Enhanced field emission properties of vertically aligned double-walled carbon nanotube arrays

Vertically aligned double-walled carbon nanotube (VA-DWCNT) arrays were synthesized by point-arc microwave plasma chemical vapor deposition on Cr/n-Si and SiO(2)/n-Si substrates. The outer tube diameters of VA-DWCNTs are in the range of 2.5-3.8?nm, and the average interlayer spacing is approximately 0.42?nm. The field emission properties of these VA-DWCNTs were studied. It was found that a VA-DWCNT array grown on a Cr/n-Si substrate had better field emission properties as compared with a VA-DWCNT array grown on a SiO(2)/n-Si substrate and randomly oriented DWCNTs, showing a turn-on field of about 0.85?V?μm(-1) at the emission current density of 0.1?μA?cm(-2) and a threshold field of 1.67?V?μm(-1) at the emission current density of 1.0?mA?cm(-2). The better field emission performance of the VA-DWCNT array was mainly attributed to the vertical alignment of DWCNTs on the Cr/n-Si substrate and the low contact resistance between CNTs and the Cr/n-Si substrate.     

Synthesis of vertically aligned carbon nanotube arrays by injection method in CVD

The well aligned multiwalled carbon nanotube arrays were synthesized by injecting the acetonitrile-ferrocene solution at regular intervals of time. The carbon nanotube arrays were deposited on quartz substrate which is placed at the centre of the CVD reactor in quartz tube. The injection method in chemical vapor deposition allows-excellent control of the catalyst to carbon ratio which facilitates the better growth of aligned carbon nanotubes. The effect of various reaction parameters such as growth temperature, catalyst concentration, gas flow rate, growth time and substrate surface on growth of carbon nanotubes have been studied. It was observed that the diameter of carbon nanotubes increases with increase in catalyst concentration and temperature of the synthesis. The SEM analysis reveals that the average growth rate of carbon nanotube film synthesis was about 1.1 microm/min when the synthesis time was one hour.     

碳纳米管阵列研究进展

在介绍CNT阵列性能的基础上,对国内外直接合成CNT阵列的方法进行了评述,重点阐述了各种方法的基本特点及CNT阵列的生长机理、结构控制和批量制备问题。进而探讨了CNT原生阵列、抽丝形成的CNT丝、以及CNT阵列分散后得到的CNTs在复合材料、力学增强、功能器件等方面的初步应用,展望了CNT阵列的发展趋势,指出低成本、大批量可控制备CNT阵列仍然是未来一段时间内国际研究热点。     

Coating alumina on catalytic iron oxide nanoparticles for synthesizing vertically aligned carbon nanotube arrays

To synthesize long and uniform vertically aligned carbon nanotube (VACNT) arrays, it is essential to use catalytic nanoparticles (NPs) with monodisperse sizes and to avoid NP agglomeration at the growth temperature. In this work, VACNT arrays were grown on chemically synthesized Fe(3)O(4) NPs of diameter 6 nm by chemical vapor deposition. Coating the NPs with a thin layer of Al(2)O(3) prior to CNT growth preserves the monodisperse sizes, resulting in uniform, thick and dense VACNT arrays. Comparison with uncoated NPs shows that the Al(2)O(3) coating effectively prevents the catalyst NPs from sintering and coalescing, resulting in improved control over VACNT growth.     

环己烷浮游催化法制备超长碳纳米管阵列

采用非芳香烃类环己烷作为碳源,通过浮游法实现了超长垂直碳纳米管(Carbon nanotube,CNT)阵列的生长。研究表明:浮游催化过程中反应温度、催化剂前体补给速度、进料速度、生长气氛等因素对CNT阵列的生长影响显著。在直径为25mm的石英反应器中,反应温度、催化剂前体二茂铁的补给速度、碳源环己烷的补给速度、反应气氛分别控制在820℃、0.24mg/min、0.12mL/min、640mL/min(H2/Ar=1∶15)的生长窗口内,实现了CNT阵列的快速协同生长。在单因素考察的基础上,通过对宏观参数的调变,可以制备出长度达5.0mm的CNT阵列。所获得的CNT取向一致,长径比大于105,纯度达到96.7%。     

Patterned arrays of vertically aligned carbon nanotube microelectrodes on carbon films prepared by thermal chemical vapor deposition

A straightforward procedure is described for preparation of arrays of microdisk electrodes comprising bundles of vertically aligned carbon nanotubes (VACNTs). The arrays are fabricated by thermal chemical vapor deposition synthesis directly on a planar carbon film support. Use of standard micro- and nanolithography procedures for patterning the bilayer catalyst spots enables arrays to be grown with controlled electrode diameters and spacings. The minimum accessible VACNT bundle diameter, and hence microelectrode diameter, is 2 microm. After insulating the arrays with SU-8 epoxy and exposing the VACNT ends by polishing or treating with O2 plasma, the microdisk electrodes exhibit attractive electrochemical properties.     

