Fabrication of ZnO colloidal photonic crystal by spin-coating method

1. Fabrication of ZnO colloidal photonic crystal by spin-coating method Speaker:Tzung-Wei Yu Advicer:Ja-Hon Lin

2. Outline • Introduction • Experimental detail • Results and discussion • Conclusions

3. Introduction • PCs(photonic crystals) are periodic dielectric structures designed to control the propagation of electromagnetic (EM) waves [1] • One of the characteristic properties of PCs is the presence of a photonic band gap (PBG) • At the PBG wavelength, propagation of photons is inhibited inside the PC structure • Colloidal self-assembly is a simpler and less costly approach to generating large-scale PCs [1] Solvothermal synthesis of 3D photonic crystals based on ZnS/opal system

4. Introduction • The self-assembly method is the large concentration of various types of intrinsic defects such as plane stacking faults, dislocations, vacancies • We have proposed a tunable PC, in which the PBG can be tuned as desired by controlling ZnO colloidal spheres size

5. Experimental The method of synthesis ZnO colloidal spheres involves two steps • (1)0.02 mol. Zinc acetate dehydrate (ZnAc) was added into 150ml diethylene glycol (DEG) Heating under 180 ℃ Discard Product : polydisperse powder supernatant Second step

6. Producing the monodisperse ZnO spheres (2)The same way as a primary reaction Heating at 170 ℃ Supernatant was added into the solution(DEG) Temperature drop After reaching 180 ℃ The reaction was stirred for 1 h, after which the heat source was removed, and the flask cooled to room temperature

7. Producing colloidal photonic crystals The suspension was dropped onto glass substrates, which were rotated at 3000RPM for 0.5min, after deposited by spin coating The films were dried at 200 ℃ for 15 min to evaporate the solvent and remove organic residuals Then the films were inserted to a furnace and annealed at 550 ℃ for 30 min

8. SEM images of ZnO PCs • Our method results in a highly ordered structure • Indeed such flawless and surface smooth region can extend over areas of about several micrometers • The colloidal spheres assemble in a hexagonal symmetry, consistent with (111) growth direction in a face-centered cubic(fcc)

9. Optical transmission of ZnO PCs with different spheres size for normal incidence • Optical transmissions are shown for different sizes of colloidal spheres • The position of the stop band changes systematically with the size of the colloidal spheres according to Bragg’s law at normal incidence

10. Bragg’s law at normal incidence λc: the position of diffraction peak d: the interplanar space neff the effective refractive index ; In the fcc lattice Eq. (1) reduces to following relation D: the diameter of the particles

11. Relation between stop band position and spheres diameter • The plot of the experimental values of λc (stop band position) versus the diameter D of the ZnO spheres • The line in the figure is the linear fit of the Eq. (2) with a single fitting parameter, neff = 1.80 [2] [2] Synthesis of thin film titania photonic crystals through a dip-infiltrating sol–gel process.

12. Results and discussion • We can estimate neff independently using the following effective medium relation: f : the filling factor of the ZnO in the photonic crystals nZnO and nair: the refractive indices of the ZnO spheres and the air voids Assuming the packing density of ZnO spheres to be 0.74, corresponding to close packed fcc crystal, we obtain neff =1.877,in a good agreement with our experimental results

13. Conclusions • ZnO colloidal photonic crystals were prepared on glass substrates by spin-coating method • The optical transmission experiments yielded orderedcrystalline structure • It is found that with the decrease of the size of colloidal spheres, the reflectance spectra of these ordered structures blue shift