**4. Applications of periodic micro/nanostructured arrays**

Based on colloidal monolayer templates, the different hierarchical micro/nanostructured arrays can be fabricated by physical deposition and their morphologies and structural parameters (sizes and interspace of micro/nanostructured unit) can be facilely controlled by periodicities of colloidal monolayers, experimental conditions such as deposition time, background gas pressure etc. Some properties, such as the surface wettability, field emission, catalytic properties are closely morphology and structural parameter-dependent. These properties can be readily optimized by changing morphologies and parameters of the periodic arrays. Their investigations supply useful theoretic foundations and are highly valuable for designing micro/nanodevices based on these special arrays.

## **4.1 Wettability**

98 Advances in Unconventional Lithography

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

Based on colloidal monolayer templates, the different hierarchical micro/nanostructured arrays can be fabricated by physical deposition and their morphologies and structural parameters (sizes and interspace of micro/nanostructured unit) can be facilely controlled by periodicities of colloidal monolayers, experimental conditions such as deposition time, background gas pressure etc. Some properties, such as the surface wettability, field emission, catalytic properties are closely morphology and structural parameter-dependent. These properties can be readily optimized by changing morphologies and parameters of the periodic arrays. Their investigations supply useful theoretic foundations and are highly

observed from the top. (B) Image with a tilt angle of 45o. The scale bar is 1 μm.

**4. Applications of periodic micro/nanostructured arrays** 

valuable for designing micro/nanodevices based on these special arrays.

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 to Wenzel's equation: 95

$$
\cos \theta\_r = r \cos \theta \tag{1}
$$

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

Physical Deposition Assisted Colloidal Lithography:

superhydrophilic surface.

equation,100

cos

A Technique to Ordered Micro/Nanostructured Arrays 101

Fig. 30. Oil (rapeseed) droplet shape on amorphous TiO2 micro/nano- structured array film.

More importantly, this amorphous TiO2 hierarchical array demonstrated very good photocatalytic activity for organic molecular degradation (e.g., effective decomposition of stearic acid under UV illumination). A combination of superamphiphilicity and photocatalytic activity can yield a self-cleaning surface. For instance, an oily liquid contaminant spreads out on a surface due to superoleophilicity, which is helpful in improving the photocatalytic efficiency under light illumination. An organic contaminant including oil gradually degrades under sunlight irradiation (sunlight contains some UV light). The self-cleaning effect can be realized after washing away contamination from the

Additionally, superhydrophobic surfaces with large water CA and small SA have a selfcleaning effect. For superhydrophobic film, the surface should be sufficiently rough and have a chemical coating with low free-energy materials in order to trap the air on the rough surface. In this case, the area fraction of a water droplet in contact with the sample surface is very small, which helps obtain a small SA. Hierarchical periodic micro/nanostructured arrays based on colloidal templates provide surfaces with regularly ordered and welldefined roughness. They may lead to enhancement from hydrophobicity to superhydrophobicity on the surface after modification with low free-energy materials97-99. For instance, Co3O4 hierarchical, hncp micro/nano- rod arrays was created by PLD assisted colloidal lithography after annealing at 450 oC for 3 h (oxygen pressures: 93.1 Pa), as shown in Figure 32 a and b.88 Such surface was chemical modification with fluorosilane, a kind of low free energy material, it presented superhydrophobicity with water CA of 152.6o and a very small SA, indicating self-cleaning effect. It can be explained by Cassie and Baxter

> θ*r*=*f1* cosθ

Here, *f1* and *f2* are the surface area fractions of the projecting solid and air (*f1* + *f2*= 1). The large fraction of air trapped in the nanorod arrays forms a cushion at the film–water interface that prevents the penetration of water droplets into the grooves. In this case,


 θ*r* is

The oil contact angle becomes 0° in 0.5 s after it was dropped onto the surface.

was greatly increased compared with that of the nanoparticle TiO2 film produced by PLD without using a colloidal monolayer. According to Wenzel's equation, wettability is enhanced from amphiphilicity to superamphiphilicity. Therefore, the superamphiphilicity of the amorphous micro/nano-structured array originates from the combination of the amphiphilicity produced by PLD and the special rough structures of hcp hierarchical arrays.

