Portland, Ore. - In nature, incredibly complex nanostructures pervade every bulk material, from macroscopic sea shells to microscopic diatoms. But reproducing such atomic accuracy in man-made materials has required costly high-temperature, high-vacuum manufacturing-and the synthetic structures are simpletons compared with nature's creations.
Now, Sandia National Laboratories researchers say they have begun to unravel nature's secrets on the road to developing a "wetware" manufacturing process that is expected to produce inexpensive, waste-free materials with more finely tuned properties than previously achievable.
"Our team has already demonstrated that we can produce materials with superior photocatalytic properties that might be useful for new types of chemical sensors," said Jim Voigt, a chemical engineer on the project at Sandia. "We are not trying to rival the incredibly fine nanostructures produced in nature, but we do think we can learn enough of nature's tricks to inexpensively manufacture materials with some of their remarkable properties."
Project leader Jun Liu, manager of chemical synthesis and nanomaterials at Sandia, said that his goal for the project is reliable and scalable production of nanoscale materials using environmentally benign chemical processes. Other members of the Sandia team are Zhengrong Tian, Matt McDermott, Randy Cygan, Louise Criscenti, Dianna Moore, Jessica Bickel and Tom Sounart.
According to Sandia, the effort's main aim is to decode nature's methodology for creating atomically precise materials. The researchers hope to mimic how nature precisely controls its materials' composition, particle size and shape, crystalline structure, orientation, and surface- and interface-chemistry, despite the "open air" conditions under which natural processes must do their work.
"If you look closely at natural materials, you will see incredibly fine structures," said Voigt. "For instance, sea shells have a structure that alternates between vertical calcite columns and closely packed aragonite nano-platelets, so there must be a mechanism there to switch between them that we might be able to learn how to use."
Liu thinks a staggering number of real-world applications could benefit from elucidation of the principles used by nature to create complex materials. He cited possible applications in microelectronic devices; chemical and biological sensing and diagnosis; catalysis; and energy conversion and storage, including photovoltaic cells, batteries, capacitors, and hydrogen storage devices, as well as light-emitting displays, drug delivery mechanisms and optical storage.
"Biominerals" is the term the team uses to describe complex natural materials that are composed of simple minerals, such as calcium, but that are organized in complex three-dimensional nanostructures. The team's first thrust was to uncover the mechanisms by which such complex crystals are induced into growing at selected sites.
For instance, the biominerals in both macroscopic seashells and microscopic diatoms are synthesized in nature when the organism extracts dissolved ions of calcium and silicate from ocean water and uses proteins to reorganize them into nanostructures.
"We've found that nature uses protein molecules to precisely control the orientation and morphology of biominerals. As a result, these materials are much stronger than normal man-made versions," said Voigt.
Proteins not only determine where the dissolved mineral ions will be deposited but also precisely how they will be structured, according to the Sandia researchers. For instance, the team found that in the marine snail called the red abalone, water-soluble proteins direct the formation of calcite columns. Then, at precisely the right moment, a second protein appears that switches the process from calcite-column formation to the creation of closely packed aragonite nanoplatelets. The result is a layered nanocomposite material that is far stronger than today's less-complex man-made materials.
How they did it
As a first step, the team re-created the "wet chemistry" of nature's nanoscale growth mechanisms. Rather than use high temperatures, high concentrations of chemicals and organic solvents, as in conventional processing, the group concocted a low-temperature, low-chemical-concentration, aqueous "soup."
The result was to reduce the by-products compared with the level of unwanted precipitations engendered by typical man-made processes.
"We found that nature controls biominerals using mechanisms that only allow their formation at specific sites," said Voigt.
Typical man-made processes uncontrollably react not only with the desired devices but also with everything nearby. For instance, man-made processes typically "coat" their own containers, a phenomenon observed in items ranging from cans of paint to chemical vapor deposition equipment.
Biominerals, on the other hand, use chemical and physical codes to induce the proper reactions only at the desired sites, virtually eliminating by-products. Sometimes the entire surface is prepared in a manner that stimulates growth in exactly the right area. Other times nature provides a nucleation "seed" from which new minerals can be formed. In both cases, ions from the surrounding solution provide the supply of raw mineral molecules.
Natural materials use such mechanisms to grow incredibly complex nanostructures, with nature-imposed controls on exactly where the structures will form, how they will be oriented and how they will align with each other and their surroundings. The Sandia team hopes to pattern future microdevices by mimicking nature's approach.
"We don't understand the processes in nature well enough to even approach their complexity, but in our experiment we did learn to control the growth of crystals with some simple organic molecules," said Voigt.
Using computer models, the team designed simple experiments using organic molecules that bind to crystals, thereby directing and controlling their growth. By proving the concept in that way, Liu's team embarked on the long journey toward understanding how nature directs organic growth and translating that into a set of general rules guiding the manufacturing of atomically perfect nanomaterials.
Currently the team is codifying its findings into a set of laboratory tools for controlling the delivery, diffusion and transport of the chemical "species" in its aqueous reaction chambers. The team plans to leverage Sandia's microfluidic platforms to provide a precise mechanism for altering the parameters of its experiments.
The team predicts that its findings will result in manufacturing methodologies that are environmentally benign but that enable superior nanoparticles, nanowires and complexly nanostructured films.