Growth Mechanism of Vertically Aligned Carbon Nanotube Arrays

Vertically aligned multi-walled carbon nanotube arrays grown on quartz substrate are obtained by co-pyrolysis of xylene and ferrocene at 850 oC in a tube furnace. Raman spectroscopy and high resolution transmission electron microscopy measurements show that the single-walled carbon nanotubes are only present on top of vertically aligned multi-walled carbon nanotube arrays. It has been revealed that isolated single-walled carbon nanotubes are only present in those floating catalyst generated materials. It thus suggests that the single-walled carbon nanotubes here are also generated by floating catalyst. Vertically alignedcarbon nanotube arrays on the quartz substrate have shown good orientation and good graphitization. Meanwhile, to investigate the growth mechanism, two bi-layers carbon nan-otube films with di erent thickness have been synthesized and analyzed by Raman spectroscopy. The results show that the two-layer vertically aligned carbon nanotube films grow “bottom-up”. There are distinguished Raman scattering signals for the second layer itself, surface of the first layer, interface between the first and second layer, side wall and bottom surface. It indicates that the obtained carbon nanotubes follow the base-growth mechanism, and the single-walled carbon nanotubes grow from their base at the growth beginning when iron catalyst particles have small size. Those carbon nanotubes with few walls (typically <5 walls) have similar properties, which also agree with the base-growth mechanism.     

Structural characterization of single-walled carbon nanotube bundles by experiment and molecular simulation

A procedure, combining molecular simulation, Raman spectroscopy, and standard nitrogen adsorption, is developed for structural characterization of single-walled carbon nanotube (SWNT) samples. Grand canonical Monte Carlo simulations of nitrogen adsorption are performed on the external and internal adsorption sites of homogeneous arrays of SWNTs of diameters previously determined by Raman spectroscopy of the sample. The results show the importance of the peripheral grooves of a nanotube bundle at low relative pressure and the insensitivity of nanotube diameter toward adsorption on the external surface of the bundle at higher pressures. Simulations also reveal that samples containing thin nanotubes have less internal adsorption capacity that saturates at lower pressure than those comprising large diameter nanotubes. The fraction of open-ended nanotubes in a sample can be estimated by scaling the simulated internal adsorption inside nanotubes to obtain a near perfect fit between simulated and experimental isotherms. This procedure allows extrapolation of adsorption properties to conditions in which all nanotubes in the sample are open-ended.     

Chirality- and diameter-dependent reactivity of NO2 on carbon nanotube walls

We report the density-functional calculations of NO2 adsorption on single-walled carbon nanotube walls. A single molecular adsorption was endothermic with an activation barrier, but a collective adsorption with several molecules became exothermic without an activation barrier. We find that NO2 adsorption is strongly electronic structure- and strain-dependent. The NO2 adsorption on metallic nanotubes was energetically more favorable than that on semiconducting nanotubes and furthermore the adsorption became less stable with increasing diameters of nanotubes. The adsorption barrier height shows similar dependence on the electronic structure and diameter to the adsorption energy. Our theoretical model can be a good guideline for the separation of nanotubes by electronic structures using various adsorbates.     

An investigation of the mechanisms of electronic sensing of protein adsorption on carbon nanotube devices

It has been reported that protein adsorption on single-walled carbon nanotube field effect transistors (FETs) leads to appreciable changes in the electrical conductance of the devices, a phenomenon that can be exploited for label-free detection of biomolecules with a high potential for miniaturization. This work presents an elucidation of the electronic biosensing mechanisms with a newly developed microarray of nanotube "micromat" sensors. Chemical functionalization schemes are devised to block selected components of the devices from protein adsorption, self-assembled monolayers (SAMs) of methoxy(poly(ethylene glycol))thiol (mPEG-SH) on the metal electrodes (Au, Pd) and PEG-containing surfactants on the nanotubes. Extensive characterization reveals that electronic effects occurring at the metal-nanotube contacts due to protein adsorption constitute a more significant contribution to the electronic biosensing signal than adsorption solely along the exposed lengths of the nanotubes.     

