Nanotechnology

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Nanotechnology

Nanotechnology describes technologies where the component parts can measure just a few atomic diameters (generally around a millionth of a millimeter). The general goal of nanotechnology research programs is to reduce complex and sophisticated machinery into very small operational units.

Nanotechnology involves the development of techniques to build machines from atoms and molecules. The name comes from nanometer, which is a length equal to one-billionth of a meter. It involves the development of new electrical devices that depend on quantum effects that arise when the dimension of a structure is only a few atoms across. Because the techniques best suited for fabricating devices on the sub-micron scale originated in semiconductor processing technology for the production of integrated circuits, nanoscale devices are all based on semiconductors.

The theory of nanotechnology was the brainchild of American engineer K. (Kim) Eric Drexler (1955–), who, as a student of genetic engineering at the Massachusetts Institute of Technology in 1971, envisioned the potential for building machines and materials with atoms as basic blocks just as living matter is constructed with deoxyribonucleic acid (DNA). Until the twentieth century, the physics of atoms and molecules were an invisible science. Atoms finally became visible in 1981 when the scanning tunnelingmicroscope was built. The microscope has the ability to scan through the clouds created by electrons around atoms so the individual atom can be seen. The doorway to nanotechnology was opened in 1985 by American chemist and physicist Richard Errett Smalley (1943–2005), a professor with Rice University, who found a way to produce carbon in a third form (the two natural forms are graphite and the diamond) that is crystalline and possesses incredible strength properties. The molecule contains 60 carbon atoms and has a shape much like a soccer ball or a geodesic dome built by American architect Buckminster Fuller (1895–1983), so the molecule was named the buckyball in Fuller’s honor.

By 1991, experiments with buckyballs led to long cylindrical strings of the molecules that are tube- or straw-like and are called buckytubes or nanotubes. Many scientists think nanotubes are a fundamental unit for building countless other nanodevices. Nanotubes can be flexed and woven and are being woven into experimental fibers for use in ultralight, bulletproof vests, as one example. Nanotubes are also perfect conductors, and they may be used to construct atomically precise electronic circuitry for more advanced computers and flat panel displays. They are important to the emerging field of molecular nanotechnology (MNT).

Nanofabrication techniques

Nanoscale devices are made using a combination of different fabrication steps. First is the growth stage, in which layers of different semiconductor material are grown on a substrate, providing structure in one dimension. Second is the lithography/pattern transfer stage, in which a pattern is imposed on a uniform layer, giving structure in the second and third dimensions. Repetition of these two stages results in the production of very complex, three-dimensionally nanoscale semiconductor devices.

Growth stage

In the growth stage, each successive layer has a different composition to impose electrical or optical characteristics on the carriers (electrons and holes). A variety of growth techniques can be used; molecular beam epitaxy (MBE) and metallo-organic chemical vapor deposition (MOCVD) have proved to be the most useful for nanostructures because they can be used to grow layers of a predetermined thickness to within a few atoms. In MBE, a gun fires a beam of molecules at a substrate upon which the semiconductor crystal is to be grown. As the atoms hit the surface, they adhere and take up positions in the ordered pattern of the crystal, so a near perfect crystal can be grown one atomic layer at a time. The mixture of material in the beam is changed to produce different layers; for example, the introduction of aluminum to the growth of gallium arsenide will result in the production of a layer of aluminum gallium arsenide.

Lithography/pattern transfer stage

Epitaxial growth allows the formation of thin planes of differing materials in one dimension only. The two-stage process of lithography and pattern transfer is used to form structures in the other two dimensions. In the lithography stage (see Figure 1), a film of radiation-sensitive material known as the resist is laid on top of the semiconductor layer where the structure is going to be made. A pattern is exposed on the resist using electrons, ions, or photons. The film is altered during the exposure step, thus allowing the resist to be chemically developed as a relief image. This image is then transferred to the semiconductor by doping (adding minute amounts of foreign elements), etching, growing, or lift-off as shown in Figure 1.

The exposure pattern can be written on the resist in a line-by-line manner using an electron or ion beam or it can be imprinted all at once using a mask, much like spray painting a letter on a wall using a template. Exposure by writing the pattern with a beam is especially useful for making prototype structures, because it avoids the expense of making a new mask for each pattern; features as small as a few nanometers can be written this way.

Writing patterns one line at a time is time consuming, however, and expensive when it comes to producing large quantities, so masks are used that expose a number of chips simultaneously. Masks can be used with electron or ion beams or with photons. It is important to note that the wavelength of light for exposing a pattern has to be less than the smallest feature being exposed.

