**2.3 Extension of PLD assisted colloidal lithography**

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 arrays by further annealing process. 85-88

Physical Deposition Assisted Colloidal Lithography:

corresponding SAED pattern (inset).

A Technique to Ordered Micro/Nanostructured Arrays 87

Fig. 16. FE-SEM and TEM images of the hierarchical hncp micro/nano particle arrays. a)-c): FE-SEM images. a) Large area hierarchical micro/nanostructured array b) Same at high magnification. c) Cross section d) TEM image of hierarchical particles (scratched from the supporting substrate by a knife and transferred to TEM copper grid for observation) and the

The formation of hncp, hierarchical micro/nano- particle array is discussed based on experimental results. Herein the explanation is given using TiO2 as an example. The sample produced by PLD assisted colloidal lithography without further heating displayed an hcp alignment (Figure 17a). TiO2 was deposited on the PS sphere surfaces and grew along the vertical direction (Figure 17b). Each particle is composed of two parts: the PS sphere at the bottom and an amorphous, porous TiO2 layer consisting of smaller particles on the top (Figure 17c, d). The amorphous materials crystallize after being annealed at high temperature. In this case, when the amorphous TiO2 with its supporting PS spheres was heated at 650 oC for 2 h, the PS spheres were burned out. Meanwhile, the TiO2 particles on top of the PS sphere were changed to anatase polycrystals composed of smaller nanoparticles of ca.30 nm and were dropped vertically down to the original position of the PS sphere. Additionally, the volume of TiO2 particles decreased during the change from the amorphous to the crystalline phase and hence an hncp hierarchical particle array was formed on the substrate, as illustrated in Figure 18. The hierarchical hncp mciro/nanoparticle array film adhered tightly to the substrate after annealing and could not be detached from the supporting substrate even when it was washed ultrasonically in water for 30 min.

PS colloidal monolayer was first fabricated on Si substrates by a self-assembly process. It was then placed in a PLD chamber for deposition at room temperature. After deposition, the sample was moved to an oven from the PLD chamber and annealed in air. The hncp, hierarchically ordered micro/nano- particle arrays were thus prepared on the substrate, as illustrated in Figure 15.85

Figure 16 shows FE-SEM images of the TiO2 hierarchical hncp micro/nano- particle array obtained by PLD assisted colloidal lithography and subsequently annealed at 650 oC for 2 h in air. One can clearly find that this particle-ordered array takes on hncp arrangement. Each particle in the periodic array exhibits a hemispherical shape with an average size of 240 nm (Figure 16c) and is composed of small nanoparticles (Figure 16d). Additionally, SEAD pattern (inset in Figure 16d.) indicates the deposited materials are changed from amorphous to anatase typed TiO2 after annealing. These results reflect that this hncp particle array possesses a hierarchical micro/nano-structure.

Fig. 15. Schematic illustration of fabrication process for the hierarchical hncp micro/nanoparticle array.

PS colloidal monolayer was first fabricated on Si substrates by a self-assembly process. It was then placed in a PLD chamber for deposition at room temperature. After deposition, the sample was moved to an oven from the PLD chamber and annealed in air. The hncp, hierarchically ordered micro/nano- particle arrays were thus prepared on the substrate, as

Figure 16 shows FE-SEM images of the TiO2 hierarchical hncp micro/nano- particle array obtained by PLD assisted colloidal lithography and subsequently annealed at 650 oC for 2 h in air. One can clearly find that this particle-ordered array takes on hncp arrangement. Each particle in the periodic array exhibits a hemispherical shape with an average size of 240 nm (Figure 16c) and is composed of small nanoparticles (Figure 16d). Additionally, SEAD pattern (inset in Figure 16d.) indicates the deposited materials are changed from amorphous to anatase typed TiO2 after annealing. These results reflect that this hncp particle array

Fig. 15. Schematic illustration of fabrication process for the hierarchical hncp micro/nano-

illustrated in Figure 15.85

particle array.

possesses a hierarchical micro/nano-structure.

Fig. 16. FE-SEM and TEM images of the hierarchical hncp micro/nano particle arrays. a)-c): FE-SEM images. a) Large area hierarchical micro/nanostructured array b) Same at high magnification. c) Cross section d) TEM image of hierarchical particles (scratched from the supporting substrate by a knife and transferred to TEM copper grid for observation) and the corresponding SAED pattern (inset).

