Versatile Tri(pyrazolyl)phosphanes as Phosphorus Precursors for the Synthesis of Highly Emitting InP/ZnS Quantum Dots

Article abstract of DOI:10.1002/anie.201705650

Tri(pyrazolyl)phosphanes (5^R1,R2) are utilized as an alternative, cheap and low-toxic phosphorus source for the convenient synthesis of InP/ZnS quantum dots (QDs). From these precursors, remarkably long-term stable stock solutions (>6 months) of P(OLA)₃ (OLAH=oleylamine) are generated from which the respective pyrazoles are conveniently recovered. P(OLA)₃ acts simultaneously as phosphorus source and reducing agent in the synthesis of highly emitting InP/ZnS core/shell QDs. These QDs are characterized by a spectral range between 530–620 nm and photoluminescence quantum yields (PL QYs) between 51–62 %. A proof-of-concept white light-emitting diode (LED) applying the InP/ZnS QDs as a color-conversion layer was built to demonstrate their applicability and processibility.

Full text of DOI:10.1002/anie.201705650

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Accepted Article
Title: Versatile Tri(pyrazolyl)phosphanes - Application as phosphorus
precursors for the synthesis of highly emitting InP/ZnS quantum
dots
Authors: Jan J. Weigand, Alexander Eychmüller, Rene Panzer, Chris
Guhrenz, Rene Hübner, Nikolai Gaponik, and Danny Haubold
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705650
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Versatile Tri(pyrazolyl)phosphanes – Application as phosphorus
precursors for the synthesis of highly emitting InP/ZnS quantum
dots
René Panzer,[a] Chris Guhrenz,[b] Danny Haubold,[b] René Hübner,[c] Nikolai Gaponik,[b] Alexander
Eychmüller,*[b] and Jan J. Weigand*[a]
Abstract: Tri(pyrazolyl)phosphanes (5R1,R2
) are utilized as an
alternative, cheap and low-toxic phosphorus source for the convenient
synthesis of InP/ZnS quantum dots (QDs). From these precursors,
remarkably long-term stable stock solutions (>6 months) of P(OLA)3
(OLAH = oleylamine) are generated from which the respective
pyrazoles are conveniently recovered. P(OLA)3 acts simultaneously
as phosphorus source and reducing agent in the synthesis of highly
emitting InP/ZnS core/shell QDs. These QDs are characterized by a
spectral range between 530–620 nm and photoluminescence
quantum yields (PL QYs) between 51–62%. A proof-of-concept white
light-emitting diode (LED) applying the InP/ZnS QDs as color
conversion layer was built to demonstrate their applicability and
processibility.
INTRODUCTION
Scheme 1. Application of 1–5 for the synthesis of InP QDs and application of
5Me,Me in P-P bond formation reactions; I) 2 Cy2PH (Cy = cyclohexyl), −2 PyrH,
In recent years InP-based QDs became the most highlighted
candidates for a less-toxic surrogate for Cd-[1] and Pb-based[2]
photoelectric devices.[3] Despite the improvements of InP-based
QDs with regard to PL QY, narrow full-width at half-maximum
(FWHM) and photo-stability,[4] the pool of phosphorus precursors
(P1-sources) is still quite limited. Especially for applications on an
industrial scale the search for a less expensive, harmless and
CH3CN; II)
3 Cy2PH, CH3CN, Pyr = 3,5-dimethyl-1-pyrazolyl; III) 5
PhPH(CH2)2PHPh, 4 5Me,Me -PhP(Pyr)(CH2)2P(Pyr)Ph, -10 PyrH, CH3CN; IV)
,
InCl3, ZnCl2, OLAH, - 3 PyrH.
