Self-assembly

Self-assembly methods (Denkov et al., 1993) appear to be a cost-effective way to create templates, from which an array of structures as shown in figure 5.1 can be formed (Ghanem et al., 2004, Zhukov et al., 2003). These structures are the motivation behind this chapter. One particular method of chemical self-assembly involves the formation of templates through the evaporation of an aqueous suspension of polystyrene latex spheres (Bartlett et al., 2002, Bartlett et al., 2003a), initiating the self-assembly. Figure 5.3 shows this process schematically.

Figure 5.4: A typical nanodot ``droplet'' geometry arranged in a hexagonal lattice (artist's impression on the left) and a schematic of the droplet (right)

Using these templates, it is possible to create magnetic structures from sizes of 50nm to 1000nm by filling the spaces between the close-packed spheres with some material through electrochemical deposition. By etching away the polystyrene spheres, another template is formed. This template can then be filled with magnetic material, and by varying the fill amount of the resulting spherical holes, connected or disconnected arrays of dots can be formed. This is known as the double-template self-assembly method.

The resulting structures have several applications, such as photonic materials (Bogomolov et al., 1997, Vlasov et al., 2001), microchip reactors (Gau et al., 1999) and biosensors (Velev and Kaler, 1999). The application in which we are interested for this work pertains to magnetism.

We present here the results for two types of nanodot geometry -- part-sphere and ``droplet''. The droplet geometry can be considered to be a part-sphere with a rounded upper; this is described in more detail in section 5.5.