Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

allows isolation of tetrapods in f82% yield (compared to 15–40% by theHPA method) at 120jC. Size control is less well developed, and particles arerelatively large compared to their HPA-derived counterparts. Nevertheless, itis the first significant report of solution-based growth of bipod and tripodmorphologies in the II–VI system and provides a more predictable methodof producing tetrapods [41].The same monosurfactant system can be applied to shape-controlledpreparation of the magnetic semiconductor MnS [42]. At low solution-growthtemperatures (120–200jC), MnS prepared from the single-source precursorMn(S 2 CNEt 2 ) 2 can nucleate in either the zinc-blende or the wurtzite phase,whereas at high temperatures (>200jC), MnS nucleates in the rock-saltphase. Low-temperature growth yields a variety of morphologies: highly anisotropicnanowires, bipods, tripods, and tetrapods (120jC), nanorods(150jC), and spherical particles (180jC). The ‘‘single pods’’ comprise wurtzitecores with wurtzite-phase arms. In contrast, the multipods comprisezinc-blende cores with wurtzite arms, where the arms grow in the [001] directionfrom the zinc-blende {111} faces, as discussed previously with respect tothe Cd–chalcogenide system. Dominance of the isotropic spherical particleshape in reactions conducted at moderate temperatures (180jC) implies ashift from predominantly kinetic control to predominantly thermodynamiccontrol over the temperature range from 120jC to 180jC [42]. Formation of1D particles at low temperatures results from kinetic control of relativegrowth rates. At higher temperatures, differences in activation barriers togrowth of different crystal faces are more easily surmounted, equalizingrelative growth rates. Finally, high-temperature growth supports only thethermodynamic rock-salt structure—large cubic crystals. Also, by combiningincreased growth times with low growth temperatures, shape evolution to‘‘higher-temperature’’ shapes is achieved [42].Extension of the ligand-controlled shape methodology to highly symmetriccubic crystalline systems is also possible. Specifically, PbS, having therock-salt structure, can been prepared as rods, tadpole-shaped monopods,multipods (bipods, tripods, tetrapods, and pentapods), stars, truncatedoctahedra, and cubes [43]. The rod-based particles, including the monopodsand multipods, retain short-axis dimensions that are less than the PbS Bohrexciton radius (16 nm) and, thus, can potentially exhibit quantum-size effects.These highly anisotropic particle shapes represent truly metastable morphologiesfor the inherently isotropic PbS system. The underlying PbS crystallattice is the symmetric rock-salt structure, the thermodynamically stablemanifestations of which are the truncated octahedra and the cubic nanocrystals.The PbS particles are prepared by pyrolysis of a single-sourceprecursor, Pb(S 2 CNEt 2 ) 2 , in hot phenyl ether in the presence of a large excessof either a long-chain alkyl thiol or amine. The identity of the coordinatingCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.