Mysteries of TOPSe revealed: Insights into quantum dot nucleation

Article abstract of DOI:10.1021/ja103805s

We have investigated the reaction mechanism responsible for QD nucleation using optical absorption and nuclear magnetic resonance spectroscopies. For typical II-VI and IV-VI quantum dot (QD) syntheses, pure tertiary phosphine selenide sources (e.g., trioctylphosphine selenide (TOPSe)) were surprisingly found to be unreactive with metal carboxylates and incapable of yielding QDs. Rather, small quantities of secondary phosphines, which are impurities in tertiary phosphines, are entirely responsible for the nucleation of QDs; their low concentrations account for poor synthetic conversion yields. QD yields increase to nearly quantitative levels when replacing TOPSe with a stoiciometric amount of a secondary phosphine chalcogenide such as diphenylphosphine selenide. Based on our observations, we have proposed potential monomer identities, reaction pathways, and transition states and believe this mechanism to be universal to all II-VI and IV-VI QDs synthesized using phosphine based methods.

Full text of DOI:10.1021/ja103805s

Published on Web 07/26/2010
Mysteries of TOPSe Revealed: Insights into Quantum Dot Nucleation
Christopher M. Evans, Meagan E. Evans, and Todd D. Krauss*
Department of Chemistry and the Institute of Optics, UniVersity of Rochester, Rochester, New York 14627
Received May 11, 2010; E-mail: krauss@chem.rochester.edu
Abstract: We have investigated the reaction mechanism respon-
sible for QD nucleation using optical absorption and nuclear
magnetic resonance spectroscopies. For typical II-VI and IV-VI
quantum dot (QD) syntheses, pure tertiary phosphine selenide
sources (e.g., trioctylphosphine selenide (TOPSe)) were surpris-
ingly found to be unreactive with metal carboxylates and incapable
of yielding QDs. Rather, small quantities of secondary phos-
phines, which are impurities in tertiary phosphines, are entirely
responsible for the nucleation of QDs; their low concentrations
account for poor synthetic conversion yields. QD yields increase
Figure 1. Optical absorption of PbSe MSCs after 40 min at 40 °C for the
to nearly quantitative levels when replacing TOPSe with a
stoiciometric amount of a secondary phosphine chalcogenide
reaction between Pb(oleate)2 and TOPSe with varying dioctylphosphine
concentrations (see SI for complete growth series). Note the negligible
such as diphenylphosphine selenide. Based on our observations,
we have proposed potential monomer identities, reaction path-
ways, and transition states and believe this mechanism to be
universal to all II-VI and IV-VI QDs synthesized using phosphine
based methods.
reactivity for the purest TOPSe source.
Table 1. Observed Compounds Resulting from the Reaction
between Diphenylphosphine Selenide and Metal Carboxylates
The significant potential of semiconductor nanocrystal quantum
dots (QDs) for photovoltaics, biolabeling/imaging, and light-
emitting diodes has been well-reported.1 Unfortunately, realizing
this potential has yet to occur, in part due to a synthetic procedure
that lacks scalability and high conversion yield. Current syntheses
can produce high-quality QDs with respect to photoluminscent
quantum yield (QY >50%) and size distribution ((5%). However,
growth temperatures are high (∼300 °C for CdSe), conversion
yields are poor (<2%), and the method suffers from well-known
irreproducibilities.2,3 We suggest that the lack of a rational synthetic
mechanism has stalled advancements in QD synthetic design.
The essential components in the synthesis of high-quality II-VI
and IV-VI QDs have remained largely unchanged for 20 years,
typically employing tertiary phosphine chalcogenides and metal salts
as the reactive precursors.4 Notable advances include removing the
need for pyrophoric compounds5 and designing ligands to obtain a
greater control over QD size, size distribution, and shape.3 Recently,
secondary phosphines were recognized as useful additives to
phosphine based syntheses demonstrating improvements in conver-
sion yield.2,6 Here, we identify a reaction pathway consistent with
previous synthetic observations and illuminate the role of precursor
reagents in the formation of QDs. Specifically, we have identified
that secondary phosphine chalcogenides are the reactive species
directly responsible for the formation of QD nuclei.
b
c
a All spectra were recorded in toulene-d8. 1H NMR. 31P NMR.
