Fragmentation of Magic-Size Cluster Precursor Compounds into Ultrasmall CdS Quantum Dots with Enhanced Particle Yield at Low Temperatures

Article abstract of DOI:10.1002/anie.202001608

Colloidal small-size CdS quantum dots (QDs) are produced usually with low particle yield, together with side products such as the particular precursor compounds (PCs) of magic-size clusters (MSC). Here, we report our synthesis of small-size CdS QDs without the coexistence of the PC and thus with enhanced particle yield. For a conventional reaction of cadmium oleate (Cd(OA)₂) and sulfur (S) in 1-octadecene (ODE), we show that after the formation of the PC in the pre-nucleation stage, the addition of tri-n-octylphosphine oxide (TOPO) facilitates the production of small-size QDs. We demonstrate that TOPO fragmentizes the PC that have formed, which enables the nucleation and growth of small-size QDs even at room temperature. Our findings introduce a new approach to making small-size QDs without the coexistence of the PC and with improved particle yield. Providing experimental evidence for the two-pathway model proposed for the pre-nucleation stage of colloidal binary QDs, the present study aids in the advance of non-classical nucleation theory.

Full text of DOI:10.1002/anie.202001608

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How to cite:
International Edition: doi.org/10.1002/anie.202001608
German Edition: doi.org/10.1002/ange.202001608
Nanoparticle Synthesis Hot Paper
Fragmentation of Magic-Size Cluster Precursor Compounds into
Ultrasmall CdS Quantum Dots with Enhanced Particle Yield at Low
Temperatures
Lijia Li, Jing Zhang, Meng Zhang, Nelson Rowell, Chunchun Zhang, Shanling Wang, Jiao Lu,
Hongsong Fan, Wen Huang, Xiaoqin Chen,* and Kui Yu*
Abstract: Colloidal small-size CdS quantum dots (QDs) are
produced usually with low particle yield, together with side
products such as the particular precursor compounds (PCs) of
magic-size clusters (MSC). Here, we report our synthesis of
small-size CdS QDs without the coexistence of the PC and thus
with enhanced particle yield. For a conventional reaction of
by various side products, which are magic size clusters (MSCs)
and/or the particular precursor compounds (PCs) of the
[
13–21]
MSCs.
The CdS MSC-related PC does not have a distinc-
tive optical absorption feature, but exhibits a matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF)
mass spectrometry (MS) peak (at ꢀ 5167 Da), which is similar
to that of the CdS MSC. For this reason, it has been assumed
that the ME PC and its corresponding ME MSC form a pair of
cadmium oleate (Cd(OA) ) and sulfur (S) in 1-octadecene
2
(
ODE), we show that after the formation of the PC in the pre-
nucleation stage, the addition of tri-n-octylphosphine oxide
TOPO) facilitates the production of small-size QDs. We
[
18–29]
polymorphs with the same ME core composition.
(
For samples extracted in sequence from the reactions, the
CdS QDs exhibit broad optical absorption peaks, which keep
red-shifting as their size increases. The CdS MSCs display
sharp optical absorption peaking at 311 and/or 322 nm, which
are referred to as MSC-311 and/or MSC-322, respectively.
Recently, advances have been made in understanding the PC
demonstrate that TOPO fragmentizes the PC that have formed,
which enables the nucleation and growth of small-size QDs
even at room temperature. Our findings introduce a new
approach to making small-size QDs without the coexistence of
the PC and with improved particle yield. Providing exper-
imental evidence for the two-pathway model proposed for the
pre-nucleation stage of colloidal binary QDs, the present study
aids in the advance of non-classical nucleation theory.
[
18]
to MSC transformation, the growth relationship between
the QDs and MSCs with a two-pathway model for the pre-
[
19]
nucleation stage of colloidal binary QDs, the thermally-
induced and chemically-induced changes between MSC-311
[
22,30]
Introduction
and MSC-322,
and how to improve the production yield
[
20]
of MSC-311. Until the present study, it has not been known
whether or not it is possible to synthesize small-size CdS QDs
with enhanced production yield, without the coproduction of
the MSCs and PCs.
Colloidal semiconductor quantum dots (QDs) with size-
dependent optical properties are promising for biomedical-
related, energy-associated, and environmental-oriented ap-
[
1–6]
plications.
