Ex Situ Synthesis of Clusters

As an alternative to in situ growth, nanocrystals can be synthesized either in solution (e.g. by colloidal synthesis) or in the gas phase, prior to dispersion in a matrix or deposition or self-assembly onto a substrate. 

Although gold colloids were known and studied very early [3.2], the synthesis technique has been much studied and greatly improved during the past few decades. The two-phase approach permits the fabrication of thiol-covered noble-metal nanoparticles [3.141]. The thiol layer prevents coalescence of the metal particles, thus stabilizing the colloid and preserving the size distribution. Motivated by the self-ordering properties of layers of nanoparticles with a narrow size distribution. 

This technique has been extended to transition metals. 6nm trioctylphosphine-coated Co particles have 

Variations of the colloidal-synthesis and size selection techniques have also been developed in parallel for the preparation of semiconductor nanoparticles, particularly II-VI compounds such as CdSe [3.149], from which self-assembled superlattices can be grown [3.150]. GaAs nanocrystals have been produced similarly [3.151]. 

Several techniques have been applied for the production of preformed clusters in the gas phase, prior to their deposition onto a substrate. The low-energy cluster beam deposition technique was developed especially for the deposition of small clusters of transition metals (Ni, Fe, Co, etc.) [3.152]. An extensive review of this vast subject can be found in [3.153]. 

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