One-step fabrication of nanowire-grid polarizers using liquid-bridge-mediated nanotransfer molding
© Park et al.; licensee Springer. 2012
Received: 24 April 2012
Accepted: 27 June 2012
Published: 27 June 2012
Ag nanowire-grid polarizers (NWGPs) were prepared by a one-step fabrication method, called liquid-bridge-mediated nanotransfer molding (LB-nTM). LB-nTM is a new direct nano-patterning method based on the direct transfer of various materials from a mold to a substrate via liquid layer. We fabricated NWGPs with Ag nanowire arrays (81 nm parallel lines and 119 nm spaces) on 2.5 in. transparent substrates by LB-nTM using an Ag nanoparticle solution. The maximum and minimum transmittances of the Ag NWGP at 800 nm were 80% and 10%, respectively.
Polarizer is an indispensable device in a wide range of optical systems, including flat panel displays, microdisplays, and optical networking. Nanowire-grid polarizers (NWGPs) have been of great interest because of their excellent polarization performance and planar structure that allows them to be integrated to other thin-film optoelectronic devices. Moreover, they show good optical stability with respect to variation of the polar angle and azimuthal rotation, and excellent durability at high temperatures and under exposure to high UV flux . The NWGP generally consists of fine grid of parallel metal nanowires with space and width less than the wavelength of light. For the light polarized parallel to the nanowire, it reflects, whereas for the light polarized perpendicular to the nanowire, it transmits. This type of polarizer shows very high extinction ratio between the reflected transverse electric-polarized light and the transmitted magnetic-polarized light over a wide wavelength range and incident angle.
There are several fabrication methods for generating the NWGPs, which include photolithography , e-beam lithography [3–6], laser interference lithography [7, 8], and nanoimprint lithography [9–17]. Among these techniques, nanoimprint lithography is a cost-effective and high-throughput method for the fabrication of metal nanowire grids over a large area, but it suffers from problems. For instance, it involves additional etching or sidewall deposition processes, and continuous fabrication can be difficult because vacuum conditions are required for metal deposition. Recently, we have developed a new direct printing method for generating nanometer-scale patterns of various materials, called liquid-bridge-mediated nanotransfer molding (LB-nTM) . LB-nTM is based on the direct transfer of various materials from a mold to a substrate through a liquid bridge between them. This new technique is capable of generating well-defined large-area nanowire patterns through one step and is well suited for use in automated direct printing machines. LB-nTM is the most efficient method for the fabrication of the wire-grid polarizers at low cost and low environmental impact.
Unless otherwise noted, all commercial materials were obtained from Aldrich Chemical Co. (St. Louis, MO, USA) and used without further purification. The Ag nanoparticle ink (DGP 40LT-15 C) was purchased from Advanced Nano Products (Chungcheongbuk-do, South Korea). The ink contained 20 wt.% silver nanoparticles, with a particle diameter of 40 to 50 nm, dispersed in methanol solvent. PUA (MINS-ERM, Minuta Tech. Co. LTD, Gyeonggi-do, Korea) was used to prepare the UV-curable hard molds. Polydimethylsiloxane (PDMS, Sylgard 184) was ordered from Dow Corning (Dow Corning, Midland, Michigan, USA). Deionized water was purified with a Millipore Milli Q plus system (Billerica, MA, USA), distilled over KMnO4, and then passed through a Millipore Simplicity system.
Preparation of substrates
The flexible substrates employed in this study were cut from polyethylene terephthalate (PET) films (i-components Inc., Seongnam, South Korea). The PET substrates were cleaned with methanol and deionized water, and finally blow-dried with nitrogen to remove the contaminants. The Si substrates used in this research were cut from n-type (100) wafers with resistivity in the range of 1 to 5 Ω·cm. The Si substrates were initially treated by a chemical cleaning process, which involves degreasing, HNO3 boiling, NH4OH boiling (alkali treatment), HCl boiling (acid treatment), rinsing in deionized water, and blow-drying with nitrogen, proposed by Ishizaka and Shiraki, to remove contaminants . A thin oxide layer was grown by placing the Si substrate in a piranha solution (4:1 mixture of H2SO4:H2O2) for 10 to 15 min. The substrate was rinsed several times in deionized water (resistivity = 18 MΩ·cm) then dried with a stream of nitrogen.
The samples were characterized by using a scanning electron microscopy (SEM, Hitachi S4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan) at 15 kV and a UV–vis spectrometer (Agilent 8453 UV–vis, Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
In summary, we described a one-step fabrication of an Ag NWGP using LB-nTM. Ag nanowire arrays, produced by LB-nTM using an Ag-particle solution, exhibited fine fidelity with a high aspect ratio (approximately 1.73). The Ag NWGPs were fabricated on 2.5 in. PET substrates and showed a high transmission and a contrast ratio for the range of visible light. This method is an ideal fabrication technique for automated direct printing machines that produce large area NWGPs on diverse substrates with no additional steps.
This work was supported by a grant from the National Research Foundation (NRF) funded by the Korea government (MEST) (No. 2011–0029811), a grant from the Global Frontier R&D Program at the Center for Multiscale Energy System funded by the National Research Foundation (No. 2011–0031562) and by a Global Ph. D. Fellowship funded by NRF (No. 2011–0007507).
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