Transformation Pathway from CdSe Magic-Size Clusters with Absorption Doublets at 373/393 nm to Clusters at 434/460 nm

Article abstract of DOI:10.1002/anie.202104986

Divergent interpretations have appeared in the literature regarding the structural nature and evolutionary behavior for photoluminescent CdSe nanospecies with sharp doublets in optical absorption. We report a comprehensive description of the transformation pathway from one CdSe nanospecies displaying an absorption doublet at 373/393 nm to another species with a doublet at 433/460 nm. These two nanospecies are zero-dimensional (0D) magic-size clusters (MSCs) with 3D quantum confinement, and are labeled dMSC-393 and dMSC-460, respectively. Synchrotron-based small-angle X-ray scattering (SAXS) returns a radius of gyration of 0.92 nm for dMSC-393 and 1.14 nm for dMSC-460, and indicates that both types are disc shaped with the exponent of the SAXS form factor equal to 2.1. The MSCs develop from their unique counterpart precursor compounds (PCs), which are labeled PC-393 and PC-460, respectively. For the dMSC-393 to dMSC-460 transformation, the proposed PC-enabled pathway is comprised of three key steps, dMSC-393 to PC-393 (Step 1), PC-393 to PC-460 (Step 2 involving monomer addition), and PC-460 to dMSC-460 (Step 3). The present study provides a framework for understanding the PC-based evolution of MSCs and how PCs enable transformations between MSCs.

Full text of DOI:10.1002/anie.202104986

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Transformation pathway from CdSe magic-size clusters with
absorption doublets at 373/393 nm to clusters at 434/460 nm
Authors: Jinming Zhu, Zhaopeng Cao, Yongcheng Zhu, Nelson Rowell,
Yan Li, Shanling Wang, Chunchun Zhang, Gang Jiang, Meng
Zhang, Jianrong Zeng, and Kui Yu
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10.1002/anie.202104986
Angewandte Chemie International Edition
RESEARCH ARTICLE
Transformation pathway from CdSe magic-size clusters with
absorption doublets at 373/393 nm to clusters at 434/460 nm
Jinming Zhu+, Zhaopeng Cao+, Yongcheng Zhu, Nelson Rowell, Yan Li, Shanling Wang, Chunchun
Zhang, Gang Jiang, Meng Zhang,* Jianrong Zeng* and Kui Yu*
Nelson Rowell
[*]
Jinming. Zhu, Yongcheng. Zhu, Gang. Jiang, Meng. Zhang, Kui. Yu
Institute of Atomic and Molecular Physics, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
E-mail: mengzhang@scu.edu.cn
Metrology Research Centre, National Research Council Canada
Ottawa, Ontario K1A 0R6 (Canada)
Shanling. Wang, Chunchun. Zhang
Analytical & Testing Center, Sichuan University
Chengdu, Sichuan, 610065 (P. R. China)
Jianrong. Zeng
Shanghai Synchrotron Radiation Facility, Shanghai Advanced
Research Institute, Chinese Academy of Sciences
Shanghai, 201204 (P. R. China)
kuiyu@scu.edu.cn
Zhaopeng. Cao, Yan. Li, Jianrong. Zeng
Shanghai Institute of Applied Physics, Chinese Academy of Sciences
Shanghai, 201800 (P. R. China)
E-mail: zengjianrong@sinap.ac.cn
Zhaopeng. Cao, Yan. Li
Kui. Yu
School of Physical Science and Technology, ShanghaiTech University
Shanghai, 201210 (P. R. China)
Zhaopeng. Cao, Yan. Li
Engineering Research Center in Biomaterials, Sichuan University
Institution
Chengdu, Sichuan, 610065 (P. R. China)
These authors contributed equally to this work.
University of Chinese Academy of Sciences
Beijing, 100049 (P. R. China)
[+]
Supporting information for this article is given via a link at the end of
the document.
Abstract: Divergent interpretations have appeared in the literature
regarding the structural nature and evolutionary behavior for
photoluminescent CdSe nano-species with sharp doublets in optical
and includes an additional step in which MSCs transform into
monomers via PCs (Scheme S1).