The effort in synthesis has been focused
For the two-pathway model of II–VI metal (M) chalco-
[19]
essentially on size control, where the QD dimensions are
mainly regulated through reaction temperature and duration
in both conventional hot-injection and non-hot-injection
genide (E) QDs, one pathway involves the formation of MÀ
[10,12,31,32]
E covalent bonds and thus monomers and fragments.
Following the lines of Lamer model of the classical nucleation
[
7–9]
[33–37]
approaches.
In such cases, small-size metal chalcogenide
theory (CNT),
the nucleation and growth of QDs occurs.
(
ME) QDs are produced only with a low production yield
The other pathway starts with the self-assembly of M and E
(
such as < 2% for CdSe), during initial stages of reactions at
precursors, which is followed by the formation of MÀE
[
10–12]
relatively low temperatures in short time periods.
size CdS QDs have been shown to be produced accompanied
Small-
covalent bonds that takes place inside the assembles forming
[
23,24]
the PCs.
The PC to MSC transformation via intra-
[
*] L. Li, J. Lu, H. Fan, X. Chen, K. Yu
Engineering Research Center in Biomaterials, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
E-mail: xqchen@scu.edu.cn
W. Huang
Laboratory of Ethnopharmacology, West China School of Medicine
Chengdu, Sichuan, 610065 (P. R. China)
and
kuiyu@scu.edu.cn
West China Hospital, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
J. Zhang, M. Zhang, K. Yu
Institute of Atomic and Molecular Physics, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
K. Yu
State Key Laboratory of Polymer Materials Engineering, Sichuan
University, Chengdu, Sichuan, 610065 (P. R. China)
N. Rowell
Metrology Research Centre, National Research Council Canada
Ottawa, Ontario K1A 0R6 (Canada)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202001608.
C. Zhang, S. Wang
Analytical and Testing Center, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
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molecular re-organization can occur at a relatively low
[
18–22,23–28]
temperature in a two-step approach
or at a relatively
[
19,20,29]
high reaction temperature in a one-step approach.
It is
useful to note that the various binary ME PCs follow the same
growth pathway. The two-pathway model has distinct sim-
ilarities with the multi-step nucleation model reported for
several other material systems including calcium-based in-
[
38–42]
[43–45]
[46–48]
[49–51]
organics,
organics,
polymers,
and metals.
Scheme S1 (Supporting Information) presents our compre-
hension on the various nucleation models proposed. It is
noteworthy that the PC is reminiscent of the oligomer
[38,41]
(
CaCO ) (n = 3–11) and (Ca (PO ) ) reported.
In par-
3
n
3
4 2 3
ticular, (CaCO ) has been used as a starting molecule (via
3
n
[38]
crosslinking) to synthesize pure monolithic CaCO3. Until
the present study, there has been no reported use of the CdS
PC molecule to synthesize small-size CdS QDs, which takes
advantage of the CdÀS covalent bonds that have already
formed.
Here, we report the fragmentation of the CdS PC by tri-n-
octylphosphine oxide (TOPO), which results in the nuclea-
tion and growth of small-size CdS QDs with enhanced particle
yield (Scheme 1). For a conventional reaction of cadmium
oleate (Cd(OOC-C H ) , Cd(OA) ) and elemental sulfur (S)
1
7
33
2
2
in 1-octadecene (ODE), we synthesize the PC first, and then
add TOPO (1.32 mmol) into a pre-nucleation stage sample.
For all the reaction batches, the feed molar ratio of Cd(OA)2
(
0.60 mmol) to S (0.15 mmol) was fixed at 4 to 1, with a S
À1
À1
concentration of 30 mmolKg (or 24 mmolL ) and a total
weight of the reaction mixture of 5.0 g. We demonstrate that
the addition of TOPO to a pre-nucleation stage sample that
contains the PC enables the nucleation and growth of small-
size QDs with enhanced particle yield and without the
complication of the coexistence of the PC and/or MSCs.
When TOPO is present prior to the PC formation, the
nucleation and growth of QDs happens at a relatively high
tmeperature (1808C) (Figure 1). When TOPO is added to
a prenucleation stage sample at room temperature (Figure 2)
or at 1208C (Figure S2-3) after the PC has formed, the
nucleation and growth of CdS QDs occurs at room temper-
ature or at 1208C, respectively. The particle yield obtained is
higher when TOPO is added after the PC formation than if
the additon is performed before the PC formation (Figure 3).
