**2. Carbon nanotubes (CNTs)**

CNTs were first discovered under TEM in 1991. It is a one-dimensional tubular material made of SP<sup>2</sup> hybrid carbon atoms. Its diameter ranges from several nanometers to tens of nanometers, and its length can reach centimeter-level at most. According to the wall layer, it can be divided into single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) (**Figure 1**). It is the most commercialized nanofiber with the highest strength and the smallest diameter [19–21]. Moreover, CNTs have good toughness, which can withstand 40% of tensile strain without brittle behavior or fracture phenomenon, thus improving the toughness of matrix composite [22]. CNTs with super high aspect ratio and excellent mechanical and physical properties, such as high strength, high thermal conductivity, high conductivity, and low thermal expansion coefficient, are regarded as the ideal functional modifier for preparing high-performance composite materials [23–25].
