Acoustic-assisted assembly of an individual monochromatic ultralong carbon nanotube for high on-current transistors

Fluidization and Application of Carbon Nano Agglomerations

Carbon nanomaterials are unique with excellent functionality and diverse structures. However, agglomerated structures are commonly formed because of small-size effects and surface effects. Their hierarchical assembly into micro particles enables carbon nanomaterials to break the boundaries of classical Geldart particle classification before stable fluidization under gas-solid interactions. Currently, there are few systematic reports regarding the structural evolution and fluidization mechanism of carbon nano agglomerations. Based on existing research on carbon nanomaterials, this article reviews the fluidized structure control and fluidization principles of prototypical carbon nanotubes (CNTs) as well as their nanocomposites. The controlled agglomerate fluidization technology leads to the successful mass production of agglomerated and aligned CNTs. In addition, the self-similar agglomeration of individual ultralong CNTs and nanocomposites with silicon as model systems further exemplify the important role of surface structure and particle-fluid interactions. These emerging nano agglomerations have endowed classical fluidization technology with more innovations in advanced applications like energy storage, biomedical, and electronics. This review aims to provide insights into the connections between fluidization and carbon nanomaterials by highlighting their hierarchical structural evolution and the principle of agglomerated fluidization, expecting to showcase the vitality and connotation of fluidization science and technology in the new era.

Monochromatic Carbon Nanotube Tangles Grown by Microfluidic Switching between Chaos and Fractals

The nature of chaos is in that elusive flow that is an advanced order out of our vision. It is wise to take advantage of chaos after recognizing or modifying its unique fractal properties. Here, a magnetron weaving strategy was developed for producing chaotic but monochromatic carbon nanotube tangles (CNT-Ts) under Kelvin-Helmholtz instability (KHI). The self-similarity characteristic facilitated individual ultralong CNTs to manipulate their entropy-driven fractal geometry, resulting in ∼10⁴ μm² CNT-Ts with variable curvature radius. In addition, based on the rate-selected mechanism, 85% metallic and ∼100% semiconducting CNT-Ts were synthesized and separated simultaneously at different length positions. After ex situ modifying their fractal into aligned CNTs with hydrogel, these CNT-Ts delivered a current of 10 μA μm^-1 in transistors with an on/off ratio >10⁷. It has provided the third route as a paradigm of applying one-dimensional nanomaterials by switching between chaos and fractal, in parallel with that of direct synthesis and postseparation.

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.

Super-durable ultralong carbon nanotubes

Fatigue resistance is a key property of the service lifetime of structural materials. Carbon nanotubes (CNTs) are one of the strongest materials ever discovered, but measuring their fatigue resistance is a challenge because of their size and the lack of effective measurement methods for such small samples. We developed a noncontact acoustic resonance test system for investigating the fatigue behavior of centimeter-long individual CNTs. We found that CNTs have excellent fatigue resistance, which is dependent on temperature, and that the time to fatigue fracture of CNTs is dominated by the time to creation of the first defect.

Chirality Pure Carbon Nanotubes: Growth, Sorting, and Characterization

Single-walled carbon nanotubes (SWCNTs) have been attracting tremendous attention owing to their structure (chirality) dependent outstanding properties, which endow them with great potential in a wide range of applications. The preparation of chirality-pure SWCNTs is not only a great scientific challenge but also a crucial requirement for many high-end applications. As such, research activities in this area over the last two decades have been very extensive. In this review, we summarize recent achievements and accumulated knowledge thus far and discuss future developments and remaining challenges from three aspects: controlled growth, postsynthesis sorting, and characterization techniques. In the growth part, we focus on the mechanism of chirality-controlled growth and catalyst design. In the sorting part, we organize and analyze existing literature based on sorting targets rather than methods. Since chirality assignment and quantification is essential in the study of selective preparation, we also include in the last part a comprehensive description and discussion of characterization techniques for SWCNTs. It is our view that even though progress made in this area is impressive, more efforts are still needed to develop both methodologies for preparing ultrapure (e.g., >99.99%) SWCNTs in large quantity and nondestructive fast characterization techniques with high spatial resolution for various nanotube samples.

Rate-selected growth of ultrapure semiconducting carbon nanotube arrays

Carbon nanotubes (CNTs) are promising candidates for smart electronic devices. However, it is challenging to mediate their bandgap or chirality from a vapor-liquid-solid growth process. Here, we demonstrate rate-selected semiconducting CNT arrays based on interlocking between the atomic assembly rate and bandgap of CNTs. Rate analysis confirms the Schulz-Flory distribution which leads to various decay rates as length increases in metallic and semiconducting CNTs. Quantitatively, a nearly ten-fold faster decay rate of metallic CNTs leads to a spontaneous purification of the predicted 99.9999% semiconducting CNTs at a length of 154 mm, and the longest CNT can be 650 mm through an optimized reactor. Transistors fabricated on them deliver a high current of 14 μA μm⁻¹ with on/off ratio around 10⁸ and mobility over 4000 cm² V⁻¹ s⁻¹. Our rate-selected strategy offers more freedom to control the CNT purity in-situ and offers a robust methodology to synthesize perfectly assembled nanotubes over a long scale.

