Jess P. Wilcoxon

Photoluminescence from nanosize gold clusters

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.

Catalytic Properties of Single Layers of Transition Metal Sulfide Catalytic Materials

Single layer transition metal sulfides (SLTMS) such as MoS2, WS2, and ReS2, play an important role in catalytic processes such as the hydrofining of petroleum streams, and are involved in at least two of the slurry‐catalyst hydroconversion processes that have been proposed for upgrading heavy petroleum feed and other sources of hydrocarbon fuels such as coal and shale oils. Additional promising catalytic applications of the SLTMS are on the horizon. The physical, chemical, and catalytic properties of these materials are reviewed in this report. Also discussed are areas for future research that promise to lead to advanced applications of the SLTMS.

Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime

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 lar...

Optical properties of gold colloids formed in inverse micelles

We discuss the formation of gold metal colloids in a variety of surfactant/solvent systems. Static and dynamic light scattering, small angle x‐ray and neutron scattering, TEM analysis, and UV‐visible absorbance are used to characterize the kinetics of formation and final colloid stability. These gold colloids exhibit a dramatic blueshift and broadening of the plasmon resonance with decreasing colloid size. Several types of reduction method are discussed and differences between micelle (water‐free) or microemulsions as reaction media are compared. Use of inverse micelles allows smaller clusters to be formed with greater long‐term stability.

Tailorable, Visible Light Emission From Silicon Nanocrystals

Crystalline, size-selected Si nanocrystals in the size range 1.8–10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced (∼2 nm) was in the blue (∼365 nm) with a radiative lifetime of ∼1 ns and is attributed to direct recombination at zone center.

Optical properties of gold and silver nanoclusters investigated by liquid chromatography

We report high-pressure liquid chromatography (HPLC) and transmission electron microscopy studies of the size-dependent absorbance properties of Au and Ag nanoclusters dispersed in organic solvents. These nanosize metal clusters are synthesized by an inverse micelle synthetic technique at room temperature in inert oils and those investigated range in diameter from 1.3–8 nm. HPLC allows us to separate the clusters from all other chemicals and size select to a resolution of ±2 A. We use an on-line photodiode array to study the size-dependent absorbance properties of these clusters. For both Au and Ag clusters in the size range d=8 to d=1.5 nm, the plasmon linewidth broadens following a 1/R linewidth size dependence whose slope is greatest for Au. The peak asymmetry in the plasmon band shape is greatest for Au and increases with decreasing size for both Au and Ag clusters. The plasmon peak energy blue shifts with decreasing size for Au clusters while in the case of Ag nanoclusters a red shift is observed.We report high-pressure liquid chromatography (HPLC) and transmission electron microscopy studies of the size-dependent absorbance properties of Au and Ag nanoclusters dispersed in organic solvents. These nanosize metal clusters are synthesized by an inverse micelle synthetic technique at room temperature in inert oils and those investigated range in diameter from 1.3–8 nm. HPLC allows us to separate the clusters from all other chemicals and size select to a resolution of ±2 A. We use an on-line photodiode array to study the size-dependent absorbance properties of these clusters. For both Au and Ag clusters in the size range d=8 to d=1.5 nm, the plasmon linewidth broadens following a 1/R linewidth size dependence whose slope is greatest for Au. The peak asymmetry in the plasmon band shape is greatest for Au and increases with decreasing size for both Au and Ag clusters. The plasmon peak energy blue shifts with decreasing size for Au clusters while in the case of Ag nanoclusters a red shift is observed.

Electron transfer dynamics in MoS2 nanoclusters: Normal and inverted behavior

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.

Applications of metal and semiconductor nanoclusters as thermal and photo-catalysts

We discuss studies of photo and thermally driven reactions which illustrate the unique catalytic features of nanoclusters which may be of great technical utility. The application of photocatalysts of nanosize MoS2 to oxidize organic pollutants to CO2 is discussed. Nanosize MoS2 shows improved rates of oxidation of organics such as phenol compared to alternative bulk powders such as TiO2 or ZnO. Selective hydrogenation of pyrene using both dispersed and supported nanoclusters of Rh, Pt, and Pd is also discussed.

Direct imaging of core-shell structure in silver-gold bimetallic nanoparticles

High-angle annular dark field imaging in the electron microscope has been exploited to reveal the internal structure of monodispersed, bimetallic gold (Au) - silver (Ag) nanoparticles of ∼4nm diameter, prepared using a seed-growth wet chemical method and passivated with an organic layer. Starting with a 3 nm Ag seed particle, deposition of Au atoms to a final overall atomic ratio of Ag:Au=1:2 leads to nanoparticles with an Ag-rich core and an Au-rich shell, as expected. If the overall atomic ratio is 2:1, the core∕shell structure is not observed. The physical significance of these observations is discussed in terms of the stability of particles of nanoscale dimensions.

Strong quantum confinement effects in semiconductors: FeS2 nanoclusters

High quality, size-selected, nano-size clusters of FeS2 have been successfully grown inside inverse micellar cages and their optical absorption studied. Remarkable structure in the absorption spectrum is observed and large blue shifts in the absorption edge and other spectral features are observed upon crossing from the weak to strong quantum confinement regimes with decreasing cluster size. The bandgap appears to remain indirect down to the smallest cluster size studied (2 nm).