High Aspect Ratio Carbon Research Articles

Fabrication of porous carbon micropillars using a block copolymer as porogen

A technique to fabricate porous carbon micropillars using a block copolymer, F127, as porogen is described. In this process, negative tone photoresist (i.e. SU-8) mixed with F127 was photopatterned and carbonized under inert atmosphere. The thermal behavior of the photoresist precursor (F127 + SU-8) during carbonization process was characterized using differential scanning calorimetry and thermogravimetric analysis. Texture analysis on the carbon surface showed a mesoporous feature distribution. Electrochemical characterization based on the reaction of Fe ( CN ) 6 3 - / Fe ( CN ) 6 4 - redox couple was utilized to study the change in the effective surface area ( A eff) of the porous carbon electrodes with different weight percentages of F127 in SU-8. These results indicated that porous carbon thin film electrodes derived from 10% F127 mixed in SU-8 had an A eff 185% compared to the conventional photoresist derived carbon electrode. This fabrication approach can be employed to produce reproducible high aspect ratio carbon microelectrodes with different shapes for various electrochemical devices.

Patterned growth of ultra long carbon nanotubes. Properties and systematic investigation into their growth process

High aspect ratio carbon nanotube arrays were grown using a hydrogen/water assisted catalytic chemical vapor deposition method using ethylene as precursor which achieves a growth rate of 25 to 30 μm carbon nanotube length per minute. The influence of the growth parameters determining the growth rate and the growth height of the carbon nanotubes arrays was studied in detail. Water and hydrogen concentration were varied systematically and it was found that they have to be in a precise ratio in order to establish ultralong growth together with high quality (no contamination with carbonaceous side products) of the arranged carbon nanotubes. For given growth conditions, the hydrogen content in the feed gas stream controls the carbon nanotube length, the purity level and the number of graphitic walls of carbon nanotubes grown. It was found that water acts as a weak oxidant keeping the mixed aluminium/iron catalyst active for ultrahigh length growth. Its concentration should be established very precisely in the process to achieve a maximum growth rate of carbon nanotubes. Furthermore a straightforward, non-costly mask technique is presented to achieve highly structured carbon nanotube growth. Finally the field emission characteristics of such ultrahigh, well arranged and structured carbon nanotube blocks was studied and this revealed low turn-on electric and threshold field values of 0.34 V μm−1 and 0.71 V μm−1 respectively.

The fabrication of high aspect ratio carbon nanotube arrays by direct laser interferencepatterning

High aspect ratio periodic carbon nanotube arrays were fabricated using direct laser interferencepatterning with a frequency tripled Nd:YAG (YAG: yttrium aluminium garnet) laser (λ = 355 nm) emitting 10 ns laser pulses. The depth of the fabricated arrays could be adjusted bycontrolling the number of laser pulses. Application of successive laser pulses (10–20)induced pattern distortion, resulting in the formation of conical organized arrays.The number of laser pulses necessary to produce this distortion is proportionalto the spatial period of the interference pattern. Raman spectroscopy analysesconfirmed that the chemical form of the carbon nanotubes was preserved afterpatterning.

End-specific strategies of attachment of long double stranded DNA onto gold-coatednanofiber arrays

We report the effective and site-specific binding of long double stranded (ds)DNAto high aspect ratio carbon nanofiber arrays. The carbon nanofibers were firstcoated with a thin gold layer to provide anchorage for two controllable bindingmethods. One method was based on the direct binding of thiol end-labeled dsDNA.The second and enhanced method used amine end-labeled dsDNA bound withcrosslinkers to a carboxyl-terminated self-assembled monolayer. The bound dsDNAwas first visualized with a fluorescent, dsDNA-intercalating dye. The specificbinding onto the carbon nanofiber was verified by a high resolution detectionmethod using scanning electron microscopy in combination with the binding ofneutravidin-coated fluorescent microspheres to the immobilized and biotinylated dsDNA.Functional activity of thiol end-labeled dsDNA on gold-coated nanofiber arrayswas verified with a transcriptional assay, whereby Chinese hamster lung cells(V79) were impaled upon the DNA-modified nanofibers and scored for transgeneexpression of the tethered template. Thiol end-labeled dsDNA demonstratedsignificantly higher expression levels than nanofibers prepared with control dsDNAthat lacked a gold-binding end-label. Employing these site-specific and robusttechniques of immobilization of dsDNA onto nanodevices can be of advantagefor the study of DNA/protein interactions and for gene delivery applications.

