Prospects of Colloidal Nanocrystals for Electronic - Computer Science

Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 403
preferred pure, all-inorganic, and highly crystalline materials,
which stimulated the development of functional all-inorganic
ligand coatings (section 4.3).
4.1. Organic Ligands with Long Hydrocarbon
Chains, Their Removal, and Exchange for Smaller
Molecules
The most common class of surface ligands used for
synthesis and processing of NCs is represented by molecules
having a head group (thio-, amino-, carboxylic, etc.) with
high affinity to the NC surface and the aliphatic tail that
provides sterical stabilization of a colloidal solution in
nonpolar solvents. There are an extensive number of studies
dedicated to NCs capped with such surface ligands, and we
refer the reader to several review articles that explicitly cover
the topic. 108,109,274 Oleic acid, trioctylphosphine oxide (TOPO),
dodecanethiol, oleylamine, hexadecylamine, and phosphonic
acids are examples of the most frequently used surfactants.
We have to admit that our current understanding of the
binding modes at the NC-ligand interface is rather incomplete.
As an example, recent X-ray diffraction studies by
Jadzinsky and co-workers revealed that “simple” binding of
thiol molecules to the surface of Au NCs can indeed be very
nontrivial. 275 It is usually believed that surface ligands form
uniform shells around the NC core. It was recently reported
that two kinds of ligand molecules can self-assemble into
complex patterns driven by segregation of different tail
groups at the NC surface. 276
Bulky ligands, convenient for colloidal synthesis, create
thick insulating barriers around each nanoparticle, blocking
the charge transport in NC solids. In addition, simple
monofunctional ligands do not have any functional end
groups, which could be used for carrier transport, conjugation
with other molecules and surfaces, etc. To overcome these
limitations, the original surface ligands can be replaced with
new ones.
The ligand exchange procedure typically involves an
exposure of colloidal NCs to the large excess of competitive
ligand, resulting in partial or complete exchange of surface
molecules. 26,39,156 This procedure can be prolonged up to a
few days or repeated multiple times to maximize the removal
of original ligands, often accelerated by gentle heating. The
ligand exchange is typically carried out without high temperature
treatments, allowing one to introduce low boiling
ligands such as pyridine26 or 1-butylamine265 and others
incompatible with high-temperature conditions. Because of
the effect of mass action, the incoming ligand may have
lower affinity to NC surface than the leaving molecule. The
exchange process is often facilitated by the phase-separation
of the products by using, for example, two-phase mixtures
of polar and nonpolar solvents. 277 The degree of ligand
exchange is usually probed by NMR156,278 and Fouriertransform
infrared spectroscopy (FTIR), 279 and X-ray photoelectron
spectroscopy (XPS). 278,280,281 In many cases, such
studies are supplemented by elemental analysis, dynamic
light scattering, and mass spectrometry. Ligand-exchange
reactions can be classified into two kinds: (i) reaction
involving the same functional group and different tails and
(ii) competition of molecules with different anchoring
functional groups.
Ligand Exchange for Molecules with the Same Head Group
Thiol-for-thiol exchange serves as a classical example.
Using NMR and FTIR techniques, Hostetler and co-workers
studied the mechanism and dynamics of ligand-exchange
reactions using Au NCs and alkanethiols. 282 They found that,
in addition to the expected concentration dependence, there
is also strong impact from the size of the alkyl tail because
the molecules have to penetrate through the existing ligand
shell. The smaller was the tail group of the leaving molecule,
the higher was the exchange rate observed. In so-called
phase-transfer reactions, hydrophilic thiols can be easily
replaced by alkylthiols 277 and vice versa, 283 allowing an easy
switch between hydrophilic and hydrophobic NC surface.
Ligand Exchange for Molecules with Different Head Groups
Many ligand manipulations involve the use of pyridine as
weakly coordinating ligand, which was first demonstrated
for CdSe NCs in a seminal paper by Murray, Norris, and
Bawendi. 26 Typically, NCs are refluxed in pyridine, which
makes the product insoluble in nonpolar aliphatic solvents
but readily dispersible in more polar solvents, aromatics,
small-molecule chlorinated solvents, and mixtures thereof.
Pyridine is often used as a ligand that can be replaced by
other molecules or removed by vacuum and/or heat treatments.
The pyridine exchange was utilized in the synthesis
of core-shell NCs like CdSe/CdS 94 or to integrate CdSe NCs
into hybrid structures of colloidal NC and bulk semiconductors.
284 In the latter case, easy removal of pyridine molecules
allows uniform growth of ZnSe capping layer by molecular
beam epitaxy (MBE). Pyridine was also used as an efficient
intermediate ligand when the direct exchange between
desired and original surface ligands was impossible. 285 In
addition to II-VI nanomaterials, III-V (InP) 286,287 and
IV-VI (PbSe) 288 semiconductor NCs can be functionalized
with pyridine molecules. In case of PbSe NCs, pyridine
treatment was found to significantly improve the charge
transport and photoconductivity. 288
In some cases, postpreparative surface modification is
required to improve chemical and colloidal stability of NCs.
Thus, Hutchison et al. demonstrated the facile phosphineto-thiol
exchange using 1.4 nm Au NCs. 289-291 Triphenylphosphine
was originally used for the synthesis of very
small Au NCs, which, however, were not stable in the long
term. Simple addition of hydrophobic or hydrophilic thiols
considerably improved stability without damaging the NC
core. Using ω-functionalized thiol, 289 Hutchison and coworkers
demonstrated that ligand exchange can tolerate a
very broad range of functional groups. At the same time,
phosphine-to-amine exchange using 1.4 nm Au clusters
occurred with lower substitution rate and was accompanied
by a slow increase of the NCs size. 290,292
A typical limitation of the solution-phase exchange of
larger ligands by smaller ones is the decrease of colloidal
stability often leading to nanoparticle aggregation. To address
this issue, the ligands exchange can be applied not to solution
of NCs, but to a film or a superlattice. This approach benefits
from convenient fabrication of high-quality NC films using
colloidal solutions stabilized by original ligands followed by
soaking the NC film in a solution containing new capping
ligands. Typical examples include treatment of CdSe or PbSe
NCs with various short-chain molecules such as methylamine,
ethylamine, butylamine, ethanethiol, sodium hydroxide,
pyridine, or hydrazine. 23,25,293-296 This approach is also
very useful for NC cross-linking using molecules with two
functional end groups such as diamines or dithiols (see
section 4.2). Such treatments significantly decrease the
interparticle spacings and improve electronic transport in NC