Scheme 1. Schematic drawing for our comprehension on the use of
TOPO to fragmentize the PC that has formed in a prenuclation stage
sample to facilitate the nucleation and growth of colloidal small-size
CdS QDs with enhanced particle yield and without the coexistence of
the PC and/or MSCs. Without TOPO, the reaction produces QDs at
1208C, together with the MSC and the PC. Regarding the two-pathway
model proposed for the prenuclation stage of QDs, one pathway leads
to QDs via monomers (M) and fragments (F). The other pathway
results in MSCs via the intramolecular transformation of their PC
molecules. The two pathways are interconnected and the growth of
[
16]
QDs in size faciliates the fragmentation of the PC. To the best of our
knoweldge, the present study is the first, reporting on the synthesis of
small-size QDs with improved particl yield using the PC formed.
We validate further when a seoncdary phosphine HPPh is
2
used to activate S, the addition of TOPO in the prenucleation
stage after the PC formation also leads to small-size CdS QDs
with enhanced particle yield, which are free of CdS MSCs and
the PC (Figure 4). Our findings introduce a new avenue to the
fragmentation of the CdS PC to effect the nucleation and
growth of small-size CdS QDs with enhanced particle yield
and without the coexistence of the PC and/or MSCs,
contributing to the advance of the non-classical multi-step
and 2208C (3, right panel). When the Cd precursor in ODE
was heated to 908C, S was added followed by TOPO
(1.32 mmol) (Batch a, top panel) or S only (Batch b, bottom
panel). Afterwards, the reaction temperature was increased to
1208C. Samples were taken after 10 min at each temperature
(increased in steps of 208C) up to a maximum of 2208C. For
the spectroscopy study, an aliquot of 25 mL of each as-
synthesized sample was dispersed in 3.0 mL of toluene (Tol),
and the spectra were collected immediately upon dispersion
[
19,21,38–51]
nucleation models.
(blue traces, 0 day) and later following a one-day incubation
in Tol (red traces, 1 day). Figures S1-1a and S1-1b show the
optical absorption spectra of the six samples from Batches
a and b, respectively, extracted in sequence during the
temperature increase from 120 to 2208C.
Results and Discussion
In Figure 1, we present optical absorption spectra of
samples that were extracted from two reactions of Cd(OA) +
S in ODE at 1808C (1, left panel), 2008C (2, middle panel),
For Batch a (top panel), nucleation and growth of QDs
occurred around 1808C. The first two samples extracted at
2
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Figure 1. Optical absorption spectra of CdS samples extracted from two conventional reactions, Batches a (top panel) and b (bottom panel).
When the Cd precursor (0.60 mmol) in ODE was heated up to 908C, the addition of S (0.15 mmol) followed by TOPO (1.32 mmol) (a) or S
(
0.15 mmol (b) was performed. From 120 to 2208C with steps of 208C, six samples were taken from each batch after 10 min had elapsed at each
temperature. Each sample (25 mL) was dispersed in 3.0 mL of Tol, and the spectra were collected upon dispersion (0 day, blue traces) and after
a one-day incubation in Tol (1 day, red traces) (Figure S1-1). Results for the last three samples from the two batches are shown here. With the
presence of TOPO (Batch a), no significant CdS MSC PC formed, and the nucleation and growth of QDs occurred at 180 instead of 2208C
(
Batch b without).
1
20 and 1408C (Figure S1-1a) exhibited nearly featureless
160 to 2008C, the optical density at 311 nm of the samples
became larger, indicating that the population of MSC-311 had
increased considerably. Figure 1b indicates clearly that for
optical absorption. However, the last three samples (Figure 1,
blue traces) displayed broad absorption peaks, which red
shifted continuously from 398 (a1) to 426 nm (a3), with peak
optical density increasing from 0.03 to 0.05. After a one-day
incubation, the optical absorption properties changed little
for the first two (Figure S1-1a) and the last three samples
the conventional reaction of Cd(OA) and S in ODE, the QD
2
product contains a side product comprised of the PC for
[
18]
MSC-311.
Thus, with the presence of TOPO (Batch a, added at
908C), the onset of nucleation and growth of QDs took place
at a relatively low temperature (1808C). Without TOPO
(Batch b), the onset of nucleation and growth of QDs
occurred at a relatively high temperature (2208C). Further-
more, after a one-day incubation, CdS MSCs were not seen in
the samples from Batch a, but were observed in the samples
from Batch b (extracted at temperatures higher than 1608C).
