Mechanistic insights into the formation of InP quantum dots

Article abstract of DOI:10.1002/anie.200905632

Figure Presented InP Kudos: The molecular mechanism of InP colloidal quantum dot (QD) syntheses was investigated by NMR spectroscopy. Unlike methods for monodisperse PbSe and CdSe, existing InP syntheses result in total depletion of molecular phosphorous species following nucleation, so QD growth is due exclusively to non-molecular ripening. Amines inhibit precursor depletion by solvation (see picture), contrary to previous reports.

Full text of DOI:10.1002/anie.200905632

Communications
DOI: 10.1002/anie.200905632
Quantum Dots
Mechanistic Insights into the Formation of InP Quantum Dots**
Peter M. Allen, Brian J. Walker, and Moungi G. Bawendi*
In memory of Peter Curtin
InP QDs (InP quantum dots)[1–3] are of increasing techno-
logical interest as a replacement for CdSe QDs in visible-light
applications. However, the synthetic methods used for InP
QDs have not produced QDs with the narrow size distribu-
tions attained for CdSe and PbSe QDs.[1–5] Studies of the
molecular mechanisms involved in the formation of QDs have
only recently been reported for CdSe and PbSe QDs,[6,7] and
the mechanisms underlying InP QD formation are essentially
unknown. We investigated the reactions involved in InP QD
formation to understand the broad size distributions in
current InP QD syntheses.
Scheme 1. Proposed mechanistic pathway for amine-inhibited InP
synthesis. Both the formation of outer-sphere complex 1 and the
irreversible formation of intermediate 2 are inhibited by increased
In a simplified view of the formation of monodisperse
colloids, two general events should occur: 1) an initial
nucleation of colloids, followed by 2) subsequent growth of
these nuclei from molecular precursors.[8,9] Studies on the
growth of CdSe and PbSe QDs have shown that these systems
fulfill both events.[6,7] For InP QDs, we have found that
molecular phosphorus precursors are completely depleted
following InP nucleation, indicating that subsequent QD
growth is due exclusively to ripening from non-molecular InP
species. The inability of InP QD syntheses to satisfy (2) owing
to depletion of molecular precursors may explain the broad
size distributions of InP QDs relative to CdSe or PbSe QDs.
Colloidal InP QDs are synthesized by the injection of
precursors into a hot solution of surfactants, or by mixing
precursors at room temperature followed by heating.[1–3] In
these reactions, indium(III) myristate, In(MA)3, reacts with
tris(trimethylsilyl)phosphine, (TMS)3P, at elevated temper-
atures to produce trimethylsilyl myristate (TMS-MA) and InP
QDs (Scheme 1). By operating at reduced temperatures with
amines, it is possible to monitor the evolution of molecular
species during InP formation. Amines inhibit precursor
decomposition, which is contrary to previous claims that
amines act as activating agents in InP QD synthesis.[2,10–13]
À
solvation. A charge-dispersion SN2 transition state for TMS X bond
À
formation and P TMS bond cleavage is inferred from the large
negative activation entropy and the large rate decrease with added
amine.
We propose the mechanism for amine-inhibited InP QD
synthesis given in Scheme 1. Initially, In(MA)3 is coordinated
to Lewis base(s), such as octylamine (OA), in the outer
(solvation) sphere. In the reversible first step, one (TMS)3P
molecule becomes incorporated into the solvation sphere (1).
À
Complex 1 then loses a myristate ligand, a stable In P bond
forms, and the coordinated phosphine loses a TMS group,
thereby irreversibly forming a molecular intermediate (2).
Intermediate 2 reacts further to form [InP] clusters and
nanocrystals. Evidence that supports this mechanism will be
described below.
To probe the evolution of molecular species during InP
QD synthesis, we used H NMR spectroscopy to investigate
species with TMS substituents. In this reaction, the TMS
group is the only likely ligand for molecular phosphines, so
the high H NMR sensitivity of the TMS group permits the
1
1
observation of any phosphorus-containing molecules or
decomposition products present at significant concentration.
Reactions were performed in sealed NMR tubes with 0.02m
In(MA)3, 0.01m (TMS)3P, 0.0–1.44m octylamine, and 0.03m
diphenylmethane as an internal standard in [D8]toluene.
[*] P. M. Allen,[+] B. J. Walker,[+] Prof. M. G. Bawendi
Department of Chemistry, Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
Fax: (+1)617-452-2708
E-mail: mgb@mit.edu
[+] These authors contributed equally to this work.
