Fabrication of Monolayer Polymer/Nanosphere Hybrid at a Water-Air Interface
Below is an illustration of preparing monolayer of close-packed NSs at water/air onterface. First, monodisperse 1 mm PS NSs are mixed with ethanol and then carefully spread onto a water/air interface. The diffraction pattern shows hexagonal symmetry and enlargement in reciprocal lattice spacing as the NSs area fraction fL increases. After introducing PEO into water, we noticed that only first-order peaks appear in the diffraction pattern which sets an upper limit to the separation d of 60 nm. This inferred further reduction of d and also the formation of close-packed NSs at the water/air interface resulting from the addition of PEO polymer.
In the figure above, the left and right panels show the images of monolayer NSs with diameter 1 mm and 220 nm, respectively, on a silicon (100) substrate prepared by this approach. The defect-free region is more than 100 mm x100 mm for 1 mm NSs and 20 mm x20 mm for 220 nm NSs. We remark that the entire substrate area larger than 1 cmx1 cm is uniformly covered by the monolayer NSs.
The measured average Zeta potential on the PS NSs logarithmically decays with PEO concentration shown in the left panel above. This suggests an effective screening of the coulomb repulsive force between NSs which further reduces the separation between NSs. When NSs are close enough, it triggers the polymer bridging effect and makes the structure of monolayer NSs robust against the surface tension during the NSs transfer process. This scenario is confirmed by the observation from the SEM images which shows PEOs residual on the surface of nanosphere and also between the nanospheres (right panel above). In addition, high speed and high resolution optical microscopy also reveals the immobility of nanospheres at the water/air interface after introducing the PEOs. The links to the movie clips are listed below, where a strong suppression of Brownian motion in PEO-bridged PS NSs is observed. This mimics the famous ancient Chinese legend of interconnected ploys (連環計) during the great Battle of Red Cliffs (赤壁之戰) to immobile the battleships on the river in the year A.D. 208.
PS NSs floating at water/air interface (click here if not shown)
PEO-bridged PS NSs floating at water/air interface (Click here if not shown)
Related publications:
1. Kun-Yang Wu, Yu-Tai Tao, Chi-Chih Ho, Wei-Li Lee, and Tsong-Pyng Perng, "High-performance space-charge-limited transistors with well-ordered nanoporous aluminum base electrode," Appl. Phys. Lett. 99, 093306 (2011).
2. Chi-Chih Ho, Po-Yuan Chen, Keng-hui Lin, Wen-Tau Juan and Wei-Li Lee, "Fabrication of Monolayer of Polymer/Nanospheres Hybrid at a Water-Air Interface," ACS Applied Materials & Interfaces 3, 204 (2011).
3. Chi-Chih Ho, Tung-Wu Hsieh, Hsiang-Hsi Kung, Wen-Tau Juan, Keng-Hui Lin, and Wei-Li Lee, "Reduced saturation magnetization in cobalt antidot thin films prepared by polyethylene oxide-assisted self-assembly of polystyrene nanospheres," Appl. Phys. Lett. 96, 122504 (2010).
4. US/Taiwan Patent pending.