Fig. 29. Time course of water-contacting behavior on the amorphous TiO2 micro/nanostructured array film.

was greatly increased compared with that of the nanoparticle TiO2 film produced by PLD without using a colloidal monolayer. According to Wenzel's equation, wettability is enhanced from amphiphilicity to superamphiphilicity. Therefore, the superamphiphilicity of the amorphous micro/nano-structured array originates from the combination of the amphiphilicity produced by PLD and the special rough structures of hcp hierarchical arrays.

Fig. 29. Time course of water-contacting behavior on the amorphous TiO2 micro/nano-

structured array film.

Fig. 30. Oil (rapeseed) droplet shape on amorphous TiO2 micro/nano- structured array film. The oil contact angle becomes 0° in 0.5 s after it was dropped onto the surface.

More importantly, this amorphous TiO2 hierarchical array demonstrated very good photocatalytic activity for organic molecular degradation (e.g., effective decomposition of stearic acid under UV illumination). A combination of superamphiphilicity and photocatalytic activity can yield a self-cleaning surface. For instance, an oily liquid contaminant spreads out on a surface due to superoleophilicity, which is helpful in improving the photocatalytic efficiency under light illumination. An organic contaminant including oil gradually degrades under sunlight irradiation (sunlight contains some UV light). The self-cleaning effect can be realized after washing away contamination from the superhydrophilic surface.

Additionally, superhydrophobic surfaces with large water CA and small SA have a selfcleaning effect. For superhydrophobic film, the surface should be sufficiently rough and have a chemical coating with low free-energy materials in order to trap the air on the rough surface. In this case, the area fraction of a water droplet in contact with the sample surface is very small, which helps obtain a small SA. Hierarchical periodic micro/nanostructured arrays based on colloidal templates provide surfaces with regularly ordered and welldefined roughness. They may lead to enhancement from hydrophobicity to superhydrophobicity on the surface after modification with low free-energy materials97-99. For instance, Co3O4 hierarchical, hncp micro/nano- rod arrays was created by PLD assisted colloidal lithography after annealing at 450 oC for 3 h (oxygen pressures: 93.1 Pa), as shown in Figure 32 a and b.88 Such surface was chemical modification with fluorosilane, a kind of low free energy material, it presented superhydrophobicity with water CA of 152.6o and a very small SA, indicating self-cleaning effect. It can be explained by Cassie and Baxter equation,100

$$
\cos\theta\_{\overline{r}} \overline{\!\_{l}} \cos\theta\_{\overline{r}} \,\_{l} f\_{2} \tag{2}
$$

Here, *f1* and *f2* are the surface area fractions of the projecting solid and air (*f1* + *f2*= 1). The large fraction of air trapped in the nanorod arrays forms a cushion at the film–water interface that prevents the penetration of water droplets into the grooves. In this case, θ*r* is

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 103

Fig. 32. FE-SEM images of Co3O4 hierarchical micro/nano- rod arrays obtained by PLD assisted colloidal lithography after annealing at 450 oC for 3 h (oxygen pressures: 93.1 Pa). (a) Top views; (b) Tilted at 45o (c) water CA after modification with low free energy

materials.

152.6o and θ is 18.8o, so a value for *f1* of 0.06 is calculated from eqn (2) (i.e. *f2* is 0.94), implying that only 6% of the observed contact area beneath a water droplet is in contact with the water droplet. High *f2* of 0.94 means that the air was well trapped into the groove among nanorod arrays and hence the water droplet kept a spherical shape.

