## 2 Fabrication Strategy for Functional Materials with 3D Periodic Nanostructures
### Infiltration of Functional Inorganics into 3D Nanostructured Polymer Templates via Electroplating  
Electroplating, also called electrodeposition, is a conventional thin film deposition process in which a thin film is grown on a conductive substrate using electrical energy through reduction reaction. Electroplating requires a simple experimental setup consisting of an electrochemical cell and accessories for applying current at a certain voltage. The cell generally contains an electrolyte, a working electrode (cathode), and a counter electrode (anode) (Figure 2a). As necessary, a three-electrode system with a reference electrode made of Ag/AgCl or Pt can be used to improve the stability of electroplating by compensating the ohmic potential (\(iR\)) drop accompanying the passage of current through the electrolyte [75]. Depending on the electrolyte recipe, a variety of metals, semiconductors, and dielectrics composed of a single element or compound can be deposited [76; 77]. When an optimal potential is applied to the cell, the alignment of water molecules occurs in a diffusion layer and the source cations dissolved in the electrolyte are reduced on the conductive substrate, which acts as the cathode, forming a thin solid film (Figure 2b) [78; 79]. At this time, the deposition rate and morphology of the thin film can be controlled by the atmosphere (e.g., pH and temperature of the bath), additives, a current density, a type of current waveform, etc. Optimization of these process parameters is even more important when there are structures on the substrate surface. In particular, when template-assisted electroplating is performed using a high-aspect-ratio microstructure, defects such as voids or seams can easily occur if the process parameters are not optimized (Figure 2c-e). There are four main reasons that electroplating is suitable when filling a nanoporous polymer template with heterogeneous materials. First, since solution-based electroplating is performed near room temperature, the thermal damage to nanostructure composed of polymer with a low glass transition temperature or melting point is negligible. Second, since electroplating exhibits a bottom-up growth behavior from the seed layer, templates with 3D nanoporous networks can be completely filled with heterogeneous materials to be electroplated. Third, since electroplating is a mature industrial technology, chemical recipes capable of electrodepositing various inorganic substances have been well developed and a batch process is available for mass production. Based on these good compatibilities between optical 3D lithography and electroplating, various 3D nanostructured inorganic materials \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Materials** & **Product Area** & **Speed** & **Resolution** & **Structures** & **Remarks** \\ \hline DLW & Photosensitive polymer or & & & & & \\ hybrid material & \textless{several mm}^{2}\) & \textasci250 μm/s & \textasci100 nm & Arbitrary & Low throughput \\ MBIL & Photosensitive polymer or & & & & & Periodic & Sensitive to \\ hybrid materials & & & & & (FCC, BCC, woodpile, etc.) & environment \\ PnP & Photosensitive polymer or & & & & & \\ hybrid materials & & & & & & \\ hybrid materials & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical comparison of unconventional optical 3D lithography. have emerged. Figure 3 summarizes representative examples of the 3D nanostructured metals and semiconductors produced by template-assisted electroplating. Various metals and alloys (Au, Cu, Ni, Ni-Sn, etc.) with 3D nanostructures have been successfully demonstrated. [PERSON] et al. reported the successful fabrication of a periodic array of tapered Au helix structures through the combination of DLW and electroplating (Figure 3a) [80]. Electroplating conditions with a constant electrical current of 0.6 mA were used to completely fill Au in a template with inverse helical structures. The polymer template was subsequently removed by air plasma treatment. [PERSON] et al.