Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation EFermi/N1/3, predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon—the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.

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Highly fluorescent noble-metal quantum dots.

A literature review of modern transition-metal nanoclusters, with an emphasis on those nanoclusters which are catalytically active, is presented in two parts. Part One presents background information on transition-metal nanoclusters, including an overview of common synthetic routes, a description of how nanoclusters are stabilized, and a brief summary of the multiple characterization techniques used (and the type of information that they can provide). In Part Two, five specific nanocluster case studies are presented, case studies which compare and contrast the syntheses, characterization approaches, and catalytic applications of transition-metal nanoclusters.

A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis

By using mass spectrometry to analyze the atomic and molecular secondary ions that are emitted from a solid surface when bombarded with ions, one obtains detailed information about the chemical composition of the surface. A time-of-flight mass spectrometer is especially suitable for the analysis of secondary ions because of its high transmission, high mass resolution, and ability to detect ions of different masses simultaneously. By using a finely focused primary ion beam it is also possible to analyze microareas and generate surface images with a lateral resolution of 0.1 μm or less. Static time-of-flight secondary ion mass spectrometry (TOF-SIMS) allows monolayer imaging and local analysis of monolayers with high sensitivity, a wide mass range, high mass resolution, and high lateral resolution. Besides information on elements and isotopes, the technique yields direct information on the molecular level and can also be used to analyze surface species of high molecular mass that are thermally unstable and cannot be vaporized. The method can be applied to practically all types of materials and sample forms, including insulators in particular. In this article the basic principles of TOF-SIMS are explained, and its analytical capabilities for both large area and imaging applications are illustrated by examples. These include silicon surfaces (both uniform and structured), thermally unstable organic molecules on surfaces, synthetic polymers, and synthetically prepared molecular surface films, particles, and fibers. Emitted neutral particles can also be analyzed by postionization with a laser, and the possibilities of this technique are discussed.

Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS)

Structural and functional evolution of metallacrowns.

Time-of-flight secondary ion mass spectrometry: techniques and applications for the characterization of biomaterial surfaces.

In many secondary ion mass spectrometry (SIMS) investigations, the total number of generated secondary ions is limited by the amount of sample material available. This is the case in surface reaction studies as well as in organic and inorganic trace analysis or imaging SIMS. In such cases a time‐of‐flight mass spectrometer has some considerable advantages: quasisimultaneous detection of all masses, unlimited mass range, and very high transmission. We have developed a high‐resolution reflectron based time‐of‐flight secondary ion mass spectrometer with a new electrodynamic mass separation and beam chopping technique based on a pulsed 90° deflection of the primary ion beam. A primary ion pulse width of less than 1.5 ns has been obtained. Second‐order energy focusing in the flight path of the secondary ions is achieved by a two‐stage reflectron. A mass resolution m/Δm=13 000 and a dynamic range of five orders of magnitude have been obtained with this instrument.

Design and performance of a reflectron based time‐of‐flight secondary ion mass spectrometer with electrodynamic primary ion mass separation

Experiments have been performed to compare secondary ion emission from polymer substrates under kiloelectronvolt ion (secondary ion mass spectrometry, SIMS) and megaelectronvolt ion (plasma desorption mass spectrometry, PDMS) bombardment. The yield of carbon cluster ions has been determined for different polymers. A positive PDMS spectrum of poly(vinylidene fluoride) showing even-numbered carbon clusters to C{sub 200} is presented. The emission of these carbon cluster ions is explained by the strong degradation of the polymer under megaelectronvolt ion bombardment. This is discussed in comparison with the results in SIMS and laser mass spectrometry.

Carbon cluster emission from polymers under kiloelectronvolt and megaelectronvolt ion bombardment

Formation of very large gold superclusters (clusters of clusters) as secondary ions up to (Au13)55 by SIMS

Comparative and complementary plasma desorption mass spectrometry/secondary ion mass spectrometry investigations of polymer materials

High-resolution (M/ΔM=7000) TOF-SIMS was used to study the mechanism of fragment-ion formation for several polystyrenes. Ions are formed from neutral fragments resulting from chain cleavage and are cationized with a silver ion. Spectra of the polystyrenes consist of repeating patterns of clusters of invidual peaks extending to m/z>3000. Fragmentation of the polymer backbone can be explained by limited chain fragmentation, consistent with mechanims proposed for other methods. Most polystyrenes show five clusters in their repeat pattern except for poly(α-methylstyrene) which differs considerably

Herbert Feld

Papers

Design and performance of a reflectron based time‐of‐flight secondary ion mass spectrometer with electrodynamic primary ion mass separation

Carbon cluster emission from polymers under kiloelectronvolt and megaelectronvolt ion bombardment

Formation of very large gold superclusters (clusters of clusters) as secondary ions up to (Au13)55 by SIMS

Comparative and complementary plasma desorption mass spectrometry/secondary ion mass spectrometry investigations of polymer materials

High-resolution TOF-SIMS studies of substituted polystyrenes