The formation of hncp, hierarchical micro/nano- particle array is discussed based on experimental results. Herein the explanation is given using TiO2 as an example. The sample produced by PLD assisted colloidal lithography without further heating displayed an hcp alignment (Figure 17a). TiO2 was deposited on the PS sphere surfaces and grew along the vertical direction (Figure 17b). Each particle is composed of two parts: the PS sphere at the bottom and an amorphous, porous TiO2 layer consisting of smaller particles on the top (Figure 17c, d). The amorphous materials crystallize after being annealed at high temperature. In this case, when the amorphous TiO2 with its supporting PS spheres was heated at 650 oC for 2 h, the PS spheres were burned out. Meanwhile, the TiO2 particles on top of the PS sphere were changed to anatase polycrystals composed of smaller nanoparticles of ca.30 nm and were dropped vertically down to the original position of the PS sphere. Additionally, the volume of TiO2 particles decreased during the change from the amorphous to the crystalline phase and hence an hncp hierarchical particle array was formed on the substrate, as illustrated in Figure 18. The hierarchical hncp mciro/nanoparticle array film adhered tightly to the substrate after annealing and could not be detached from the supporting substrate even when it was washed ultrasonically in water for 30 min.

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 89

Fig. 19. (a, b) FE-SEM images of a nanorod array by PLD using a colloidal template

(SAED) pattern (inset).

monolayer with 350 nm PS spheres in O2 at a pressure of 6.7 Pa for 60 min and subsequent annealing in air. a) A top-view FE-SEM image. (b) An FE-SEM image of a sample tilted to 45o. (c) TEM image of a nanorod and the corresponding selective area electron diffraction

Fig. 17. (a, b): FE-SEM images of the as-deposited sample produced by the PLD in ambient atmosphere without heating. a) Top-view image. b) Cross-sectional image. c) TEM image of single unit. d) SAED pattern of deposited materials.

Fig. 18. Formation of hierarchical *hncp* micro/nanostructured particle arrays.

Fig. 17. (a, b): FE-SEM images of the as-deposited sample produced by the PLD in ambient atmosphere without heating. a) Top-view image. b) Cross-sectional image. c) TEM image of

Fig. 18. Formation of hierarchical *hncp* micro/nanostructured particle arrays.

single unit. d) SAED pattern of deposited materials.

Fig. 19. (a, b) FE-SEM images of a nanorod array by PLD using a colloidal template monolayer with 350 nm PS spheres in O2 at a pressure of 6.7 Pa for 60 min and subsequent annealing in air. a) A top-view FE-SEM image. (b) An FE-SEM image of a sample tilted to 45o. (c) TEM image of a nanorod and the corresponding selective area electron diffraction (SAED) pattern (inset).

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 91

Fig. 21. SEM images of Fe2O3 micro/nanostructured array based on colloidal monolayer template and PLD at the oxygen pressure of 6 Pa. (a, b) Before (c-f) after annealing 450 oC for 3 h in air. (c) Top-surface image. (d) 45o tilted view. (e) Cross-section image. (f) Higher

In this route, the interspace between two neighboring rods can be controlled by changing the background gas pressures in PLD process. Usually, the porosity and specific surface area of the rods in arrays can be tuned by varying the background gas pressure in PLD deposition and they increase with increase of background gas pressure (Figure 22a–c). Therefore, the interspace between neighboring anatase rods can be tuned with different background gas pressures in an hncp array after annealing. The interspace will increase

with increasing background gas pressure (Figure. 22a'–c' and a''–c'').

magnification.

