**3.1 Zero-dimensional nanostructures**

The significant progress has been made in the field of zero-dimensional nanostructures. A rich variety of physical and chemical methods have been proposed to synthesize zero-dimensional nanostructures. Recently, zero-dimensional nanostructures such as uniform particle arrays (quantum dots), heterogeneous particle arrays, core-shell quantum dots, onions, and hollow spheres are being obtained by several research groups [18].
