**2.2 Formation mechanism of hierarchical micro/nanostructured arrays by PLD**

The formation process was traced by PLD using colloidal clusters with different PS spheres as templates. Herein, the TiO2 was selected as desired material and colloidal monolayer with PS sphere size of 350 nm as template to demonstrate the formation process of hierarchical micro/nanostructured arrays. The colloidal clusters with different PS sphere were fabricated by spin-coating with a higher rotation speed (2000 rpm) and lower concentration (1.0 wt%) of PS colloidal microsphere suspension. For example, a single PS sphere or PS sphere clusters with different sphere numbers (2, 3, 4,...) can be easily created on the substrate by above route, as indicated in column A of Table 1. After PLD, morphologies observed from the top compared to those before PLD, as demonstrated in column B of Table 1. For a single PS microsphere, the shape kept spherical but that the size increased from 350 nm (PS sphere size) to 500 nm after PLD. For the PS sphere-clusters with sphere number from two to six, each unit size in the sphere-cluster still increased, but could not maintain the spherical shape after PLD. Growth of deposited TiO2 was restricted at the contact point of two neighboring PS spheres, the contact between the neighboring units changed from a quasidot contact to a facet contact before (PS sphere-cluster) and after (PS sphere-cluster with deposited materials on the surface) PLD. If a PS sphere in sphere-cluster was completely surrounded by others, e.g., the central sphere in a hexagonal close packed (hcp) spherecluster of seven, its size after deposition was almost the same as before PLD and the morphology was slightly changed from spherical shape to hexagonal one. A section of a PS sphere-cluster of 10 spheres with hcp arrangement after PLD displays that hierarchical micro/nanorods have formed on the two spheres completely surrounded by the others and that hierarchical rod cannot be formed on the spheres at the edge of the sphere-cluster. This implies that a hierarchical micro/nanostructured array will be easily produced after PLD if a colloidal monolayer with a large-scale is applied in the PLD process. Additionally, if the desired materials are deposited on a bare silicon substrate without any PS spheres by PLD, nanocolumns grow vertically on the Si substrate, as seen in Figure 11.

Generally, nanocolumns prefer to grow in the normal direction on the substrate during the PLD process.75 In the PLD process, the desired target (TiO2) is irradiated by a laser beam using an energy level exceeding its threshold in vacuum environment, plasma including ions (Ti4+, O2-, etc.), molecules, electrons and clusters are released into the PLD chamber from the target. However, if a background gas with high pressure is introduced into the chamber, the movement direction of ions or electrons will be changed from an almost uniform direction to multidirection due to collisions between the ions, electrons, molecules and clusters of the ejected species and the background gas. According to the above facts, the formation mechanism of hierarchical hcp nanocolumn arrays can be easily understood, as displayed in Figure 12. If a substrate without PS spheres is used in the PLD process, a film consisting of vertical nanocolumns of small diameter will be formed. If a single PS sphere exists on the substrate, a composite of a PS sphere at the bottom and a shell composed of TiO2 radiation-shaped nanobranches on sphere top will turn up, due to preferential vertical growth along the normal direction of the supporting surface and multidirectional deposition. For a PS sphere cluster (more than one sphere) on the substrate, a shadow effect will be produced in the deposition between any two neighboring spheres. If one sphere in the sphere-cluster is completely surrounded by six other spheres as in the case of hcp arrangement, one rod with hierarchical micro/nanostructure will grow on this sphere top. If a colloidal monolayer with a large scale is adopted, this route can easily fabricate an hcp hierarchical micro/nanostructured array. In this strategy, an off-axis configuration is adopted where the target and substrate are perpendicularly placed. It is similar to the glancing angle deposition (GLAD) or oblique angle deposition in which there is a large

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 83

Fig. 11. FE-SEM images of the TiO2 nanocolumns deposited by PLD directly on a bare substrate without colloidal spheres. (a) Observation from top. (b) Section view.

