On the mechanism of lead chalcogenide nanocrystal formation

Article abstract of DOI:10.1021/ja062626g

On the basis of evidence from ³¹P NMR spectroscopy, and using PbSe as a model, we propose two simultaneous mechanisms through which monomers are formed in preparations of lead chalcogenide nanocrystals (NCs). In one mechanism, selenium is delivered as a Se^2- species, whereas in the other, Se⁰ reacts with metal already reduced by the organophosphine. This latter mechanism helps explain the sensitivity of NC preparations to the purity of organophosphines and allows the rational modification of batch NC reactions to increase yield. Copyright

Full text of DOI:10.1021/ja062626g

Published on Web 09/16/2006
On the Mechanism of Lead Chalcogenide Nanocrystal Formation
Jonathan S. Steckel, Brian K. H. Yen, David C. Oertel, and Moungi G. Bawendi*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received April 19, 2006; E-mail: mgb@mit.edu.
Significant advances in the synthesis of semiconductor nanoc-
rystals (NCs) have made possible the preparation of samples with
narrow size distributions and high quantum yields. Most procedures
involve the rapid injection of precursors into a heated mixture of
solvents and ligands; organo-phosphines, organo-phosphine oxides,
and carboxylic acids are often used during the preparation of II-
VI, III-V, and IV-VI NCs.1 The formation of NCs in solution is
driven by the supersaturation of “monomers” which undergo a
nucleation and growth process. Despite the large number of recipes
developed that produce high quality particles, progress toward
understanding nucleation and growth has been limited to measure-
ment of the size, particle number, and yield.2 Very little specific
information is known regarding the mechanism by which monomers
are generated in solution. In this study, we used 31P NMR
spectroscopy to monitor key compounds (alkylphosphines and
alkylphosphine oxides) involved in the formation of PbSe NCs.
On the basis of the NMR data, we propose that two competing
mechanisms occur simultaneously to generate monomers in the
preparation of PbSe NCs.
Figure 1. 31P NMR spectra of four PbSe NC samples grown over 40 min
at 170 °C with the corresponding reaction yields (from flame atomic-
absorption spectroscopy) shown in parentheses. No free TOP was observed
in the injection solution or throughout the run. The inset shows the detail
view of the TOPO region.
Scheme 1. Proposed Mechanism for Reaction 1a
Lead selenide NCs are typically synthesized by rapidly injecting
TOPSe into a heated solution of lead precursor, Pb(oleate)2.3
Because of relatively low injection temperatures, the PbSe system
is ideal for mechanistic studies aimed at gaining insight into NC
chemistries. We propose that the following two mechanisms occur
during the formation of PbSe NCs.
a R3PdSe (TOPSe) and metal carboxylates (Pb(oleate)2) react to
form monomer ([PbSe]). The [PbSe] species is likely stabilized by the
presence of free ligands in solution (R3PdO and R3PdSe). Ra
(CH2)7CHdCH(CH2)7CH3, Rb ) octyl
)
It is possible that analogous mechanisms occur in other NC
syntheses using metal salts and phosphine chalcogenides. Because
of the weak P-X (X ) Se, Te) bond,4 R3PdX can be regarded
either as a source of X0 or X2-. In the first mechanism (1), tri-n-
octylphosphine selenide (TOPdSe) delivers selenium as a Se2-
species, resulting in the production of monomer, tri-n-octylphos-
phine oxide (TOPdO), and an anhydride. In the second mechanism
(2), TOPdSe can be regarded as delivering Se0 to a reduced Pb0
species, resulting in the formation of monomer, an anhydride, and
free TOP. Once the monomer concentration increases above the
solubility limit (supersaturation), the monomers combine by a
nucleation and growth process to form NCs. The monomer is a
transient species that is likely a PbSe unit stabilized by a number
of ligands present in solution, and we label it as [PbSe] in our
mechanisms.
to elucidate reaction 1 in which our model system was held at an
elevated temperature for four lengths of time (10, 20, 30, and 40
min). The growth solution for each time span was then quenched
thermally and by dilution. The 31P NMR spectra reveal that the
TOPO concentration increased over time (figure inset). We attribute
the slight shift in TOPO chemical shift to a change in the local
environment as NCs are being formed in large amounts. Flame
atomic absorption spectroscopy was also performed on these same
samples, and PbSe reaction yield was found to increase correspond-
ingly with the TOPO concentration.
Scheme 1 shows in more detail our proposed mechanism for
reaction 1 in which TOPSe reacts directly with the Pb2+ center to
produce TOPO, an oleate anhydride, and PbSe monomer. The Pb
center remains Pb2+, while the Se is delivered formally as a Se2-
species. It is important to note that only TOPO and the anhydride
are formed in Scheme 1, consistent with the observation that no
free TOP is generated (Figure 1).
The basic model system we used to study the formation of PbSe