Monte Carlo simulations of hydrogen adsorption in alkali-doped single-walled carbon nanotubes

Monte Carlo simulations and Widom's test particle insertion method have been used to calculate the solubility coefficients (S) and the adsorption equilibrium constants (K) in single-walled (10,10) armchair carbon nanotubes including single nanotubes, and nanotube bundles with various configurations with and without alkali dopants. The hydrogen adsorption isotherms at room temperature were predicted by following the Langmuir adsorption model using the calculated constants S and K. The simulation results were in good agreement with experimental data as well as the grand canonical Monte Carlo simulation results reported in the literature. The simulations of nanotube bundle configurations suggest that the gravimetric hydrogen adsorption increases with internanotube gap size. It may be attributed to favorable hydrogen-nanotube interactions outside the nanotubes. The effect of alkali doping on hydrogen adsorption was studied by incorporating K+ or Li+ ions into nanotube arrays using a Monte Carlo simulation. The results on hydrogen adsorption isotherms indicate hydrogen adsorption of 3.95 wt% for K-doping, and 4.21 wt% for Li-doping, in reasonable agreement with the experimental results obtained at 100 atm and room temperature.     

Integrated single-walled carbon nanotube/microfluidic devices for the study of the sensing mechanism of nanotube sensors

A method to fabricate integrated single-walled carbon nanotube/microfluidic devices was developed. This simple process could be used to directly prepare nanotube thin film transistors within the microfluidic channel and to register SWNT devices with the microfludic channel without the need of an additional alignment step. The microfluidic device was designed to have several inlets that deliver multiple liquid flows to a single main channel. The location and width of each flow in the main channel could be controlled by the relative flow rates. This capability enabled us to study the effect of the location and the coverage area of the liquid flow that contained charged molecules on the conduction of the nanotube devices, providing important information on the sensing mechanism of carbon nanotube sensors. The results showed that in a sensor based on a nanotube thin film field effect transistor, the sensing signal came from target molecules absorbed on or around the nanotubes. The effect from adsorption on metal electrodes was weak.     

The effect of atomic hydrogen adsorption on single-walled carbon nanotubes properties

We investigated the adsorption of hydrogen atoms on metallic single-walled carbon nanotubes using ab initio molecular dynamics method. It was found that the geometric structures and the electronic properties of hydrogenated SWNTs can be strongly changed by varying hydrogen coverage. The circular cross sections of the CNTs were changed with different hydrogen coverage. When hydrogen is chemisorbed on the surface of the carbon nanotube, the energy gap will be appeared. This is due to the degree of the sp³ hybridization, and the hydrogen coverage can control the band gap of the carbon nanotube.     

Probing chiral selective reactions using a revised Kataura plot for the interpretation of single-walled carbon nanotube spectroscopy

Raman spectroscopy on surfactant-dispersed, aqueous suspensions of single-walled carbon nanotubes is used to verify the energies of interband transitions and validate the spectral assignments of semiconducting and metallic nanotubes determined by spectrofluorimetry for the former and Raman excitation profiles for the latter. The results are compiled into an experimentally based mapping of transition versus nanotube diameter to revise those previously employed using single-electron theoretical treatments. Because this mapping provides the transitions associated with a precise chiral wrapping of a particular nanotube, it allows the monitoring of reaction pathways that are selective to the nanotube chirality vector. This is demonstrated using a model electron-transfer reaction of 4-chlorobenzenediazonium shown to be selective for metallic over semiconducting carbon nanotubes via charge-transfer stabilization of complexes at the surfaces of the former.     

