SWNT Arrays Research Articles

Pave the way to the batch production of SWNT arrays for carbon-based electronic devices

Pave the way to the batch production of SWNT arrays for carbon-based electronic devices

Common salts directed the growth of metal-free horizontal SWNT arrays.

Acquiring metal-free horizontal single-walled carbon nanotube (SWNT) arrays is of paramount importance for the development of stable nanodevices. However, the majority of SWNTs are prepared with transition metal-based catalysts, which will inevitably leave metallic residuals and deteriorate the device performance. Here, green and low-cost NaCl is developed as a metal-free catalyst. By employing a strategy of rapid nucleation at a higher temperature followed by steady growth at a lower temperature, the production of a well-defined NaCl catalyst capable of growing metal-free horizontal SWNT arrays with an average density of ∼100 tubes per 100 μm is realized. Besides, we prove that the as-grown metal-free SWNT arrays have a unique advantage in preparing stable devices for eliminating the potential risk of local mass catalyst residuals. Hence, the current study can offer a feasible solution to promote practical applications of SWNT-based next-generation nanodevices.

Photochlorination to prepare semiconducting single-walled carbon nanotube and its intramolecular junction

As the key to manufacturing large scale integrated circuits, horizontally aligned semiconducting single-walled carbon nanotube (s-SWNT) arrays play a key role in the next-generation electronic device. Though various techniques have been developed, obtaining pure s-SWNT horizontal arrays is still the greatest challenge. Here, we propose an effective and facile method to produce s-SWNT arrays from a diverse perspective by implementing photochemical chlorination on the as-grown aligned SWNT arrays. The electrical measurements demonstrate a high fraction of s-SWNTs (>98.1%) after photochlorination, and its exceptional performance as a field-effect transistor with an on-off ratio up to 3730. Meanwhile, through the well-controlled mask-assisted photochlorination, a type of metallic-semiconducting SWNT intramolecular junction with rectifying diode and non-linear transport characteristics has also been successfully obtain. Our work provides a new method for the preparation of high-purity s-SWNT arrays and SWNT intramolecular junction, which has great significance for the carbon-based nano-electronic devices in applications.

Laser-induced phenylation reaction to prepare semiconducting single-walled carbon nanotube arrays.

Single-walled carbon nanotube (SWNT) arrays are the key to making integrated circuits smaller than 10 nanometers. Herein, a brand-new approach is proposed to efficiently prepare semiconducting (s-) SWNT arrays by implementing a simple phenylation to modulate the metallic SWNT bandgap through the radical reaction between SWNTs and benzoyl peroxide molecules. Electrical measurement indicates that the percentage of s-SWNTs in the functionalized arrays could be higher than 97.8% after phenylation, promoting its exceptional performance as a field-effect transistor with an on-off ratio of 11 300. Our work paves a new avenue for the design and synthesis of high-purity s-SWNT arrays, which are highly important for future applications in carbon-based nano-electronic devices.

Methylation-Induced Reversible Metallic-Semiconducting Transition of Single-Walled Carbon Nanotube Arrays for High-Performance Field-Effect Transistors

Acquirement of aligned semiconducting single-walled carbon nanotube (s-SWNT) arrays is one of the most promising directions to break Moore's Law, thus developing the next-generation electronic devices. Despite that widespread approaches have been developed, it is still a great challenge to facilely prepare s-SWNT arrays with tunable electronic properties. Herein, a different perspective is proposed to produce s-SWNT arrays by implementing reversible methylation reactions on the as-grown aligned SWNT arrays. In this way, the metallic single-walled carbon nanotubes (m-SWNTs) are selectively and reversibly methylated to acquire semiconducting properties, to afford tunable electronic properties of the as-obtained SWNT arrays in a highly controllable and simple manner. Electrical measurements suggest a high fraction of s-SWNTs is attained (>97.5%) after methylation, facilitating its exceptional performance as a field-effect transistor (FET) with an on-off ratio of up to 17543. This method may provide a new way for the preparation of s-SWNT arrays with tunable electronic properties and impressive prospects toward the fabrication of high-performance FETs.

