Introduction

The prefix nano in the word nanotechnology means a billionth (1 x lO-9). Nanotechnology deals with various structures of matter having dimensions of the order of a billionth of a meter. While the word nanotechnology is relatively new, the existence of functional devices and structures of nanometer dimensions is not new, and in fact such structures have existed on Garth as long as life itself. The abalone, a mollusk, constructs very strong shells having iridescent inner surfaces by organizing calcium carbonate into strong nanostructured bricks held together by a glue made of a carbohydrate-protein mix. Cracks initiated on the outside are unable to move through the shell because of the nanostructured bricks. The shells represent a natural demonstration that a structure fabricated from nanoparticles can be much stronger. We will discuss how and why nanostructuring leads to stronger materials in Chapter 6.

It is not clear when humans first began to take advantage of nanosized materials. It is known that in the fourth-century A.D. Roman glassmakers were fabricating glasses containing nanosized metals. An artifact from this period called the Lycurgus cup resides in the British Museum in London. The cup, which depicts the death of King Lycurgus, is made from soda lime glass containing sliver and gold nanoparticles. The color of the cup changes from green to a deep red when a light source is placed inside it The great varieties of beautiful colors of the windows of medieval cathedrals are due to the presence of metal nanoparticles in the glass.

Introduction to Nanotechnology, by Charles P. Poole Jr. and Frank J. Owens. ISBN 0-471-07935-9. Copyright © 2003 John Wiley & Sons, Inc.

The potential importance of clusters was recognized by the Irish-born chemist Robert Boyle in his Sceptical Chymist published in 1661. In it Boyle criticizes Aristotle's belief that matter is composed of four elements: earth, fire, water, and air. Instead, he suggests that tiny particles of matter combine in various ways to form what he calls corpuscles. He refers to "minute masses or clusters that were not easily dissipable into such particles that composed them."

Photography is an advanced and mature technology, developed in the eighteenth and nineteenth centuries, which depends on production of silver nanoparticles sensitive to light. Photographic film is an emulsion, a thin layer of gelatin containing silver halides, such as silver bromide, and a base of transparent cellulose acetate. The light decomposes the silver halides, producing nanoparticles of silver, which are the pixels of the image. In the late eighteenth century the British scientists Thomas Wedgewood and Sir Humprey Davy were able to produce images using silver nitrate and chloride, but their images were not permanent. A number of French and British researchers worked on the problem in the nineteenth century. Such names as Daguerre, Niecpce, Talbot, Archer, and Kennet were involved. Interestingly James Clark Maxwell, whose major contributions were to electromagnetic theory, produced the first color photograph in 1861. Around 1883 the American inventor George Eastman, who would later found the Kodak Corporation, produced a film consisting of a long paper strip coated with an emulsion containing silver halides. He later developed this into a flexible film that could be rolled, which made photography accessible to many. So technology based on nanosized materials is really not that new.

In 1857 Michael Faraday published a paper in the Philosophical Transactions of the Royal Society, which attempted to explain how metal particles affect the color of church windows. Gustav Mie was the first to provide an explanation of the dependence of the color of the glasses on metal size and kind. His paper was published in the German Journal Annalen der Physik (Leipzig) in 1908.

Richard Feynman was awarded the Nobel Prize in physics in 1965 for his contributions to quantum electrodynamics, a subject far removed from nanotechno-logy. Feynman was also a very gifted and flamboyant teacher and lecturer on science, and is regarded as one of the great theoretical physicists of his time. He had a wide range of interests beyond science from playing bongo drums to attempting to interpret Mayan hieroglyphics. The range of his interests and wit can be appreciated by reading his lighthearted autobiographical book Surely You're Joking, Mr. Feynman. In 1960 he presented a visionary and prophetic lecture at a meeting of the American Physical Society, entitled "There is Plenty of Room at the Bottom," where he speculated on the possibility and potential of nanosized materials. He envisioned etching lines a few atoms wide with beams of electrons, effectively predicting the existence of electron-beam lithography, which is used today to make silicon chips. He proposed manipulating individual atoms to make new small structures having very different properties. Indeed, this has now been accomplished using a scanning tunneling microscope, discussed in Chapter 3. He envisioned building circuits on the scale of nanometers that can be used as elements in more powerful computers. Like many of present-day nanotechnology researchers, he recognized the existence of nanostructures in biological systems. Many of Feynman's speculations have become reality. However, his thinking did not resonate with scientists at the time. Perhaps because of his reputation for wit, the reaction of many in the audience could best be described by the title of his later book Surety You 're Joking, Mr. Feynman. Of course, the lecture is now legendary among present-day nanotechnology researchers, but as one scientist has commented, "it was so visionary that it did not connect with people until the technology caught up with it."

There were other visionaries. Ralph Landauer, a theoretical physicist working for IBM in 1957, had ideas on nanoscale electronics and realized the importance that quantum-mechanical effects would play in such devices.