Fig. 31. Water and oil CAs on a TiO2 film on a silicon wafer prepared by PLD without using a PS colloidal monolayer. (a) Water CA: 15 degrees. (b) Oil (rapeseed) CA: 27 degrees.

implying that only 6% of the observed contact area beneath a water droplet is in contact with the water droplet. High *f2* of 0.94 means that the air was well trapped into the groove

Fig. 31. Water and oil CAs on a TiO2 film on a silicon wafer prepared by PLD without using a PS colloidal monolayer. (a) Water CA: 15 degrees. (b) Oil (rapeseed) CA: 27 degrees.

among nanorod arrays and hence the water droplet kept a spherical shape.

is 18.8o, so a value for *f1* of 0.06 is calculated from eqn (2) (i.e. *f2* is 0.94),

152.6o and

θ

Fig. 32. FE-SEM images of Co3O4 hierarchical micro/nano- rod arrays obtained by PLD assisted colloidal lithography after annealing at 450 oC for 3 h (oxygen pressures: 93.1 Pa). (a) Top views; (b) Tilted at 45o (c) water CA after modification with low free energy materials.

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 105

Fig. 33. FE properties of periodic hncp nanorod array by PLD using a colloidal monolayer template with 350nm PS spheres in O2 at a pressure of 6.7 Pa for 60 min and subsequent annealing in air. a) FE current density–electric field (J–E) curves measured for an hncp TiO2 nanorod array at an anode–cathode distance of 60 μm. b) Corresponding Fowler–Nordheim

(FN) plot.

#### **4.2 Field emission**

Field-emission (FE) properties have recently attracted so much attention due to great commercial interest in flat-panel displays and other microelectronic devices.101 Besides carbon nanotubes, semiconductors have also attracted great interest in field emitters owing to their good mechanical stability, low work function, and high electrical and thermal conductivities.102 FE properties are usually decided by the nature of the cathode materials as well as geometry and size of them. By well designing the geometry and size of cathode, for example, to introduce nanostructures on it, good FE performances have been achieved including faster turn-on time, compactness, and sustainability during the field emission compared to conventional bulky material forms. More importantly, researchers have found that cathodes composed of periodic regular arrays on the surface are very highly helpful for producing a low operating voltage and a stable current because of the elimination of the shield effect on densely packed 1D nanostructured arrays in field emission.

Periodic TiO2 micro/nano-rod arrays with hncp arrangement can be synthesized by combining a colloidal monolayer template with pulsed laser deposition (PLD) followed by annealing in ambient air, as described before.86 By this route, the periodicity of such special nanorods can be easily tuned by changing the colloidal sphere size in the colloidal monolayer template. While a distance between neighboring nanorods can be controlled by varying the background gas pressure during the PLD process if periodicity is fixed for a nanorod array**.** The well tunable periodicity and distance between neighboring nanorods are very useful for investigating and optimizing their FE performance.86

The periodic hncp TiO2 nanorod array was fabricated by PLD using a colloidal monolayer template with 350nm PS spheres at 6.7 Pa O2 for 60 min and subsequent annealing at 650 oC for 2 h in air (Figure 19). It demonstrated a low turn-on field of about 5.6 V μm-1 (here the turn-on field was defined as the value of electric field when an emission current density was 4.5 nA cm-2) according to the FE current density–applied electric field curve (J–E) at a working distance of 60 μm from the anode to the nanorod array serving as the cathode (Figure 33). This FE current–voltage characteristics can be expressed by a simplified Fowler– Nordheim (FN) equation and a field-enhancement factor, β can be defined as Bφ3/2/κ according to the FN equation (here φ: the work function of cathode material; κ: slope in FN plot).103 This hncp TiO2 nanorod array showed a field-enhancement factor β of 8.38×102. However, a TiO2 nanorod array with top aggregation for several neighboring nanorods caused by longer deposition displayed a much higher turn-on field of 15.8 V μm-1 and lower field-enhancement factor b 3.34×102 (Figure 34). This result indicates that the good FE properties of a periodic TiO2 hncp nanorod array are mainly attributable to the aligned and periodic hncp nanorod morphology.