Where nanoscale dimensions are involved, this necessitates the use of vacuum ultraviolet light or x rays. Penetration of the x rays into the semiconductor must be avoided to prevent damage to the crystal, so wavelengths of 1.3 nm or 4.5 nm are preferred because the polymeric resist exhibits an absorption depth of about 1 micron at these wavelengths.

The requirement on the wavelength of the x rays and the obvious need to have the x ray beam directed precisely necessitates the use of a well-controlled x ray source, such as a synchrotron. Laser-based x ray sources are currently being considered as a less expensive alternative to the synchrotron.

Theoretical methods

In order to progress in the fabrication and diagnostics of nanostructures and surfaces, scientists are developing more advanced spectroscopic and imaging methods. Highly charged ion surface interaction using an electron beam ion trap (EBIT) facility is one such method being examined by researchers in the Department of Physics at the University of Nevada and at the Lawrence Livermore National Laboratory. Researchers deposited a large amount of potential energy from single ions at the surface leading to localized surface defects of subnanosize. The comparison of the computer simulation with experimental x-ray spectra showed the formation of subsurface hollow atoms.

Conventional nanoscale devices

In conventional semiconductors, the carrier velocity is limited to about 4⁷ in/sec (10⁷ cm/sec), which results in limitations of current density and turn-on time, or frequency response. Because of the need for faster devices (it takes less time for a carrier to cross a smaller device) and the desire to squeeze more devices onto a single chip, e.g. a processor chip for computers,

significant advances have been made since the 1970s in fabricating smaller devices.

Additional advantages have been found for operating devices in the nanoscale regime. The carrier velocity limit of 4⁷ in/sec (10⁷ cm/sec) is set in large-scale semiconductors by collisions with crystalline defects, photons, etc., and can be characterized by a mean free path between collisions: the longer the mean free path, the fewer collisions and the higher the carrier velocity. It has been found that in nanoscale devices, the device itself can be considerably shorter than the mean free path, in which case a carrier injected at a high velocity, say 4⁸ in/sec (10⁸ cm/sec), never suffers any collisions and, therefore, does not slow down. This phenomenon is termed ballistic transport and allows semiconductors to operate far faster than was possible before, reaching frequencies of 200 Ghz (gigahertz).

There are complexities associated with the manufacturing of conventional nanoscale devices that appear to be major obstacles on the way to producing large quantities of these types of devices. This difficulty in manufacturing, coupled with the fact that ballistic transport devices perform best at low signal levels (as opposed to conventional electronics that operate well at high power levels) suggests that these new devices will not become widely available for use in computer chips in the near future.

Quantum-effect nanoscale devices

According to the laws of quantum mechanics, free carriers in a metal or semiconductor can only take on specific values of energy, as defined by the crystal structure; that is, the energy is quantized. For most practical purposes, there are so many closely spaced energy levels, it appears that the carriers have a continuum of possible energies, except for the well-defined gaps characteristic of semiconductors. When the carrier is confined to a region where one or more of the dimensions reach the range of less than 100 nm, the quantum energy levels begin to spread out and the quantum nature becomes detectable. This reduction in size can take place in one, two, or three dimensions, using the fabrication techniques discussed earlier, yielding structures known respectively as superlattices, quantum wires, and quantum dots.

When electrons are introduced into a semiconductor structure, they migrate to those positions where their energy is lowest, much like a ping-pong ball will come to rest in a dimple on a waffled surface. If the nanostructure is engineered correctly, then the electrons will settle in the nanostructure itself and not in the adjacent layers. These carriers will then exhibit the quantum effects imposed on them by the nanostructure. The ability to engineer artificial atoms and molecules in semiconductors using nanofabrication techniques has resulted in a powerful new tool in creating novel semiconductor devices, such as quantum dots where the number of carriers trapped by the dot can be controlled by an external voltage.

It appears possible that nanoscale quantum-effect devices may become widely used in complex electronic systems, such as a neural array of quantum dots spaced only a few 100 nm apart, but this will only take place after significant progress has been made in fabrication and tolerance.

Scientists see a universe of potential in nanotechnology, following years or perhaps decades of research and development. Some of the applications they foresee are as follows: surgical instruments of molecular scale that are guided by computers of the same size; rocket ships for the individual made of shatterproof materials created by nanomachines; synthesis of foods by molecules and an end to famine; pollution-free manufacturing and molecular devices that clean up existing pollution without human intervention; consumer goods that assemble themselves; reforming of soil (termed terraforming) both on the Earth and other planets where soil and rock may not exist; and computers capable of more computations in one second than all the existing semiconductor devices in the world combined. Nanodevices may create smart matter that, when used to build a bridge or a high-rise building, knows when and how to repair itself; diamonds of perfect quality and any size may be built atom by atom to suit industrial needs or an individual’s ideal; injectable molecular robots that enter the bloodstream on seek-and-destroy missions for cancer, AIDS (acquired immune deficiency syndrome), invading bacteria or viruses, and arterial blockages. Similarly, nanoparticles might carry vaccines and drugs directly to the source of the ailment.

