Building Of Three Dimensional Structures Using CdS Spheres As Building Block And Their Applications

Recently,there have been increasing interests for the research to high-refractive-index and moderate-refractive index nanospheres?>2.0?because of there strong,optically induced electric and magnetic Mie resonances.But the application of the high-refractive-index?>3.0?dielectric nanospheres has not been expanded broadly because of the complicated physical preparation technology.From the viewpoint of practical applications,the chemical synthesis of moderate-refractive index nanospheres provide a simpler method,so the intensive study of the moderate-refractive index nanospheres is necessary for theory and application.In this paper,the ordered and disordered CdS films were fabricated by Cd S spheres with the theoretical refractive index of 2.5 using the methods of evaporation-induced self-assembly and inkjet printing respectively to explore optical properties and their potential for applications.The monodisperse CdS spheres with different diameters were synthesized by using one-step method.The molar ratio of Cd and thiourea precursor was kept to be 1:1 in the preparation process and reaction time was 6 h.To obtain CdS spheres with diameters of 190nm,240 nm,270 nm,290 nm,320 nm and 350 nm,the concentration of Cd precursor was66.7 mM?80 mM?100 mM?106.7 mM?120 mM and 133.3 mM,respectively.The crystalline phase of products have higher purity,which characterized by XRD and the pattern of the TEM image clearly verified the polycrystalline nature of the products.The CdS films with different diameters were obtained by evaporation-induced self-assembly method on hydrophobic substrates.The reflectance spectra of the films at different angles were measured.Although the photonic stop band has located in infrared region,when the diameter of the CdS sphere is greater than 265 nm,the structural color is obviously observable because of the electromagnetic resonance.Moreover,the resonances exhibit red shifts with the increase of the nanosphere diameter.Excitingly,the brilliant cyan,green,and orangey-red colors matched with the peak position of 490 nm,523 nm and 603 nm were achieved using 265,310,and 375 nm CdS spheres,respectively.In this paper,we propose a new mechanism to generate structural color.On the basis of the new mechanism,we developed a very simple method similar to the process we usually use to print in our daily life with inkjet printing structural color patterns on commercially available paper without any pretreatment.The essence of this strategy is the ink,which is prepared using monodisperse CdS spheres.Single-and double-colored structural color patterns were designed on computer and printed using inks containing 270,290,315,and 335 nm CdS spheres,respectively.It is clear that when the incident angle is fixed at 10°,the brightness of the structural color is highest during the viewing angle from 8°to 15°,nearly the specular angle.However,when the viewing angle is larger than 20°,the brightness of the structural color is weaken.It is worth noting that by this method,the prepared patterns can appear clearly and disappear by simply changing the viewing angle.Switching between the invisible and visible patterns of the anticounterfeiting structural color requires no external stimuli,and the printed structures maintain their original form.A sandwiched structure composing a NaYF₄:Yb³⁺/Er³⁺UCNPs layer between two layers of photonic crystals with bandgap at 530 nm and 660 nm was fabricated with CdS spheres of190 nm and 245 nm and demonstrated to effectively separate and enhance the red and green emissions of NaYF₄:Yb³⁺/Er³⁺UCNPs.The highly ordered structures and high refractive index contrast give rise to a strong modulation effect on the emission intensity of UCL.In this structure,each photonic crystal act as a nanofilter to remove one emission band and at the same time to enhance the other emission band.As a result,pure and enhanced red or green emission can be observed from the top side or bottom side of the structure,respectively.Furthermore,CdS spheres with diameter of 365 nm were synthesized to fabricate photonic crystal,which possesses a bandgap overlapped with the 980 nm excitation laser on the bottom of UCNPs layer.The results indicate that the intensity of green and red emissions is enhanced of 10-fold and 5.1-fold respectively due to the increased interaction of 980 nm laser with UCNPs and electromagnetic resonance.