When a periodicity of hncp nanorod array increased from 350 nm to 750nm and 1 μm by choosing the colloidal monolayers with different PS sphere sizes during the PLD at the same background gas pressure as before and followed by the same annealing, as presented in Figure 35. The field-enhancement factor decreased with increasing periodicity of the hncp nanorod array (Figure 36a). This is mainly attributed to a decreasing number density of nanorods with an increase in the hncp array periodicity. When the periodicity of the hncp nanorod array was increased from 350 to 750 nm, the turn-on field also increased from 5.6 to 13.0 Vμm-1. When a periodicity further increased to 1 μm, the turn-on field remained about 13.0 Vμm-1. It is evident that the hncp nanorod array with the smallest periodicity of 350nm exhibited the best FE properties in this investigation.

Field-emission (FE) properties have recently attracted so much attention due to great commercial interest in flat-panel displays and other microelectronic devices.101 Besides carbon nanotubes, semiconductors have also attracted great interest in field emitters owing to their good mechanical stability, low work function, and high electrical and thermal conductivities.102 FE properties are usually decided by the nature of the cathode materials as well as geometry and size of them. By well designing the geometry and size of cathode, for example, to introduce nanostructures on it, good FE performances have been achieved including faster turn-on time, compactness, and sustainability during the field emission compared to conventional bulky material forms. More importantly, researchers have found that cathodes composed of periodic regular arrays on the surface are very highly helpful for producing a low operating voltage and a stable current because of the elimination of the

Periodic TiO2 micro/nano-rod arrays with hncp arrangement can be synthesized by combining a colloidal monolayer template with pulsed laser deposition (PLD) followed by annealing in ambient air, as described before.86 By this route, the periodicity of such special nanorods can be easily tuned by changing the colloidal sphere size in the colloidal monolayer template. While a distance between neighboring nanorods can be controlled by varying the background gas pressure during the PLD process if periodicity is fixed for a nanorod array**.** The well tunable periodicity and distance between neighboring nanorods are

The periodic hncp TiO2 nanorod array was fabricated by PLD using a colloidal monolayer template with 350nm PS spheres at 6.7 Pa O2 for 60 min and subsequent annealing at 650 oC for 2 h in air (Figure 19). It demonstrated a low turn-on field of about 5.6 V μm-1 (here the turn-on field was defined as the value of electric field when an emission current density was 4.5 nA cm-2) according to the FE current density–applied electric field curve (J–E) at a working distance of 60 μm from the anode to the nanorod array serving as the cathode (Figure 33). This FE current–voltage characteristics can be expressed by a simplified Fowler– Nordheim (FN) equation and a field-enhancement factor, β can be defined as B

plot).103 This hncp TiO2 nanorod array showed a field-enhancement factor β of 8.38×102. However, a TiO2 nanorod array with top aggregation for several neighboring nanorods caused by longer deposition displayed a much higher turn-on field of 15.8 V μm-1 and lower field-enhancement factor b 3.34×102 (Figure 34). This result indicates that the good FE properties of a periodic TiO2 hncp nanorod array are mainly attributable to the aligned and

When a periodicity of hncp nanorod array increased from 350 nm to 750nm and 1 μm by choosing the colloidal monolayers with different PS sphere sizes during the PLD at the same background gas pressure as before and followed by the same annealing, as presented in Figure 35. The field-enhancement factor decreased with increasing periodicity of the hncp nanorod array (Figure 36a). This is mainly attributed to a decreasing number density of nanorods with an increase in the hncp array periodicity. When the periodicity of the hncp nanorod array was increased from 350 to 750 nm, the turn-on field also increased from 5.6 to 13.0 Vμm-1. When a periodicity further increased to 1 μm, the turn-on field remained about 13.0 Vμm-1. It is evident that the hncp nanorod array with the smallest periodicity of 350nm

φ3/2/κ

: the work function of cathode material; κ: slope in FN

shield effect on densely packed 1D nanostructured arrays in field emission.

very useful for investigating and optimizing their FE performance.86

φ

according to the FN equation (here

periodic hncp nanorod morphology.

exhibited the best FE properties in this investigation.

