# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 2 Preparation of 3D Polymer Nanonetworks  
The 3D nanonetwork-structured polymer template can be divided into two types depending on the regularity of the structure (Figure 1b,c). For example, electrospinningning is a typical technique to fabricate irregular 3D polymer nanonetworks over large areas [37; 38; 39; 40]. Although electrospinningning is an old technique, it is still the most powerful technique for the mass production of polymer nanonetworks. Most nanofibrous fabrics are made using this technique. The electrospinningning system generally consists of a syringe pump that can push out a polymer solution, a DC high-voltage generator, a needle for extracting nanofibers, and a grounded collecting plate (Figure 2a). The polymer solution located at the end of the capillary forms a hemispherical droplet based on the equilibrium between gravity and surface tension. When an electric field is applied, a charge or dipole orientation is induced at the interface between the air and the droplet, creating an electrostatic repulsive force opposite to the surface tension. Thus, the hemispherical surface of the solution at the tip of the capillary stretches into a conical shape known as a Taylor cone. At the threshold voltage, the electrostatic repulsive force overcomes the surface tension, and a jet of polymer solution is ejected from the Taylor cone. In the case of a polymer solution with a sufficiently high viscosity, the charged jet ejects towards the collecting plate without collapsing. The jet trajectory can bend or change direction during flight. The jet is tapered in flight and the initial jet is split into several smaller filaments by electrostatic repulsion because the charge is concentrated on the surface. The solvent is evaporated during the flight and solid-phase polymer nanofibers are continuously deposited on the collecting plate. Electrospinning can control the diameter and density of 3D polymer nanonetworks by adjusting process parameters such as viscosity or surface tension of the solution, distance from the capillary tip to the collecting plate, applied voltage, and spinning time. A typical electrospun product is a nonwoven mat of randomly oriented fibers due to whipping instability. In some cases, extra auxiliary forces are used to align the nanofibers in a two-dimensional (2D) plane [41]. However, accurate vertical alignment between layers for constructing regular 3D nanonetworks is still difficult to achieve through electrospinningning. Polymer templates with regular or periodic 3D nanonetwork structures can be prepared using unconventional lithographic techniques. First, BCPs have been extensively investigated in the interest of generating polymer templates because of their ability to self-assemble into various types of periodic nanostructures, depending on their composition and molecular weight (Figure 2b) [42]. In particular, a gyroid structure can be created by using a BCP designed to self-assemble in 3D. For example, a polystyrene-based gyroid structure can be fabricated from the self-assembly of degradable block copolymers and polystyrene-b-poly(L-lactide) (PS-PLLA), followed by the hydrolysis of PLLA blocks [43]. BCP is advantageous in that it can easily manufacture periodic 3D polymer nanonetworks without expensive or complicated facilities. However, because it is based on a solution process, the precision and reproducibility of the structure are inferior. Next, DLW based on two-photon polymerization is a well-known 3D printing technology that allows the construction of nanostructures with sub-100-nm resolution (Figure 2c) [44]. An ultrafast high-power laser beam is tightly focused within the volume of a transparent photopolymer, and the photopolymer is locally polymerized by absorbing one or more photons simultaneously. As the laser beam travels along the path designed using computer-aided design (CAD) or computer-aided manufacturing (CAM), a 3D ultrafine object is created by the solvent after the development process.