Nanocrystals Synthesized in Colloidal Self-Assemblies
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During this last decade, because of the emergence of a new generation of high-technology materials, the number of groups studying nanomaterials has increased exponentially. The electrical, optical, and magnetic properties of inorganic nanomaterials vary widely with their sizes and shapes. Nanomaterials are used in several domains such as chemistry, electronics, high-density magnetic recording media, sensors, and biotechnology. This is, in part, because of their novel material properties, which differ from both the isolated atoms and the bulk phase. An ultimate challenge in materials research is the creation of perfect nanometer-scale crystallites identically replicated in unlimited quantities in a state than can be readily handled and can behave as pure macromolecular substances. Thus the ability to systematically manipulate these is an important goal in modern materials chemistry. Optimizing this ability requires an understanding of nanocrystal growth, which turns out to be a complex process. The essential first step in the study of their physical properties and the use of nanomaterials in various technologies is their production. Several approaches to manipulate inorganic nanocrystals have been undertaken. The major contribution was to produce spherical nanocrystals with a very low size distribution. Deposition processes include use of microwave plasma, low-energy cluster beam deposition, inorganic chemistry, ball milling, sonochemical reactions, sol–gel, and flame by vapor phase reaction and condensation. In 1986, we developed a method based on reverse micelles (water-in-oil droplets) to prepare nanocrystals. Normal micelles make it possible to produce ferrite magnetic fluids.
To control the shapes of nanocrystals, several procedures are now being studied. Hard templates are employed to direct 1-D nanostructure growth. The nanometer-sized pores in membranes and zeolites are utilized to confine the growth of wires. Alternatively, lithography and deposition are combined to create quantum wires on single-crystal surfaces. Electrochemical synthesis is used to produce well-defined nanorods. In 1993 and again in 1995, we were able to partially control the shape of nanocrystals by using colloidal solutions as templates.
In the following, we will concentrate mainly on nanocrystal growth in colloidal self-assemblies and describe discrepancies in the control of size and shape.