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A Matter of Size: Triennial Review of the National Nanotechnology Initiative
WHAT IS SELF-ASSEMBLY?
In the broadest sense, self-assembly describes the natural tendency of physical systems to exchange energy with their surroundings and assume patterns or structures of reduced free energy. Random thermal motions bring constituent particles together in various configurations, so that stable configurations (those with significant binding energy) form, tend to persist, and eventually become predominant. Through this simple operation of physical law, pattern or structure arises in a bounded system with the input of relatively little information from outside. The information on how to assemble the structure is embodied in the structures of the individual components. A system slowly approaching equilibrium will assume a simple repetitive structure, while a dynamic system may generate structures of great complexity. For example, molecules in a cooling bucket of water will self-assemble as simple ice crystals, while the same molecules in a turbulent cloud with ever-changing temperature and humidity will self-assemble as complex snowflakes in enormous variety. Many fascinating structures in the natural world around us are self-assembled.
Chemists and biologists often use the term self-assembly in a more restricted sense to describe structure formation in a fluid containing various types of molecules, particularly organic molecules that form weak chemical bonds with a strength that depends sensitively on molecular shape and orientation. The strongest bond between such molecules often occurs when the molecules fit together in a “lock and key” fashion. Biological molecules such as proteins are particularly suited to forming complex higher-order structures. For example, the bacterial ribosome—a complex molecular machine consisting of about 55 different protein molecules and several ribosomal RNA molecules—will, under appropriate conditions, self-assemble in a test tube.2
MOLECULAR SELF-ASSEMBLY AS A MANUFACTURING TECHNOLOGY
For the Manufacture of Materials
Relatively complex materials such as semi-permeable membranes are manufactured every day by processes that exemplify molecular self-assembly. In a broad sense, fabrication and manufacturing processes for many common materials are exercises in self-assembly—quenching, solidification and crystallization, solution-and vapor-phase chemical reactions, and polymerization. The properties of the resulting materials—for example, the strength of metals or the electron mobility of semiconductors—depend exquisitely on the self-assembly of atoms and molecules to form the atomic and molecular structure of the finished material. The trick