Improved density in aligned arrays of single-walled carbon nanotubes by sequential chemical vapor deposition on quartz

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.

Effect of surfactant concentration on the evaporation-driven deposition of carbon nanotubes: from coffee-ring effect to strain sensing

Carbon nanotubes (CNTs) as electrically conductive materials are of great importance in the fabrication of flexible electronic devices and wearable sensors. In this regard, the evaporation-driven self-assembly of CNTs has attracted increasing attention. CNT-based applications are mostly concerned with the alignment of CNTs and the density of CNT films. In the present work, we focus on the latter by trying to achieve an optimal evaporation-driven deposition with the densest CNT ring. Although surfactants are used for effective dispersion and colloidal stabilization of CNTs in the aqueous phase, their excessive usage induces Marangoni eddies in the evaporating sessile droplets, leading to poor ring depositions. Thus, there is an optimum surfactant concentration that contributes to CNTs deagglomeration and results in the densest ring-like deposition with relatively high thickness. We report that this optimum concentration for sodium dodecyl sulfate (SDS) as a surfactant can be approximately considered as much as the concentration of multi-walled carbon nanotubes (MWCNTs) as the colloidal nanoparticles. Optimal depositions show the lowest electrical resistances for each CNT concentration, making them suitable for electronic applications. We also propose the multiple depositions method in which a new droplet is printed after the complete evaporation of the previous droplet. This method can lead to denser rings with a higher conductivity using lower concentrations of CNTs. Lastly, we fabricate strain sensors based on the optimal evaporation-driven deposition of CNTs which show higher gauge factors than the commercial strain gauges, corroborating the applicability of our method.

Two dimensional semiconducting materials for ultimately scaled transistors

Two dimensional (2D) semiconductors have been established as promising candidates to break through the short channel effect that existed in Si metal-oxide-semiconductor field-effect-transistor (MOSFET), owing to their unique atomically layered structure and dangling-bond-free surface. The last decade has witnessed the significant progress in the size scaling of 2D transistors by various approaches, in which the physical gate length of the transistors has shrank from micrometer to sub-one nanometer with superior performance, illustrating their potential as a replacement technology for Si MOSFETs. Here, we review state-of-the-art techniques to achieve ultra-scaled 2D transistors with novel configurations through the scaling of channel, gate, and contact length. We provide comprehensive views of the merits and drawbacks of the ultra-scaled 2D transistors by summarizing the relevant fabrication processes with the corresponding critical parameters achieved. Finally, we identify the key opportunities and challenges for integrating ultra-scaled 2D transistors in the next-generation heterogeneous circuitry.

Universal Map of Gas-Dependent Kinetic Selectivity in Carbon Nanotube Growth

Single-walled carbon nanotubes have been a candidate for outperforming silicon in ultrascaled transistors, but the realization of nanotube-based integrated circuits requires dense arrays of purely semiconducting species. In order to directly grow such nanotube arrays on wafers, control over kinetics and thermodynamics in tube-catalyst systems plays a key role, and further progress requires a comprehensive understanding of seemingly contradictory reports on the growth kinetics. Here, we propose a universal kinetic model that decomposes the growth rates of nanotubes into the adsorption and removal of carbon atoms on the catalysts, and we provide its quantitative verification by ethanol-based isotope labeling experiments. While the removal of carbon from catalysts dominates the growth kinetics under a low supply of precursors, resulting in chirality-independent growth rates, our kinetic model and experiments demonstrate that chiral angle-dependent growth rates emerge when sufficient amounts of carbon and etching agents are cosupplied. The kinetic maps, as a product of generalizing the model, include five types of kinetic selectivity that emerge depending on the absolute quantities of gases with opposing effects. Our findings not only resolve discrepancies existing in the literature but also offer rational strategies to control the chirality, length, and density of nanotube arrays for practical applications.

Building a Bridge for Carbon Nanotubes from Nanoscale Structure to Macroscopic Application

Through 30 years of research, researchers have gained a deep understanding of the synthesis, characteristics, and applications of carbon nanotubes (CNTs). However, up to now, there are still few industries using CNT as the leading material. The difficulty of CNTs to be applied in industry is the gap between the properties of CNT-based aggregates and those of a single carbon nanotube. Therefore, how to maintain the intrinsic properties of CNTs when they are assembled into aggregates is of great significance. Herein, we summarize and analyze the research status of CNT materials applied in different fields from proven techniques to potential industries, including energy storage, electronics, mechanical and other applications. For each application, the intrinsic properties of CNTs and the real performances of their aggregates are compared to figure out the key problems in CNT synthesis. Finally, we give an outlook for building a bridge for CNTs from nanoscale structure to macroscopic application, giving inspiration to researchers making efforts toward the real application of carbon nanotubes.

