where h is Planck's constant (6.626*10 34 J s), Eg b is the bulk band gap, R is the radius, m* is the effective mass of the specimen (m*= 29.15*10 31 kg for ZnO), and Eg n is the bandgap at strong absorption edge in our case it is 4.32nm. In our case, the absorbance value of 310 nm corresponds to a computed radius (R) of 2.84nm.