1
In the absence of octylamine, a H NMR spectrum taken
within three minutes of mixing In(MA)3 and (TMS)3P at
room temperature showed quantitative conversion of
(TMS)3P into TMS-MA (Figure 1a,b). The rapid decompo-
sition of (TMS)3P is due to the direct approach of (TMS)3P to
the indium center, circumventing outer-sphere equilibria en
[**] This work was supported in part by the MIT-Harvard NIH CCNE
(1U54-CA119349) and the US ARO through the ISN (W911NF-07-D-
0004).This work also made use of the DCIF (CHE-980806, DBI-
9729592). B.J.W. was supported by a NSF Graduate Research
Fellowship. Special thanks to Alejandro Lichtscheidl and Peter Reiss
for helpful discussions, and Jeffrey Simpson for assistance with
HMBC measurements.
À
route to the production of a stable Si O bond in the TMS-MA
product. The exceedingly fast conversion of (TMS)3P into
TMS-MA at room temperature occurs on a timescale that is
not practical for monodisperse QD synthesis or kinetic
analysis by NMR.[6–9]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905632.
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Figure 1. 1H NMR spectra of a) (TMS)3P at 208C before injection,
b) the reaction mixture of In(MA)3 and (TMS)3P in [D8]toluene three
minutes after mixing at 258C, and c) the reaction mixture of (TMS)3P
and 6:1 octylamine/In(MA)3 in 1,2-[D4]dichlorobenzene after heating at
1788C for one minute. Both (b) and (c) show quantitative conversion
of (TMS)3P into TMS-MA shortly after the reactions are initiated.
Using literature procedures at elevated temperatures
containing octylamine, we find that no molecular precursors
remain shortly after reaction initiation at high temperature. A
sealed NMR tube with a composition similar to those
previously reported was heated at the published reaction
temperature (1788C)[2] and showed no (TMS)3P after 1 min
(Figure 1c). Our findings, with and without octylamine,
demonstrate that previously reported reaction conditions
for InP QDs are based on ripening of non-molecular InP
species, similar to those used for InAs,[14] and do not fulfill the
requirements for the formation of monodisperse colloids.
However, when octylamine-containing InP reaction mix-
tures were run at 408C, the reaction rate slowed sufficiently to
permit kinetic analysis. From time-resolved 1H NMR spectra
(Figure 2), four species account for all of the TMS groups
throughout the reaction. (TMS)3P and TMS-MA together
account for over 90% of all TMS groups; the remaining TMS
groups are either associated with the intermediate 2 (Fig-
ure 2a, inset) or the silated octylamine (TMS-OA) that occurs
in less than 5% yield under standard conditions. The relative
rates of (TMS)3P depletion and of TMS-MA growth are
consistent with near quantitative conversion of (TMS)3P into
TMS-MA (Figure 2b). Linear evolution at early times
facilitated the analysis of initial rates.
1
Figure 2. a) Time-resolved H NMR spectra at 408C, showing evolution
of (TMS)3P (blue) and TMS-MA signals (red) during standard con-
ditions for InP QD synthesis with amines. Inset: Detail of the
spectrum at 15 minutes. The doublet at d=0.26 ppm is assigned to 2.
b) Concentration (c) profiles of (TMS)3P and TMS-MA protons deter-
mined at 1 min intervals by integration of spectra represented in
Figure 2a. c) Concentration profiles of (TMS)3P for InP QD syntheses
with varying amine concentrations at 408C, normalized at t=0.
~
!
&
*
[amine]/[In]: 36:1, 18:1, 12:1, 6:1. Reaction rate decreases
with increasing amine concentration; reactions without amines reach
completion too rapidly to be resolved with our method.
(Figure 2c); thus amines inhibit rather than activate[2,10–13] InP
synthesis. As a change in amine ratio influences the rate even
at high concentrations, and as the rates do not have a clear
power dependence on octylamine concentration in either this
or prior studies,[13] the amine does not have a well-defined
stoichiometry during the rate-determining step. For ratios of
octylamine to indium of greater than 18:1, we observed a
diminishing change in the rate with added amine. The
reactions with high octylamine content also yielded an
increase in the TMS-OA product, which complicates kinetic
analysis.