碳纳米管内N2吸附行为及等量吸附热的GCMC分子模拟研究

碳纳米管及石墨烯具有高比表面积、高化学稳定性以及高耐蚀性等优点,被认为是一种理想的吸附材料。分子模拟技术的发展和应用丰富了人们对吸附机理研究的方式,而简单气体吸附体系的吸附机理研究对吸附理论的发展有着重要的推动作用。本文以单壁碳纳米管（SWCNT）-N₂吸附体系为研究对象,首先通过透射扫描电镜和氮气吸/脱附测试对所选用碳纳米管的微观孔形貌及吸/脱附等温线进行了表征,然后根据对应孔径参数采用巨正则蒙特卡罗方法对该体系的吸附过程进行了分子模拟,并详细研究了碳纳米管孔径和温度对该体系吸附行为的影响。结果显示,SWCNT孔径越小,吸附能力则越强;孔半径为0.746nm的SWCNT的吸附体系发生凝聚相变的临界温度为66K。通过对等量吸附热进行计算发现,孔半径0.746、1.15、1.56和1.83 nm的SWCNT-N₂吸附体系对应的初始固-液等量吸附热分别为10.9、9.2、8.6和8.4 kJ/mol。67.5K时,孔半径1.56和1.83 nm的吸附体系的等量吸附热有热峰出现。     

UV-light mediated decomposition of a polyester for enrichment and release of semiconducting carbon nanotubes

Purification of single-walled carbon nanotubes using conjugated polymers to selectively disperse either semiconducting or metallic nanotubes is effective and has received significant attention. However, the interaction between the conjugated polymer and the nanotube surface is very strong, making it difficult to remove the adsorbed polymer. Here, we report a poly(carbazole-co-terephthalate) polymer that is not only selective for semiconducting carbon nanotubes but can also be largely removed from the nanotube surface via irradiation with UV light. Irradiation of the polymer-nanotube dispersion causes degradation of ester linkages in the polymer backbone, effectively cutting the polymer into fragments that no longer bind strongly to the nanotube surface. Characterization of the electronic nature of the samples was carried out via the combination of absorption, Raman, and fluorescence spectroscopy. In addition, thermogravimetric analysis allowed determination of the amount of polymer left on the nanotube surface after irradiation and indicated that a large proportion of the polymer is removed. The reported methodology opens new possibilities for purification of semiconducting single-walled carbon nanotubes and their isolation from the polymeric dispersant.     

The effects of the lengths and orientations of single-walled carbon nanotubes on the electrochemistry of nanotube-modified electrodes

The influence of both nanotube orientation and length on the electrochemical properties of electrodes modified with single-walled carbon nanotubes was investigated. Gold electrodes were modified with either randomly dispersed or vertically aligned nanotubes to which ferrocenemethylamine was attached. Electron transfer kinetics were found to depend strongly on the orientation of the nanotube, with electron transfer between the gold electrode and the ferrocene moiety being 40 times slower through randomly dispersed nanotubes than through vertically aligned nanotubes. The difference is hypothesized to be due to electron transfer being more direct through a single tube than that with electrodes modified with randomly dispersed nanotubes. With the vertically aligned nanotubes the rate constant for electron transfer varied inversely with the mean length of the nanotubes. The results indicate there is an advantage in using aligned carbon nanotube arrays over randomly dispersed nanotubes for achieving efficient electron transfer to bound redox active species such as in the case of bioelectronic or photovoltaic devices.     

Sensor activity with respect to alkali metals of a carbon nanotube edge-modified with amino group

The work is devoted to the theoretical study of sensor activity of nanosystems based on a carbon nanotube modified with a functional amino group, with respect to certain metal atoms and ions. The calculations were performed within the molecular cluster model using the semiempirical MNDO scheme and density functional theory DFT. The mechanism of attachment of an amino group to the open edge of zigzag single-walled carbon nanotubes possessing cylindrical symmetry was studied to design a chemically active sensor based on them. The key geometric and electron-energy characteristics of the constructed systems have been determined. The interaction of the sensors thus constructed with atoms and ions of some metals—potassium, sodium, and lithium—has been studied. The scanning of arbitrary surfaces containing selected atoms or ions has been modeled; from the interaction energies, and the activity of the single-walled carbon nanotube + amino group probe system has been determined with respect to the selected elements to be initialized. Analysis of the charge state of the system has established that the sensor action mechanism is realized, as a result of which the number of charge carriers in the resulting nanotubular system serving as a sensor probe changes, which provides the appearance of conductivity in the system. The presence of metallic atoms can be experimentally detected by the change in the potential in a probe system based on a nanotube with a functional group. The theoretical studies have proved the possibility of creating highly sensitive sensors based on the most promising nanomaterial— carbon nanotubes functionalized with active chemical groups, including the amino group NH₂.     