Compression behaviour of thick vertically aligned carbon nanotube blocks

Blocks of vertically aligned multiwall carbon nanotubes were prepared by thermal chemical vapor deposition starting from camphor and ferrocene precursors. The blocks, having a thickness of approximately 2 mm and composed of nanotubes with diameter ranging between 30 and 80 nm, were submitted to compression tests. The results were analyzed accordingly with a simple model consisting in a parallel array of nanotubes under compression and bending suffering microscopic instability and compaction. The model mostly fits the experimental stress-strain curves, with a small deviation attributed to dissipative phenomena, such as frictional forces and nanotube wall breakage.     

Mechanical compliance of photolithographically defined vertically aligned carbon nanotube turf

We describe the micro-mechanical properties of vertically aligned carbon nanotubes (VACNTs) fabricated using a photolithographically patterned iron catalyst prepared using sol–gel techniques. The carbon nanotubes (CNTs) were grown via chemical vapor deposition. The relative mechanical stiffness of the resultant structure was measured using nanoindentation based techniques and is shown to be related to the number of contact sites between tubes. Elastic deformation occurs during compression at large strains, however energy is dissipated during deformation, likely through tube–tube interactions. The effective elastic modulii are depth dependent, due to the compression of pre-buckled geometries. The effective elastic modulii range between 0.03 and 0.08 GPa for a low number of contact sites and 0.1 and 0.3 GPa for a high number of contact sites.     

Super-long aligned TiO2/carbon nanotube arrays

5?mm long aligned titanium oxide/carbon nanotube (TiO(2)/CNT) coaxial nanowire arrays have been prepared by electrochemically coating the constituent CNTs with a uniform layer of highly crystalline anatase TiO(2) nanoparticles. While the presence of the TiO(2) coating was confirmed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and x-ray diffraction, the resultant TiO(2)/CNT coaxial arrays were demonstrated to exhibit minimized recombination of photoinduced electron-hole pairs and fast electron transfer from the long TiO(2)/CNT arrays to external circuits. This, in conjunction with the aligned macrostructure, facilitates the fabrication of TiO(2)/CNT arrays for various device applications, ranging from photodetectors to photocatalytic systems. Thus, the millimeter long TiO(2)/CNT arrays represent a significant advance in the development of new macroscopic photoelectronic nanomaterials attractive for a variety of device applications beyond those demonstrated in this study.     

Real-time imaging of vertically aligned carbon nanotube array growth kinetics

In situ time-lapse photography and laser irradiation are applied to understand unusual coordinated growth kinetics of vertically aligned carbon nanotube arrays including pauses in growth, retraction, and local equilibration in length. A model is presented which explains the measured kinetics and determines the conditions for diffusion-limited growth. Laser irradiation of the growing nanotube arrays is first used to prove that the nanotubes grow from catalyst particles at their bases, and then increase their growth rate and terminal lengths.     

Fabrication of large arrays of high-aspect-ratio single-crystal silicon columns with isolated vertically aligned multi-walled carbon nanotube tips

This paper describes the fabrication of large arrays (10(6) units in 1?cm(2)) of 100?μm tall, single-crystal silicon columns with submicron tip cross-sections. The columns are formed using thin film deposition and growth, reactive ion etching, and deep reactive ion etching. The columns can be either slightly tapered or have pencil-like morphology with nanoscaled tip diameter (41?nm). Conformal thin film coating was used to substantially and uniformly modify the porous structure and, thus, vary by orders of magnitude the fluid permeability of the structure. Gaps between the vertical pillars were varied between 9?μm and 50?nm. Isolated 45?nm diameter, 5?μm tall plasma enhanced chemical vapour deposited multi-walled carbon nanotubes (MWNTs) were grown on the top surface of the columns using a 7?nm thick evaporated Ni film as catalyst. Field emission characterization of the resulting structure was conducted and it is in agreement with scanning electron micrographs of the MWNTs.     

One-step catalytic growth of carbon nanofiber arrays vertically aligned on carbon substrate

Vertically aligned carbon nanofiber (VA-CNF) arrays on carbon substrate have been synthesized via one-step chemical vapor deposition process on copper foil, by using acetylene as carbon resource. Three types of carbon nanostructures, viz. bare carbon films, CNFs and VA-CNFs grown on carbon substrate, could be selectively synthesized by only modulating the concentration of C₂H₂ and NH₃ in the feeding gases. It was found that NH₃ was a key factor in the growth of VA-CNF arrays, which could increase the diffusion capability of copper atoms in carbon materials, therefore promote forming larger spherical Cu NPs catalysts for the growth of VA-CNFs. Furthermore, a growth mechanism in different feeding gas compositions was proposed.     

