Made to Measure

New Materials for the 21st Century

Philip Ball

• Éditeur: Princeton University Press
• Année: 2021
• ISBN:
  9781400865338
• Sujet: Physical Sciences
• Sous-sujet: Chemistry

Made to Measure introduces a general audience to one of today's most exciting areas of scientific research: materials science. Philip Ball describes how scientists are currently inventing thousands of new materials, ranging from synthetic skin, blood, and bone to substances that repair themselves and adapt to their environment, that swell and flex like muscles, that repel any ink or paint, and that capture and store the energy of the Sun. He shows how all this is being accomplished precisely because, for the first time in history, materials are being "made to measure": designed for particular applications, rather than discovered in nature or by haphazard experimentation. Now scientists literally put new materials together on the drawing board in the same way that a blueprint is specified for a house or an electronic circuit. But the designers are working not with skylights and alcoves, not with transistors and capacitors, but with molecules and atoms.
This book is written in the same engaging manner as Ball's popular book on chemistry, Designing the Molecular World, and it links insights from chemistry, biology, and physics with those from engineering as it outlines the various areas in which new materials will transform our lives in the twenty-first century. The chapters provide vignettes from a broad range of selected areas of materials science and can be read as separate essays. The subjects include photonic materials, materials for information storage, smart materials, biomaterials, biomedical materials, materials for clean energy, porous materials, diamond and hard materials, new polymers, and surfaces and interfaces.

CHAPTER ONE

Light Talk

PHOTONIC MATERIALS

Every day you play with the light of the universe.

—Pablo Neruda

The next revolution in information technology will dispense with the transistor and use light, not electricity, to carry information. This change will rely on the development of photonic materials, which produce, guide, detect, and process light.

BY A few years into the twenty-first century, the whole world will be “online.” Just about every nation on Earth will be linked up to a communications network in which information can flow in the blink of an eye between computer terminals in Denver and Beijing, Mombasa and Copenhagen. This is the information superhighway, a web of information channels that knows no territorial, cultural, or political barriers. That it will coexist with the most appalling poverty in some parts of the world, with wars and ethnic conflicts, is a stark reminder that information alone solves no human problems. Yet however you regard it, a communications system of this sort will be like nothing we have seen before, and it will change our lives.

The flow of data that this system will have to support is immense. Many millions of individual messages will be routed along the superhighway’s arteries, simultaneously and without interfering with one another. Their transmission must take place over long distances without deterioration of the signal. Computer networks like the Internet create an ever-expanding demand for efficient communications systems, and already threaten to strain existing systems to overload. The nascent digital video technology will add to the pressure; sending digital video data “down the line” so that distant viewers can receive live pictures from a video camera requires around five hundred times more data-transmission capacity than a telephone call.

All this is simply the latest development in a succession that has led from the telegraph of the early nineteenth century to the telephone, the television, the communications satellite, and the fax machine. Until the early 1970s, the demand on long-distance communications could be met by the electronics industry. But it has become ever more clear that electronic transmission of information will be unable to accommodate the growth in data flow that the future promises to bring. A new technology is needed.

That technology is with us already, but only in a form comparable to that of the early days of electronic communications. It is called photonics, and it replaces electrical currents with light: instead of being conveyed by electrons in a copper wire, information is borne by photons, the particles of light. The first long-distance photonic transmission cable was laid down in 1988; today such cables are replacing copper telecommunications cables in just about all long-distance and most short-distance applications. These cables, made from glass optical fibers, can carry many thousands of times more information than electrical wires, and at lower power consumption.

At present, just about all of the data handling at each end of a fiber-optic transmission cable is still done by electronics. So it has been necessary to devise ways of converting an electronic signal into a series of light pulses that are fed into the optical cable, and to turn those pulses back into electricity at the other end. This integration of optical and electronic data processing is called optoelectronics.

Optoelectronic circuits are now an essential part of information technology. The practical difficulties of making optoelectronic devices that can be integrated with silicon-based circuits on a single microchip are far from trivial, however, and are still being tackled. Quite aside from this integration problem, the use of electronics will ultimately set a speed limit on the rate with which data can be handled—photonics alone could do it much faster. So engineers are now asking whether this cumbersome method of converting a signal first to one form and then to another is really the best way of going about the problem. Why not, they suggest, do it all with light? That is to say, why not dispense with electronics altogether and make chips that perform data processing purely by photonic means?