Near-infrared fluorescence microscopy of single-walled carbon nanotubes in phagocytic cells

The uptake of pristine single-walled carbon nanotubes into macrophage-like cells has been studied using the nanotubes' intrinsic near-infrared fluorescence. Macrophage samples that have been incubated in growth media containing suspended single-walled nanotubes show characteristic nanotube fluorescence spectra. The fluorescence intensities increase smoothly with incubation time and external nanotube concentration. Near-infrared fluorescence microscopy at wavelengths above 1100 nm provides high contrast images indicating localization of nanotubes in numerous intracellular vesicles. Nanotube uptake appears to occur through phagocytosis. Population growth of macrophage cultures is unaffected by exposure to single-walled nanotube concentrations of ca. 4 mug/mL for up to 96 h.     

Template-free directional growth of single-walled carbon nanotubes on a- and r-plane sapphire

We report high-throughput growth of highly aligned single-walled carbon nanotube arrays on a-plane and r-plane sapphire substrates. This is achieved using chemical vapor deposition with ferritin as the catalyst. The nanotubes are aligned normal to the [0001] direction for growth on the a-plane sapphire. They are typically tens of micrometers long, with a narrow diameter distribution of 1.34 +/- 0.30 nm. In contrast, no orientation was achieved for growth on the c-plane and m-plane sapphire, or when Fe films, instead of ferritin, were used as the catalyst. Such orientation control is likely related to the interaction between carbon nanotubes and the sapphire substrate, which is supported by the observation that when a second layer of nanotubes was grown, they followed the gas flow direction. These aligned nanotube arrays may enable the construction of integrable and scalable nanotube devices and systems.     

Preferential destruction of metallic single-walled carbon nanotubes by laser irradiation

Upon laser irradiation in air, metallic single-walled carbon nanotubes (SWNTs) in carbon nanotube thin film can be destroyed in preference to their semiconducting counterparts when the wavelength and power intensity of the irradiation are appropriate and the carbon nanotubes are not heavily bundled. Our method takes advantage of these two species' different rates of photolysis-assisted oxidation, creating the possibility of defining the semiconducting portions of carbon nanotube (CNT) networks using optical lithography, particularly when constructing all-CNT FETs (without metal electrodes) in the future.     

Electrical transport characteristics of surface-conductance-controlled, dielectrophoretically separated single-walled carbon nanotubes

Alternating current dielectrophoresis has attracted considerable attention as a possible candidate to separate single-walled carbon nanotubes according to electronic types. Recently, the significant effect of surface charge on the polarizability of semiconducting nanotubes was demonstrated using comparative Raman spectroscopic studies. Here we present electrical transport characteristics of surface-charge-controlled, dielectrophoretically deposited nanotube arrays. The surface charge was controlled using cationic/anionic surfactant mixtures. Complete separation between metallic and semiconducting species was achieved at the electric field frequency of 10 MHz only when the surface charge of nanotubes was neutralized, which is consistent with previous Raman investigation. A theoretical analysis, using zeta potential information as input, further supported the experimental observation.     