applied successfully for the synthesis of highly emitting InP/ZnS
core/shell QDs so far.[10] Tessier et al.[11] and Buffard et al.[12]
reported independently that 4 undergoes an in situ transamination
reaction with OLAH as key solvent for the InP core synthesis to
yield P(OLA)3 and HNMe2. The formed P(OLA)3 acts
simultaneously as phosphorus source and reducing agent during
the formation of the InP QDs.[11-12]
more environmental friendly phosphorus precursor is of great
[7]
interest. PH3 (1)[5], (K/Na)3P (2)[6] and P4
are used as
phosphorus source for the synthesis of III-V QDs, however, show
a series of severe drawbacks. Nowadays most commonly used is
the hazardous and expensive tris(trimethylsilyl)phosphane
In our continuous efforts to use pyrazolyl-substituted phosphorus
compounds as efficient P1-source we demonstrated that the
methyl-derivative of 5R1,R2 (R1,2 = Me) acts as a remarkably
efficient P1-building block in P–P bond forming reactions.[13] Thus,
the reaction of 5Me,Me with Cy2PH (Scheme 1) gives access to
triphosphane 6 (I) and isotetraphosphane 7 (II) depending on the
applied stoichiometries.[14] We also reported the reaction of 5Me,Me
8]
((Me3Si)3P, 3)[4b,
which also requires special handling and
storage precautions (Scheme 1). Intensive research deals with
5e, 6b, 7b, 9]
the exploration of other suitable P1-sources[5d,
but
besides
these
efforts,
it
appears
that
only
tris(dimethyl)aminophosphane (P(NMe2)3, 4) has been
with
1,2-bis(phenylphosphino)ethane
which
yields
hexaphosphane 8 and 1,2-bis(phenylpyrazolylphosphino)ethane
in an unexpected P–N/P–P bond metathesis reaction (III).[15]
Those versatile reactivities are attributed to the labile P–N bonds
of the pyrazolyl-subsituted phosphanes. Thus, we envisioned that
derivatives 5R1,R2 should be suitable phosphorus precursors for
the synthesis of high-quality InP QDs (IV). Herein, we report on
the facile access to a variety of tri(pyrazolyl)phosphanes 5R1,R2
which are suitable for the generation of a long-term stable stock
solution of P(OLA)3 (>6 months) and their application for the
synthesis of high-quality InP QDs.
[a] Prof. Dr. Jan J. Weigand, Rene Panzer, TU Dresden,
Professur für Anorganische Molekülchemie
01062 Dresden (Germany)
E-mail: jan.weigand@tu-dresden.de
[b] Prof. Dr. Alexander Eychmüller, Prof. Dr. Nikolai Gaponik, Chris
Guhrenz, Dr. Danny Haubold, TU Dresden
Professur für Physikalische Chemie
01062 Dresden (Germany)
E-mail: alexander.eychmueller@chemie.tu-dresden.de
[c] Dr. René Hübner
Institute of Ion Beam Physics and Materials Research
Helmholtz-Zentrum-Dresen-Rossendorf
01328 Dresden (Germany)
Supporting Information
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Scheme 2. Synthesis of tri(pyrazolyl)phosphanes 5R1,R2 according to method I)
neat, 16 h, RT or II) THF, 5iPr,tBu (16 h, RT) and for 5tBu,tBu (24 h, reflux).
Tri(pyrazolyl)phosphanes 5R1,R2 are obtained via a modified
procedure of Fischer et al.[16] from the reaction of PCl3 with either
trimethylsilylpyrazoles 9R1,R2 (Scheme 2, I; Table 1, entries 1–6)
for less sterically demanding substituted phosphanes or from the
sodium salts 10R1,R2 for sterically encumbered derivatives
(Scheme 2, II; Table 1, entries 7 and 8). Both reaction pathways
provide high yields (88–98%) and the compounds can be
obtained on a multi-gram scale. The reaction involving derivatives
of 9R1,R2 with PCl3 is best performed under solvent-free conditions
and analytically pure products are conveniently obtained after
removal of all volatiles under vacuum. Dropwise addition of PCl3
to a cooled solution of sodium pyrazolates 10R1,R2 in THF affords
after stirring at room temperature for 16 h (10iPr,tBu) or refluxing for
24 h (10iPr,tBu) the sterically demanding phosphanes 10iPr,tBu and
10tBu,tBu, respectively.[17]
Scheme 3. Proposed reaction pathway for the synthesis of InP QDs applying
tri(pyrazolyl)phosphanes 5R1,R2, R = C18H35; I) 50°C, vacuum; II) 9R1,R2 (12 NEt3,
12 TMSCl, Et2O, RT), 10R1,R2(12 NaH, THF, RT).
Tessier et al.[11] and Buffard et al.[12] propose different reaction
pathways, but both point out that 11 is the reactive species for the
formation of InP QDs. According to our investigations the
dissolution of 5Me,Me under reduced pressure yields a remarkably
stable solution of 11. The formed by-product 3,5-dimethylpyrazole
is conveniently and completely removed via sublimation and can
be recovered in high yields (>92%). The obtained clear, colorless
solution is stable for several months (>6 months) when stored
under inert atmosphere. Thus, it represents a handy stock
solution which only contains minor amounts of the tautomer 11´
and the amino-bridged diphosphane 13 (Figure 1).
