**3. Sputtering assisted colloidal lithography**

In this method, a PS colloidal monolayer on a substrate is placed into the chamber of radio frequency (RF) magnetron sputtering for material deposition at room temperature and Ar is introduced as the background gas.89 A unique hncp hierarchical micro/nanostructured array is formed due to PS-templated plasma etching/deposition in a relatively high vacuum (0.06 Pa). The features of this technique are (1) low-pressure sputtering; (2) PS-templated sputtering, which guarantees a periodic arrangement, and (3) plasma etching/deposition, which eventually produces the unique hncp hierarchical micro/nano-structure.

Figure 23 presents FE-SEM and TEM images of hierarchical alumina mciro/nanostructured arrays by sputtering target alumina using a PS colloidal template with a sphere size of 750 nm in a relatively high vacuum (0.06 Pa). They show the following three unique features. First, it is a periodic nanocolumn array cushioned by a semi-shell in an hncp arrangement. Each nanocolumn is composed of two parts: a semishell- shaped cushion at the bottom and a vertical nanocolumn on the top of the cushion. Such nanocolumn possesses a very rough

Fig. 22. FE-SEM images of an amorphous hcp TiO2 nanorod array and anatase hncp TiO2 nanorod arrays obtained by PLD under different background gas pressures and subsequent annealing. PLD was performed in oxygen (a) at 2.0 Pa for 200 min; (b) at 16.8 Pa for 43 min; (c) at 26.8 Pa for 30 min. (a–c) before annealing, (a'–c') top views after annealing, (a''–c'')

In this method, a PS colloidal monolayer on a substrate is placed into the chamber of radio frequency (RF) magnetron sputtering for material deposition at room temperature and Ar is introduced as the background gas.89 A unique hncp hierarchical micro/nanostructured array is formed due to PS-templated plasma etching/deposition in a relatively high vacuum (0.06 Pa). The features of this technique are (1) low-pressure sputtering; (2) PS-templated sputtering, which guarantees a periodic arrangement, and (3) plasma etching/deposition,

Figure 23 presents FE-SEM and TEM images of hierarchical alumina mciro/nanostructured arrays by sputtering target alumina using a PS colloidal template with a sphere size of 750 nm in a relatively high vacuum (0.06 Pa). They show the following three unique features. First, it is a periodic nanocolumn array cushioned by a semi-shell in an hncp arrangement. Each nanocolumn is composed of two parts: a semishell- shaped cushion at the bottom and a vertical nanocolumn on the top of the cushion. Such nanocolumn possesses a very rough

which eventually produces the unique hncp hierarchical micro/nano-structure.

tilted view with 45 degrees after annealing.

**3. Sputtering assisted colloidal lithography** 

structure on the surface and seems to be composed of many minicolumns, indicating that the sample possesses a hierarchical, porous structure and hence has a high surface area. Secondly, the periodicity was 750 nm, matching well with the initial size of the PS spheres. It is very evident that the sizes of the cushion and the central columns were reduced by about 15% and 45% compared to the original size of the PS template.

