Aligned Single-walled Carbon Nanotubes Arrays Research Articles

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.

A Comparison Between Reduced and Intentionally Oxidized Metal Catalysts for Growth of Single‐Walled Carbon Nanotubes

In this article, the authors report a detailed comparative study on the growth of single‐walled carbon nanotubes (SWNTs) from monometallic and its oxide (intentionally oxidized) catalysts. The difference and findings are summarized in the following three aspects. 1) High‐quality SWNTs are also grown from intentionally oxidized Co nanoparticles, but the yield of SWNTs significantly decreases, resulting in random network rather than vertically aligned SWNT arrays. 2) The authors clarify from in‐plane transmission electron microscopy that Co oxide nanoparticles grow SWNTs through an in situ reduction process, which means Co oxide becomes metallic Co after the introduction of ethanol; direct growth from an oxide phase is not observed. 3) Compared to the SWNTs grown by pre‐reduced catalysts, SWNTs grown from intentionally oxidized catalysts show narrower diameter distribution and smaller average diameter from Raman spectra. The strategy also shows similar results in the Ni catalysts. The findings are helpful to improve the controllability of SWNT diameter and morphology, as well as the reproducibility and robustness of CVD process.

Anisotropic Raman‐Enhancement Effect on Single‐Walled Carbon Nanotube Arrays

AbstractThe charge transfer between molecules and materials can modulate the polarizability tensor of the molecules and lead to an enhancement of the Raman scattering. Surface‐enhanced Raman scattering on in‐plane anisotropic layered materials has suggested the crystalline‐axis‐dependent charge interactions between molecules and materials. However, the full understanding of the anisotropic charge transfer process is still lacking. The rigorous anisotropic nature and structural diversity of single‐walled carbon nanotube (SWNT) provide an ideal platform to systematically study the anisotropic charge transfer process. The present work reports the anisotropic Raman enhancement effect of molecules on horizontally aligned SWNT arrays and attribute it to the charge transfer efficiency that depends on the laser polarization direction and the resonance of SWNTs. The Raman signal of probe molecules on SWNT arrays is enhanced and reaches the maximum intensity when the incident laser is polarized along the SWNT axial direction, and the intensity is the minimum if they are perpendicular to each other. The different efficiencies of charge transfer are further confirmed by polarized optical absorption measurements and the energy alignment analysis. The present work provides a sensitive way to study the tunable charge interactions between molecules and anisotropic low‐dimensional materials, which are also important for polarization‐controlled optoelectronic applications.

Preparation of Horizontal Single-Walled Carbon Nanotubes Arrays.

The synthesis of SWNTs has achieved great success with the development of synthetic methodologies. From the viewpoint of exploiting the exceptional electrical properties of single-walled carbon nanotubes (SWNTs) in advanced applications, one of the most difficult challenges is how to assemble the SWNTs with high degrees of alignment and purity in electronic conducting (mainly semiconducting) behavior into functional nanodevices. Numerous approaches have been developed to reach this goal, which could be divided into two categories. One is direct preparation of SWNT arrays on the substrate, and the other is self-assembly of pre-sorted SWNTs from solution. The former one obtains SWNT arrays via chemical vapor deposition (CVD) growth, with the sorting realized by either selective growth or post-growth treatment; the latter one assembles SWNT into arrays from solution, with the sorting process occurring before the aligning process in most cases. This review will highlight both in situ and post-synthetic approaches for preparing samples of aligned arrays of SWNTs with well-defined electronic properties-including the working mechanism for directional growth of SWNTs, growth/sorting methods like catalyst engineering, cloning/cap engineering, in situ etching, and ex situ selective removal/etching for surface-grown SWNT sample, and assembling technologies from SWNT solution such as dielectrophoresis, adsorption on lithographically patterned and/or chemically functionalized substrates, Langmuir-Blodgett and Langmuir-Schaefer techniques, and evaporation-driven self-assembly-and research efforts towards direct growth of arrays of complex SWNT structures.

