Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB₃ (MmNi₃, MmFe_1.5Ni_1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm² at 0.9 V/μm.     

Transverse elasticity of multi-walled carbon nanotubes

A discrete shell model is proposed to describe the radial deformation of carbon nanotubes under a hydrostatic pressure and the radial Young's modulus of (single- or multi-walled) nanotubes is obtained. It is found that the radial modulus decreases with increasing tube diameter while increases with increasing number of layers. The computational results agree well with the previous results of SWNTs and indicate that the radial modulus of carbon nanotubes is independent of the Poisson's ratio.     

Influence of cesium atoms on the field electron emission from multi-walled carbon nanotubes

It has been shown that the deposition of cesium atoms on multi-wall carbon nanotubes abruptly increases the current of the field electron emission, decreases the threshold electric field by a factor of three (to 0.8 V/m), and decreases the work function to 2.1–2.3 eV. It has been found that the flowing of the large emission current I ≥ 2 × 10^?6 A leads to a change in the current-voltage characteristics and a decrease in the emission current. This effect has been explained by escape of cesium atoms from the tips of most nanotubes into the nanotube depth due to desorption or intercalation. At the same time, the low work function is retained for some nanotubes, probably, due to the stronger bonding of Cs atoms with these nanotubes.     

Nitrogen-doped multi-walled carbon nanotubes for paracetamol sensing

A novel sensor consisting of nitrogen-doped multi-walled carbon nanotubes was fabricated by means of chemical vapor deposition technique with decomposition of acetonitrile onto oxidized silicon wafer using ferrocene as catalyst. The electrochemical response of carbon nanotubes-based sensor towards oxidation of paracetamol to N-acetyl-p-quinone imine was investigated in phosphate buffer solution (pH 7.0) by means of standard electrochemical techniques. A quasi-reversible response for oxidation of paracetamol was identified on carbon nanotubes-based sensor with detection limit and sensitivity of 0.485 μM and 0.8406 A M^?1 cm^?2, respectively. It was found that the nitrogen doping in carbon nanotubes enhances the sensor's detection ability. Namely, electrochemical studies performed on film consisting of pristine carbon nanotubes reveal as well quasi-reversible response towards oxidation of paracetamol but nevertheless poorer detection ability and sensitivity (0.950 μM; 0.601 A M^?1 cm^?2). The findings strongly suggest the application of nitrogen-doped carbon nanotubes in biosensing.     

XPS study of fluorinated carbon multi-walled nanotubes

Arc-produced carbon multi-walled nanotubes (MWNTs) were fluorinated at 420 °C in a flow of diluted F₂ gas containing small admixture of HF gas. Fluorinated materials (F-MWNTs) with 10–55 wt.% fluorine content were studied by XPS. It was shown that fluorination begins at the external layers of nanotubes and the reaction front propagates inside the multi-layer particles in concert with structural deterioration of graphene layers. The C₂F stoichiometry still allows MWNT wall integrity, similar to known for SWNTs. The fluorine contents in the product can noticeably exceed this higher fluorine limit for tube stability. The position of the F 1s line at 688.2 eV does not depend on the fluorine concentration. Nearly covalent C–F bonds dominate the F-MWNT samples, with a small quantity (2–9%) of ionic bonds also present. Fluorinated carbon tends to spatially separate from non-fluorinated carbon.     

Guided waves characteristics of multi-walled carbon nanotubes

The theoretical analysis of propagation of guided waves in the multi-walled carbon nanotubes is presented within the framework of the classical electrodynamics. Electronic excitations of each wall of the system are modeled as an infinitesimally thin cylindrical layer of the π-electrons, whose dynamics are described by means of the fluid theory. General expressions of dispersion relations are obtained for the electromagnetic wave with the transverse magnetic and transverse electric modes, respectively, by solving Maxwell and fluid equations with appropriate boundary conditions.     

Properties of hydrothermally processed multi-walled titania nanotubes

We report here, the production principle and optical characteristics of TiO₂ nanotubes (TNTs) grown via a hydrothermal route. As compared to TiO₂ nanoparticles (TNPs), X-ray diffraction study of TNTs exhibited weak diffraction signals along with a characteristic peak at 2θ=9.2°. The morphological study of TNTs was carried out by high resolution transmission electron microscopy (HRTEM) which revealed that each of the TNTs is made of 5–7 walls having an average wall-spacing of 0.36 nm. From the UV–vis optical absorption spectroscopy studies, the optical band gap was estimated to be 3.16 eV, for TNTs and 3.22 eV, for TNPs. The Fourier transform infrared spectroscopy study of TNTs has indicated the presence of Ti–O–Ti stretching vibrations in the range 400–800 cm⁻¹. The electron–phonon coupling parameter was found to be smaller for TNTs (S=0.7) than TNPs (S=1.3). The production of submicron sized long multiwall nanotubes would find scope in nanofluidic and other hybrid devices.     

