Nanotube Ensembles Research Articles

Optical determination of the flexural rigidity of carbon nanotube ensembles

We demonstrate two simple and consistent optical methods for quantitatively determining the flexural rigidity EI (where E is the elastic modulus and I the moment of inertia), a quantity of practical importance in determining the deflection and buckling characteristics of carbon nanotubes (CNTs). This is done through monitoring the deflection of patterned arrays of CNTs, which are subject to fluid flow. In addition to mechanical characterization of filamentous nanostructures, the implications of our work extend to the monitoring of nanoscale fluid flows for tactile and shear force sensors and the characterization of the mechanosensor response of cilia in physiology.

Free-Standing, Erect Ultrahigh-Aspect-Ratio Polymer Nanopillar and Nanotube Ensembles

Free-standing polymer (poly(methyl methacrylate) or cyclic olefin copolymer) nanopillar and nanotube ensembles with previously unreported, ultrahigh aspect ratios (300 to >1600) were fabricated via anodic aluminum oxide (AAO) template-based methods that utilize a dilute, aqueous H3PO4 template etchant followed by freeze drying removal of the aqueous medium. Good replication of the AAO template by either solutions of the polymeric materials or molten polymer was achieved by using ultrasonic degassing and vacuum conditions. Classical surface wetting and viscoelastic fluid rheology theories were applied to explain the formation of polymer nanopillars and nanotubes in the aluminum oxide templates. The utilization of dilute H3PO4 for etching the AAO template and freeze-drying removal of the environmental liquid allows for the preparation of free-standing, erect, and ordered polymeric nanopillars or nanotubes that show much promise for use in biological microelectromechanical systems that target biological analyses.

Artificial introduction of defects into vertically aligned multiwalled carbon nanotube ensembles: Application to electrochemical sensors

Carbon nanotubes (CNTs) inevitably contain defects that exert a significant influence on their physical, electrical, and electrochemical properties. In this study, we subject vertically aligned multiwalled CNT ensembles to argon and hydrogen ion irradiation, to artificially introduce defects into the structure. Subsequently, Raman spectroscopy in conjunction with electrochemical analyses was used to characterize the amount and nature of disorder within the CNTs. While an increased disorder with ion irradiation was generally observed, argon and hydrogen exhibit different effects on the Raman intensity spectra. Argon irradiation seems to cause charged defects, e.g., in the form of dangling bonds, and increases the in-plane correlation length (La), while hydrogen irradiation passivates residual defects and decreases the La. It was noted that hydrogen treated CNTs could serve as electrochemical sensors with quicker response time.

Linear optical properties of zigzag single-walled BN nanotube ensembles from a model calculation

Linear optical properties of zigzag single-walled BN nanotube ensembles from a model calculation

Photoluminescence from Intertube Carrier Migration in Single-Walled Carbon Nanotube Bundles

Photoluminescence (PL) identifies spectroscopic signatures of intertube transfer of optically pumped carriers in single-walled carbon nanotube (SWNT) ensembles. Resonant photoexcitation of large band gap SWNTs produces strong PL from smaller band gap SWNTs. Magnetic alignment measurements associate the energy-transfer PL peaks with the formation of SWNT bundles, suggesting that efficient coupling results from physical contact.

Computational Model for Transport in Nanotube-Based Composites With Applications to Flexible Electronics

Thermal and electrical transport in a new class of nanocomposites composed of random isotropic two-dimensional ensembles of nanotubes or nanowires in a substrate (host matrix) is considered for use in the channel region of thin-film transistors (TFTs). The random ensemble of nanotubes is generated numerically and each nanotube is discretized using a finite volume scheme. To simulate transport in composites, the network is embedded in a background substrate mesh, which is also discretized using a finite volume scheme. Energy and charge exchange between nanotubes at the points of contact and between the network and the substrate are accounted for. A variety of test problems are computed for both network transport in the absence of a substrate, as well as for determination of lateral thermal and electrical conductivity in composites. For nanotube networks in the absence of a substrate, the conductance exponent relating the network conductance to the channel length is computed and found to match experimental electrical measurements. The effective thermal conductivity of a nanotube network embedded in a thin substrate is computed for a range of substrate-to-tube conductivity ratios. It is observed that the effective thermal conductivity of the composite saturates to a size-independent value for large enough samples, establishing the limits beyond which bulk behavior obtains. The effective electrical conductivity of carbon nanotube-organic thin films used in organic TFTs is computed and is observed to be in good agreement with the experimental results.

Phase diagrams of magnetic nanotubes

Analytical expressions for the total magnetic energy of two characteristic internal configurations of nanometric tubes are calculated. A magnetic phase diagram with respect to the aspect ratio of the tubes is obtained which allows a discussion about the possibility of getting ensembles of nanotubes with low coercive fields. A comparison with recently reported coercive fields of three different cobalt nanotube arrays agrees well with the phase diagram derived here.

