Wafer scale synthesis of dense aligned arrays of single-walled carbon nanotubes

Here we present an easy one-step approach to pattern uniform catalyst lines for the growth of dense, aligned parallel arrays of single-walled carbon nanotubes (SWNTs) on quartz wafers by using photolithography or polydimethylsiloxane (PDMS) stamp microcontact printing (μCP). By directly doping an FeCl₃/methanol solution into Shipley 1827 photoresist or polyvinylpyrrolidone (PVP), various catalyst lines can be well-patterned on a wafer scale. In addition, during the chemical vapor deposition (CVD) growth of SWNTs the polymer layers play a very important role in the formation of mono-dispersed nanoparticles. This universal and efficient method for the patterning growth of SWNTs arrays on a surface is compatible with the microelectronics industry, thus enabling of the fabrication highly integrated circuits of SWNTs.     

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.     

High-performance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have many exceptional electronic properties. Realizing the full potential of SWNTs in realistic electronic systems requires a scalable approach to device and circuit integration. We report the use of dense, perfectly aligned arrays of long, perfectly linear SWNTs as an effective thin-film semiconductor suitable for integration into transistors and other classes of electronic devices. The large number of SWNTs enable excellent device-level performance characteristics and good device-to-device uniformity, even with SWNTs that are electronically heterogeneous. Measurements on p- and n-channel transistors that involve as many as approximately 2,100 SWNTs reveal device-level mobilities and scaled transconductances approaching approximately 1,000 cm(2) V(-1) s(-1) and approximately 3,000 S m(-1), respectively, and with current outputs of up to approximately 1 A in devices that use interdigitated electrodes. PMOS and CMOS logic gates and mechanically flexible transistors on plastic provide examples of devices that can be formed with this approach. Collectively, these results may represent a route to large-scale integrated nanotube electronics.     

Random networks and aligned arrays of single-walled carbon nanotubes for electronic device applications

Singled-walled carbon nanotubes (SWNTs), in the form of ultrathin films of random networks, aligned arrays, or anything in between, provide an unusual type of electronic material that can be integrated into circuits in a conventional, scalable fashion. The electrical, mechanical, and optical properties of such films can, in certain cases, approach the remarkable characteristics of the individual SWNTs, thereby making them attractive for applications in electronics, sensors, and other systems. This review discusses the synthesis and assembly of SWNTs into thin film architectures of various types and provides examples of their use in digital electronic circuits with levels of integration approaching 100 transistors and in analog radio frequency (RF) systems with operating frequencies up to several gigahertz, including transistor radios in which SWNT transistors provide all of the active functionality. The results represent important steps in the development of an SWNT-based electronics technology that could find utility in areas such as flexible electronics, RF analog devices and others that might complement the capabilities of established systems. This article is published with open access at Springerlink.com     

Biocompatibility of immobilized aligned carbon nanotubes

In vivo host responses to an electrode-like array of aligned carbon nanotubes (ACNTs) embedded within a biopolymer sheet are reported. This biocompatibility study assesses the suitability of immobilized carbon nanotubes for bionic devices. Inflammatory responses and foreign-body histiocytic reactions are not substantially elevated when compared to negative controls following 12 weeks implantation. A fibrous capsule isolates the implanted ACNTs from the surrounding muscle tissue. Filamentous nanotube fragments are engulfed by macrophages, and globular debris is incorporated into the fibrous capsule with no further reaction. Scattered leukocytes are observed, adherent to the ACNT surface. These data indicate that there is a minimal local foreign-body response to immobilized ACNTs, that detached fragments are phagocytosed into an inert material, and that ACNTs do not attract high levels of surface fouling. Collectively, these results suggest that immobilized nanotube structures should be considered for further investigation as bionic components.     

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。     

An attempt to prepare carbon nanotubes by carbonizing polyphosphazene nanotubes with high carbon content

Polyphosphazene nanotubes with about 20 nm in inner diameter and 100-200 nm in outer diameter were fabricated easily and then carbonized at 800 °C in a nitrogen atmosphere. Scanning electron microscopy and transmission electron microscope results showed that the bulk morphology of polyphosphazene nanotubes was retained after carbonization. The carbon content of the carbonized samples reached 93.28%. X-ray diffraction and Raman spectrum showed that the carbonized samples had low graphitization state. The present method can be used for a mass production of carbon nanotubes.     

