Bottom-up vs Top-down (in the context of nanoparticle formation)

There are two approaches for the manufacturing of nanomaterials:

• The “top-down” approach, which involves the breaking down of large pieces of material to generate the required nanostructures from them.
• The “bottom-up” approach, which implies assembling single atoms and molecules into larger nanostructures.

Source: GreenFacts

Nanoparticle - Nanotechnology