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Results and Discussion

在文檔中 中 華 大 學 (頁 160-171)

Chapter 7 Patterns of Polycarbonate Ordered Antireflective Nanopillars Arrays

7.3 Results and Discussion

accuracy of the nanohole array’s structure.

Fig 7-4 (a) shows the AFM of 2D topography surface of the nanomould of CrN ordered nanohole arrays fabricated by colloidal templates and by RIE processing using O2 plasma of 50 W for 20 min on Si substrates. The CrN nanohole’s structure with a period of 540 nm was determined by the initial diameter of PS nanospheres. The depth profiles showed the mean values of depth of nanoholes to be around 100 ± 6 nm. Fig 7-4 (b) shows the X-ray diffraction (XRD) pattern of CrN film growen on Si (100) substrates. The result indicates two peaks which show preferred orientations indexed to (111) and (200) planes. The (200) peak is much larger than the other peaks and has the lowest surface free energy [126].

For the fabrication of ordered CrN nanohole arrays of anti-sticking nanomoulds, it is necessary to create a hydrophobic surface (CA > 90°) and lower the surface free energy.

Hence, in this study, we used contact angle values of the three test liquids, distilled water (θW), ethylene glycol (θE), and di-iodomethane (θD) to calculate the surface free energy for the CrN nanomould. Fig 7-5 (a) shows the water static droplet in contact with ordered CrN nanohole array’s patterned surfaces. The drop image was processed using image analysis that calculated both left and right contact angles from the drop’s shape with ± 0.1° accuracy. This measured the contact angle between the surface and a line that is tangential to a drop of liquid on the CrN pattern surface. A water droplet forms to minimize interfacial energy if the surface has a low surface energy. Figure 5 (b) represents the relationship between the diameters of ordered CrN nanohole array’s structure and contact angles. It can be seen that the contact angle increased with decreasing diameter of nanoholes. The water contact angles measured on diameters of 540 nm periodic of sizes-varied CrN nanoholes changed from 91.6° to around 102.2°, indicating a surface free energy decrease at the interface.

Generally, for the surface roughness, Wenzel presented a model describing the water dewetting behavior on nanohole films; the following equation is obtained [111]:

where r is the roughness factor, and θr and θ are the contact angles on a nanoholes film and a native film, respectively. According to Wenzel’s equation, obviously, high roughness can enhance both the hydrophobicity of hydrophobic surfaces and the hydrophilicity of hydrophilic surfaces. The surface roughness of periodic CrN nanohole array’s structure was higher than that of flat CrN films, therefore, the roughness itself was insufficient to render the surfaces hydrophobic. It is the combination of the ordered CrN nanohole arrays structure and the roughness of nanohole that produced the high contact angles. The Wenzel model can explain why these surfaces demonstrate more hydrophobic behavior.

Fig 7-6 indicates the relationship between the diameter of ordered CrN nanoholes and the surface free energy. The solid surface free energy can be calculated the surface energy by measuring fluid contact angle. This measurement method on Young's equation, it 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.

To obtain the solid surface free energy an estimate of must be obtained. According to the contact angle values of the three test liquids, the surface free energy of the samples were calculated using the Owens–Wendt geometric mean approach [131], this extended the Fowkes equation by including the hydrogen bonding term, and using the geometric mean to combine the dispersion force and hydrogen bonding components giving the following equation:

(6.6) From the Young Eq. (6.5), it follows that

(6.7) To obtain and 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 room temperature on the nanomould surface of ordered CrN nanohole arrays. From the results, it was found that the linear surface free energy tended to decrease with decreasing diameter of nanohole. This can be attributed to the contact angle increasing with the decreasing the roughness of nanohole. The surface free energy decreased from 46.02 to around 35.72 mN/m with decreasing roughness of nanohole.

The hierarchical nanohole arrays of the CrN nanomould have a low surface energy at the interface when produced by the magnetron sputtering method using NSL based technology, and can eliminate the problem of the sticking to the nanomould surface during demolding.

AFM images in Fig 7-7 represent the order of the tapered antireflective nanopillar arrays on the PC film surface fabricated by the thermal nanoimprint procedure. The evaluation of the period, diameter and height of the nanoimprinted nanopillars was performed by AFM analysis.