13C NMR. e Verified by addition of authentic sample. Isolated
d
f
diffraction quality single crystals from the reaction. g Chemiker Zeitung
1982, 391-395. h Chem. Commun. 2005, 2692-2694.
addition of a secondary phosphine (diisopropylphosphine) to high
purity tertiary phosphine chalcogenide precursors, reaction rates
in all cases accelerated rapidly yielding PbSe QDs. Typical tertiary
phosphine sources (tributylphosphine and trioctylphosphine (TOP))
used for QD syntheses are highly impure, and all contain measurable
quantities of secondary phosphines (dibutylphosphine (31P δ -69.5
ppm) and dioctylphosphine (31P δ -69.7 ppm) respectively), as
shown in Supporting Information (SI) Figures S1 and S2. The
importance of secondary phosphines is further emphasized by Figure
1, which shows that growth kinetics and yield for PbSe magic-
sized clusters (MSCs)7 are dependent upon dioctylphosphine
concentration (i.e., [(DOP)]) rather than [TOP] or [TOPSe]. In fact,
dioctylphosphine selenide (DOPSe) can be observed as an impurity
in neat TOPSe (31P δ 4.7 ppm, 725 Hz 1J(31P - 77Se)) but disappears
immediately upon combination with a metal carboxylate at 300 K
suggesting that it plays a critical part in QD nucleation.
We studied reactions performed between high purity tertiary
phosphine chalcogenides (e.g., triethylphosphine selenide, triiso-
propylphosphine selenide, and triphenylphosphine selenide) and
Pb(oleate)2 heated to 120 °C, which were monitored by both optical
absorption spectroscopy and 31P NMR. In all cases, no reactivity
was observed even after 5 h of heating, in stark contrast to
commercially obtained trioctylphosphine selenide (TOPSe), which
reacts with Pb(oleate)2 in minutes, forming a dark product. Upon
In order to further elucidate the role of secondary phosphines in
the formation of chalcogenide QDs we studied the reaction between
9
10.1021/ja103805s  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 10973–10975 10973

C O M M U N I C A T I O N S
Scheme 1. Possible Reaction Mechanism for Metal-Chalcogenide
Bond Formation
Scheme 2. Reaction of Secondary Phosphines with Metal
Carboxylates
reagent ratio dependence illustrates that at least two concurrent
reaction pathways compete for the formation of product: (1)
reactions rich in metal carboxylates produce monomers (1 and/or
6) that polymerize yielding MSe MSCs and eventually QDs; (2)
reactions rich in DPPSe induce termination pathways yielding a
stable metal-selenide complex and minimal MSe product.
Previously proposed roles for secondary phosphines do not fit
with our observations (Scheme 2). Applying heat (140 °C) to a
mixture of diphenylphosphine and Pb(oleate)2 did result in Pb0, but
the reaction took hours, much longer than the reaction for a typical
QD synthesis, which takes several minutes at equivalent temper-
atures. We also found that the metal product of the above reaction,
Pb0, is inert to commonly used selenium sources such as TOPSe
and thus is likely uninvolved in QD formation. Therefore, we
suggest that the role of secondary phosphines cannot simply be to
reduce metal carboxylates to metal but rather to promote the
dissociation of oleate and formation of a metal-phosphine complex
(10, 11).9
To test our mechanism under QD reaction conditions typically
employed in the literature, we studied the evolution of product
species in a reaction between Pb(oleate)2 and TOPSe when adding
15% (mol,mol) of DPP. Prior to combination of reagents, DOP (δ
-69.1 ppm), DOPSe (δ 4.3 ppm), and DPPSe (δ 5.9 ppm) are
observed by 31P NMR, in addition to the many impurities normally
found in TOP (Figure 2A). However, in agreement with our studies
of QD nucleation with pure secondary phosphine species, these
species also disappear immediately after combination with
Pb(oleate)2 which supports our hypothesis that secondary phosphine
impurities are entirely responsible for QD growth (Figure 2B).
Additional peaks are observed after combination of reagents and
are assigned to tetraphenyldiphosphine (δ -14.0 ppm, 12) and
9-octadecenoxydiphenylphosphine (δ 98.9 ppm, 3) which directly
corresponds to the products of our model reactions between DPPSe
and M(oleate)2.
Even in QD syntheses solely based upon TOPSe we observe
byproducts indicative of the reaction between secondary phosphine
chalcogenides (i.e., DOPSe, DPPSe) and metal carboxylates.
