Nanotechnologies

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NANOTECHNOLOGIES

Nanotechnology is the technology of building things at a molecular scale, that is, where objects are measured in nanometers. Nanotechnology has the potential to improve both energy production technology, and the efficiency of end–use technology. Before this is possible, scientists must develop techniques to manipulate atoms and molecules well enough to build machines and structures. This means that the design of an object must specify each atom in the object, and all the chemical bonds between them. Such a design or object is called "atomically precise." In plants and animals, molecular biology has this capability, with certain substantial limitations. In the laboratory, atoms and molecules can be moved with the tip of a scanning probe, such as an STM (scanning tunneling microscope) or an AFM (atomic force microscope), but with other limitations. Other approaches include manipulation with electron beams, light waves ("optical tweezers"), and deposition in thin films (sputtering, molecular beam epitaxy).

HISTORY

In the 1950s, biologists (notably Francis Crick and James Watson) discovered the molecular basis for information coding in DNA and established that the workings of cells were molecular machines with understandable structure and function. Mathematician John von Neuman developed a mathematical theory of self-reproducing machines based on the biological theories.

In the 1960s, physicists such as Richard Feynman and Carver Mead showed that the speed and power efficiency of machines can improve with decreasing scale, particularly for solid-state electronics. A trend toward miniaturization in electronics was accelerated by the space program. Biologists began to decipher the genetic code.

In the 1970s, computers on a chip (microprocessors) appeared. Electron microscopes neared molecular resolution. By the end of the decade, transistors could be made the size of a human cell (10 microns).

In the 1980s, biologists began manipulating ("recombining") DNA–they could now read and write information at the molecular scale. Physicists (notably Gerd Binnig and Heinrich Rohrer) invented "scanning probe microscopes" able to image individual atoms. Technologists (notably William Trimmer) began to build "micromachines" using techniques from microelectronics. Futurists (notably Eric Drexler) began describing nanomachines and using the term "nanotechnology." At the end of the decade, transistors could be made the size of a bacterium (1 micron).

In the 1990s, nanotechnology emerged as a distinct field, with its own journals, conferences, and funding programs. MIT conferred the first Ph.D. in nanotechnology (to Drexler). Micromechanics became a burgeoning commercial field. Although most of the activity was still nanoscience as opposed to nanoengineering, the explosion of computational power made designing and simulating molecular machines feasible. Drugs and industrial catalysts were routinely designed and simulated in this fashion. Genetically modified organisms were commercial technology. Nanotechnologists (notably Nadrian Seeman) built controllable, albeit very rudimentary, molecular machines. By the end of the decade, commercial electronics had one-tenth micron (100 nanometer) transistors, but switching with single molecules had been done in laboratories.

ENERGY

As the field of nanotechnology matures, the ability to manipulate atoms and molecules will develop into the ability to build machines that do so. This will revolutionize the generation, storage, transmission, and use of energy. For example, a major source of energy is the oxidation of fossil fuels. If the fuel is burned, the energy is thermalized, and must be recovered by a heat engine. The laws of thermodynamics together with engineering constraints give this process a poor efficiency, typically 30 to 50 percent. Efficiency is even worse for small (e.g., hand-held) engines. Nanotechnology gives us the prospect of a different approach. Molecules of oxygen and fuel could be positioned rigidly and moved mechanically through the oxidation process, so that it becomes reversible (both practically and in the technical thermodynamic sense). This would raise the efficiency to near 100 percent, even in small engines.

Reversing the process yields an efficient method of energy storage. The process inputs combustion products such as carbon dioxide and water, and energy in the form of electricity or shaft power, and outputs oxygen and fuel (typically hydrogen or hydrocarbons).

Reversible fuel oxidation is as yet not well understood, and can be considered a major goal of nanotechnology in the energy area. Other forms of reversible conversion, such as shaft power to electricity and vice versa (generators and motors), are better understood and there are existing designs awaiting only construction capability. When of reversible oxidation becomes a reality, nanotechnology will enable the construction of heat engines that are small, powerful, clean, and as efficient as thermodynamics permits. One early application that is the subject of research is storage of hydrogen as a fuel.