Thermal management for purification of aligned arrays of single-walled carbon nanotubes based on thermocapillary flow by pulsed heating

Semiconducting single-walled carbon nanotube (s-SWNT) arrays attract much attention due to the ideal configuration for high performance electronics. A recent approach based on the nanoscale thermocapillary flow enabled the achievement of aligned arrays of highly purified s-SWNTs by removing the metallic SWNTs (m-SWNTs) in as-grown arrays by direct pulsed current injection. Thermal management of this process is critically important since the cumulative heating from the m-SWNT array yields a high temperature increase to cause the difficult control of thermocapillary flow. In this paper, an analytical heat transfer model combining a two-dimensional transient thermal model and a three-dimensional steady state thermal model is established for predicting the temperature increase of the SWNT array under a pulsed current injection with a period step function. The analytical predictions agree well with three-dimensional finite element analysis. Effects of parameters (such as duty cycle, period, spacing, etc.) on the temperature distributions and the maximum temperature increase are investigated systematically. These results can help guide the purification process to reduce the adverse thermal effects to obtain highly purified s-SWNTs.

Water-assisted self-sustained burning of metallic single-walled carbon nanotubes for scalable transistor fabrication

Although aligned arrays of semiconducting single-walled carbon nanotubes (s-SWNTs) are promising for use in next-generation electronics owing to their ultrathin bodies and ideal electrical properties, even a small portion of metallic (m-) counterparts causes excessive leakage in field-effect transistors (FETs). To fully exploit the benefits of s-SWNTs for use in large-scale systems, it is necessary to completely eliminate m-SWNTs from as-grown SWNT arrays and thereby obtain purely semiconducting large-area arrays, wherein numerous FETs can be flexibly built. In this study, we performed electrical burning of m-SWNTs assisted by water vapor and polymer coating to eliminate m-SWNTs over a long length for the scalable fabrication of transistors from the remaining s-SWNT arrays. During the electrical-breakdown process, the combination of water vapor and the polymer coating significantly enhanced the burning of the SWNTs, resulting in a self-sustained reaction along the nanotube axis. We found that m-SWNT segments partially remaining on the anode side resulted from one-way burning from the initial breakdown position, where Joule-heating-induced oxidation first occurred. The s-SWNT-enriched arrays obtained were used to fabricate multiple FETs with a high on-off current ratio. The results indicate the advantages of this approach over conventional electrical breakdown for the large-scale purification of s-SWNTs.

Electron spectrum for aligned SWNT array in a magnetic field

Solvable model of single-walled nanotube array of cross-bar architecture in a magnetic field is constructed. The model is based on the theory of self-adjoint extensions of symmetric operators. Results of numerical analysis of the energy spectrum for the system is presented. The ”flux-energy” and ”angle-energy” diagrams for the Bloch bands are obtained. Evidence for a fractal structure of these diagrams is given.

DNA-Directed Self-Assembly of Highly Ordered and Dense Single-Walled Carbon Nanotube Arrays.

Single-walled carbon nanotubes (SWNT or CNT) have unique and well-known high-performance material properties that can enable revolutionary increases in the performance of electronic devices and architectures. However, fabrication of large-scale SWNT-based ICs is an enormously challenging, unsolved problem, and self-assembly is likely needed for critical steps. Over the past several years, methods have been introduced to created ordered carbon nanotube structures using DNA guided self-assembly. In this chapter, we briefly review the challenges involved in using DNA to assemble SWNT nanostructures, and then give detailed methods to assemble dense, aligned SWNT arrays. In particular, we discuss the preparation of DNA wrapped single-walled nanotubes (DNA-CNTs) using commercial carbon nanotube products that are suitable for electronics applications. Then, we discuss methods to characterize DNA-CNTs using fluid mode atomic force microscopy (AFM). Finally, we give detailed procedures for assembly of DNA-CNTs into dense parallel arrays via linker induced surface assembly (LISA).

Growth of Horizontal Semiconducting SWNT Arrays with Density Higher than 100 tubes/μm using Ethanol/Methane Chemical Vapor Deposition.

Horizontally aligned semiconducting single-walled carbon nanotube (s-SWNT) arrays with a certain density are highly desirable for future electronic devices. However, obtaining s-SWNT arrays with simultaneously high purity and high density is extremely challenging. We report herein a rational approach, using ethanol/methane chemical vapor deposition, to grow SWNT arrays with a s-SWNT ratio over 91% and a density higher than 100 tubes/μm. In this approach, at a certain temperature, ethanol was fully thermally decomposed to feed carbon atoms for Trojan-Mo catalysts growing high density SWNT arrays, while the incomplete pyrolysis of methane provided appropriate active H radicals with the help of catalytic sapphire surface to inhibit metallic SWNT (m-SWNT) growth. The synergistic effect of ethanol/methane mixtures resulted in enriched semiconducting SWNTs and no obvious decrease in nanotube density due to their milder reactivity and higher controllability at suitable growth conditions. This work represents a step forward in large-area synthesis of high density s-SWNT arrays on substrates and demonstrates potential applications in scalable carbon nanotube electronics.

