Field emission from a selected multiwall carbon nanotube

The electron field emission characteristics of individual multiwalled carbon nanotubes were investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental set-up ensures a precise evaluation of the geometric parameters (multiwalled carbon nanotube length and diameter and anode-cathode separation) of the field emission system. For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour was obtained, with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of ～30?V?μm(-1) and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1?μm were evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities was extensively investigated.     

硅基底上热解法生长CNT薄膜的强流脉冲发射特性

采用酞菁铁高温热解方法在直径为5cm的硅基底上生长了定向CNT薄膜,并对其强流脉冲发射特性进行了表征.测试结果表明,在单脉冲条件下,当宏观场强为11.7V/μm时,发射脉冲电流的峰值约为109.4A;而在双脉冲模式下,当第一脉冲峰值宏观场强为8.6V/μm,第二脉冲峰值宏观场强为5.4V/μm时,第一脉冲和第二脉冲峰值...     

Effect of gas pressure on electron field emission from carbon nanotube forests

This article quantifies the effect of the operating pressure of the H2 + C2H4 gas mixture on the current density and threshold voltage of the electron emission from dense forests of multiwalled carbon nanotubes synthesized using thermal catalytic Chemical Vapor Deposition under near atmospheric pressure process conditions. The results suggest that in the pressure range of interest 400-700 Torr the field emission properties can be substantially improved by operating the process at lower gas pressures when the nanostructure aspect ratios are higher. The obtained threshold voltage approximately 1.75 V/microm and the emission current densities approximately 10 mA/cm2 offer competitive advantages compared with the results reported by other authors.     

Formation of free-standing carbon nanotube array on super-aligned carbon nanotube film and its field emission properties

Flexible and free-standing well-aligned carbon nanotube arrays have been synthesized on super-aligned carbon nanotube films. The combined structure of the carbon nanotube array and carbon nanotube film was formed during chemical vapor deposition on a quartz substrate which had previously been covered with a super-aligned carbon nanotube film. It was found that the growing carbon nanotube array could support up the super-aligned carbon nanotube film entirely, and the top of the array became densely entangled with the super-aligned carbon nanotube film. The carbon nanotube array with the super-aligned carbon nanotube film could be easily peeled off from the quartz substrate as a whole, giving a flexible and free-standing structure with good mechanical properties. The bottom of the array was also exposed after being peeled off and was used as a field emitter. The combined structure of the carbon nanotube array with the carbon nanotube film allowed adsorbent-free field emission by passing a heating current through it. Furthermore, due to the fast thermal response of the structure and the long time needed for re-adsorption of adsorbates in vacuum, it was found that pulsed heating with a 10% duty ratio was sufficient for adsorbent-free field emission. The heating power necessary to sustain the adsorbent-free state can be lowered in this way.

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Electrochemical nanoneedle biosensor based on multiwall carbon nanotube

We report the fabrication and analytical functions of a biosensor based on a nanoneedle consisting of a multiwall carbon nanotube attached to the end of an etched tungsten tip. The devised electrode is the smallest needle-type biosensor reported to date. The nanoneedles prepared in this work are 30 nm in diameter and 2-3 microm in length. Dopamine and glutamate, which are physiologically important neurotransmitters, were successfully detected using these nanoneedles. Bare nanoneedles detected dopamine in the range from 100 to 1000 microM by differential pulse voltammetry, and enzyme-modified nanoneedles were able to respond to glutamate in the 100-500 microM range by potentiostatic amperometry.     

The influence of organophilic clay on field electron emission uniformity and lifetime of screen printed carbon nanotube film

The effect of adding clay, modified by a silane coupling agent, into the carbon nanotube (CNT) paste on the field electron emission and a lifetime of screen printed CNT film was studied. The composition of organophilic clay and CNTs was characterized by a combination of X-ray diffraction, and scanning electron microscopy. The incorporation of clay improved the dispersivity, adhesiveness, and conductive networking of paste, therewith enhancing the field emission's uniformity and stability. The improvement of this gas barrier on the CNT film by clay loading prevents emission degradation.     

Light harvesting with multiwall carbon nanotube/silicon heterojunctions

We report on a significant photocurrent generation from a planar device obtained by coating a bare n doped silicon substrate with a random network of multiwall carbon nanotubes (MWCNTs). This MWCNT/n-Si hybrid device exhibits an incident photon to current efficiency reaching up to 34% at 670 nm. We also show that MWCNTs covering a quartz substrate still exhibit photocurrent, though well below than that of the MWCNTs coating the silicon substrate. These results suggest that MWCNTs are able to generate photocurrent and that the silicon substrate plays a fundamental role in our planar device. The former effect is particularly interesting because MWCNTs are generally known to mimic the electronic properties of graphite, which does not present any photocurrent generation. On the basis of theoretical calculations revealing a weak metallic character for MWCNTs, we suggest that both metallic and semiconducting nanotubes are able to generate e-h pairs upon illumination. This can be ascribed to the presence of van Hove singularities in the density of states of each single wall carbon nanotube constituting the MWCNT and to the low density of electrons at the Fermi level. Finally, we suggest that though both MWCNTs and Si substrate are involved in the photocurrent generation process, MWCNT film mainly acts as a semitransparent electrode in our silicon-based device.     