We attribute the doublet at d = 0.26 ppm (3JHP = 4.4 Hz)
to 2,[15] as this signal suggests a phosphorus-containing species
that retains at least one TMS group. We corroborated this
assignment with a heteronuclear multiple bond coherence
We find no evidence for other molecular phosphorus
À
compounds, such as P H-containing species. The only reso-
1
(HMBC) experiment, which showed that H resonances of
nances in the 31P NMR spectra correspond to (TMS)3P and 2
(Supporting Information, Figure S11). When monitoring the
depletion of (TMS)3P at various octylamine concentrations,
(TMS)3P and 2 were cross-coupled to respective 31P reso-
1
nances (Supporting Information, Figure S10). The H NMR
1
resonance of 2 has a linewidth comparable to that of
molecular (TMS)3P and TMS-MA noted above, thus indicat-
ing that the species has a distinct molecular structure. Finally,
the time evolution of the 2 resonance shows the characteristic
growth and depletion of a reaction intermediate (Supporting
Information, Figure S1).
we do not observe doublets in the H NMR spectrum that
would indicate the formation of P H species, so this reaction
À
does not appear to be a significant pathway in the decom-
position of (TMS)3P.
To account for the mechanism of amine inhibition, we
measured the initial reaction rate at various temperatures;
these data are summarized in an Eyring plot (Figure 3). The
activation entropy indicates that the reaction proceeds via an
Upon increasing the octylamine concentration from 6:1 to
18:1, the rate of (TMS)3P depletion continued to decrease
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apparent that the challenges in the synthesis of III–V
(Group 13/15) QDs are not due to differences in the bonding
between II–VI or IV–VI and III–V semiconductors, but
instead result from the depletion of molecular precursors
following QD nucleation in current III–V QD syntheses.
Experimental Section
Reactions were performed in 600 MHz J-Young NMR tubes. [D8]-
toluene, [D4]-1,2-dichlorobenzene (C.I.L.), diphenylmethane, octyl-
amine (Fluka), and all the reaction solutions were stored over 4 ꢀ
molecular sieves prior to use. (TMS)3P (Strem) was used without
further purification, and In(MA)3 was prepared as previously
reported.[17] NMR samples were prepared in [D8]toluene by mixing
0.35 mL of 0.04m In(MA)3 , 0.06m diphenylmethane, and 0.0–1.44m
octylamine with 0.35 mL of 0.02m (TMS)3P in a nitrogen-filled
glovebox, under minimal lighting, and then immediately transferred
into a variable-temperature 500 MHz Varian NMR for 1H and 31P
spectroscopic analysis. A detailed explanation of the kinetic analysis
and additional experimental details can be found in the Supporting
Information.
Figure 3. Eyring plot for amine-based synthesis of colloidal InP QDs,
with DH° =51.9Æ1.3 kJmolÀ1 and DS° =À126Æ4 JmolÀ1 KÀ1. Each
data point was acquired three times, and temperature error was
negligible.
Received: October 7, 2009
Published online: December 18, 2009
ordered transition state, which is likely from an arrangement
of two or more species. The activation enthalpy (DH° =
(51.9 Æ 1.3) kJmolÀ) is about 10 kJmolÀ1 less than that
measured by Liu et al. for CdSe nanocrystal formation,[7]
which is plausible for the highly reactive precursors used
during InP QD synthesis. The addition of octylamine may
decrease the rate (and increase DG°) by stabilizing the
enthalpy of precursors (increasing DH°), but octylamine may
also influence the activation entropy.
The amine inhibition of InP synthesis results from
solvation effects during the steps that lead to complex 1, 2,
or both. As the concentration of a competing Lewis base
increases, the sterically hindered (TMS)3P molecule is less
likely to approach the indium center. Therefore, an increase
in amine concentration decreases the formation of 1, and the
observed saturation behavior may arise from crowding of the
In(MA)3 ligand sphere.
Keywords: indium phosphide · reaction mechanisms ·
.
nanocrystals · nucleation · quantum dots
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The large negative DS° = (À126 Æ 4) JmolÀ1 KÀ1 and the
more than thousandfold decrease in rate upon addition of
amines are also consistent with solvation changes during
nucleophilic substitution.[16] Strong solvation effects are rarely
observed for isopolar transition states (e.g. for pericyclic
reactions), so it is unlikely that the route from 1 to 2 occurs in
a single step. A potential step for this reaction is via a SN2
transition state that results in charge dispersion. During
charge-dispersive SN2 reactions, charged species react to form
uncharged products. Because octylamine can hydrogen-bond
more strongly with the attacking nucleophiles than with 2, an
increase in amine concentration hinders the formation of the
transition state, and thus decreases the formation of 2.
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syntheses are based on non-molecular ripening processes. We
demonstrated that amines inhibit precursor decomposition,
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