Therefore, we conclude that the PC of MSCs was not present
(
Figure 1, red traces). The 1608C sample (Figure S1-1a)
appeared to give rise to an absorption doublet peaking at
340/352 nm, which disappeared with the presence of a broad
ꢀ
absorption bump after the one-day incubation.
For Batch b (bottom panel), the first five samples
exhibited almost featureless optical absorption (Figures 1b
and S1-1b). Nucleation and growth of QDs took place at
2
208C, a sample of which displayed a broad absorption
[18]
peaking at 384 nm, with a peak optical density of 0.12. After
a one-day incubation, the optical absorption of the first two
samples changed little. For the remaining four samples, an
absorption peaking at ꢀ 311 nm evolved, suggesting the
presence of MSC-311. With the temperature increase from
in the former batch, but was contained in the latter batch.
Figure S1-2 presents the optical absorption spectra of samples
from two reactions with the use of the 0.5TOPO to 1.0Cd and
1.0TOPO to 1.0Cd feed molar ratios. The formation of a small
amount of the PC was observed for the former batch. Also,
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Figure 2. Optical absorption spectra collected (offset) from one CdS prenucleation stage product (obtained from the reaction of Cd(OA) and S in
2
ODE which was heated at 1608C for 30 min). After a sample (1.0 mL) at room temperature was mixed with a mixture of TOPO (0.1 g) and ODE
(
1.0 g) (1, dark blue traces) or with ODE (1.0 g) (2, light blue traces), 25 mL of each of the two resulting mixtures was dispersed in 3.0 mL of Tol.
The spectra were collected immediately after dispersion (left, a) and later after a five-day incubation in Tol (middle, b). Also, the two sample
mixtures were incubated at room temperature for five days, and gelation was observed for Mixture 2 but not for Mixture 1, as demonstrated by
the two images (insets) (right, c). Again, two dispersions were prepared in the same way and the spectra were collected without delay (right, c).
At room temperature, the presence of TOPO seemed to result in the fragmentation of the PC and promoted the nucleation and growth of QDs.
when TOPO (in Figure 1 Batch a) was replaced by the same
amount of oleic acid, Figure S1-3 shows that the formation of
the PC was not suppressed. It seems that TOPO suppresses
the presence of the PC of MSCs and enables the nucleation
and growth of QDs at a relatively low temperature.
To explore the effect of TOPO on the PC, we decided to
perform an addition of TOPO at room temperature (Figure 2)
and at 1208C (Figure S2-3), after the formation of the PC at
and growth of QDs taking place at room temperature during
the two incubations, and the growth seemed to be favored by
a relatively high concentration. Parts a1 and a2 of Figure S2-
1 illustrate the temporal evolution of QDs, showing the red
shifts of the broad absorption bumps during the incubation.
The redshift behavior specifies an increase in size of the CdS
QDs at room temperature.
For Mixture 2 (light blue traces), the optical absorption
spectrum collected right after dispersion is similarly featur-
eless (a). After a 5-day incubation, the dispersion displayed
an absorption peaking at 311 nm (b), which indicates the
1
608C (with the growth period of 30 min) for the conven-
tional reaction of Cd(OA) and S in ODE. In Figure 2, we
2
show the optical absorption spectra collected from two
[
18–22]
mixtures. After the Cd(OA) and S reaction had been heated
presence of CdS MSC-311.
Mixture 2 exhibited sharp absorption peaking at
ꢀ 322 nm (c), which suggests the evolution of CdS MSC-
322. It has been demonstrated elsewhere that MSC-311
and MSC-322 form a pair of isomers, with respect to the CdS
After a 5-day incubation,
2
at 1608C for 30 min and then cooled to room temperature,
a portion (1.0 mL) of the sample was mixed with a mixture of
TOPO (0.1 g) and ODE (1.0 g) giving us Mixture 1) TOPO/
ODE (dark blue traces), and another portion (1.0 mL) was
mixed with ODE (1.0 g) providing us Mixture 2) ODE (light
blue traces). For Mixtures 1 and 2, 25 mL of each was
dispersed in 3.0 mL of Tol, and spectra were collected upon
dispersion (a left) and after a five-day incubation (b middle).
Also, Mixtures 1 and 2 were incubated for five days at room
temperature; afterwards, two visual images were taken (c
right, inset), for which two corresponding dispersions were
prepared and spectra were collected upon dispersion (c right).