Synthesis of High Aspect-Ratio Carbon Nanotube “Flying Carpets” from Nanostructured Flake Substrates

We present a robust method for synthesis of aligned, single-walled carbon nanotube (CNT) "flying carpets" from nanostructured alumina flakes. Roll-to-roll e-beam deposition is utilized to produce the flakes, and hot filament chemical vapor deposition is utilized to grow dense, aligned carbon nanotubes from the flakes with remarkably high CNT yields. The flakes are captured inside a mesh cage and freely suspended in the gas flow during growth. Optical characterization indicates the presence of high quality, small diameter single-walled carbon nanotubes.

Ultrasharp and high aspect ratio carbon nanotube atomic force microscopy probes forenhanced surface potential imaging

The resolution of scanning surface potential microscopy (SSPM) is mainly limited bynon-local electrostatic interactions due to the finite probe size. Here we present highresolution surface potential imaging with ultrasharp and high aspect ratio carbon nanotube(CNT) atomic force microscopy (AFM) probes fabricated via dielectrophoresis.Enhancement of surface potential contrast by several factors is reported for integratedcircuit structures and purple membrane fragments for these CNT AFM probesas compared to conventional probes. In particular, ultrahigh lateral resolution(∼2 nm) surface potential images of self-assembled bacteriorhodopsin proteins are reported atambient conditions, with the implication of label-free protein detection by SSPMtechniques.

Carbon post-microarrays for glucose sensors

A novel design and fabrication method of glucose sensors based on high aspect ratio carbon post-microarrays is reported in this paper. Apart from the fact that carbon has a wide electrochemical stability window, a major advantage of using carbon post-microarrays as working electrodes for an amperometric glucose sensor is the large reactive surface per unit footprint substrate area, improving sensitivity of the glucose sensor. The carbon post-microarrays were fabricated by carbon-microelectromechanical systems (C-MEMS) technology. Immobilization of enzyme onto the carbon post-electrodes was carried out through co-deposition of glucose oxidase (GOx) and electrochemically polymerized polypyrrole (PPy). Sensing performance of the glucose sensors with different post-heights and various post-densities was tested and compared. The carbon post-glucose sensors show a linear range from 0.5 mM to 20 mM and a response time of about 20 s, which are comparable to the simulation result. Sensitivity per unit footprint substrate area as large as 2.02 mA/(mM cm 2) is achieved with the 140 μm high (aspect ratio around 5:1) carbon post-samples, which is two times the sensitivity per unit footprint substrate area of the flat carbon films. This result is consistent with the hypothesis that the number of reaction sites scales with the reactive surface area of the sensor. Numerical simulation based on enzymatic reaction and glucose diffusion kinetics gives the optimum geometric design rules for the carbon post-glucose sensor. Glucose sensors with even higher sensitivity can be achieved utilizing higher carbon post-microarrays when technology evolution will permit it.

Effect of particle dimension on biocompatibility of carbon nanomaterials

With various emerging applications ranging from medicine to materials and electronics, the risk of exposure to nanomaterials is rapidly increasing. Several routes of exposure to nanomaterials exist; the most important being dermal contact and inhalation. In this dermal toxicity study, the cellular effects of carbon-based materials with diameters ranging from micro- to nano-dimension were investigated using mouse keratinocytes (HEL-30). The carbon materials tested included carbon fibers (CF; 10 μm diameter), carbon nanofibers (CNF; 100 nm diameter), multi-walled carbon nanotubes (MWCNT; 10 nm diameter), and single-walled carbon nanotubes (SWCNT; 1 nm diameter). CF and CNF did not significantly affect cell viability; however, MWCNT and SWCNT reduced cell viability in a time-dependent manner up to 48 h, with full recovery of mitochondrial function by the 72 h time point. After a 24 h exposure, cells exposed to MWCNT produced up to 3-fold higher increase in reactive oxygen species than those exposed to SWCNT. The results of this study suggest that high-aspect ratio carbon material toxicity is dependent on dimension and composition.

Viscoelastic behavior and electrical properties of flexible nanofiber filled polymer nanocomposites. Influence of processing conditions

Two different types of high aspect ratio flexible nanofibers, cellulose nanofibrils and carbon nanotubes, were dispersed in an amorphous thermoplastic polymer matrix. The mechanical and (in the case of carbon nanotubes filled composites) electrical properties of these composites were investigated. Dynamic mechanical analysis highlighted the influence of entanglements between fibers and of fiber/fiber contact properties on the composite mechanical reinforcement in the rubbery state. In the case of cellulose filled nanocomposites a large mechanical reinforcement effect was observed. This effect was explained by the formation of a rigid nanofibril network linked by strong hydrogen bonds. The formation of this network was assumed to be governed by a percolation mechanism. Conversely, when such bonds between cellulose fibrils were prevented by the process, a lower mechanical reinforcement is observed and can be modeled by a classical mean field approach. On the other hand, both types of composites filled with carbon nanotubes (where no strong interactions are possible) highlighted the fact that entanglements are responsible for a strong increase in thermo-mechanical stability but do not influence the mechanical reinforcement. However, carbon nanotubes are good conductive objects and for such nanocomposites, electrical percolation properties were found. The influence of the process on these electrical properties was highlighted and discussed in term of modification of tube–tube contact electrical properties.