Confined Fe Catalysts for High-Density SWNT Arrays Growth: a New Territory for Catalyst-Substrate Interaction Engineering

Great efforts have been devoted to searching for efficient catalytic systems to produce ultra-high density single-walled carbon nanotube (SWNT) arrays, which lay the foundation for future electronic devices. However, one major obstacle for realizing high-density surface-aligned SWNT arrays is the poor stability of metal nanoparticles in chemical vapor deposition catalytic processes. Recently, Trojan catalyst has been reported to yield unprecedented high-density SWNT arrays with 130 SWNTs per µm on the a-plane (11-20) of the sapphire substrate. Herein, a concept of catalyst confinement effect is put forward to revealing the secret of remarkable growth efficiency of SWNT arrays by Trojan catalyst. Combined experimental and theoretical studies indicate that confinement of catalyst nanoparticles on discrete a-plane strips plays a key role in stabilizing the small nanoparticles. The highly dispersive and active states of catalysts are maintained, which promote the growth of super-dense SWNT arrays. By rationally designing the substrate reconstruction process, large areas of dense SWNT arrays (130 SWNTs per µm) covering the entire substrate are obtained. This approach may provide novel ideas for the synthesis of various high-density 1D nanomaterials.

Growth of Homogeneous High-Density Horizontal SWNT Arrays on Sapphire through a Magnesium-Assisted Catalyst Anchoring Strategy

In-situ growth of high-density single-walled carbon nanotube (SWNT) arrays with homogeneity is highly desirable for integrated circuits. However, disastrous migration and aggregation of catalyst nanoparticles on substrate has greatly limited the area of as-grown SWNT arrays. Herein, we develop a magnesium-assisted catalyst anchoring strategy to restrain catalyst nanoparticles sintering on substrate. Magnesium modification ameliorates sapphire surface by high temperature solid reaction and thus provides a stronger metal-support interaction (SMSI). Hereby, we realize the direct growth of high-density SWNT arrays that fully cover an entire 10×10 mm² substrate with the local highest density of ≈110 tubes μm^-1 using iron as catalyst. This strategy was also proven universal when employing solid carbide catalysts.

Horizontal Single-Walled Carbon Nanotube Arrays: Controlled Synthesis, Characterizations, and Applications

Single-walled carbon nanotubes (SWNTs) emerge as a promising material to advance carbon nanoelectronics. However, synthesizing or assembling pure metallic/semiconducting SWNTs required for interconnects/integrated circuits, respectively, by a conventional chemical vapor deposition method or by an assembly technique remains challenging. Recent studies have shown significant scientific breakthroughs in controlled SWNT synthesis/assembly and applications in scaled field effect transistors, which are a critical component in functional nanodevices, thereby rendering the horizontal SWNT array an important candidate for innovating nanotechnology. This review provides a comprehensive analysis of the controlled synthesis, surface assembly, characterization techniques, and potential applications of horizontally aligned SWNT arrays. This review begins with the discussion of synthesis of horizontally aligned SWNTs with regulated direction, density, structure, and theoretical models applied to understand the growth results. Several traditional procedures applied for assembling SWNTs on target surface are also briefly discussed. It then discusses the techniques adopted to characterize SWNTs, ranging from electron/probe microscopy to various optical spectroscopy methods. Prototype applications based on the horizontally aligned SWNTs, such as interconnects, field effect transistors, integrated circuits, and even computers, are subsequently described. Finally, this review concludes with challenges and a brief outlook of the future development in this research field.

Selectively Patterned Regrowth of Bilayer Graphene for Self-Integrated Electronics by Sequential Chemical Vapor Deposition

There is a critical demand for the highly qualified synthesis of graphene with precisely controlled thickness over a large coverage area. Selective growth can be considered as one method of preparing a vertically stacked graphene, but it usually requires elaborately alloyed substrates for chemical vapor deposition (CVD). Here, we report on a newly developed synthesis strategy for a selectively patterned grown graphene sheet in a spatially defined multithickness scale, exhibiting single- and bilayer graphene produced by a conventional CVD process. In particular, a sequential CVD growth technique on a single Cu substrate was used to produce highly ordered and alternatively patterned single- and bilayer graphene, maintaining its continuous configuration in a simplified and scalable manner. Our regrowth process did not require multiple transfer procedures or an alloying catalytic substrate to satisfy the properties of graphene associated with the needs for various applications. We also investigated the most valid mechanisms for our regrowth CVD process, which suggests that it is useful for the cost-effective synthetic approach into a built-in heterostructured single- and bilayer graphene. Finally, we demonstrated the possible accesses of transparent flexible electrodes and monolithically self-integrated all-graphene-based thin-film transistors to fully utilize regrown graphene.

