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-

We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.

Metal nanoclusters have physical properties differing significantly from their bulk counterparts. Metallic properties such as delocalization of electrons in bulk metals which imbue them with high electrical and thermal conductivity, light reflectivity and mechanical ductility may be wholly or partially absent in metal nanoclusters, while new properties develop. We review modern synthetic methods used to form metal nanoclusters. The focus of this critical review is solution based chemical synthesis methods which produce fully dispersed clusters. Control of cluster size and surface chemistry using inverse micelles is emphasized. Two classes of metals are discussed, transition metals such as Au and Pt, and base metals such as Co, Fe and Ni. The optical and catalytic properties of the former are discussed and the magnetic properties of the latter are given as examples of unexpected new size-dependent properties of nanoclusters. We show how classical surface science methods of characterization augmented by chemical analysis methods such as liquid chromatography can be used to provide feedback for improvements in synthetic protocols. Characterization of metal clusters by their optical, catalytic, or magnetic behavior also provides insights leading to improvements in synthetic methods. The collective physical properties of closely interacting clusters are reviewed followed by speculation on future technical applications of clusters. (125 references).

Near the sol-gel transition, gelling systems exhibit an extremely slow relaxation of thermally driven density Auctuations. %'e have made a detailed quasielastic light scattering study of the decay of density fluctuations in reacting silica sol-gels in the pre-and post-gel regimes, and at the gel point. In the pre-gel regime the dynamic structure factor S(q, t) for the branched polymer melt has a stretched exponential tail whose characteristic time diverges at the gel point. This critical slowing down is due to the divergence of the average cluster size and is distinct from the usual critical slowing down observed in second-order thermodynamic phase transitions, since the initial decay rate of S(q, t) is nondivergent at the gel point. In fact, at the gel point, S(q, t) becomes a power law, indicating a fractal time set in the scattered field. These observations are accounted for by considering the dynamics of percolation clusters, and in this connection the analogy to viscoelasticity is described. Beyond the gel point S (q, t) remains a power law, but the amplitude of the relaxing part of the intensity autocorrelation function diminishes. Finally, the dynamics of clusters diluted from the reaction bath is studied, and a crossover from power law to stretched exponential decay of S (q, t) is observed. It is shown that at infinite dilution the long-time tail of the correlation function describes the internal modes of a single percolation cluster.

No abstract

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.

No abstract

The dynamics of the sol-gel transition is probed by use of quasielastic light scattering. A type of critical dynamics is observed that is associated with a divergent friction, rather than a singularity in a thermodynamic quantity. Several novel effects are reported, including power-law time decay of the intensity autocorrelation function, critical slowing down of the average relaxation time, and observation of a fractal time set in the scattered field.PACS numbers: 82.70. Gg, 61.41.+e A gelling solution at the sol-gel transition is a unique state of matter that is neither liquid nor solid, but rather is in transition between these states. For example, the viscosity of the incipient gel is infinite, but the modulus is zero. Recently we have come to appreciate the unusual dynamics of this transition state by using the technique of quasielastic light scattering x to probe the relaxation of density fluctuations of wave vector q by the autocorrelation function of the scattered intensity, l(q,t) =(1(0)/(/)). In many systems the decay of density fluctuations can be described by a single relaxation time (exponential time decay). More complex materials, such as polymeric melts near the glassy transition temperature, exhibit a spectrum of relaxation times that gives a "stretched" exponential decay, exp[ -(t/r) b ] where 0 < b < 1. Regardless of the form, all known I(q,t) can be described by some characteristic time T. In this paper we demonstrate that in a gelling solution the characteristic time diverges at the sol-gel transition. This observation is unexpected and must not be confused with the usual critical slowing down in second-order thermodynamic phase transitions, since scattered-intensity measurements show that the compressibility does not diverge at the gel point. The fact that a critical slowing down should not be observed in quasielastic light scattering at the gel point has been elaborated by de Gennes, 2 who points out that the longitudinal modes observed in a quasielastic light-scattering experiment should be insensitive to the formation of a weak gel phase.The observation of an infinite characteristic time implies two possible modes of decay. First, the decay can be described by a function that is scaled by a divergent characteristic time, e.g., an exponential decay with r-• <». This is precisely the description of critical slowing down in second-order phase transitions. Second, the form of the decay can be independent of the time scale, at least on times short compared with the characteristic time. This is possible if the decay is described by a function that does not contain a time scale-a power law. In fact, we will show that at the gel point a decay of the form I(q,t)~-\/t 021 is observed over the experimentally accessible 5 decades in time. Before the gel point, this power-law decay is truncated by a stretched-exponential tail, at a certain divergent characteristic time. A simple description of these phenomena is proposed, and we show that the detected photons divide the time axis in a selfsimilar wa...