Colloid Systems: Micelles, Nanocrystals, and Nanocrystal Superlattices
Read full article onlineFull Article
Colloids, which are basically a dispersion of one of the states of matter, i.e., gas, liquid or solid, in the other, are important for a variety of reasons. From natural colloids such as milk, blood, and egg white, etc. to synthetic ones such as paints, inks, creams, sprays, and fire extinguishers, we encounter colloidal systems daily in one way or the other. Generally, the usage of the term colloids would refer to the dispersion of particles (usually polydisperse) in the size range 1 µm–1 nm in another medium without any order. The class of colloids called micelles are formed when molecules consisting of long hydrocarbon tails and polar head groups self-assemble into an ordered structure beyond a critical concentration. Another class of colloid systems, where the particles are highly monodisperse in nature especially in the size range of few nanometer (1 nm = 10− 9 m or 10− 7 cm), have recently attracted renewed interest because of the enormous promise these materials hold for future technologies. These materials—generally called nanoparticles or, if they consist single crystalline phase, nanocrystals—are envisioned to be the building blocks of every new technology from here onward. The control of size and shape of these particles with a penchant to form long-range ordered structures is expected to be very crucial for the fields of electronics, optoelectronics, and smart chips for future devices. Unlike the traditional atomic or molecular crystal lattices, these superlattices consist of a two-tier structure. Here each nanocrystal results from the regular arrangement of the atoms or molecules of the constituent material, in which the crystal sizes are restricted to a few nanometers only. These nanocrystals then become the building blocks of the superlattices. The properties of the resulting superlattices are controlled both by the constituent materials and their structural order.
The purpose of this article is to introduce the reader to the field of colloids, focusing on two subclasses of these systems—micelles and nanocrystals and the superlattices of nanocrystals. Among these, micelles have been studied for a long period of time and there are excellent reviews and books that are dedicated to these systems. Hence only a brief introduction to these systems is given here. The reader is encouraged to refer to more elaborate treatises on this subject for further details. On the other hand, nanocrystals and their superlattices are relatively new entries to this field, and a great deal of attention is being given to them recently because of their applications to various fields, either already established or envisioned. Here we present the recent advances in the synthesis of monodisperse nanocrystals and their arrangement into 2-D or 3-D superlattices. Factors governing their arrangement into different types of superlattices are presented and discussed.