Digital Isotope Coding to Trace the Growth Process of Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are attracting increasing attention as an ideal material for high-performance electronics through the preparation of arrays of purely semiconducting SWCNTs. Despite significant progress in the controlled synthesis of SWCNTs, their growth mechanism remains unclear due to difficulties in analyzing the time-resolved growth of individual SWCNTs under practical growth conditions. Here we present a method for tracing the diverse growth profiles of individual SWCNTs by embedding digitally coded isotope labels. Raman mapping showed that, after various incubation times, SWCNTs elongated monotonically until their abrupt termination. Ex situ analysis offered an opportunity to capture rare chirality changes along the SWCNTs, which resulted in sudden acceleration/deceleration of the growth rate. Dependence on growth parameters, such as temperature and carbon concentration, was also traced along individual SWCNTs, which could provide clues to chirality control. Systematic growth studies with a variety of catalysts and conditions, which combine the presented method with other characterization techniques, will lead to further understanding and control of chirality, length, and density of SWCNTs.

An air gap moderates the performance of nanowire array transistors

Solution-processed nanowires are promising for low-cost and flexible electronics. When depositing nanowires from solution, due to stacking of the nanowires, an air gap exists between the substrate and much of the active material. Here, using confocal Raman spectroscopy, we quantify the thickness of the air gap in transistors comprising organic semiconductor nanowires. The average air gap thickness is found to be unexpectedly large, being at least three times larger than the nanowire diameter, leading to a significant impact on transistor performance. The air gap acts as an additional dielectric layer that reduces the accumulation of charge carriers due to a gate voltage. Conventional determination of the charge carrier mobility ignores the presence of an air gap, resulting in an overestimate of charge carrier accumulation and an underestimate of charge carrier mobility. It is shown that the larger the air gap, the larger the mobility correction (which can be greater than an order of magnitude) and the larger the degradation in on-off current ratio. These results demonstrate the importance of minimizing the air gap and of taking the air gap into consideration when analyzing the electrical performance of transistors consisting of stacked nanowires. This finding is applicable to all types of stacked one-dimensional materials including organic and inorganic nanowires, and carbon nanotubes.

Horizontally aligned carbon nanotube arrays: growth mechanism, controlled synthesis, characterization, properties and applications

This review summarizes the growth mechanism, controlled synthesis, characterization, properties and applications of horizontally aligned carbon nanotube arrays.

Horizontal carbon nanotube alignment

The production of horizontally aligned carbon nanotubes offers a rapid means of realizing a myriad of self-assembled near-atom-scale technologies - from novel photonic crystals to nanoscale transistors. The ability to reproducibly align anisotropic nanostructures has huge technological value. Here we review the present state-of-the-art in horizontal carbon nanotube alignment. For both in and ex situ approaches, we quantitatively assess the reported linear packing densities alongside the degree of alignment possible for each of these core methodologies.

Teslaphoresis of Carbon Nanotubes

This paper introduces Teslaphoresis, the directed motion and self-assembly of matter by a Tesla coil, and studies this electrokinetic phenomenon using single-walled carbon nanotubes (CNTs). Conventional directed self-assembly of matter using electric fields has been restricted to small scale structures, but with Teslaphoresis, we exceed this limitation by using the Tesla coil's antenna to create a gradient high-voltage force field that projects into free space. CNTs placed within the Teslaphoretic (TEP) field polarize and self-assemble into wires that span from the nanoscale to the macroscale, the longest thus far being 15 cm. We show that the TEP field not only directs the self-assembly of long nanotube wires at remote distances (>30 cm) but can also wirelessly power nanotube-based LED circuits. Furthermore, individualized CNTs self-organize to form long parallel arrays with high fidelity alignment to the TEP field. Thus, Teslaphoresis is effective for directed self-assembly from the bottom-up to the macroscale.

