pletion of the derivatization is marked by a vivid interchange of the colors from the
aqueous layer to the hydrocarbon layer. The advantage of this method is that wellcharacterized metal particles can be easily thiol-derivatized in a nonaqueous medium. A variety of hydrosols of Au, Ag and Pt has been thiolized by this procedure.
A simple modification of this technique is shown to be effective in the case of CdS
nanocrystals [97].
4.3
Programmed Assemblies
Like molecular systems, nanocrystals capped with suitable ligands spontaneously
assemble into ordered aggregates. That such self-assembly can occur through a
variety of weak forces is being recognized. Cooperative assemblies of ligated metal
and semiconductor as well as of colloidal polymer spheres seem to occur through
the mediation of electrostatic and capillary forces [98–100]. The forces that govern
the nanocrystal assembly, however, are different in many ways. Surface tension for
example, plays an important role [37] because in a nanocrystal, a large fraction
of atoms are present at the surface. Surfactant molecules which self-assemble on
solid surfaces have proved to be the best means of obtaining ordered arrays of
nanocrystals [100].
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).
4.3.1
One-Dimensional Arrangements
Hornayak and coworkers [102] used the ordered channels of porous alumina as
templates to obtain linear arrangements of Au nanocrystals. By varying the pore
size, the diameter of the nanowire could be controlled. A linear arrangement has
also been obtained by coordinating Au particles (@1.4 nm) stabilized with phosphine ligands to single stranded DNA oligonucleotide of the desired length and
specific sequence [103, 104]. Liquid crystalline phases of a genetically engineered
virus–ZnS nanocrystal hybrid material was used as a template to obtain linear
arrays of ZnS nanocrystal aggregates [105]. Similarly, Pt nanocrystals in the form
of ribbons have been obtained using a cholesteric liquid crystalline template
[106]. Organization of particles in a 1D lattice has met with limited success. Heath
and coworkers [107] have fabricated wires of Ag nanocrystals by compressing a
dispersion of Ag (4.5 nm) nanocrystals in toluene (Figure 4.9). The wires were
one nanocrystal thick, a few nanocrystals wide and extended in length from 20–
300 nm. The interwire separation distance and the alignment of the wires could
4.3 Programmed Assemblies 61
aqueous layer to the hydrocarbon layer. The advantage of this method is that wellcharacterized metal particles can be easily thiol-derivatized in a nonaqueous medium. A variety of hydrosols of Au, Ag and Pt has been thiolized by this procedure.
A simple modification of this technique is shown to be effective in the case of CdS
nanocrystals [97].
4.3
Programmed Assemblies
Like molecular systems, nanocrystals capped with suitable ligands spontaneously
assemble into ordered aggregates. That such self-assembly can occur through a
variety of weak forces is being recognized. Cooperative assemblies of ligated metal
and semiconductor as well as of colloidal polymer spheres seem to occur through
the mediation of electrostatic and capillary forces [98–100]. The forces that govern
the nanocrystal assembly, however, are different in many ways. Surface tension for
example, plays an important role [37] because in a nanocrystal, a large fraction
of atoms are present at the surface. Surfactant molecules which self-assemble on
solid surfaces have proved to be the best means of obtaining ordered arrays of
nanocrystals [100].
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).
4.3.1
One-Dimensional Arrangements
Hornayak and coworkers [102] used the ordered channels of porous alumina as
templates to obtain linear arrangements of Au nanocrystals. By varying the pore
size, the diameter of the nanowire could be controlled. A linear arrangement has
also been obtained by coordinating Au particles (@1.4 nm) stabilized with phosphine ligands to single stranded DNA oligonucleotide of the desired length and
specific sequence [103, 104]. Liquid crystalline phases of a genetically engineered
virus–ZnS nanocrystal hybrid material was used as a template to obtain linear
arrays of ZnS nanocrystal aggregates [105]. Similarly, Pt nanocrystals in the form
of ribbons have been obtained using a cholesteric liquid crystalline template
[106]. Organization of particles in a 1D lattice has met with limited success. Heath
and coworkers [107] have fabricated wires of Ag nanocrystals by compressing a
dispersion of Ag (4.5 nm) nanocrystals in toluene (Figure 4.9). The wires were
one nanocrystal thick, a few nanocrystals wide and extended in length from 20–
300 nm. The interwire separation distance and the alignment of the wires could
4.3 Programmed Assemblies 61