Compared to the reaction involving 4 as P1-source, the dissolution
of tri(pyrazolyl)phosphanes 5R1,R2 (Table 1, entries 1–7) in OLAH
yields aminophosphanes 11 and the corresponding pyrazoles
(Scheme 3; SI, Scheme S1). No reaction is observed in the case
of 5tBu,tBu (Table 1, entry 8; SI, Figure S2) even after prolonged
heating. We attribute this observation to the steric demand of the
tBu substituents. A substantial improvement compared to the
synthesis with 4 is the drastically reduced toxicity of the starting
material and that most of the liberated pyrazoles can be recycled
and reused for the synthesis of 5R1,R2. This contributes positively
towards a desirable waste-management. The prevention of high
amounts of gaseous, corrosive and flammable dimethylamine
reduces the hazardous and environmental risks drastically as well.
For derivatives 5,CF3 and 5,CF3 the recovery of the corresponding
pyrazoles is problematic since their increased acidity leads to an
adduct formation with the solvent OLAH (SI, Figure S1).[17]
However, the redox-reaction of 11 with InCl3 yields three
equivalents of the phosphonium salt 12[Cl] and InP (Scheme 3).
Figure 1. 31P-NMR spectra of the P(OLA)3 stock solution after 1 day, 2 weeks,
3 and 6 months showing the long-term stability of the stock solution.
Similarly to 4, 5Me,Me can also be applied for the in situ synthesis
of InP QDs. For the hot injection technique we can now either
rapidly inject a hot solution of phosphane 5Me,Me in OLAH under
static vacuum (synthetic route 1, SR1) or inject the previously
prepared 0.5 mmol stock solution of aminophosphane P(OLA)3 in
OLAH (synthetic route 2, SR2).[17] Both methods have been
performed in our labs and are compared in the following
discussion. We found, that the application of the P(OLA)3 stock
solution (SR2) contributes more positively to the development of
a well defined LaMer-type growth for InP QDs, approaching more
conveniently the narrow FWHM of the established II-VI and IV-VI
semiconductor QDs. The growth of the QDs can be evidenced for
both cases by the red-shift in the absorption spectra and the color
change of the QD solutions (Figure 2A). As can be seen from the
absorption spectra in Figure 2A the characteristic excitonic
feature of the InP QDs at ~450 nm starts to develop after 10 min
using static vacuum for the removal of the pyrazole (SR1). The
growth of the QDs slows down after 60 min reaching a maximum
at ~560 nm, which exhibits generally the complete consumption
Table 1. Yields and 31P-{1H} NMR chemical shifts of tri(pyrazolyl)phosphanes
5R1,R2 obtained according to Scheme 2.
5R1,R2
entry
Yield
[%]
(31P)
[ppm]
Method
R1
R2
1a
2b
3c
4f
I
I
H
Me
iPr
Ph
H
H
92
98
98
88
96
95
84
90
61.1
71.9
69.4
74.8
63.9
76.4
66.8
80.6
Me
iPr
I
I
Ph
5g
6h
7d
8e
I
CF3
CF3
tBu
tBu
I
Me
iPr
tBu
II
II
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of the phosphorus precursor. However, monitoring the reaction by
31P-NMR spectroscopy reveals that after 90 min reaction time
there is still compound 11 present (Figure 2B), possibly implying
an interfering effect of the released pyrazole. In contrast, applying
the prior synthesized stock solution (SR2), the reaction proceeds
remarkably clean (Figure 2D), indicating a better separation of
nucleation and growth and, therefore, a more LaMer-type
reaction.[18] The results for the first absorption peak maximum
(Figure 2C) are comparable with prior publications using
P(NMe2)3,[10b, 11-12] but show slower reaction kinetics. The resulting
InP QDs are not luminescent as already observed for the
synthesis using P(NMe2)3 as P1-source. However, in order to get
highly fluorescent QDs a passivating ZnS shell can be grown on
the core QDs. To guarantee a successful ZnS overcoating of the
InP cores and to exhibit a high stability and performance an
excess of zinc chloride, elevated temperatures (260°C) and a long
shelling time (4 h) were chosen.[17] Due to an undesired side
reaction with the couple product [P(OLA)4]Cl (12), 1-
Dodecanethiol (DDT) is used in excess, as sulfur source. Detailed
31P-NMR investigations revealed that DDT reacts with the
phosphonium salt 12 to thiophosphorictrioleylamine 14 under the
harsh reaction conditions (Figure 2D, (III) and SI, Figure S3).[17]
After successful ZnS shelling highly emitting QDs displaying a
green to red emission are obtained. As can be seen from Figure
3 the emission wavelength can be tuned between ~535–620 nm
with a FWHM of 66–81nm. The corresponding absorption spectra
of the InP/ZnS QDs are given in Figure S5[17] showing an increase
of the optical density at ~400 nm resulting from the ZnS shell.
