Possibilities


The global excitement over nanotechnology has resulted from several discoveries late in the twentieth century. Primary among them is the ability to manipulate individual atoms in a controlled fashion, a sort of atomic brick-laying made possible by techniques such as scanning probe microscopy (SPM). The ability to produce significant amounts of nanoparticles, such as silver and gold nanoparticles, has also fed the excitement. And the underlying fact that materials and devices on the scale of atoms and molecules have new and useful propertiesdue in part to quantum and surface effectscan also be cited. Nanoscale quantum dots with remarkable optical properties are a prime example.

Another major contributor to the interest in nanotechnology was the discovery of carbon nanotubes (CNTs), which are extremely narrow, hollow cylinders made up of carbon atoms. With the possibility of single- and multiple-walled varieties, each with a myriad of potential applications, carbon nanotubes offer many exciting prospects. The single-walled versions can have various geometries, as shown in Figure 14-1. Depending upon the exact orientation of the carbon atoms, the nanotubes can exhibit either conducting (metallic) or semiconducting properties. This fact, plus the ability to grow CNTs at specific locations and to manipulate them after growth, leads to optimism that CNTs will be important for electronics and sensors, either alone or integrated with each other.

Figure 14-1. Carbon nanotubes can exist in a variety of forms, which can be either metallic or semiconducting in nature. (Courtesy of NASA Ames Research Center, Moffett Field, CA.)


For these and other reasons, the funding for nanotechnology has increased more than a factor of 5 in the period 1997 to 2003, and it is still increasing.[3] With the major increase in funding and the resulting scientific breakthroughs, there is great potential for improvements in existing sensors and for the development of really new sensors. One market study of nanoenabled sensors predicts sales near $0.6 billion by 2009.[4] Another is much more aggressive, saying that the nanotechnology sensor market will generate global revenues of $2.7 billion by 2008.[5] Whatever the actual outcome, it seems clear that the sales of nano-enabled sensors will grow significantly, and the growth is largely a result of the research investment in nanoscience and nanotechnology.

Relentless Integration

Historically, there has been a relatively clean separation between the materials, device, and systems levels. However, in recent decades, integration has been a major theme of technology. First, transistors were made into integrated circuits. Then such microelectronics were integrated with microoptics and micromechanics. The microtechnology devices were packaged individually and mounted on printed circuit boards. The recent use of flip chipswhere the chip is the packageand the placement of passive components within printed circuit boards are blurring the classical distinction between devices and systems. The integration of nanomaterials into MEMS devices is now a major research topic. Nanotechnology makes it possible to integrate the different levels to the point that the (very smart) material essentially is the device and possibly also the system. Larry Bock, founder of Nanosys, recently noted, "Nanotech takes the complexity out of the system and puts it in the material."[6]

Now the vision of sensing the interaction of a small number of molecules, processing the data, transmitting the information using a small number of electrons, and then storing information in nanometer-scale structures can be seriously contemplated. Fluorescence and other means of single-molecule detection are being developed. Data storage systems, which use proximal probes to make and read indentations in polymers, are being developed by IBM and others. They promise read and write densities near one trillion bits per square inch, far in excess of current magnetic storage densities.[7]

The integration of such nanoscale capabilities, once they are adequately developed, is challenging, but it would result in capable smart sensors that are small, use low power, and can be used cheaply in large numbers. These advances will enable, for example, more in-situ sensing of structural materials, more sensor redundancy in systems, and increased use of sensors in size-and weight-constrained systems, such as satellites and space platforms.

The production of nano-enabled sensors depends on the availability of nanomaterials and processes to grow them in place or else the practice of placing previously grown materials in the right positions in the desired amounts and geometries. Hence, there is a three-dimensional space that interrelates nanomaterials, processes for making or handling them, and specific sensors. This is sketched in Figure 14-2. What materials are used, how they are employed, and why a particular sensor is being produced all come into consideration. A given material might be made or handled by a variety of processes and made into different sensors. A particular process might be germane to various materials and sensors.

Figure 14-2. The schematic relation between nanomaterials, processes for making or emplacing them, and the sensors that are thus enabled.


Advances in Processing

Recent progress in "top-down" manufacturing processes has helped enable microtechnologies as well as nanotechnologies. Leading-edge integrated circuits are made by top-down processes in which lithography, etching, and deposition techniques are used to sculpt a substrate, such as silicon, and then build up structures on it. Using these approaches, conventional microelectronics has approached the nanometer scale as the line widths in chips surpass the 100-nm level and are continuing to shrink. MEMS devices are constructed in a similar top-down process. As these processes continue to shrink to smaller and smaller dimensions, they can be used for making a variety of nanotechnology components, much as a large lathe can be used to make small parts in a machine shop.

In the nanotechnology arena, such processes are being joined by a collection of "bottom-up" methods in which individual atoms and molecules are used to construct useful structures. Under the right conditions, the atoms, molecules, and larger units can self-assemble.[8] Alternatively, directed assembly techniques can be employed.[9]

The control of both self-assembly and directed assembly presents major technical challenges. Processes for getting nanomaterials to grow in the right places in the desired geometries require development. Overall, the combination of nanoscale top-down and bottom-up processes gives the designer of materials and devices a wide variety of old and new tools. It is also possible to combine micro- and nanotechnologies to enable the development of new sensor systems.

Diverse Nanomaterials

Nanomaterials and structures offer other possibilities for sensors. There are two often separable functions within many sensors, especially those for chemicals and biological materials. They are recognition of the molecule of interest, and transduction of that recognition event into a useful signal. Nanotechnology offers new possibilities in both of these fundamental arenas in a manner that offers benefits far beyond the advantages offered by MEMS and other microsensors.

Most of the nanomaterials that have been produced and used to date contain only one kind of nanostructure and chemistry. There are a few exceptions, but the situation for nanomaterials is now similar to that for macromaterials before composites, such as fiberglass, were formulated. There is no reason that composites of two or more nanomaterials should not be made, tested, and exploited.

As with large-scale materials, the combinatorics of making composite materials results in a very large number of potential new materials. This is illustrated in Figure 14-3 for composite materials made of two nanomaterials. Making nanocomposites from different kinds of nanomaterials, which can have a wide range of chemistries, presents many processing challenges. It is expected that the production and use of composite nanomaterials for sensors and other applications will increase steadily in the coming years.

Figure 14-3. Binary composites made of nanomaterials taken two at a time. The top row contains zero-dimensional (0-D) materials, the second row 1-D materials, the third row 2-D materials, and the bottom row 3-D materials. The binaries represent 28 possible composite materials.


New Tools

Another aspect of the development of nanotechnology-based sensors deserves emphasis. Because of experimental tools now availablenotably synchrotron X-radiation and nuclear magnetic resonancethe atomic structures of many complex molecules are now known. But knowledge of the structure on the nanometer scale is not enough. It is the interactions of atoms and molecules that are important in the recognition, and sometimes the transduction, stage of sensing. The availability of powerful computers and algorithms for the simulations of interactions on the nanoscale makes it possible to design nano-tech-based sensors computationally, and not just experimentally. Molecular dynamics codes and calculations are already a fundamental tool in nano-science, and they can be applied to the development of nanotechnology-based sensors.[10]




Nanotechnology. Science, Innovation, and Opportunity
Nanotechnology: Science, Innovation, and Opportunity
ISBN: 0131927566
EAN: 2147483647
Year: 2003
Pages: 204

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