## 1 Introduction  
Around the 2000s, with the explosive increase in interest in the experimental realization of three-dimensional (3D) photonic crystals in the academic world, various unconventional techniques optimized to manufacture nanostructured materials with 3D structural periodicity at the sub-micron level were developed. The proposed 3D nanofabrication techniques can be broadly classified into two types, non-optical and optical approaches. Non-optical approaches include colloidal self-assembly [1; 2; 3; 4; 5; 6], block copolymer lithography [7; 8], direct ink writing [9], and layer-by-layertransfer printing [10; 11]. These methods have the advantage of being able to easily create 3D periodic nanostructures with a simple experimental setup, but they inherently have limitations in industrial aspects such as patterning resolution, defect control, and mass production. On the other hand, optical approaches derived from photolithography, which is a mature technology in the semiconductor industry, can precisely control 3D periodic nanostructures with high resolution and have great potential to advance into industrial technologies. The high resolution of optical manufacturing technologies is due to the nature of light that can be highly focused and precisely controlled through delicate optical components. Various interesting approaches such as direct laser writing (DLW) [12; 13; 14; 15; 16; 17; 18; 19; 20; 21], multi-beam interference lithography (MBIL) [22; 23; 24; 25; 26; 27], and proximity field nanopatterning (PnP) [28; 29; 30; 31] have emerged as optical 3D nanofabrication techniques and have successfully demonstrated their ability to implement different classes of 3D periodic nanostructures. Recently, the research trend related to 3D nanofabrication and nanostructures has started to change slightly. The mainstream of previous research has focused on optical applications including 3D photonic crystals based on the structural periodicity of 3D nanostructures fabricated through 3D nanofabrication techniques. In addition to the unique optical properties that stem from structural periodicity, many scientists and engineers have begun to pay attention to the porous nature of structures created through 3D nanofabrication techniques. In particular, optical approaches such as DLW, MBIL, and PnP have a differentiated ability to precisely control pore size at the nanoscale and to align sub-micron pores three-dimensionally. Through this, it is possible to design the porosity of the material and to control the microscopic flow of liquid/gaseous substances or electron/phonon, which may have a profound impact on the overall performance of the electronic and electrochemical systems [42; 43; 44]. Many existing electrochemical devices facing the limit of performance improvement seek to employ such well-designed porous materials with large surface-to-volume ratio (SVR) because their performance is primarily dependent on the degree of surface chemical reaction. Moreover, the porous network of a 3D periodic nanostructure allows it to serve as a matrix capable of continuously carrying heterogeneous materials to realize multifunctional materials [31; 38; 45; 46]. Such types of bicontinuous nanocomposites have the advantage of being able to independently control heterogeneous physical functions that cannot be achieved through traditional mixture-based nanocomposite systems [45]. In response to these technical demands, research on the development of functional materials with 3D periodic nanostructures for non-photonic applications is actively progressing by combining unconventional optical 3D lithography with conventional thin film deposition techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), sol-gel reaction, and electrochemical deposition.