The scientific underpinnings of an all-photonic technology are already in place: we know how to make miniaturized components that guide beams of light and use them to perform logical operations—the central steps of computation. A photonic transistor, a device that is still in the early stages of development, would be switchable much more quickly than the electronic varieties, and this might allow a photonic computer to run a thousand times more speedily than modern electronic computers. Moreover, photonic devices permit engineers to contemplate entirely new types of circuit design and architecture. Optical circuit components should in principle contain fewer constituent parts than their electronic counterparts, making them cheaper and easier to package onto chips. All in all, photonics should be a cleaner, faster, more compact, and more versatile form of information processing.

None of these developments can happen without the right materials. For optical communications, the optical properties of glass fibers have been honed to an astonishing degree. Optoelectronics has been wholly dependent on the identification of suitable materials for making the solid-state lasers that act as light sources and photodetectors for converting light back to electricity. Performing information processing with light requires materials whose response to light is highly unusual and very different from that of our everyday experience. When the photonic era arrives, it will be materials scientists who will act as the midwife.

A REVOLUTION WRITTEN IN SILICON

Telecommunications—literally, long-distance discourse—became an instant affair only with the advent of the electronic age. First came the telegraph, tapped out in code in the manner beloved of movies of the Old West; then in the 1870s Alexander Graham Bell’s telephone, regarded in its early days with almost superstitious awe; and in the 1890s Guglielmo Marconi’s “wireless telegraph,” which showed that words could be sent through the air rather than through copper wire. Electronic communications, then and now, use modulated electrical signals—a current that varies in time—to carry information down copper wires. By the 1970s, the U.S. telecommunications industry was consuming around 200,000 tons of copper per year in cabling.

As the traffic of information grew, the task of processing it—modulating the signal at the transmitting end, routing the data correctly, and interpreting it at the receiving end—became ever more challenging. The turning point in electronic data processing came in 1947 with the invention of the transistor by John Bardeen and Walter Brattain at Bell Telephone Laboratories. Previously, the modulation and amplification of electrical signals were performed by vacuum tubes, which were fragile, cumbersome, and consumed a lot of power. Transistors did away with all of these problems in a single swoop—they are compact and robust and consume a minuscule fraction of the power of vacuum tubes (even the very first transistor ran on a millionth of the power of a tube). What is more, they are much faster and more reliable switches. It is no coincidence that the invention of the transistor was soon followed by a rapid growth in the power and commercialization of computers—automated devices for handling and processing electronic information. The earliest computers, such as the ENIAC device developed by engineers at the University of Pennsylvania in the 1940s, were tube-driven analog machines that occupied entire rooms and were of questionable reliability. Today many thousands of transistors and other electronic devices can be carved into semiconducting materials on a single chip no more than a millimeter square (fig. 1.1), and computers can fit into a briefcase.

The transistor’s central place in modern electronics has been gained only through diligent research on the materials from which it is made, of which the most important is silicon. It is hard to think of any other industry that has become more intimately associated with the material on which it depends. We hear talk of the silicon revolution and of silicon chips pouring out of America’s heartland of information technology, Silicon Valley in California. So closely has silicon become linked with “thinking” machines that it is the staple of science-fiction writers searching for plausible life forms not based on carbon.

FIGURE 1.1 A silicon microchip manufactured by Digital Equipment Corporation. This chip, the Alpha 21164, is the world’s fastest single-chip microprocessor, able to execute over one billion instructions per second. (Photograph courtesy of Digital Equipment Corporation.)

The key to silicon’s central role in microelectronics is the fact that it is a semiconductor—a material whose electrical properties can be influenced in a variety of subtle ways. A material’s electrical conductivity is determined by its electronic structure, by which I mean the disposition of its electrons. The chemical bonds that hold materials together are formed by overlap of the veils of electrons (called orbitals) that surround atoms; these are called covalent bonds.¹ In solids these overlapping electron orbitals give rise to extended networks of “electron density” throughout the material; in general, different networks can be ascribed to the overlap of different sets of atomic orbitals. The energies of electrons in these extended states, or “bands,” are restricted by quantum mechanics to a certain range of values, and so the electronic structure of solids can be depicted as electronic bands separated by gaps of forbidden energies, called band gaps (fig. 1.2a).

