It is of great importance to develop new micro-actuators with high performance by optimizing the structures and materials. Here we develop a VO₂/Al₂O₃/CNT eccentric coaxial nanofiber, which can be potentially applied as a micro-actuator. The specific eccentric coaxial structure was efficiently fabricated by conventional thin film deposition methodology with individual CNT templet. Activated by thermal and photothermal stimuli, the as-developed actuator delivers a bidirectional actuation behavior with large amplitudes and an ultra-fast response, ~ 2.5 mS. A tweezer can be further made by assembling two such nanofibers symmetrically onto a tungsten probe. Clamping and unclamping can be realized by laser stimulus. More experimental and simulation investigations indicated that the actuation behaviors could be attributed to the nanostructured eccentric coaxial geometry, the thermal coefficient mismatch between layers and the fast phase transition of VO₂. The micro-actuators will have potentials in micro manipulators, nanoscaled switches, remote controls and other autonomous systems. Furthermore, a large variety of coaxial and eccentric coaxial nanofibers with various functions can also be developed, giving the as-developed methodology more opportunities.

Scanning electron microscopy (SEM) plays an indispensable role in nanoscience and nanotechnology because of its high efficiency and high spatial resolution in characterizing nanomaterials. Recent progress indicates that the contrast arising from different conductivities or bandgaps can be observed in SEM images if single-walled carbon nanotubes (SWCNTs) are placed on a substrate. In this study, we use SWCNTs on different substrates as model systems to perform SEM imaging of nanomaterials. Substantial SEM observations are conducted at both high and low acceleration voltages, leading to a comprehensive understanding of the effects of the imaging parameters and substrates on the material and surface-charge signals, as well as the SEM imaging. This unified picture of SEM imaging not only furthers our understanding of SEM images of SWCNTs on a variety of substrates but also provides a basis for developing new imaging recipes for other important nanomaterials used in nanoelectronics and nanophotonics.

Single-walled carbon nanotube (SWCNT) films with a high density exhibit broad functionality and great potential in nanodevices, as SWCNTs can be either metallic or semiconducting in behavior. The films greatly benefit from characterization technologies that can efficiently identify and group SWCNTs based on metallic or semiconducting natures with high spatial resolution. Here, we developed a facile imaging technique using scanning electron microscopy (SEM) to discriminate between semiconducting and metallic SWCNTs based on black and white colors. The average width of the single-SWCNT image was reduced to ~9 nm, ~1/5 of previous imaging results. These achievements were attributed to reduced surface charging on the SiO₂/Si substrate under enhanced accelerating voltages. With this identification technique, a CNT transistor with an on/off ratio of > 10⁵ was fabricated by identifying and etching out the white metallic SWCNTs. This improved SEM imaging technique can be widely applied in evaluating the selective growth and sorting of SWCNTs.

Aligned carbon nanotube films coated with amorphous carbon were developed into novel templates by atomic layer deposition. Freestanding macroscopic metal-oxide nanotube films were then successfully synthesized by using these templates. The reactive amorphous carbon layer greatly improved the nuclei density, which ensured the high quality of the films and allowed for precise control of the wall thickness of the nanotubes. Using template-synthesized alumina nanotube films, we demonstrate a humidity sensor with a high response speed, a transmission electron microscopy (TEM) grid, and a catalyst support. The cross-stacked assembly, ultrathin thickness, chemical inertness, and high thermal stability of the alumina nanotube films contributed to the excellent performance of these devices. In addition, it is expected that the metal-oxide nanotube films would have significant potential owing to their material richness, macroscopic appearance, flexibility, compatibility with the semiconducting technologies, and the feasibility of mass production.

Transfer printing of nanomaterials onto target substrates has been widely used in the fabrication of nanodevices, but it remains a challenge to fully avoid contamination introduced in the transfer process. Here we report a metal-film-assisted method to realize an ultra-clean transfer of single-walled carbon nanotubes (SWCNTs) mediated by poly(methyl methacrylate) (PMMA). The amount of PMMA residue can be greatly reduced due to its strong physical adhesion to the metal film, leading to ultra-clean surfaces of both the SWCNTs and the substrates. This metal-film-assisted transfer method is efficient, nondestructive, and scalable. It is also suitable for the transfer of graphene and other nanostructures. Furthermore, the relatively low temperature employed allows this technique to be compatible with nanomaterial-based flexible electronics.

Spinning carbon nanotube (CNT) yarns from super-aligned carbon nanotube (SACNT) arrays is a promising approach to fabricate high-strength fibers. However the reported tensile strengths of the as-prepared fibers are far below that of an individual CNT. It is therefore still a challenge to improve their mechanical strengths. Here we report that the tensile strengths and Young's moduli can be further improved to 2.2 GPa and 200 GPa respectively, if we first treat the SACNT array with oxygen plasma by using a reactive ion etching (RIE) facility, then dry spin yarns from it and make composite fibers with polyvinyl alcohol. According to the experimental results obtained using scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), the improvement is attributed to the oxygen RIE process, as it can create functional groups on the outer walls of CNTs and thus improve the interaction between the CNTs and the polymer molecules.