**4.2 Field emission** 

Fig. 33. FE properties of periodic hncp nanorod array by PLD using a colloidal monolayer template with 350nm PS spheres in O2 at a pressure of 6.7 Pa for 60 min and subsequent annealing in air. a) FE current density–electric field (J–E) curves measured for an hncp TiO2 nanorod array at an anode–cathode distance of 60 μm. b) Corresponding Fowler–Nordheim (FN) plot.

Physical Deposition Assisted Colloidal Lithography:

A Technique to Ordered Micro/Nanostructured Arrays 107

Fig. 35. FE-SEM images of hncp nanorod arrays with different periodicities: 750nm for (a) and (b); 1μm for (c) and (d). PLD was performed in 6.7 Pa of oxygen for 130 min. (a) and (c)

While a distance between neighboring nanorods can be tuned by changing the background gas pressure during the PLD process if periodicity is fixed to 350 nm for a nanorod array (Figure 22)**.** With increasing this distance of neighboring ones, the field-enhancement factor increased and the turn-on field decreased (Figure 36a). The sample with a small nanorod distance of 20 nm exhibited a relatively low field-enhancement factor of 5.04×102 and a high turn-on field of 9.7 V μm-1. When the nanorod distance increased to 50 nm, the FE properties showed enhanced performance with a high field-enhancement factor of 8.38 ×102 and a low turn-on field of 5.6 V μm-1. When the nanorod distance further increased to 110 nm, the best FE properties with a field-enhancement factor of 9.39 ×102 and a turn-on field of 5.3 V μm-1 were obtained. (Figure 36b) The above results suggest that optimized FE properties can be achieved by increasing the nanorod distance by controlling the experimental parameters. The increased field-enhancement factor b with the increase of nanorod distance can easily be understood as follows if the periodicity of hncp nanorod array is fixed. The field enhancement factor β is generally related to geometry of an emitter and can be expressed as β∝*h*/*r*, where *h* is the height and *r* is the curvature radius of an emitting center. With an increase in the nanorod distance, the effective diameter of an individual nanorod and a curvature radius *r* would decrease (Figure 37), resulting in an increase of β according to the

are observed from the top, (b) and (d) are observed with a tilt angle of 458.

above relationship.

Fig. 34. Top aggregation of a TiO2 nanorod array obtained PLD assisted colloidal lithography with a longer PLD time (80 min) and subsequent annealing. a) FE-SEM image; b) FE J–E curves measured at an anode–cathode distance of 60 mm for a top-aggregated TiO2 nanorod array; c) Corresponding FN plot.

Fig. 34. Top aggregation of a TiO2 nanorod array obtained PLD assisted colloidal

TiO2 nanorod array; c) Corresponding FN plot.

lithography with a longer PLD time (80 min) and subsequent annealing. a) FE-SEM image; b) FE J–E curves measured at an anode–cathode distance of 60 mm for a top-aggregated

Fig. 35. FE-SEM images of hncp nanorod arrays with different periodicities: 750nm for (a) and (b); 1μm for (c) and (d). PLD was performed in 6.7 Pa of oxygen for 130 min. (a) and (c) are observed from the top, (b) and (d) are observed with a tilt angle of 458.

