Fabrication And Properties Of Cu₂O And CdS Photonic Crystal Films With Low Angle-dependent And Highly Stable Structures

Three-dimensional photonic crystals with brilliant colors have shown great potential in many application areas,such as display devices and sensors.However,the angle dependence and the intrinsic instability severely restrict their practical applications.In this thesis,aimed to solve above scientific issues,monodisperse Cu2O and CdS colloidal spheres with high refractive index were prepared and selected as the building blocks for fabricating three-dimensional photonic crystals.The angle dependence of the observed structural colors was suppressed by increasing the effective refractive index of the whole photonic crystal structure.Meanwhile,a one-step in-situ embedding strategy was developed for fabricating composite photonic crystal films with highly stable and brilliant structural colors.The nucleation rate of Cu2O nanocrystal was controlled based on the temperature-dependent reducing power of diethylene glycol.And the nucleation rate of CdS nanocrystal was controlled based on the temperature-dependent decomposition rate of thiourea.The nucleation and aggregation growth processes were separated by using a two-step method and tuning reaction temperatures and concentrations of reactants.First,nanocrystals were prepared through fast reactions at relatively high temperatures.The obtained nanocrystals served as seeds for controlling the diameter distribution of the spherical aggregates.Then at relatively low temperatures,nanocrystals aggregated to form spheres through slow reaction processes,which lead to monodisperse Cu2O spheres(diameter:140?190 nm)and CdS spheres(diameter:170?220 nm)with suitable diameters for fabricating photonic crystal structural colored materials.Photonic crystal films with various colors were fabricated with Cu2O and CdS spheres with high refractive index.By increasing the effective index of the photonic crystal structure,the angle dependence of the obtained structural colors has been effectively suppressed.Brilliant colors were obtained due to Bragg diffraction from the highly ordered arrays.The saturation of the obtained colors was high due to the absorption of visible light from spheres.As the incident angle and viewing angle were changed from 10° to 45°,the green,golden yellow and red colors of the Cu2O photonic crystal films obtained from 145 nm,165 nm and 187 nm Cu2O spheres remained almost the same,with reflection spectra peak shifts being 14 nm,15 nm and 23 nm,respectively.Monodisperse SiO2,PS,CdS and Cu2O spheres with similar diameters were selected as building blocks to fabricate photonic crystal films for comparison.The results reveal that,with the increase of the refractive index of building blocks,the angle dependence of the obtained structural colors decreased gradually.The results prove that the angle dependence of the structural colors from three-dimensional photonic crystals can be effectively suppressed by using spheres with high refractive index as building blocks.Nitrocellulose varnish films were selected to combine with CdS photonic crystals due to their good mechanical properties.The films containing proper amount of solvent(5?40 wt%)were used as substrates and CdS spheres were selected as building blocks of photonic crystal.Ethanol,which can be miscible with the solvent of polymer to form good solvents for the polymer substrate,was selected to disperse CdS spheres.When the dispersion was dropped onto the polymer film,polymer dissolved into the mixed solvent and CdS spheres dropped into the dissolved polymer.With the evaporation of solvents,CdS spheres self-assembled into highly ordered arrays with polymer precipitating at the interstice among spheres simultaneously.By using this in-situ embedding strategy,composite photonic crystal films with brilliant colors and high mechanical strength can be fabricated in one-step,with hardness and modulus of the film being about 0.23 GPa and 6 GPa,respectively.