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Fig. 2 Nanocrystalline film of Au formed at the toluene-water interface (middle). Gold is introduced as a toluene solution of Au(PPh3)Cl while partially hydrolyzed THPC (tetrakishydromethylphosphoniumchloride) in water acts as a reducing agent. The film is obtained when the two layers are allowed to stand for several hours. When dodecanethiol is added to the toluene layer, the film breaks up forming an organosol (left) while mercaptoundecanoic acid added to water produces a hydrosol (right). Shown below are the corresponding TEM images showing nanocrystals. The scale bars correspond to 50 nm. Films of CdS nanocrystals could also be prepared by adopting the same methods.

nanocrystals can be prepared using a water-toluene interface.[55,56] A typical film of Au nanocrystals is shown in Fig. 2. Traditionally, clusters of controlled sizes have been generated by ablation of a metal target in vacuum followed by mass selection of the plume to yield cluster beams.[57,58] Such cluster beams could be subject to in situ studies or be directed on to solid substrates. To obtain nanocrystals in solution, Harfenist et al.[59] steered a mass-selected Ag cluster beam through a toluene solution of thiol and capped the vacuum prepared particles.

Colloids of alloys have been made by the chemical reduction of the appropriate salt mixture in the solution phase. In the case of semi-conductor nanocrys-tals, a mixture of salts is subject to controlled precipitation. Thus, Ag-Pd and Cu-Pd colloids of varying composition have been prepared by alcohol reduction of mixtures of silver nitrate or copper oxide with palladium oxide.[60] Fe-Pt alloy nanocrystals have been made by thermal decomposition of the Fe and Pt acetylace-tonates in high boiling organic solvents.[61] Au-Ag alloy nanocrystals have been made by co-reduction of silver nitrate and chloroauric acid with sodium borohydride.[62,63] Semiconductor nanocrystals of the form CdxMn1_xS, CdSxSe1_x have been obtained by the inverted micelle methods as well as in glasses by sol-gel methods.[22,64] Alloys of controlled composition are also made by thermal decomposition of carefully chosen precursors, to achieve homogeneity. For example, Mn2(m-SeMe)2(CO)8 was used as selenium source to obtain Cd1_xMnSe nanocrystals.[65] Au-Ag alloying and segregation has been brought about by the use of lasers on Au-Ag layered particles.[66,67]

Size Control

The successful synthesis of nanocrystals involves three steps: nucleation, growth, and termination by the capping agent or ligand.[34-36] Though the reaction temperature and reagent concentrations provide a rudimentary control of the three steps, it is often impossible to independently control them and so the

Fig. 3 Metal nanocrystals in closed-shell configurations with magic number of atoms.

obtained nanocrystals usually exhibit a distribution in size. Typically, the distribution is log-normal with a standard deviation of 10%.[36] Given the fact that properties of the nanocrystals are size-dependent, it is significant to be able to synthesize nanocrystals of precise dimensions with minimal size-distributions. This can be accomplished to a limited extent by size selective precipitation either by centrifugation or by use of a miscible solvent-non-solvent liquid mixture to precipitate nanocrystals. However, single crystals of large clusters of semiconducting material such as Cu147Se73(PEt3)22,[68]

Cd32Si4(SC6H5)36DMF4,[69'70]

[CdioS4(SPh)16]4-,[69'70] Cdi7S4(SCH2CH2OH)26,L69;/UJ Cd32Si4(SCH2CH(CH3) OH)36,[71] Hg32Sei4(SeC6H5)36[72J have been obtained. Solutions of such clusters possess optical properties similar to those of the sols. Schmid[73J and Zamaraev[74J succeeded in preparing truly mono-disperse nanocrystals, which they called ''cluster compounds.'' These cluster compounds are like macromolecules with a core containing metal-metal bonds yet obtainable in definite stoichio-metries, typical examples being [Pt^CO^HJ2- and Au55(PPh3)12Cl6. The enhanced stability of Au55 was recently demonstrated clearly by Boyen et al.[75J who exposed a series of Aun nanocrystals to oxidation. These nanocrystals are bequeathed with special stability because they consist of a magic number of metal atoms, which enables the complete closure of successive shells of atoms in a cubic close-packed arrangement. The magic numbers 13, 55,147,309, and 561 correspond to the closure of 1,2, 3, 4, and 5 shells, respectively.[76] A schematic illustration of magic nuclearity nanocrystals is shown in Fig. 3. Since the breakthrough, several magic nuclearity nanocrystals have been prepared including PVP-stabilized Pd561 nano-crystals.[77,78] In Fig. 4 are shown scanning tunnelling and transmission electron microscopic (TEM) images of polymer-protected Pd561 nanocrystals.

Shape Control

Since the properties of the nanocrystals follow from the confinement of the electrons to the physical dimensions of the nanocrystals, it would be interesting to vary the shape of the nanocrystals and study the effect of confinement of electrons in such artificial shapes.[79J For example, it is predicted that light emitted from a nanorod would be linearly polarized along the growth-axis.[22J Such predictions have led to the revival

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