angle between the deposition direction and the normal direction of the substrate.76-84 In the traditional GLAD method, atoms from the target obliquely arrive and condense on the substrate, and the tilted and separated nanowire or nanopillar array with a porous structure are gradually produced due to the shadow effect of the initial deposited nanoparticles under high-vacuum conditions. The critical difference between this route and GLAD is the background gas pressure during deposition, which converts the directional flow of ejected species in a vacuum into a multidirectional one at higher pressure. Therefore, this multidirection deposition and shadow effect are a principal reason why a vertical hierarchical micro/nanostructured array with hcp alignment is formed on the colloidal monolayer. This can be further verified by varying the angles between substrates and target in PLD process, as seen in Figure 13 and 14. If these experiments were carried out in a vacuum, tilted rod-like structured arrays with different angles would be obtained on the different substrates. However, from these results, the rod-like morphologies are independent of the angle between the substrate and target but the growth rates are different for different angles because of the plume shape in PLD , they always grow vertically on the substrate due the multiple direction deposition combined with shadow effect of neighboring colloidal sphere (Figure 14).


Table 1. Morphologies of before and after PLD on the PS sphere surface (Scale bars are 500 nm)

angle between the deposition direction and the normal direction of the substrate.76-84 In the traditional GLAD method, atoms from the target obliquely arrive and condense on the substrate, and the tilted and separated nanowire or nanopillar array with a porous structure are gradually produced due to the shadow effect of the initial deposited nanoparticles under high-vacuum conditions. The critical difference between this route and GLAD is the background gas pressure during deposition, which converts the directional flow of ejected species in a vacuum into a multidirectional one at higher pressure. Therefore, this multidirection deposition and shadow effect are a principal reason why a vertical hierarchical micro/nanostructured array with hcp alignment is formed on the colloidal monolayer. This can be further verified by varying the angles between substrates and target in PLD process, as seen in Figure 13 and 14. If these experiments were carried out in a vacuum, tilted rod-like structured arrays with different angles would be obtained on the different substrates. However, from these results, the rod-like morphologies are independent of the angle between the substrate and target but the growth rates are different for different angles because of the plume shape in PLD , they always grow vertically on the substrate due the multiple direction deposition combined with shadow effect of neighboring

Table 1. Morphologies of before and after PLD on the PS sphere surface (Scale bars are 500

colloidal sphere (Figure 14).

nm)

Fig. 11. FE-SEM images of the TiO2 nanocolumns deposited by PLD directly on a bare substrate without colloidal spheres. (a) Observation from top. (b) Section view.

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 85

Fig. 14. FESEM cross-section images of the samples at different positions in Figure 10.

The hexagonal close-packed (hcp) micro/nanostructured arrays can be obtained by PLD using colloidal monolayer template. Such PLD assisted colloidal lithography can be extended to prepare hexagonal non-close-packed (hncp) micro/nano- particle or nanorod

Images A to E are from the samples at positions D1 to D5.

**2.3 Extension of PLD assisted colloidal lithography** 

arrays by further annealing process. 85-88

Fig. 12. Schematic illustration of formation mechanism of hcp hierarchical micro/nanostructured arrays.

Fig. 13. Schematic illustration of multi-substrate experiment. D1: 8 mm. D2: 23 mm. D3: 35 mm. D4: 50 mm. D5: 64 mm. In this experiment, substrate rotation: 0 rpm. PS sphere size in colloidal monolayer: 350 nm.

Fig. 12. Schematic illustration of formation mechanism of hcp hierarchical

Fig. 13. Schematic illustration of multi-substrate experiment. D1: 8 mm. D2: 23 mm. D3: 35 mm. D4: 50 mm. D5: 64 mm. In this experiment, substrate rotation: 0 rpm. PS sphere size in

micro/nanostructured arrays.

colloidal monolayer: 350 nm.

Fig. 14. FESEM cross-section images of the samples at different positions in Figure 10. Images A to E are from the samples at positions D1 to D5.