Large-Scale Direct Patterning of Aligned Single-Walled Carbon Nanotube Arrays Using Dip-Pen Nanolithography

The strength of dip-pen nanolithography (DPN) is the ability to create nano- or microarrays of organic compounds and nanomaterials in a nondestructive and direct-write manner. However, transporting large-sized ink materials, such as carbon nanotubes (CNTs), has been a significant challenge. We report a direct-write patterning of aligned single-walled carbon nanotube (SWNT) arrays on silicon oxide using DPN. The patterned SWNT arrays show a high degree of alignment and controllable width ranging from 2 μm down to 8 nm. Furthermore, field-effect transistors based on these SWNT arrays show p-type characteristic. High-throughput patterning of the aligned SWNTs over a large area was also achieved via polymer pen lithography (PPL). The reported technique will further expand the application of SWNTs to diverse nanoelectronic devices.

Aligned Single-Walled Carbon Nanotube Arrays from Rhodium Catalysts with Unexpected Diameter Uniformity Independent of the Catalyst Size and Growth Temperature

Diameter control in the synthesis of single-walled carbon nanotube (SWNT) arrays is vital to the control of their properties and their integration into practical devices. Controlling the size of catalyst nanoparticles (NPs) and following a tangential nucleation mode has been shown to be an effective approach to diameter control of SWNTs. The size distribution of SWNTs, however, was much wider than that of the catalyst NPs because of the evolution of catalysts at high growth temperatures. Here, we demonstrate that rhodium (Rh) nanoparticles serve as highly active catalysts for the synthesis of dense and aligned arrays of SWNTs and offer unexpected diameter control via a perpendicular nucleation mode. The diameter distribution of SWNTs grown from the Rh catalysts is much narrower than that from conventional iron and copper catalysts and is independent of the size and shape of the catalyst NPs. More importantly, the Rh catalysts have remarkable catalytic activity at reduced temperatures (<800 °C) and exhibit...

Bolometric Properties of Semiconducting and Metallic Single-Walled Carbon Nanotube Composite Films

Single-walled carbon nanotubes (SWNTs) have shown interesting bolometric properties, making them good candidates for the detection of infrared and terahertz radiation. However, little has been reported on the bolometric characteristics of SWNT as a function of their chirality or the possible influence of composite morphology on these properties. The separation of SWNTs based on chirality allows for almost purely semiconductive or metallic SWNTs to be studied. The current study focuses on the bolometric performance of self-assembled composite films of SWNTs. The dependence of these properties on the chirality of the SWNTs was evaluated. To this end, metallic, semiconducting, and a 1:1 mixture of metallic and semiconductive were studied. Also, a theoretical model based on the Wiedemann–Franz law is used to explain the resistance of the SWNT composite films as a function of temperature. Results show that the composite morphology has a significant impact on bolometer performance, with cracked composite films containing highly aligned SWNT arrays suspended over a silicon substrate showing superior responsivity values due to higher thermal isolation. Uncracked composite films showed superior thermal coefficient of resistance values (α = −6.5%/K), however, the responsivity was lower due to lower thermal isolation.

Growth of high-density horizontally aligned SWNT arrays using Trojan catalysts.

Single-walled carbon nanotube (SWNT)-based electronics have been regarded as one of the most promising candidate technologies to replace or supplement silicon-based electronics in the future. These applications require high-density horizontally aligned SWNT arrays. During the past decade, significant efforts have been directed towards growth of high-density SWNT arrays. However, obtaining SWNT arrays with suitable density and quality still remains a big challenge. Herein, we develop a rational approach to grow SWNT arrays with ultra-high density using Trojan catalysts. The density can be as high as 130 SWNTs μm(-1). Field-effect transistors fabricated with our SWNT arrays exhibit a record drive current density of -467.09 μA μm(-1) and an on-conductance of 233.55 μS μm(-1). Radio frequency transistors fabricated on these samples exhibit high intrinsic fT and fMAX of 6.94 and 14.01 GHz, respectively. These results confirm our high-density SWNT arrays are strong candidates for applications in electronics.

Charge trapping in aligned single-walled carbon nanotube arrays induced by ionizing radiation exposure

The effects of near-interfacial trapping induced by ionizing radiation exposure of aligned single-walled carbon nanotube (SWCNT) arrays are investigated via measurements of gate hysteresis in the transfer characteristics of aligned SWCNT field-effect transistors. Gate hysteresis is attributed to charge injection (i.e., trapping) from the SWCNTs into radiation-induced traps in regions near the SWCNT/dielectric interface. Self-consistent calculations of surface-potential, carrier density, and trapped charge are used to describe hysteresis as a function of ionizing radiation exposure. Hysteresis width (h) and its dependence on gate sweep range are investigated analytically. The effects of non-uniform trap energy distributions on the relationship between hysteresis, gate sweep range, and total ionizing dose are demonstrated with simulations and verified experimentally.

