Resist-free antireflective nanostructured film fabricated by thermal-NIL
© Kang et al.; licensee Springer 2014
Received: 31 March 2014
Accepted: 13 April 2014
Published: 20 May 2014
Resist-free antireflective (AR) nanostructured films are directly fabricated on polycarbonate (PC) film using thermal-nanoimprint lithography (T-NIL) and the moth-eye shape of AR nanostructure is elaborately optimized with different oxygen reactive ion etching conditions. Anodic aluminum oxide (AAO) templates are directly used as master molds of T-NIL for preparation of AR nanostructures on PC film without an additional T-NIL resist. AR nanostructures are well arranged with a period of about 200 nm and diameter of about 150 nm, which corresponds to those of the AAO template mold. The moth-eye AR nanostructures exhibit the average reflectance of 2% in wavelength range from 400 to 800 nm. From the results, highly enhanced AR properties with simple direct imprinting on PC film demonstrate the potential for panel application in the field of flat display, touch screen, and solar cells.
Recently, an antireflection (AR) property has been a great interest due to the demands for the enhanced performance in electronic devices such as solar cells and displays . Especially, needs for AR property of plastic substrate have been increasing for the rapid growth of flexible electronic fields. Many research groups have already investigated various AR techniques such as single or multi-layer coating, surface texturization, and fabricating the periodic nanostructures . AR nanostructure, so-called moth-eye structure, can efficiently reduce the light reflectance attributed to the gradual change in refractive index between the air and substrate by altering the volume fraction of structures, which in turn reduce Fresnel reflection . There are many techniques available for fabricating AR nanostructure, such as electron beam lithography , thermal imprinting , hybrid nano-patterning lithography , dip pen nanolithography  and so on. However, these techniques still require high cost, complex process, and sophisticated equipments. In order to overcome these drawbacks, we employ the nanoimprint lithography (NIL) which is one of the most potential nano-scale patterning techniques with advantages of rapidness, low cost, and reproducibility. In NIL process, molds or stamps are normally produced using various materials, such as silicon, glass, and metal. However, it is difficult to prepare the periodically uniform nano-sized patterns on large area using molds from these materials. Therefore, an anodic aluminum oxide (AAO) template can be good candidate for mold material due to the advantages of the facile large scale synthesis via electrochemical method and periodical nano-sized patterns with high aspect ratio .
In this study, we report a facile method for the preparation of AR nanostructure on PC film using AAO template as a master mold through the T-NIL process. Furthermore, the shape of AR nanostructures is elaborately optimized with different reactive ion etching (RIE) conditions and the optical properties of the fabricated AR films are investigated.
Prior to T-NIL process, AAO templates were coated with the anti-adhesion layer to decrease the surface free energy of AAO for the easy separation of the AAO templates from PC film. A liquid phase deposition of 1H, 1H, 2H, 2H-perfluorooctyl-trichlorosilane was used to form the self-assembled monolayer (SAM) based anti-adhesion layer. The formation of hydrophobic anti-adhesion layer on the surface of the AAO templates was confirmed by contact angle measurement which was determined to be 150° as shown in Figure 1(b).
Results and discussion
We successfully demonstrate that the resist-free AR nanostructures are directly fabricated on PC film using T-NIL and the shape of AR nanostructures is elaborately optimized with different oxygen RIE conditions. AAO templates are directly used as master molds of T-NIL for preparation of AR nanostructures on PC film without an additional T-NIL resist. AR nanostructures are well arranged with a period of about 200 nm and diameter of about 150 nm, which corresponds to those of the AAO template mold. The average reflectance of the PC film with AR nanostructures was obtained to be about 2% in the broadband wavelength range of 400–800 nm. Comparing to bare PC film and the fabricated AR film, the average reflectance of AR film is reduced to approximately 11% from 13% to 2% in the visible light region. As a result, moth-eye AR nanostructured PC film using T-NIL and oxygen RIE can efficiently reduce the light reflectance at the visible light region. We suggest that the prepared AR film is useful for panel application in the field of flat display, touch screen, and solar cells.
Young Hun Kang is currently a senior researcher in Korea Research Institute of Chemical Technology, Daejeon, Korea. He received BS from Sungkyunkwan University in 2006 and MS from University of Science & Technology in 2009. He has recently researched development of solution processable organic & inorganic hybrid materials for application to thin film transistors and thermoelectric devices based on various printing technologies.
Jae Hyung Han is a researcher of the Creative Research Center of Graphene Electronics in ETRI, Korea. He received MS from the Department of Advanced Device Technology in University of Science and Technology (UST) in 2012.
Dr. Song Yun Cho is currently working for Korea Research Institute of Chemical Technology as a principal research scientist. He received MS in Polymer Science & Engineering from Inha University in 1998 and Ph. D. in Chemistry at the Pennsylvania State University in 2004. His recent research is specialized in the organic or oxide semiconducting materials for electronic and energy applications.
Dr. Choon-Gi Choi received Ph. D. from Université d’Orléans (France) in 1996. He is currently a director at the Creative Research Center for Graphene Electronics in Electronics and Telecommunications Research Institute (ETRI) and an adjunct professor at the Department of Advanced Device Technology in University of Science and Technology (UST), Korea. He is also an associate editor of the journal Nano Convergence with Springer publishing. His research interests are graphene electronics and optoelectronics, metamaterials, nanophotonics, nano-imprint lithography.
This research was supported by the Nano R&D Program (No. 2012–0006204) and GNT R&D Program (No. 2012–0006657) through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning of Korea.
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