Nanotechnology Paves Way for Coming Scientific Revolution
Science of submicroscopic substances begins to reach marketplace with the first nanobased tools, solutions.
Scientists at Sandia National Laboratories, Albuquerque, New Mexico, are conducting ground-breaking research into super-small structures that has led to prototype devices such as ultraminiaturized chemical sensors and analyzers, tiny medical devices, super-strong alloys, and catalysts for destroying hazardous materials. Future applications could include filters that selectively admit or seal out substances through molecule-sized valves, medical devices that precisely monitor patient health and deliver exact doses of medication based on that data, and clothing that knows when the wearer is hot or cold and then admits air or becomes an insulator accordingly.
Experts predict that the study and development of very small machines and materials built up atom by atom will continue to open the possibilities for engineering, medicine, physics and chemistry. A step beyond microengineering and the microelectronics most commonly associated with semiconductors, nanotechnology involves objects that are a thousand times smaller than those of the microworld. Nano-sized items measure in the billionths of a meter and are created through the manipulation of atoms, molecules or molecular clusters to form structures that range in size from 1 to 100 nanometers.
Terry Michalske, senior manager at Sandia National Laboratories, explains that nanoscience as a field of study began in the late 1980s. The availability of new tools such as the scanning tunneling microscope helped spark interest in researching extremely small objects because it allowed scientists to begin probing and measuring individual nanostructures. The ability to observe coupled with the development of fabrication techniques led to the field’s growth, he adds.
The science allows researchers to tailor a nanoscale structure in a very regular and precise manner. The ability to control both the structure and the chemistry is a key development in nanoscience that Michalske believes will lead to many technological applications. For example, Sandia is researching structures that can sort, detect or identify individual types of molecules and determine if they are biological or chemical agents. The ability to build materials on the scale of specific molecules will allow more efficient methods of chemical and biological detection, he says.
According to S. Thomas Picraux, director of the physical and chemical sciences center at Sandia National Laboratories, and Michalske, some of the first nanotechnology applications to enter the market will be in chemical and biological detectors and in micromechanical applications. Sandia’s µChemLab project is an example of this trend. The device is a prototype sensing microsystem designed to detect chemical and biological substances such as explosives and biological warfare agents. µChemLab relies on the integration of chemical, electronic, micromechanical and photonic devices into a single microsystem. Measuring only 1 centimeter on each side, nanostructured materials are incorporated into the chemical preconcentrators used to collect selected molecules. Detection is based on integrated optical fluorescence and piezoelectric wave detectors.
Nanotechnology can also be applied to a number of existing materials or be used to create entirely new substances, Picraux says. “I can make aluminum that’s as hard as steel, and I can make nickel—which is typically soft—extremely hard by adding in nanoprecipitates in the 1-to-5 nanometer scale. I can build structures that use nanoporous materials and use these as a front end, a preconcentrator in a microchemical laboratory called µChemLab on a chip. That has essentially increased sensitivity by a factor of 500,” he explains. With chemical and biological agents, the preconcentrators greatly increase detection capability by being tailored to accept these materials. These structures can also be integrated with a molecular coating designed to preferentially capture specific molecules, Picraux adds.
Nanoscale structures can be created in two primary ways: atomic-level manufacturing and self-assembly. In atomic-level construction, atoms are carefully deposited in layers. This process allows researchers to create materials that do not normally exist naturally or that cannot be made by conventional materials processing methods. A new class of microlasers called vertical cavity surface-emitting lasers (VCSELs) is an example of this approach. These devices are constructed from multiple nanometer-thick layers of different semiconducting materials that are sandwiched together. The specific wavelength and intensity of the emitted light can be tuned by controlling the spacing between material layers in the fabrication process. According to Sandia scientists, VCSELs are more powerful, efficient, coherent and cost-effective than conventional microlasers and have great potential in high-bandwidth, optics-based telecommunications networks.
Self-assembly takes advantage of the inherent tendency of molecules and molecular clusters to interact and organize themselves into larger-scale structures. This process appears in nature because molecules have built-in preferences to combine with certain other molecules in specific ways. Natural nanofabrication processes include the formation of structural proteins by living cells to make cytoskeletons, keratins and membranes. They are self-assembled from simpler molecules with the aid of highly specific enzymes.
Sandia scientists note that one of the advantages of this method is that structures can be formed without using strong external forces such as heat or pressure. This makes the self-assembly process more energy efficient and potentially more environmentally friendly than standard manufacturing processes, experts say. Self-assembly is also a very quick process, taking place in seconds or minutes as the molecules arrange themselves. Previous nanocomposite assembly methods involved a tedious process of depositing multiple layers of substances.
Unlike atomic layering techniques, self-assembly permits the creation of three-dimensional structures. However, the principles controlling this process are not well understood, Michalske observes. One research goal is to find ways to influence self-assembly down to the interaction of individual molecules to control the way structures are formed. Michalske believes that directed self-assembly will become a key element in nanoscience. “We are looking at what happens when you supply an electric or magnetic field to that collection [of molecules], where one can bias or in some way guide the assembly process. We’re marveling at its capabilities and trying to ask questions that will allow us to get in and actually dial in the parameters that can be used to select the ultimate structure that it produces. We’re certainly not there yet,” he says.
