane grids that are compatible with high resolution transmission electron microscopy (HRTEM). Resist-based electron beam lithography is used to fabricate electrodes on 50 nm thin silicon nitride membranes and focused-ion-beam milling is used to cut out a 200 nm gap across a gold electrode to produce the viewing window for HRTEM. Spin-coating and AC electrophoresis are used as methods to deposit small bundles of carbon nanotubes across the electrodes. We demonstrate the viability of this approach by performing both electrical measurements and HRTEM imaging of solution-processed CNTs in a device.
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Synthesis of carbon nanotubes with and without catalyst particles.
Nanoscale Res Lett
PUBLISHED: 04-07-2011
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The initial development of carbon nanotube synthesis revolved heavily around the use of 3d valence transition metals such as Fe, Ni, and Co. More recently, noble metals (e.g. Au) and poor metals (e.g. In, Pb) have been shown to also yield carbon nanotubes. In addition, various ceramics and semiconductors can serve as catalytic particles suitable for tube formation and in some cases hybrid metal/metal oxide systems are possible. All-carbon systems for carbon nanotube growth without any catalytic particles have also been demonstrated. These different growth systems are briefly examined in this article and serve to highlight the breadth of avenues available for carbon nanotube synthesis.
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Carbon-nanotube-based stimuli-responsive controlled-release system.
Chemistry
PUBLISHED: 03-22-2011
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A stimuli-responsive controlled-release delivery system based on carbon nanotubes is demonstrated. Through TEM, FTIR spectroscopic, and thermogravimetric analysis, functional groups have been successfully attached to the open ends of the tubes, thereby enabling functionalized silica spheres to preferentially attach to the ends. This, in essence, plugs the ends of the tube. Controlling release of encapsulated materials within the tubes is illustrated by fluorescein-filled carbon nanotubes. The discharge process can be triggered by exposure to 1,4-dithiothreitol (DTT) or at elevated temperature. Moreover, both triggering systems, DTT and temperature, provide rate of release control through increased DTT concentration or temperature choice, respectively. This delivery system paves the way for the development of a new generation of site-selective, controlled-release, drug-delivery systems, and interactive nanosensor devices.
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Atomic resolution imaging of the edges of catalytically etched suspended few-layer graphene.
ACS Nano
PUBLISHED: 02-23-2011
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Nanostructured graphene and graphene nanoribbons have been fabricated by catalytic hydrogenation, and the edge smoothness has been examined via direct imaging with atomic resolution. When abstaining from solvents during sample preparation, the prepared nanoribbons possess clean edges ready for inspection via transmission electron microscopy (TEM). Edges with subnanometer smoothness could be observed. A method has been developed to make catalytic hydrogenation experiments compatible with TEM, which enables monitoring of the nanoparticles prior to and after hydrogenation. In this way, etching of free-standing few-layer graphene could be demonstrated. Our results enable evaluation of the degree of edge control that can be achieved by means of catalytic hydrogenation.
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Electron paramagnetic resonance investigation of purified catalyst-free single-walled carbon nanotubes.
ACS Nano
PUBLISHED: 11-17-2010
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Electron paramagnetic resonance of single-walled carbon nanotubes (SWCNTs) has been bedevilled by the presence of paramagnetic impurities. To address this, SWCNTs produced by laser ablation with a nonmagnetic PtRhRe catalyst were purified through a multiple step centrifugation process in order to remove amorphous carbon and catalyst impurities. Centrifugation of a SWCNT solution resulted in sedimentation of carbon nanotube bundles containing clusters of catalyst particles, while isolated nanotubes with reduced catalyst particle content remained in the supernatant. Further ultracentrifugation resulted in highly purified SWCNT samples with a narrow diameter distribution and almost no detectable catalyst particles. Electron paramagnetic resonance (EPR) signals were detected only for samples which contained catalyst particles, with the ultracentrifuged SWCNTs showing no EPR signal at X-band (9.4 GHz) and fields < 0.4 T.
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High-performance field effect transistors from solution processed carbon nanotubes.
