Close-packed layers networks

Sequence of close-packed layers Network formed by A atoms... 

The electron transport properties described earlier markedly differ when the particles are organized on the substrate. When particles are isolated on the substrate, the well-known Coulomb blockade behavior is observed. When particles are arranged in a close-packed hexagonal network, the electron tunneling transport between two adjacent particles competes with that of particle-substrate. This is enhanced when the number of layers made of particles increases and they form a FCC structure. Then ohmic behavior dominates, with the number of neighbor particles increasing. In the FCC structure, a direct electron tunneling process from the tip to the substrate occurs via an electrical percolation process. Hence a micro-crystal made of nanoparticles acts as a metal. 

Figure 3.9. A close-packed layer (a) and (b) with black atoms at centers of hexagons and hexagonal networks corresponding to L2/3 (c) and L1 /3 (d) for partially occupied layers.

Layered silicates reveal two types of filled close-packed layers of oxide ions. Those bonded to Al3+ or Mg2+ ions in octahedral sites are the usual close-packed layers, oxide ions form a network of hexagons with an oxide ion at the centers. The oxide layers forming the bases of tetrahedra have oxide ions which form smaller hexagons without oxide ions at the centers. This is also the pattern found for layers 2/3 filled, but those oxide layers in silicates are filled. For many of the layered silicates the repeating units, based on packing positions, requires stacking as many as three unit cells. 

The prime cause of the surface shear viscosity is friction between surfactant molecules the cause of surface shear elasticity is attractive forces between those molecules, leading to a more or less continuous two-dimensional network. For a closely packed layer, the effects may be substantial. For layers of small-molecule surfactants, however, the values of rif are generally immeasurably small, about 10 5 N s m 1 or less. For adsorbed polymers, values between 10 3 and lN-m-s-1 have been reported. 

Globular proteins form close-packed monolayers at fluid interfaces. Hence a large contribution to the adsorbed layer viscoelasticity arises from short-range repulsive interactions between hard-sphere particles. In addition to, or instead of, this glass-like5 structure from hard spheres densely packed in two dimensions, many adsorbed proteins can exhibit attractive interactions leading to a more gel-like5 network structure. Hence the mechanical properties of an adsorbed layer depend on many... 

The element carbon occurs in nature in two so-called allotropic forms, different crystal structures with the same chemical formula. In Fig. 3.13 the crystal structure of diamond and graphite have been represented. In diamond the C atoms are closely packed and each C atom is linked with four other C atoms. Thus a tight network of atoms is formed which, together with the binding strength, is responsible for the extreme hardness of diamond. Graphite has a layered structure and the space between the layers is relatively large. 

FIGURE 15.3. Of the many theoretically possible hquid crystal structures, five are most commonly encountered in surfactant systems. The lamellar phase (a) is simply alternating layers of surfactant molecules. The hexagonal phases (fe,c) are infinite hexagonal close-packed structures of normal and inverted cylindrical micelles. The most complicated, and difficult to visualize and shown schematically here, are the cubic bicontinuous (or interpenetrating) network d) and the cubic close packed ellipsoidal or finite cylindrical arrays (e). 

---