Considering that pure tertiary phosphine selenides are inert to metal
carboxylates and the observation of identical reaction products for
reaction with pure secondary phosphine selenide precursors, we
suggest that TOPSe’s role in nucleation may simply be as a
homogeneous source for selenium. In support of this hypothesis
we note that selenium liberated from TOPSe can exchange either
to secondary phosphines or to intermediate metal phosphine
complexes (10, 11).
In order to generate a II-VI or IV-VI QD monomer species
several pieces of evidence suggest that Se exchange is indeed
occurring between an unobserved metal-phosphine complex (10,
11) and TOPSe. Chalcogenide exchange is a well-known and useful
tool for phosphine protection strategies in organic synthesis. Rapid
equilibration of Se substitution is often observed in mixtures of
phosphines with the more basic phosphorus nuclei representing the
preferred substitution.10 Thus, literature precedent suggests that
diphenylphosphine selenide (DPPSe) and Pb(oleate)2 via NMR
spectroscopy utilizing 1H, 13C, and 31P nuclei. The combination of
toluene solutions of DPPSe and Pb(oleate)2 results in an immediate
reaction producing PbSe MSCs with complete conversion of DPPSe
(verified through 31P NMR) at room temperature (see SI for
experimental details). Over the course of the reaction, we identified
eight reaction products in addition to the PbSe species (see Table
1 and NMR spectra in the SI). Monitoring the temporal progression
of the various reactants and products over the course of the reaction
allowed for the postulation of a hypothetical mechanism for
monomer formation consistent with our observations (Scheme 1).
Significantly, we also found the organic byproducts were identical
for both the CdSe and PbSe QD syntheses, leading us to conclude
that this mechanism is universal to all phosphine-based syntheses
of both II-VI and IV-VI semiconductor QDs.
A QD monomer species is represented by structure 1 and is
produced through a ligand disproportionation yielding oleic acid.
Intermolecular reactions between multiple monomers (1) produce
3 while enlarging the PbSe unit. While 1 could not be isolated due
to its predicted self-reactivity, 3 was observed in good quantity
and shown to produce both 4 and 5 by reaction with oleic acid
(see SI). An interesting parallel can be made between intermediates
in condensation polymerizations (e.g., nylon) and 1 whereby
asymmetrically terminated monomers combine to lengthen the
repeat unit. 4 and 5 have been observed in prior mechanistic studies
using the standard TOPSe QD synthesis2,8 and likely arise from
the presence of secondary phosphine impurities (i.e., DOPSe and
DOP).
We varied the ratio of DPPSe/Cd(oleate)2 from 10:1 to 1:2 and
observed that the number and relative abundance of inorganic
products depended heavily on the initial proportion of reagents.
Reaction conditions with a large excess (>5:1) of phosphine
chalcogenide (DPPSe) yield X-ray quality crystals of 9 without
production of any larger PbSe species such as MSCs. In addition
to 9, several other P-containing species are observed to predominate
only under DPPSe rich conditions. Conversely, reactions performed
at 300 K with an excess of Pb(oleate)2 (also observed for
Cd(oleate)2) rapidly yield large quantities of PbSe (CdSe) MSCs7
and oleic acid with only two major P-containing species (3 and
12). After 24 h at room temperature both 3 and oleic acid disappear
in favor of diphenylphosphine oxide (4) and oleic anhydride (5)
without significant changes to the size of the PbSe (CdSe) MSC
product. This reaction appears to be an organic side reaction
independent of monomer formation and growth. Altogether, the
9
10974 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

C O M M U N I C A T I O N S
Figure 2. 31P{1H} NMR for (A) neat TOPSe with 15% (mol,mol) DPP and (B) 30 min after combining with Pb(oleate)2 at 40 °C. The major products from
this reaction (3 (98.9 ppm) and 12 (-14.5 ppm)) are consistent with a reaction between sec-phosphine chalcogenides and metal carboxylates.
TOPSe represents a soluble organic Se source capable of efficient
Se exchange to more reactive species during a QD synthesis. Indeed,
we have observed direct Se exchange between TOPSe and impurity
quantities of dioctylphosphine via NMR spectroscopy; in this case,
the exchange equilibrium is weighted toward TOPSe but nonetheless
results in an observable exchange. Altogether, the strong effect of
secondary phosphines on QD synthetic yield, the importance of
the DOPSe impurity to reactivity, and the selenium exchange
equilibrium present during the reaction strongly suggest that DOPSe
represents the reactive species responsible for QD formation.
