Micro- or nano-patterns of nanoparticles have recently attracted considerable interest due to their possible applications in high-density magnetic storage media (S. Sun et al. 2000), magneto-optical devices (H. Akinaga et al. 2000), and quantum dot displays (K. Suzuki et al. 1999). To obtain these particle patterns, various methods have been successfully applied such as selective wetting on a patterned self-assembled monolayer (SAM) (S. Palacin et al. 1996), layer-by-layer (LbL) self-assembly on a patterned SAM (H. Zheng et al. 2002; and I. Lee et al. 2002), LbL self-assembly combined with lift-off and/or metal mask techniques (F. Hua et al. 2002a; F. Hua et al. 2002b), electron beam lithography after deposition of a nanoparticle layer (X. M. Lin et al. 2001; M. H. V. Werts et al. 2002), and micro-contact printing (μCP) of nanoparticles using soft poly(dimethylsiloxane) (PDMS) network stamps (P. C. Hidber et al. 1996 and Q. Guo et al. 2003). However, the first (S. Palacin et al. 1996) and the second methods (H. Zheng et al. 2002; and I. Lee et al. 2002) need pre-patterned SAM layers on a substrate typically prepared by μCP with PDMS stamps, which involves additional fabrication steps of original masks and stamps. The third method (F. Hua et al. 2002a; F. Hua et al. 2002b) involves many steps such as conventional lithography to prepare photoresist patterns and lift-off and/or metal mask after LbL self-assembly. In the fourth technique (X. M. Lin et al. 2001; M. H. V. Werts et al. 2002) it is crucial to prepare dense mono- or multilayers of nanoparticles on a substrate surface, which may not be easy in a large area fabrication. The last method (P. C. Hidber et al. 1996 and Q. Guo et al. 2003) also needs dense mono- or multilayers of nanoparticles on a PDMS stamp and sometimes additional adhesion promoters are needed for smooth pattern transfer to another substrate. With the exception of e-beam lithography (M. H. V. Werts et al. 2002) the size and periodicity of patterns obtained by the above methods are typically on the order of microns.
Recently, a few research groups used the capillary forces of a receding liquid front to self-assemble particles into physically templated wells on both micrometer and nanometer scales (Y. Yin et al. 2001, J. P. Spatz et al. 2002, and M. J. Misner et al. 2003). Particularly, Spatz et al. (2002) reported an ordering of single polystyrene-block-poly(2-vinylpyridine) (PS-PVP) micelle loaded with tetrachloroauric acid into each regularly-spaced hole of photoresist patterns with an aspect ratio of a=0.4˜2.7 prepared by e-beam lithography. They observed a circular depletion zone without nanoparticles around a hole, which was consistent with the capillary force effect. Misner et al. (2003) also reported the self-assembly of nanoparticles using capillary forces into block copolymer templates of perpendicularly oriented cylindrical wells obtained by UV irradiation. They found that nanoparticles with a diameter larger than 10 nm could not be accommodated perfectly in the cylindrical wells due to the small diameter (˜20 nm) of the wells.