Ruthenium porphyrin functionalized single-walled carbon nanotube arrays--a step toward light harvesting antenna and multibit information storage

Ruthenium porphyrin functionalized single-walled carbon nanotube arrays have been prepared using coordination of the axial position of the metal ion onto 4-aminopyridine preassembled single-walled carbon nanotubes directly anchored to a silicon(100) surface (SWCNTs-Si). The formation of these ruthenium porphyrin functionalized single-walled carbon nanotube array electrodes (RuTPP-SWCNTs-Si) has been monitored using infrared spectroscopy (IR), differential pulse voltammetry (DPV), atomic force microscopy (AFM), laser desorption time-of-flight mass spectroscopy (LDI-TOF-MS), UV-vis spectroscopy, fluorescence spectroscopy, and cyclic voltammetry. Electrochemical results show two successive one-electron reversible redox waves. The surface concentration of the ruthenium porphyrin molecules is 3.44 x 10 (-8) mol cm (-2). Optical results indicate that the immobilization of ruthenium porphyrin enhances the light absorption of SWCNTs-Si surfaces in the visible light region. Moreover mixed assembly of ferrocene/porphyrin onto carbon nanotube arrays has been achieved by altering the ratio of two redox-active species in the deposition solution. These results suggest the ruthenium porphyrin modified electrodes are excellent candidates for molecular memory devices and light harvesting antennae.     

Fabrication of carbon nanotube sheets and their bilirubin adsorption capacity

Bilirubin adsorption on carbon nanotube surfaces has been studied to develop a new adsorbent in the plasma apheresis. Powder-like carbon nanotubes were first examined under various adsorption conditions such as temperatures and initial concentrations of bilirubin solutions. The adsorption capacity was measured from the residual concentrations of bilirubin in the solution after the adsorption process using a visible absorption spectroscopy. We found that multi-walled carbon nanotubes (MWCNTs) exhibit greater adsorption capacity for bilirubin molecules than that of single-walled carbon nanotubes (SWCNTs). To guarantee the safety of the adsorbents, we fabricated carbon nanotube sheets in which leakage of CNTs to the plasma is suppressed. Since SWCNTs are more suitable for robust sheets, a complex sheet consisting of SWCNTs as the scaffolds and MWCNTs as the efficient adsorbents. CNT/polyaniline complex sheets were also fabricated. Bilirubin adsorption capacity of CNTs has been found to be much larger than that of the conventional materials because of their large surface areas and large adsorption capability for polycyclic compound molecules due to their surface structure similar to graphite.     

Effect of Single-walled Carbon Nanotubes on Cellulose Phenylcarbamate Chiral Stationary Phases

Single-walled carbon nanotubes(SWNTs)have a high adsorption ability and nanoscale interactions.Cellulose trisphenylcarbamates possess high enantioseparation ability in high-performance liquid chromatography(HPLC).Single-walled carbon nanotubes mixed with cellulose trisphenylcarbamate are coated on the silica gel as chiral stationary phases and higher enantioseparation factors are obtained.After a single-walled carbon nanotube is linked to the 6-position of cellulose 2,3-bisphenylcarbamate,its enantioseparation resolution increases compared to that of the cellulose trisphenylcarbamate.It is the first time that SWNTs have been applied to enantioseparation.The results indicate that the single-walled carbon nanotubes are good promoters of chiral recognition.This method can be used to improve the enantioseparation efficiency of the polysaccharide chiral stationary phases.     

Ab initio simulations of doped single-walled carbon nanotube sensors

The interactions between oxygen and nitrogen atoms with single-walled carbon nanotubes were investigated for nanotubes with two different geometrical configurations using first-principle calculations within the framework of the density functional theory. We introduced a new type of toxic gas sensor that can detect the presence of H₂, Cl₂, CO, and NO molecules. We also demonstrated that the sensitivity of this device can be controlled by the concentration of the dopants on the surface of the nanotube. In addition, the transport properties of the doped nanotube were studied for different concentrations of oxygen or nitrogen atoms that were randomly distributed on the surface of the single-walled carbon nanotube. We observed that small amounts of dopants can modify the electronic and transport properties of the nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with either oxygen or nitrogen atoms.