Prospects of Colloidal Nanocrystals for Electronic - Computer Science

396 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al.
lasing 16 applications. The confinement regime can depend
on the diameter of the core and thickness of the shell. For
example, ZnSe/CdSe core-shell nanoparticles can exhibit
either type-I or type-II behavior, depending on CdSe shell
thickness. 165,169 If core and shell materials have a difference
in lattice parameters (i.e., lattice mismatch) larger than a few
percent, the addition of an intermediate “wetting” layer helps
to relax the interfacial strain and prevent accumulation of
structural defects at the interface. 11,170
The seeded growth can be used to fabricate a large variety
of core-shell structures, including some very unusual
material combinations such as Co/CdSe, 160 FePt/CdSe, 172
PtFe/CdS 172 magnetic core-semiconductor shell, or Au/PbS
plasmonic core-semiconductor shell 24 (Figure 6c). Depending
on the interfacial energy, lattice matching, and the
reaction conditions, multicomponent nanostructures can range
from uniformly covered core-shells to dumbbells (Figure
6d) and even highly anisotropic heterostructures like tetrapods
shown in Figure 6b.
The use of seeds other than spherical nanoparticle, such
as nanorods and tetrapods, may result in highly anisotropic
structures where the new phase is selectively deposited on
the tips of nanorods or tetrapod arms. Thus, PbSe/CdSe/
PbSe, 173 CdTe/CdSe/CdTe, 35 and metal-semiconductor-metal
Au/CdSe/Au heterostructures 38 (Figure 6e) were synthesized
from CdSe nanorods.
Robinson et al. used partial exchange of Cd 2+ ions in CdS
nanorods with Ag + via cation exchange reaction 175 to prepare
beautiful periodic CdS/Ag2S nanoheterostructures (Figure 6f).
This approach is considered as a promising route to create
modulated structures, interesting for thermoelectronic applications.
Among other examples of multicomponent nanostructures,
Au/PbS, 24 CoPt3/Au dumbbells, 174 PbSe/Aux, 176
gold/iron oxide dumbbells, 177 solid core-shell 178 and
core-hollow shell structures, 32 TiO2/Fe2O3, 179 PtFe/iron
oxide, 180 and ZnO/Ag, 181 and many others have been reported
during the last several years. The recent advances in the
synthesis of hybrid structures were reviewed by Cozzoli et
al. 37 and Zeng et al. 36
The toolbox of synthetic techniques is quickly expanding,
and many novel structures are reported every year. No doubt,
this progress will further continue, providing scientists and
engineers with novel potential functional building blocks for
electronics and optoelectronics.
3. Nanocrystal Solids
When considering chemically synthesized nanoparticles
for electronic and optoelectronic applications, we should keep
in mind that the actual active element of most devices will
be not individual nanoparticles but their macroscopic arrays.
In analogy with conventional solids, the assemblies of
nanoparticles should conduct charge carriers and perform
other useful actions such as light absorption and carrier
separation in solar cells, light emission in LEDs, etc. The
ability to tailor size, shape, and compositions of individual
nanoparticles provides means for fine-tuning material properties.
At the same time, the behavior of nanoparticle ensembles
depends not only on the properties of individual elements,
but on the electronic and optical communication between
the nanoparticles, on the interparticle medium, packing
density, and mutual orientations of NCs, etc. All of these
issues add new levels of complexity to the problem of
designing electronic materials from nanoparticle building
blocks. At the same time, understanding and utilizing self-
Figure 7. (a) High-resolution cross-section SEM image of a dropcast
film assembled from PbSe nanocrystals. Thickness of SiO2
layer is 100 nm. Reprinted with permission from ref 23. Copyright
2005 American Association for Advancement of Science. (b) SEM
micrographs of a glassy solid prepared from 5.6 nm CdSe
nanoparticles. Reprinted with permission from ref 39. Copyright
2000 Annual Reviews.
assembly of nanoparticles can provide routes to multifunctional
materials of unprecedented precision, complexity, and
aesthetic beauty. 182
NCs can be brought together in a form of an amorphous
(glassy) or ordered periodic structures. Glassy NC solids can
be defined as isotropic materials with only short-range order
among the NCs (Figure 7). Several factors, such as polydispersity
of NCs, their poor solubility in a given solvent,
and fast solvent evaporation, favor the formation of disordered
structures or assemblies with short-range order (Figure
7). Also, if repulsive forces dominate the particle-particle
interactions and other interactions are weak, there is no
significant energy driving the formation of an ordered
lattice. 39
Ordered NC solids (also referred to as “superlattices”) are
anisotropic materials that are characterized by threedimensional
periodicity with or without preferential orientation
of individual nanoparticles. 39,183 In the case of periodic
assemblies, the positions of particles and, as a result, packing
density and chemical composition are uniform throughout
the entire structure. This uniformity makes periodic nanoparticle
arrays (films, 3D crystals) especially attractive for
practical applications and for fundamental studies. However,
there are a number of technical challenges in reproducible
preparation of large nanoparticle superlattices, such as lack
of control over their dimensions and structural defects.