However, an area of self-assembly that is already showing promise is the construction of porous membranes. These materials are currently used in applications such as µChemLab as chemical preconcentrators. Because scientists can precisely control pore size, the resulting film is tailored to the size of specific molecules, which makes it very useful in chemical and biological sensors. For example, in a chemical sensor, the surface area of the film could be set to be highly sensitive to specific agents such as sarin gas. According to Sandia researchers, under test conditions, a sensor equipped with this film detected 200 parts per billion of a sarin gas facsimile.
Another potential application of porous nanoscale membranes is the development of molecular valves. Michalske notes that scientists are learning how to assemble organic layers within the pores so that they expand and contract when an electric field is applied. Valve expansion closes off the pores and contraction reopens them. Such materials could be used as sophisticated filters. One possible application would be in a smart gas mask that could react to specific types of chemical and biological agents by sealing itself off, Picraux observes.
Biological systems are a major source of inspiration for self-assembled materials, Michalske maintains. An important model scientists are investigating is the cell wall. These membranes have a tremendous ability to detect specific molecules and convey chemical signals through a complex system of self-assembly. Sandia scientists are exploring some of these principles for use in new chemical and biological detectors, he notes.
Both Michalske and Picraux believe that in addition to biological and chemical applications, nanoscale devices will initially play a role in micromechanical and microelectronic engineering. While scientists are adept at miniaturizing mechanical devices such as gears and levers, materials behave differently at microscopic levels. One important behavioral difference involves a phenomenon called stiction—the tendency of very small objects such as microscopic gears to adhere to each other when at rest. In applications where this tendency is a problem, one possible solution is to apply a nanoscale molecular coating to the surfaces of micromechanical devices. Picraux notes that the molecules of this self-assembling substance possess two different properties—one end adheres to the surface of the device, while the other end creates a slick surface that serves as a nanoscale lubricant.
Other uses of nanotechnology are for electronics and optics. The quantum transistor is a prototype device capable of trillions of operations per second at extremely low power—tens of millivolts and microamps. The device gets its speed from its design and quantum effects that occur at that scale. The chip consists of two gallium arsenide layers 150 angstroms thick separated by a 125-angstrom-thick aluminum gallium arsenide barrier. Normally, the barrier acts as a wall between both layers. But when a current is applied, electrons can in effect “tunnel” through this barrier with extreme speed. This technology can be fabricated in a large-scale industrial process and has potential applications in satellites, smart missiles and optical detectors. Picraux notes that the devices can now operate at 32 degrees Fahrenheit. This marks a considerable improvement in the technology. When the chips first appeared in 1998, they required an operating temperature at or below –321 degrees Fahrenheit.
A potential optics technology being examined is the photonic lattice. This device consists of silicon bars at the 1.5-micron scale that cross each other like Lincoln logs. Photonic lattices act like flawless mirrors that can turn light with extreme efficiency, Picraux says. Potential applications include chemical sensing where the interaction of substances with the photonic surface itself is analyzed in a direct optical readout.
Nanotechnology can also be used in chemistry. Picraux notes that Sandia researchers are making spherical cages to encapsulate other particles. Molybdenum disulfide particles are one family of these. When made in the 3-nanometer scale, these particles are extremely effective as catalysts for breaking down organic contaminants—such as pentachlorophenols—with sunlight. This photocatalytic destruction operates when the nanospheres trap a molecule and expose it to sunlight. Because the nanocages absorb light through the entire solar spectrum, they are better catalysts than cadmium sulphide powders and titanium dioxide particles, Picraux observes. This efficiency stems from an effect called quantum confinement. Because the spheres are so small, they absorb light over a wider spectrum, he points out.
Smart materials research is another avenue for nanotechnology development. Michalske notes that applications include porous films and molecular valves that could result in new methods for drug delivery in medical settings. By developing devices capable of sensing their immediate chemical environment—a patient’s body for example—specific amounts of pharmaceuticals could be released when needed.
Further into the future, the ability to control porosity based on environmental or chemical cues could lead to changes in clothing. Michalske envisions a material that operates like a smart Goretex by allowing air in until it receives a chemical signal to no longer be porous. This would allow clothing to be worn in a greater variety of temperature and weather conditions, he states.
Michalske notes that one application of nanotechnology that is currently being used in the commercial sector involves drill bits for manufacturing circuit boards. This is a high-wear environment; however, by coating the bits with carbide nanoparticles, wear resistance is greatly improved, he says.
While much of the work is still in the research stage, products based on the ability to structure items entirely at the nanoscale level are now entering the market, Michalske says. Concerning future applications, however, he believes that these early discoveries only represent the tip of the iceberg. “We are now developing the tools and understanding to really exploit the potential,” he reveals.
The Clinton administration has given nanoscience an important financial boost with the National Nanotechnology Initiative. Launched in January, the goal of the program is to increase funding for related research and development by 83 percent to $495 million by fiscal year 2001. The initiative is also designed to strengthen scientific disciplines and create critical interdisciplinary opportunities. Participating agencies include the National Science Foundation, the U.S. Defense Department, the U.S. Energy Department, National Institutes of Health, National Aeronautics and Space Administration, and the U.S. Commerce Department’s National Institute of Standards and Technology.