Multiple Twinned Particles Research Articles

Favored Structure of Ag Nanoparticles Embedded in SiO2 by Implantation: Single Crystal with Contracted (111) Lattice

The structure of Ag nanoparticles, embedded in crystalline SiO2 by high-dose implantation, was investigated. It was found that single crystal is favored over multiple-twinned particles. In addition, the contracted (111) lattice spacing of the Ag nanocrystals was measured by x-ray diffraction.

Spontaneous dissolution at room temperature of gold nanoparticles.

We have studied gold nanoparticles evaporated on NaCl. We examined the sample before and after an annealing at room temperature for seven months. We found that most of the small particles with square shape disappear after the annealing. Many of the particles remaining correspond to multiple twinned particles (MTP), i.e., particles with icosahedral or decahedral shapes. We also found that the size of the icosahedral and decahedral particles did not change with the room-temperature annealing. The only consistent explanation is a spontaneous dissolution of the square-shaped nanoparticles at room temperature. This indicates that the binding energy of the atoms in such particles is much lower than the bulk value. The reason why such a particle would rather be dissolved instead of undergoing quasimelting is not clear. Room-temperature dissolution of square-shaped particles is another example of nanoparticles instability.

On the equilibrium shape of multiple-twinned particles

The shape of well-annealed gold multiple-twinned particles of the icosahedral type is studied using weak beam dark field and high resolution TEM. It is shown that well-annealed particles develop extra facets in agreement with the strong faceting model for their equilibrium shape recently put forward by Marks and Howie.

Direct lattice imaging of small metal particles

Observations on small vapour-deposited particles of silver and gold with direct lattice imaging in the Cambridge University 600 kV high-resolution electron microscope have established unambiguously the presence of dislocations in particles as small as 15nm in diameter. The detailed structure of more complicated ' polyparticles ' can also be characterized directly. Knowledge of the internal and surface structure of small metallic particles is basic to an understanding of the nucleation of thin metal films and any explanation of their important role in heterogeneous cataIysis. Observations using electron diffraction and electron microscopy have proved invaluable in elucidating their crystallography, in particular in establishing the existence of multiple twinning within many particles (In0 1966, Ino and Ogawa 1967, Allpress and Sanders 1967). These so-called multiple-twinned particles (MTPs) are known to be composed of either five or twenty tetrahedra which make up decahedra or icosahedra. (The icosahedral MTPs have a similar topological structure to spherical viruses (Caspar and Klug 1961) and geodesic domes (Fuller 1963).) However, such arrangements of tetrahedra are not completely space-filling, and some form of Iattice distortion or imperfection is required. Lattice imaging with axial illumination at 100 kV has been used previously to observe evaporated gold particles (Komoda 1968). Typically however, with an objective lens defocus, AZ, appropriate for best visibility of (111) lattice fringes of about 0.2-0-4 pm (given by A2 = Cs81112, where C, is the spherical aberration coefficient of the objective lens and 0111 is the diffraction angle), information about lattices of other periodicities cannot be seen unless the coherence of the incident illumination is adequate, and it is then considerably misplaced. At operating voltages of 500 kV or more the diffraction angles are much smaller than at 100 kV, substantially reducing the fringe displacements. Furthermore, the increased spatial coherence of the illumination afforded by the high-brightness lanthanum hexaboride electron gun

Mise en évidence du passage de la structure icosaédrique à la structure c.f.c. dans les petits agrégats d'or condensés sur NaCl

The structure of very small gold particles condensed onto an NaCl cleavage plane was examined by electron diffraction and electron microscopy; it was found that beyond a certain size (about 80 Å) the icosahedral particles exhibit a gap which can be interpreted as proof that they become multiple twinned particles.