## 2 Morphology Design of Graphene-Based Composites
### Graphene-Based Photonic Crystals  
Photonic crystals (PCs) are periodic optical nanostructures with unique characteristics that affect the propagation of photons, resulting in efficient light harvesting of solar radiation (UV, vis and IR). The most important property of PCs is the formation of photonic bandgap (PBG), at which electromagnetic waves are strictly forbidden to propagate at  
Figure 10: Schematic illustration of the synthesis steps for r-GO/AuNP/m-TiO\({}_{2}\) sandwich-like hollow spheres. Reprinted with permission from [45]. Copyright (2015) ACS. Figure 9: Schematic drawings of: (**a**) titania NPs aggregated on graphene; (**b**) titania nanowires dispersed uniformly on graphene. Reprinted with permission from [44]. Copyright (2012) ACS. certain frequencies, resulting from the periodicity of refractive index (n) within the materials [46]. Consequently, the effect of slow photons (reduction of photons' group velocity) is observed at both shorter and longer edges of PBG wavelengths due to the multiple scattering of photons within the nanostructure. In recent years, the composites of PCs with graphene-based materials (G, GO, rGO) have been explored for numerous applications, such as switch [47], electronic and optoelectronic devices [48; 49; 50; 51], sensors [52; 53], surface-enhanced Raman scattering (SERS) [54], and photocatalysis [47; 55; 56], as exemplary shown in Table 1.  
Various nanostructures of graphene-based PCs have been designed and fabricated, including one-dimensional (1D), e.g., graphene nanolayers stacked and embedded between dielectric materials [57], two-dimensional (2D), e.g., nanocavities [58], periodic cylindrical holes in multilayered graphene [30], and three-dimensional (3D), e.g., inverse opal PCs (IO-PCs). According to various theoretical models that have been reported, PBG positions could be varied according to the thickness of dielectric layer [59] and by altering the chemical potential of graphene [9; 30] for both 1D and 2D graphene-based PCs. Usually, 1D-PC structures consist of layers made from material with high-refractive index arranged periodically over the 1D structure of material with lower refractive index. The 1D PCs are commonly utilized in biosensing for biomolecular detections due to their distinct changes in resonance behavior once the binding/interaction between the target compound and PC occurs [60; 61]. Graphene-based 1D PC has been identified as a good biosensor candidate due to its strong interaction with light, as proved by a theoretical study conducted by [PERSON] et al., where about 14-fold higher sensitivity has been obtained for graphene-based PC than that for SPR-based biosensor [53] since the latter might lose the sensitivity due to damping in optical waves [62]. In recent theoretical study by [PERSON] and [PERSON], it has been revealed that dielectric coupled with layers of graphene shows additional stopband apart from Bragg's PBG, known as \"graphene PBG\" [51]. Interestingly, the changes in incident angle of irradiation of the 1D PC do not exhibit significant differences in the PBG width, which is in contrast to any 3D inverse opal (IO) PC, where the stop band position varies in dependence on the angle of incident light [63; 64]. However, in contradiction to studies by [PERSON] and [PERSON], it has been shown that graphene layer deposited on SiO\({}_{2}\) 1D PC exhibits an enhanced absorption [65].