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The structure of a carbon nanotube is key to its remarkable properties. A single-walled carbon nanotube (SWCNT) can be conceptualized by taking a sheet of graphene and rolling it into a seamless cylinder. The way the sheet is rolled is defined by a chiral vector, [latex]vec{C}_h = nvec{a}_1 + mvec{a}_2[/latex], where [latex]vec{a}_1[/latex] and [latex]vec{a}_2[/latex] are the graphene lattice vectors and (n, m) are integers. This chiral vector dictates the nanotube’s diameter and its electronic properties. If n = m, the nanotube is called “armchair” and is always metallic. If n – m is a multiple of 3, the nanotube is a narrow-gap semiconductor or quasi-metallic. Otherwise, it is a moderate-gap semiconductor. This extreme sensitivity of electronic properties to subtle structural changes is a unique feature of CNTs.

Multi-walled carbon nanotubes (MWCNTs) consist of multiple concentric cylinders of graphene, nested like Russian dolls. They are generally easier and cheaper to produce than SWCNTs but have more complex properties due to inter-wall interactions. Their mechanical strength is exceptional. The strong sp² carbon-carbon bonds make CNTs one of the stiffest and strongest materials ever discovered, with a Young’s modulus over 1 TPa and tensile strength up to 100 GPa, far exceeding that of steel. Their thermal conductivity along the tube axis can also surpass that of diamond. These properties make them ideal reinforcement agents in composite materials. However, challenges remain in achieving uniform dispersion within a matrix and in producing CNTs with a single, selected chirality on a large scale, which has limited their application in electronics where purity is paramount.