The formation of such hierarchical micro/nanostructured arrays is traced by the different sputtering deposition time, as demonstrate in Figure 24. With increase of deposition time (Figure 24 A–D), the PS sphere size gradually decreases and the alumina columns grow vertically in the center, finally the columnar structures and a salver-shaped semi-shell are formed. Generally, thin alumina continuous film is formed on bare substrate without PS spheres due to the strong ion energy and subsequent rapid surface migration under such a low sputtering pressure. In the case with PS sphere array on the substrate, alumina components sputtered from the target are impinged and implanted into the PS due to the strong ion energy and soft nature of PS. The part of PS sphere is also continuously etched away by argon ions and part of deposited alumina is also etched away, but remaining part will gradually form a structure. Additionally, the PS colloidal monolayer supplies the periodic array template. Merging these two aspects into one forms a unique hierarchical micro/nano-structured arrays. The PS spheres become smaller with plasma etching and a salver-shaped semi-shell gradually appear. Further sputtering causes the PS spheres to be etched more significantly, and the species generated from the target deposit perpendicularly onto the template (both the center and the semi-shell part), thus forming a column structure and salver-shaped semi-shell. Implanted components of aluminium and oxygen into PS sphere will be linked together by continuous etching of PS. But at the side edge of the spheres, the amount of PS is not much and easily etched away to form aluminium oxide film, resulted in the cushion shell. The amount of PS at the center part is much more and even by continuous etching a film cannot be formed and rod-like structures are generated. Further sputtering continues etching the PS sphere until the final unique hncp hierarchical structure forms. The formation process of this unique hierarchical mciro/nano-structure is schematically illustrated in Figure 25. In order to further confirm this process, the pressures of Ar were adjusted from low level (0.06 Pa) to 0.13 and final 6.7 Pa. The FE-SEM images of the samples are presented in Figure 26. With increase of the background gas pressure from 0.06 Pa to 0.13 and 6.7 Pa, the collision probability between the ejected species and Ar molecules increases, thus the PS spheres are more significantly etched and no semi-shell can form. Therefore, only columnar structures are obtained (Figure 26). The amounts of deposited materials in the inter-columnar structures are negligible probably due to the blocking effects of gaseous species emitted from decomposed PS spheres during sputtering. These results firmly prove that a relatively high vacuum condition subsequently induces mild plasma etching/deposition. Besides hncp alumina micro/nano-structured arrays with a periodicity of 750 nm, novel hierarchical arrays with periodicities of 350 nm, 1 μm, and 2 μm were also created by colloidal monolayers with different PS sphere sizes during sputtering at the same pressure of Ar as in Figure 27. Besides alumina, hierarchical arrays of other materials including Au/Al2O3 composite, CuO, and NiO can also be fabricated by the presented one-step plasma etching. Some of the results are presented in Figure 28. In this method, only the inorganic materials can be used as the deposited materials. Otherwise, the deposition cannot be guaranteed because of the subsequent etching.

Physical Deposition Assisted Colloidal Lithography:

figure.

micro/nano-structure.

A Technique to Ordered Micro/Nanostructured Arrays 95

Fig. 24. FE-SEM images of samples obtained with different deposition times. (A) 10 min. (B) 25 min. (C) 30 min. (D) 60 min. The inset is high-magnification image of one unit in each

Fig. 25. Schematic illustration of the formation process of unique hncp hierarchical

Fig. 23. FE-SEM (A, B) and TEM (C) images of a sample obtained by sputtering using a PS colloidal monolayer as the substrate (PS sphere size: 750 nm; deposition time: 2 h). (A) Top view. (B) Titled view with 45o angle. The scale bar in A, B: 1 μm. (C) TEM image of one unit.

Fig. 23. FE-SEM (A, B) and TEM (C) images of a sample obtained by sputtering using a PS colloidal monolayer as the substrate (PS sphere size: 750 nm; deposition time: 2 h). (A) Top view. (B) Titled view with 45o angle. The scale bar in A, B: 1 μm. (C) TEM image of one unit.

Fig. 24. FE-SEM images of samples obtained with different deposition times. (A) 10 min. (B) 25 min. (C) 30 min. (D) 60 min. The inset is high-magnification image of one unit in each figure.

Fig. 25. Schematic illustration of the formation process of unique hncp hierarchical micro/nano-structure.

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 97

Fig. 27. FESEM images of a sample obtained by sputtering using a PS colloidal monolayer

template with different PS sphere sizes at 0.06 Pa. (A) 350 nm. (B) 1 μm. (C) 2 μm.

Fig. 26. FESEM images of a sample obtained by sputtering using a PS colloidal monolayer as the substrate at Ar pressures of (A) 0.13 Pa and (B) 6.7 Pa (PS sphere size 750 nm; deposition time 2 h). These images are observed with a tilt angle of 45°. The scale bar is 1 μm.

Fig. 26. FESEM images of a sample obtained by sputtering using a PS colloidal monolayer as the substrate at Ar pressures of (A) 0.13 Pa and (B) 6.7 Pa (PS sphere size 750 nm; deposition

time 2 h). These images are observed with a tilt angle of 45°. The scale bar is 1 μm.