ACS Nano
PUBLISHED: 10-19-2010
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Nanoelectronic field effect transistors (FETs) are produced using solution processed individual carbon nanotubes (CNTs), synthesized by both arc discharge and laser ablation methods. We show that the performance of solution processed FETs approaches that of CVD-grown FETs if the nanotubes have minimal lattice defects and are free from surface contamination. This is achieved by treating the nanotubes to a high-temperature vacuum annealing process and using 1,2-dichloroethane for dispersion. We present CNT FETs with mobilities of up to 3546 cm(2)/(V s), transconductance of 4.22 ?S, on-state conductance of 9.35 ?S and on/off ratios as high as 10(6). High-resolution transmission electron microscopy is used to examine the presence of catalyst particles and amorphous carbon on the surface and Raman spectroscopy is used to examine the lattice defects, both of which lead to reduced device performance.
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Self-assembly formation of multi-walled carbon nanotubes on gold surfaces.
Nanoscale
PUBLISHED: 09-27-2010
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We report on the observation of self-assembled carbon nanostructures on a standard transmission electron microscopy (TEM) Au substrate formed via thermal chemical vapor deposition. Multi-walled carbon nanotubes (MWNTs) and other carbon nanostructures (CNs), such as carbon nanofibers and carbon nanoparticles (NPs), could be fabricated through structural transformation of metastable carbon layers on the Au surface during 800-850 °C with the thermal decomposition of ethylene. At these temperatures, we found that Au NPs will form immediately through the structural transformation of the Au grid surface in helium atmosphere. The Au NPs work as active centers to trigger the decomposition of ethylene into carbon atoms, which form metastable carbon layers or amorphous carbon nanobugs, and then form CNs via self-assembling. The growth of CNs was characterized by field-emission scanning electron microscopy (SEM), high-resolution TEM and RAMAN spectroscopy. The transformation of amorphous carbon nanobugs by electron beam irradiation is also recorded by in situ monitoring of TEM.
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In situ observations of fullerene fusion and ejection in carbon nanotubes.
Nanoscale
PUBLISHED: 08-16-2010
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We present in situ experimental observations of fullerenes seamlessly fusing to single-walled carbon nanotubes. The morphing-entry of a fullerene to the interior of a nanotube is also captured. The confined (1D) motion of the newly-encapsulated fullerene within its host attests to the actual change from the exterior to interior.
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Examining the stability of folded graphene edges against electron beam induced sputtering with atomic resolution.
Nanotechnology
PUBLISHED: 07-19-2010
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Low energy electron beam irradiation of the edges of graphene can lead to rearrangement of the carbon atomic structure. We demonstrate the ability to distinguish intrinsic edges of graphene from edges formed by back folding based upon atomic structure and their susceptibility to sputtering. We examine how the atomic structure of the edges of graphene and few layer graphene sheets influences their stability under electron beam irradiation at a low electron accelerating voltage of 80 kV using aberration-corrected high-resolution transmission electron microscopy. We demonstrate that edges of few layer graphene produced by back folding are extremely robust against electron beam induced sputtering as compared to intrinsic edges that are naturally formed. The stability of the folded edges in few layer graphene is shown to increase with increasing number of layers. The higher stability of fold edges is related to the absence of any unsaturated carbon atoms that can be sputtered by electron beam irradiation.
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Direct low-temperature nanographene CVD synthesis over a dielectric insulator.
ACS Nano
PUBLISHED: 07-01-2010
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Graphene ranks highly as a possible material for future high-speed and flexible electronics. Current fabrication routes, which rely on metal substrates, require post-synthesis transfer of the graphene onto a Si wafer, or in the case of epitaxial growth on SiC, temperatures above 1000 degrees C are required. Both the handling difficulty and high temperatures are not best suited to present day silicon technology. We report a facile chemical vapor deposition approach in which nanographene and few-layer nanographene are directly formed over magnesium oxide and can be achieved at temperatures as low as 325 degrees C.
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Atomic resolution imaging and topography of boron nitride sheets produced by chemical exfoliation.
ACS Nano
PUBLISHED: 02-13-2010
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Here, we present a simple method for preparing thin few-layer sheets of hexagonal BN with micrometer-sized dimensions using chemical exfoliation in the solvent 1,2-dichloroethane. The atomic structure of both few-layer and monolayer BN sheets is directly imaged using aberration-corrected high-resolution transmission electron microscopy. Electron beam induced sputtering effects are examined in real time. The removal of layers of BN by electron beam irradiation leads to the exposure of a step edge between a monolayer and bilayer region. We use HRTEM imaging combined with image simulations to show that BN bilayers can have AB stacking and are not limited to just AA stacking.