During these incubations, spectra were also collected after
one day and after five days. As shown in Figure S2-1, there
were a total of 10 optical absorption spectra collected from
Mixtures 1 and 2.
a
[
20–22]
[
20–22,30]
core composition.
Whether MSC-311 or MSC-322
evolves in a prenucleation stage sample is predominantly
dictated by the sample incubation temperature, with the
former preferring relatively low temperatures and the latter
[
20,22]
relatively high temperatures.
Parts b1 and b2 of Fig-
ure S2-1 show the temporal evolution of MSC-311 and MSC-
322 during the incubation, respectively.
We have shown that the presence of TOPO in a CdS
prenucleation sample, in which the PC had formed, has
a substantial influence on the nucleation and growth of CdS
QDs (free of CdS MSCs) during incubation (Figure 2). For
Mixture 1 with TOPO, the nucleation and growth of CdS QDs
took place even at room temperature (without MSCs). For
Mixture 2 without TOPO, the evolution of CdS MSCs
occurred (without CdS QDs). It is reasonable to assume that
TOPO causes the PC to break into monomers and fragments,
the presence of which results in the nucleation and growth of
QDs at room temperature. The fragmentation of the PC by
TOPO is supported by the matrix-assisted laser desorption/
For Mixture 1 (deep blue traces), the optical absorption
spectrum collected immediately upon dispersion was almost
featureless (a). After a 5-day incubation, the dispersion
exhibited an optical absorption bump around ꢀ 320 nm (b).
When Mixture 1 was incubated for five days, its optical
absorption spectrum exhibited a similar bump but around
ꢀ
330 nm (c). These features are indicative of the nucleation
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Figure 3. Optical absorption spectra of CdS samples from two reaction batches, with the addition of TOPO after (Part a, top panel) and before
Part b, bottom panel) the formation of the PC. For the former, we repeated the reaction of Batch a shown in Figure S2-3, but increased the
(
temperature from 120 to 2208C in steps of 208C, after the addition of TOPO at 1208C. For the latter, we repeated the reaction of Batch a shown
in Figure 1, but decreased the temperature to 1208C, after the reaction had been heated at 1608C for 30 min. Also, we increased the temperature
from 120 to 2208C in steps of 208C; from each reaction, we extracted six samples at each temperature after 10 min had elapsed. An aliquot
(
25 mL) of each sample was dispersed in 3.0 mL of Tol. The spectra were collected immediately upon dispersion (0 day, blue traces) and after one
day in Tol (1 day, red traces). Results for the last three samples from the two batches are shown here. Apparently, the particle yield of the samples
of Batch a was larger than that of Batch b.
ionization time-off-light mass spectrometry (MALDI-TOF
MS) result shown in Figure S2-2. Here, we do not distinguish
the fragments which were produced by the PC fragmentation
with those formed directly from the Cd and S precursor
reaction. We have argued that the formation of M-E covalent
bonds leads first to the formation of the PC (due to a high
concentration that resulted from the self-assembly of the M
tion spectra collected from samples taken from two reaction
batches at 1208C. For Batch a with TOPO added at 1208C
(top panel), the nucleation and growth of QDs took place.
From the 20 to 60 min samples, the relatively broad absorp-
tion peaks redshifted from 334 to 345 nm. The growth in size
of the QDs was accompanied by a small decrease of peak
optical density from 0.09 to 0.08. The optical absorption
properties changed little before and after a one-day incuba-
tion. For Batch b at 1208C without TOPO (bottom panel), the
nucleation and growth of QDs did not take place, and the
corresponding absorption spectra are essentially featureless
(blue traces). After a one-day incubation (red traces), an
optical absorption peaking at 311 nm was observed for each
of the six samples, suggesting the presence of CdS MSC-311.
To validate further the hypothesis that the nucleation and
growth of QDs at 1208C is due to the fragmentation of the PC
formed at 1608C, we repeated the two reactions shown in
Figure 1, but at a constant temperature of 1208C (Figures S2-
4a and S2-4b). After as much as 60 min, the nucleation and
[
24,29]
and E precursors) and then to monomers and fragments.
It is remarkable that TOPO results in the nucleation and
growth of CdS QDs at room temperature, for a prenucleation
stage sample. The nucleation and growth of CdS QDs via the
fragmentation of the PC seems to resemble the one used in
the construction of monolithic CaCO from the prenucleation
3
[38]
intermediate, the oligomer (CaCO ) .