While MSCs exhibit characteristic optical absorption at specific
wavelengths, their counterpart PCs do not absorb at these or longer
wavelengths.[1-17] Here the MSCs and their PCs exist as quasi
isomers, which can reversibly interchange via intramolecular
reorganization. Furthermore, some transformations display
isosbestic points, such as between CdS MSC-311 and 322,[7] among
CdTe MSC-371, 417, and 448,[10-12] and between binary CdTe MSC-
371 and ternary CdTeSe MSC-399.[14] The numbers represent the
peak positions in wavelength (nanometers (nm)) of the sharp
optical absorption singlets of the MSCs. Following the concept that
the MSCs evolve from their own unique PCs, the pathway for these
apparent transformations between two types of MSCs has been
hypothesized to be PC-enabled, with substitution reactions of
monomers that transform the reactant MSC PC into the product
MSC PC (Figure S1a).[1,7,8,10-12,14]
In addition to the MSCs that display sharp optical absorption
singlets,[3-15,18-26] there is another type of nano-species that exhibits
sharp optical absorption doublets.[16,17,27-44] An example
semiconductor is photoluminescent (PL) CdSe peaking at around
373/393 (Species-393) or 433/460 nm (Species-460).[16,27-30]
Different assignments have been made,[35-39] with 0-dimension (0D)
dMSC-393 and/or 2D nanoplates NPL-393 for Species-393, and
dMSC-460 and/or NPL-460 for Species-460. Meanwhile, the dMSCs
(such as dMSC-460 with 3D quantum confinement) and their
corresponding NPLs (such as NPL-460 with 1D quantum
confinement) exhibit different polarized photoluminescence (PL)
but with similar optical absorption.[16,38,39]
absorption. Here, we report
a comprehensive description of the
transformation pathway from one CdSe nano-species displaying an
absorption doublet at 373/393 nm to another species with a doublet at
433/460 nm. These two nano-species are shown to be as zero-dimension
(0D) magic-size clusters (MSCs) with 3D quantum confinement, and are
labelled dMSC-393 and dMSC-460, respectively. Synchrotron-based small
angle X-ray scattering (SAXS) returns a radius of gyration of 0.92 nm for
dMSC-393 and 1.14 nm for dMSC-460, and indicates that both types are
disc-shaped with the exponent of the SAXS form factor equal to 2.1. The
MSCs develop from their unique counterpart precursor compounds (PCs),
which are labelled PC-393 and PC-460, respectively. For the dMSC-393 to
dMSC-460 transformation, the proposed PC-enabled pathway is comprised
of three key steps, dMSC-393 to PC-393 (Step 1), PC-393 to PC-460 (Step 2
involving monomer addition), and PC-460 to dMSC-460 (Step 3). The
present study provides a framework for understanding the PC-based
evolution of MSCs and how PCs enable transformations between MSCs.
Introduction
It has been shown that magic-size clusters (MSCs) have their own
unique precursor compounds (PCs).[1,2] This concept has recently
been demonstrated in the synthesis of various II-VI metal
chalcogenide (ME) MSCs, which include CdS,[3-7] CdSe,[8,16] CdTe,[9-
12,17] CdTeSe,[13,14] and ZnSe.[15] In all these cases binary PCs start to
form before conventional binary ME quantum dots (QDs) nucleate
and grow. This pre-nucleation stage is also known as the QD
induction period. The growth relationship between the binary ME
MSCs and corresponding QDs can be adequately elucidated with a
recently developed two-pathway model.[1,2] The pathway from
MSCs to QDs in this model is similar to that previously proposed in
which the MSCs provide a reservoir of monomers for the QDs,[18]
As shown by Figure S1b, the NPLs appear to be comprised of n
layers of Cd and n - 1 layers of Se.[40,41] In this scheme, three layers
of Cd and two layers of Se make up NPL-393, while four layers of Cd
and three layers of Se form NPL-460. Thus, NPL-393 has one fewer
CdSe monolayer (ML) than NPL-460. Various formation schemes
have been proposed for these NPLs, including the self-assembly of
0D dMSC-460 to 2D NPL-460 (Figure S1c).[38,42-44] For reaction
1
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10.1002/anie.202104986
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RESEARCH ARTICLE
mixtures of various Cd and Se precursors, the evolution of both
Species-393 and Species-460 has been reported, with a decrease of
the former and an increase of the latter at relatively late points in
the reaction (Figure S1d).[16,27-30] Although there were indications
that dMSC-393 and dMSC-460 formed from transformations of
their respective counterpart PC-393 and PC-460,[16] the present
study provides the first exposition of their growth relationship.