多壁碳纳米管光限幅特性的研究

用化学气相沉积法制备了多壁碳纳米管,并将其溶解在甲苯溶液中.用波长为1064nm的皮秒脉冲激光测量该样品的透过率,发现了非常明显的光限幅特性.当入射光强较小时,透射光强度随入射光强度的增大而增大,输出与输入为线性关系;随着入射光强的增大,透射光强增长的速度明显变慢,并逐渐趋于饱和.当入射光强度较小时,样品的透过率接近100%;而当入射光强为8GW/cm²时,非线性透过率达到30%.根据三光子吸收理论计算,理论拟合与实验结果非常符合,说明多壁碳纳米管的三光子吸收产生了光限幅效应.实验测 关键词： 多壁碳纳米管 光限幅 三光子吸收     

Optimization of field emission properties from multi-walled carbon nanotubes using ceramic fillers

The field emission properties of multi-walled carbon nanotubes were examined using a screen-printed thick film with a diode-type configuration in a vacuum. The effects of various concentrations of two different ceramic fillers, indium tin oxide (ITO) powder and a glass frit, on the emission current density and turn-on field were evaluated. The emission properties of both pastes were dependent on the amount of filler. Considerably enhanced emission properties were obtained with the paste containing 5–10 wt.% of either ITO or the glass frit compared with those without a filler. The paste containing the ceramic filler showed enhanced emission properties compared with that containing the 5 wt.% Ag conventionally used, which confirmed the importance of the filler. The paste containing 10 wt.% ITO represented an emission current density of 176.4 μA/cm² at 5 V/μA, a turn-on field of 1.87 V/μA for an emission current density of 1 μA/cm² and a field enhancement factor of 7580. The paste formulation was also found to be suitable for fine patterning using UV-lithography techniques. A long-term stability test for 110 h of a paste containing 10 wt.% ITO revealed a half-life of approximately 30000 h, which is appropriate for commercial applications.     

Field emission from multi-walled carbon nanotubes and its application to electron tubes

Received: 5 December 1997     

Covalent functionalization of multi-walled carbon nanotubes by lipase

Lipase from Candida rugosa was covalently anchored onto acid-treated multi-walled carbon nanotubes (MWNTs) through a self-catalytic mechanism. A variety of characterization techniques including FTIR, Raman spectroscopy, and XPS were employed to demonstrate the formation of the ester linkage between lipase and MWNTs. The MWNTs-lipase biocomposites showed significantly increased solubility in some common-used organic solvents, such as THF, DMF and chloroform. This study may offer a novel and facile route for covalent modification of carbon nanotubes, and expand the potential utilization of both lipases and MWNTs in the fields of biocatalyst and biosensor.     

Field electron emission images of multi-walled carbon nanotubes

Field electron emission microscope images from multi-walled carbon nanotubes can typically be characterized by the presence of five pentagons surrounding a sixth central pentagon. The observations of bright line centered interference patterns between adjacent pentagons in the field electron emission microscope images of multi-walled carbon nanotubes have been reported in the literature. We have observed a shift from bright to dark line centered interference patterns and associated this with the presence of surface adsorption. In order to identify the origin of the contaminant, multi-walled carbon nanotubes were dosed with H₂, H₂O, CO and O₂ and then imaged in the field electron emission microscope. Only the samples exposed to O₂ showed a shift from a bright line centered pattern between adjacent pentagons of a clean surface to a dark line centered pattern when one pentagon was contaminated or a bright line centered pattern when both adjacent pentagons become contaminated. The results of the experimental studies and the modeling of the changes in the field emission pattern as phase shifts in the wave function of the tunneling electrons due to modifications in the surface work function are presented.     

Low voltage STEM imaging of multi-walled carbon nanotubes

We present high magnification STEM images of multi-walled carbon nanotubes recorded with a 5 keV electron beam using a Helios Dual Beam microscope and a dedicated multi-segment transmission (STEM) detector. Images of carbon nanotubes recorded with bright-field (BF), annular dark-field (ADF) and high angle annular dark-field (HAADF) signals all show high contrast features, with internal structures 1-2 nm in width clearly revealed in the STEM images. Thicker regions of the nanotubes appear to show an unusual contrast reversal when comparing ADF and HAADF images. An understanding of the image contrast, and its dependence on thickness, is obtained by computing simulations of the ADF and HAADF images using Monte-Carlo software taking into account electron scattering in the nanotube.     