The effects of nitrogen and boron doping on the optical emission and diameters of single-walled carbon nanotubes

Using TEM, absorption and photoluminescence-excitation spectroscopy we have shown that nitrogen and nitrogen/boron doping of single-walled carbon nanotubes produce significant changes in both the optical properties and the diameter distribution of nanotubes produced by the arc-discharge method. Smaller diameter tubes are preferentially formed in the presence of boron. In addition the presence of nitrogen is found to significantly affect the emission properties of the nanotube ensemble, causing a shift in the dominant emission to lower energies, possibly due to changes in the bundling structure of the nanotubes in solution, but only very small changes are observed in the emission energies for individual nanotubes.

Synthesis, Properties, and Applications of Ferromagnetic‐Filled Carbon Nanotubes

AbstractFerromagnetic‐filled carbon nanotubes are new nanostructured materials with many possible applications. They can be synthesized using the thermal decomposition of metallocenes of the iron triad. Two different methods (solid and liquid source CVD) are suitable for producing, at very high filling rates, filled nanotubes on precoated Si substrates. The diameters of deposited filled nanotubes are particularly dependent on the size of catalyst particles on the substrate, while the lengths depend more on the sublimation and decomposition rate of metallocene. The growth mechanism of filled carbon nanotubes is based on the root growth mode. Multiwalled carbon nanotubes, filled with body‐centered cubic Fe, show unusual magnetic properties. Aligned‐growth nanotube ensembles can reach coercivities up to 130 mT (bulk iron 0.09 mT). Ferromagnetic‐filled carbon nanotubes can be successfully used both as cantilever tips in magnetic force microscopy and as a nanocontainer for new therapies in medicine.

Photoelectron spectroscopy and microscopy of carbon nanotubes

In this review we will focus our attention on the characterization of carbon nanotubes by X-ray photoelectron spectroscopy. In contrast to other spectroscopic techniques, photoelectron spectroscopy allows to obtain information on the overall electronic structure of the sample in a wide energy range, which makes it a unique technique. We will discuss the most recent and the most significant results on the intrinsic electronic properties of carbon nanotubes obtained with photoelectron spectroscopy. Furthermore, we will discuss in detail the application of photoelectron microscopy, i.e. photoelectron spectroscopy with high lateral resolution, to the study of carbon nanotubes. This technique allows to obtain laterally resolved spectroscopic information from ensembles of carbon nanotubes, but it even allows to perform photoelectron spectroscopy on single carbon nanotubes.

Carbon Nanotube Micro-Opto-Mechanical Grippers

ABSTRACTWe report the integration of single wall carbon nanotube ensembles into micro-mechanical systems to realize a new carbon nanotube micro-optomechanical system (CNT-MOMS). CNT-MOM grippers were fabricated with CMOS compatible techniques involving nanotube film formation, wafer bonding, photo-lithography, plasma etching and dry release. MOM-grippers displacement of ∼24μm was obtained from a gripper of 430μm in length under infra-red laser stimulus and continuous operation of more than 100,000 cycles was acquired. The optical power consumption of the gripper operation was estimated to be as small as ∼240μW. This study is a good example of how nano-materials could be integrated into CMOS compatible techniques for applications in high performance MEMS and nanoscale actuation technologies.

Enhanced magnetism in Fe-filled carbon nanotubes produced by pyrolysis of ferrocene

By optimization of the synthesis of ferromagnetic-filled carbon nanotube ensembles on Si substrates (catalytic decomposition of ferrocene) and following annealing at 645°C, marked hysteresis loops can be measured by the alternating-gradient method. Unusually high coercivities and strong anisotropies with an easy magnetic axis parallel to the alignment of the nanotubes are observed from the as-grown samples, whereas an enhanced magnetic saturation moment (up to a factor of 2) and a decreased anisotropy are realized after annealing at 645°C. The increase of the magnetic saturation moment of the Fe-filled carbon nanotube ensembles is caused by the entire transformation within the tubes of the γ-Fe and Fe3C phases to ferromagnetic α-Fe and graphite. X-ray diffraction with different glancing incidence shows that the γ-Fe is predominantly at the tips of the nanotubes, while the iron carbide resides closer to the substrate. However, after the annealing process only α-Fe is found. At an annealing temperature of 675°C the nanotube structures are destroyed and the magnetic characteristics are dramatically altered (viz., the disappearance of anisotropy and reduction in coercivity).

Conductance in multiwall carbon nanotubes and semiconductor nanowires

Electronic transport in an ensemble of multiwall carbon nanotubes and semiconductor nanowires was compared. The nanotubes and nanowires are obtained by template synthesis and are contacted in a current perpendicular to the plane geometry by using different methods. In all cases, the nonohmic behavior of the conductance, the so-called zero-bias anomaly, shows a temperature dependence that scales with the voltage dependence. This robust scaling law describes the conductance $G(V,T)$ by a single coefficient $\ensuremath{\alpha}$. A universal behavior as a function of $\ensuremath{\alpha}$ is found for all samples. Magnetoconductance measurements furthermore show that the conduction regime is weak localization. The observed behavior can be understood in terms of the Coulomb blockade theory, providing that a single tunnel barrier is present. This hypothetical tunnel barrier would have a resistance of the order of $2500\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}$ and a typical energy of about $40\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for all samples.

