[Show abstract][Hide abstract] ABSTRACT: We review modern synthetic methods, emphasizing the use of inverse micelles to form nanoparticles. Control of particle size and surface chemistry is emphasized. Homoatomic nanoparticles composed of transition metals such as Ag, Au, Mo, Pd, Pt, Rh and base metals such as Co and Fe are discussed. Synthesis and optical characterization of nanoalloys of these elements and core/shell nanostructures are also discussed. Classical surface science methods of characterization such as transmission electron microscopy are augmented by chemical analysis methods such as size exclusion chromatography, providing feedback for improvements in synthetic protocols. The size-dependent optical properties of Ag, Au and binary core/shell and nanoalloy particles provide important structural insights, and new phenomena such as metal photoluminescence are observed. Growth of nanoparticles from atoms and etching of nanoparticles indicate certain sizes are especially favoured. The magnetic response of superparamagnetic Co nanoparticles is shown to change with sample age becoming larger than that of bulk Co. We find that the addition of surfactants or other magnetic (Co, Fe) or non-magnetic (Ag, Pt) atoms to the surface of nanoparticles of Co or Fe can be used to either quench or increase the magnetic response showing the important role of surface magnetism in such small nanoparticles. Finally, we report the catalytic activity and selectivity of nanoparticles of FeS2, MoS2, Pd, Pt and Rh emphasizing pyrene hydrogenation and redox reactions important to technologies such as coal liquefaction, crude oil hydrotreating and production of industrially important chemicals such as H2, CH4 and NH3 by thermo- or photocatalytic processes using simple abundant molecules like water, carbon dioxide or nitrogen.
[Show abstract][Hide abstract] ABSTRACT: Highly crystalline nanoclusters of MoSâ were synthesized and their optical absorption and photoluminescence spectra were investigated. Key results include: (1) strong quantum confinement effects with decreasing size; (2) preservation of the quasiparticle (or excitonic) nature of the optical response for clusters down to â¼ 2.5 nm in size which are only two unit cells thick; (3) demonstration that 3-D confinement produces energy shifts which are over an order of magnitude larger than those due to 1-D confinement; (4) 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; and (5) observation of photoluminescence due to both direct and surface recombination. Application is to photocatalysts for solar fuel production and detoxification of chemical waste.
[Show abstract][Hide abstract] ABSTRACT: The optical absorption and photoluminescence properties of nanosize PbS clusters synthesized in inverse micelles are presented. Relative to bulk crystals, the bandgap of PbS nanoclusters is significantly blue shifted from the near-IR into the visible and near-UV region with decreasing cluster size. The optical absorption spectra for clusters with a diameter of approx.2.5-3 nm contain a molecular-like feature at 284 nm (4.37eV) which matches the absorption of diatomics PbS. The main photoluminescence band occurs at 520 nm and is due to recombination at surface states. The PLE spectrum of the 520 nm band has a minimum which coincides with the absorption peak. The absorption line is interpreted to arise from a pre-dissociative excitation of PbS molecules on the surface. The PLE band is interpreted to be an excitation molecular-like levels in the interior of the cluster. Evidence from size, stoichiometry and surface alloy (Au) dependent properties supports this interpretation.
[Show abstract][Hide abstract] ABSTRACT: The authors report a real-time, two-dimensional light scattering study of the evolution of structure of a two component nonionic micelle system undergoing phase separation. The micelles act like molecular slug-a-beds whose domain growth is lethargic (i.e. slower than the cube root of time prediction for simple binary fluids). In fact, the growth kinetics can be empirically described as a stretched exponential approach to a pinned domain size. Although the kinetics are not yet understood, the anomalous behavior may be due to the ability of the spherical micelles to reorganize into more complex structures.
[Show abstract][Hide abstract] ABSTRACT: The authors report the fabrication and electrical characterisation of nanowires created via direct electron beam writing in films of passivated gold nanocrystals. Charge transport measurements yield room temperature resistances in the range 10<sup>5</sup>-10<sup>8</sup> . Variable temperature measurements yield two distinct sets of characteristics: activated conduction (high resistance) consistent with weakly coupled metal nanocrystals separated by alkyl thiol tunnel barriers and quasi-localised behaviour (low resistance) consistent with stronger coupling between granular metallic islands in a carbonaceous matrix. The data indicate that electron beam writing is a promising method for local manipulation of inter-nanocrystal coupling in nanocrystal arrays.