Infrared extinction performance of high aspect ratio carbon nanoparticles

Infrared volume extinction coefficients have been theoretically calculated for high aspect ratio carbon microparticles and nanoparticles. Two principal types of carbon material were modelled: layered crystalline graphite and microcrystalline graphite. The infrared extinction performance of each material was assessed and compared for two high aspect ratio particle geometries, thin lamellar flakes and acicular fibres. Optimum dimensions for both geometries of each material for maximum volume efficient extinction performance have been identified. The volume extinction coefficient calculations were performed using the finite difference time domain method (FDTD), the T-matrix method (TMM) and the infinite cylinder solution (ICS). Phase functions were calculated for all particle geometries and morphologies modelled. Transmission calculations were also performed using the six-flux method of radiative transfer. Finally, an attempt was made to compare the theoretical calculations to an experimental measurement.

Patterning lyotropic liquid crystals as precursors for carbon nanotube arrays

We report on a simple patterning technique using lyotropic liquid-crystal solutions to fabricate organized arrays of high aspect ratio carbon nanostructures. An automated pen writing system is used in conjunction with nanochannel alumina templates to produce ordered arrays of nanotubes and nanofibers in a variety of shapes, widths, and carbon crystal structures set by surface anchoring phenomena. The nanostructure arrays are characterized with optical and electron microscopy and Raman spectroscopy to establish the morphology and crystal structures of the carbon nanoform arrays.

A novel method for the fabrication of high-aspect ratio C-MEMS structures

A novel fabrication process was developed to create high aspect ratio (>10:1) carbon posts, all-carbon suspended bridges and wires, self-organized bunches of carbon posts, and carbon plates supported by carbon beams. The structures are all made from a two-step pyrolysis process with SU-8 photoresist as the starting material. In this paper we describe the fabrication of these various new C-MEMS structures and detail an important application of the high aspect ratio carbon posts arrays. The carbon post arrays can be reversible charged/discharged with Li ions, an application that may greatly impact the application of C-MEMS in three-dimensional microbatteries. Complex suspended C-MEMS structures, such as wires, plates, ribbons, and self-organized bunches of posts, were built. Methods to accurately and repeatedly fabricate all the above 3-D C-MEMS structures are given.

Fabrication of individual aligned carbon nanotube for scanning probe microscope

This paper demonstrates the fabrication of a micro-cantilever equipped with a high aspect ratio carbon nanotube (CNT) tip to perform surface characterization with high spatial resolution. A single vertical CNT, synthesized on a freestanding tip-lessmicro cantilever, is intended to be used for scanning probe microscopy (SPM). E-beam lithography and lift-off technique was adopted to define a nano-sized catalyst particle. An individual CNT was then synthesized by using an inductively coupled plasma chemical vapor deposition (ICP-CVD) system. The freestanding individual aligned CNT probe was then used to characterize a silicon nano pillar structure. The surface morphology was scanned in a contact mode and 0.1 nN set-point force. The ultra fine resolution can be characterized without wearing the probe and sample. The CNT probe shows a great potential as a high sensitive sensor operating in an optimally adjusted atomic force microscopy (AFM) system.

From MEMS to NEMS with carbon

Our work in carbon-microelectromechanical systems (C-MEMS) suggests that C-MEMS might provide a very interesting material and microfabrication approach to battery miniaturization, active DNA arrays and a wide variety of chemical and biological sensors. In C-MEMS, photoresist is patterned by photolithography and subsequently pyrolyzed at high-temperatures in an oxygen-free environment. We established that it is possible to use C-MEMS to create very high-aspect ratio carbon structures (e.g. posts with an aspect ratio >10), suspended carbon plates and suspended carbon nanowires (C-NEMS). By changing the lithography conditions, soft and hard baking times and temperatures, additives to the resist, pyrolysis time, temperature and environment, C-MEMS permits a wide variety of interesting new MEMS and NEMS applications that employ structures having a wide variety of shapes, resistivities and mechanical properties. We also demonstrate that arrays of high-aspect ratio carbon posts can be charged/discharged with Li and this enables the fabrication of a smart switchable array of batteries.

