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Preface

Electronic states and probabilities of optical transitions in molecules and crystals are determined by the properties of atoms and their spatial arrangement. Electron in an atom possesses a discrete set of states, resulting in a corresponding set of narrow absorption and emission lines. Elementary excitations in an electron subsystem of a crystal, i.e., electrons and holes, possess many properties of a gas of free particles. In semiconductors, broad bands of the allowed electron and hole states separated by a forbidden gap give rise to the characteristic absorption and emission features that have nothing similar to atomic spectra. It is therefore reasonable to pose a question: What happens on the way from atom to crystal? The answer to this question can be found in the studies of small particles with the number of atoms ranging from a few atoms to several hundreds of thousands atoms. The evolution of the properties of the matter from atom to crystal can be described in terms of the two steps: from atom to cluster and from cluster to crystal.

The main distinctive feature of clusters is the discrete set of the number of atoms organized in cluster. These so-called "magic numbers" determine unambiguously the spatial configuration, electronic spectra and optical properties of clusters. Sometimes a transition from a given magic number to the neighboring one results in a drastic change in energy levels and optical transition probabilities. As the particle size grows, starting from a certain size the properties can be described in terms of the particle size and shape instead of dealing with the particular number of atoms and spatial configuration. This type of microstructures can be referred to as mesoscopic structures as their size is always larger than the crystal lattice constant but comparable to the de Broglie wavelength of the elementary excitations. They are often called "quantum crystallites", "quantum dots" or "quasi-zero-dimensional structures". As the size of these crystallites ranges from one to tens nanometers, the word "nanocrystals" is widely used as well. This term refers to the crystallites size only, whereas the other terms contain a certain hint at the interpretation of their electron properties in terms of the quantum confinement effects.

From the standpoint of a solid state physicist, nanocrystals are nothing else but a kind of a low-dimensional structure complementary to quantum wells (two-dimensional structures) and quantum wires (one-dimensional structures). However, the finiteness of quasi-zero-dimensional species results in a number of specific features that are not inherent in the two- and one-dimensional structures. Quantum wells and quantum wires still possess a translational symmetry in two or one dimensions, and a statistically large number of electronic excitations can be created. In nanocrystals, the translational symmetry is totally broken and only a finite number of electrons and holes can be created within the same nanocrystal. Therefore, the concepts of the electron-hole gas and quasi-momentum fail in nanocrystals. Additionally, a finite number of atoms in nanocrystals promotes a variety of photoinduced phenomena like persistent and permanent photophysical and photochemical phenomena that are known in atomic and molecular physics but do not occur in solids. Finally, nanocrystals are fabricated by means of techniques borrowed from the glass technology, colloidal chemistry, and other fields that have nothing common with the crystal growth.

From the viewpoint of molecular physics, nanocrystals can be considered as a kind of large molecules. Similar to molecular ensembles, nanocrystals dispersed in a transparent host environment (liquid or solid) exhibit a variety of guest-host phenomena known for molecular structures. Moreover, every nanocrystal ensemble has inhomogeneously broadened absorption and emission spectra due to distribution of sizes, defect concentration, shape fluctuations, environmental inhomogeneities, and other features. Therefore, the most efficient way to examine the properties of a single nanocrystal which are smeared by the inhomogeneous broadening is to use numerous selective techniques developed in molecular and atomic spectroscopy.

Additionally, as the size of crystallites and their concentration grow, the heterogeneous medium "matrix-crystallites" becomes a subject of the optics of ultradisperse media, thus introducing additional aspects to the optical properties of nanocrystal ensembles.

Because of the above-mentioned features, studies of the optical properties of nanocrystals form a new field bordering solid state physics, optics, molecular physics, and chemistry.

Despite matrices colored with semiconductor nanocrystals are known in the human practice for a number of centuries as stained-glasses, the systematic studies of their physical properties have begun not long ago. Probably, first investigations on quasi-zero-dimensional structures were the pioneering works by Froelich (1937) and Kubo (1962) in which non-trivial properties of small metal particles were predicted due to a discreteness of electron spectra. The systematic studies of size-dependent optical properties of semiconductor nanocrystals have been stimulated by impressive advances in the quantum confinement approach for fine semiconductor layers (quantum wells) and needle-like structures (quantum wires).

The St.-Petersburg school in Russia, which included solid state physics, optical spectroscopy, and glass technology (Ekimov et al. 1980, 1982; Efros and Efros 1982) and independently the Murray Hill group in the USA (Rossetti et al. 1983) were the first to outline the size-dependent properties of nanocrystals due to the quantum confinement effect. Since then, a great progress in the field has been achieved due to extensive studies performed by thousands of researchers over the world. The advances in the theory of semiconductor quantum dots have been described thoroughly in the nice book by Banyai and Koch (1993). The present book is aimed to summarize the progress in experimental studies of semiconductor nanocrystals.

Chapters I-IV contain a brief description of the theoretical results on electron states in an idealized nanocrystal, a sketch of the growth techniques and structural properties, and a survey of the selective optical techniques and relevant optical effects known for other spectrally inhomogeneous media. These Chapters are designed to provide an introductory overview which seems to be reasonable taking into account an interdisciplinary nature of the field. Chapter V contains the systematic analysis of the size-dependent absorption and emission processes that can be described in terms of creation or annihilation of a single electron-hole pair within the same nanocrystal. The materials under consideration are II-VI (CdSe, CdS, ...), III-V (GaAs, InAs), and I-VII (CuCl, CuBr, AgBr) compounds, and nanocrystals of group IV elements (Si and Ge). In Chapter VI, a variety of many-body effects will be considered resulting in the intensity-dependent, i.e., nonlinear optical, phenomena. A variety of crystallite-matrix interface processes that are responsible for the majority of photoinduced persistent and permanent effects like, e.g., stable spectral hole-burning or photodarkening, will be a subject of Chapter VII. In Chapter VIII we consider the recent advances in the fabrication and description of spatially ordered ensembles of nano- and microcrystals which is a challenging way towards artificial materials like three-dimensional superlattices of crystallites. The most intriguing kind of these structures is the so-called photonic crystal which is to photons as an ordinary crystal to electrons. In some respects, this field combined with nanocrystal optics leads to the photonic engineering providing structures with desirable spectrum, lifetime, and the propagation conditions.

The presentation style of this book was chosen to provide an introduction to and an overview of the field in the form understandable for the senior and graduate students specialized in physics and chemistry and interested in solid state optics and engineering.