Nanotechnology

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Bottom-up Approach

As a result, scientists have become interested in another vastly different approach to creating structures at the nanoscale, known as the bottom-up approach. The bottom-up approach involves the manipulation of atoms and molecules to form nanostructures. The bottom-up approach avoids the problem of having to create an ever-finer method of reducing material to the nanoscale size. Instead, nanostructures would be assembled atom by atom and molecule by molecule, from the atomic level up, just as occurs in nature. However, assembly at this scale has its own challenges.

In school, children learn about some of these challenges when they study the random Brownian motion seen in particles suspended in liquids such as water. The particles themselves are not moving. Rather, the water molecules that surround the particles are constantly in motion, and this motion causes the molecules to strike the particles at random. Atoms also exhibit such random motion due to their kinetic energy. Temperature and the strength of the bonds holding the atoms in place determine the degree to which atoms move. Even in solids at room temperature—the chair you may be sitting on, for example—atoms move about in a process called diffusion. This ability of atoms to move about increases as a substance changes from solid to liquid to gas. If scientists and engineers are to successfully assemble at the atomic scale, they must have the means to overcome this type of behavior.

A clear example of such a challenge occurred in 1990 when scientists from the International Business Machines Corporation (IBM) used a scanning probe microscope tip to assemble individual xenon atoms so that they formed the letters IBM on a nickel surface. To prevent the atoms from moving away from their assigned locations, the nickel surface was cooled to temperatures close to absolute zero, the lowest temperature theoretically possible and characterized by the complete absence of heat. (Absolute zero is -273.15°C [-459.67°F].) At this low temperature, the atoms possessed very little kinetic energy and were essentially frozen.

Achieving this temperature, however, is impractical and uneconomical for the operation of commercial devices. Nevertheless, the ability of scientists to manipulate atoms was one of the first indications that the bottom-up approach might work. It also signaled the emergence of nanotechnology as an experimental science.

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IV

The Emergence of Nanotechnology

The concept of nanotechnology originated with American physicist Richard P. Feynman. In a talk to the American Physical Society in December 1959, entitled “There’s Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics,” Feynman provided examples of the benefits to be obtained by producing ultrasmall structures. Feynman calculated that the entire content of Encyclopædia Britannica could be reduced to fit on the head of a pin, and he estimated that all of printed human knowledge could be reduced to fit on 35 normal-sized pages.

Although he did not coin the term nanotechnology, the visionary Feynman predicted key aspects of today’s nanotechnology, such as the importance of advanced microscopes and the development of new fabrication methods. He also emphasized the importance of combining the knowledge, tools, and methodologies used by physicists, chemists, and biologists. He pointed to the natural world as an example of how much information and function can be packed into a tiny volume. A single cell, for example, can move, perform biochemical processes, and contains within its DNA molecule the complete knowledge of the design and function of the complex organism of which it is part.

Feynman believed the creation of nanoscale devices was possible within the boundaries set by the laws of physics. He specifically cited the possibility of atom-by-atom assembly—that is, building a structure (a molecule or a device) from individual atoms precisely joined by chemical forces. This possibility led to the concept of a “universal assembler,” a robotic device at nanoscale dimensions that could automatically assemble atoms to create molecules of the desired chemical compounds. Such a device, for example, could assemble carbon atoms to form low-cost, large diamonds, a potentially important industrial material, now used only in limited quantities due to the high cost of mining and synthesis. Such synthetic diamonds could have many industrial and consumer applications because they are lightweight and yet extremely hard, and are electrically insulating but excellent conductors of heat. The idea of a nanoscale robotic assembler continues to be promoted by some researchers, although there is considerable debate whether such a device is indeed possible within the known laws of chemistry, physics, and thermodynamics.

Nanotechnology began being promoted as a key component of future technology in the late 1970s. The term nanotechnology was first used in 1974 by Japanese scientist Norio Taniguchi in a paper titled “On the Basic Concept of Nanotechnology.” However, the term was also used by American engineer K. Eric Drexler in the book Engines of Creation (1986), which had a greater impact and helped accelerate the growth of the field. By this time, major breakthroughs had been achieved in industry, such as the formation of nanoparticle catalysts made of nonreactive metals and used in catalytic converters found in automobiles. These catalysts chemically reduced noxious nitrogen oxides to benign nitrogen and simultaneously oxidized poisonous carbon monoxide to form carbon dioxide.

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The Tools of Nanotechnology

The scientific community began serious work in nanoscience when tools became available in the late 1970s and early 1980s—first to probe and later to manipulate and control materials and systems at the nanoscale. These tools include the transmission electron microscope (TEM), the atomic force microscope (AFM), and the scanning tunneling microscope (STM). See also Microscope.

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Transmission Electron Microscope (TEM)

The TEM uses a high-energy electron beam to probe material with a sample thickness of less than 100 nm. The electron beam is directed onto the object to be magnified. Some of the electrons are absorbed by or bounce off the object, while others pass through the object and form a magnified image of the material. A photographic plate, fluorescent screen, or digital camera placed behind the material records the magnified image. TEMs can magnify an object up to 30 million times. By contrast a conventional optical microscope can magnify objects up to 1,000 times. TEMs are suitable for imaging objects with dimensions of less than 100 nm, and they yield information on the size of the nanostructure, its composition, and its crystal structures.

The TEM is a popular and powerful instrument within the nanoscience community. Most of the images published in scientific journals on nanocrystals found in semiconductors were recorded with this instrument. TEMs can easily visualize individual atoms within semiconductor nanocrystals.

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Atomic Force Microscope (AFM)

An AFM uses a tiny silicon tip, usually less than 100 nm in diameter, as a probe to create an image of a sample material. As the silicon probe moves along the surface of the sample, the electrons of the atoms in the sample repel the electrons in the probe. The AFM adjusts the height of the probe to keep the force on the sample constant. A sensing mechanism records the up-and-down movements of the probe and feeds the data into a computer, which creates a three-dimensional image of the surface of the sample. Thus, the exact surface topography can be recorded with precise height information, and individual atoms in the surface can be imaged. The lateral resolution of this technique, however, is sometimes poor.