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Investigating the outskirts of Fe and Co catalyst particles in alumina-supported catalytic CVD carbon nanotube growth.
ACS Nano
PUBLISHED: 01-22-2010
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Using thermal CVD, the synthesis of multi-walled carbon nanotubes exhibiting roots anchored directly onto alpha-alumina supports, rather than the catalyst particle, is reported. At such roots, the alignment of the graphitic planes with the support lattice fringes depends on the support crystal structure and orientation. Surface defects may alter the reactivity of the surface or control the anchoring of supported atoms or nanoparticles. We argue this surface defect is provided by the catalyst particles edge interaction with the support, in other words its circumference. The development of oxide-based catalysts is attractive in that they potentially provide an appropriate solution to directly integrate the synthesis of carbon nanotubes and graphene into silicon-based technology.
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Investigating the graphitization mechanism of SiO(2) nanoparticles in chemical vapor deposition.
ACS Nano
PUBLISHED: 11-14-2009
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The use of SiO(2) as a catalyst for graphitic nanostructures, such as carbon nanotubes and graphene, is a new and rapidly developing catalyst system. A key question is whether carbide phases form in the reaction. We show the formation of SiC from SiO(2) nanoparticles for the synthesis of graphitic carbon nanostructures via chemical vapor deposition (CVD) at 900 degrees C. Our findings point to the carbothermal reduction of SiO(2) in the CVD reaction. The inclusion of triethyl borate apparently accelerates the process and leads to improved yields. The study helps better understand the growth mechanisms at play in carbon nanotube and carbon nanofiber formation when using SiO(2) catalysts.
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Unravelling the mechanisms behind mixed catalysts for the high yield production of single-walled carbon nanotubes.
ACS Nano
PUBLISHED: 11-04-2009
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The use of mixed catalysts for the high-yield production of single-walled carbon nanotubes is well-known. The mechanisms behind the improved yield are poorly understood. In this study, we systematically explore different catalyst combinations from Ni, Co, and Mo for the synthesis of carbon nanotubes via laser evaporation. Our findings reveal that the mixing of catalysts alters the catalyst cluster size distribution, maximizing the clusters potential to form a hemispherical cap at nucleation and, hence, form a single-walled carbon nanotube. This process significantly improves the single-walled carbon nanotube yields.
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Capturing the motion of molecular nanomaterials encapsulated within carbon nanotubes with ultrahigh temporal resolution.
ACS Nano
PUBLISHED: 09-12-2009
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We use in situ low-voltage aberration corrected high resolution transmission electron microscopy with a temporal resolution of 80 ms to track the motional dynamics of nanostructures encapsulated within carbon nanotubes. Two different nanostructures are examined and both are produced by electron beam irradiation of peapods containing La@C(82) metallofullerenes. The first novel nanostructure consists of a LaC(2) metal cluster attached to carbon nanotube inside a nanotube host. It exhibits repeated nanopiston-like behavior over a 5 min duration, driven by energy supplied by electron beam irradiation. Interaction of the metal cluster with the nanotube host is also examined, revealing that the metal cluster can open up the nanotube sidewall, exit, and then seal the hole in the wall back up with carbon from the surrounding region. Finally, the intrinsic motional dynamics of an isolated single fullerene within a SWNT is captured and we report velocities up to 112 nm/s.
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Investigating the diameter-dependent stability of single-walled carbon nanotubes.
ACS Nano
PUBLISHED: 05-26-2009
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We investigate the long-standing question of whether electrons accelerated at 80 kV are below the knock-on damage threshold for single-walled carbon nanotubes (SWNTs). Aberration-corrected high-resolution transmission electron microscopy is used to directly image the atomic structure of the SWNTs and provides in situ monitoring of the structural modification induced by electron beam irradiation at 80 kV. We find that SWNTs with small diameters of 1 nm are damaged by the electron beam, and defects are produced in the side walls that can lead to their destruction. SWNTs with diameters of 1.3 nm and larger are more stable against degradation, and stability increases with diameter. The effect of diameter, defects, and exterior contamination on the inherent stability of SWNTs under electron beam irradiation is investigated.