Theoretical and experimental studies of Schottky diodes that use aligned arrays of single-walled carbon nanotubes

We present theoretical and experimental studies of Schottky diodes that use aligned arrays of single-walled carbon nanotubes. A simple physical model, taking into account the basic physics of current rectification, can adequately describe the single-tube and array devices. We show that for as-grown array diodes, the rectification ratio, defined by the maximum-to-minimum-current-ratio, is low due to the presence of metallic-single-walled nanotube (SWNT) shunts. These tubes can be eliminated in a single voltage sweep resulting in a high rectification array device. Further analysis also shows that the channel resistance, and not the intrinsic nanotube diode properties, limits the rectification in devices with channel length up to 10 μm.     

p-n homo-junction arrays of aligned single walled carbon nanotubes fabricated by selective patterning of polyethyleneimine film

We describe the fabrication and electrical performance of p-n homo-junction diode arrays of horizontally aligned single walled carbon nanotubes (SWCNTs). Horizontally aligned SWCNTs grown on stable temperature-cut quartz with a density of ～ 6 SWCNTs μm(-1) were transferred onto a SiO(2)/Si substrate. After the electrical breakdown, aligned SWCNT field effect transistors (FETs) showed unipolar p-type characteristics with a large current on/off ratio of 10(6) at 1 V and a hole mobility per tube of 1500 cm(2) V(-1) s(-1). Spin-coating of polyethyleneimine (PEI) onto p-type SWCNT FETs showed the n-type transfer characteristics. Patterning of spin-coated PEI film enabled the fabrication of p-n homo-junction arrays of aligned SWCNTs in an easy way, where the rectifying behavior was observed with a rectification ratio of ～ 10(4) at ± 2 V. A comparative study with a p-n homo-junction of random networks of SWCNTs confirmed the advantage of aligned SWCNTs for applications in high performance electronic devices.     

A rapid and efficient method to prepare aligned SnO2 nanorod arrays for field-emission application

Aligned tin dioxide (SnO₂) nanorods have been synthesized by high-frequency inductive heating. Nanorods were grown on silicon substrates vertically in less than 3 min, using SnO₂ and graphite as the source powder. Scanning electron microscopy and transmission electron microscopy showed nanorod with diameters from 25 to 50 nm. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.6 V/μm. This type of SnO₂ nanorods can be applied as field emitters in displays as well as vacuum electric devices.     

Controlled two-dimensional pattern of spontaneously aligned carbon nanotubes

We report a simple solution process to form controlled patterns of aligned single-walled carbon nanotubes on solid substrates. The essential step of the process is to deposit a dilute solution of DNA-wrapped carbon nanotubes (DNA-CNTs) on a SiO(2) surface covered with a thin hydrophobic layer. This leads to deposition of fully aligned CNTs. The alignment pattern can be controlled by metal electrodes in the deposition region and can be quantitatively modeled by the behavior of a quasi-two-dimensional DNA-CNT nematic phase near the solution/SiO(2) interface. These results point to the possibility of rational design and economical fabrication of CNT alignment patterns on solid substrates.     

Highly oriented carbon nanotube papers made of aligned carbon nanotubes

Paper-like carbon nanotube (CNT) materials have many important applications such as in catalysts, in filtration, actuators, capacitor or battery electrodes, and so on. Up to now, the most popular way of preparing buckypapers has involved the procedures of dispersion and filtration of a suspension of CNTs. In this work, we present a simple and effective macroscopic manipulation of aligned CNT arrays called 'domino pushing' in the preparation of the aligned thick buckypapers with large areas. This simple method can efficiently ensure that most of the CNTs are well aligned tightly in the buckypaper. The initial measurements indicate that these buckypapers have better performance on thermal and electrical conductance. These buckypapers with controllable structure also have many potential applications, including supercapacitor electrodes.     