3
n
To test further the conjecture that the nucleation and
growth of QDs at unusually low temperatures (such as below
1
808C) is promoted by the fragmentation of the PC, we added
TOPO at a higher temperature, this time at 1208C instead of
at room temperature. Figure S2-3 shows the optical absorp-
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Figure 4. Comparison of optical absorption properties of CdS samples from two reaction batches at 1208C. The reaction was Cd(OA) + S +
2
À1
HPPh with the feed molar ratio of 4Cd(OA) (0.60 mmol) to 1S (0.15 mmol) to 4HPPh (0.57 mmol) and a S concentration of 30 mmolKg in
2
2
2
ODE. After the addition of S followed by HPPh at 608C, the reaction temperature was increased to 1208C rapidly, and TOPO (1.32 mmol) was
2
added to Batch a (dark blue traces) but not to Batch b (light blue traces). The reactions were kept at 1208C, and samples were extracted after
certain periods of 10 (a), 20 (b), 30 (c), 40 (d), 50 (e), and 60 (f) min had elapsed. An aliquot (10 mL) of each sample was dispersed in 6.0 mL of
Tol. The spectra were collected upon dispersion and after 1 day in Tol as shown in Figure S4. The 1-day data are compared here. Again, the
presence of TOPO after the PC formed appeared to fragmentize the PC and thus to facilitate the nucleation and growth of QDs at 1208C.
growth of QDs had not taken place, and neither had the
formation of the PC. Thus, for Figure S2-3 Batch a at 1208C
with TOPO, the PC was fragmented by TOPO, and the
nucleation and growth of QDs took place. After the
incubation, no MSCs were seen. For Figure S2-3 Batch b at
ature was increased to 1608C for 30 min, it was decreased to
1208C; following a similar pattern, the temperature was
increased from 120 to 2208C in steps of 208C (Figure 3b).
During the course of the temperature increase, we
extracted six samples after 10 min had elapsed at each
temperature. In the same way, dispersions were prepared
and spectra were collected. Figures S3-1a and S3-1b present,
respectively the optical absorption spectra of the six samples
for the two batches. For the last three samples from the two
batches extracted at 1808C (left panel), 2008C (middle panel),
and 2208C (right panel), Figure 3 shows their optical absorp-
tion spectra collected right after the preparation of the
dispersions (blue traces, 0 day) and after a one-day incubation
(red traces, 1 day). Parts a (top panel) and b (bottom panel)
address the former and latter reactions, respectively, for which
TOPO was present after and before the formation of the PC.
For Batches a and b, the nucleation and growth of QDs
took place at 1208C. The optical absorption properties
changed little after a one-day incubation in Tol, similar to
1
208C without TOPO, the nucleation and growth of QDs did
not occur at 1208C. During the incubation, the PC trans-
formed to MSC-311. These results are in agreement with
those shown in Figure 2. As expected, the QDs produced at
1
208C (Figure S2-3 Batch a) are somewhat larger than those
obtained at room temperature (Figure 2 Mixture 1).
Now we discuss how the QD production yield was
affected by the addition of TOPO, which was performed
after (Figure 3a) and before (Figure 3b) the formation of the
PC. For this investigation we repeated the reaction of Batch a
shown in Figure S2-3; however, following the addition of
TOPO at 1208C, we increased the temperature from 120 to
2
208C in steps of 208C (Figure 3a). Also, we repeated the
reaction of Batch a shown in Figure 1, but after the temper-
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what was seen for the samples from Batch a in Figure S2-3
and from Batch a in Figure 1. For Batch a, the 2208C sample
exhibited an absorption peaking at 426 nm (a3) with a peak
optical density of 0.12. For Batch b, the 2208C sample
displayed an absorption peaking at 411 nm (b3) with a peak
optical density of 0.02. Table S1 summarizes the peak optical
density and peak positions for the samples from these two
For Batch a samples in Tol after the one-day incubation
(deep blue traces), the absorption spectra changed little, as
also shown in Figure S4-1a. Importantly, these samples, which
contained small-size CdS QDs, did not have the PC and/or
MSCs. From the 10 to 60 min samples, the broad optical
absorption peaks redshifted from 337 to 343 nm. Therefore,
the growth in size at 1208C over time seemed to be limited,
and the optical density of the broad absorption peak is 0.11 Æ
0.01. The Batch b samples contained both MSCs and QDs, as
also shown in Figure S4-1b. After a one-day incubation (light
blue traces), the absorption spectra changed little for the
QDs, but showed that CdS MSC-322 changed to CdS MSC-
311. These two features are very similar to those report-
À1
batches, together with the QD production yield (molL ).