Dramatically enhancing the yield of carbon nanotubes by simply adding oxygen-containing molecules in solid-state synthesis

Commercially available oxygen-containing molecules are utilized to enhance the yield of carbon nanotubes in the solid-state pyrolysis of organometallic precursors.

Recent Progress in Obtaining Semiconducting Single-Walled Carbon Nanotubes for Transistor Applications

High purity semiconducting single-walled carbon nanotubes (s-SWCNTs) with a narrow diameter distribution are required for high-performance transistors. Achieving this goal is extremely challenging because the as-grown material contains mixtures of s-SWCNTs and metallic- (m-) SWCNTs with wide diameter distributions, typically inadequate for integrated circuits. Since 2000, numerous ex situ methods have been proposed to improve the purity of the s-SWCNTs. The majority of these techniques fail to maintain the quality and integrity of the s-SWCNTs with a few notable exceptions. Here, the progress in realizing high purity s-SWCNTs in as-grown and post-processed materials is highlighted. A comparison of transistor parameters (such as on/off ratio and field-effect mobility) obtained from test structures establishes the effectiveness of various methods and suggests opportunities for future improvements.

Photoluminescence enhancement of aligned arrays of single-walled carbon nanotubes by polymer transfer

The photoluminescence of as-grown, aligned single-walled carbon nanotubes (SWNTs) on quartz is strongly quenched and barely detectable. Here we show that transferring these SWNTs to another substrate such as clean quartz or glass increases their emission efficiency by up to two orders of magnitude. By statistical analysis of large nanotube arrays we show at what point of the transfer process the emission enhancement occurs and how it depends on the receiving substrate and the employed transfer polymer. We find that hydrophobic polystyrene (PS) as the transfer polymer results in higher photoluminescence enhancement than the more hydrophilic poly(methyl methacrylate) (PMMA). Possible mechanisms for this enhancement such as strain relief, disruption of the strong interaction of SWNTs with the substrate and localized emissive states are discussed.

Growth of close-packed semiconducting single-walled carbon nanotube arrays using oxygen-deficient TiO2 nanoparticles as catalysts

For the application of single-walled carbon nanotubes (SWNTs) in nanoelectronic devices, techniques to obtain horizontally aligned semiconducting SWNTs (s-SWNTs) with higher densities are still in their infancy. We reported herein a rational approach for the preferential growth of densely packed and well-aligned s-SWNTs arrays using oxygen-deficient TiO2 nanoparticles as catalysts. Using this approach, a suitable concentration of oxygen vacancies in TiO2 nanoparticles could form by optimizing the flow rate of hydrogen and carbon sources during the process of SWNT growth, and then horizontally aligned SWNTs with the density of ∼ 10 tubes/μm and the s-SWNT percentage above 95% were successfully obtained on ST-cut quartz substrates. Theoretical calculations indicated that TiO2 nanoparticles with a certain concentration of oxygen vacancies have a lower formation energy between s-SWNT than metallic SWNT (m-SWNT), thus realizing the preferential growth of s-SWNT arrays. Furthermore, this method can also be extended to other semiconductor oxide nanoparticles (i.e., ZnO, ZrO2 and Cr2O3) for the selective growth of s-SWNTs, showing clear potential to the future applications in nanoelectronics.

Polyfluorene-sorted, carbon nanotube array field-effect transistors with increased current density and high on/off ratio

Challenges in eliminating metallic from semiconducting single-walled carbon nanotubes (SWCNTs) and in controlling their alignment have limited the development of high-performance SWCNT-based field-effect transistors (FETs). We recently pioneered an approach for depositing aligned arrays of ultra-high-purity semiconducting SWCNTs, isolated using polyfluorene derivatives, called dose-controlled floating evaporative self-assembly. Here, we tailor FETs fabricated from these arrays to achieve on-conductance (G(on)) per width and an on-off ratio (G(on)/G(off)) of 261 μS/μm and 2 × 10(5), respectively, for a channel length (L(ch)) of 240 nm and 116 μS/μm and 1 × 10(6), respectively, for an L(ch) of 1 μm. We demonstrate 1400× greater G(on)/G(off) than SWCNT FETs fabricated by other methods, at comparable G(on) per width of ∼250 μS/μm and 30-100× greater G(on) per width at comparable G(on)/G(off) of 10(5)-10(7). The average G(on) per tube reaches 5.7 ± 1.4 μS at a packing density of 35 tubes/μm for L(ch) in the range 160-240 nm, limited by contact resistance. These gains highlight the promise of using ultra-high-purity semiconducting SWCNTs with controlled alignment for next-generation semiconductor electronics.

