Jeff Brinker (left) and Hongyou Fan
Size Matters
February / March 2005
Volume 3 Number 1
Imagine trying to build a house if you didn't understand the properties of wood for the framing or PVC pipes for the plumbing or metal conduit for the electrical wires. Can you drive a metal nail into wood? Will the wood shatter? Will the nail? Can you really make a watertight seal between a plastic pipe and a clay pipe? Will the wires get too hot in the conduit? Will they melt and merge with the conduit? If they do, will the lights still come on or will the house burn down?
Before you can build something useful from any material you have to know how the material is going to perform under a wide range of conditions. The scientists and engineers at Sandia National Laboratories are studying the properties and behaviors of materials at almost unimaginably small scales to apply chemistry, mechanics, optics and other technical disciplines to new devices that will operate at the nanoscale. To understand the difficulties in simply seeing what is happening at this small scale, it may help to get an idea of the size of things at the atomic level. A nanometer is one-billionth of a meter. For comparison, the smallest features on current computer chips measure about 200 nanometers, and a human hair is 100,000 nanometers thick.
Moving beyond the observation of nanoscale processes and behavior made possible by powerful new tools such as scanning, tunneling and atomic force microscopes, scientists are now poised to make practical advances in nanotechnology, the creation of materials, devices and systems through the control of matter at the atomic level. By understanding and controlling the way molecules organize into nanoscale patterns, scientists are discovering new phenomena and learning to design materials with vastly different sets of properties. Design possibilities are limited only by one's imagination.
First You Observe, Then You Build
Sandia is concentrating its efforts in four areas of nanotechnology: nanochemistry, nanomechanics, nano-optics and self-assembled monolayers. In the search for cleaner air, nanoclustered chemical catalysts could help destroy pollutants using the energy from sunlight. Controlling the size of the nanoclusters determines the ability of the catalyst to decompose pollutants. Effects that come into play at the nanoscale greatly increase the effectiveness of catalysts. Also, these same catalysts have the potential to produce fuels—€”like hydrogen—€”by the decomposition of water and hydrogen sulfide, using sunlight. Cleaning up our water and creating an inexhaustible, nonpolluting fuel are two potential ways to apply nanochemistry to real-world problems.
In nanomechanics, Sandia already has used ion-implantation techniques to create lightweight aluminum composite surfaces that are as strong as the best steel available. Beams of energetic particles produce surface layers, which reduce friction and wear of metals for long-term use in micromechanical devices. These beams might be used to create unimagined performance of nanomaterials for tiny components. Also, just because the technique is applied to the thinnest imaginable layer does not mean that those layers do not impact the surface behavior of macroscale devices. Nanomechanics could also have application in the development of stronger, lighter materials that are more economical to produce.
Someday the fender of your Chevy or Honda might have been made lighter and stronger through nanomechanics.
Nanostructures also allow scientists to manipulate light in ways not previously possible. Based on nanofabricating several 100-nanometer-sized bars into regular structures, photonic lattices allow the steering of light beams around corners and reflect those beams with nearly 100 percent efficiency. This capability represents the new generation of nanocomponents that may bring about an age of optical computing. Already, structuring semiconductors at the nanoscale led to the development of the vertical-cavity surface-emitting laser (VCSEL), the most efficient, low-power light source available and one that is fast becoming a key 21st century technology for communications.
Simple, efficient methods to organize molecules and molecular clusters into precise, pre-determined structures are another important area of nanotechnology exploration. Nature provides many examples of intricately organized architectures, such as sea shells, whose self-assembly is choreographed by molecular interactions. Researchers are applying similar strategies to spontaneously organize new nanocomposite materials.
Self-Assembled Nanocrystals as Cancer Fighters
Sandia scientists are blending the technological building blocks of nanoscale chemistry, mechanics, optics and self-assembly in an effort to use their growing understanding of nanomaterial behaviors to create practical applications. One such application is the self-assembling durable nanocrystal array.