宏观超长定向碳纳米管阵列的制备

采用二甲苯和二茂铁作为碳源和催化剂的化学气相沉积法连续制备定向碳纳米管阵列,在反应6h内获得长度为6mm的宏观定向碳纳米管阵列。当反应时间超过6h后,定向碳纳米管阵列的生长速度明显下降,并有停止生长的趋势,反应16h,定向碳纳米管阵列的厚度仅为6．7mm。本文研究了采用多次连续进给碳源和催化剂的方式制备定向碳纳米管阵列,成功的制备了8．9mm厚的定向碳纳米管阵列,阵列中的碳纳米管具有高的定向性和良好的界面结合。     

Ultrahigh density modulation of aligned single-walled carbon nanotube arrays

Controlling the densities of aligned single-walled carbon nanotube arrays (SWNTs) on ST-cut quartz is a critical step in various applications of these materials. However the growth mechanism for tuning SWNT density using the chemical vapor deposition (CVD) method is still not well understood, preventing the development of efficient ways to obtain the desired results. Here we report a general “periodic” approach that achieves ultrahigh density modulation of SWNT arrays on ST-cut quartz substrates—with densities increased by up to ∼60 times compared with conventional methods using the same catalyst densities—by varying the CH₄ gas “off” time. This approach is applicable to a wide range of initial catalyst densities, substrates, catalyst types and growth conditions. We propose a general mechanism for the catalyst size-dependent nucleation of SWNTs associated with different free carbon concentrations, which explains all the observations. Moreover, the validity of the model is supported by systematic experiments involving the variation of key parameters in the “periodic” CVD approach.      

Tritrichomonas foetus adhere to superhydrophilic vertically aligned multi-walled carbon nanotube surface

For the first time, we show that Tritrichomonas foetus can adhere on superhydrophilic vertically aligned carbon nanotubes (VACNT) films. Scanning electron microscopy shows an unusual adhesion with a higher membrane filopodium projection in all directions, directly attached to superhydrophilic VACNT tips.     

Fatigue resistance of aligned carbon nanotube arrays under cyclic compression

Structural components subject to cyclic stress can succumb to fatigue, causing them to fail at stress levels much lower than if they were under static mechanical loading. However, despite extensive research into the mechanical properties of carbon nanotube structures for more than a decade, data on the fatigue behaviour of such devices have never been reported. We show that under repeated high compressive strains, long, vertically aligned multiwalled nanotubes exhibit viscoelastic behaviour similar to that observed in soft-tissue membranes. Under compressive cyclic loading, the mechanical response of the nanotube arrays shows preconditioning, characteristic viscoelasticity-induced hysteresis, nonlinear elasticity and stress relaxation, and large deformations. Furthermore, no fatigue failure is observed at high strain amplitudes up to half a million cycles. This combination of soft-tissue-like behaviour and outstanding fatigue resistance suggests that properly engineered nanotube structures could mimic artificial tissues, and that their good electrical conductivity could lead to their use as compliant electrical contacts in a variety of applications.     

Infiltration of highly aligned carbon nanotube arrays with molten polystyrene

Highly aligned carbon nanotube/polystyrene (HACNT/PS) composites were prepared conveniently via polymer impregnation of aligned arrays, avoiding conventional solution or melt processes that involve high-shear blending. Detailed scanning electron microscopy (SEM), X-Ray diffraction (XRD) and differential scanning calorimetry (DSC) studies show that the alignment of the multi-wall nanotubes is retained after polymer impregnation. A range of key parameters were investigated, including the amount of molten polymer required for complete infiltration, the maximum mass fraction of CNTs in dense composites, and the effect of CNTs on the glass transition temperature (T_g) of the polymer. This approach may be adapted to the fabrication of large scale, highly anisotropic, thin or multilayer CNT composites using other molten polymers.     

Effective growth of vertically aligned carbon nanotube turfs on flexible Al foil

Vertically aligned multi-walled carbon nanotube turfs (VACNTs) have been synthesized onto commercially available aluminum foil without the use of a pre-deposited catalyst. We used ferrocene as a precursor powder catalyst and acetylene as a carbon source to grow VACNTs using three-temperature-zone chemical vapor deposition at 550–950 °C. The greatest average height of the VACNTs was 124.5 ± 1.7 μm, attained with heating at 750 °C for 10 min. The diameters of the VACNTs were in the range of 5.25–11.85 nm with growth at temperatures of 600–850 °C. The scalable deposition method allows large area coverage on flexible metal foils, which is well suited for integration of vertically aligned carbon nanotubes into plastic electronics.     

Growth of vertically aligned arrays of carbon nanotubes for high field emission

Vertically aligned multi-walled carbon nanotubes have been grown on Ni-coated silicon substrates, by using either direct current diode or triode plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas has been used as the carbon source while ammonia and hydrogen have been used for etching. However densely packed (∼ 10⁹ cm^− 2) CNTs were obtained when the pressure was ∼ 100 Pa. The alignment of nanotubes is a necessary, but not a sufficient condition in order to get an efficient electron emission: the growth of nanotubes should be controlled along regular arrays, in order to minimize the electrostatic interactions between them. So a three dimensional numerical simulation has been developed to calculate the local electric field in the vicinity of the tips for a finite square array of nanotubes and thus to calculate the maximum of the electron emission current density as a function of the spacing between nanotubes. Finally the triode plasma-enhanced process combined with pre-patterned catalyst films (using different lithography techniques) has been chosen in order to grow regular arrays of aligned CNTs with different pitches in the micrometer range. The comparison between the experimental and the simulation data permits to define the most efficient CNT-based electron field emitters.     

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.