SWNTs

Single-walled carbon nanotubes (SWNTs) are cylindrical nanostructures composed of a single layer of carbon atoms arranged in a hexagonal lattice (graphene) rolled into a seamless tube. Diameters are typically 0.4 to 2 nanometers, and lengths can reach micrometers or longer. The tube’s circumference is characterized by a pair of integers (n,m) that define how the graphene sheet is rolled; this chiral vector also determines the nanotube’s chirality and electronic properties. Armchair tubes (n = m) are typically metallic, while zigzag and chiral tubes fall along a spectrum from metallic to semiconducting depending on whether (n − m) is a multiple of 3.

Electronic and mechanical properties: SWNTs exhibit exceptional mechanical strength and stiffness, with a high Young’s modulus around 1 TPa and tensile strengths in the tens of GPa. Electrically, they can behave as metals or semiconductors depending on chirality; metallic SWNTs conduct with low resistance, while semiconducting ones have band gaps that depend on diameter, typically in the range of 0.6–1.5 eV for common sizes. Thermal conductivity along the tube axis is among the highest for any material, reaching several thousand watts per meter-kelvin.

Synthesis and purification: SWNTs are produced by arc discharge, laser ablation, and chemical vapor deposition (CVD). Each method yields mixtures of chiralities and diameters, necessitating purification and separation to obtain uniform samples. Separation techniques include density gradient ultracentrifugation, DNA-assisted separation, and selective chemistry to enrich for specific chiralities or electronic types.

Applications and challenges: Potential uses span nanoelectronics (transistors, interconnects), sensors, reinforcement for composites, and energy storage. Key challenges include scalable, selective synthesis of desired chirality, batch variability, and integration into fabrication technologies.

History: SWNTs were first observed in 1991 by Sumio Iijima, who reported multi-walled nanotubes; subsequent work identified single-walled variants and established their unique properties. Ongoing research focuses on controlled synthesis, chirality sorting, and device integration.