Figure 1: Types of anisotropy in particles
|Figure 2: Template free electroless coating approaches to patchy and Janus particles|
Small particles are widely exploited in a broad range of materials and devices, ranging from dense, closely packed layers to a dilute dispersion in a matrix. The interactions utilized to form such arrangements are generally isotropic and the properties of the resulting material can often be regarded as equivalent to those of an effective, isotropic medium. There is however, a rapidly growing interest in the development of particles with anisotropic properties. There are two principal reasons for this trend. On the one hand, anisotropy enables particles to take the role of "intelligent" building blocks, combining together according to asymmetrically directed interactions to form hierarchical structures with features unlike anything possible with isotropic particles (see for instance the recent impressive work on biphasic particle assembly by the Granick Group at UIUC). On the other hand, exciting technological possibilities are opened up due to the novel or enhanced properties of the anisotropic particles or structures derived from them. Examples of promising applications can be found in many diverse fields including displays, solar cells, electronic devices, security labelling, medical diagnostics and therapeutics.
A particle can be anisotropic in a number of different ways (See figure below). In general we can identify three principal anisotropic dimensions: shape, volume or surface (Figure 1). From the point of view of experimental studies, the first two of these have by far enjoyed the most prolific exploration. This is due to the fact that both of these anisotropic dimensions can be realised in single phase particles formed by relatively simple approaches i.e. colloidal synthesis, lithography etc.. Indeed, the current literature reveals that a myriad of particles with different shapes and sizes can now be synthesised, and likewise, routes to particles of materials with permanent or inducible dipoles are widespread. Despite these advances, there is presently quite a deficit regarding the practical exploration of surface anisotropic, or, as they are usually known, "patchy", particles. In particular there is an almost total absence of studies of systems which have realistic potential for exploitation on application scales. However, the development of strategies to scalably and tunably incorporate heterogeneities onto otherwise isotropic particle surfaces are essential if the full potential of anisotropy referred to above can be realised.
We have recently reported that silver patches can be formed on non-functionalized silica core particles using a carefully controlled electroless deposition approach. This method relies on the fact that silver ions become sufficiently concentrated at the silica surface due to electrostatic interactions that when reducing conditions are generated (by addition of formaldehyde and a base, ammonia), heterogeneous nucleation and surface growth is preferred over homogeneous nucleation.
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