Prospects of Colloidal Nanocrystals for Electronic - Computer Science

394 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al.
faces. 106 Obtained aerogels retain a much stronger resemblance
to their original gel network structure than do
xerogels. For more details on sol-gel synthesis, we address
the reader to several recent review articles. 105,107-111
Brock et al. reported the synthesis of xerogels and aerogels
assembled from CdS, ZnS, PbS, CdSe, and CdSe/ZnS core/
shell nanoparticles. 112,113 Primary nanoparticles were aggregated
to the wet gels and then dried under ambient
conditions or in supercritical CO2 to produce nanoparticle
xerogels and aerogels, respectively.
2.2. Nanoparticle Shape and Morphology
Engineering
In analogy with bulk crystals, the nanoparticles are
terminated by facets that expose different crystallographic
planes. Selective adhesion of surfactant molecules allows for
tuning the growth kinetics of different crystal facets and
tailoring the NC shape from nearly spherical to highly
anisotropic 30 (Figure 4). Strong binding of capping molecules
suppresses the growth of certain facets, leading to a variety
of NC shapes. The multicomponent mixtures of stabilizing
agents are often employed to provide the difference in growth
rate in different crystallographic directions. For example,
depending on the length of alkyl chain and concentration of
alkylphosphonic acid and the heating regime, the rod-,
arrow-, rise-, teardrop-, and tetrapod-shaped CdSe NCs can
be synthesized. 4,32 Possible nanoparticle shapes are determined
by symmetry of underlying crystal lattice; for example,
PbSe NCs with rocksalt atomic lattice can be synthesized in
the form of spheres, cubes (Figure 4a), octahedrons, or starshaped
NCs. 31
Tetrapods of ZnO, 114 iron oxide, 115 Pt, 116 CdSe, 35,117,118
CdTe (Figure 4b), 119 ZnSe, 120,121 and ZnS 121 are the examples
of nanomaterials with higher level of morphological complexity.
Two models were adopted to explain the formation
Figure 4. Examples of inorganic nanomaterials with different
shapes and morphologies synthesized by colloidal chemistry:
(a) PbSe cubes; 31 (b) CdTe tetrapods; 119 (c) PbSe nanowires; 31 and
(d) hollow iron oxide nanoparticles. Reprinted with permission
from ref 31. Copyright 2005 American Chemical Society. Reprinted
with permission from ref 119. Copyright 2003 Nature Publishing
Group.
of the tetrapods. The nucleation of seeds with zinc-blende
crystal structure followed by the growth of four wurtzitephase
arms was originally proposed to explain the morphology
of CdTe nanotetrapods. 117,119,122 The alternative octahedral
multiple twin model is based on the assumption that
the center of tetrapod consists of eight tetrahedral domains
enclosed by low-index, high-atomic-density planes and three
twin planes at the common edges. 114,121,123-125 The further
growth of octa-twin core results in decohesion of the twin
boundaries to relieve the strain energy accumulated at the
common edges of the twins. Preferential growth along the
c-axis for wurzite domains leads to the formation of a
tetrapod. Formation of the twin planes is also known to be
a reason of complex faceting and unusual anisotropic
structures for noble metal nanostructures, for example, Au
and Ag nanowires with 5-fold cross sections. 126
There are a number of other techniques that can be applied
to engineer the shape of colloidal NCs. Thus, long uniform
colloidal nanowires (Figure 4c) can be synthesized by the
oriented attachment 127-130 or solution-liquid-solid 131 growth
mechanisms. In oriented attachment, individual nanoparticles
attach and fuse along identical crystal faces forming oriented
chains. 128 Such anisotropic assembly along one direction
followed by subsequent fusion and annealing of surface
defects has been reported for a number of systems including
Ag, 132 CdTe, 133 CdSe, 134 PbSe, 31 ZnO, 135 ZnS, 136 ZnTe, 137
and In2O3. 138 To explain anisotropic growth, the concept of
dipole-dipole interparticle interactions as a driving force of
chain formation was proposed. 31,127,133 Synthesis through the
oriented attachment allows one to control the nanowire
morphology. In addition to straight wires, zigzag, helical,
branched, and tapered nanowires could all be prepared by
adjustment of the reaction conditions. 31 Solution liquid solid
growth (SLS) of nanowires 131,139 involves the stages analogous
to vapor-liquid-solid growth, 140-142 but nanowires
grow in a colloidal solution in the presence of surfactant
molecules, which prevent aggregation and precipitation of
nanowires. In this approach, metal nanoparticles are used as
catalysts to initiate and direct the nanowire growth; the droplets
of melted metal function as a liquid phase catalyst for the growth
of solid phase through intermediate formation of supersaturated
eutectic solution of semiconductor in metal. 131 Nanoparticles
of low-melting point metals such as Bi or In were used as
the catalysts for solution-phase synthesis of different group
IV, II-VI, III-V, and IV-VI semiconductor nanowires,
freely suspended in solution. 139,143-146
Hollow nanomaterials (Figure 4d) were synthesized through
the nanoscale Kirkendall effect based on the difference in
diffusion rates of two species resulting in accumulation and
condensation of vacancies. 147 During gentle oxidation of
metal nanoparticles, the outward diffusion of metal is much
faster in formed oxide layer than the inward diffusion of
oxygen, leading to the formation of a nanoscale void in the
center of a nanoparticle. 148 This phenomenon was originally
discovered by Yin et al. 148 and was observed for a variety
of materials including hollow magnetic iron oxide (Figure
4d) 149 and cobalt sulfide 148 nanoparticles. Galvanic displacement
reactions have been also employed for the synthesis
of hollow nanostructures. 150 Thus, Xia et al. reported the
formation of hollow Au nanoboxes by reacting ∼100 nm
Ag cubes with Au 3+ ions. 151,152 The hollow Au structure
formed because in the course of galvanic displacement
reduction of each Au 3+ ion required the oxidation and
dissolution of three silver atoms in the nanoparticle core.