Diameter-Specific Growth of Semiconducting SWNT Arrays Using Uniform Mo2C Solid Catalyst.

Semiconducting single-walled nanotube (s-SWNT) arrays with specific diameters are urgently demanded in the applications in nanoelectronic devices. Herein, we reported that by using uniform Mo2C solid catalyst, aligned s-SWNT (∼90%) arrays with narrow-diameter distribution (∼85% between 1.0 and 1.3 nm) on quartz substrate can be obtained. Mo2C nanoparticles with monodisperse sizes were prepared by using molybdenum oxide-based giant clusters, (NH4)42[Mo132O372(H3CCOO)30(H2O)72]·10H3CCOONH4·300H2O(Mo132), as the precursor that was carburized by a gas mixture of C2H5OH/H2 during a temperature-programmed reduction. In this approach, the formation of volatile MoO3 was inhibited due to the annealing and reduction at a low temperature. As a result, uniform Mo2C nanoparticles are formed, and their narrow size-dispersion strictly determines the diameter distribution of SWNTs. During the growth process, Mo2C selectively catalyzes the scission of C-O bonds of ethanol molecules, and the resultant absorbed oxygen (Oads) preferentially etches metallic SWNTs (m-SWNTs), leading to the high-yield of s-SWNTs. Raman spectroscopic analysis showed that most of the s-SWNTs can be identified as (14, 4), (13, 6), or (10, 9) tubes. Our findings open up the possibility of the chirality-controlled growth of aligned-SWNTs using uniform carbide nanoparticles as solid catalysts for practical nanoelectronics applications.

Graphoepitaxial effect in the guided growth of SWNT arrays on quartz

We demonstrated that water vapor introduced during the catalyst annealing process can change the alignment mode of the SWNTs on the quartz surface by modifying the surface status of both the catalyst and the substrate. Zigzag shaped nanotube arrays can be obtained by enabling both epitaxial and graphoepitaxial alignment modes simultaneously.

Sorting out Semiconducting Single‐Walled Carbon Nanotube Arrays by Washing off Metallic Tubes Using SDS Aqueous Solution

Semiconducting single-walled carbon nanotube (s-SWNT) arrays are produced via a procedure analogous to a surfactant-assisted decontamination process. Aligned individual SWNT arrays grow on a quartz surface as a mixture of metallic SWNTs (m-SWNTs) and s-SWNTs. They are immersed in a sodium dodecyl sulfate (SDS) solution, and the SDS molecules selectively adsorb onto m-SWNTs. This SDS coating minimizes the interaction between m-SWNTs and the substrate, thus the m-SWNTs are easily washed off during ultrasonication while the s-SWNT arrays remain on the substrate. The percentage of s-SWNTs in the arrays can be higher than 90%.

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. Open image in new window

Regrowth of Carbon Nanotubes Array on Al Layer Coated Substrate

Carbon nanotube (CNT) arrays have been synthesized by a repeated growth method using a custom‐fabricated plasma‐enhanced thermal chemical vapor deposition (PE‐thermal CVD) apparatus. The initial catalyst is a layered structure prepared by depositing 10 nm of Al followed by 3 nm of Fe on an oxidized silicon substrate. Following CNT growth, the CNT arrays are removed using an ultrasonic cleaner, and another CNT array is grown on the remaining Fe‐Al bimetalic nanoparticles without the addition of more catalyst. Annealing the catalytic substrate in air between growth cycles results in the removal of residual amorphous carbon along with the CNTs, and oxidation of the Fe‐Al nanoparticles. The diameter of CNTs is reduced with repeated growth‐annealing cycles, an effect of which is attributed to the diminishing size of the catalytically active nanoparticles with each cycle. After two growth cycles, SWNTs with the extraordinarily narrow diameter of 0.86 nm are synthesized. The ID/IG ratio derived from the Raman spectrum of these of the SWNT arrays shows the remarkably low value of 0.22.

Direct Growth of Semiconducting Single-Walled Carbon Nanotube Array

By introducing the UV beam into our homemade chemical vapor deposition system, we had obtained a well aligned SWNT array on an ST-cut quartz substrate. After transfer onto a SiO(2)/Si substrate, the SWNT array was detected by Raman spectroscopy and electrical measurement, which showed that over 95% of the SWNTs were semiconducting ones. It is proposed that the selection process took place at the very beginning of the SWNT formation rather than destroying the metallic SWNTs after growth. This approach has solved one of the most important problems in SWNT application.

