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  • Abstract

Nanoscale Materials: How Small is Big

Nano—it is the prefix that became a word and cultural phenomenon of our times. A casual search of that ubiquitous social barometer, the Internet, reveals a bewildering array of products, services, and science all claiming to be nano. This is despite a 2004 survey of public opinion commissioned by the British government finding that only 29% of respondents had heard the term “nanotechnology” and, of those, only 19% could offer a definition [1]. In these politically correct times, it is perhaps ironic that a prefix derived from the ancient Greek word for “dwarf” has escaped scientific circles and developed its own verve.

Defined in the SI system of units to indicate one-billionth of a quantity, “nano” has been swept up in the rush of miniaturization that has dominated the last half-century of science. Popularly traced to a lecture delivered by Nobel laureate Richard Feynman in 1959, “There's Plenty of Room at the Bottom,” the drive to control matter on ever decreasing scales is best embodied by the progress of the electronics industry under Moore's prophetic vision. Reaching nanometer length scales presents something of a crossover point on this crusade, where the properties of bulk materials start to break down and atomic character emerges.

The nanometer is not an intuitive length scale; one-billionth of a meter, a distance covered by perhaps only a few atoms end to end. This is a length that, compared to a soccer ball, is a soccer ball compared to the earth, or roughly how far your hair might grow between strokes of an electric razor. It is the control of shape and size at this scale to which the term nanotechnology was first applied by Norio Taniguchi in 1974. Today, while fluid in definition, the phrase is most often used for structures with at least one length scale between single atomic and 100 nm, these materials being “nanoscale.”

Though the synthesis and application of what science now calls nanoscale materials is nothing new—one historic example being the centuries old practice of using nanoscale gold to color glass—direct investigation and measurement of nanoscale materials is. Catalyzed by newly developed techniques such as scanning probe microscopy (SPM), the nano technology revolution has spent the last few decades snowballing. Worldwide scientific investment in this revolution was valued at nearly $8 billion as far back as 2004, while the market place for products of the revolution is predicted to top $1 trillion by 2011 [1].

All this however leads one to ask: Why nano? Aren't we just beating up a continued miniaturization that will soon see trendsetters and grant applicants moving onto the next smallest prefix (pico)? The answer lies in the fundamental importance of the nanoscale alluded to earlier. While the nanometer might not be an intuitive scale in our macro world, it holds the key to much of significance in the world of the atom; atomic radii, mean free paths, coherence lengths, wavelengths, and more. It just so happens that when the physical dimensions of a system begin to approach these other critical nanoscale lengths, some very unique physical phenomena present. Suddenly we can control material properties including chemical, thermodynamic, optical, and electrical simply by changing the physical size of a system. In this way platinum, a well-known noble metal, can become a catalyst, aluminium can melt at the temperature of boiling water, yellow gold can become anything from red to blue, and silicon, the well-known semiconductor, can be made a conductor.

The first key explanation for these changes is the increased significance of surface. Consider the geometry of a sphere; surface area varies with the square of radius while volume the cube. For a solid soccer ball, the ratio of atoms on the surface to bulk may be an insignificant one in a billion. For a nanosized cluster, however, there may actually be more surface atoms than bulk. As atoms on the surface behave very differently to those in the bulk, so nanoscale materials will exhibit very different physical properties to those of the bulk.

The second key explanation for nanoscale phenomena are quantum effects. The now famous duality result of quantum mechanics allows particles and quasi-particles to be expressed in terms of wavelength. Where this wavelength approaches or exceeds the physical dimensions of a nanoscale material, bizarre quantum effect ensue. Carriers begin to require shorter wavelengths and therefore higher energies, while tunnelling allows the passage of particles such as electrons through apparent barriers.

As a meeting point between atomic and bulk, the nanoscale is also a significant crossover point for device fabrication—where top-down becomes bottom-up. Top-down describes the traditional approach to miniaturization, whereby features are introduced into a bulk material with ever increasing precision. By far, the majority of current electronics industry practice, techniques such as lithography and milling, are proven but rapidly approaching fundamental physical and economic limits. An alternative is found in the bottom-up approach, where the process is reversed and fabrication begins with fundamental units of material—atoms and molecules.

Broadly divided into three categories—chemical synthesis, positional assembly, and self-assembly—the concept of bottom-up may not be new, but the tools to exploit it are. Chemical synthesis represents the more basic routes that have been, in principle, with us for centuries. Utilizing controlled chemical reactions, often developed more by trial and error than real nanoscale understanding, methods such as deposition and precipitation can deliver nanosizedproducts. Positional assembly offers a more elegant solution employing recently developed tools such as optical tweezers and SPM to manipulate individual atoms and molecules into place. The process does, however, remain laborious to the power of 23 and is hence reserved for more fundamental work. Combing the best of both worlds, self-assembly makes use of specially functionalized molecules that do the work of arrangement themselves through chemical and physical interactions. Mastered by nature, self-assembly promises efficient fabrication and replication of macro sized nanostructured devices. The stuff of science fiction's “gray goo,” a material that replicates uncontrollably, consuming everything in its path, the design of complex self-assembly processes remains, perhaps thankfully, some way into the future.

