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.

The trap-to-trap relaxation and recombination dynamics of photogenerated electron/hole pairs in MoS 2 nanoclusters have been studied. Static and time-resolved emission experiments have been performed on 3.0 and 4.5 nm diameter nanoclusters in ternary inverse micelles, acetonitrile, and octane at room temperature and at 20 K. The results indicate that, following synthesis in ternary inverse micelles, the nanoclusters have both shallow and deep traps. The deep traps are retained upon extraction into acetonitrile and passivated upon charge neutralization and reextraction into octane. The emission kinetics show that trap-to-trap relaxation is fast (<40 ps) at room temperature and slows (∼200 ps) at 20 K. A distributed kinetics model is presented that quantitatively describes electron/hole recombination. The trapped electron Bohr radius is found to be 2.0-2.5 nm in all cases. Charge neutralization and reextraction into octane passivates both the deep and the shallow traps on some nanoclusters, resulting in indirect band edge emission at 20 K.

The photophysics and electron transfer (ET) dynamics of quantum confined MoS2 nanoclusters have been studied using static and time resolved emission spectroscopy. The MoS2 nanoclusters consist of a single S–Mo–S trilayer, having diameters of ∼2.5 or 4.5 nm. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2′-bipyridine (bpy) and 4,4′,5,5′-tetramethyl-2,2′-bipyridine (TMB). The ET reaction exothermicities may be varied by changing the electron acceptor or by varying the size of the MoS2 nanocluster. TMB is harder to reduce, and thus has a smaller ET driving force than bpy. The smaller nanoclusters have a higher energy conduction band, and thus have a larger ET driving force. In all cases, the ET driving force may be calculated from bulk MoS2 properties and quantum confinement theory. Both ‘‘normal’’ and ‘‘inverted’’ behaviors are observed. A reorganization energy of 0.40 eV is calculated from energy dependent ET rates.

Sputter deposited Al (1-x) Sc x N thin films with a Sc content from x ¼ 0 to 43 at% are investigated by electron microscopy in order to study and explain the formation and growth of abnormally oriented grains (AOG). It is found that the latter did not nucleate at the interface with the substrate, but at high energy grain boundaries, at which systematically higher Sc concentrations are detected. The AOGs are thus formed during the growth of c-textured grains. They grow faster than those, and finally protrude from the c-textured film surface, having at their end a pyramidal shape with three facets of a hexagonal wurtzite crystal: one (0001) and two (11 20) facets. Process conditions favoring less compact grain boundaries, and lower surface diffusion across grain boundaries are thought to promote nucleation of AOGs. Finally, a 4-step growth mechanism explaining the nucleation from a Sc-rich complexion and proliferation of AOGs with increasing film thickness is proposed.

The synthesis and characterization of PtS2 nanoclusters synthesized in AOT/hexanol/heptane inverse micelles are reported. Electron diffraction and optical spectroscopy have been used to characterize these nanoclusters. The electron diffraction results show that the nanoclusters have the same crystal structure as bulk PtS2 and are consistent with the nanoclusters being a single S−Pt−S trilayer. Absorption spectroscopy shows that these nanoclusters have an indirect band gap of 1.58 eV as compared to 0.87 eV for bulk PtS2. The nanoclusters can be grown such that their mass is doubled, resulting in a band gap of 1.27 eV. PtS2 nanoclusters doped with 1−5% Eu3+ were also synthesized in AOT/hexanol/heptane and tridodecylmethylammonium chloride (TDAC)/hexanol/octane inverse micelles. The mj structure and relative intensities of Europium emission lines are indicative of the symmetry of the local environment and hence the location of the Eu3+ ion. It is concluded that synthesis of doped nanoclusters in TDAC/hexanol/octane results in Eu3+ ions that are situated in the near-octahedral holes of the PtS2 lattice, while an AOT/hexanol/heptane synthesis results in a Eu3+ ion on the nanocluster edges. The emission and fluorescence excitation spectra show that 4.0 eV optical excitation of the nanocluster results in energy transfer and subsequent luminescence of the europium dopant. Since the europium excited state is at a higher energy than the band gap, it is concluded that energy transfer to the dopant competes with energy relaxation of the electron/hole pair. Passivation of the nanocluster surface trap states is observed to increase the intensity of europium luminescence, and we conclude that trapping also competes with electron/hole energy relaxation.

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The dynamics of photoinduced electron transfer between 3.0 nm diameter MoS2 nanoclusters and adsorbed electron acceptors have been studied using static and time resolved optical spectroscopy. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2'-bipyridine and 4,4',5,5'-tetramethyl-2,2'-bipyridine. The results reported here focus on the interpretation of the strongly non-exponential electron transfer kinetics. Electron/hole recombination kinetics have previously been analyzed in terms of a simple distributed kinetics model, and this model is extended to include the case of interfacial electron transfer.

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