The comparison of the QD production yield is further
highlighted in Figure S3-1c. Clearly, the particle yield of the
samples from Batch a is larger than that from Batch b. For
instance, the QD-381 yield from Batch a (1608C) is about five
times larger than the QD-381 yield from Batch b (1208C).
Also, the QD-402 yield from Batch a (1808C) is about five
times larger than the QD-398 yield from Batch b (1808C). For
this reason, an approach based on the addition of TOPO after
the PC has formed would provide a better means for
synthesizing small-size CdS QDs with enhanced particle yield
at relatively low temperatures.
We also repeated Batch a without TOPO, in which case
the result is shown in Figure S3-2. Again, the nucleation and
growth of QDs occurred around 2008C. The first four samples
exhibited almost featureless absorption spectra, while the last
two samples (extracted at 200 and 2208C) displayed absorp-
tion peaking at 367 and 394 nm, respectively. The redshift
indicates a size increase for the QDs. After a one-day
incubation in Tol, CdS MSC-311 evolved in each sample, as
was seen for the samples from Batch b in Figure S2-3. The
population of CdS MSC-311 increased with the sample
temperature from 120 to 2008C, indicating an increase of
the PC as the reaction progressed. The decrease of CdS MSC-
[18–22]
ed.
For the 10 to 60 min samples, the broad optical
absorption peaks redshifted from 346 to 363 nm, and the peak
optical density was 0.06 Æ 0.01.
It is clear that the particle yield of the CdS QDs in the
Batch a samples is about twice as large as that of the Batch b
samples. The relatively high QD concentrations of the
Batch a samples with relatively small sizes could be reason-
ably attributed to the fragmentation of the PC that provided
relatively high concentrations of monomers and fragments,
which in turn result in the nucleation and growth of QDs. The
higher concentration of monomers and fragments is, the more
nuclei tend to be formed, and thus the fewer atoms that are
contained in each QD. Table S2 summarizes the estimated
concentrations and sizes of the CdS QDs produced in the two
batches; also, Figure S4-1c highlights the comparison of the
former. With the use of HPPh , the particle yield of Figure 4
2
Batch a is larger than that of Batch a shown in Figures S2-3
and 3 without HPPh . Figure S4-2 shows X-ray diffraction
2
3
11 in the samples from 200 to 2208C can be attributed to
(XRD) and transmission electron microscopy (TEM) of one
60 min sample (purified), which was from another Figure 4
Batch a reaction. The small-size QDs seem to have a cubic-
like structure that readily self-assemble during the TEM
sample preparation. Thus, it is not possible to define the
a corresponding and competing growth in size of the
QDs.
two-pathway model, whereby the growth of the QDs results in
the fragmentation of the PC.
After seeing the effectiveness of TOPO in enabling the
nucleation and growth of QDs via the fragmentation of the
PC, we decided to investigate further for reactions with the
use of a secondary phosphine, diphenyl phosphine (HPPh ). It
has been shown that the reaction of Cd(OA) + S + HPPh2
leads to conventional CdS QDs, together with CdS MSC-
3
S precursor, SPPh H, and the QD yield was promoted.
Figure 4, we present the optical absorption spectra collected
after a one-day incubation of CdS samples in Tol. The samples
were extracted from two reactions of Cd(OA) + S + HPPh2
in ODE, which had a feed molar ratio of 4Cd(OA) to 1S to
4
[
18–20]
This phenomenon can be understood using the
[18–20]
[
22,52,53]
crystal structure and size from the TEM results.
Furthermore, a crystal structure identification for small-size
QDs based on XRD is unreliable due to the limited number
[7]
of atoms in such QDs. Figure S4-3 shows the optical
absorption spectra of the 60 min sample, suggesting that the
synthetic reproducibility of the Figure 4 Batch a approach is
high.
2
2
[20]
22. HPPh interacts with S to result in a much more active
2
[
20,32]
In
As illustrated by Figures S4-1c and S3-1c, respectively, the
addition of TOPO after the PC formation facilitates the
production of small-size CdS QDs with enhanced particle
yield and without the coproduction of the MSC PC or MSCs.