In the PLD assisted colloidal lithography, if the deposition time increases to a appropriate time, an hncp micro/nano rod array can be obtained after PLD and subsequently annealing.86 Figure 19 presents FE-SEM images of the hierarchical micro/nano rod array obtained by PLD assisted colloidal lithography (PS sphere size: 350 nm; background O2 pressure: 6.7 Pa; longer deposition time: 60 min) after annealed at 650 oC for 2 h in air. Figure 19 a and b indicate that a periodic nanorod array takes an hncp arrangement and that each nanorod consists of many nanoparticles. The TEM image (Figure 19c) of a single nanorod shows that it has an aspect ratio of ca. 2:1 and is composed of small nanoparticles, and the PS sphere templates were entirely removed during annealing. The hncp micro/nano- rod arrays originated from amorphous, hcp micro/nanostructured array produced by PLD at room temperature without annealing, as displayed in Figure 20. Beside amorphous materials deposited by PLD, some crystalline materials, e.g. Fe2O3. their hncp micro/nanostructured arrays can be also achieved by PLD assisted colloidal lithography after annealing. Figure. 21a, b present SEM images of hierarchical, hcp, crystalline Fe2O3 micro/nanostructured arrays by PLD at oxygen pressures of 6 Pa at room temperature. Such sample was annealed in air at 450 oC for 3 h and, PS colloidal monolayer template was completely decomposed and hierarchical hncp micro/nano-structured array was formed, as shown in Figure 21 c-f.

Fig. 20. FE-SEM images of an amorphous TiO2 hcp nanorod array on the colloidal monolayer before annealing. a) Observation from top, b) Cross section. Inset in (b): a TEM image of a single amorphous TiO2 nanorod with PS sphere.

Physical Deposition Assisted Colloidal Lithography: A Technique to Ordered Micro/Nanostructured Arrays 91

90 Advances in Unconventional Lithography

In the PLD assisted colloidal lithography, if the deposition time increases to a appropriate time, an hncp micro/nano rod array can be obtained after PLD and subsequently annealing.86 Figure 19 presents FE-SEM images of the hierarchical micro/nano rod array obtained by PLD assisted colloidal lithography (PS sphere size: 350 nm; background O2 pressure: 6.7 Pa; longer deposition time: 60 min) after annealed at 650 oC for 2 h in air. Figure 19 a and b indicate that a periodic nanorod array takes an hncp arrangement and that each nanorod consists of many nanoparticles. The TEM image (Figure 19c) of a single nanorod shows that it has an aspect ratio of ca. 2:1 and is composed of small nanoparticles, and the PS sphere templates were entirely removed during annealing. The hncp micro/nano- rod arrays originated from amorphous, hcp micro/nanostructured array produced by PLD at room temperature without annealing, as displayed in Figure 20. Beside amorphous materials deposited by PLD, some crystalline materials, e.g. Fe2O3. their hncp micro/nanostructured arrays can be also achieved by PLD assisted colloidal lithography after annealing. Figure. 21a, b present SEM images of hierarchical, hcp, crystalline Fe2O3 micro/nanostructured arrays by PLD at oxygen pressures of 6 Pa at room temperature. Such sample was annealed in air at 450 oC for 3 h and, PS colloidal monolayer template was completely decomposed and hierarchical hncp micro/nano-structured array was formed, as

Fig. 20. FE-SEM images of an amorphous TiO2 hcp nanorod array on the colloidal

image of a single amorphous TiO2 nanorod with PS sphere.

monolayer before annealing. a) Observation from top, b) Cross section. Inset in (b): a TEM

shown in Figure 21 c-f.

Fig. 21. SEM images of Fe2O3 micro/nanostructured array based on colloidal monolayer template and PLD at the oxygen pressure of 6 Pa. (a, b) Before (c-f) after annealing 450 oC for 3 h in air. (c) Top-surface image. (d) 45o tilted view. (e) Cross-section image. (f) Higher magnification.

In this route, the interspace between two neighboring rods can be controlled by changing the background gas pressures in PLD process. Usually, the porosity and specific surface area of the rods in arrays can be tuned by varying the background gas pressure in PLD deposition and they increase with increase of background gas pressure (Figure 22a–c). Therefore, the interspace between neighboring anatase rods can be tuned with different background gas pressures in an hncp array after annealing. The interspace will increase with increasing background gas pressure (Figure. 22a'–c' and a''–c'').

Physical Deposition Assisted Colloidal Lithography:

15% and 45% compared to the original size of the PS template.

deposition cannot be guaranteed because of the subsequent etching.

A Technique to Ordered Micro/Nanostructured Arrays 93

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

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

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'') tilted view with 45 degrees after annealing.