State of the art of single-walled carbon nanotube synthesis on surfaces

Single-walled carbon nanotubes (SWNTs) directly synthesized on surfaces are promising building blocks for nanoelectronics. The structures and the arrangement of the SWNTs on surfaces determine the quality and density of the fabricated nanoelectronics, implying the importance of structure controlled growth of SWNTs on surfaces. This review summarizes the recent research status in controlling the orientation, length, density, diameter, metallicity, and chirality of SWNTs directly synthesized on surfaces by chemical vapor deposition, together with a session presenting the characterization method of the chirality of SWNTs. Finally, the remaining major challenges are discussed and future research directions are proposed.

Selective removal of metallic single-walled carbon nanotubes in full length by organic film-assisted electrical breakdown

An organic film-assisted electrical breakdown technique is proposed to selectively remove metallic (m-) single-walled carbon nanotubes (SWNTs) in full length towards creation of pure semiconducting SWNT arrays which are available for the large-scale fabrication of field effect transistors (FETs). The electrical breakdown of horizontally aligned SWNT arrays embedded in organic films resulted in a maximum removal length of 16.4 μm. The removal of SWNTs was confirmed using scanning electron microscopy and Raman mapping measurements. The on/off ratios of FETs were improved up to ca. 10,000, similar to that achieved for in-air breakdown. The experimental results suggest that exothermic oxidation of organic films induces propagation of oxidation reaction, hence the long-length removal of m-SWNTs.

Flexible carbon nanotube films for high performance strain sensors

Compared with traditional conductive fillers, carbon nanotubes (CNTs) have unique advantages, i.e., excellent mechanical properties, high electrical conductivity and thermal stability. Nanocomposites as piezoresistive films provide an interesting approach for the realization of large area strain sensors with high sensitivity and low manufacturing costs. A polymer-based nanocomposite with carbon nanomaterials as conductive filler can be deposited on a flexible substrate of choice and this leads to mechanically flexible layers. Such sensors allow the strain measurement for both integral measurement on a certain surface and local measurement at a certain position depending on the sensor geometry. Strain sensors based on carbon nanostructures can overcome several limitations of conventional strain sensors, e.g., sensitivity, adjustable measurement range and integral measurement on big surfaces. The novel technology allows realizing strain sensors which can be easily integrated even as buried layers in material systems. In this review paper, we discuss the dependence of strain sensitivity on different experimental parameters such as composition of the carbon nanomaterial/polymer layer, type of polymer, fabrication process and processing parameters. The insights about the relationship between film parameters and electromechanical properties can be used to improve the design and fabrication of CNT strain sensors.

Carbon nanotube circuit integration up to sub-20 nm channel lengths

Carbon nanotube (CNT) field-effect transistors (CNFETs) are a promising emerging technology projected to achieve over an order of magnitude improvement in energy-delay product, a metric of performance and energy efficiency, compared to silicon-based circuits. However, due to substantial imperfections inherent with CNTs, the promise of CNFETs has yet to be fully realized. Techniques to overcome these imperfections have yielded promising results, but thus far only at large technology nodes (1 μm device size). Here we demonstrate the first very large scale integration (VLSI)-compatible approach to realizing CNFET digital circuits at highly scaled technology nodes, with devices ranging from 90 nm to sub-20 nm channel lengths. We demonstrate inverters functioning at 1 MHz and a fully integrated CNFET infrared light sensor and interface circuit at 32 nm channel length. This demonstrates the feasibility of realizing more complex CNFET circuits at highly scaled technology nodes.

Dose-controlled, floating evaporative self-assembly and alignment of semiconducting carbon nanotubes from organic solvents

Arrays of aligned semiconducting single-walled carbon nanotubes (s-SWCNTs) with exceptional electronic-type purity were deposited at high deposition velocity of 5 mm min(-1) by a novel "dose-controlled, floating evaporative self-assembly" process with excellent control over the placement of stripes and quantity of s-SWCNTs deposited. This approach uses the diffusion of organic solvent on the water-air interface to deposit aligned s-SWCNT (99.9%) tubes on a partially submerged hydrophobic substrate, which is withdrawn vertically from the surface of water. By decoupling the s-SWCNT stripe formation from the evaporation of the bulk solution and by iteratively applying the s-SWCNTs in controlled "doses", we show through polarized Raman studies that the s-SWCNTs are aligned within ±14°, are packed at a density of ∼50 s-SWCNTs μm(-1), and constitute primarily a well-ordered monodispersed layer. The resulting field-effect transistor devices show high performance with a mobility of 38 cm(2) V(-1) s(-1) and on/off ratio of 2.2 × 10(6) at 9 μm channel length.