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.

Enhanced etching of silicon didioxide guided by carbon nanotubes in HF solution

This paper describes a new method to create nanoscale SiO2 pits or channels using single-walled carbon nanotubes (SWNTs) in an HF solution at room temperature within a few seconds. Using aligned SWNT arrays, a pattern of nanoscale SiO2 channels can be prepared. The nanoscale SiO2 patterns can also be created on the surface of three-dimensional (3D) SiO2 substrate and even the nanoscale trenches can be constructed with arbitrary shapes. A possible mechanism for this enhanced etching of SiO2 has been qualitatively analysed using defects in SWNTs, combined with H3O+ electric double layers around SWNTs in an HF solution.

Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates

We demonstrate the fabrication of solid-state dielectric energy storage materials from self-assembled, aligned single-walled carbon nanotube arrays (VA-SWNTs). The arrays are transferred as intact structures to a conductive substrate and the nanotubes are conformally coated with a thin metal-oxide dielectric and a conductive counter-electrode layer using atomic layer deposition. Experimental results yield values in agreement with those obtained through capacitive modeling using Al 2O 3 dielectric coatings ( C > 20 mF/cm 3), and the solid-state dielectric architecture enables the operation of these devices at substantially higher frequencies than conventional electrolyte-based capacitor designs. Furthermore, modeling of supercapacitor architectures utilizing other dielectric layers suggests the ability to achieve energy densities above 10 W h/kg while still exhibiting power densities comparable to conventional solid-state capacitor devices. This device design efficiently converts the high surface area available in the conductive VA-SWNT electrode to space for energy storage while boasting a robust solid-state material framework that is versatile for use in a range of conditions not practical with current energy storage technology.

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

Sorting out semiconducting single-walled carbon nanotube arrays by preferential destruction of metallic tubes using water

The coexistence of metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWNTs) in grown samples remains an obstacle in the application of SWNTs in nano-electronics. We report herein a rational approach to prepare well-aligned s-SWNTs using water vapor as a weak oxidant to etch the m-SWNTs during or after SWNTs growth. Water vapor with controlled concentration is carried into the furnace by argon gas during or after the SWNTs growth. It is found that the oxidation temperature and the concentration of water have a clear effect on the destruction of m-SWNTs. During SWNT growth (temperature above 800 °C), the introduction of water at a certain concentration only etches the SWNTs with small diameters and shows no selectivity between m-SWNTs and s-SWNTs. After SWNT growth, the water can etch m-SWNTs effectively with optimized oxidation temperatures and water concentrations. Micro-Raman spectra and electrical transport characterization confirm the selective etching effects. The mechanism of the selective etching of SWNTs with water is discussed based on the electronic structures of SWNTs. Using this method, densely packed and well aligned semiconductor SWNT arrays can be obtained. We believe this selective etching approach would largely broaden the application of SWNTs, especially for future nano-electronic and molecular detection devices.

Unexpected Hole Transfer Leads to High Efficiency Single-Walled Carbon Nanotube Hybrid Photovoltaic

We report surprisingly efficient photocurrent generation at individual single-walled carbon nanotube (SWNT) /poly(3-hexylthiophene-2,5-diyl) (P3HT) junctions. Contrary to previous prediction, both semiconducting SWNTs (s-SWNTs) and metallic SWNTs (m-SWNTs) function as efficient hole acceptors. By active tuning of SWNTs' Fermi level, we confirm that P3HT p-dopes both s-SWNT and m-SWNT, and the work function difference between the nanotube and P3HT leads to a built-in voltage driving the efficient exciton dissociation and hole transfer. We further demonstrate square millimeter scale SWNT/P3HT bilayer photovoltaics using horizontally aligned SWNT arrays. Importantly, the devices exhibit greater than 90% effective external quantum efficiency. These key findings will not only enhance our knowledge of photocurrent generation at nanoscale interfaces, but also make selective omission of m-SWNT redundant, promising carbon nanomaterial-based, low-cost, high-efficiency hybrid photovoltaics.

Preferential Growth of Semiconducting Single-Walled Carbon Nanotubes on Substrate by Europium Oxide.

