A double-source evaporator which can be set in the specimen chamber of a high voltage electron microscope has been constructed, With the use of the evaporator, isolated gold clusters of 5 ~ 10 nm in size were first prepared on a supporting film of amorphous carbon, and then copper atoms were deposited onto the film (and of course onto the gold clusters as well), which was kept at room temperature. Upon the deposition of copper, the gold clusters changed into highly concentrated, homogeneously mixed Au—Cu alloy clusters. The copper atom diffusivity in gold clusters deduced from the present observations is at least 9 orders of magnitude faster than the copper solute diffusion in bulk gold.

Spontaneous alloying of copper into gold atom clusters

It has been established for some time [1, 2] that small supported metal particles can be reproducibly prepared in a controlled way by ultrahigh vacuum (UHV) deposition onto planar supports. Such planar deposits have been employed in the past to study the physical [3–11] and chemical [12–20] properties of mesoscopic metal aggregates. In these studies, the emphasis has usually been placed on pointing out the differences in particle properties with respect to the bulk properties of the same material. Although it has often been found that the electronic properties reach bulk values at surprisingly small cluster sizes of about 30 to 50 atoms, it has also been observed that the chemical properties can change over a much wider particle size range [21] for structure-sensitive reactions [22]. Furthermore, it is generally realized that particle size alone is usually not the only important parameter but that the influence of the support in terms of electronic, structural, chemical, and morphological particl...

Nucleation, Growth, and TEM Analysis of Metal Particles and Clusters Deposited in UHV

Effects of Pd Particle Size and Ceria Loading on NO Reduction with CO

Publisher Summary The energy gap between the valence and conduction band is of fundamental importance for the properties of a solid. Most of a material's behavior—such as intrinsic conductivity, optical transitions, or electronic transitions—depends on it. Any change of the gap may significantly alter the material's physics and chemistry. This occurs when the size of a solid is reduced to the nanometer length scale. Therefore, the science and the technology of nano materials need to take into account a band gap, which is different from that of the bulk. This chapter determines experimentally and theoretically the dependence of the energy gap for particles, which are reduced in size to the nanometer range. In addition, effects such as structural changes, lattice contraction, atomic relaxation, surface reconstruction, surface passivation, or strain induced by a host material can change the gap. For clusters of simple metals, two models are used for their description: the tight-binding model and the electron-shell model. They can lead to different band gaps for small clusters and different critical sizes for the cluster-to-bulk transition. For divalent metals, a band gap is expected to open with decreasing particle size because of narrowing and shift of energy bands.

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The energy gap of clusters, nanoparticles, and quantum dots

Debye's scattering equation (DSE) has spanned a century of scientific development, from the dawn of quantum mechanics and the investigation of the structure of atoms and molecules to the era of nanotechnology, paving the way to total scattering methods. The formulation offers the most accurate representation of the intensity scattered by randomly oriented atomic aggregates, constructed by superimposing the signal from each atomic distance in the molecule. The present paper reviews some of the milestone applications, from the interpretation of the intensity curves from gases and vapours, to aggregates of increasing size and more extended order. Important developments, aimed at mitigating the prohibitive computational complexity of the DSE, and state-of-the-art methods for the characterization of static and dynamic displacements are also discussed.

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100 years of Debye's scattering equation.

The crystallographic structure of small Pd particles formed during the earliest stages of vapor deposition onto refractory oxide supports was studied by energy filtered in situ UHV scanning transmission electron diffraction (STED). Although only the average structure of particles can be obtained by this high‐precision, large area diffraction technique, its main advantage over high‐resolution transmission electron microscopy (TEM) is the drastic reduction of radiation damage effects. On‐line processed experimental diffraction data were compared to calculated elastic diffraction intensities and radial distribution functions obtained by Fourier transformation. The results show conclusive evidence that small Pd clusters of less than about 2 nm size prefer non‐fcc icosahedral packaging over the standard fcc structure, which confirms earlier theoretical predictions for minimum energy cluster configurations. Standard fcc structures always dominate in later stages of growth with increasing particle size.

M. Avalos‐Borja

Papers

The determination of structure in small supported metal particles by scanning transmission electron diffraction