The nanoimprinted nanopillars were obtained using the ordered CrN nanohole arrays of anti-sticking nanomould fabricated by colloidal templates using the RIE process. Fig 7-7 (a) and (b) indicate that the 540 nm periodic arrays of the diameter at full width half maximum (FWHM) of the tapered nanopillar were approximately 300 ± 14 nm and 200 ± 5 nm individually. The height profiles show the mean values of height of nanopillars to be around 85 ± 5 nm. From the results, the tapered nanopillars on the PC film surface replicated from CrN nanomould were successfully de-molded with fine features. It can be seen that the antireflective nanopillars did not follow the semicircular profiles of the CrN nanoholes. They formed tapered shapes with an ordered distribution on the PC film surface. We deduced that these tapered antireflective nanopillars were formed due to the viscous effect of the PC film near the melting temperature. The ordered pattern with tapered shapes makes these nanopillars a good antireflective layer.

Fig 7-8 shows the transmission spectra of the 540 nm periodic tapered antireflective nanopillar array’s structure with different diameters on the PC film surface, the transmittance

was observed that the transmittance of the patterned PC substrate was higher than that of the un-patterned bare PC substrate due to a decrease of reflection on the PC surface by the tapered antireflective nanostructure patterns. The transmittance of the tapered antireflective nanopillars with an aspect ratio at the wavelength of 1200–1600 nm was around 92.9 %, and at the wavelength of 1800–2100 nm was around 91.2 %. Compared with a bare PC film, the improvement of transmittance is 2.7 % at a wavelength of 1800–2100 nm, and around 2.8 % at a wavelength of 1800–2100 nm. However, compared with a bare PC film, the improvement of transmittance of the tapered antireflective nanostructure with diameter of 300 nm at a wavelength of 1200–1600 nm was around 0.5 %, and at a wavelength of 1800–2100 nm was around 0.5 %. This can be attributed to the abrupt change of the refractive index between the PC and the tapered antireflective nanostructures due to changes in the diameter and aspect ratio of the nanostructures. From the result, it was found that the antireflective properties were strongly dependent on the shape and diameter of the tapered antireflective nanopillars.

Fig. 7-2 SEM images of (a) and (b) top view of the polystyrene nanosphere colloid with a 540 nm diameter ion etched by O2 RIE of 50 W for 5 and 20 min, respectively. Images (c) and (d) show the cross-sectional view of the polystyrene nanospheres, respectively.

Fig. 7-3 SEM images of size-varied ordered CrN nanohole arrays and diameter statistics of nanomould. Images (a), (b), and (c) were obtained by the colloidal template and thinned by the RIE process at 50W for 5 min, 10 min, and 15 min, respectively. Image (d) shows the diameter of nanoholes statistics of a series of samples.

Fig. 7-4 (a) AFM 2D topography surface of the nanomould of ordered CrN nanohole arrays which were fabricated by colloidal templates using the RIE process with O2 plasma of 50 W for 20 min on Si substrates. The depth profiles show the mean values of depth of nanoholes to be around 100 ± 6 nm. (b) XRD spectrum of the ordered CrN nanohole arrayed structures fabricated by using a closed field unbalanced magnetron sputtering ion plating system.

Fig. 7-5 (a) Water droplet in contact with ordered CrN nanohole array’s patterned surfaces.

(b) Relationship between the diameters of ordered CrN nanohole array’s structure and contact angles with 540 nm initial diameter of nanosphere.

Fig. 7-6 The relationship between the diameters of ordered CrN nanohole array’s structure and surface free energy.

Fig. 7-7 AFM images of the order of the tapered antireflective nanopillar arrays on the PC film surface fabricated by nanoimprint lithography. Images (a) and (b) were obtained by the ordered CrN nanohole arrays of anti-sticking nanomould using the RIE process with O2

plasma of 50 W for 5 and 20 min on Si substrates, respectively. The height profiles show the mean values of nanopillars with height around 85 ± 5 nm.

Fig. 7-8 Transmission spectra of the 540 nm periodic the tapered structure of antireflective nanopillar arrays with different diameters on a PC film surface.

在文檔中 中 華 大 學 (頁 160-171)

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