Sergej A. Nepijko

Pentagonal Symmetry and Disclinations in Small Particles

A comprehensive review is given on experimental studies of small particles with fivefold symmetry accompanied by an in-depth theoretical description of their characteristics and computer modeling. The cases of uniform and nonuniform deformations (disclination model), stability and relaxation of elastic stresses in pentagonal particles and needle-like crystals, models of their formation are discussed.

Composition and lattice structure of fivefold twinned nanorods of silver

Multiply twinned silver particles of rod-like shape and nanometer dimensions prepared by inert-gas aggregation technique have been studied by high-resolution electron microscopy. These pentagonal nanorods exhibit aspect ratio between 1.8 and 6 with the length of their fivefold axis ranging from 22 to 132 nm. Digital image processing and evaluation was utilised to characterise composition and lattice structure of the nanorods. Measuring the spacings of lattice plane fringes of nanorod subunits in various orientations revealed no deviation from the face centred cubic lattice type. There was also no indication of extended lattice defects found. Instead, a certain extent of non-regular lattice distortions recognised in the surface regions of the nanorods apparently is an effective means to achieve sufficient space filling.

Growth of rodlike silver nanoparticles by vapor deposition of small clusters

A beam of silver clusters of 3 nm average diameter is produced by the gas-aggregation technique and deposited onto the NaCl (100) surface. Subsequent agglomeration leads in part to the growth of rodlike particles with the structure of truncated decahedra of typical dimensions 70 nm long and 10 nm diameter and a fivefold symmetry, as shown in the picture. A growth mechanism is inferred from a detailed analysis of electron microscopy data.

The surface plasmon resonance of free and embedded Ag-clusters in the size range 1,5 nm D 30 nm

It is experimentally proven that the size dependence of the surface plasmon resonance of large spherical silver clusters is related to the surface optical properties of Ag. It is also demonstrated that there is a minimum size, below hich this approach fails as expected, and that there is a maximum size, above which the surface contribution can be neglected such that classical optics applies.

Multiply twinned particles beyond the icosahedron

Multiply twinned particles with shapes beyond that of the complete icosahedron have been observed in island layers obtained by physical vapour deposition on alkali halide substrate. Image analysis applied to lattice plane images of one distinctive example of such particles by employing Fourier processing routines allowed to set up a detailed model of MTPs composed of two icosahedra stacked into each other. These icosahedra being in twin position share a central decahedron that does not exhibit external faces. So, the particle comprises 35 tetrahedral subunits, but has only 30 outer faces. Comparison of the image contrast calculated according to this model with the experimental electron microscopy image showed rather good agreement. The formation of such particles is assumed to proceed via successive growth twinning. ( 2000 Elsevier Science B.V. All rights reserved.

The light emission observed from small palladium particles during passage of electronic current

Light emission spectra were measured from separate Pd particles excited by the passage of electrical current through them. As the particle size decreases or input power increases the intensities of peaks increase more strongly in the high-energy part of the spectra than in the low-energy part. Moreover, new peaks can appear in the high-energy region. The color of an emission center changes from red to blue upon an increase in electric power fed into the Pd particle. This process is reversible and can be observed visually. On a further increase in feeding power, the Pd particle can be brought to a premolten state and can even melt. The integral intensity of light emission then increases dramatically, and the light emission spectrum changes and corresponds to thermal radiation. This latter process is irreversible.

Influence of vacuum conditions on the light emission caused by passing a current through a silver cluster film

Performed experiments showed that silver clusters prepared by different technique, such as the thermal evaporation in vacuum and the gas aggregation method, had similar spectra of the light emission. The integral intensity of the light emission decreases as pressure of the residual gases increases. This change is reversible. The spectrum shape remains invariable; however, spectral lines of excitation of residual gases arise in the spectrum.

Electroluminescence spectra of porous silicon as a function of the applied voltage

Abstract The spectra of light emission from a separate emission center in porous silicon contained in “sandwich” structures were measured under the passage of electrical current as a function of applied voltage. The light intensity increases quicker in the short-wave part of the spectrum than in the long-wave part on increasing the voltage. Such a feature of the spectrum change has been observed earlier for island metal films.