Reactions between DPPSe and metal carboxylates, at room
temperature, completely consume both reagents rapidly yielding
very small MSe MSCs. Increasing the temperature for the above
reaction (to 80 °C for PbSe and 200 °C for CdSe) produced PbSe
or CdSe QDs, respectively, with near-quantitative conversion yields
(yield >90% based upon literature extinction coefficients11). QDs
made from the above process can often possess optical properties
and size distributions comparable to the best literature methods
without the use of postsynthetic processing (examples are shown
in the SI). Further, the addition of excess ligand was not necessary
for growth but its presence limited interparticle aggregation which
became evident after several hours. On the other hand, we found
QD size control using pure secondary phosphine selenide is not
well understood and depends upon a complicated set of interrelated
parameters including reaction temperature, surface capping ligand,
and reagent stoichiometry. A detailed study is underway evaluating
these many parameters and their individual roles in producing
monodisperse QDs with a tunable size.
In summary, we have developed a rational synthetic mechanism
for QD monomer formation that accounts for previous observations
of several organic species and identifies the likely structure of the
reactive intermediate. The extremely high conversion yield of QD
reactions based on secondary phosphine chalcogenides, and at a
reduced temperature, stresses the important role of small quantities
of DOPSe impurities in the conventional synthesis. Further, our
results may explain the low yields in TOPSe based procedures
(<2%) and the notorious inconsistencies and irreproducibilities often
seen in QD syntheses. In addition to formation of MSe QD species,
we anticipate similar reactivity for secondary phosphine sulfide and
telluride reagents operating under an identical nucleation mechanism
as detailed above with equivalent improvements to conversion yield.
A complete kinetic analysis is necessary to prove this particular
mechanism; however, our study represents an important step toward
understanding the complex chemistry associated with both nucle-
ation and growth. Our expectation is that a better understanding of
the QD reaction mechanism should allow for future use of cheaper
and more environmentally benign reagents in the synthesis of a
wide array of QD materials.
Acknowledgment. Funding was provided by the Camille and
Henry Dreyfus Foundation, the Rochester Human Immunology
Center supported by the National Institutes of Health, and the
National Science Foundation. We also thank William Jones, Robert
Boeckman, Mary Lenczewski, and Pu Luo for helpful discussions.
We acknowledge assistance from William Brennessel at the X-ray
Crystallographic Facility of the Department of Chemistry at the
University of Rochester.
Supporting Information Available: Experimental details regarding
QD synthesis, table of observed reaction products, NMR spectra, and
the X-ray crystal structure of species 9. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Nozik, A. J. Annu. ReV. Phys. Chem. 2001, 52, 193. (b) Colvin, V. L.;
Schlamp, M. C.; Alivisatos, A. P. Nature 1996, 370, 354. (c) Alivisatos,
P. Nat. Biotechnol. 2004, 22 (1), 47.
(2) Steckel, J. S.; Yen, B. K. H.; Oertel, D. C.; Bawendi, M. G. J. Am. Chem.
Soc. 2006, 128 (40), 13032.
(3) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124 (13), 3343.
(4) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115
(19), 8706.
(5) Qu, L.; Peng, A.; Peng, X. Nano Lett. 2001, 1 (6), 333.
(6) Joo, J.; Pietryga, J. M.; McGuire, J. A.; Jeon, S.-H.; Williams, D. J.; Wang,
H.-S.; Klimov, V. I. J. Am. Chem. Soc. 2009, 131 (30), 10620.
(7) Evans, C. M.; Guo, L.; Peterson, J. J.; Maccagnano-Zacher, S.; Krauss,
T. D. Nano Lett. 2008, 8 (9), 2896.
(8) Liu, H.; Owen, J. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2007, 129 (2),
305.
(9) Driess, M.; Janoschek, R.; Pritzkow, H.; Rell, S.; Winkler, U. Angew. Chem.,
Int. Ed. Engl. 1995, 34 (15), 1614.
(10) Brown, D. H.; Cross, R. J.; Keat, R. J. Chem. Soc., Dalton Trans. 1980, 6,
871.
(11) Dai, Q.; Wang, Y.; Li, X.; Zhang, Y.; Pelligrino, D. J.; Zhao, M.; Zou, B.;
Seo, J.; Wang, Y.; Yu, W. W. ACS Nano 2009, 3 (6), 1518.
JA103805S
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 10975