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One-dimensional confined motion of single metal atoms inside double-walled carbon nanotubes.
Phys. Rev. Lett.
PUBLISHED: 05-14-2009
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Peapods containing La@C_{82} metallofullerenes are transformed into double-walled carbon nanotubes (DWNTs) using in situ electron beam irradiation at 80 kV. Using this low accelerating voltage we find no damage to the outer single-walled carbon nanotube (SWNT) within this time period and the complete formation of an inner nanotube within 5-7 min of irradiation. The La metal atoms are restricted to the interior of the inner SWNT and remain trapped. We demonstrate that energy from electron beam irradiation can drive the lateral confined motion of single La atoms. The interplay between two La atoms confined within the interior of a DWNT is examined and we find large La-La separation unique to this 1D environment. We also demonstrate the formation of TWNTs from DWNT peapods.
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Structural transformations in graphene studied with high spatial and temporal resolution.
Nat Nanotechnol
PUBLISHED: 04-08-2009
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Graphene has remarkable electronic properties, such as ballistic transport and quantum Hall effects, and has also been used as a support for samples in high-resolution transmission electron microscopy and as a transparent electrode in photovoltaic devices. There is now a demand for techniques that can manipulate the structural and physical properties of graphene, in conjunction with the facility to monitor the changes in situ with atomic precision. Here, we show that irradiation with an 80 kV electron beam can selectively remove monolayers in few-layer graphene sheets by means of electron-beam-induced sputtering. Aberration-corrected, low-voltage, high-resolution transmission electron microscopy with sub-ångström resolution is used to examine the structural reconstruction occurring at the single atomic level. We find preferential termination for graphene layers along the zigzag orientation for large hole sizes. The temporal resolution can also be reduced to 80 ms, enabling real-time observation of the reconstruction of carbon atoms during the sputtering process. We also report electron-beam-induced rapid displacement of monolayers, fast elastic distortions and flexible bending at the edges of graphene sheets. These results reveal how energy transfer from the electron beam to few-layer graphene sheets leads to unique structural transformations.
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Direct imaging of rotational stacking faults in few layer graphene.
Nano Lett.
PUBLISHED: 03-31-2009
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Few layer graphene nanostructures are directly imaged using aberration corrected high-resolution transmission electron microscopy with an electron accelerating voltage of 80 kV. We observe rotational stacking faults in the HRTEM images of 2-6 layers of graphene sheets, giving rise to Moir patterns. By filtering in the frequency domain using a Fourier transform, we reconstruct the graphene lattice of each sheet and determine the packing structure and relative orientations of up to six separate sets. Direct evidence is obtained for few layer graphene sheets with packing that is different to the standard AB Bernal packing of bulk graphite. This has implications toward bilayer and few layer graphene electronic devices and the determination of their intrinsic structure.
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The polycyclic aromatic hydrocarbon concentrations in soils in the Region of Valasske Mezirici, the Czech Republic.
Geochem. Trans.
PUBLISHED: 03-06-2009
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The polycyclic aromatic hydrocarbon (PAH) contamination of urban, agricultural and forest soil samples was investigated from samples obtained in the surroundings of Valasske Mezirici. Valasske Mezirici is a town located in the north-east mountainous part of the Czech Republic, where a coal tar refinery is situated. 16 PAHs listed in the US EPA were investigated. Organic oxidizable carbon was also observed in the forest soils. The PAH concentrations ranged from 0.86-10.84 (with one anomalous value of 35.14) and 7.66-79.39 mg/kg dm in the urban/agricultural and forest soils, respectively. While the PAH levels in the urban/agricultural soils are within the range typically found in industrialized areas, the forest soils showed elevated PAH concentrations compared to other forest soils in Western and Northern Europe. The PAH concentrations and their molecular distribution ratios were studied as functions of the sample location and the meteorological history. The soils from localities at higher altitudes above sea level have the highest PAH concentrations, and the PAH concentrations decrease with increasing distance from the town.
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van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.
ACS Nano
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van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 ?m(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation.