Growth of high-density-aligned and semiconducting-enriched single-walled carbon nanotubes: decoupling the conflict between density and selectivity

Single-walled carbon nanotubes (SWNTs) are highly desired for future electronic applications due to the excellent electrical, mechanical, and thermal properties. However, the density and the selectivity in the growth of aligned semiconducting nanotubes do not coexist previously: when the selectivity is high, the density is low and vice versa. In the present work, we found that random carbon nanotubes (CNTs) in the catalyst area block the growth of aligned SWNTs along the lattice structure on the quartz surface, thus significantly reducing the density of nanotubes during growth. More interestingly, it was shown that the random CNTs can be selectively removed through appropriate treatments using water vapor as an in situ etchant while the aligned SWNTs survive even after long-time water vapor treatment. To obtain high-density semiconducting SWNT arrays, we designed an improved multiple-cycle growth method, which included the treatment of SWNTs with water vapor after each growth cycle without cooling the system. Using this method, we have successfully obtained dense semiconducting SWNTs (∼10 SWNTs/μm) over large areas and with high uniformity.

Aligned carbon nanotubes: from controlled synthesis to electronic applications

Single-wall carbon nanotubes (SWNTs) possess superior geometrical, electronic, chemical, thermal, and mechanical properties and are very attractive for applications in electronic devices and circuits. To make this a reality, the nanotube orientation, density, diameter, electronic property, and even chirality should be well controlled. This Feature article focuses on recent achievements researchers have made on the controlled growth of horizontally aligned SWNTs and SWNT arrays on substrates and their electronic applications. Principles and strategies to control the morphology, structure, and properties of SWNTs are reviewed in detail. Furthermore, electrical properties of field-effect transistors fabricated on both individual SWNTs and aligned SWNT arrays are discussed. State-of-the-art electronic devices and circuits based on aligned SWNTs and SWNT arrays are also highlighted.

Single-walled carbon nanotubes for high-performance electronics

Single-walled carbon nanotubes (SWNT) could replace silicon in high-performance electronics with their exceptional electrical properties and intrinsic ultra-thin body. During the past five years, the major focus of this field is gradually shifting from proof-of-concept prototyping in academia to technology development in industry with emphasis on manufacturability and integration issues. This article reviews recent advances, starting with experimental and modeling works that evaluate the potential of adopting SWNTs in ultimately scaled transistors. Techniques to separate nanotubes according to their electronic types and assemble them into aligned arrays are then discussed, followed by a description of the engineering aspects in their implementation in integrated circuits and systems. A concluding discussion provides some perspectives on future challenges and research opportunities.

High-performance partially aligned semiconductive single-walled carbon nanotube transistors achieved with a parallel technique

Single-walled carbon nanotubes (SWNTs) are widely thought to be a strong contender for next-generation printed electronic transistor materials. However, large-scale solution-based parallel assembly of SWNTs to obtain high-performance transistor devices is challenging. SWNTs have anisotropic properties and, although partial alignment of the nanotubes has been theoretically predicted to achieve optimum transistor device performance, thus far no parallel solution-based technique can achieve this. Herein a novel solution-based technique, the immersion-cum-shake method, is reported to achieve partially aligned SWNT networks using semiconductive (99% enriched) SWNTs (s-SWNTs). By immersing an aminosilane-treated wafer into a solution of nanotubes placed on a rotary shaker, the repetitive flow of the nanotube solution over the wafer surface during the deposition process orients the nanotubes toward the fluid flow direction. By adjusting the nanotube concentration in the solution, the nanotube density of the partially aligned network can be controlled; linear densities ranging from 5 to 45 SWNTs/μm are observed. Through control of the linear SWNT density and channel length, the optimum SWNT-based field-effect transistor devices achieve outstanding performance metrics (with an on/off ratio of ~3.2 × 10(4) and mobility 46.5 cm(2) /Vs). Atomic force microscopy shows that the partial alignment is uniform over an area of 20 × 20 mm(2) and confirms that the orientation of the nanotubes is mostly along the fluid flow direction, with a narrow orientation scatter characterized by a full width at half maximum (FWHM) of <15° for all but the densest film, which is 35°. This parallel process is large-scale applicable and exploits the anisotropic properties of the SWNTs, presenting a viable path forward for industrial adoption of SWNTs in printed, flexible, and large-area electronics.