Here we report our exploration of the pathway for the
transformation from PL CdSe dMSC-393 to dMSC-460. Based on
what we have seen in our experiments, we propose that the PC-
enabled pathway as illustrated in Scheme 1 is responsible for the
transformation from smaller dMSC-393 to larger dMSC-460; the
two types of PL CdSe dMSCs are disc-shaped. The three key steps
labelled in Scheme 1 can be represented by three equations, which
are
shaped CdSe cluster.[24] Notably, the monomer addition takes place
on PC-393 (Step 2, Equation 2), instead of on dMSC-393 directly.
The present understanding on the PC-enabled pathway (Scheme 1)
provides a more comprehensive explanation for a number of
phenomena. These include the direct evolution of dMSC-460
during a reaction without the presence of dMSC-393 (Part A of
Figure S1d),[27] the formation of NPLs (Figure S1c),[38,42-44] and the
transformation from one type of NPLs to the next thicker NPLs with
an additional ME monolayer (Part B of Figure S1c and Scheme
S2).[43] A common assumption for the growth is the direct addition
of monomers, but our study (Scheme 1) does not support this
assumption.
Results and Discussion
dMSC-393  PC-393
PC-393 + CdSe monomers  PC-460 (2)
PC-460  dMSC-460 (3)
Steps 1/4 and Steps 3’/3 represent the reversible transformations
between dMSC-393 and PC-393 and between dMSC-460 and PC-
(1)
This section is primarily comprised of two parts. The first part
addresses optical absorption and PL (with excitation wavelength of
350 nm) properties (Figures 1-3), and the second part deals with
synchrotron-based small angle X-ray scattering (SAXS) (Figure 4).
Reactions of cadmium myristate Cd(MA)2 and Se were carried out
in 1-octadecene (ODE), with acetic acid (HOAc) added at 110 C
when the reaction was in the induction period occurring prior to
nucleation and growth of CdSe QDs. The effect of HOAc addition in
the induction period has been reported elsewhere.[16,38] When the
reaction temperature was kept at 190 C for a period of 5 min,
Sample A (light yellow in color) was obtained, while Sample B (deep
yellow in color) came from the reaction at 230 C for 30 min. The
two samples, purified by toluene (Tol), contained mainly dMSC-393
(white) and dMSC-460 (deep yellow), respectively.
460, respectively. Step
2 denotes the PC-393 to PC-460
transformation, which occurs as a result of the addition reaction of
monomers. Thus, this PC-enabled pathway is different from that
proposed before with the substitution reaction of
monomers,[1,7,8,10-12,14] and can also account for the addition of a
CdSe monolayer on one of four identical facets of a tetrahedrally-
dMSC-393 (purified Sample A) was heated directly (without
dilution in ODE) (Figure 1) and in ODE (Figure 2), with a similar
heating procedure which was employed also for the reaction of
Cd(MA)2 and Se in ODE with HOAc added at 110 C (Figure 3). While
the dMSC-393 to dMSC-460 transformation occurred at the lower
temperature of 210 C, an evolution of dMSC-460 was the principal
process observed at 230 C (Figure 1, Parts a and b of Figure 2, and
Part a of Figure 3). An almost featureless optical absorption around
210 C was observed for purified Sample A (Figures 1 and 2), but
not for the two reaction mixtures of Cd(MA)2 and Se (Figure 3). This
result can be explained as PC-393 and CdSe monomers (Scheme 1)
being generated differently in purified Sample A (Figures 1 and 2)
and in the reaction mixtures (Figure 3). Synchrotron-based small
angle X-ray scattering (SAXS) delivers the morphological
information that dMSC-393 and dMSC-460 have the exponent of
the SAXS form factor close to 2, together with the respective radius
of gyration (Rg) of 0.92 and 1.14 nm (Figure 4). In this regard, they
are disc-shaped and not spherical, and dMSC-393 (Sample A) is
smaller than dMSC-460 (Sample B). The SAXS form factor (within
the high-q region) has a power law behavior, where the exponent
is the signature of the shape of the object. The value of the
exponent varies from 0 to 4, with 1.0 for long rods, 2.0 for discs,
and 4.0 for smooth-surface spheres.[45] On the basis of the
experimental observations, we propose that the dMSC-393 to
dMSC-460 transformation is PC-enabled (Scheme 1).
