Methods for making nanomaterials are divided into top-down methods, which break larger materials into smaller pieces, and bottom-up methods, which assemble atoms and molecules. Which is a bottom-up method?

Answer: C. Reducing metal salts or decomposing precursors in solution so that nanoparticles form by nucleation and growth

Reduction or thermal decomposition in solution (chemical liquid-phase synthesis) is a bottom-up method in which atoms and molecules gather to form nuclei that grow into particles. The sol-gel method, condensation from the vapor phase and CVD growth of carbon nanotubes are also bottom-up methods. Ball milling is a top-down method that mechanically crushes large particles into smaller ones; the equipment is simple and suited to mass production, but the size distribution tends to be broad, and contamination from the milling media and introduction of strain occur. Lithography and etching are also top-down methods, removing unwanted parts from a larger film, with the advantage of precise control over shape and position. Bottom-up methods make it easy to control size and shape at the atomic level but face challenges in mass production and positional control.