In this paper, we have demonstrated that europium oxide (Eu2O3) is a new type of active catalyst for single-walled carbon nanotubes (SWNTs) growth under suitable conditions. Both random SWNT networks and horizontally aligned SWNT arrays are efficiently grown on silicon wafers. The density of the SWNT arrays can be altered by the CVD conditions. This result further provides the experimental evidence that the efficient catalyst for SWNT growth is more size dependent than the catalysts themselves. Furthermore, the SWNTs from europium sesquioxides have compatibly higher quality than that from Fe/Mo catalyst. More importantly, over 80% of the nanotubes from Eu2O3 are semiconducting SWNTs (s-SWNTs), indicating the preferential growth of s-SWNTs from Eu2O3. This new finding could open a way for selective growth of s-SWNTs, which can be used as high-current nanoFETs and sensors. Moreover, the successful growth of SWNTs by Eu2O3 catalyst provides new experimental information for understanding the preferential growth of s-SWNTs from Eu2O3, which may be helpful for their controllable synthesis.

Orthogonal Growth of Horizontally Aligned Single-Walled Carbon Nanotube Arrays

Direction-controlled growth of horizontally aligned single-walled carbon nanotubes (SWNTs) on r-plane sapphire substrates and their alignment mechanisms are demonstrated. On a flat r-plane substrate, anisotropic nanotube−substrate interaction is known to align SWNTs parallel to the [1101] direction of the sapphire. We find that the introduction of a slight miscut (−1° inclined to the [1101] direction) on the substrate changed the SWNT growth direction by 90°, aligning perpendicular to the [1101] direction. This dramatic change of the growth direction is explained by the contribution of newly proposed one-dimensional surface atomic rows and/or atomic steps appeared on the r-plane. Annealing the substrate in hydrogen atmosphere prior to SWNT growth recovers the original nanotube growth direction, while annealing in air deteriorates the alignment. The direct growth of an orthogonally aligned SWNT array is achieved through optimized surface treatment. Site-selective directional control of aligned SWNTs ...

Parametric Study of Alcohol Catalytic Chemical Vapor Deposition for Controlled Synthesis of Vertically Aligned Single-Walled Carbon Nanotubes

In this study we examine catalyst preparation and chemical vapor deposition (CVD) parameters related to synthesis of single-walled carbon nanotubes (SWNTs) by alcohol catalytic CVD. We show that modifying the catalyst recipe considerably changes the average SWNT diameter, and vertically aligned arrays with an average diameter of 1.5 nm were obtained. The height of vertically aligned SWNT arrays can be significantly enhanced by surface modification of the substrate prior to dip-coating, although this produces SWNTs with larger diameters at the root of the array. We demonstrate patterned growth by combining this method with suppression of SWNT synthesis by formation of a hydrophobic surface. We also consider the effects of ethanol flow rate and thermal decomposition on the chemical environment at the substrate. The growth process is considerably altered by the extent of ethanol decomposition, with sudden termination of the growth occurring in the extreme low-flow (complete decomposition) case.

Transfer and Alignment of Random Single-Walled Carbon Nanotube Films by Contact Printing

We present a simple method to transfer large-area random single-walled carbon nanotube (SWCNT) films grown on SiO(2) substrates onto another surface through a simple contact printing process. The transferred random SWCNT films can be assembled into highly ordered, dense regular arrays with high uniformity and reproducibility by sliding the growth substrate during the transfer process. The position of the transferred SWCNT film can be controlled by predefined patterns on the receiver substrates. The process is compatible with a variety of substrates, and even metal meshes for transmission electron microscopy (TEM) can be used as receiver substrates. Thus, suspended web-like SWCNT networks and aligned SWCNT arrays can be formed over the grids of TEM meshes, so that the structures of the transferred SWCNTs can be directly observed by TEM. This simple technique can be used to controllably transfer SWCNTs for property studies, for the fabrication of devices, or even as support films for TEM meshes.

The controlled growth of horizontally aligned single-walled carbon nanotube arrays by agas flow process

High-density horizontally aligned single-walled carbon nanotubes (SWCNTs)have been grown by annealing Fe catalyst in air and optimizing catalystthickness in the gas flow process. The aligned SWCNT density reaches>60 nanotubes per100 µm and the length can be a few millimeters. Interestingly, the growth of aligned SWCNTarrays can be extended from the catalyst substrate to a downstream substrate across thegap between them by gas flow when the two substrates are put close to eachother, and thus an aligned SWCNT array has been achieved on an isolated cleansubstrate.