While a distance between neighboring nanorods can be tuned by changing the background gas pressure during the PLD process if periodicity is fixed to 350 nm for a nanorod array (Figure 22)**.** With increasing this distance of neighboring ones, the field-enhancement factor increased and the turn-on field decreased (Figure 36a). The sample with a small nanorod distance of 20 nm exhibited a relatively low field-enhancement factor of 5.04×102 and a high turn-on field of 9.7 V μm-1. When the nanorod distance increased to 50 nm, the FE properties showed enhanced performance with a high field-enhancement factor of 8.38 ×102 and a low turn-on field of 5.6 V μm-1. When the nanorod distance further increased to 110 nm, the best FE properties with a field-enhancement factor of 9.39 ×102 and a turn-on field of 5.3 V μm-1 were obtained. (Figure 36b) The above results suggest that optimized FE properties can be achieved by increasing the nanorod distance by controlling the experimental parameters. The increased field-enhancement factor b with the increase of nanorod distance can easily be understood as follows if the periodicity of hncp nanorod array is fixed. The field enhancement factor β is generally related to geometry of an emitter and can be expressed as β∝*h*/*r*, where *h* is the height and *r* is the curvature radius of an emitting center. With an increase in the nanorod distance, the effective diameter of an individual nanorod and a curvature radius *r* would decrease (Figure 37), resulting in an increase of β according to the above relationship.

Physical Deposition Assisted Colloidal Lithography:

**4.3 Enhanced catalytic properties** 

A Technique to Ordered Micro/Nanostructured Arrays 109

Fig. 37. Schematic illustration of defined nanorod distance in periodic hncp nanorod array.

The hierarchical micro/nanostructured arrays possess a large specific surface area and hence they might have important application in catalytic fields. For instance, the hcp amorphous TiO2 micro/nanostructured array on a colloidal monolayer obtained by PLD assisted colloidal lithography demonstrated an enhanced photocatalytic activity (Figure 2). Its photocatalytic performance was estimated based on the decomposition of organic molecules, stearic acid under UV illumination by monitoring the FT-IR spectra.104,105 The frequencies of 2919 and 2849 cm-1 reflect the methylene group asymmetric (*ν*asymmCH2) and symmetric (*ν*symmCH2) stretching modes of stearic acid. These values for the methylene group stretching mode are close to those of a crystalline alkane and are typically taken as evidence of the formation of a dense, well-ordered, self-assembled monolayer of stearic acid on the oxide surface.106-108 Therefore, the photodegradation of stearic acid can be monitor by observing density of these two frequencies. With increasing the UV irradiation time, the vibrational bands of the methylene group gradually decreased and almost completely disappeared after 25 min, as shown in Figure 38a. The decrease in C-H vibrational bands reflects that the stearic acid is gradually photodegraded by such TiO2 hierarchical micro/nanostructured array films under UV irradiation. Figure 38b shows that degradation curves of a stearic acid film on a silicon wafer, an amorphous TiO2 film by PLD without using a colloidal monolayer, an hcp amorphous TiO2 hierarchical micro/nano-rod array on the colloidal monolayer, and an anatase TiO2 rod array (obtained by annealing hcp amorphous TiO2 hierarchical micro/nano-rod array on the colloidal monolayer at 650 °C for 2 h. These results indicate that TiO2 exhibited efficient degradation for stearic acid and that the hcp amorphous TiO2 hierarchical micro/nano-rod array on a colloidal monolayer demonstrated the best performance compared to the amorphous film and the anatase rod array. Anatase is usually deemed to be more photocatalytically active than the rutile and

Fig. 36. (a) Field-enhancement factor β changing with increasing periodicity of an hncp nanorod array, (b) Change in field-enhancement factor and turn-on field with varying neighboring nanorod distance in an hncp array.

Fig. 36. (a) Field-enhancement factor β changing with increasing periodicity of an hncp nanorod array, (b) Change in field-enhancement factor and turn-on field with varying

neighboring nanorod distance in an hncp array.

(b)

(a)

Fig. 37. Schematic illustration of defined nanorod distance in periodic hncp nanorod array.
