# Photonic Crystal-Based Sensors for Detecting Alcohol Concentration
## 3 Results and Discussion  
### Characterization of the PS Opal and TiO\({}_{2}\) Inverse Opal Structures  
Figure 3 shows the surface micro profile of the monodispersed PS microarray by AFM. The line profile of the a-b direction exhibited that the average feature size of the PS hemispherical part and the spacing is 244 nm in width. AFM images also noted that a close-packed and ordered arrangement of the synthesized PS microsphere array could be obtained by the self-assembly dip-drawing method. Figure 4a,b respectively show the FE-SEM image of the surface topography and the cross-sectional image of the self-assembled PS opal structure at dip-drawing rate of 1 \(\upmu\)m/s. Though some local defects and vacancies existed in the PS opal structure, most of PS microspheres are uniformly dispersed and  
Figure 3: The AFM image of the PS opal structure: (**a**) Average diameter of PS microspheres was labeled; (**b**) A profile line a–b was exhibited on the top surface of the PS microspheres array; (**c**) 3D image of the ordered close-packed PS opal structure. Figure 2: (**a**) Detail of the apparatus geometry and experimental; (**b**) The top view of the variable angle multifunctional optical characteristic measuring system (MFS-630). arranged in a compact-ordered periodic structure. The orderly arrangement of the PS microspheres was due to van der Waals attraction and electrostatic repulsion between the microspheres, which were balanced at the air/suspension interface [24]. Figure 5a,b are the FE-SEM images of TiO\({}_{2}\) structure after 2 h sintering at 500 and 600 \({}^{\circ}\)C, respectively. From Figure 5, we found that after high temperature sintering, the PS microspheres that had been covered by TiO\({}_{2}\) sol were removed. The titania sintered at 600 \({}^{\circ}\)C has dense porous structure packing, the average dimension of the air-hollows is \(\sim\)200 nm. This value is smaller than the diameter of the synthesized PS microspheres (244 nm) used to form the template, which demonstrates that shrinkage occurs during the sintering process. This fact is not surprising given that the PS microspheres are mesoporous and, in the early stages of sintering, probably decrease in size as water vapor is released from the pores [10]. Figure 6a,b illustrated the XRD patterns of TiO\({}_{2}\) sintered at 500 and 600 \({}^{\circ}\)C, respectively. The amorphous phase of titania could be transformed to the anatase crystalline phase when titania was sintered at 600 \({}^{\circ}\)C [27]. For the sample sintered at 600 \({}^{\circ}\)C, the characteristic peaks associated with the 500 \({}^{\circ}\)C structure (d\({}_{112}\), d\({}_{105}\), and d\({}_{220}\)) disappeared, and a new characteristic peak (d\({}_{204}\)) was detected. The anatase peaks were weakened, and the characteristic peaks of the rutile phase were enhanced, demonstrating that a phase transformation occurred in titania during the sintering process at 600 \({}^{\circ}\)C.  
Figure 4: The (**a**) top view and (**b**) cross sectional scanning electron microscope (SEM) images of the PS opal structure. Figure 5: The SEM images of the TiO\({}_{2}\) inverse opal structure sintered at (**a**) 500 \({}^{\circ}\)C and (**b**) 600 \({}^{\circ}\)C. The dark and hollow regions are air pores, and the bright regions are TiO\({}_{2}\). The reproducibility of the PS opal and TiO\({}_{2}\) inverse opal structures is affected by the following factors: (1) Environmental interference: a slight shaky vibration should be avoided during preparing the opal films, since the PS microarray is prepared by a slow self-assembled dip-drawing method. Undoubtedly, the dip-drawing rate is also an important fabricating parameter for preparing a perfect microarray; (2) Pre-treatment of glass substrate: The PS microspheres cannot be easily attached on the glass substrate owing to the hydrophobic surface. Therefore, oxygen plasma treatment is required; (3) The quality of the PS suspension: The particle size distribution of the synthesized PS microspheres should be very uniform in size.