Arrays of Metal Nanocrystals

Gold organosols using alkane thiols as surfactants were first prepared by Schiffrin and co-workers [109] by phase transferring gold ions and carrying out reduction in the presence of thiols. Several workers have adopted this procedure to obtain thiolized metal nanocrystals [110-113]. 

---

OF-KEDF s with the DD AWF emerged, " and a scheme to treat highly inhomogeneous systems like realistic surfaces was tested with semiquantitative success. " An immediate application to the study of the metal-insulator transition in a 2-dimensional array of metal nanocrystal quantum dots (with 498 A1 atoms per simulation cell) further magnifies its promise. ... 

Electrical transport measurements on layer-by-layer assemblies of nanocrystals on conducting substrates have been carried out with a sandwich configuration [691-693]. Nanocrystalline films with bulk metallic conductivity have been realized with Au nanocrystals of 5 and llnm diameter spaced with ionic and covalent spacers [692, 693]. The conductivity of monolayered two-dimensional arrays of metal nanocrystals has been examined with patterned electrodes [694-699], Structural disorder and interparticle separation distance are found to be key factors that determine the conductivity of such layers [694-697]. The conductivity of the layers can be enhanced by replacing the alkane thiol with an aromatic thiol in situ [698,699]. The interaction energy of nanocrystals in such organizations can be continually varied by changing the interparticle distance. 

Mesoscale self-assembly of metal nanocrystals into ordered arrays and giant clusters... 

The very first report of two-dimensional arrays was of Fe304 nanocrystals [128]. Bentzon et al. observed that the ferrofluid obtained by thermolysis of iron penta-carbonyl upon drying (over a period of several weeks) yielded well ordered two-dimensional arrays of Fe304 nanocrystals. Since then, easier methods have been devised to obtain arrays of Fc304 nanocrystals [129]. Two-dimensional arrays of amine-capped metal oxide nanocrystals such as C03O4 have been obtained by start-... 

Fig. 4.10. Normalized density of states (DOS) measured from arrays of Ag nanocrystals of diameter 2.6 nm capped (a) decanethiol and (b) hexanethiol at various temperatures. The temperature dependence of DOS near 0 V for decanethiol capped particles indicates that the films are nomnetallic. In the case of hexanethiol capped nanocrystals, the DOS around 0 V is temperature independent reveaUng the metallic nature of the film (reproduced with permission from [700])...

Surfactant molecules that self-assemble on metal surfaces have proved to be the best means of obtaining ordered arrays of nanocrystals [12], The way in which... 

The giant clusters could be reproducibly formed starting from Pd561 nanocrystals in water, ethanol and ethanol-water mixtures and from sols with very different concentrations of the nanocrystals. It is possible that the formation of the giant clusters is facilitated by the polymer shell that encases them. Unlike Pd nanocrystals coated with alkanethiols, which self-assemble to form ordered arrays, the polymer shell effectively magnifies the facets of the metallic core, thereby aiding a giant assembly of the nanocrystals. The surface properties of the polymer-coated nanocrystals are clearly more favorable in that the interparticle interaction becomes sufficiently attractive. 

The way in which the nanocrystals organize themselves depends critically on the core diameter, the nature of the ligand, substrate and even the dispersive medium used [101]. Thiolized metal nanocrystals readily arrange into two-dimensional arrays on removal of the solvent [29]. Using suitable methods, they can also be put into one-dimensional organization in the form of strings or assembled in a stepwise fashion in a three-dimensional superlattice (see Figure 4.8). 

---