Absolute PL QYs of the core/shell QDs are reaching 51–62%. In
comparison, applying P(Pyr)3 in situ (SR1) results also in the
formation of non-fluorescing InP QDs which, after passivation with
ZnS and development of a type-I band alignment, gain PL QYs
ranging between 33–58% covering the same spectral range
(530–620 nm, SI, Figure S6).[17] The broader variation in PL QYs
and FWHMs (59–90 nm) may be explained by the lower reaction
rates causing formation of a larger number of trap states. The
results for both synthetic techniques are summarized in Table
S1.[17] The successful growth of the ZnS shell was evidenced by
Figure 2. Temporal evolution of the absorption spectra of the InP QDs using
SR1 (A) or SR2 (C). Inset B shows the stacked 31P-NMR spectra after 30, 60,
and 90 min reaction time applying SR1. Inset D shows the stacked 31P-NMR
spectra following SR2: stock solution (I), after 15 min reaction time (II), after
shelling with DDT (III). 14 occurs from a side reaction of DDT with 12.
fringes throughout the whole nanoparticle which indicates a high
crystallinity of the final core/shell material. The measured lattice-
fringe distance of 0.32 nm corresponds well with the (111) lattice
plane of the ZnS zinc blende structure (JCPDS # 5-566). The
comparison of the X-ray diffraction (XRD) patterns of the InP and
the InP/ZnS QDs exhibits also the successful ZnS shelling of the
InP cores (Figure 4, left). In particular, the XRD pattern of the InP
QDs depicts the 111, 220, and 311 Bragg reflections of the cubic
zinc blende structure. After shelling Bragg reflexes of the thick
ZnS coating superimpose the diffraction maxima of the InP cores
(Figure 4, left). Hence, only the most prominent 111 reflex of the
InP/ZnS QDs shows a shoulder at lower diffraction angles to the
peak maximum (enlargement in SI, Figure S7).[17] The additional
shift of the InP 111 reflex to higher 2 theta values might result from
the incorporation of zinc into the InP lattice leading to an
intermixed In(Zn)P core structure. Previous reports already
mentioned that during the growth of InP QDs zinc can be
implemented into the QD lattice.[19] ICP-OES measurements of
our InP QDs revealed an element composition of In : P : Zn =
0.47 : 0.46 : 0.07 indicating no major assembly of zinc before
shelling. Nevertheless, an alloyed In(Zn)P QD core allows better
transmission electron microscopy (TEM). In Figure
3
representative bright-field TEM images of the pure InP cores (A)
and the resulting InP/ZnS core/shell QDs (B-C) are displayed.
According to TEM, the average size of the pure InP QDs is
between ~3.2 nm (15 min growth time) and ~6.5 nm (60 min
growth time). As a result of ZnS shelling the spherical InP QDs
(Figure 3A) obtain a tetrahedral shape (Figures 3B–C). HRTEM
measurements of the InP/ZnS QDs show continuous lattice
lattice engineering for the subsequent strain-free, epitaxial shell
[19b]
growth as demonstrated for ZnSezSz-1
.
Figure 3. PL spectra of InP/ZnS QDs synthesized via SR2 (left) and TEM bright-
field-images of (A) InP and (B–C) InP/ZnS QDs.
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Figure 4. XRD of InP and InP/ZnS QDs (left); HAADF-STEM image of the
synthesized InP/ZnS QDs and the corresponding EDXS data for the distribution
of In, Zn, P, and S (right).
Additional XAS (X-ray absorption spectroscopy) measurements
by Cho et al.[21] revealed that the In–Zn bond becomes more
pronounced with increasing ZnS shelling time. This was attributed
to additional bonding between the slow growing InP/ZnS
core/shell interface.
In particular, the implemented zinc reduces the lattice mismatch
resulting in higher PL QYs of the final core/shell structure. Figure
Due to the lower toxicity in comparison to commonly used Cd-
based materials, highly emitting InP/ZnS QDs are very attractive
for solid state lighting applications.[4b, 22] As suitable demonstrator
we produced a proof-of-concept white LED and mounted it into a
conventional pocket torch (SI, Figure S8).[17]According to a
previous publication we incorporated the resulting green- and
orange-emitting InP/ZnS QDs into a protective KCl salt matrix.[23]
Pressing the NC-loaded salt under cold flow conditions at 2.2 GPa
results in composite materials which can be used as color
conversion layers when stacked on top of a commercially
available blue-emitting LED (Figure 5, bottom). The
corresponding PL spectrum and CIE diagram (x = 0.337, y =
0.333) of the white LED are shown in Figure 5 (top).