The understanding of the electronic properties of nanostructures is one of the most rapidly advancing areas in science. This has two major implications: first, it will lead to the construction of nanocircuitry and nanocomputers that will use considerably less power than current computers while being faster and smaller; and second, it will lead to increasing efficiency and decreasing cost of photovoltaic power conversion ("solar energy").

ATOMIC IMAGING AND MANIPULATION

A scanning-probe microscope consists of a sharply pointed object, preferably so sharp that its tip is a single atom, mounted on a block of piezoelectric material such as a quartz crystal. A voltage is applied to the block, which warps in response. This warping can be controlled to atomic dimensions, allowing the tip to be steered across a molecular sample. The proximity of the tip to atoms of the sample can be sensed by various means, allowing a computer to build up a picture of the sample by scanning the tip across it. Kinds of scanning probe microscope include STM (scanning tunneling microscope) in which a current tunneling from the sample to the probe is measured, the AFM (atomic force microscope) in which the probe presses on the sample and the resulting force is measured, near-field optics in which the probe is an optical funnel focusing or detecting photons with much greater precision than their free-space behavior would allow, and many others.

Scanning probes can also be used to manipulate atoms and molecules individually, placing the tip in contact with the subject atom and pushing or pulling (atoms stick to the tip by virtue of the van der Waals force).

COMPUTATIONAL NANOTECHNOLOGY

Computer-aided design (CAD) and simulation of molecular structures is a rapidly advancing and widely applicable field. Molecules can be designed with molecular CAD programs; early programs allowed the user to specify the type and place of each atom or of substructures ("moieties") from a library. Research in molecular CAD is now focusing on the automation of parts of the process, following in the footsteps of a similar development in design software for digital electronics (e.g., for microprocessors).

Simulation of molecules can be done at the quantum mechanical level, as is necessary to determine the electronic properties of molecules, to analyze covalent bonds or simulate bond formation and breaking. However, quantum mechanical simulation is extremely computationally intensive and is too time-consuming for all but the smallest molecular systems.

A more practical approach for larger systems is molecular dynamics. In this method, the properties of bonds are determined through a combination of quantum-mechanical simulation and physical experiments, and stored in a database called a (semiempirical) force field. Then a classical (non-quantum) simulation is done where bonds are modeled as spring-like interactions. Molecular dynamics simulations are appropriate for studies involving the properties of molecules as physical structures and shapes (including "docking" and the catalytic properties of biomolecules in solution, and the structural properties and energy dissipation mechanisms of nanomachine parts in operation).

EARLY COMMERCIAL NANOTECHNOLOGY

Commercial nanotechnology in the 1990s was limited by the lack of a general synthetic capability. It characterized by a plethora of techniques for building nanostructures, including a number of methods based on gas-phase nucleation (e.g., laser pyrolysis, sputtering), methods from synthetic chemistry, including dendrimers and fullerenes, and methods from molecular biology, including DNA synthesis and the use of DNA as a structural material, and protein engineering. In general, the limitations on DNA/protein methods are in the ability to design and predict structures, where the limitations on the other methods are on their physical synthetic capabilities.

Methods capable of detailed atomic manipulation were confined in the 1990s to the laboratory, since they were generally incapable of producing commercially useful amounts of product. There are a few exceptions to this, in applications where a few carefully constructed molecules can be useful, such as chemical sensors, laboratory equipment, and so forth.

THE FUTURE OF NANOTECHNOLOGY

The field of nanotechnology is advancing rapidly, so it is not practical or useful to make short-term predictions about its specific form and capabilities. At most it can be noted that one of the most rapidly advancing subfields is nanoelectronics (sometimes referred to as "molectronics," for molecular electronics).

For longer-term projections, a common practice is to compare the field with others that are advancing as rapidly. Computers, both hardware and software, and biotechnology are cases in point. Both fields bear on nanotechnology and contribute to its advance. In computers, the watershed capability was the storedprogram von Neumann architecture. In nanotechnology, it is expected to be another von Neumann design, the self-reproducing system. In practical terms, this simply means that nanomachines need to be capable of making parts for nanomachines, just as a conventional machine shop is capable of making parts for its own machines. Once this is achieved, it will be possible to build commercial quantities of product with atomic precison, and in particular, to have commercial quantities of product that consists of working nanomachines.

J. Storrs Hall