States of advanced excitement

A prominent place in the progress of science should be reserved for those who can take a large body of phenomenological knowledge, often depending on a huge number of variables, and distil from it a coherent understanding of the underlying principles. This was how Philip Bowden and David Tabor elucidated the processes involved in friction, lubrication and wear, and so created the academic discipline of tribology.

Noriaki Itoh and Marshall Stoneham have done a similar service for the effect of electronic excitation on materials. There was already a mass of information on many different systems but the common ground had not been identified. Starting with key concepts such as energy and charge localisation, together with energy transfer and storage, Itoh and Stoneham elucidate the similarities among the many excitation-enhanced processes that can be used to change the properties of materials, and then go on to illustrate how they can be used to modify materials.

Electronic excitation can take many forms. It can be concentrated or spread out in space and in time, and it may excite whole atoms or just their electrons. The book's first four chapters cover basic concepts of the nature and behaviour of electronic excitations in materials. The first concept to be introduced is the polaron, which describes a carrier and its associated local polarisation and deformation of the atomic lattice. It is hard to escape the lattice terminology associated with crystalline materials, but the polaron concept is also important in amorphous and molecular materials. There follow chapters on energy deposition and redistribution in solids, electron-lattice coupling and its consequences, and self-trapping.

We are used to thinking of the electrons in metals as being nearly free and interacting only weakly with the atomic lattice except in the vicinity of the Brillouin zone boundary. But in many solids, the interaction can be strong and a charge carrier (which may be an electron or a hole) can cause a strong local distortion of the lattice and an associated local electrical polarisation. These polarons can be transported through a lattice, but they can also become self-trapped. The same is true of excitons, which are an electron-hole pair bound to one another rather like the proton and electron of a hydrogen atom. Although these quasi-particles are often introduced in the context of crystalline lattices, they are also important in amorphous materials such as glass and molecular materials such as polymers.

With the foundation laid, accounts of the modification of materials by electronic excitation begin in earnest in chapter five. The details depend on the type of material. The extraordinary sensitivity of photographic emulsion depends on the way the reduction of two or three atoms of silver from the halide can catalyse the chemical reduction of a whole crystallite containing perhaps 109 atoms. Since this requires the absorption of only about ten photons per grain, it means that about 100 million silver atoms are produced per photon absorbed. Oxides have significantly higher thermal formation energies for defect pairs, with cations that can exist in several valence states, only weak self-trapping, and much higher dislocation densities.

The complex effects of electronic excitation present a challenge to any tidy scheme of mechanisms. In semiconductors, the smaller bandgap means that individual excitations generally have insufficient energy to create defects, and it is usually pre-existing defects such as impurities that are affected. Thermal effects of excitation become important, making possible designer diffusion of selected species with spatial control. Amorphous materials lack long-range order. Photo-induced excitons can weaken bonds, resulting in phenomena as diverse as luminescence, defect formation and photo-darkening. Laser excitation of surfaces opens the possibility of removing atoms through mechanisms that depend on whether the bandgap is bigger or smaller than the photon energy.

One of the delights of materials science is the freedom to reach out to very fundamental science in one direction and to cutting-edge applications in the other. The book ends with a crescendo of opportunities for new technology. In chemically amplified resists for lithography, each photon stimulates a photo-acid generator to form an acid that catalyses changes in the resist material, with enormous potential for future nanolithography. Such developments have multiplied even since the book went to press, and will undoubtedly contribute to new fields. The best basis for technological advance is fundamental understanding, and this Itoh and Stoneham offer in rich measure.

With more than 1,600 references stretching back before the reach of online databases, the authors present encyclopedic breadth of knowledge, with the critical judgement crucial for any useful literature survey.

Andrew Briggs is professor of materials, University of Oxford.