Results and Discussion

by CrN deposition and then lift-off PS nanosphere processes. The 3D view of the CrN nanohole array in image (a) shows that the structure was uniform in an area of 20 µm². Image (b) shows the side view and a corresponding line scan of the 2D topography surface and depth profiles, indicating that the nanohole depth was approximately 100 ± 5.6 nm.

Figure 6-5 shows the XRD measurements of the CrN nanohole structure grown on Si(100) substrates in the range of 10–90 (2θ). The XRD pattern of the CrN nanohole indicates two peaks that show preferred orientations indexed to the (111) and (200) planes. The CrN films were indexed to a NaCl-type structure. CrN (200) has the lowest surface-free energy and (111) the lowest strain energy [126].

This study shows the measurement of the contact angles of the three test liquids, including distilled water (θW), ethylene glycol (θE), and diiodomethane (θD), on the CrN ordered nanohole arrays of the nanomold surface. When the contact angles decreased the nanohole diameter increased (Fig. 6-6). The dependence curve between the nanohole diameter and contact angles showed significant variations in the contact angle values on the ordered CrN nanohole array structure. For the surface roughness, Wenzel presented a model describing the water dewetting behavior of nanohole films; the following equation is obtained [111]:

(6.1) where r is the roughness factor, and θr and θ are the contact angles on the nanohole film and native film, respectively. According to Wenzel’s equation, high roughness can enhance both the hydrophobicity of the hydrophobic surface and the hydrophilicity of the hydrophilic surface.

Based on the morphology and topography of the nanomold of the ordered CrN nanohole array structure, these ordered CrN nanohole array films are substantially rougher than the relatively flat surface. The roughness of ordered CrN nanohole array films increased as the nanohole size increased because of the numerous protuberances on the nanohole walls. Thus,

this hydrophilicity increased as the nanohole size increased. Conversely, this indicates that the roughness of the ordered CrN nanohole array films decreased as the nanohole size decreased, producing certain protuberances on the nanohole walls of a large nanohole size. Equation (1) shows that θr should increase, which agrees with the current results ((Fig. 6-3 (a-c)).

This study investigates the surface free energy rather than the contact angle, and therefore, presents the results in terms of the surface free energy. In general, the CrN coatings with a larger contact angle value imply lower surface free energy. Therefore, the CrN coatings have a good anti-adhesive effect, and can be applied to molding components [125]. Three test liquids were used as a probe for the surface-free energy calculations at a surface temperature of 20 ^oC:

distilled water, ethylene glycol, and diiodomethane. The surface tension of distilled water at 20 ^oC was calculated using data from Vargaftik et al. [127]: = 72.8, = 21.8, and = 51.0 mN/m. The surface tension of distilled water was estimated using the following equation:

(6.2) where T(^◦C) is the distilled water temperature.

The surface tension of ethylene glycol at 20 ^oC was obtained using data from Jho and Carreras [128]: = 48.0, = 29.0, and = 19.0 mN/m. The surface tension of ethylene glycol was estimated using the following equation:

(6.3) where T(^oC) is ethylene glycol temperature. The surface tension of diiodomeyhane at 20 ^oC was obtained using the data from Zhao et al. [129]: = 50.8, = 50.8, and = 0 mN/m.

The surface tension of diiodomeyhane was estimated using the following equation:

(6.4) where T(^oC) is the diiodomethane temperature. According to these three empirical equations, the contact angle of at least two liquids with known surface tension components ( , , )

on the solid must be determined. The data for the test liquid surface tension and surface tension components at a surface temperature of 20 ^oC were obtained using these three empirical equations. The solid surface free energy can be calculated the surface energy by measuring fluid contact angle. This measurement method is based on Young's equation, which is expressed in a balanced formula of solid - liquid interface as follows [130]:

(6.5) where is the experimentally determined surface tension of the liquid, θ the contact angle,

the surface-free energy of the solid, and is the solid–liquid interfacial energy.

However, an estimate of must be obtained to obtain solid surface-free energy . The contact angle values of the surface-free energy of the samples were calculated using the Owens–Wendt geometric mean approach [131]. This approach extends the Fowkes equation by including the hydrogen bonding term, and uses a geometric mean to combine the dispersion force and hydrogen bonding components giving the following equation:

(6.6) The Young equation (6.5) shows that

(6.7) To obtain the and values of a thin film, the contact angle of at least two liquids with known surface tension components ( , , ) on the solid must be determined. The surface-free energy was calculated using the contact angle test at 20 ^oC on the nanomold surface of the ordered CrN nanohole arrays. Figure 6-7 shows the relationship between the nanohole diameter and the surface-free energy. The surface-free energy decreases linearly as the nanohole diameter increases because the contact angle can enhance the decrease in nanohole roughness. Thus, the water contact angle increases as the nanohole diameter decreases (Fig. 6-6), which implies a lower surface-free energy.

ordered CrN nanohole structure surface has low surface energy at the interface. This eliminates the sticking problem regarding the nanomold surface during demolding. Therefore, a low surface-free energy in the nanomold is required to meet the ever-increasing demands of NIL. In this study, the diameter of nanohole decreases from 304 ± 10.4 to approximately 136

± 4 nm and the surface-free energy decreases from 40.83 to approximately 24.58 mN/m. The ordered CrN nanohole structure obtained the lowest surface free energy. Therefore, this study reports the fabrication of a 2D nanomold of low surface-free energy, large-scale, well-ordered, periodic nanoholes for NIL using modified NSL. The nanomold surface when the diameter of nanohole decreased with decreasing the surface-free energy. The ordered CrN nanohole array of the nanomold has an anti-sticking property that eliminates the problems of friction and sticking during demolding. This new approach of fabricating an ordered CrN nanohole structure can be used to form a 2D nanomold for NIL.

Fig. 6-2. SEM images for various size of nanosphere arrays. (a) The top view of the spin coated monolayer PS nanosphere arrays with 540 nm diameter on a clean glass substrate. (b) The top view of the PS nanosphere colloidal ion etched by O2 RIE at 50 W for 20 min. The inset shows a cross-sectional view of the PS nanosphere.

Fig. 6-3. SEM images of sizes-varied ordered CrN nanohole arrays and size statistics of the nanomold. Images (a), (b), and (c) were produced by the colloidal templates and thinned by the RIE process at 50 W for 20, 25, and 35 min, respectively. The inset shows (a) a high-magnification cross-sectional view of the CrN nanohole. Image (d) shows the diameter of the nanohole and spacing statistics for a series of samples.

Fig. 6-4. The AFM images and depth profiles of the ordered CrN nanohole arrays of a nanomold fabricated by colloidal templates using O2 RIE at 50 W for 10 min on Si substrates, followed by CrN deposition and lift-off PS nanosphere processes.

Fig. 6-5. The XRD spectrum of the ordered CrN nanohole array structure fabricated using a closed-field unbalanced magnetron sputtering ion plating system.

Fig. 6-6. The relationship between the nanohole diameter and the contact angles of three test liquids.

Fig. 6-7. The relationship between the nanohole diameter and the surface-free energy.