Park et al. Nanoscale Research Letters 2012, 7:351 http://www.nanoscalereslett.com/content/7/1/351 Page 2 of 5 Figure 1 Schematic outline. Schematic outline of the procedure to fabricate an Ag nanowire-grid polarizer by LB-nTM. protruding surfaces of the mold as a result of the selective inking. The mold with solidified ink is then brought into contact with a substrate surface covered by a thin polar li- quid layer. Due to the solidification of the ink solution, LB- nTM does not suffer from surface diffusion and can gener- ate nanometer-scale metal lines well below 100 nm. The polar liquid layer on the substrate forms a liquid bridge (a capillary bridge) [20-23] between the substrate and the mold that contains recessed patterns. The liquid bridge allows good conformal contact between the solidified ink and the substrate. As the liquid evaporates, the attractive capillary force gradually increases, pulling the two surfaces into contact, and providing good conformal contact be- tween them with no additional pressure to the mold. After drying, the separation of the mold from the substrate then results in the formation of the well-defined Ag nanowire grids onto the substrate. Methods Materials 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, Syl- gard 184) was ordered from Dow Corning (Dow Corn- ing, 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-compo- nents Inc., Seongnam, South Korea). The PET substrates were cleaned with methanol and deionized water, and fi- nally blow-dried with nitrogen to remove the contami- nants. 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 [24]. A thin oxide layer was grown by placing the Si substrate in a piranha solu- tion (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. Sample characterization 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 spec- trometer (Agilent 8453 UV–vis, Agilent Technologies Inc., Santa Clara, CA, USA). Results and discussion For fabrication of NWGPs, Ag nanowire patterns were made on a variety of substrates such as oxidized Si(100),