Barn [b]
A barn (b) is a very small unit of area used mainly in nuclear and particle physics to measure the cross-sectional area of atomic nuclei and particles.
1 barn = 10⁻²⁸ square meters (m²).
The name "barn" humorously refers to something “as big as a barn” in the tiny world of atomic scales, despite its extremely small size.
Square Nanometer [nm²]
A square nanometer (nm²) is a unit of area equal to a square with sides 1 nanometer long (1 nm = 1 billionth of a meter, or 10⁻⁹ m).
It’s used to measure extremely tiny areas, like the surface of molecules or nanomaterials.
1 nm² = 1 × 10⁻¹⁸ m².