Although Feynman presented his visionary lecture in 1960, there was experimental activity in the 1950s and 1960s on small metal particles. It was not called nanotechnology at that time, and there was not much of it. Uhlir reported the first observation of porous silicon in 1956, but it was not until 1990 when room-temperature fluorescence was observed in this material that interest grew. The properties of porous silicon are discussed in Chapter 6. Other work in this era involved making alkali metal nanoparticles by vaporizing sodium or potassium metal and then condensing them on cooler materials called substrates. Magnetic fluids called ferrofluids were developed in the 1960s. They consist of nanosized magnetic particles dispersed in liquids. The particles were made by ballmilling in the presence of a surface-active agent (surfactant) and liquid carrier. They have a number of interesting properties and applications, which are discussed in Chapter 7. Another area of activity in the 1960s involved electron paramagnetic resonance (EPR) of conduction electrons in metal particles of nanodimensions referred to as colloids in those days. The particles were produced by thermal decomposition and irradiation of solids having positive metal ions, and negative molecular ions such as sodium and potassium azide. In fact, decomposing these kinds of solids by heat is one way to make nanometal particles, and we discuss this subject in Chapter 4. Structural features of metal nanoparticles such as the existence of magic numbers were revealed in the 1970s using mass spectroscopic studies of sodium metal beams. Herman and co-workers measured the ionization potential of sodium clusters in 1978 and observed that it depended on the size of the cluster, which led to the development of the jellium model of clusters discussed in Chapter 4.

Groups at Bell Laboratories and IBM fabricated the first two-dimensional quantum wells in the early 1970s. They were made by thin-film (epitaxial) growth techniques that build a semiconductor layer one atom at a time. The work was the beginning of the development of the zero-dimensional quantum dot, which is now one of the more mature nanotechnologies with commercial applications. The quantum dot and its applications are discussed in Chapter 9.

However, it was not until the 1980s with the emergence of appropriate methods of fabrication of nanostructures that a notable increase in research activity occurred, and a number of significant developments resulted. In 1981, a method was developed to make metal clusters using a high-powered focused laser to vaporize metals into a hot plasma. This is discussed in Chapter 4. A gust of helium cools the vapor, condensing the metal atoms into clusters of various sizes. In 1985, this method was used to synthesize the fullerene (C60). In 1982, two Russian scientists, Ekimov and Omushchenko, reported the first observation of quantum confinement, which is discussed in Chapter 9. The scanning tunneling microscope was developed during this decade by G. K. Binnig and H. Roher of the IBM Research Laboratory in Zurich, and they were awarded the Nobel Prize in 1986 for this. The invention of the scanning tunneling microscope (STM) and the atomic force microscope (AFM), which are described in Chapter 3, provided new important tools for viewing, characterizing, and atomic manipulation of nanostr uctures. In 1987, B. J. van Wees and H. van Houten of the Netherlands observed steps in the current-voltage curves of small point contacts. Similar steps were observed by D. Wharam and M. Pepper of Cambridge University. This represented the first observation of the quantization of conductance. At the same time T. A Fulton and G. J. Dolan of Bell Laboratories made a single-electron transistor and observed the Coulomb blockade, which is explained in Chapter 9. This period was marked by development of methods of fabrication such as electron-beam lithography, which are capable of producing IG-nm structures. Also in this decade layered alternating metal magnetic and nonmagnetic materials, which displayed the fascinating property of giant magne-toresistance, were fabricated. The layers were a nanometer thick, and the materials have an important application in magnetic storage devices in computers. This subject is discussed in Chapter 7.

Although the concept of photonic crystals was theoretically formulated in the late 1980s, the first three-dimensional periodic photonic crystal possessing a complete bandgap was fabricated by Yablonoviteh in 1991. Photonic crystals are discussed in Chapter 6. In the 1990s, Iijima made carbon nanotubes, and superconductivity and ferromagnetism were found in Qo structures. Efforts also began to make molecular switches and measure the electrical conductivity of molecules. A field-effect transistor based on carbon nanotubes was demonstrated. All of these subjects are discussed in this book. The study of self-assembly of molecules on metal surfaces intensified. Self-assembly refers to the spontaneous bonding of molecules to metal surfaces, forming an organized array of molecules on the surface. Self-assembly of thiol and disulfide compounds on gold has been most widely studied, and the work is presented in Chapter 10.

In 1996, a number of government agencies led by the National Science Foundation commissioned a study to assess the current worldwide status of trends, research, and development in nanoscience and nanotechnology. The detailed recommendations led to a commitment by the government to provide major funding and establish a national nanotechnology initiative. Figure 1.1 shows the growth of U.S. government funding for nanotechnology and the projected increase due to the national nanotechnology initiative. Two general findings emerged from the study.

The first observation was that materials have been and can be nanostructured for new properties and novel performance. The underlying basis for this, which we discuss in more detail in later chapters, is that every property of a material has a characteristic or critical length associated with it. For example, the resistance of a material that results from the conduction electrons being scattered out of the direction of flow by collisions with vibrating atoms and impurities, can be

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