ABSTRACT
Figure 8.1 A SEM image of a PS colloidal monolayer on a glass substrate. Abbreviation: SEM, scanning electron microscopy. It was found the as-prepared PS colloidal monolayer has good transferability from its substrate to other ones, including silicon, silica, mica, a transmission electron microscopy (TEM) grid, and even other hydrophobic and curved surfaces. This is of great importance for templating applications on any desired substrate. We can thus design and fabricate micro/nano-structured arrays on any substrate according to desires in devices [24]. Figure 8.2 is a demonstration for such transferability. First, a PS colloidal monolayer on a glass slide is put into distilled water in a cup slowly. Then the monolayer can be peeled off from the glass due to surface tension of water, and it floats onto the water surface retaining its integrity. Finally, the monolayer on the water surface is picked up with a desired substrate. After drying, the monolayer is thus obtained on a new substrate. It
should be mentioned that if a colloidal monolayer on the original substrate was placed for a long time (say, several weeks), it could not be transferred because of binding between colloidal spheres and the original substrate. A B C
Figure 8.2 Photos depicting transferring a centimeter-square-sized PS colloidal monolayer from a glass substrate onto a silicon substrate. (A) The monolayer on a glass substrate; (B) lift-off onto water; and (C) pick-up of the monolayer with a silicon substrate. It should be mentioned that the as-prepared colloidal monolayer is of close packing due to the self-assembly drive. However, in many applications, the colloidal monolayer with non-close-packing is also desired to realize micro/nano-structured arrays in controllable size and morphology. Nowadays, there also exist some routes to obtain non-close-packed colloidal monolayers in controlled spacing between neighboring spheres. Jiang et al. [25] developed a changed spin-coating technique to embed non-close-packed silica colloidal crystals in a poly-(ethoxylated trimethylolpropane triacrylate) (PETPTA) matrix; after removal of PETPTA, a non-close-packed colloidal monolayer was thus obtained. In addition, a non-close-packed colloidal monolayer can also be fabricated by insetting a close-packed colloidal monolayer into an extendable polydimethylsiloxane (PDMS) film. If extending in different ways, the morphology of the non-close-packed colloidal monolayer can also be controlled. Our group also obtained a non-close-packed PS colloidal monolayer by plasma-etching the corresponding close-packed one. Figure 8.3 shows two typical samples after etching a 2,000 nm PS close-packed colloidal monolayer for different times. It can be seen, after etching for 30 minutes, PSs were reduced to 1,820 nm in diameter, but still connected with necks, due to the area contact between PSs induced by heating treatment (see Fig. 8.3A). If etching for 45 minutes, PSs were completely isolated from each other with size reduced to 1,620 nm in diameter (Fig. 8.3B). The tilted images show that the etched PSs are still spherical in shape (see Fig. 8.3C, D),
owing to a nearly isotropic etching process. Obviously, spacing between the etched PSs (or size of the etched PSs) can be easily controlled by the etching time.
