A Programmable Matter(TM) smart material is any bulk substance whose physical properties can be adjusted in real time through the application of light, voltage, electric or magnetic fields, etc. Primitive forms may allow only limited adjustment of one or two traits (e.g., the "photodarkening" or "photochromic" materials found in light-sensitive sunglasses), but there are theoretical forms which, using known principles of electronics, should be capable of emulating a broad range of naturally occurring materials, or of exhibiting unnatural properties which cannot be produced by other means.
Back to Top
Wellstone(TM) was a hypothetical form of smart material first proposed by Wil McCarthy in his novella "Once Upon a Matter Crushed" (Science Fiction Age, May 1999), consisting of nanoscopic semiconductor threads covered with quantum dots. These threads can be woven together to form a bulk solid with real-time adjustable properties. The terms "Wellstone(TM)" and "Programmable Matter(TM)" are occasionally incorrectly used interchangeably, although many other forms of smart materials exist.
Back to Top
No. Various forms of smart material have appeared in fiction, but are in many cases based on technologies which exist today, or on reasonable extrapolations from them.
Back to Top
Various aspects of smart materials (including quantum dots, electrochromic materials, magnetoreheologic materials, and various kinds of fiber-based circuitry) are under investigation in labs all over the world. Major players include (but are by no means limited to) IBM, Nippon Telehone and Telegraph, Fujitsu, Delft University, MIT, Harvard, Stanford, Princeton, Cornell, CalTech, and The University of California at Santa Barbara. Wellstone(TM), Wafflestone(TM), and Gridwell(TM), using quantum dots incorporated into fibers, ribbons, and plates are under explicit investigation at the Programmable Matter(TM) Corporation.
Back to Top
Yes and no. The word "nanotechnology" simply means "technology on the scale of nanometers," or billionths of a meter, i.e. technology on the molecular scale. Most forms of Programmable Matter(TM) smart materials rely on nano-circuitry, designer molecules, or both, so in this literal sense they are nanotechnology. However, as originally coined by K. Eric Drexler in the 1980s and as commonly used by lay persons today, the word nanotechnology implies nanoscale machinery, more properly known as molecular nanotechnology or MNT. While bulk materials incorporating MNT may have programmable properties, they also have moving parts. Our smart materials do not rule out such materials, but more typically refers to substances whose properties can be adjusted in the solid state, with no moving parts other than photons and electrons.
Back to Top
No. Micro Electromechanical Systems, or MEMS, are microscopic machines crafted using standard methods for the manufacture of microchips. MEMS have many useful applications in the real world, but are far too large to exhibit the quantum effects necessary to affect the bulk properties of matter. However, the "Utility Fog" substance proposed by J. Storrs Hall in the early 1990s, consisting of millions or billions of MEMS micromachines -- each with with 12 retractable, linkable arms -- has numerous adjustable bulk properties and can thus be considered a crude, mechanical form of smart materials. Also, The Programmable Matter(TM) Corporation is exploring the possible uses of Wellstone(TM) smart materials to enhance the properties of MEMS.
Back to Top
Yes, although technically speaking, a cellular automaton can only contain virtual smart material, whereas physical examples which meet the definition are available in the real world.
Back to Top
An LCD screen's optical properties can be dramatically altered by the application of electrical signals. Thus, it is clearly a form of smart material, albeit a simple one. A transistor can switch between an electrically conductive state and an electrically insulative one, but is properly a "device" rather than a substance or material. However, a bulk material fashioned from transistors (transistronium?) would be electrically switchable between these two states, and possibly numerous intermediate states. This meets (trivially) the definition for smart material stated above. In general, the more capable forms of Programmable Matter(TM) smart materials rely on the doping effects of "artificial atoms" or "quantum dots" inside a bulk material.
Back to Top
Doping is the addition of impurities (dopants) to a bulk material (the substrate) in order to adjust its electrical, thermal, optical, or magnetic properties. The addition of one dopant atom per million atoms of substrate is often sufficient to cause major changes in the material's behavior, and impurities in the parts-per-billion can disrupt the expected behavior of a pure crystal.
Back to Top
Quantum confinement is the trapping of electrons or electron "holes" (charge carriers) in a space small enough that their quantum (wavelike) behavior dominates over their classical (particle-like) behavior. In quantum mechanical terms, for quantum confinement to occur the dimension of the confining device or particle must be comparable to, or smaller than, the de Broglie wavelength of the carriers, and also the carrier inelastic mean free path IMFP and electron-hole Bohr radius of the material it's made from. Under cryogenic conditions, this typically occurs with dimensions of 1000 nm (0.001 mm) or less. At room temperature, depending on the materials, confinement dimensions of 20 nm or smaller are typically required.
