Binary superlattices

Colloidal crystals . At the end of Section 2.1.4, there is a brief account of regular, crystal-like structures formed spontaneously by two differently sized populations of hard (polymeric) spheres, typically near 0.5 nm in diameter, depositing out of a colloidal solution. Binary superlattices of composition AB2 and ABn are found. Experiment has allowed phase diagrams to be constructed, showing the crystal structures formed for a fixed radius ratio of the two populations but for variable volume fractions in solution of the two populations, and a computer simulation (Eldridge et al. 1995) has been used to examine how nearly theory and experiment match up. The agreement is not bad, but there are some unexpected differences from which lessons were learned. 

Figure 10.14. Binary superlattices self-assembled from various combinations of magnetic, semiconducting, and metallic nanoparticles, (a-e) TEM micrographs of (001) planes of binary superlattices isostructural with the A1B2 compound, (f) A sketch of the A1B2 unit cell. (g,h) Depictions of the front and side views of the superlattice (001) plane, correspondingly. Reproduced from Ref. 24, Copyright 2006, with permission from the American Chemical Society.

Redl, F. X., Cho, K. S., Murray, C. B. O Brien, S. Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots. Nature (London) 423, 968—971 (2003). 

Similar methods were later successfully applied for the fabrication of PbS, PbSe, PbTe, and y-Fe203 superlattices [26-28]. Moreover, binary superlattices built from variations of PbS, PbSe, CoPts, Fc203, Au, Ag, and Pd nanocrystals exhibiting one of... 

Bartlett P and Campbell AI. 2005. Three-dimensional binary superlattices of oppositely charged colloids. Physical Review Letters 95 128302. 

Luminescent magnetic materials. They can be useful in combined detection and separation in bioanalytical assays. For instance, polymer coated 7-Fe203 cores and CdSe/ZnS shells with immobilized anticycline E antibodies have been used for the separation and observation of MCF-7 breast cancer cells. " Materials joining potentially tunable optical and magnetic properties have been proposed that are composed of three-dimensional binary superlattices of self-assembled 7-Fe20j and PbSe nanoparticles. ... 

Bartlett P, Ottewill R FI and Pusey P N 1992 Superlattice formation in binary mixtures of hard-sphere colloids Phys. Rev. Lett. 68 3801-4... 

Heterostructures and Superlattices. Although useful devices can be made from binary compound semiconductors, such as GaAs, InP, or InSb, the explosive interest in techniques such as MOCVD and MBE came about from their growth of ternary or quaternary alloy heterostmctures and supedattices. Eor the successful growth of alloys and heterostmctures the composition and interfaces must be accurately controlled. The composition of alloys can be predicted from thermodynamics if the flow in the reactor is optimised. Otherwise, composition and growth rate variations are observed... 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

Urban, J. I Talapin, D. V. Shevchenko, E. V. Kagan, C. R. Murray, C. B. 2007. Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films. Nature Mater. 6 115-121. 

Efforts have been made to eliminate diffusion-control of solid-solid reactions by using superlattices of nanometric dimensions as reactants. Formation of Cu MOgSeg from the superlattices of Cu, Mo and Se is one such example (Fister et al., 1994). The results reveal that superlattice reactant geometry could be used to kinetically trap the ternary phases which are thermodynamically unstable with respect to the binary phases. 

When either of the semicrystalline forms of the asymmetric alkane are cooled, they transform to a double-layer and a triple-layer crystalline structure, respectively. These 1-d superlattices are described as ABAB... and ABAABA... stacks of crystalline layers as depicted in Figure 25, panels c and e. Note that the structure in Figure 25e is related to the mixed integer folded-extended structure in some pure n-alkanes (Figure 7c) and in binary mixtures of long n-alkanes (Section II.J, Figure 30b). 

Figure 30. Schematic structures of (a) the high-temper-ature semicrystalline phase, and (b) the low-temperature triple-layer superlattice in binary long alkanes. Model electron density profiles are shown on the right, with density increasing from left to right (from ref 176 with permission of the American Physical Society).

Hachisu, S. and Yoshimura, S., Optical demonstration of crystalline superlattices in binary mixtures of latex globules. Nature (London), 283, 188, 1980. 

Eldridge M D, Madden P A and Frenkel D 1993 Entropy-driven formation of a superlattice in a hard-sphere binary mixture Mol. Phys. 79 105-20... 

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