Fuelling the fire of the nanotechnology revolution has been science's enthusiastic, even bewildering, output of nanoscale materials. As varied as their applications, one fundamental method of classification is by the number of orthogonal dimensions greater than nanoscale. In this way, thin films maybe termed two-dimensional (2-D), nanowires, or similarly high aspect ratio nanomaterials one-dimensional (1-D) and nanoparticles zero-dimensional. As the upper definition of nanoscale (100 nm) is usually far too large to appreciate quantum effects, a similar system may also be used to indicate how many orthogonal directions are greater than “quantum scale.”

Nanoscale thin films are a somewhat mature technology with a broad base of applications across scientific and engineering fields. Produced by a similarly mature array of technologies including vapor deposition, electoylsis, spincoating, and molecular beam Epitaxy (MBE), the two major applications of nanofilms have been as surface modifiers or electronically discrete layers. Sometimes containing nanoscale particles themselves, surface modification films are regularly used to tailor surface area and activity, finding use in applications ranging from barrier layers to fuel cells and self-cleaning windows. While some applications simply reflect miniaturization and not true exploitation of nanoscale effects, quantum effects are now regularly exploited in a range of commercial electronic products such as the humble CD laser. Further development of 2-D nanostructures is likely to come in the form of advanced materials such as the organic nanosheet graphene and precision fabrication techniques such as atomic layer deposition.

One-dimensional or high aspect ratio nanostructures, while most likely the least commercialized of all nanoscale geometries, have arguably occupied the greatest amount of research interest. This is for one good reason—the Carbon NanoTube (CNT), a nanoscale material commonly fabricated from graphite, the material at the end of “lead” pencils. Despite these modest beginnings, CNTs develop some of the highest strength, stiffness, and thermal conductivity values of any known material type in a package that can be mechanically and electrically functionalized. Possible applications range from structural reinforcement in composites to cancer drug delivery and electrical devices. Similar hopes are held for other 1-D nanostructures unfairly hidden in the CNT aura. A large variety of metals, semiconductors, and insulators have been fabricated into a similarly wide variety of nanoshapes; nanowires, nanorods, nanopillars, and even nanopyramids. One particularly promising application is in nanoscale biological sensing, where the long awaited “lab on a chip” could provide your local doctor with real-time diagnosis from a drop of blood.

Zero-dimensional nanostructures—nanoparticles, nanopowders, and nanocrystals—perhaps represent some of the more fundamental nanotechnology research. Derived primarily from the chemical synthesis route, or sometimes even mechanical action, nanoparticles have been fabricated from most material types in work that has focused to the greater part on high surface area and optical effects. Further research is likely to center around shape control, organic nanoparticles termed fullerenes and electronic application as “quantum dots.” Already sold commercially in goods such as catalytic converters, sunglasses, and even cosmetics, it is nanoparticles that have opened one of the first ethical battlegrounds of the nanotechnology revolution. Not unreasonably, concern has arisen where these products are free to enter the environment or human body, a common example being zinc oxide nanoparticles in some sunscreens. Although current chemical regulations provide a partial legislative framework, many developed countries have placed a priority on assessing the risks associated with nanotechnology before further commercialization.

Despite some authors' seemingly suffering nanotechnology fatigue, science is really just looking into the rabbit hole. Generation one of nanotechnology is where we are now, and the word being bandied about by some commentators is “passive.” By this it is meant that current nanotechnology really just represents further miniaturization of existing technology or, at best, singular use of nanoscale effects. The real excitement of the previous few decades has been in the development of nanotechnology tools and the potential that has brought. That potential is now being realized in generation two or “active” nanotechnology—structures that are functionalized on the nanoscale to perform complex tasks. This includes mechanical devices such as actuators, electrical devices such as transistors, and bioactive devices such as targeted drug delivery systems. Beyond these dreams are integrated systems of functionalized nanotechnology; molecular manufacturing, quantum computing, and eventually even self-replicating nanorobots. If you think that all sounds far fetched, just take a look at yourself. Nature in the form of biological systems mastered nanotechnology a while ago—and see where that got us.▪

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References

1. The Royal Society

Nanoscience and Nanotechnologies: Opportunities and Uncertainties, London, U.K., 2004

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