In summary, with the advantageous synthesis of a library of novel
tri(pyrazolyl)phosphanes 5R1,R2, we introduced versatile P1-
building blocks for the direct synthesis of InP core QDs. Most
importantly the combination of a convenient access to a long-term
stable P(OLA)3 stock solution (> 6 month) with a high-yield
recovery of the pyrazole (> 90 %) provides an auspicious new
approach for the synthesis of InP QDs. A possible extension to
other phosphide-based QDs such as AlP, GaP, Zn3P2 or Cd3P2 is
currently addressed. After shelling the resulting InP/ZnS
core/shell QDs are covering a broad spectral range from 530–620
nm showing high PL QYs up to 60%. Applying the highly emitting
InP/ZnS QDs as color conversion layers, we manufactured a
proof-of-concept white LED pocket torch showing the applicability
and processibility of the core/shell QDs. The innovative access to
a storable P(OLA)3 stock solution is given and leads to the
development of a defined LaMer-type growth for InP QDs.
Moreover, an extension to As(Pyr)3 is of great interest and will be
in the focus of our forthcoming paper.
4, right shows
a high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) image of the
resulting InP/ZnS QDs. From the corresponding EDXS (energy-
dispersive X-ray spectroscopy) data, obtained for the rectangular
region marked in the HAADF-STEM image, successful ZnS
shelling is evidenced. It is, however; not possible to observe
separated InP cores. Rather indium and zinc seem to be
distributed over the whole inner QD core. Besides true intermixing
of both elements, as discussed above, 2-dimensional data
recording in the TEM from the 3-dimensional tetrahedrally shaped
QDs leads to simultaneous acquisition of In and P from the core
region as well as of Zn and S from the corresponding shell volume
in front and behind the core. Furthermore it should be mentioned
that next to phosphorus rich regions high amounts of indium and
zinc are observed. Due to the high reaction temperature (260°C),
interdiffusion and partial ion exchange of In3+ and Zn2+ during ZnS
shelling is resulting in an alloyed interface layer between the
In(Zn)P core and the pure ZnS shell. This is in agreement with
published XPS (X-ray photoelectron spectroscopy) data following
a two-step approach for the synthesis of InP/ZnS QDs.[20]
Acknowledgements
R.P. and J.J.W. gratefully acknowledge financial support from the
European Research Council (ERC) (SynPhos, project number
307616). C.G., N.G. and A.E. gratefully acknowledge financial
support from the M-ERA.NET Network (ICENAP, project number
GA 1289/3-1) and the European Research Council (AEROCAT,
ERC-2013-AdG). D.H. and A.E. gratefully acknowledge the
financial support from the Graduate Academy of Technische
Figure 5. PL spectra of the manufactured white LED (top left) with the
corresponding CIE 1931 plot (top right); LED under ambient conditions (bottom
left) and in operation (bottom right).
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Universität Dresden as well as the Excellence Initative by the
German Federal and by the Deutsche Forschungsgemeinschaft
through the research training group Nano- and biotechnologies
for packaging of electronic systems (project number DFG 1401/2).
Furthermore, the use of HZDR Ion Beam Center TEM facilities
and the funding of TEM Talos by the German Federal Ministry of
Education of Research (BMBF), Grant No. 03SF0451 in the
framework of HEMCP are gratefully acknowledged.
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10.1002/anie.201705650
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
René Panzer,[a] Chris Guhrenz,[b]
Tri(pyrazolyl)phosphanes are utilized
for the convenient synthesis of long-
Danny Haubold,[b] René Hübner,[c]
Nikolai Gaponik,[b] Alexander
term stable stock solutions
of
=
Eychmüller,*[b] and Jan J. Weigand*[a]
P(OLA)3 (>6 months, OLAH
Page No. – Page No.
oleylamine) for the preparation of
InP/ZnS quantum dots in the spectral
range between 530–620 nm reaching
photoluminescence quantum yields
between 51–62%. The liberated
pyrazoles can easily be recovered
and reused contributing positively to a
sustainable waste-management.
Versatile Tri(pyrazolyl)phosphanes –
Application
as
phosphorus
precursors for the synthesis of highly
emitting InP/ZnS quantum dots
This article is protected by copyright. All rights reserved.