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Understanding high-yield catalyst-free growth of horizontally aligned single-walled carbon nanotubes nucleated by activated C60 species.
ACS Nano
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Our understanding of the catalyst-free growth of single-walled carbon nanotubes by chemical vapor deposition is limited. Toward improving our knowledge base, we conducted systematic investigations into the initial preparation of C(60) fullerenes as nucleation precursors for single-wall and even double-wall carbon nanotube fabrication. The role of the dispersing media is shown to be crucial and is related to the initial fullerene cluster size. Oxygen-based groups, in particular, epoxy groups, are shown to be vital prior to actual growth. Moreover, the presence of oxygen groups during the growth phase is necessary for tube development. We also demonstrate the possibility of fabricating the tubes in crossbar configurations with bespoke crossing angles in a single synthesis step, unlike other routes which require at least two synthesis steps. The systematic studies significantly advance our understanding of the growth mechanisms involved in all-carbon catalyst-free growth of single- and double-walled carbon nanotubes.
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Programmable sub-nanometer sculpting of graphene with electron beams.
ACS Nano
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Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without C(s) correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains.
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Lattice expansion in seamless bilayer graphene constrictions at high bias.
Nano Lett.
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Our understanding of sp(2) carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bilayer graphene at high bias, little to nothing is known. To this end, we investigated bilayer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bilayer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 × 10(9) A cm(-2) and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5% and an isotropic component showing an expansion exceeding 1%. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states, thus weakening bonds. The altered character of C-C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bilayer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.
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Size and shape control of colloidal copper(I) sulfide nanorods.
ACS Nano
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Many physical and chemical properties of semiconducting nanocrystals strongly depend on their spatial dimensions and crystallographic structure. For these reasons, achieving a high degree of size and shape control plays an important role with respect to their application potential. In this report we present a facile route for the direct colloidal synthesis of copper(I) sulfide nanorods. A high reactivity of the starting materials is essential to obtain nanorods. We achieve this by using a thiol that thermally decomposes easily and serves as the sulfur source. The thiol is mixed in a noncoordinating solvent, which acts as the reaction medium. Adjustment of the nucleation temperature makes it possible to tailor uniform nanorods with lengths from 10 to 100 nm. The nanorods are single crystalline, and the growth direction is shown to occur along the a-axis of djurleite. The growth process and character of the nanorods were investigated through UV-vis and NIR absorption spectroscopy, transmission electron microscopy, and powder X-ray diffraction measurements.
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CVD-grown horizontally aligned single-walled carbon nanotubes: synthesis routes and growth mechanisms.
Small
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Single-walled carbon nanotubes (SWCNTs) have attractive electrical and physical properties, which make them very promising for use in various applications. For some applications however, in particular those involving electronics, SWCNTs need to be synthesized with a high degree of control with respect to yield, length, alignment, diameter, and chirality. With this in mind, a great deal of effort is being directed to the precision control of vertically and horizontally aligned nanotubes. In this review the focus is on the latter, horizontally aligned tubes grown by chemical vapor deposition (CVD). The reader is provided with an in-depth review of the established vapor deposition orientation techniques. Detailed discussions on the characterization routes, growth parameters, and growth mechanisms are also provided.
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A universal transfer route for graphene.
Nanoscale
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Often synthetic graphene requires transfer onto an arbitrary substrate prior to use because the substrate it was originally synthesized on is inappropriate for either electrical measurement or characterization. While a variety of routes have been developed they are substrate dependant and often involve the use of harsh treatments. Here we present a facile and cheap route that can be applied to graphene over any substrate. This universal transfer route is based on a wet chemical reaction producing gaseous species which can intercalate between the substrate and the graphene and thus gently delaminate the two.
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JoVE Visualize is a tool created to match the last 5 years of PubMed publications to methods in JoVE's video library.

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In developing our video relationships, we compare around 5 million PubMed articles to our library of over 4,500 methods videos. In some cases the language used in the PubMed abstracts makes matching that content to a JoVE video difficult. In other cases, there happens not to be any content in our video library that is relevant to the topic of a given abstract. In these cases, our algorithms are trying their best to display videos with relevant content, which can sometimes result in matched videos with only a slight relation.