crystals [29, 30]. Subsequent layers can be introduced by dipping the substrate sequentially into the respective spacer molecule solution and the nanocrystal dispersion, with intermediate steps involving washing and drying. The formation of the
multilayer assembly can be monitored using a variety of spectroscopy and microscopy tools as illustrated in Figure 4.19. Thus, employing Au substrates and dithiols
as spacers, multilayer assemblies of several nanocrystals such as Au or CdS have
been accomplished [141]. One such example of layer-by-layer deposition of Pt
(5 nm) nanocrystals is shown in Figure 4.19. Brust et al. [142] have reported the
formation of multilayers of Au nanoparticles using dithiols. These workers have
confirmed the layer-by-layer deposition of particle arrays by employing UV–vis
spectroscopy and ellipsometry. Multilayers with CdS nanocrystals prepared by the
reverse micelle technique and spaced with alkane were made and measurable
photocurrents were generated by these assemblies. Three-dimensional superlattices involving nanocrystals of different metals (e.g. Pt, Au) and of metals and
semiconductors (e.g. Au, CdS) have also been prepared and characterized [141].
Such assemblies can be made with polyelectrolytes such as poly(diallyldimethylammonium chloride) (PDDA), Polyethyleneimine (PEI) [143, 144], Poly(allylamine
hydrochloride) (PAH) and also polymers such as poly-phenylenevinylene (PPV)
[145, 146]. Thus, mulilayers such as those of CdTe nanocrystals spaced with
PDDA, CdSe spaced with PPV, have been prepared.
4.3.4
Superclusters
It has been proposed that self-similarity in metal nanocrystal organization would
manifest in the form of a giant cluster whose shape and size are direct consequences of the nanocrystals themselves [147]. The invariance of the shell effects
in metal nanocrystals with scaling is shown schematically in Figure 4.20. Thus,
Pd 561 nanocrystals would be expected to self-aggregate into a giant cluster of the
type (Pd 561 ) 561 under suitable conditions. The monodisperse nature of the nanocrystals is thought to be important in assisting the self-aggregation process. Formation of such clusters was observed in the mass spectra of magic nuclearity Au 55
nanocrystals. Secondary ion mass spectrometry indicated the presence of species
with large m/z values and these were attributed to (Au 13 ) 55 giant clusters [148]. The
giant clusters so obtained have, however, not been isolated or imaged. One such
observation was made in the case of Pd 561 nanocrystals where the PVP covered
nanocrystals aggregated to form giant clusters [149]. The TEM image in Figure
4.21 is revealing. There are regions where the nanocrystals are densely packed in
the form of giant aggregates with estimated nanocrystal nuclearities corresponding
to various magic numbers. It is possible that the formation of the giant clusters is
facilitated by the polymer shell that encases them. Unlike in the case of 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.
4.3 Programmed Assemblies 73
multilayer assembly can be monitored using a variety of spectroscopy and microscopy tools as illustrated in Figure 4.19. Thus, employing Au substrates and dithiols
as spacers, multilayer assemblies of several nanocrystals such as Au or CdS have
been accomplished [141]. One such example of layer-by-layer deposition of Pt
(5 nm) nanocrystals is shown in Figure 4.19. Brust et al. [142] have reported the
formation of multilayers of Au nanoparticles using dithiols. These workers have
confirmed the layer-by-layer deposition of particle arrays by employing UV–vis
spectroscopy and ellipsometry. Multilayers with CdS nanocrystals prepared by the
reverse micelle technique and spaced with alkane were made and measurable
photocurrents were generated by these assemblies. Three-dimensional superlattices involving nanocrystals of different metals (e.g. Pt, Au) and of metals and
semiconductors (e.g. Au, CdS) have also been prepared and characterized [141].
Such assemblies can be made with polyelectrolytes such as poly(diallyldimethylammonium chloride) (PDDA), Polyethyleneimine (PEI) [143, 144], Poly(allylamine
hydrochloride) (PAH) and also polymers such as poly-phenylenevinylene (PPV)
[145, 146]. Thus, mulilayers such as those of CdTe nanocrystals spaced with
PDDA, CdSe spaced with PPV, have been prepared.
4.3.4
Superclusters
It has been proposed that self-similarity in metal nanocrystal organization would
manifest in the form of a giant cluster whose shape and size are direct consequences of the nanocrystals themselves [147]. The invariance of the shell effects
in metal nanocrystals with scaling is shown schematically in Figure 4.20. Thus,
Pd 561 nanocrystals would be expected to self-aggregate into a giant cluster of the
type (Pd 561 ) 561 under suitable conditions. The monodisperse nature of the nanocrystals is thought to be important in assisting the self-aggregation process. Formation of such clusters was observed in the mass spectra of magic nuclearity Au 55
nanocrystals. Secondary ion mass spectrometry indicated the presence of species
with large m/z values and these were attributed to (Au 13 ) 55 giant clusters [148]. The
giant clusters so obtained have, however, not been isolated or imaged. One such
observation was made in the case of Pd 561 nanocrystals where the PVP covered
nanocrystals aggregated to form giant clusters [149]. The TEM image in Figure
4.21 is revealing. There are regions where the nanocrystals are densely packed in
the form of giant aggregates with estimated nanocrystal nuclearities corresponding
to various magic numbers. It is possible that the formation of the giant clusters is
facilitated by the polymer shell that encases them. Unlike in the case of 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.
4.3 Programmed Assemblies 73