Single-Walled Carbon Nanotube Pillars: A Superhydrophobic Surface

A series of SWNT arrays have been constructed by disproportionation of carbon monoxide on Co-Mo catalyst films deposited on flat Si wafer substrates. The arrays include a two-dimensional SWNT thin film prepared by spraying a SWNT suspension, as well as random SWNT networks, and vertically aligned SWNT. In addition, a novel SWNT array has been developed and termed SWNT "pillars". These pillars are prepared by controlled synthesis of SWNT over a templated substrate. When water droplets are deposited on these surfaces, it is observed that the static contact angle changes dramatically with the type of SWNT array. The higher is the degree of surface roughness (at the nano and microscales) of these structures, the higher is the contact angle. The novel SWNT pillars exhibit superhydrophobicity (i.e., contact angle approximately 160 degrees). To quantify the effect of surface roughness, two simple models have been employed. The Wenzel model can be used to quantify the behavior of the SWNT thin films while the Cassie-Baxter model is used for the SWNT arrays with three-dimensional structure (grass, forest, and pillars).

Sorting out Semiconducting Single-Walled Carbon Nanotube Arrays by Preferential Destruction of Metallic Tubes Using Xenon-Lamp Irradiation

We report herein a simple and reliable approach to prepare densely packed, well-aligned individual semiconducting single-walled carbon nanotube (s-SWNT) arrays using long-arc xenon-lamp (Xe-lamp) irradiation. The densely packed, well-aligned individual SWNT arrays (the mean density was ∼20 tubes/μm) were first grown on a-plane sapphire using ethanol containing 3 wt % water as the carbon source. It is found that water plays a key role in the improvement of the density and the alignment of the SWNT arrays. The densely packed aligned SWNT arrays were then irradiated by a long-arc Xe lamp. Atomic force microscopy manipulation, Raman spectroscopy, and electrical transport measurement revealed that the SWNTs with small diameters (d < 1 nm) and the metallic SWNTs (m-SWNTs) in the arrays can be preferentially destroyed, thus leaving s-SWNT arrays on the surface. In general, for the SWNTs with diameters in the region of 1.15−1.55 nm, the percentage of s-SWNTs increased from ca. 50% to 95% after 60 min of irradiati...

Amine Basicity (pKb) Controls the Analyte Binding Energy on Single Walled Carbon Nanotube Electronic Sensor Arrays

A wide range of analytes adsorb irreversibly to the surfaces of single walled carbon nanotube electronic networks typically used as sensors or thin-film transistors, although to date, the mechanism is not understood. Using thionyl chloride as a model electron-withdrawing adsorbate, we show that reversible adsorption sites can be created on the nanotube array via noncovalent functionalization with amine-terminated molecules of pKa < 8.8. A nanotube network comprising single, largely unbundled nanotubes, near the electronic percolation threshold is required for the effective conversion to a reversibly binding array. By examining 11 types of amine-containing molecules, we show that analyte adsorption is largely affected by the basicity (pKb) of surface groups. The binding energy of the analyte is apparently reduced by its adsorption on the surface chemical groups instead of directly on the SWNT array itself. This mediated adsorption mechanism is supported by X-ray photoelectron spectroscopy (XPS) and molecular potential calculations. Reversible detection with no active regeneration at the parts-per-trillion level is demonstrated for the first time by creating a higher adsorption site density with a polymer amine, such as polyethyleneimine (PEI). Last, we demonstrate that this transition to reversibility upon surface functionalization is a general phenomenon.

Electric-Field-Enhanced Assembly of Single-Walled Carbon Nanotubes on a Solid Surface

We report a novel electric-field-enhanced chemical assembly approach for fabricating highly aligned SWNT arrays on a solid surface with remarkably improved efficiency and packing density, which is very important for the real applications of carbon nanotube arrays. With the enhancement of the electric field, the assembling kinetics of SWNTs is remarkably speeded up to effectively decrease the assembling time, and the packing density can even exceed the saturated density of conventional assembly method by four times within only half an hour. The molecular dynamics simulation results illustrated the alignment of SWNTs with their long axes along the electric flux in solution, leading to the increase of packing density and efficiency through overcoming the steric hindrance of the "giant" carbon nanotubes.