Crystal structures monolayers

The term monolayer (ML) must be defined clearly. In the work presented here, two definitions are used for surface studies, one ML indicates one adsorbate for each surface atom. For studies of compound formation, a monolayer is a slice of the compound s crystal structure, composed of one atomic layer of each of the constituent atoms. This does not necessarily mean a one unit-cell thick deposit is formed, as most compounds have larger unit cells from the point of view of crystallography, dependent on the orientation (Figure 8). 

Atomic force microscopy (AFM) can be used to obtain high-resolution imagery of molecular orientation and ordering for materials adsorbed onto substrates. Early AFM studies on gluconamides were hampered by the tendency of the fibers to unravel on substrates forming bilayer sheets.41 These layers showed the head-to-tail packing of a monolayer which is similar to the crystal structure reported for anhydrous gluconamides.38 A procedure to retain the fiber networks on surfaces with the addition of a small fraction of... 

In Chapter 13 the characterization of thin films was described and the concepts of edges, steps, and similar surface sites were introduced (see Fig. 13.1). In the present chapter we discuss the crystal structure of solids, so that, for instance, the number of atoms per square centimeter on a surface and the height /i of a monolayer may be determined. The values of and h depend on the crystallographic structure and the orientation of the surface plane. 

A well researched and popular class of monolayers is based on the strong adsorption of thiols (R - SH), disulfides (R - S - S - R) and sulfides (R - S - R) onto metal surfaces. Although thiols, disulfides, and sulfides strongly align with a number of different metals Hke gold, silver, platinum, or copper, gold is usually the substrate of choice because of its inert properties and the formation of a well-defined crystal structure. 

Grazing incidence x-ray diffraction (GID) measurements have indicated that both precollapse and collapsed state monolayers at the air-water interface can be crystalline (Birdi, 1989). A general procedure was delineated that could provide near-atomic resolution of two-dimensional crystal structures of -triacontanoic acid (C29H59COOH). A monolayer composed of rod-like molecules would generally pack in such a way that each molecule has six nearest neighbors, that is, hexagonal cell. 

Multilayers condensed upon the ordered monolayers maintained the same orientation and packing as found in the monolayers. Thus, the monolayer structure determines the growth orientation and the surface structure of the growing organic crystal. This phenomenon is called pseudomorphism and as a result the surface structures of the growing organic crystals do not correspond to planes in the reported bulk crystal structures. The exception appears to be n-octane on the Ag(l 11) surface that is deposited with the (lOl) orientation of its bulk crystal structure. 

Cyclohexane forms a (9 X 9) surface structure on the Ag(l 11) crystal face and a (j j) surface structure on the Pt(l 11) crystal face at around 200 K. This latter surface structure corresponds to the (001) surface orientation of the monoclinic bulk crystal structure of the molecule. On heating the platinum crystal face to 450 K a ( ) surface structure forms that is identical to the surface structure formed by cyclohexene monolayers at the same temperature. It appears that cyclohexane dehydrogenates at elevated temperatures on platinum to form the same species or that of cyclohexene. 

The EBSD technique measures the top few monolayers of a sample to determine its crystal structure. However, most polishing techniques leave a damaged layer on the samples. Even 1 or 0.25 pm diamond polish is not sufficient. For EBSD work, a final polish with colloidal silica (grain size 0.05 pm) is often required to remove the damaged surface layers. 

Determination of lateral periodicities in the self-assembled layer is an important goal in surface analysis. 2D surface crystal structures are best studied with low energy electrons, since their escape depth, contrary to X-rays, is basically limited to the top-most atomic layers. Consequently, LEED has become the most important method in surface monolayer crystallography. However, single-crystalline substrates are required. Via this technique, 2D supramolecular chiral lattice structures on single crystal surfaces had already been observed in 1978 [19]. 

Fig. 8. Crystal structure of a zinc blende compound. A monolayer of the compound is defined.

An interesting analogous in vitro experiment was performed by Xu el al. [92] in which the presence of a polyelectrolyte in solution resulted in a layer of amorphous calcium carbonate forming under a structured monolayer. This subsequently transformed into a thin layer of polycrystalline calcite. The crystal growth in this in vitro system occurs by phase transition of amorphous calcium carbonate into calcite and not by dissolution of amorphous phase and reprecipitation of calcite crystals. 

This simple model is sufficient to reproduce properties such as crystal structure, vibrational frequencies, dispersion curves, and elastic constants [40-43], These calculations were applied also to other organized media such as monolayer gaseous films on graphite [44,45], liquid crystals, and Langmuir Blodgett films. 

On the other hand, the crystal structures of the corresponding more-soluble salts of 13a with Af-protected amino acids show that such a monolayer of 13a H+ molecules, found in the less soluble salts, is not formed.136 These... 

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