Carbon nanotube electronics--moving forward

Single-walled carbon nanotubes (SWNTs) possess fascinating electrical properties and offer new entries into a wide range of novel electronic applications that are unattainable with conventional Si-based devices. The field initially focused on the use of individual or parallel arrays of nanotubes as the channel material for ultra-scaled nanoelectronic devices. However, the challenge in the deterministic assembly has proven to be a major technological barrier. In recent years, solution deposition of semiconductor-enriched SWNT networks has been actively explored for high performance and uniform thin-film transistors (TFTs) on mechanically rigid and flexible substrates. This presents a unique niche for nanotube electronics by overcoming their limitations and taking full advantage of their superb chemical and physical properties. This review focuses on the large-area processing and electronic properties of SWNT TFTs. A wide range of applications in conformal integrated circuits, radio-frequency electronics, artificial skin sensors, and displays are discussed--with emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required. The demonstrations show SWNTs' immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent device performances.

A review of fabrication and applications of carbon nanotube film-based flexible electronics

Flexible electronics offer a wide-variety of applications such as flexible circuits, flexible displays, flexible solar cells, skin-like pressure sensors, and conformable RFID tags. Carbon nanotubes (CNTs) are a promising material for flexible electronics, both as the channel material in field-effect transistors (FETs) and as transparent electrodes, due to their high intrinsic carrier mobility, conductivity, and mechanical flexibility. In this feature article, we review the recent progress of CNTs in flexible electronics by describing both the processing and the applications of CNT-based flexible devices. To employ CNTs as the channel material in FETs, single-walled carbon nanotubes (SWNTs) are used. There are generally two methods of depositing SWNTs on flexible substrates-transferring CVD-grown SWNTs or solution-depositing SWNTs. Since CVD-grown SWNTs can be highly aligned, they often outperform solution-processed SWNT films that are typically in the form of random network. However, solution-based SWNTs can be printed at a large-scale and at low-cost, rendering them more appropriate for manufacturing. In either case, the removal of metallic SWNTs in an effective and a scalable manner is critical, which must still be developed and optimized. Nevertheless, promising results demonstrating SWNT-based flexible circuits, displays, RF-devices, and biochemical sensors have been reported by various research groups, proving insight into the exciting possibilities of SWNT-based FETs. In using carbon nanotubes as transparent electrodes (TEs), two main strategies have been implemented to fabricate highly conductive, transparent, and mechanically compliant films-superaligned films of CNTs drawn from vertically grown CNT forests using the "dry-drawing" technique and the deposition or embedding of CNTs onto flexible or stretchable substrates. The main challenge for CNT based TEs is to fabricate films that are both highly conductive and transparent. These CNT based TEs have been used in stretchable and flexible pressure, strain, and chemical and biological sensors. In addition, they have also been used as the anode and cathode in flexible light emitting diodes, solar cells, and supercapacitors. In summary, there are a number of challenges yet to overcome to optimize the processing and performance of CNT-based flexible electronics; nonetheless, CNTs remain a highly suitable candidate for various flexible electronic applications in the near future.

Arrays of single-walled carbon nanotubes with full surface coverage for high-performance electronics

Single-walled carbon nanotubes have exceptional electronic properties and have been proposed as a replacement for silicon in applications such as low-cost thin-film transistors and high-performance logic devices. However, practical devices will require dense, aligned arrays of electronically pure nanotubes to optimize performance, maximize device packing density and provide sufficient drive current (or power output) for each transistor. Here, we show that aligned arrays of semiconducting carbon nanotubes can be assembled using the Langmuir-Schaefer method. The arrays have a semiconducting nanotube purity of 99% and can fully cover a surface with a nanotube density of more than 500 tubes/µm. The nanotube pitch is self-limited by the diameter of the nanotube plus the van der Waals separation, and the intrinsic mobility of the nanotubes is preserved after array assembly. Transistors fabricated using this approach exhibit significant device performance characteristics with a drive current density of more than 120 µA µm(-1), transconductance greater than 40 µS µm(-1) and on/off ratios of ∼1 × 10(3).