Fig. 27. FESEM images of a sample obtained by sputtering using a PS colloidal monolayer template with different PS sphere sizes at 0.06 Pa. (A) 350 nm. (B) 1 μm. (C) 2 μm.

Physical Deposition Assisted Colloidal Lithography:

**4.1 Wettability** 

to Wenzel's equation: 95

A Technique to Ordered Micro/Nanostructured Arrays 99

Wettability is generally related to the surface morphologies, roughness and free energy of materials surface and it is evaluated by the water or oil contact angle. A special surface with self-cleaning effect is usually defined as a surface that has the ability to remove dirt or contaminants that are on it when water droplets slide along the surface. Self-cleaning is closely related to surface wettability90-94. The self-cleaning effect is normally attributed to superhydrophobicity (water contact angle (CA) exceeding 150◦ and sliding angle (SA) less than 10◦) or superhydrophilicity (water CA less than 10◦) of the surface. For superhydrophobicity with a self-cleaning effect, contaminants adhere to the water droplet surface and are removed after the water droplet slides off the solid surface with a small tilted angle, due to large water CA and low surface free energy. For superhydrophilic surfaces, contaminants can easily be swept away by adding water droplets on them, due to very low water CA. Wettability can be enhanced by increasing surface roughness, according

 cos *θ<sup>r</sup>* = *r* cos*θ* (1) where *r* is the roughness factor, defined as the ratio of total surface area to projected area on the horizontal plane; *θr* is the CA of film with a rough surface; and *θ* is the CA of film with a smooth surface. Obviously, increased roughness can enhance the hydrophobicity and/or hydrophilicity of hydrophobic and/or hydrophilic surfaces. The hierarchical micro/nanostructured arrays based on colloidal monolayers are actually rough films at the micro/nano-scale level. It is expected that such hierarchical micro/nanostructured arrays could induce surface superhydrophilicity or superhydrophobicity with a self-cleaning effect, due to their high roughness. Amorphous, porous hierarchical TiO2 micro/nanostructured arrays were prepared by PLD assisted colloidal lithography (Figure 3)73. These arrays exhibited strong superhydrophilicity. When a small water droplet was dropped on a hierarchical structured array, the droplet spread out rapidly on the surface and displayed a water CA of 0◦ in a 0.225 s (Figure 29). Additionally, this hierarchical array film exhibited superoleophilicity when a small oil droplet was placed on the nanorod surface and the oil CA became 0◦ in 0.5 s (Figure 30). These results suggest that this amorphous hierarchical micro/nano-structured array had superamphiphilicity with 0◦ of both water CA and oil CA. A TiO2 film with superamphiphilicity can generally be obtained by UV irradiation, due to hydroxyl ions generated by oxygen defects or dangling bonds on its surface, induced by photochemical processes96. However, the TiO2 hierarchical micro/nano- structured array film possessed superamphiphilicity without further UV irradiation. The ions (e.g., Ti4+, and O2−) and electrons are released into the PLD chamber and some oxygen species are lost in the vacuum environment in PLD after a TiO2 target absorbs energy from laser irradiation by exceeding its threshold. Oxygen vacancies are produced in the deposited TiO2 during PLD, converting relevant Ti4+ sites to Ti3+ sites that are favorable for dissociative water adsorption. Therefore, these defect sites microscopically form hydrophilic domains on the TiO2 surface. However, the other parts surrounding the hydrophilic domain remain oleophilic on the surface. A composite TiO2 surface having hydrophilic and oleophilic domains on a microscopically distinguishable scale demonstrates macroscopic amphiphilicity on the TiO2 surface96. Additionally, a TiO2 nanoparticle film prepared by PLD without a PS colloidal monolayer exhibited a water CA of 15◦ and an oil CA of 27◦ (Figure 31). The roughness of the hcp TiO2 hierarchical micro/nano-structured array film

Fig. 28. FESEM images of periodic Au/Al2O3 nanocomposite arrays obtained by cosputtering multiple targets consisting of an Al2O3 target and Au sheets and using a PS colloidal monolayer as the substrate (PS sphere size 750 nm; deposition time 2 h). (A) Image observed from the top. (B) Image with a tilt angle of 45o. The scale bar is 1 μm.