Thermal conductivities of alumina-based multiwall carbon nanotube ceramic composites

Composites incorporating various vol.% (0.0, 1.1, 6.4, and 10.4) of multiwall carbon nanotubes (MWCNTs) in alumina were consolidated by the spark plasma sintering. Their thermal transport properties were investigated over the temperature range 300–800 K as a function of nanotube contents. It was observed that the temperature-dependent effective thermal conductivity decreases with the addition of MWCNTs in alumina. This behavior was analyzed in terms of phonon mean free path, elastic modulus, average sound speed, and interface thermal resistance. Compared with 1/T behavior for pristine alumina, a subtle decrease in temperature dependence of the thermal conductivity of the composites with the addition of MWCNTs is observed, indicating the presence of extra phonon scattering mechanism beyond the intrinsic phonon–phonon scattering. Simulation of experimental results with theoretical model shows that the large interfacial thermal barrier between MWCNTs and alumina plays a dominant role in controlling thermal transport properties of the composites. In addition to dominant interface thermal resistance other secondary factors such as nanotube agglomeration, processing defects, porosity also contribute for low thermal conductivity at the higher volume fraction of MWCNTs in the composite.     

Preparation and investigation of multiwall carbon nanotube—polymer composites

In our research we focus on thermoplastic composites of multiwall carbon nanotubes. Different composition of carbon nanotubes and polymers were produced by a special mixing unit called Infinitely Variable Dynamic Shear Mixer (IDMX) using ABS and polycarbonate polymers as matrix materials. Polycarbonate/multiwall carbon nanotube masterbatch was used in the preparation of different compositions. Concentration series were manufactured and investigated. The nanotube composites were granulated and test pieces were injection moulded. The prepared materials were characterised by scanning electron microscopy. Mechanical, electrical properties of the materials were also determined. Correlation was found between the yield stress and the nanotube contents. Impact strength of the composites decreased with the nanotube content, showing more rigid structure than the original pure matrix material. Glass transition of the composites was determined by DSC method. It was found that the characteristic temperatures change by the ratio of the pure materials. Slight change also was found as the carbon nanotube content changes.     

Polypyrrole/multiwall carbon nanotube nanocomposites electropolymerized on copper substrate

Polypyrrole/multiwall carbon nanotube (PPy/MWCNT) nanocomposites were successfully synthesized by electropolymerization of MWCNT-dispersed pyrrole solution on the surface of copper electrodes. The obtained nanocomposites were characterized with scanning electron microscopy (SEM), linear sweep voltammetry (LSV) and thermal gravimetric analysis (TGA). Polypyrrole structures which embraced the MWCNTs led to the formation of nanocomposite striated parallel walls. MWCNTs acted as appropriate substrates for electrodeposition of polypyrrole particulate structures and high yield synthesis of PPy was observed on them. Smooth PPy/MWCNT nanocomposite films were obtained on Cu electrodes by decreasing the potential scan rate. Thermogravimetric analysis showed that MWCNTs increased the thermal stability of polypyrrole.     

Secondary electron emission from nanocomposite carbon films

The secondary electron emission (SEE) properties of sulfur-incorporated nanocomposite carbon (n-C) films were studied. Maximum SEE yield (δ _max) values obtained ranged from 3.27 to 6.98, which are between those for graphite (δ _max ~ 1) and high purity diamond films in their as-grown condition (δ _max ~ 9), and are consistent with the composite nature of the films. It was found that δ _max values of n-C films are mostly determined by the atomic oxygen concentration on the surface of the films, which appears to control the probability of escape of the secondary electrons from the surface of the films, as inferred by employing Ascarelli’s model for SEE (J Appl Phys 89:689, 2001). Also, mean escape depth values for the secondary electrons were obtained using this model, and their significance as bulk parameters for the films is discussed.     

Phonon-assisted electron emission from individual carbon nanotubes

A question of how electrons can escape from one-atom-thick surfaces has seldom been studied and is still not properly answered. Herein, lateral electron emission from a one-atom-thick surface is thoroughly studied for the first time. We study electron emission from side surface of individual electrically biased carbon nanotubes (CNTs) both experimentally and theoretically and discover a new electron emission mechanism named phonon-assisted electron emission. A kinetic model based on coupled Boltzmann equations of electrons and optical phonons is proposed and well describes experimentally measured lateral electron emission from CNTs. It is shown that the electrons moving along a biased CNT can overflow from the one-atom-thick surface due to the absorption of hot forward-scattering optical phonons. A low working voltage, high emission density, and side emission character make phonon-assisted electron emission primarily promising in electron source applications.     