[Show abstract][Hide abstract] ABSTRACT: The oldest topic in nanoscience is the size-dependent optical properties of gold and silver colloids or nanoparticles, first investigated scientifically by Michael Faraday in 1857. In the modern era, advances in both synthesis and characterization have resulted in new insights into the size-dependent absorbance of Au and Ag nanoparticles with sizes below the classical limit for Mie theory. In this paper we discuss the synthesis and properties of core/shell and nanoalloy particles of Au and Ag, compare them to particles of pure gold and silver, and discuss how alloying affects nanoparticle chemical stability. We show that composition, size, and nanostructure (e.g., core/shell vs quasi-random nanoalloy) can all be employed to adjust the optical absorbance properties. The type of nanostructure--core/shell vs alloy--is reflected in their optical absorbance features.
The Journal of Physical Chemistry B 04/2009; 113(9):2647-56. DOI:10.1021/jp806930t · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three types of bimetallic AgAu nanoparticles, with mean size of 4-5 nm, Ag(core)Au(shell), Au(core)Ag(shell) and alloyed AgAu, have been synthesized using an inverse micelle method. To image these small size nanoparticles, quantitative high angle annular dark field imaging using scanning transmission electron microscopy was successfully applied. Our results show that good control of nanoparticle size dispersion and composition modulation was achieved. Optical properties of the nanoparticles are correlated with direct internal structure analysis. The structural stability is discussed, based on thermodynamic considerations.
[Show abstract][Hide abstract] ABSTRACT: Nanopartciels comprising two different metallic elements offer additional degrees of freedom for altering their physical properties
by varying the atomic composition and atomic arrangement. This could potentially enable a wide range of applications for discovering
materials with novel physical properties. However, our present understanding of fundamental issues has been hindered by the
lack of knowledge of the precise chemical composition and atomic arrangement within a given bimetallic nanoparticle. Here,
we report our recent systematic study on structural and optical properties of core/shell structured AgAu nanoparticles of
4–5 nm diameter by a combination of high resolution scanning transmission electron microscopy (STEM) with UV-visible absorption
[Show abstract][Hide abstract] ABSTRACT: Nanosized semiconductors (semiconductor clusters) have the potential to revolutionize the fields of photooxidation and photocatalysis through the combined effects of quantum confinement and their unique surface morphologies. Photocatalytic oxidation as applied to environmental reme-diation (i.e., detoxification of chemical wastes), green/sustainable chemistry, as well as alternative energy paths (i.e., splitting of H 2 O to produce H 2) has already experienced improvements in ac-tivity, efficiency, and stability through the use of semiconductor nanoclusters based on materials such as TiO 2 , MoS 2 , WS 2 , MoSe 2 , FeS 2 , and SnO 2 . Issues such as improved control of size and surface chemistry play an important role in the success of these semiconductor nanocatalysts. This review explores the effect of advances in the fields of nanoscience and photocatalysis for current and future applications.
Critical Reviews in Solid State and Material Sciences 01/2008; 30(3):153-182. DOI:10.1080/10408430500200981 · 6.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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).
Chemical Society Reviews 12/2006; 35(11):1162-94. DOI:10.1039/b517312b · 33.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Frequency downconversion is an effective method for the detection of ionizing radiation. We demonstrate the utilization of thin films of chemically synthesized CdSe quantum dots for this purpose. A high quantum yield of ∼ 0.2 is obtained at room temperature, which does not depend on the excitation wavelength in the range from 32 to 114 nm. The measured decay time is short, in the nanosecond regime, and the visible light output is stable. A significant advantage of such semiconductor quantum dots is the potential to tune the emission wavelength via selection of the nanoparticle size.
[Show abstract][Hide abstract] ABSTRACT: An alternative route followed for the characterization of nanoparticle assemblies by using the high-angle annular dark-field (HAADF) imaging technique in scanning TEM (STEM), was discussed. The method is also known as Z-contrast imaging since the electron scattering cross section is sensitive to the atomic number, Z. It was found that the incoherent electron imaging technique has great potential in characterizing the growth of ultrafine-nanoparticle thin films. The results show that the technique offered the possibility of directly visualizing the 3D topography of colloidal thin films at the nanometer scale.
[Show abstract][Hide abstract] ABSTRACT: 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 ∼ 4 nm 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.
[Show abstract][Hide abstract] ABSTRACT: We report studies of the magnetic response of dilute frozen solutions of nanocrystalline Co particles grown in inverse micelles. Crystalline nanoclusters which initially exhibit only a small fraction of the bulk saturation moment restructure in solution without any change in cluster size or blocking temperature over a period of ∼30–60 days, finally yielding a moment/atom which exceeds that of bulk Co. The saturation magnetism maintains its enhanced value for temperatures up to the melting point of the solvent matrix, but is strongly dependent on surface active additives and molecular oxygen.
Physical Review B 05/2004; 69(17). DOI:10.1103/PhysRevB.69.172402 · 3.74 Impact Factor