CATALYTIC PRODUCTION OF GRAPHITIC NANORODS VIA THE GAS PHASE DECOMPOSITION OF ETHYLENE OVER SUPPORTED NICKEL

The catalytic growth of high aspect ratio carbon nanorods by ethylene decomposition over nickel/silica is presented as a viable low-cost selective route to a high-value product. This study focuses on the role of catalyst preparation in determining carbon yield and structural characteristics and considers the application of a 10% w/w Ni loading prepared by impregnation and precipitation/deposition. The latter is characterized by a narrower dispersion of smaller Ni particles (average diameter = 2.4 nm) but lower carbon yields. Doping this catalyst with KBr resulted in a 40-fold increase in carbon production with >95% having rod diameters <10 nm; doping the impregnated catalyst had little effect on catalyst performance. High-resolution transmission electron microscopy and temperature programmed oxidation were employed to characterize the catalysts and the carbon product; the effect of temperature (673–873 K) on carbon yield is also reported.

C-MEMS for the Manufacture of 3D Microbatteries

We have demonstrated that carbon-microelectromechanical systems (C-MEMS), in which patterned photoresist is pyrolyzed in inert environment at high temperature, constitutes a powerful approach to building 3D carbon microelectrode arrays for 3D microbattery applications. High aspect ratio carbon posts (>10:1) are achieved by pyrolyzing SU-8 negative photoresist in a simple one step process. Lithium can be reversibly charged and discharged into these C-MEMS electrodes. Because of the additional volume of the posts, higher capacities are achieved with the 3D array electrodes as compared to unpatterned carbon films with the same projected electrode area. These novel electrode arrays represent one of the critical components for 3D batteries, which may be interconnected with C-MEMS leads to enable smart power management schemes.

Field emission from individual vertically carbon nanofibers

Individual high-aspectratio carbon nanofibers(CNFs) were grown on W wire by plasma-enhanced hot filament chemical vapor deposition method, using a gas mixture of methane and hydrogen. The average diameter and length of carbon nanofibers are 60—100 nm and 6—30 μm, respectively, and the density of carbon nanofibers is less than 10.6cm-2. The field emission properties from individual carbon nanofibers have been measured using a movable W probe with a low-curvature radius. The results indicated that the CNFs showed a turn on field of about 5 V/μm and the field-emission current density of 20μA/cm2 at 5 V/μm.The experimental data also indicated that the length of CNFs and the position of a CNF are responsible for the properties of field emission related to the parameters such as the work function of materials, applied field and the field amplification factor. Furthermore, the electron scattering induced by defects in CNFs is also a key factor on the field-emission current.

Mini features

DuPont in Europe has introduced chemical‐ and creep‐resistant machined finished parts and stock shapes in “Zymaxx”, a high temperature Teflon matrix with a specially treated high aspect ratio carbon fibre reinforcement.

Investigation of fabrication parameters for the electron-beam-induced deposition of contamination tips used in atomic force microscopy

Electron-beam deposition in a field-emission electron microscope is used to grow high aspect ratio carbon contamination tips suitable for imaging in atomic force microscopy. Tip lengths of up to several microns, tip diameters between 100 and 400 nm and cone half-angles of 3 degrees -50 degrees are achieved with typical radii of curvature between 20 and 40 nm. The influence of the major deposition parameters, i.e. electron energy, beam current, deposition time and working distance, on the growth rate and shape (length, cone angle and cone length) of the tip, are investigated systematically. It is found that increasing beam current leads to a decrease in length growth probably due to enhanced thermal desorption of adsorbates and reduced sticking coefficients. Electron energy mainly determines cone angles and cone length while the variation of working distance only has a small influence on the tip growth. Imaging capabilities of the tips are verified on vertically walled, lithographically fabricated patterns.

Processing effects on the morphology of titanium carbide in self-propagating high-temperature synthesis

AbstractThe morphology of titanium carbide through self-propagating high-temperature synthesis (SHS) was studied to obtain uniform surfaces and high-aspect ratios in titanium carbide fibres. It was found that the initial carbon shape, mixing, and compaction method can significantly influence the morphology of the final products. Non-fibrous titanium carbides were produced after ignition when carbon fibres and titanium powders were mixed in an inert argon atmosphere and cold isostatically pressed. However, when the reactants were mixed in n-hexane, dried and cold isostatically pressed, the final products were found to be fibrous titanium carbides having a smooth surface. High aspect ratios for titanium carbide fibre can be obtained through SHS processing by initially choosing a high aspect-ratio carbon fibre, optimizing the mixing method, the chemistry of the fibre precursors, and use of reaction additives.