## 2 Fabrication Strategy for Functional Materials with 3D Periodic Nanostructures
### Key Considerations for 3D Nanostructured Template-Assisted Electroplating  
There are some special considerations needed when filling inorganic materials inside the polymer template with a 3D periodic nanostructure through electroplating. First, since the 3D nanostructured template to be formed on the substrate is a polymer-based insulator, optical 3D lithography introduced in the previous chapter must be preceded on a conductive substrate to enable electroplating. Second, plating conditions must be precisely controlled to ensure the complete filling of the material to be deposited in the 3D nanoporous network without voids or seams. Third, in order to extend the utility of the 3D nanostructured materials, it is desirable to be able to form a freestanding membrane separated from the substrate as needed. Regarding the first issue, the most preferred approach is to perform optical 3D lithography (e.g., DLW, MBIL, or PnP) on a transparent and conductive substrate such as ITO or fluorine-doped tin oxide (FTO) glass coated with photoresist. At this time, it is very important to clean the substrate to improve the adhesion between the conductive substrate and the photoresist. Despite meticulous pretreatment of the substrate, 3D nanostructured films with high SVR formed by optical 3D lithography can be easily peeled off from ITO or FTO substrates. The reason is that the area of the surface structure formed by optical 3D lithography in contact with the substrate is small, so inherently poor adhesion between the substrate and the 3D nanostructured film cannot withstand the large swelling and shrinkage of the photoresist. The SU-8, which is a well-known epoxy-based negative-tone photoresist for the micro-electromechanical system (MEMS), generally suffers from 20-30% shrinkage during the development, rinsing, and drying steps. In particular, this problem often occurs as the result of MBIL and PnP. The resulting surface geometry of 3D periodic nanostructures generally consists of sharp and high-aspect-ratio nanopillars that form point contact rather than surface contact with the substrate, resulting in very poor adhesion with the substrate [85]. For this reason, in most cases, a hard-baked relatively thin (<2 um) homogeneous photoresist film (e.g., SU-8) is pre-coated on the conductive seed layer-deposited substrate as an adhesive layer to maximize the bonding force between the substrate and the 3D nanostructured film with a thickness of 10 um or more. The seed layer usually consists of highly conductive Cr/Au or Cu formed by sputtering or thermal evaporation. However, in template-assisted electroplating, the adhesive layer, which is a solid insulator, impedes the flow of current from the electrolyte to the seed layer (cathode) and does not allow electroplating. This is a significant practical concern that has not been dealt with so far. To solve this, a \"window\" process was developed and used (Figure 4). This process consists of three steps. (1) Through traditional photolithography, the adhesive layer is patterned to create an exposed part of the conductive seed layer. (2) Next, the entire substrate area is coated with photoresist and optical 3D lithography is performed. At this time, since the adhesive layer outside the \"window\" tightly holds the 3D nanostructured film well, delamination from the substrate does not occur. (3) Through electroplating, thin film growth occurs only in the area where the seed layer is exposed, filling the 3D nanostructured template with inorganic materials. This strategy makes it possible to perform the template-assisted electroplating with 3D nanostructured templates prepared by optical 3D lithography on an ad hoc basis [38; 82; 86]. However, the effect of light reflection from the metal seed layer on the formation of the 3D nanostructure has not yet been significantly considered. Ultimately, the development of an approach that fundamentally solves the problem of adhesion between the substrate and the 3D nanostructure is required.