Understanding high-yield catalyst-free growth of horizontally aligned single-walled carbon nanotubes nucleated by activated C60 species

Evolution of catalyst design for controlled synthesis of chiral single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) possess superb properties originating from their unique chiral structures. However, accurately controlling the structure of SWCNTs remains challenging due to the structural similarities of their chiral structures, which hinders their widespread application in various fields, particularly in electronics. In recent years, much effort has been devoted to preparing single chiral SWCNTs by adopting three constructive strategies, including growth condition control for structurally unstable liquid catalysts, employing stable solid catalyst design, and pre-synthesis of carbon seeds with a well-defined shape. This review comprehensively discusses the state-of-the-art developments in these approaches as well as their advantages and disadvantages. Moreover, insights into the key challenges and future directions are provided for acquiring chirally pure SWCNTs.

Synthesis, Sorting, and Applications of Single-Chirality Single-Walled Carbon Nanotubes

The synthesis of high-quality chirality-pure single-walled carbon nanotubes (SWCNTs) is vital for their applications. It is of high importance to modernize the synthesis processes to decrease the synthesis temperature and improve the quality and yield of SWCNTs. This review is dedicated to the chirality-selective synthesis, sorting of SWCNTs, and applications of chirality-pure SWCNTs. The review begins with a description of growth mechanisms of carbon nanotubes. Then, we discuss the synthesis methods of semiconducting and metallic conductivity-type and single-chirality SWCNTs, such as the epitaxial growth method of SWCNT ("cloning") using nanocarbon seeds, the growth method using nanocarbon segments obtained by organic synthesis, and the catalyst-mediated chemical vapor deposition synthesis. Then, we discuss the separation methods of SWCNTs by conductivity type, such as electrophoresis (dielectrophoresis), density gradient ultracentrifugation (DGC), low-speed DGC, ultrahigh DGC, chromatography, two-phase separation, selective solubilization, and selective reaction methods and techniques for single-chirality separation of SWCNTs, including density gradient centrifugation, two-phase separation, and chromatography methods. Finally, the applications of separated SWCNTs, such as field-effect transistors (FETs), sensors, light emitters and photodetectors, transparent electrodes, photovoltaics (solar cells), batteries, bioimaging, and other applications, are presented.

Synthesis, purification, properties and characterization of sorted single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) have attracted significant attention due to their outstanding mechanical, chemical and optoelectronic properties, which makes them promising candidates for use in a wide range of applications. However, as-produced SWCNTs have a wide distribution of various chiral species with different properties (i.e. electronic structures). In order to take full advantage of SWCNT properties, highly purified and well-separated SWCNTs are of great importance. Recent advances have focused on developing new strategies to effectively separate nanotubes into single-chirality and/or semiconducting/metallic species and integrating them into different applications. This review highlights recent progress in this cutting-edge research area alongside the enormous development of their identification and structural characterization techniques. A comprehensive review of advances in both controlled synthesis and post-synthesis separation methods of SWCNTs are presented. The relationship between the unique structure of SWCNTs and their intrinsic properties is also discussed. Finally, important future directions for the development of sorting and purification protocols for SWCNTs are provided.

Confirming the key role of Ar+ ion bombardment in the growth feature of nanostructured carbon materials by PECVD

In order to confirm the key role of Ar⁺ ion bombardment in the growth feature of nanostructured carbon materials (NCMs), here we report a novel strategy to create different Ar⁺ ion states in situ in plasma enhanced chemical vapor deposition (PECVD) by separating catalyst film from the substrate. Different bombardment environments on either side of the catalyst film were created simultaneously to achieve multi-layered structural NCMs. Results showed that Ar⁺ ion bombardment is crucial and complex for the growth of NCMs. Firstly, Ar⁺ ion bombardment has both positive and negative effects on carbon nanotubes (CNTs). On one hand, Ar⁺ ions can break up the graphic structure of CNTs and suppress thin CNT nucleation and growth. On the other hand, Ar⁺ ion bombardment can remove redundant carbon layers on the surface of large catalyst particles which is essential for thick CNTs. As a result, the diameter of the CNTs depends on the Ar⁺ ion state. As for vertically oriented few-layer graphene (VFG), Ar⁺ ions are essential and can even convert the CNTs into VFG. Therefore, by combining with the catalyst separation method, specific or multi-layered structural NCMs can be obtained by PECVD only by changing the intensity of Ar⁺ ion bombardment, and these special NCMs are promising in many fields.

Current Progress in the Chemical Vapor Deposition of Type-Selected Horizontally Aligned Single-Walled Carbon Nanotubes

Exciting electrical properties of single-walled carbon nanotubes show promise as a future class of electronic materials, yet the manufacturing challenges remain significant. The key challenges are to determine fabrication approaches for complex and flexible arrangements of nanotube devices that are reliable, rapid, and reproducible. Realizing regular array structures is an important step toward this goal. Considerable efforts have and are being made in this vein, although the progress to date is somewhat modest. However, there are reasons to be optimistic. Positive steps of being able to control not only the spatial location and diameter of the tubes but also their electronic type (chiral control) are being made. Two primary approaches are being exploited to address the challenges. Tube deposition techniques, on the one hand, and direct growth of the desired tube at the target location are being explored. While this review covers both approaches, the emphasis is on recent developments in the direct fabrication of type-selected horizontally aligned single-walled carbon nanotubes by chemical vapor deposition.

Nearly exclusive growth of small diameter semiconducting single-wall carbon nanotubes from organic chemistry synthetic end-cap molecules

The inability to synthesize single-wall carbon nanotubes (SWCNTs) possessing uniform electronic properties and chirality represents the major impediment to their widespread applications. Recently, there is growing interest to explore and synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable advantage of such processes is that no metal catalysts are used, and the produced nanotubes will be free of metal contamination. Many of these methods, however, suffer shortcomings of either low yield or poor controllability of nanotube uniformity. Here, we report a brand new approach to achieve high-efficiency metal-free growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic characterization, electrical transport measurements, and density functional theory calculations. Our strategy combines bottom-up organic chemistry synthesis with vapor phase epitaxy elongation. We identify a strong correlation between the electronic properties of SWCNTs and their diameters in nanotube growth. This study not only provides material platforms for electronic applications of semiconducting SWCNTs but also contributes to fundamental understanding of the growth mechanism and controlled synthesis of SWCNTs.

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.

Spatial recognition of defects and tube type in carbon nanotube field effect transistors using electrostatic force microscopy

The exceedingly high current capacity of single wall carbon nanotubes (SWCNTs) and the high currents that SWCNT thin films can sustain are driving significant efforts to fabricate high quality horizontally aligned SWCNTs. Dielectrophoresis is being increasingly used to prepare aligned nanotubes. However, the aligned nanotubes are generally of low quality since the processing involved can introduce defects. In addition, in arrays of tubes, tube-tube contact resistance can be high. Electrostatic force microscopy is a technique similar to atomic force microscopy, but in this case a bias voltage between the conductive tip and sample enables one to probe the long range electrostatic force. In this study we confirm that the technique can be successfully used to spatially determine defects, crossover points between tubes and nanotube electronic type.