Calorimetric study on mechanically milled aluminum and multiwall carbon nanotube composites

Pure aluminum reinforced with carbon nanotube (CNT) composites have been prepared by high energy attritor milling up to 48 hrs. Differential Scanning Calorimetry (DSC) has been carried out to investigate apparent activation energy and order of the reaction between carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal conditions. The DSC results clearly reveal that an exothermic reaction occurs before the melting of aluminum. The effect of milling time on the initiation of this exothermic reaction has been studied. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy was found to get doubled after milling for 36 hrs compared to 24 hrs milled samples. The mechanism of the reaction kinetics which depends on reaction order is instantaneous nucleation and one dimensional growth for both samples. Formation of Al₄C₃ was confirmed by X-ray diffraction (XRD) of as-milled powders and after performing DSC of the milled powders.     

Enhanced mechanical properties of polyacrylonitrile/multiwall carbon nanotube composite fibers

The use of multiwall carbon nanotubes (MWNTs) as a reinforcing phase in a polyacrylonitrile (PAN) fiber matrix was investigated with the goal of producing a PAN-derived carbon/MWNT composite fiber with enhanced physical properties. MWNTs were dispersed in a PAN/DMAc (dimethylacetamide) solution and spun into composite fibers containing up to 5 wt.% MWNTs, with the use of a lab-scale dry-jet wet spinline. The spinning process resulted in alignment of the MWNTs parallel with the fiber axis. Three types of chemical vapor deposition (CVD)-derived, high-purity MWNTs were used: as produced, graphitized (heat treated to 2800 degrees C), and NaCN-treated (chemically treated to attach CN groups to the nanotube surface). Tensile tests were performed to measure yield stress/strain, initial modulus, break stress/strain, and energy to yield and energy to break. Significant mechanical property increases were recorded for the composite fibers compared with the control samples with no MWNT reinforcement: break strength +31%, initial modulus +36%, yield strength +46%, energy to yield +80%, and energy to break +83%.     

Polyaniline/multiwall carbon nanotube nanocomposite for detecting aromatic hydrocarbon vapors

In this paper, polyaniline/multiwall carbon nanotube (PANI/MWCNT) composites were fabricated through an in situ polymerization method and were applied for the detection of aromatic hydrocarbon vapors. The composites were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, and Raman spectroscopy. The SEM results showed that the PANI was uniformly coated on the MWCNT, and the coating thickness was dependent on the mass ratio of aniline monomer to MWCNT. The response to aromatic hydrocarbon vapors was investigated in several hundreds ppm ranges. The sensor showed an increase in conductivity, and the maximum response measured at 1000 ppm was several tens of percent.     

Enhanced field emission from aligned multistage carbon nanotube emitter arrays

In this work we report on the synthesis and field emission properties of carbon nanotube multistage emitter arrays grown on porous silicon by catalytic thermal chemical vapor deposition. The vertically oriented multistage array structures consisted of SWNTs and thin MWNTs grown on MWNTs, confirmed by TEM and Raman analysis. Higher field emission current ～32 times and low threshold field ～1.5 times were obtained for these structures in comparison to only MWNT arrays. The enhanced field emission results for these multistage emitters are a consequence of higher field concentration, which is ～3 times more than MWNTs.     

Field emission characteristics of an individual carbon nanotube inside a field emission-scanning electron microscope

Field emission (FE) properties of individual single-walled carbon nanotubes (SWCNT) were investigated inside a field emission-scanning electron microscopy. The individual SWCNT turned on a voltage of 23 V defined to produce a current of 10 pA, and was saturated at around 43 V and 880 nA. The FE characteristic of individual SWCNT also followed a conventional Fowler-Nordheim (F-N) theory in which a single linear slope in the F-N plots is measured below their limit of current level corresponding to the saturation regime of emission current. Energy-dispersive X-ray spectroscopy analysis showed that carbon atoms were deposited on the anode surface by the local heating of SWCNT tip during the FE processes and indicated about atomic 83% of carbon atoms. The carbon atoms were newly found to be evaporated and deposited on the anode surface during the FE process such that it was assumed that the degradation of FE was caused by evaporation and deposition of carbon atoms during the FE process.     

双掺杂碳纳米管电子场发射性能的密度泛函研究

采用第一性原理的密度泛函理论(DFT)研究了(5,5)碳纳米管(CNT)顶端硼(B)、氮(N)、硅(Si)等元素双掺杂体系的电子场发射性能.结果表明,在外电场下,各种双掺杂CNT帽端态密度(DOS)向价带移动.电子轨道分布变化显著,电荷分布明显局城化.根据电子态密度、差分电荷密度、最高分子占据轨道(HOMO)/最低分子非占据轨道(LUMO)分布等计算结果可预期Si双掺杂后更有利于场致电子发射.