High-rate low-temperature growth of vertically aligned carbon nanotubes

We report the low-temperature growth of vertically aligned carbon nanotubes (CNTs) at high growth rates by a photo-thermal chemical vapour deposition (PTCVD) technique using a Ti/Fe bilayer film as the catalyst. The bulk growth temperature of the substrate is as low as 370?°C and the growth rate is up to 1.3 μm min(-1), at least eight times faster than the values reported by traditional thermal CVD methods. Transmission electron microscopy observations reveal that as-grown CNTs are uniformly made of highly crystalline 5-6 graphene shells with an approximately 10 nm outer diameter and a 5-6 nm inner diameter. The low-temperature rapid growth of CNTs is strongly related to the unique top-down heating mode of PTCVD and the use of a Ti/Fe bimetallic solid solution catalyst. The present study will advance the development of CNTs as interconnects in nanoelectronics, through a CMOS-compatible low-temperature deposition method suitable for back-end-of-line processes.     

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.     

Coherent phonon anisotropy in aligned single-walled carbon nanotubes

By time-resolved reflectivity measurements with sub-10 fs laser pulses at 395 nm, the coherent phonons of aligned bundles of single-walled carbon nanotubes are observed for various polarization directions of the pump and probe pulses. In the isotropic reflectivity measurement, we observe the radial breathing modes, G, and even D modes, while in the anisotropic reflectivity mode, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity is explained by the superposition of G band phonons with different symmetries.     

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。     

Guided growth of large-scale, horizontally aligned arrays of single-walled carbon nanotubes and their use in thin-film transistors

A convenient process for generating large-scale, horizontally aligned arrays of pristine, single-walled carbon nanotubes (SWNTs) is described. The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on miscut single-crystal quartz substrates. Studies of the growth reveal important relationships between the density and alignment of the tubes, the CVD conditions, and the morphology of the quartz. Electrodes and dielectrics patterned on top of these arrays yield thin-film transistors that use the SWNTs as effective thin-film semiconductors. The ability to build high-performance devices of this type suggests significant promise for large-scale aligned arrays of SWNTs in electronics, sensors, and other applications.     

Re-grown aligned carbon nanotubes with improved field emission

In this work, a simple technique to improve the field emission property of multi-walled carbon nanotubes is presented. Re-grown multi-walled carbon nanotubes are grown on the same substrates after the as-grown multi-walled carbon nanotubes are transferred to other substrates using polydimethylsiloxane as intermediation. For the duration of the synthesis of the re-grown multi-walled carbon nanotubes, similar synthesis parameters used in growing the as-grown multi-walled carbon nanotubes are utilized. As a form of possible application, field emission studies show -2.6 times improvement in field enhancement factor and more uniform emission for the re-grown multi-walled carbon nanotubes. In addition, the turn-on field is reduced from 2.85 V/microm to 1.40 V/microm. Such significant improvements are attributed to new emission sites comprising of sharp carbonaceous impurities encompassing both tip and upper portion of the multi-walled carbon nanotubes. As such, this technique presents a viable route for the production of multi-walled carbon nanotubes with better field emission quality.     

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.      

A novel approach to fabricate high volume fraction nanocomposites with long aligned carbon nanotubes

Conventional micro-fiber-reinforced composites provide insight into critical structural features needed for obtaining maximum composite strength and stiffness: the reinforcements should be long, well aligned in a unidirectional orientation, and should have a high reinforcement volume fraction. It has long been a challenge for researchers to process CNT composites with such structural features. Here we report a method to quickly produce macroscopic CNT composites with a high volume fraction of millimeter long, well aligned CNTs. Specifically, we use the novel method, shear pressing, to process tall, vertically aligned CNT arrays into dense aligned CNT preforms, which are subsequently processed into composites. Alignment was confirmed through SEM analysis while a CNT volume fraction in the composites was calculated to be 27%, based on thermogravimetric analysis data. Tensile testing of the preforms and composites showed promising mechanical properties with tensile strengths reaching 400 MPa.