A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional ($\mathrm{DFT}+U$, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials.

First-principles calculations for point defects in solids

The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

X-ray absorption and dichroism of transition metals and their compounds

As early as the 1930s, it had been observed that the absorption cross section in the X-ray regime had a complex structure as a function of energy extending as far as 1000 eV above an absorption threshold.1 These measurements were made by using a low‐power conventional X-ray tube, a dispersive Bragg spectrometer, and film as a detector. After some correct and incorrect speculation concerning the origin of this extended X-ray absorption fine structure (EXAFS), the field remained essentially dormant through the 1950s. In the late 1960s the measurements were revived,2 this time using modern counting and automation techniques. The previous interpretation3–5 of the structure as arising from the backscattering of the ejected photoelectron from the atom near the absorbing site was strongly supported. The idea that this phenomenon might be used as a local structural probe was born. However, even with the experimental advances the measurements were difficult and time-consuming because of the low power of t...

Sexafs: New horizons in surface structure determinations

This paper describes the theory of the production of coherent XUV and soft x‐rays using a Transverse Optical Klystron (TOK) A TOK uses a high powered laser in conjunction with an undulator magnet to produce laser‐like output of XUV radiation from the relativistic stored electron beam in the NSLS VUV electron storage ring

The transverse optical klystron experiment at NSLS: Laser harmonics into the XUV

Coherent radiation produced by a laser-bunched relativistic electron beam (the optical klystron) has been proposed as a tunable source of laser harmonics in the XUV. This paper presents a brief introduction to the physics of optical klystron laser harmonic generation and a discussion of the limitations of the technique. The effects of finite electron beam emittance, energy spread and random undulator field errors on the performance of the optical klystron are discussed. Finally, possible methods for dealing with the limitations of the technique are discussed.

Laser harmonic generation using the optical klystron and the effects of wiggler errors

This work explores the effects of random undulator field errors on the output radiation. An approximate theory of these effects is summarized, along with limiting conditions on its validity, resulting in a description of the loss of radiated intensity by two universal functions. The functions' arguments are simple combinations of the basic undulator parameters. Numerical results are compared with the predictions of the universal functions. Finally, possible solutions to the problem of random errors in undulators are discussed.

B. M. Kincaid

Papers

Sexafs: New horizons in surface structure determinations

The transverse optical klystron experiment at NSLS: Laser harmonics into the XUV

Laser harmonic generation using the optical klystron and the effects of wiggler errors

Random Errors in Undulators and their Effects on the Radiation Spectrum