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Spontaneous organization of matter into hierarchical ordered structures is ubiquitous in nature. From a single strand of DNA in a biological cell to a colony of king penguins on an Antarctic island, self-assembled structures exist not only on molecular level, but also on a macroscopic length scale. Under a strict definition, self-assembly is a process whereby a large quantity of pre-existing components undergo spontaneous organization to form larger structures driven by internal interactions or external constraints. The assembled structures may retain the properties of their constituent building blocks, but more importantly, can demonstrate new collective phenomena due to the interactions between their components.
Under this definition, crystallization of atoms driven by the formation of covalent or ionic bonds is a self-assembling process. However, it is the self-assembly process driven by noncovalent interaction that has attracted much more attention lately. This is largely because the energy associated with noncovalent bonds is comparable to thermal energy. Thus, a slight variation of interaction or temperature can drive the same system into vastly different structures. This is partly responsible for the existing complexity of nature. Understanding the mechanism for self-assembly is not only of fundamental interest, but may also lead to many technological applications. While the vast majority of self-assembled structures exist in biological systems, this review is from the material's point of view with only a few exceptions. This is largely due to our own research interests as well as the practical consideration that it is impossible to cover every aspect of these research activities in a short review. Nevertheless, we hope to capture the essence of self-assembly, and refer interested readers to several other excellent review articles and monographs published in this encyclopedia and elsewhere.