Carbon nanotubes are considered promising materials for microelectronics, but it is challenging to separate semiconducting tubes from their metallic counterparts. Here, the authors report a self-purification growth process that allows them to obtain long, highly pure semiconducting carbon nanotubes.

Storage of Mechanical Energy Based on Carbon Nanotubes with High Energy Density and Power Density

Energy storage in a proper form is an important way to meet the fast increase in the demand for energy. Among the strategies for storing energy, storage of mechanical energy via suitable media is widely utilized by human beings. With a tensile strength over 100 GPa, and a Young's modulus over 1 TPa, carbon nanotubes (CNTs) are considered as one of the strongest materials ever found and exhibit overwhelming advantages for storing mechanical energy. For example, the tensile-strain energy density of CNTs is as high as 1125 Wh kg^-1 . In addition, CNTs also exhibit great potential for fabricating flywheels to store kinetic energy with both high energy density (8571 Wh kg^-1 ) and high power density (2 MW kg^-1 to 2 GW kg^-1 ). Here, an overview of some typical mechanical-energy-storage systems and materials is given. Then, theoretical and experimental studies on the mechanical properties of CNTs and CNT assemblies are introduced. Afterward, the strategies for utilizing CNTs to store mechanical energy are discussed. In addition, macroscale production of CNTs is summarized. Finally, future trends and prospects in the development of CNTs used as mechanical-energy-storage materials are presented.

A 3D nanoelectrokinetic model for predictive assembly of nanowire arrays using floating electrode dielectrophoresis

Floating electrode dielectrophoresis (FE-DEP) presents a promising avenue for scalable assembly of nanowire (NW) arrays on silicon chips and offers better control in limiting the number of deposited NWs when compared with the conventional, two-electrode DEP process. This article presents a 3D nanoelectrokinetic model, which calculates the imposed electric field and its resultant NW force/velocity maps within the region of influence of an electrode array operating in the FE-DEP configuration. This enables the calculation of NW trajectories and their eventual localization sites on the target electrodes as a function of parameters such as NW starting position, NW size, the applied electric field, suspension concentration, and deposition time. The accuracy of this model has been established through a direct quantitative comparison with the assembly of manganese dioxide NW arrays. Further analysis of the computed data reveals interesting insights into the following aspects: (a) asymmetry in NW localization at electrode sites, and (b) the workspace regions from which NWs are drawn to assemble such that their center-of-mass is located either in the inter-electrode gap region (desired) or on top of one of the assembly electrodes (undesired). This analysis is leveraged to outline a strategy, which involves a physical confinement of the NW suspension within lithographically patterned reservoirs during assembly, for single NW deposition across large arrays with high estimated assembly yields on the order of 87%.

Carbon nanotube bundles with tensile strength over 80 GPa

Carbon nanotubes (CNTs) are one of the strongest known materials. When assembled into fibres, however, their strength becomes impaired by defects, impurities, random orientations and discontinuous lengths. Fabricating CNT fibres with strength reaching that of a single CNT has been an enduring challenge. Here, we demonstrate the fabrication of CNT bundles (CNTBs) that are centimetres long with tensile strength over 80 GPa using ultralong defect-free CNTs. The tensile strength of CNTBs is controlled by the Daniels effect owing to the non-uniformity of the initial strains in the components. We propose a synchronous tightening and relaxing strategy to release these non-uniform initial strains. The fabricated CNTBs, consisting of a large number of components with parallel alignment, defect-free structures, continuous lengths and uniform initial strains, exhibit a tensile strength of 80 GPa (corresponding to an engineering tensile strength of 43 GPa), which is far higher than that of any other strong fibre.

Highly Dispersible Buckled Nanospring Carbon Nanotubes for Polymer Nano Composites

We propose the unique structure of highly dispersible single-walled carbon nanotubes (SWCNTs) in various solvents and polymers using the ZnO nano particle template. Buckled nanospring-shaped carbon nanotubes (NS-CNTs) were synthesized by a chemical reaction of ZnO nanoparticles with acid-treated SWCNTs and then dissolving ZnO through chemical etching. The unique structure of distorted hexagonal NS-CNTs encircled around ZnO nanoparticles was formed by the bending of SWCNTs caused by the agglomeration of chemically adsorbed Zn(OH)₂, which is further crystallized as the polycrystalline ZnO inner core. The highly dispersible NS-CNTs could be incorporated in the poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)] copolymer, one of widely studied ferro- and piezo-electric polymer, up to the value of 15 wt% as nanofillers. The relative dielectric constant (K) of polymer nanocomposite, at 1 kHz, was greatly enhanced from 12.7 to the value of 62.5 at 11 wt% of NS-CNTs, corresponding to a 492% increase compared to that of pristine P(VDF-TrFE) with only a small dielectric loss tangent (D) of 0.1.

