Random lattice disorder produced by chemical substitution in ABO3 perovskites can lead to the formation of dipolar impurities and defects that have a profound influence on the static and dynamic properties of these materials that are the prototypical soft ferroelectric (FE) mode systems. In these highly polarizable host lattices, dipolar entities form polar nanodomains whose size is determined by the dipolar correlation length, rc, of the host and that exhibit dielectric relaxation in an applied ac field. In the very dilute limit (< 0.1at.%) each domain behaves as a non-interacting dipolar entity with a single relaxation time. At higher concentrations of disorder, however, the domains can interact leading to more complex relaxational behaviour. Among the manifestations of such behaviour is the formation of a glass-like relaxor (R) state, or even an ordered FE state for a sufficiently high concentration of overlapping domains. After a brief discussion of the physics of random-site electric dipoles in dielectrics, this review begins with the simplest cases, namely the relaxational properties of substitutional impurities (e.g., Mn, Fe and Ca) in the quantum paraelectrics KTaO3 and SrTiO3. This is followed by discussions of the relaxational properties of Li-and Nb-doped KTaO3 and of the strong relaxors in the PbMg1/3Nb2/3O3 and La-substituted PbZr1−xTixO3 families. Some emphasis will be on the roles of pressure and applied dc biasing electric fields in understanding the physics of these materials including the R-to-FE crossover.

Highly crystalline, size-selected silicon (Si) nanocrystals in the size range 2-10 nm were grown in inverse micelles and their optical absorption and photoluminescence (PL) properties were studied. High resolution TEM and electron diffraction results show that these nanocrystals retain their cubic diamond stuctures down to sizes -4 nm in diameter, and optical absorption data suggest that this structure and bulk-like properties are retained down to the smallest sizes produced (-1.8 nm diameter containing about 150 Si atoms). High pressure liquid chromatography techniques with on-line optical and electrical diagnostics were developed to purify and separate the clusters into pure, monodisperse populations. The optical absorption revealed features associated with both the indirect and direct bandgap transitions, and these transitions exhibited different quantum confinement effects. The indirect bandgap shifts from 1.1 eV in the bulk to -2.1 eV for nanocrystals -2 nm in diameter and the direct transition at T(Tz -TIS) blue shifts by 0.4 eV from its 3.4 eV bulk value over the same size range.Tailorable, visible, room temperature PL in the range 700-350 nm (1.8 -3.5 eV) was observed from these nanocrystals. The most intense PL was in the violet region of the spectrum (-400 nm) and is attributed to direct electron-hole recombination. Other less intense PL peaks are attributed to surface state and to indirect bandgap recombination. The results are compared to earlier work on Si clusters grown by other techniques and to the predictions of various model calculations.Currently, the wide variations in the theoretical predictions of the various models along with considerable uncertainties in experimental size determination for clusters less than 3-4 nm, make it difficult to select among competing models.-1-

Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to the layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the states at these points; and (5) the observation of photoluminescence due to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysis.

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