High-Throughput Determination of Statistical Structure Information for Horizontal Carbon Nanotube Arrays by Optical Imaging

Assembly and Alignment of High Packing Density Carbon Nanotube Arrays Using Lithographically Defined Microscopic Water Features

High packing density aligned arrays of semiconducting carbon nanotubes (CNTs) are required for many electronics applications. Past work has shown that the accumulation of CNTs at a water-solvent interface can drive array self-assembly. Previously, the confining interface was a large-area, macroscopic feature. Here, we report on the CNT assembly on microscopic water features. Water microdroplets are formed on 10-100 μm wide hydrophilic stripes patterned on a substrate. Exposure to CNTs dispersed in solvent accumulates CNTs at the microdroplet-solvent interface, driving their alignment and deposition at the microdroplet-solvent-substrate contact line. Compared with macroscopic methods in which the contact line uncontrollably moves across the substrate as it is pulled out of the liquids, the hydrophilic patterns and microdroplets allow pinning of the contact line. As CNTs deposit, the contact line self-translates, allowing for dense CNT packing. We realize monolayer CNT arrays aligned within ±3.9° at density of 250 μm-1 and field effect transistors with a high current density of 1.9 mA μm-1 and transconductance of 1.2 mS μm-1 at -0.6 V drain bias and 60 nm channel length.

Soft-lock drawing of super-aligned carbon nanotube bundles for nanometre electrical contacts

The assembly of single-walled carbon nanotubes (CNTs) into high-density horizontal arrays is strongly desired for practical applications, but challenges remain despite myriads of research efforts. Herein, we developed a non-destructive soft-lock drawing method to achieve ultraclean single-walled CNT arrays with a very high degree of alignment (angle standard deviation of ~0.03°). These arrays contained a large portion of nanometre-sized CNT bundles, yielding a high packing density (~400 µm^-1) and high current carrying capacity (∼1.8 × 10⁸ A cm^-2). This alignment strategy can be generally extended to diverse substrates or sources of raw single-walled CNTs. Significantly, the assembled CNT bundles were used as nanometre electrical contacts of high-density monolayer molybdenum disulfide (MoS₂) transistors, exhibiting high current density (~38 µA µm^-1), low contact resistance (~1.6 kΩ µm), excellent device-to-device uniformity and highly reduced device areas (0.06 µm² per device), demonstrating their potential for future electronic devices and advanced integration technologies.

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.

Real-Time Observation of Carbon Nanotube Etching Process Using Polarized Optical Microscope

Controllable synthesis of carbon nanotubes (CNTs) is of great importance in its further application, which attracts broad attention. As growth and etching are the two sides in the process of material crystallography and the control of the competition between them forms the foundation for modern technology of materials design and manufacture, the understanding on etching process of carbon nanotubes is still very unclear because technically it is of great challenge to characterize the dynamics in such small one-dimensional (1D) scale. Here the real-time investigation on the etching process of CNTs is reported, by the hot-wall chemical reactor equipped with a polarized optical microscope. It is discovered that the CNT etching behavior in air is totally of random, including the etching sites, termination sites, and structure dependence. Combining with the dynamic simulation, it is revealed that the random behavior reflects the unique "self-termination" phenomenon. A structure-independent etching propagation barrier of 2.4 eV is also obtained, which indicates that the etching propagation process still follows the conventional Kinetic Wulff construction theory. The results represent the new knowledge on the etching process in carbon nanotube and can contribute to its selective enrichment. Furthermore, the "self-termination" phenomenon may be a universal behavior in 1D process.

High-Throughput Optical Imaging and Spectroscopy of One-Dimensional Materials

Direct visualization of one-dimensional (1D) materials under an optical microscope in ambient conditions is of great significance for their characterizations and applications. However, it is full of challenges to achieve such goal due to their relative small size (ca. 1 nm in diameter) in the optical-diffraction-limited laser spot (ca. 1 μm in diameter). In this Concept article, we introduce a polarization-based optical homodyne detection method that can be used as a general strategy to obtain high-throughput, real-time, optical imaging and in situ spectroscopy of polarization-inhomogeneous 1D materials. We will use carbon nanotubes (CNTs) as an example to demonstrate the applications of such characterization with respect to the absorption signal of individual nanotubes, real-time imaging of individual nanotubes in devices, and statistical structure information of nanotube arrays.

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.