Top-down approach:
1. It involves the reduction of particle size to nanometer size.
2. The process is used only for hard and brittle materials
3. Not all particles in the precursor may decompose to the required particle size.
4. Nanocrystalline materials prepared by this process may be contaminated by grinding tools and the atmosphere.
5. Ex. Ball milling,

An in-depth approach:
1. It involves the growth of nanoparticles from atomic-sized particles.
2. The process is also used for gases, liquids and solids.
3. More control over particle size.
4. Less risk of contamination.
5. Ex. Sol gel method, gas condensation method

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