Every solid material is made up of atoms and atoms have electrons. The arrangement of electrons in a material determines the properties of the material. In bulk materials (when the size of the material is big enough, let’s say more than 100 nm), the electrons are free to move in all the three dimensions (similar to us, as we also live in 3D) and at this level classical physics can explain all the behaviour of the materials. However, if we reduce the size of the material to microscopic scale, where electron movement is restricted at least in once dimension, the properties of the material change dramatically