Controlling of optical energy gap of Co-ferrite quantum dots in poly (methyl methacrylate) matrix

•Composites of quantum dots of Co-nanoferrite with different sizes in PMMA polymer matrix could be prepared.•There is a blue shift in the energy gap of Co-ferrite with decreasing the crystallite size.•The effective mass and the exciton radius of Co-ferrite particle could be determined from the optical absorption measurements.

Different crystallite sizes of Co-ferrite nanoparticles were prepared and dispersed in the matrix of poly (methyl methacrylate) (PMMA) polymer. The effect of crystallite size on the structure and optical energy gap of Co-nanoferrite/PMMA composite has been studied. The optical energy gap of Co-ferrite was greatly affected by the crystallite size. This result was discussed in terms of the formation of electron–hole exciton using particle in a box model. The effective mass and the Bohr radius of the formed exciton have been calculated from the spectroscopic measurements.