Tangible advances

A research team led by Chad Mirkin, Charles E. and Emma H. Morrison Professor of Chemistry and director of Northwestern’s Center for Nanotechnology (Evanston, Illinois), developed a method of writing nanostructures with what they have termed “the world’s smallest plotter.” The device writes multiple lines of molecules, each line only 15 nm or 30 molecules wide and only 5 nm or about 200 billionths of an inch apart. The plotter is based on the researcher’s dip-pen nanolithography (DPN) technique. The dip-pen nano-lithography draws tiny lines with a single ink or type of molecule. The nano-plotter prints multiple inks, or four different kinds of molecules, side by side while retaining the chemical purity of each line. DPN, described in the January 29, 1999, issue of Science, turns an atomic force microscope (AFM) into a writing instrument. Researchers first apply an oily ink of octadecanethiol (ODT) to the AFM’s tip. When the tip is brought into contact with a thin sheet of gold paper, the ODT molecules are transferred to the gold’s surface via a tiny water droplet that forms naturally at the tip. The new nanoplotter multiplies this technique, laying down a series of molecular lines with extreme accuracy. While the microfabrication of electronic circuits and other products currently use solid-state or inorganic materials, innovations such as the nanoplotter will direct future technologies toward the use of organic and even biological materials. Since its introduction, DPN has provided high-resolution patterning capabilities for many molecular and biomolecular inks on several substrates, such as semiconductors, metals, and monolayer functionalized surfaces. As of 2006, the Mirkin Group at Northwestern uses DPN to discover new information within surface science and to create technologically unique nanostructures.

Coupling the organic and inorganic, biological engineers at Cornell University (Ithaca, New York) demonstrated the feasibility of small, self-propelled bionic motors that do their builders’ bidding in plant, animal, or human cells. Such machines could travel through the body, functioning as mobile pharmacies dispensing precise doses of chemotherapy drugs exclusively to cancer cells, for example. The device, the result of integrating a living molecular motor with a fabricated device at the nano scale, is a few billionths of a meter in size. The first integrated motor, a molecule of the enzyme ATPase coupled to a metallic substrate with a genetically engineered handle, ran for 40 minutes at three to four revolutions per second.

Researchers at Rensselaer Polytechnic Institute (Troy, New York) are working with materials comprising

Key Terms

Ballistic transport —Movement of a carrier through a semiconductor without collisions, resulting in extraordinary electrical properties.

Carriers —Charge-carrying particles in semiconductors, electrons, and holes.

Epitaxy —The growth of crystalline layers of semiconducting materials in a layered structure.

Integrated circuits —Complex electronic circuits fabricated using multiple growth and lithography/pattern transfer stages to produce many miniature electronic elements on a monolithic device.

common atoms arranged in grains less than 100 nm in diameter—10,000 times smaller than grains in conventional materials. These grains are used as building blocks to create materials with entirely new properties, which the researchers predict could revolutionize everything from drug delivery to sunscreens.

Although it is still in the early stage of development, not unlike that of computer and information technology in the 1950s, nanotechnology is a rapidly expanding scientific field.

Defense programs in many countries are now concentrating on nanotechnology research programs that will facilitate advances in programs such as those designed to create secure but small messaging equipment, allow the development of smart weapons, improve stealth capabilities, develop specialized sensors (including bio-inclusive sensors), create self-repairing military equipment, and improve the development and delivery mechanisms for medicines and vaccines.

See also Microtechnology.

Resources

BOOKS

Mulhall, Douglas. Our Molecular Future: How Nanotechnology, Robotics, Genetics, and Artificial Intelligence Will Change Our World. Amherst, NY: Prometheus Books, 2002.

Ratner, Mark A., and Daniel Ratner. Nanotechnology: A Gentle Introduction to the Next Big Idea. Upper Saddle River, NJ: Prentice Hall Publishers, 2002.

PERIODICALS

Bennewitz, R., et al. “Atomic scale memory at a silicon surface.” Nanotechnology 13 (2000): 499–502.

Lee, K. B., et al. “Protein Nanoarrays Generated by Dip-Pen Nanolithography.” Science. vol. 2002, 295(5560), 1702-1705.

Lim, J-H, et al. “Direct-Write Dip-Pen Nanolithography of Proteins on Modified Silicon Oxide Surfaces.” Angew Chem. Int. Ed. vol 2003, 20, 24311-2414.

Zhang, H., et al. “Biofunctionalized Nanoarrays of Inorganic Structures Prepared by Dip-Pen Nanolithography.” Nanotechnology. vol. 2003, 14, 1114-1117.