NCs consisted of Pb(oleate)2 and TOPSe (pure TOPSe, with no
free TOP) in 1-octadecene.5 Concentrations and reaction times in
our experiments were optimized for 31P NMR signal and not size
distribution. Figure 1 shows the results of an experiment designed
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J. AM. CHEM. SOC. 2006, 128, 13032-13033
10.1021/ja062626g CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Results of Three Batch PbSe NC Preparations Using 0,
0.08, and 0.15 mmols of Diphenylphosphine (DPP) Doped into the
1 M TOPSe Injection Solution
reacn yield
[%]
first abs peak
[nm]
DPP/Pb
no. of NCs
2.3
11.6
16.2
1.81 × 1016
3.09 × 1016
4.53 × 1016
1358
1736
1716
0.08
0.15
Dialkyl phosphines can be impurities in TOP, and they can also
be generated in situ during the synthesis through a â-hydride
elimination mechanism. The mechanism in Scheme 2 can help
explain the empirical observation that different lots of TOP or
specific heating times sometimes have profound effects on the
reaction yield, size, and size distribution of resulting NCs.
With an understanding of the role of each species in the
chemistry, we can begin to rationally modify batch NC reactions.
For instance, by changing the reducing agent, the rate of reaction
2 can be varied to provide more control over the system. Table 1
shows the results of three batch preparations of PbSe NCs in which
DPP was added to TOPSe before injection, increasing markedly
the number of NCs formed and the overall reaction yields (see
Supporting Information for synthesis details). The presence of DPP
increases the supersaturation rate of monomers, thereby increasing
the nucleation rate.
Figure 2. 31P NMR spectrum of the growth solution of PbSe NCs
synthesized using TOPSe doped with diphenylphosphine. The inset shows
the 31P NMR spectrum of the TOPSe/DPP solution before combining and
reacting with the Pb(oleate)2 solution. Reaction performed at 170 °C for
10 min.
Scheme 2. Proposed Mechanism for Reaction 2a
In summary, a PbSe NC synthesis was used as a model system
to study a mechanism of monomer formation in systems based on
lead carboxylates and organo-phosphines. On the basis of our
experimental results, we propose that two mechanisms occur
simultaneously. One practical example of this understanding is our
ability to increase significantly the reaction yield of the PbSe
synthesis while maintaining the size distribution.
a R3PdSe (TOPSe), a metal carboxylate (Pb(oleate)2), and a reducing
agent (diphenylphosphine) react to form monomer ([PbSe]). The [PbSe]
species is likely stabilized by the presence of free ligands in solution (R3P,
R2HP, R2PdO, and R3PdSe). Ra ) (CH2)7CHdCH(CH2)7CH3, Rb ) octyl,
and Rc ) phenyl.
Acknowledgment. We thank Joseph P. Sadighi for helpful
discussions, Robert W. Edwards for performing FAA spectroscopy,
and Jean-Michel Caruge for PbSe cross-section measurements. This
work was funded in part by the U.S. Army through the Institute
for Soldier Nanotechnologies, under Contract DAAD-19-02-0002
with the U.S. Army Research Office, and NSF Grants 9808061-
CHE and 9729592-DBI. D.C.O. was supported by the Fannie and
John Hertz Foundation.
Semiconductor NC preparations often involve a significant
amount of free organo-phosphine, which has been shown to act as
a reducing agent. For example, triphenylphosphine reduces divalent
palladium in the complex Pd(OAc)2 through oxidation to triph-
enylphosphine oxide.6 In a second experiment, designed to probe
proposed mechanism 2, diphenylphosphine (DPP) was added to
our model system, and products and reaction yield were studied.
When diphenylphosphine was added to the TOPSe solution during
the NC synthesis (Pb/Se/DPP mole ratio of 1:1:0.3), we observed
the complete oxidation of DPP to DPPdO (Figure 2) and a resulting
significant increase in the PbSe NC reaction yield (54% compared
to 2.8% with no DPP (Figure 1)). The 31P NMR spectra in Figure
2 also show peaks corresponding to TOP and TOPO (small), the
latter of which indicates that the reaction in Scheme 1 is proceeding
to some extent, as well.
Scheme 2 describes our second proposed mechanism for reaction
2 in more detail. Pb2+ is reduced by DPP, giving a Pb0 species and
DPPO. The Pb0 species then reacts with TOPSe to liberate TOP
and form PbSe. In contrast to Scheme 1, the Se can be regarded as
being delivered to Pb0 as a Se0 species (TOP: +Se0). Though TOP
is a weaker reducing agent than DPP, TOP likely plays the role of
DPP (Scheme 2) to some extent because it is often present in large
excess in NC syntheses. Heating Pb(oleate)2 with either TOP or
DPP provided further evidence that organo-phosphines serve as
reducing agents in NC synthesis. We found that substantial amounts
of solid Pb0 forms at 250-320 °C in the presence of TOP and at
∼180 °C in the DPP case.
Supporting Information Available: Experimental details, char-
acterization of Pb(oleate)2, and PbSe nanocrystal absorption cross
section determination. This material is available free of charge via the
Internet at http://pubs.acs.org.
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