Dispersion of multi-walled carbon nanotubes in biocompatible dispersants

Owing to their phenomenal electrical and mechanical properties, carbon nanotubes (CNT) have been an area of intense research since their discovery in 1991. Different applications for these nanoparticles have been proposed, among others, in electronics and optics but also in the medical field. In parallel, emerging studies have suggested potential toxic effects of CNT while others did not, generating some conflicting outcomes. These discrepancies could be, in part, due to different suspension approaches used and to the agglomeration state of CNT in solution. In this study, we described a standardized protocol to obtain stable CNT suspensions, using two biocompatible dispersants (Pluronic F108 and hydroxypropylcellulose) and to estimate the concentration of CNT in solution. CNT appear to be greatly individualized in these two dispersants with no detection of remaining bundles or agglomerates after sonication and centrifugation. Moreover, CNT remained perfectly dispersed when added to culture medium used for in vitro cell experiments. We also showed that Pluronic F108 is a better dispersant than hydroxypropylcellulose. In conclusion, we have developed a standardized protocol using biocompatible surfactants to obtain reproducible and stable multi-walled carbon nanotubes suspensions which can be used for in vitro or in vivo toxicological studies.     

Preparation and characterization of amphiphilic multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs, average diameter 8 nm) functionalized by N-vinyl pyrrolidone (NVP) were synthesized by radical polymerization and characterized by Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis and transmission electron microscopy. These NVP–MWCNTs exhibit remarkable solubility in water, ethanol and dimethyl formamide. The polyvinyl pyrrolidone can be attached onto the surface of the MWCNTs and the degree of functionalization is affected by NVP content. The functionalization causes possible grafting reaction and solid physical coating between MWCNTs and PVP.     

Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes

We demonstrate that the hydrogen storage capacity of multi-walled carbon nanotubes can be enhanced by polyvinylpyrrolidone functionalization. The polyvinylpyrrolidone acts as a stabilizing agent for Pd-nanoparticles, reduces their size and facilitates their uniform and enhanced loading onto multi-walled carbon nanotubes. According to sorption studies, the polyvinylpyrrolidone capping and consequent nanostructural modification enables 2.3 times more hydrogen adsorption than mere Pd-functionalization of multi-walled carbon nanotubes. Corresponding morphological changes before and after polyvinylpyrrolidone capping, characterized using Raman Spectroscopy, X-ray diffraction, TEM and thermal analysis techniques, are also presented. The results contribute towards increasing the efficiency of hydrogen based sustainable energy sources.     

Chemical vapor deposition of multi-walled carbon nanotubes from nickel/yttria-stabilized zirconia catalysts

Multi-walled carbon nanotubes (MWNT) were produced by chemical vapor deposition using yttria-stabilized zirconia/nickel (YSZ/Ni) catalysts. The catalysts were obtained by a liquid mixture technique that resulted in fine dispersed nanoparticles of NiO supported in the YSZ matrix. High quality MWNT having smooth walls, few defects, and low amounts of by-products such as amorphous carbon were obtained, even from catalysts with large Ni concentrations (>50 wt. %). By adjusting the experimental parameters, such as flux of the carbon precursor (ethylene) and Ni concentration, both the MWNT morphology and the process yield could be controlled. The resulting YSZ/Ni/MWNT composites can be interesting due to their mixed ionic-electronic transport properties, which could be useful in electrochemical applications. PACS 61.46.Fg; 81.15.Gh; 82.45.Jn     

Inter-collisional cutting of multi-walled carbon nanotubes by high-speed agitation

High-speed agitation by a mixing blade has efficiently achieved the cutting of a large diameter (100-150 nm) of multi-walled carbon nanotubes. The cutting process is caused by an inter-collision of the nanotubes with high transfer energy. The collision-induced cutting allows for the shortening of the nanotubes without serious damage of the original graphitic layers due to the cutting effect being limited to the collision points. Furthermore, the operation under ambient atmosphere introduces oxygen-containing functional groups to the cut nanotubes. The estimated length distribution has indicated that high-speed agitation achieves a large cutting effect during a short duration of several minutes.     

Effect of different microwave-based treatments on multi-walled carbon nanotubes

Three new methods for the functionalization of multi-walled carbon nanotubes (MWCNTs) are reported using microwave (MW) energy and water as a mild chemical agent. In the first method we reported the effect of MW irradiation on a dispersion of MWCNTs in deionized water, in the second method we studied the exposing of MWCNT to microwave irradiation in the presence of steam, and in the third method we used microwave oven-generated plasma for the functionalization of MWCNTs. We also performed thermal oxidation and acid treatment as two conventional methods for oxidative functionalization of carbon nanotubes, to compare their effect with our results. Transmission electron microscopy (TEM) and Raman spectroscopy results showed that although these microwave methods introduced some defects to the carbon nanotubes, the damage was less severe than conventional treatments. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the functionalization of carbon nanotubes by these methods favored hydroxyl groups, which are useful when further functionalization is required.