Nanotechnologists are working to develop a simple, inexpensive means of self-assembling nanocrystals (also known as quantum dots) into robust, orderly arrangements. Like the mortar between bricks in a wall, an insulating layer of silicon dioxide (SiO2) could be linked to compatible semiconductor devices. The trapped nanocrystals might function as the medium through which laser beams travel, or as a very fine catalyst with an unusually large surface area, or perhaps as a memory device that is tunable by particle size and composition.
Also, if the nanocrystals could be prevented from clumping together, they could be used as devices in solid-state lighting. The challenge confronting the scientists today is how to make the connections from the macroscale device to the nanoscale monolayer.
Jeff Brinker, Sandia Fellow and University of New Mexico chemical engineering professor, and his Sandia and UNM colleagues are approaching the self-assembly of monolayers in a manner that allows nanocrystal arrays to be integrated into devices using standard microelectronic processing techniques, thus bridging huge gaps in scale. One possible application for the self-assembly process is to use quantum dots for bio-labeling and bio-sensing, perhaps by creating light-emitting tagging mechanisms to locate cancer cells. According to Sandia's Hongyou Fan, who initiated the effort to use the nanocrystals for those purposes, "Our approach makes quantum dots both water-soluble and bio-compatible, which are two essential qualities for in vivo imaging. The functional organic groups on the quantum dots can link with a variety of peptides, proteins, DNA, antibodies, etc., so that the dots can bind to and help locate targets such as cancer cells."
Sandia has applied for a patent, which should aid attempts to identify individual cancer cells before they increase in number. Researchers have found that, in the nanoscopic realm, changing the size of a material changes the frequency it emits when "pumped" by outside energy. Thus, quantum dots of particular sizes and material will emit at predictable frequencies, which makes them useful in locating and identifying particular cancer molecules.
Mimicking Photosynthesis to CHANGE Platinum
In another application of self-assembly, researchers have developed a way of changing the properties of platinum by manipulating the metal at the nanoscale in a way that mimics the action of photosynthetic proteins. Smaller and/or more active catalysts, sensors, and other devices can be built from the manipulated material
The idea for the technique is similar to photosynthesis, a process in which plants use the energy from sunlight to produce sugar. But instead of manufacturing sugar, the new method changes platinum ions to neutral metal atoms. Certain molecules mimic these photosynthetic proteins, repeatedly converting metal ions each time light is absorbed and depositing the metal atoms as desired at the nanoscale.
The method involves putting porphyrin molecules—€”the active part of photosynthetic proteins—€”along with the platinum salt in a solution of water and ascorbic acid at room temperature. When light is shined on the solution, the porphyrins excite, becoming catalysts for platinum reduction and deposition. As this occurs, the metal grows. The platinum nanostructures take on a different form when they are prepared under different conditions—€”for example they may look like three-dimensional Koosh—„ balls or lace-like sheets, depending on the nature of the material on which the metal grows. Since the porphyrin remains attached to the platinum nanostructure and active in the presence of light, it can also perform other functions besides growing itself. For example, when illuminated with light, the platinum nanostructure separates hydrogen from water. This is half the reaction that would produce hydrogen fuel from water using solar energy.
Investigations of the new technique are continuing at Sandia. "We see the possibility of manipulating the nanoscale structure of platinum so that we can have control over the size, porosity, composition, surface properties, stability, and other functional properties of these metal nanostructures," says John Shelnutt, a distinguished member of the technical staff at Sandia and an adjunct professor of chemistry at the University of Georgia . He adds that while research groups have reported a few platinum nanostructures—€”including nanoparticles, nanowires, nanosheets, and others—€”the addition of new types of nanostructures is "highly desirable and potentially technologically important."
All of these advances are coming from the work of multidisciplanary teams. Their ability to understand behaviors at the nanoscale and to bring together and integrate microscale devices is key part of the nanotechnology success story. Scientists and engineering probing the nanoscale are not yet at the point of knowing as much about how things work in their atomic realm as carpenters, plumbers and electricians know about building our houses, but they are making large leaps in understanding this very tiny world.
Margaret Lovell is a senior technical writer at Technically Write, assigned to Sandia National Laboratories.
Copyright © 2012 | Innovation America