Fabrication of Triangular-Shaped Cr Nanodot and Porous Nanoring Arrays Structures

of the formation of a triangular-shaped nanodot is illustrated in Fig. 5-2 (a)-(b). This is expected considering the formation of a triangular-shaped nanodot due to use the directional sputtering without the three-axis satellite rotation holder, allows the active Cr atoms/ions to fill in the triangular interstices of the PS nanospheres directly. Hence, no or little under-deposition around the bottom of the colloid nanospheres would be expected, and recently some reports in the literature are focused on the development of nanodot or nanohole arrays using the PVD process using NSL [61, 110]. Therefore, the Cr triangular-shaped nanodot structure arrays were formed using the directional sputtering process while using NSL based technology. Fig. 5-2 (f) shows the SEM image from a Cr nanoring array and corresponds to the result indicated in Fig. 5-2 (e). The nanoring array was formed between the PS nanospheres. The lateral size of these nanorings was found to be 275 ± 17 nm and the inside diameter of the nanorings was 164 ± 9 nm, the wall thickness at full width half maximum (FWHM) of the nanoring was 55.5 ± 3.3 nm, and the inter-particle spacing between two adjacent rings of the lateral size changes from 247 ± 10.1nm. We deduce such the formation of nanoring due to the samples were mounted on the three-axis satellite rotation holder using a CFUBMIP system, so the active Cr atoms/ions have more chance to diffuse into the crevices between the triangular interstices of the PS nanospheres, aggregation of these Cr atoms/ions formed a ring-shaped structures tend to move around the bottom of each PS nanosphere which favors the formation of ring-shaped structures as illustrated in Fig. 5-2 (d)-(e). After the PS nanospheres were lifted off by dichloromethane, the Cr nanoring array remained.

The size of nanosphere can be controlled using the isotropic RIE technique with oxygen plasma as shown in Fig. 4-5 (a)-(n), respectively. The RIE procedure with RF power is composed of 50W for 5 min followed by 30W for 4, 8, 12, 16 and 20 min respectively. Fig.

4-6 was plotted the distribution of diameter of nanospheres versus cumulative etching time. It

increasing cumulative etching time. This can be attributed to the oxygen ion will degrade the surface of PS nanospheres due to the hydrogen abstraction on the polymer chain and C–C bond scission by ion bombardment which occur in the near-surface layer, then removal of the weak boundary near-surface layers during ion interaction with the Polymer [95]. Hence, the PS nanosphere sizes can be controlled by varying the etching time. The period of Cr nanoring array is determined by the initial diameter of PS nanosphere.

Fig. 5-3 shows the SEM images of Cr nanorings array with a certain separation and size statistics. The metallic Cr nanorings were arranged hexagonally, and the interparticle spacing between adjacent ring centers was always equal to the diameter of the PS nanospheres (Fig.

5-3 (a), (b), (c) and (d)). The ordered patterns of the colloidal template were transferred well to the porous Cr nanoring arrays. By varying the oxygen RIE etching time of colloidal monolayers, the lateral size, the inside diameter and the wall thickness of nanoring can be well controlled as shown in Fig. 5-4 (a). When the oxygen RIE etching time is increased from 5 to 25 min the lateral size decreases from 220 ± 15 to 160 ± 9 nm, while the inner size changes from 117 ± 6 to 58 ± 3.9 nm, and the corresponding ring-wall thickness (FWHM) changed from 52 ± 3 to around 50 ± 3 nm. Subsequently, the inter-particle spacing between two adjacent rings of the lateral size changed from 264 ± 12.7 to around 362 ± 29.3 nm as shown in Fig. 5-4 (b). The construction of a linear relationship between the size as well as the spacing of nanorings and the etching time will help to control the dimensional accuracy of nanorings and the pattern of the array. To compare the formation of ring in the chemical process that probably due to the easy motion of the liquid under the colloid nanosphere, it can easily lead to change the center-to-center (CTC) spacing between two adjacent rings [107, 108]. Therefore, the size-tunable periodic ring-shaped nanostructure with controlled diameter, distinct ring walls and inter-particle spacing can be prepared using this approach.

Fig. 5-5 (a) shows the side view and a corresponding line scan of the two-dimensional topography surface that shows that the measured the height of nanorings was around 25 ± 2

nm. AFM analysis showed that the separating inter-particle spacing between two adjacent Cr nanoring centers approached 540 nm. Fig. 5-5 (b) depicts a three-dimensional view of the Cr nanoring arrays nanostructure was uniform in measured area of 10 square micrometers. In principle, the size and the height of nanorings can be fine-tuned by changing the RIE etching time and the magnetron sputtering at a constant deposition rate.

