Prospects of Colloidal Nanocrystals for Electronic - Computer Science

392 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al.
Figure 1. Examples of colloidally synthesized metal and metal
alloy nanoparticles.
dodecanethiol), amines, or fatty acids. This approach works
quite well for noble metals (Au, 50-53 Ag, 54-56 Pt, 57-59 Pd 60 )
and their alloys (e.g., Au-Ag 61 ). Current developments in
the synthesis of these materials have been extensively
reviewed by Xia et al., 44 Yang et al., 62 and Liz-Marzan. 63
More reactive transition and main group metals (Co, 4,64
Fe, 4,65-67 Cr, 68 Bi, 69 Pb 70 ) require rigorous control of the
reaction conditions and are synthesized under inert atmosphere.
High temperatures, up to 300 °C, are often necessary
to anneal out the defects in crystalline lattice and form highly
crystalline particles. 47 High boiling solvents are used to
provide a wide window of reaction temperatures. The choice
of solvent limits the range of possible molecular precursors.
For example, solvents with low polarity such as dioctyl ether
or 1-octadecene require precursors soluble in this medium.
Among metal salts, acetylacetonates or carboxylates are
frequently used 27,29,47 in the synthesis of metal nanoparticles.
Alternatively, metal NCs can be generated by thermal
decomposition of molecules containing zerovalent metal,
such as metal carbonyls. 4,27,47 For example, cobalt nanoparticles
can be synthesized either by reduction of Co(II) ions
in solution or by thermal decomposition of Co2(CO)8. 4 Longchain
carboxylic acids or alkylamines are the typical surfactants
for metal NCs. In addition to size and shape,
precursors and surfactants can control the crystalline phase
of the nanoparticle. Thus, three different polymorphs of
cobalt NCs (hexagonal close-packed, face-centered cubic,
and epsilon phase) were selectively synthesized by adjusting
reaction parameters. 47 Combining two or more metal precursors,
for example, Pt(acac)2 and Fe(CO)5, in a one-pot
reaction allows one to obtain nanoparticles of metal alloys. 27,71
Among a variety of possible metal combinations, the alloys
of magnetic metals (Fe, Co, and Ni) with Pt and Pd have
been studied most extensively because in these systems
alloying helped to overcome a serious drawback of elemental
magnetic nanoparticles, their low stability against oxidation.
Several methods can be used to tune the size of metal
nanoparticles during synthesis. Usually, the metal nanoparticles
nucleate and grow very quickly, and it is difficult to
terminate the reaction at the desired stage of particle growth.
Instead, the particle size can be adjusted by first making small
nanoparticles, which are then used as seeds for further growth
by adding controllable amounts of molecular precursors. For
example, this method was applied to vary the size of FePt27 NCs. Another approach is based on tuning the relative rates
of nanoparticle nucleation and growth. If the total amount
of molecular precursors used in the reaction is constant, the
fast nucleation provides high concentration of nuclei and
yields smaller NCs. Instead, slow nucleation provides low
concentration of seeds consuming the same amount of
precursors, thus resulting in larger particles. 47 The balance
between nucleation and growth rates can be tuned via
changing of the reaction temperature because the activation
energy for the homogeneous nucleation is usually much
higher than that for particle growth. 72 Typically, the increase
of reaction temperature allowed one to obtain smaller NCs
for different materials. 47 Besides the reaction temperature,
there are a number of other methods to control particle size,
such as nature and concentration of capping molecules, molar
ratios of precursors, etc. For example, lowering the concentration
of surfactants in reaction mixture can also lead to
smaller nanoparticles. 47
Significant success was achieved in the synthesis of NCs
of different technologically important semiconductors. Some
examples of colloidal semiconductor NCs are shown in
Figure 2. Typical reactions used for the synthesis of II-VI
(CdSe, CdTe, CdS), 26,49,73-77 III-V (InP, InAs), 78-81 and
IV-VI (PbS, 82-86 PbSe, 23 PbTe87 ) semiconductor NCs are
outlined by reactions 1-3. The syntheses are carried out at
high temperatures, and in the presence of long-chain alkylphosphines
(e.g., trioctylphosphine, TOP), alkylphosphine
oxides (e.g., trioctylphosphine oxide, TOPO), alkylamines
(e.g., hexadecylamine, HDA), and alkylphosphonic acids as
the stabilizing agents.
300°C, HDA-TOPO-TOP
Cd(CH3 ) 2 + (C8H17 ) 3PSe98 CdSe nanocrystals (1)
260°C, TOP
InCl3 + [(CH3 ) 3Si] 3P98 InP nanocrystals (2)
180°C, oleic acid
Pb(CH3COO) 2 + (C8H17 ) 3PSe98 PbSe nanocrystals (3)
In many cases, the size of semiconductor NCs can be tuned
by adjusting the concentrations of surfactants, reaction
temperature, and duration of the particle growth. In a