An electrical current corresponds to the flow of electrons (or sometimes of other charged particles). Although electronic bands are notionally extended throughout a solid, the mobility of the electrons that each contains depends on the extent to which the band is filled. Each band has only a limited electron capacity; once a band is filled, additional electrons in the material have to go into the band of next highest energy. Electrons in filled bands are relatively immobile, being constrained to stay more or less in the vicinity of individual atoms. Electrons in bands that are only partially filled, on the other hand, can move throughout the solid when a voltage is applied across it. So solids with only fully filled electronic bands cannot conduct—they are insulators—whereas those with partly filled bands (a category that includes most metals) are electrical conductors.

In all solids, the fully filled electronic band that has the highest energy is called the valence band. (Valence electrons are those that are available for forming chemical bonds; this naming of the uppermost filled band reflects the fact that it is these higher-energy electrons that are primarily responsible for the bonds between neighboring atoms.) The next band above the valence band is called the conduction band; in insulators this is empty, in metals it is partly filled (fig. 1.2a). A voltage applied across a material makes the electrons’ energies vary in space; they are lower in energy close to the positive terminal and higher close to the negative terminal. So a voltage introduces a tilt to the band structure (fig. 1.2b), and electrons that are free to move (that is, those that are in a partially filled band) flow down the slope.

Semiconductors typically have an electrical conductivity somewhere between metallic conductors such as copper and insulators such as diamond. This suggests that they have some mobile charge carriers, but far fewer than metals. The electronic band structure of pure semiconductors like silicon is of the same type as that of insulators: the uppermost electronic band (the valence band) is completely filled, and a band gap separates this from a completely empty conduction band. But the crucial distinction between a semiconductor like silicon and an insulator like diamond is the size of this gap: in silicon it is small enough that a few electrons can pick up enough thermal energy to hop up into the conduction band, where they are free to move (fig. 1.2a). This hopping leaves behind an electron vacancy—a hole—in the valence band, which can be conveniently regarded as a kind of virtual particle with an electrical charge opposite to that of an electron. So in a semiconductor like silicon, electrical current is carried by a few energetic electrons in the conduction band moving in one direction, and by positively charged holes in the valence band moving in the other.

FIGURE 1.2 (a), The overlap of electron clouds around atoms in solids gives rise to “electronic bands” in which the electrons’ energies lie between well-defined values. Each band has a certain capacity for electrons, and so the electrons in the solid fill bands of successively higher energies. Electrons in fully filled bands are not mobile and so cannot carry an electrical current. The fully filled band of highest energy is called the valence band, and the next highest band is the conduction band. If the conduction band is partly filled with electrons, these can move through the solid and the material is a metal. If the conduction band is empty, the material is an insulator—unless the valence band below is close enough in energy for a few electrons to be thermally excited into the conduction band, in which case it is a semiconductor. The difference in energy between the top of the valence band and the bottom of the conduction band is the band gap. (b), When an electric field is applied across a material, mobile charge carriers will move in the direction of the field. The energies of electrons closest to the positive terminal are lowered and those closest to the negative terminal are increased, so the overall effect of the field is to tilt the band structure. Crudely speaking, the electrons can then be considered to “flow downhill.”

In truth, the characteristic that defines a semiconductor more formally is not its absolute conductivity but the fact that this increases as the temperature rises. This is because the charged particles that give rise to the electric current in a semiconductor are thermally excited. The hotter the material, the more charge carriers there are. This situation contrasts with that in metals, where heat degrades the conductivity by causing the atoms of the material to vibrate more vigorously, making them larger obstacles to the motion of charge carriers through the solid. (This thermal jostling occurs in semiconductors too, but there it is more than compensated by the increase in charge carriers.)

The conductivity of silicon can be enhanced by adding to it certain foreign atoms that provide additional charge carriers. These atoms are called dopants, and it is this ability to fine-tune the electronic properties of silicon by doping that makes it of such value to the microelectronics industry. If we insert into the silicon crystal lattice an atom of arsenic in place of silicon, the lattice acquires a surplus electron. Each atom of silicon has four valence electrons, which together fill up the valence band. But arsenic has five valence electrons, so there is not room in the valence band for the extra electron. It therefore sits in an energy state of its own within the band gap; physically, we can consider that the electron remains close to the arsenic dopant atom. This electron has to acquire even less energy to reach the conduction band than those in the valence band, and so it readily becomes a thermally excited charge carrier. Because t...