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

Abstract

Carbon-based metal-free catalysts possess desirable properties such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, strong stability in acidic/alkaline conditions, and environmental friendliness. Because of these properties, these catalysts have recently received increasing attention in energy and environmental applications. Subsequently, various carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy and environmental protection through metal-free electrocatalysis. This article provides an up-to-date review of this rapidly developing field by critically assessing recent advances in the mechanistic understanding, structure design, and material/device fabrication of metal-free carbon-based electrocatalysts for clean energy conversion/storage and environmental protection, along with discussions on current challenges and perspectives.

Graphical Abstract

Single-Carbon-Nanotube Manipulations and Devices Based on Macroscale Anthracene Flakes

Because of the outstanding mechanical and electrical properties of carbon nanotubes (CNTs), a CNT-based torsion pendulum is demonstrated to show great potential in nano-electromechanical systems. It is also expected to achieve various manipulations for further characterization and increase device sensitivity using ultrlong CNTs and macroscale moving parts. However, the reported top-down method limits the CNT performance and device size by introducing inevitable contamination and destruction, which greatly hinders the development of single-molecule devices. Here, a bottom-up method is introduced to fabricate heterostructures of anthracene flakes (AFs) and suspended CNTs, providing a nondamaging CNT mechanical measurement before further applications, especially for the twisting behavior, and providing a controllable and clean transfer method to fabricate CNT-based electrical devices under ambient conditions. Based on the unique geometry of CNT/AF heterostructures, various complex manipulations of single-CNT devices are conducted to investigate CNT mechanical properties and prompt novel applications of similar structures in nanotechnology. The AF-decorated CNTs show high Young's modulus (≈1 TPa) and tensile strength (≈100 GPa), and can be considered as the finest and strongest torsional springs. CNT-based torsion balance enables to measure fN-level forces and the torsional spring constant is two orders of magnitude lower than previously reported values.

Observation of Van Hove Singularities and Temperature Dependence of Electrical Characteristics in Suspended Carbon Nanotube Schottky Barrier Transistors

A Van Hove singularity (VHS) is a singularity in the phonon or electronic density of states of a crystalline solid. When the Fermi energy is close to the VHS, instabilities will occur, which can give rise to new phases of matter with desirable properties. However, the position of the VHS in the band structure cannot be changed in most materials. In this work, we demonstrate that the carrier densities required to approach the VHS are reached by gating in a suspended carbon nanotube Schottky barrier transistor. Critical saddle points were observed in regions of both positive and negative gate voltage, and the conductance flattened out when the gate voltage exceeded the critical value. These novel physical phenomena were evident when the temperature is below 100 K. Further, the temperature dependence of the electrical characteristics was also investigated in this type of Schottky barrier transistor.

Centimeter-Long Single-Crystalline Si Nanowires

The elongation of free-standing one-dimensional (1D) functional nanostructures into lengths above the millimeter range has brought new practical applications as they combine the remarkable properties of nanostructured materials with macroscopic lengths. However, it remains a big challenge to prepare 1D silicon nanostructures, one of the most important 1D nanostructures, with lengths above the millimeter range. Here we report the unprecedented preparation of ultralong single-crystalline Si nanowires with length up to 2 cm, which can function as the smallest active material to facilitate the miniaturization of macroscopic devices. These ultralong Si nanowires with augmented flexibility are of emerging interest for flexible electronics. We also demonstrate the first single-nanowire-based wearable joint motion sensor with superior performance to reported systems, which just represents one example of novel devices that can be built from these nanowires. The preparation of ultralong Si nanowires will stimulate the fabrication and miniaturization of electric, optical, medical, and mechanical devices to impact the semiconductor industry and our daily life in the near future.

Universal Selective Dispersion of Semiconducting Carbon Nanotubes from Commercial Sources Using a Supramolecular Polymer

Selective extraction of semiconducting carbon nanotubes is a key step in the production of high-performance, solution-processed electronics. Here, we describe the ability of a supramolecular sorting polymer to selectively disperse semiconducting carbon nanotubes from five commercial sources with diameters ranging from 0.7 to 2.2 nm. The sorting purity of the largest-diameter nanotubes (1.4 to 2.2 nm; from Tuball) was confirmed by short channel measurements to be 97.5%. Removing the sorting polymer by acid-induced disassembly increased the transistor mobility by 94 and 24% for medium-diameter and large-diameter carbon nanotubes, respectively. Among the tested single-walled nanotube sources, the highest transistor performance of 61 cm²/V·s and on/off ratio >10⁴ were realized with arc discharge carbon nanotubes with a diameter range from 1.2 to 1.7 nm. The length and quality of nanotubes sorted from different sources is compared using measurements from atomic force microscopy and Raman spectroscopy. The transistor mobility is found to correlate with the G/D ratio extracted from the Raman spectra.

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.