Fabrication of Size-Tunable Hierarchical Porous Cr Nanoring Arrays Patterns

be greatly improved by optimizing the self-assembly conditions including volume and concentration of nanosphere colloidal suspensions, and the parameters of spin-coating. The 540 nm PS nanospheres were not completely removed from the glass substrate, as shown on the left in Fig. 5-9 (d). The right side of the image (Fig. 5-9 (d)) shows that the Cr nanorings were formed at the bottom of the PS nanospheres. The reason for the formation of nanorings can be attributed to the more active Cr atom/ion penetration into the crevices of triangular interstices of the PS nanospheres using the CFUBMIP system. It is thought that a periodic ring-shaped nanostructure with controlled diameter and inter-particle spacing can be produced using this approach.

The AFM image in Fig. 5-10 shows the 2D topography surface patterns and height profiles of the periodic Cr nanoring structures. It is clearly observed that all ordered patterns of the polystyrene template are transferred to the Cr surface and the Cr nanoring arrays are uniform over a large area. The measured average height of the Cr nanorings is around (a) 20 ± 1.2 nm, (b) 30 ± 1.6 nm, (c) 40 ± 2.2 nm and (d) 50 ± 2.8 nm. In principle, the size and the height of Cr nanorings can be fine-tuned by changing the parameters in both the RIE and magnetron sputtering processes.

The wettability of a solid surface is an important property which depends on the surface structure. In this study, the wettability of periodic Cr porous nanoring films with varying pore size and height was studied. Fig. 5-11 (a) and (b) show the relationship between the size of Cr nanoring array structures and the contact angle. It was observed that the water contact angle increased with decreasing diameter of Cr nanorings. The water contact angles, measured on diameters of 960 nm and 540 nm periodic of sizes-varied Cr nanorings, changed from 82 ± 1°

to around 88 ± 2° and from 91 ± 2° to around 98 ± 2°, respectively. It was observed that the contact angle increased with decreasing diameter of Cr nanoring due to a more pronounced decrease in the surface roughness.

surface roughness, Wenzel reported a model describing the water dewetting behavior of porous film structures; the following equation was obtained [111]:

(5.6) where r is the roughness factor, and θr and θ are the water contact angles on the rough film surface and the native film surface, respectively. According to Wenzel's equation the surface roughness can enhance both the hydrophilicity of a hydrophilic surface (water contact angle

＜ 90°) and the hydrophobicity of a hydrophobic surface (water contact angle ＞ 90°). The surface roughness of periodic Cr nanoring array structures was higher than that of flat Cr film, therefore, the Wenzel model can explain why these surfaces demonstrate more hydrophobic behavior.

Next, by varying the height of the 540 nm periodic Cr nanoring structures (see Fig. 5-10) the relationship with water contact angle was investigated. It was found that the contact angle increased with increasing height of nanorings, as shown in Fig. 5-11 (c). Increasing the height in Cr nanoring arrays significantly enhanced the hydrophobicity due to allowing more air to be trapped in the porous nanoring structure. In this case, the Cassie-Baxter equation was used to explain the wettability as follows [114]:

(5.7) where θr and θ are the water contact angle on the porous Cr nanoring array films and a native film respectively, f1 is the area fraction of a water droplet in contact with the surface, and f₂ is the area fraction of a water droplet in contact with air (i.e., f1 + f2 = 1). Obviously, increasing f₂ can lead to larger θr, that is, the area fraction of porous surface is important for the hydrophobicity for a Cassie type surface. Hence, indicating that the periodic porous nanoring structures surface produces a large amount of air trapped in the pores which is mainly caused by the unique height variation of the porous nanorings.

The optical transmission spectra of the 960 nm and 540 nm periodic of hierarchical porous Cr nanoring array structures with different fill-factors (ratio of the area of porous Cr nanoring

array divided by the total area) is presented in Fig. 5-12. The diameters of the 960 nm and 540 nm periodic Cr porous nanorings can be changed from 453 ± 27 to around 400 ± 24.4 nm and 205 ± 14.6 nm to around 191± 12.5 nm, respectively. The corresponding fill-factors of periodic Cr porous nanorings can be turned from 21.2 % to around 17.2 % and 13.1 % to around 11.4 %, respectively. The flat Cr thin film with a thickness of 15 nm (fill-factor = 0) has the lowest transmittance. It can be shown that the transmittance increased with increasing fill-factor for both the 960 nm and 540 nm periodic Cr porous nanoring structures.

This can be attributed to the rapid increase in transmission with increasing size of the hole by the conventional aperture theory for single holes [115]. On the other hand, it was observed that the optical transmission spectra of the 960 nm periodic Cr porous nanoring array structures that the plasmon wavelength was red-shift. This is due to momentum conservation in the interaction between the surface plasmon and incident light on the periodic metal array structure, the following equation is obtained [3]:

(5.8) where ksp is the surface plasmon wave vector, kx is the component of the incident wave vector that lies in the plane, G_x and G_y are the reciprocal lattice vectors, and i and j are integers.

Therefore, the surface plasmon resonant effect is highly sensitive to the fill-factor, leading to the red-shift of the recorded spectra as the fill-factor increased. This tunability of hierarchical porous Cr nanoring array structures are ideal multifunctional plasmonic substrates that can be exploited in the design of plasmonic waveguides, and potential applications in optical devices may be worth exploring.

Fig. 5-8 SEM images for various size of PS nanosphere arrays monolayer. (a) Top view of the 960nm monolayer PS nanosphere arrays on the glass substrate. (b) Top view of the PS nanosphere thinned by RIE process with 60 W for 7 min and then etched with 40 W for 12 min. (c) Top view of the 540nm monolayer PS nanosphere arrays on the glass substrate. (d) Top view of the PS nanosphere thinned by RIE process with 50 W for 5 min and then etched with 30 W for 12 min. The inset shows a cross-sectional view of the PS nanosphere respectively.

Fig. 5-9 SEM images of sizes-varied Cr porous nanoring array structures. Images (a), (b), (c) and (d) were obtained by the templates shown in Fig. 2(a), (b), (c) and (d), respectively.

Images (d) shows the 540 nm PS nanospheres were not completely removed on the glass substrate. The inset shows (c) the height profiles of periodic Cr nanoring structure is around 20 ± 1.2 nm.

Fig. 5-10 AFM image of the 2D topography surface patterns and height profiles of varying the ordered Cr nanoring structures, arrayed with height of (a) 20 ± 1.2 nm, (b) 30 ± 1.6 nm, (c) 40

± 2.2 nm and (d) 50 ± 2.8 nm.

Fig. 5-11 Relationship between the diameter of nanoring and contact angles with (a) 960 nm and (b) 540 nm initial diameter of the nanospheres. (c) Relationship between the height of Cr

Fig. 5-12 Transmission spectra of the 960 nm and 540 nm periodic Cr porous nanoring arrays structure with different fill-factors.