Influence of cobalt-metal concentration on the microstructure and optical limiting properties of PVA

The optical limiting characteristics of the Cobalt (Co) metal-doped PVA films have been measured using two laser beams at 632.8 nm and 533 nm wavelength. Different weight percentages of Co metal were stirred with PVA solution to get polymeric composite films by casting method. The crystallinity degree and the intermolecular structure of the samples have been tested by X-ray diffraction (XRD) and FT-IR spectroscopy. The semi-crystalline order of the pure PVA reduced with more adding of Co-metal due to the interaction of dopant ions with one pair of electrons of oxygen in the hydroxyl (-OH) group of the matrix as confirmed by the FTIR spectra. Moreover, the formation of the Co clusters with different size and non-uniform distribution on the composite films surface morphology has been presented by SEM. The thermal stability was carried out via DTA (Differential thermal analysis) and revealed that the Co-metal/PVA composites are more stable than PVA pure. The doped PVA film absorption increased, while the absorption edge shifted from 5.33 eV to 4.61 eV with the Co metal ratio due to the formation of localized states in the forbidden gap during the interaction between the -OH groups in the matrix of PVA and Co-metals. Also, the Urbach energy of the present films has been estimated. The direct electronic transitions have been observed only at doping >= 7.5%, as established from the results obtained from Tauc's model and optical dielectric loss. The film with ten wt.% of Co-metal shows a small energy gap and significant absorption properties. Thus, the present polymeric composite films with high Co-metal concentrations are suitable for laser attenuation and optical limiting in photonic devices.