Figure 2 shows the FE-SEM images of the nanoimprinted AR nanostructure on PC film by T-NIL. Although the pore shape of AAO templates was clearly observed on the surface of PC films, AR nanostructure is roughly formed at a pressure of 8 bar, as shown in Figure 2(a) and (b). It is attributed that the periodic nano-sized pores of AAO templates were difficult to be filled with PC at too low process temperature. Whereas, AR nanostructure fabricated at 160°C shows a period and height of 200 and 300 nm, respectively with a diameter about 150 nm, as shown in Figure 2(c) and (d). These pattern properties on PC well correspond with those of AAO template. This result means that nanoimprinting process for AR nanostructure on PC is highly dependent on processing temperature and AR nanostructures can be completely replicated on PC film at 160°C. However, there exist the problem of separation between AAO template and PC film for high temperature process over 160°C due to the intense adhesion between inner walls of AAO nano-pores and the melted PC film, which results in the structural damage of AR nanostructures.
To evaluate the AR properties of nanoimprinted PC films, the reflectance was measured by UV-visible spectroscopy in the wavelength range from 400 nm to 1,000 nm, as shown in the Figure 3. It is clearly observed that the AR nanostructures prepared at 160°C exhibits the average reflectance of about 5%. In comparison with that of the bare PC film, the average reflectance was considerably reduced from 13% to 5% in the visible light range. At 140 and 150°C, the average reflectance of the AR nanostructures is about 7% and 9%, respectively because periodic AR nanostructures were not clearly formed and separated.
In order to optimize the shape of AR nanostructures and reduce the reflectance, AR nanostructures fabricated at 160°C by T-NIL was etched by the oxygen RIE process with the various etching time. It is observed that oxygen RIE process has great influence on the surface morphology of AR nanostructures, as shown in Figure 4. The AR nanostructures which were formed by RIE process for 60 s obviously represents the pillar shape, as shown in Figure 4(a). However, with the increase of the etching time over 60 s, AR nanostructures are collapsed due to the prolonged ion bombardment during the RIE process. At 180 s of the etching time, as shown in Figure 4(c), the period of AR nanostructures is increased and the pillar shape is changed with the decrease of height and width.
Figure 5 shows the reflectance of AR nanostructures after oxygen RIE process. The average reflectance of AR nanostructures after oxygen RIE process with 60 s etching time exhibits about 2% in the visible light range from 400 nm to 800 nm. In compared with that of the AR nanostructures for the etching time of 180 s, the average reflectance is also decreased from 5% to 2%. Therefore, the average reflectance of AR nanostructures with additional 60 s RIE etching time is significantly decreased from 5% to 2% in the 400–800 nm visible range in comparison with that of the nanoimprinted AR nanostructures at 160°C.
The reflectance difference between the bare PC film and PC film with oxygen RIE processed AR nanostructures is demonstrated visually in Figure 6. The film on the left-hand side is the bare PC film and the film on the right-hand side is the AR nanostructured PC film. The bare PC film has strong surface reflection because of no AR on the surface. On the contrary, no noticeable surface reflection can be observed with the AR nanostructured PC film. Therefore, it is observed that PC film with oxygen RIE processed AR nanostructures can obviously transmit the text of printed page due to the significant reduction of light reflectance. These results highlight the potential of fabricating a high performance AR film by forming moth-eye nanostructures on polymer films.