Fig. 5-6 shows the relationship between the lateral size of Cr nanoring arrays and water contact angle. It was observed that there was a tendency to a linear increase in water contact angle with decreasing diameter of Cr nanorings. The water contact angles measured on Cr nanorings, with 540 nm initial diameter, changed from 91 ± 2° to around 100.5 ± 2°. It can be found that the linear increase in contact angle with the decreasing of diameter of Cr nanorings.

This result confirms that the water dewetting behavior on porous film structures is well-described by the Wenzel model which gives the following equation [111]:

(5.3) where r is the roughness factor, and θr and θ are the water contact angles on a rough film surface and a native film surface, respectively. According to Wenzel’s equation the surface roughness can enhance both hydrophilic (water contact angle ＜ 90°) and hydrophobic (water contact angle ＞ 90°) behavior. The surface roughness of periodic Cr nanoring arrays is higher than that of a flat Cr film, hence, the Wenzel model can explain why these solid surfaces show a more hydrophobic behavior.

Fig. 5-7 depicts the optical transmission spectra of the 540 nm periodic hierarchical porous Cr nanoring arrays with different lateral size. The RIE procedure with RF power is composed of 50W for 0 min and 5 min, then 50W for 5 min followed by 30W for 16 and 20 min respectively. It was observed that the transmittance for the porous Cr nanoring arrays tended to increase at long-wavelengths, and the optical transmission spectra for the porous Cr nanoring arrays with lateral size around 160 ± 9 nm that shows a broad maximum at the

broader than those observed in Ag and Au [1, 4]. This can be attributed to a variation of the complex dielectric constants of the metals which gives the following equation:

(5.4) At the relevant wavelengths the dielectric constants of the three metals

, and . The imaginary part ,

which largely determines the linewidth [66]. Therefore, the Cr is far larger than in Ag or Au, whereas the real part is very small. The transmittance for the porous Cr nanoring arrays which apparently tend to long-wavelength had an increase. On the other hand, it is apparent that the extraordinary transmission of the porous Cr nanoring arrays with lateral size is smaller than 170 ± 11 nm at the wavelength of 1000–2500 nm. This can be attributed to the tiny hole in metal films, with sizes smaller than the wavelength of incident light, and leads to a strongly enhanced transmission of light through the smaller perforated metal films [112, 113].Due to the momentum conservation in the interaction between the surface plasmon and incident light on the periodic metal array structure, the following equation is obtained [3]:

(5.5) 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 enhancement of transmission is due to the surface plasmon resonant effect, when the wavelength of incident light was higher than the nanoholes. This tunability of hierarchical porous Cr nanoring array structures are ideal multifunctional plasmonic substrates due to the surface plasmon resonant effect is highly sensitive to the ring-shaped nanostructures leading to the enhancement of optical transmission. The plasmon frequencies of ring-shaped nanostructures are highly tunable and depend both on the lateral and inner size and the ring-wall thickness [8]. It is expected that the porous nanoring arrays nanostructures can be applied in sensing applications due to their ability to contain high volumes of molecules and provide uniform electric fields inside the cavity [9], and can be exploited in the

design of plasmonic waveguides and potential applications in optical telecommunication band [100]. Therefore, this new approach will surely facilitate further exploration of metal nanorings for potential applications in electro-optic devices.

Fig. 5-2 Schematic illustration of fabrication process for Cr ordered high-porosity periodic nanostructured films by NSL. (a) Directional sputtering deposition using CFUBMIP system.

(b) The formation of Cr triangular-shaped nanodot patterns by lift off. (c) Plan-view SEM morphology of the Cr triangular-shaped nanodot arrays. (d) Non-directional sputtering deposition using three-axis rotation of CFUBMIP system. (e) The porous Cr nanoring arrays by lift off. (f) Plan-view SEM morphology of the porous Cr nanoring arrays.

Fig. 5-3 SEM images of sizes-varied Cr nanoring arrays and size statistics. Images (a), (b), (c) and (d) were obtained by the templates shown in Fig. 4-5 (g-h) 50 W, 5 min + 30 W, 8 min (i-j) 50 W, 5 min + 30 W, 12 min (k-l) 50 W, 5 min + 30 W, 16 min, and (m-n) 50 W, 5 min + 30 W, 20 min, respectively.

Fig. 5-4 (a) The nanoring sizes were fabricated at various cumulative etching time. (b) The inter-particle spacing statistics of a series of samples at various cumulative etching time.

Fig. 5-5 AFM images of the Cr nanoring arrays fabricated by colloidal template using RIE with an oxygen source with 50 W for 5 min and 30W for 4 min on glass substrates. (a) Two-dimensional topography surface of the nanoring array has a period of approximately 540 nm. (b) Three-dimensional topography surface of prepared nanoring arrays clearly presented that the original PS nanosphere array is well copied into the surface of Cr film.

Fig. 5-6 Relationship between the lateral size of nanoring and contact angles with 540 nm initial diameter of the PS nanospheres.

Fig. 5-7 Transmission spectra of the porous Cr nanoring arrays with different lateral size.