The nucleation and growth of small-size CdS QDs promoted
at low temperatures by the TOPO-induced PC fragmentation
provides insight into the relationship between the intermedi-
ate PC and the nuclei (of the two-pathway model proposed
for the nucleation and growth of semiconductor II–VI ME
QDs and for the evolution of ME MSCs) (Scheme S1). We
point out that, for the non-semiconductor systems such as
2
2
2
À1
HPPh and a S concentration of 30 mmolKg in a total
2
weight of 5.0 g. The addition of TOPO (1.32 mmol) was
performed for Batch a at 1208C (deep blue traces) and not for
Batch b at 1208C (light blue traces). Afterwards, the reactions
were kept at this temperature, and samples were taken after
elapsed times of 10 (a), 20 (b), 30 (c), 40 (d), 50 (e), and
[
38–42]
[43–45]
[46–48]
6
0 min (f). For the spectroscopy study, an aliquot (10 mL) of
calcium-based inorganics,
and metals,
organics,
polymers,
[
49–51]
each sample was dispersed in 6.0 mL of Tol. For the results of
Batches a and b shown in Figures S4-1a and S4-1b, respec-
tively, the spectra were collected immediately upon disper-
sion and after a one-day incubation in Tol.
the presence of liquid-like intermediates
has been addressed in the multi-step model of the non-
classical nucleation theory, while the one-step model of the
classical nucleation theory (CNT) does not account for such
&
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Research Articles
Chemie
[
33–37]
intermediates.
However, as illustrated by Scheme S1, the Acknowledgements
details of the relationship between those intermediates and
nuclei have not been established; for example, as to how
many nuclei form in one dense liquid droplet.
K.Y. thanks to the National Natural Science Foundation of
China (NSFC) 21773162, the State Key Laboratory of
Supramolecular Structures and Materials of Jilin University
for SKLSSM 201935, and the State Key Laboratory of
Polymer Materials Engineering of Sichuan University for
Grant No. sklpme2018-2-08. H.F. and W.H. appreciate the
National Major Scientific and Technological Special Project
Conclusion
In summary, we have demonstrated, for the first time, that
the fragmentation of the CdS PC results in the nucleation and
growth of small-size CdS QDs with enhanced particle yield at
low temperatures such as room temperature. For the conven-
for
“Significant
New
Drugs
Development”
(2019ZX09201005-005-002 and 2019ZX09201005-005-001).
We are grateful to Analytical & Testing Center, Sichuan
University, for TEM and XRD and NMR.
tional reaction of Cd(OA) (0.60 mmol) and S (0.15 mmol) in
2
ODE with a total weight of 5.0 g, the PC forms at 1608C
(
Figure 1 Batch b). When TOPO (1.32 mmol) was present
with S at 908C, which was prior to the formation of the PC in Conflict of interest
the prenucleation stage, the nucleation and growth of QDs
was accelerated to occur at a lower temperature of ꢀ 1808C
The authors declare no conflict of interest.
(
Figure 1 Batch a) instead of at 2208C (Figure 1 Batch b,
without TOPO). Moreover, the PC was not produced in the
Batch a samples, but was in the Batch b samples. After the PC
had formed (in the prenucleation stage at 1608C), the
nucleation and growth of QDs could occur at room temper-
ature (Figure 2) or at 1208C (Figure S2-3) without any of the
PC left, upon the addition of TOPO at room temperature
Keywords: cadmium sulfide · crystal growth ·
magic-size clusters · nucleation · quantum dots
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Version of record online: && &&
,
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Angewandte
Research Articles
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Research Articles
Nanoparticle Synthesis
Small-size CdS quantum dots are syn-
thesized without the coexistence of the
precursor compound and thus with
enhanced particle yield. It is shown that
after the formation of the precursor
compound in the pre-nucleation stage,
the addition of tri-n-octylphosphine oxide
facilitates the production of small-size
QDs.
L. Li, J. Zhang, M. Zhang, N. Rowell,
C. Zhang, S. Wang, J. Lu, H. Fan,
W. Huang, X. Chen,*
K. Yu*
&&&& — &&&&
Fragmentation of Magic-Size Cluster
Precursor Compounds into Ultrasmall
CdS Quantum Dots with Enhanced
Particle Yield at Low Temperatures
Angew. Chem. Int. Ed. 2020, 59, 2 – 11
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&&&&
These are not the final page numbers!