Assemblies, popular defects

As the structure of a nanoparticle assembly depends mostly on interparticle interactions, the shape of a particle will alter such interactions and affect the position of the nanoparticles in a superlattice structure. HRTEM analysis on all nm Co nanoparticle assembly shows that the particles in the assembly have three different types of shapes square-like, hexagon-like and square [45]. Evaporation of the hexane solvent from the dispersion results in an assembly that is different from the spherical ones. These different shapes break the normal six-fold symmetry in a typical hep superlattice assembly, resulting in two popular defects, twins and stacking faults. 

As materials chemistry has developed, it has come to pay more and more attention to that archetypal concern of materials scientists, microstructure. That concern came in early when the defects inherent in non-stoichiometric oxides were studied by the Australian. I.S. Anderson and others (an early treatment was in a book edited by Rabenau 1970), but has become more pronounced recently in the rapidly growing emphasis on self-assembly of molecules or colloidal particles. This has not yet featured much in books on materials chemistry, but an excellent recent popular account of the broad field has a great deal to say on self-assembly (Ball 1997). The phenomenon of graphoepitaxy outlined in Section 10.5.1.1 is a minor example of what is meant by self-assembly. 

Machine vision, also referred to as computer or robot vision, is a term that describes the many techniques by which machines visually sense the physical world. These techniques, used primarily for monitoring industrial manufacturing, are becoming increasingly popular as today s manufacturing environments become more automated and quality control standards increase. Whether the task is to sort and assemble a group of machined parts, to determine if a label has been placed properly on a soda bottle, or to check for microscopic defects in an automotive door panel, machine vision plays an essential role. 

Popular approaches to molecular self-assembly, which can give structures in the nanometer to millimeter range, are based on SAMs and LBL deposition of electrolytes. Self-assembly leads to equilibrium structures that are close to the thermodynamic minimum and result from multiple weak, reversible interactious betweeu subuuits which include hydrogen bonds, ionic bonds, and van der Waals forces. As information is already coded in the building blocks, this is a means to avoid defect formation in aggregate formation. SAMs are molecular assemblies of long chain alkanes that chemisorb on the patterned and unpat-temed surfaces of appropriate solid materials. The structures of SAMs, effectively 2D-crystals with controllable chemical functionality, make them a means to modify substrates to direct protein adsorption and cell attachment, surface passivation, ultrathin resists and masks and sensor development. 

---