Evaluation of field-effect mobility and contact resistance of transistors that use solution-processed single-walled carbon nanotubes

Solution-processed single-walled carbon nanotubes (SWNTs) offer many unique processing advantages over nanotubes grown by the chemical vapor deposition (CVD) method, including capabilities of separating the nanotubes by electronic type and depositing them onto various substrates in the form of ultradensely aligned arrays at low temperature. However, long-channel transistors that use solution-processed SWNTs generally demonstrate inferior device performance, which poses concerns over the feasibility of using these nanotubes in high-performance logic applications. This paper presents the first systematic study of contact resistance, intrinsic field-effect mobility (μ(FE)), and conductivity (σ(m)) of solution-processed SWNTs based on both the transmission line method and the Y function method. The results indicate that, compared to CVD nanotubes, although solution-processed SWNTs have much lower μ(FE) for semiconducting nanotubes and lower σ(m) for metallic nanotubes due to the presence of a higher level of structural defects, such defects do not affect the quality of electric contacts between the nanotube and metal source/drain electrodes. Therefore, solution-processed SWNTs are expected to offer performance comparable to that of CVD nanotubes in ultimately scaled field-effect transistors, where contacts will dominate electron transport instead of electron scattering in the channel region. These results show promise for using solution-processed SWNTs for high-performance nanoelectronic devices.

CVD-grown horizontally aligned single-walled carbon nanotubes: synthesis routes and growth mechanisms

Single-walled carbon nanotubes (SWCNTs) have attractive electrical and physical properties, which make them very promising for use in various applications. For some applications however, in particular those involving electronics, SWCNTs need to be synthesized with a high degree of control with respect to yield, length, alignment, diameter, and chirality. With this in mind, a great deal of effort is being directed to the precision control of vertically and horizontally aligned nanotubes. In this review the focus is on the latter, horizontally aligned tubes grown by chemical vapor deposition (CVD). The reader is provided with an in-depth review of the established vapor deposition orientation techniques. Detailed discussions on the characterization routes, growth parameters, and growth mechanisms are also provided.

3-Orders-of-magnitude density control of single-walled carbon nanotube networks by maximizing catalyst activation and dosing carbon supply

Tailoring the density of random single-walled carbon nanotube (SWCNT) networks is of paramount importance for various applications, yet it remains a major challenge due to the insufficient catalyst activation in most growth processes. Here we report on a simple and effective method to maximise the number of active catalyst nanoparticles using catalytic chemical vapor deposition (CCVD). By modulating short pulses of acetylene into a methane-based CCVD growth process, the density of SWCNTs is dramatically increased by up to three orders of magnitude without increasing the catalyst density and degrading the nanotube quality. In the framework of a vapor-liquid-solid model, we attribute the enhanced growth to the high dissociation rate of acetylene at high temperatures at the nucleation stage, which can be effective in both supersaturating the larger catalyst nanoparticles and overcoming the nanotube nucleation energy barrier of the smaller catalyst nanoparticles. These results are highly relevant to numerous applications of random SWCNT networks in next-generation energy, sensing and biomedical devices.

Monolithic integration of arrays of single-walled carbon nanotubes and sheets of graphene

Sheets of graphene and arrays of single-walled carbon nanotubes (SWNTs) are formed separately using chemical vapor deposition techniques onto different optimized growth substrates. Techniques of transfer printing provide a route to integration, yielding two terminal devices and transistors in which patterned structures of graphene form the electrodes and the SWNTs arrays serve as the semiconducting channels.

A statistical-based material and process guidelines for design of carbon nanotube field-effect transistors in gigascale integrated circuits

Carbon nanotube field-effect transistors (CNFETs) show great promise as building blocks of future integrated circuits. However, synthesizing single-walled carbon nanotubes (CNTs) with accurate chirality and exact positioning control has been widely acknowledged as an exceedingly complex task. Indeed, density and chirality variations in CNT growth can compromise the reliability of CNFET-based circuits. In this paper, we present a novel statistical compact model to estimate the failure probability of CNFETs to provide some material and process guidelines for the design of CNFETs in gigascale integrated circuits. We use measured CNT spacing distributions within the framework of detailed failure analysis to demonstrate that both the CNT density and the ratio of metallic to semiconducting CNTs play dominant roles in defining the failure probability of CNFETs. Besides, it is argued that the large-scale integration of these devices within an integrated circuit will be feasible only if a specific range of CNT density with an acceptable ratio of semiconducting to metallic CNTs can be adjusted in a typical synthesis process.