Back to Top
A quantum dot is any device capable of the quantum confinement of electrons (for holes, it becomes an "antidot"). Once the electrons are confined, they repel one another and also obey the Pauli Exclusion Principle, which forbids any two electrons from having the same quantum state. Thus, the electrons in a quantum dot will form shells and orbitals highly reminiscent of (though larger than) the ones in an atom, and will in fact exhibit many of the optical, electrical, thermal, and (to some extent) chemical properties of an atom. This electron cloud is therefore referred to as an artificial atom. In their various forms, quantum dots may be referred to as single-electron transistors, controlled potential barriers, Coulomb islands, zero dimensional electron gases, colloidal nanoparticles or semiconductor nanocrystals.
Back to Top
A quantum well is a device for confining electrons in one dimension, such that their quantum (wavelike) behavior dominates over their classical (particle-like) behavior along the confined axis, while classical behavior dominates along the other two axes, permitting the electrons to flow two-dimensionally through the material like billiard balls on a table. A typical quantum well may be fashioned from an N-type semiconductor, doped with electron donor atoms, trapped between two layers of P-type semiconductor, doped with electron borrower atoms. Other arrangements, such as a metal layer sandwiched between two insulators, are also possible. A quantum well is the primary component of miniature laser pointers.
Back to Top
Programmable Matter(TM) smart materials are composed of manmade objects too small to perceive directly with the human senses. This may include microscopic or nanoscopic machines, but more typically refers to fixed arrangements of conductors, semiconductors, and insulators designed to trap electrons in artificial atoms.
Back to Top
Current forms of quantom dot smart materials fall into three types: colloidal films, bulk crystals, and quantum dot chips which confine electrons electrostatically. Quantum dots can be grown chemically as nanoparticles of semiconductor surrounded by an insulating layer. These particles can then be deposited onto a substrate, such as a semiconductor wafer patterned with metal electrodes, or they can be crystalized into bulk solids by a variety of methods. Either substance can be stimulated with electricity or light (e.g., lasers) in order to change its properties.
Electrostatic quantum dots are patterns of conductor (usually a metal such as gold) laid down on top of a quantum well, such that varying the electrical voltage on the conductors can drive electrons into and out of a confinement region in the well -- the quantum dot. This method offers numerous advantages over nanoparticle ("colloidal") films, including a greater control over the artificial atom's size, composition, and shape. Numerous quantum dots can be placed on the same chip, forming a semiconductor material with a programmable dopant layer near its surface. A number of fabrication technologies exist whose resolution is sufficient to produce room-temperature quantum dot devices. Rolling such quantum dot chips into cylindrical fibers produces Wellstone(TM) smart material, a hypothetical woven solid whose bulk properties are broadly programmable.
Back to Top
Yes. Artificial atoms can easily be constructed which mimic the properties of any natural atom, except that they are larger and their electrons are bound more loosely. However, these artificial atoms have negligible mass, and can exist only inside the quantum-dot substrate which generates them, usually a semiconductor. Thus, the final properties of the material are a blend of the simulated element and the underlying substrate. Note that the color of an artificial element made of oversized atoms would be redshifted as compared with the equivalent natural element.
Back to Top
Yes and no. An artificial atom of pseudo-lead (atomic number 82), trapped permanently inside a semiconductor material, can be converted to an artificial atom of pseudo-gold (atomic number 79) by the subtraction of three electrons. Sufficient numbers of these pseudoatoms may overwhelm the natural behavior of the semiconductor to produce a metal-like material similar to lead or gold, except for its mass, ductility, and probably color. Artificial atoms designed to mimic the colors of lead or gold might have other properties (e.g., electrical or thermal conductivity) which do not match the original metal.
Back to Top
Yes. An artificial atom can contain any number of electrons, from 1 to over 1000. The form and properties of highly transuranic atoms (atomic number >> 92) are dramatically different from those of natural atoms.
Back to Top
Electrons in an atom are confined by their attraction to the nucleus, and the nuclei of highly transuranic elements are unstable. However, an artificial atom does not have a nucleus of its own, relying instead on geometry, insulative barriers, and/or electrostatic repulsion to confine its electrons inside a semiconductor substrate. Thus, transuranic artificial atoms are stable as long as the device containing the electrons continues operating. The only atomic nuclei present are those of the metal and/or semiconductor atoms which make up the quantum dot. Because the confined electrons cannot affect the properties of these nuclei, they cannot be used to trigger or modify nuclear reactions. An artificial atom with 92 electrons in it is not "real" uranium, and will not be radioactive.