Percolating Conduction in Finite Nanotube Networks

The percolating conductance of a new class of nanocomposite thin-film transistors, with channels composed of isotropic ensembles of nanotubes or nanowires, is analyzed as a function of wire/tube density and channel length. The conductance exponents are validated against analytical results for short channel transistors, and against available experimental data for longer channel devices. Our plots of conductance exponents as a function of tube-to-tube coupling strength provide a unified framework to interpret future experiments and should help design better nanocomposite transistors.

Soluble carbon nanotube ensembles for light-induced electron transfer interactions

Soluble carbon nanotube ensembles having ferrocene units covalently introduced by 1,3-dipolar cycloaddition of azomethine ylides have been synthesized and studied for light-induced electron transfer interactions. Additionally, supramolecular means have been utilized to immobilize a positively charged water-soluble pyrene derivative to purified single-walled carbon nanotube (SWNT), which was further coupled by attractive electrostatic interactions to an oligoanionic porphyrin. Electron transfer from the photoexcited states were probed by fluorescence and transient absorption spectroscopy measurements.

Nanostructuring electrodes with carbon nanotubes: A review on electrochemistry and applications for sensing

Carbon nanotubes have begun to attract enormous interest in electrochemistry because of their small size and good electrochemical properties. The vast majority of studies thus far have used ensembles of carbon nanotubes to nanostructure macroscopic electrodes either with randomly dispersed nanotubes or with aligned carbon nanotubes. The resultant nanotube modified electrodes have most frequently been used for electro-analytical purposes such as the development of biosensors. This review introduces carbon nanotubes and approaches to nanostructuring electrodes with carbon nanotubes, discusses what is known of their electrochemical properties and briefly outlines some of the exciting applications to which they are being targeted for use.

Third-order optical nonlinearities of carbon nanotubes in the femtosecond regime

Femtosecond pump–probe experiments have been carried out on an ensemble of single-wall carbon nanotubes deposited on a glass substrate. Measurements of transient changes of transmission and reflection provide an estimate of the real and imaginary parts of the second-order hyperpolarizability of carbon nanotubes. These values are compared with previous measurements and are discussed in the light of a simple model of the optical nonlinearities near the optical band-gap.

Low temperature emission spectra of individual single-walled carbon nanotubes: multiplicity of subspecies within single-species nanotube ensembles.

Low-temperature photoluminescence (PL) studies of individual semiconducting single-walled carbon nanotubes reveal ultranarrow peaks (down to 0.25 meV linewidths) that exhibit blinking and spectral wandering. Multiple peaks appear within bands previously assigned to nanotubes of certain chiralities, indicating the existence of numerous subspecies within single-chirality specimens. The sharp PL features show two types of distinctly different shapes (symmetric versus asymmetric) and temperature dependences (weak versus strong), which we attribute to the presence of unintentionally doped nanotubes along with undoped species.

Magnetic and electronic properties of multiwall carbon nanotubes

Multiwall carbon nanotubes were studied by dc magnetization and electron spin resonance (ESR) measurements. Hole doping is inferred from both the high-field dc susceptibility and ESR parameters complying favorably with the band model of two-dimensional graphite. Paramagnetic deviations are evinced on the diamagnetic susceptibility at weak fields and low temperatures, conforming qualitatively with the Aharonov-Bohm effect on the energy gap for magnetic field parallel to the tube axis. Comparison with theoretical predictions for ensembles of carbon nanotubes reveals appreciable differences, indicative of the diverse distribution of nanotubes as well as the presence of active doping.

Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection.

Arrays of electrical devices with each comprising multiple single-walled carbon nanotubes (SWNT) bridging metal electrodes are obtained by chemical vapor deposition (CVD) of nanotubes across prefabricated electrode arrays. The ensemble of nanotubes in such a device collectively exhibits large electrical conductance changes under electrostatic gating, owing to the high percentage of semiconducting nanotubes. This leads to the fabrication of large arrays of low-noise electrical nanotube sensors with 100% yield for detecting gas molecules. Polymer functionalization is used to impart high sensitivity and selectivity to the sensors. Polyethyleneimine coating affords n-type nanotube devices capable of detecting NO2 at less than 1 ppb (parts-per-billion) concentrations while being insensitive to NH3. Coating Nafion (a polymeric perfluorinated sulfonic acid ionomer) on nanotubes blocks NO2 and allows for selective sensing of NH3. Multiplex functionalization of a nanotube sensor array is carried out by microspotting. Detection of molecules in a gas mixture is demonstrated with the multiplexed nanotube sensors.