Self-Assembling Nanoparticles into Complex Nanostructures

Nanotubes

In 1992, seminal nanotechnology pioneer Dr. K. Eric Drexler introduced the term "molecular manufacturing," which he defined as the "chemical synthesis of complex structures by mechanically positioning reactive molecules, not by manipulating individual atoms.” (See the h+ article “How Close Are We to Real Nanotechnology?” in Resources) Drexler described nanofactories in which nanomachines (resembling molecular assemblers, or industrial robot arms) combine molecules to build larger atomically precise parts. These parts, in turn, can be assembled by positioning mechanisms of assorted sizes to build macroscopic (visible) but still atomically-precise products. The concept is that a functioning nanofactory will create virtually any product at the cost of only the input raw material and energy. Here’s an animated video that illustrates potential nanofactory operations:

“Nanomanufacturing” refers to the production of structures "bottom up" from nanoparticles (materials at the nanoscale of 10-9 meters) or "top down" in steps for high levels of precision. Unlike molecular manufacturing, it doesn’t necessarily require chemical synthesis. Nanoparticles provide numerous possibilities for applications in nanotechnology due to their amazing properties. However, realizing their potential versatility requires assembly of nanoparticles in regular patterns on surfaces and at interfaces. Assembling nanoparticles generates new nanostructures, which in turn have unforeseen collective, intrinsic physical properties. These properties can be exploited for multipurpose applications in nanoelectronics, spintronics, sensors, and so forth.

Recently, a team led by Dr. Ting Xu at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory made an important advance towards this nanotechnology goal. They found a simple and yet powerful way to induce nanoparticles to assemble themselves into complex arrays. By adding specific types of small molecules to mixtures of nanoparticles and polymers, Dr. Xu’s group directed the self-assembly of the nanoparticles into arrays of one, two and three dimensions with no chemical modification of either the nanoparticles or the block copolymers. In addition, they found that the application of external stimuli – light and/or heat – can be used to further direct the assemblies of nanoparticles for even finer and more complex structural details.

This video produced by Northern California’s KQED describes some of the ongoing nanotechnology research at the Lawrence Berkeley National Laboratory:

Small as they are, nanoparticles are essentially all surface. According to the Berkeley Lab News Release on Dr. Xu’s research, any process that modifies the surface of a nanoparticle can profoundly change the properties of that particle. Precisely arranging these nanoparticles is critical to tailoring the macroscopic properties during nanoparticle assembly. While chemical DNA can be used to induce self-assembly of nanoparticles with a high degree of precision, it only works well for organized arrays that are limited in size – it is impractical for large-scale fabrication. Dr. Xu’s approach is to use block copolymers – long sequences or blocks of one type of monomer molecule bound to blocks of another type of monomer molecule. Like soldiers lining up in formation, the block copolymers assemble at densities of 10 trillion bits per square inch. Dr. Xu’s technique promises to revolutionize the data storage industry, eventually leading to the contents of hundreds of DVDs — or its equivalent — fitting into a space the size of a thumbnail.

By adding specific types of small molecules to mixtures of nanoparticles and polymers, Dr. Xu’s group directed the self-assembly of the nanoparticles

Dr. Xu, an assistant professor of materials science and engineering and of chemistry at UC Berkeley, is being honored as one of the "Brilliant 10" young researchers in the November 2009 issue of Popular Science. Her group is now working on applying the nanoparticle self-assembly technique to paper-thin, printable solar cells, and ultra-small electronic devices. "We’ve advanced the technique to make the nanocomposites responsive to light, which could enable the development of photovoltaic cells that are more energy efficient," says Xu.

The nanofactories of tomorrow will likely require both molecular manufacturing as envisioned by Dr. Drexler using chemical synthesis and nanomanufacturing techniques like Dr. Xu’s. Nanoparticles can now be induced to self-assemble non-chemically using block copolymers in regular patterns on surfaces and at interfaces to provide better data storage, solar cells, and tiny electronics.

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