Back to Top
Probably not. The binding energy of artificial atoms cannot exceed the binding energy of the semiconductor substrate. However, using diamond fibers or fullerenes as a substrate should allow for some very tough smart materials. Also, changes in the magnetic behavior of a material can affect its stiffness and tensile/compressive strength in useful ways.
Back to Top
Unlike natural atoms, artificial atoms can be square, pyramidal, two-dimensional, highly transuranic, composed of charged particles other than electrons (e.g., "holes"), and can even be asymmetrical. Their size, energy, and shape are variable quantities. Thus, artificial atoms exhibit optical, electrical, thermal, magnetic, mechanical, and (to some extent) chemical behaviors which do not occur in natural materials. This variety is bounded but infinite, in sharp contrast to the 92 stable atoms of the periodic table.
Back to Top
Artificial atoms can exist only inside a semiconductor substrate. They are charge discontinuities rather than physical objects, so they don't "feel" like anything. However, their doping effects can dramatically alter the properties of the substrate, causing it to feel different. For example, a dramatic increase in thermal and electrical conductivity would make the semiconductor feel (in terms of thermal response) like a metal.
Back to Top
Almost anything. They can improve the efficient collection, storage, distribution, and use of energy from environmental sources. They can be used to create novel sensors and computing devices, probably including quantum computers. They can create materials which are not available by other means, and which change their apparent composition on demand. Currently, the design of new materials is a time- and labor-intensive process; with Programmable Matter(TM) smart materials, it becomes a real-time issue, similar to the design and debugging of software.
Back to Top
Wil McCarthy, an aerospace engineer, is a contributing editor for WIRED magazine, the science columnist for the SciFi channel web site www.scifi.com, and an author of numerous book-length works of science fact and science fiction. He has written extensively about quantum dots and smart materials, and faces a consistent set of questions, objections, and misconceptions when presenting this material. This FAQ is intended to promote intelligent discussion of smart materials and quantum dots by increasing awareness of their underlying issues and principles.
Back to Top
Single-electron transistors, a form of quantum dot, were first proposed by A.A. Likharev in 1984 and constructed by Gerald Dolan and Theodore Fulton at Bell Laboratories in 1987. The first semiconductor SET, a type of quantum dot sometimes referred to as a designer atom, was invented by Marc Kastner and John Scott-Thomas at MIT in 1989. The term "artificial atom" was coined by Kastner in 1993. However, Wil McCarthy was the first to use the term "Programmable Matter(TM)" in connection with quantum dots, and to propose a mechanism for the precise, 3D control of large numbers of quantum dots inside a bulk material. The most interesting forms of this device or substance -- known as "quantum dot fiber", "programmable dopant fiber", or "Wellstone(TM)" -- are under development at The Programmable Matter(TM) Corporation. The term "Wellstone(TM)" was coined by McCarthy's business associate, Gary E. Snyder.
Back to Top
Quantum dots are a new field with much basic research still remaining, so the ultimate properties of bulk quantum-dot materials cannot be known with precision at this time. However, the principles underlying quantum confinement are fairly well understood, and the experimental evidence overwhelmingly indicates that programmable quantum-dot smart materials are feasible, and will play an important role in future technology. Many issues have been considered, and many more are under investigation.
Back to Top
Wil McCarthy and Gary E. Snyder hold pending U.S. patents on the concept, one entitled "Fiber Incorporating Quantum Dots as Programmable Dopants", filed 13 August 2001.
Back to Top
The best online reference for lay readers is "Ultimate Alchemy," a 7,000-word article from WIRED magazine available (minus the pictures) at:
http://www.wired.com/wired/archive/9.10/atoms.html.
Offline lay-references include Richard Turton's THE QUANTUM DOT: A Journey into the Future of Microelectronics (Oxford University Press, 1996, ISBN 0-195-10959-7).
http://www.amazon.com/exec/obidos/tg/detail/-/0195109597.
and Wil McCarthy's HACKING MATTER: Levitating Chairs, Quantum Mirages, and the Infinite Weirdness of Programmable Atoms (Basic Books, 2003, ISBN 0-465-04429-8).
http://www.amazon.com/exec/obidos/tg/detail/-/046504428X.
For serious theoreticians, Paul Harrison's Quantum Wells, Wires, and Dots (Wiley, 2000, ISBN 0-471-98495-7) provides equations and computer code for estimating the behavior of confined electrons.
More technical discussions can be found in the (searchable) annals of science News, Science, Nature, Physics Today, and related journals, as well as web pages at many of the research centers listed above.
Back to Top
|