Scheme 1. Schematic drawing of the proposed pathway for the PC-
enabled transformation from dMSC-393 to dMSC-460. The
pathway involves three sequential steps, which are isomerization
from dMSC-393 to PC-393 (Step 1 Eq 1), the reaction with CdSe
monomer (Mo) addition that transforms PC-393 into PC-460 (Step
2 Eq 2), and isomerization from PC-460 to dMSC-460 (Step 3 Eq 3).
When dMSC-393 (contained in purified Sample A by toluene) was
heated (Figures 1 and 2), Step 1 occurred, and monomers were
then generated from the fragmentation of PC-393 and Step 2 took
place. In the reaction of Cd(MA)2 and Se in ODE (Figure 3), dMSC-
393 and dMSC-460 preferred relatively high and low Se feed
concentrations, respectively; this can be explained by the
competition between Steps 4 and 2 due to the direct production of
PC-393 and monomers, which is different from that when dMSC-
393 was heated (Figures 1 and 2). Synchrotron-based SAXS suggests
that dMSC-393 is smaller than dMSC-460, both of which are disc-
shaped (Figure 4). The proposed PC-enabled pathway allows for the
addition of a CdSe monolayer on one of the four identical facets of
a CdSe tetrahedra cluster.[24]
In Figure 1 we present the evolution of optical absorption and PL
properties for purified Sample A at various temperatures. This
purified sample was almost white in color at room temperature,
and Figure S1-1 shows the optical properties of Sample A (as-
synthesized and purified), with corresponding TEM and XRD results
shown in Figure S1-2. It is evident that Sample A (after purification
with Tol) still contained mostly 0D dMSC-393. When dMSC-393
2
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absorption and emission spectra for the two species have similar
full width at half maximum (FWHM) of ~10 nm. Furthermore, we
noticed that purified Sample A was quite viscous even at 210 C
with very light yellow color. At 230 C, it became deep yellow with
its viscosity decreased a little. It is reasonable that a sample
containing dMSC-393 would be light yellow in color, while one
containing dMSC-460 would be deep yellow, consistent with a size
increase from the former to the latter.
Therefore, a dMSC-393 to dMSC-460 transformation around 210
C is indicated by both the color change and the featureless
absorption spectrum at that temperature. At approximately this
critical temperature, purified Sample
A contained a barely
detectable concentration of dMSC-393, and a small amount of
dMSC-460. We therefore propose that at this stage, dMSC-393
transforms to its PC-393 (Step 1), followed by the transformations
from PC-393 to PC-460 (Step 2) and to dMSC-460 (Step 3). The PC-
460 to dMSC-460 transformation (Step 3) is apparently more
efficient at higher temperatures such as 230 C.
For the PC-393 to PC-460 transformation (Step 2), we expected
that it is affected by the amount of dMSC-393. Thus, we diluted
dMSC-393 by ODE. In Figure 2 we present the evolution of optical
absorption and PL for dMSC-393 diluted in ODE and subjected to a
similar thermal processing to that for Figure 1. The concentrations
of purified Sample A (identical to that used for Figure 1) in ODE
were 10.0 (a), 5.0 (b) and 2.5 mg∙mL-1 (c), which are smaller than
that in Figure 1. From 150 to 250 C (with temperature steps of 20
C and 5 min at each step), six samples were taken. For the optical
measurements, a volume of 50 (a), 50 (b), and 200 μL (c) of each
extracted sample was dispersed in 3.0 mL of Tol.
The optical properties of dMSC-393 (purified Sample A) in ODE
shown in Parts a and b of Figure 2 evolved in a similar fashion to
that shown in Figure 1. When the concentration of Sample A was as
high as 10.0 mg∙mL-1 (Part a), dMSC-393 increased in strength from
150 to 170 C, and then decreased from 170 to 210 C. It is
reasonable that the increase is due to more dMSC-393 being
dissolved at 170 C than at 150 C. At 210 C, Sample A displayed
relatively featureless optical absorption and PL spectra, as
compared to at the other five temperatures. From 210 to 250 C,
dMSC-460 increased in strength. When the concentration of
Sample A was 5.0 mg∙mL-1 (Part b), dMSC-393 decreased in strength
from 150 to 210 C. This purified sample displayed relatively
featureless optical absorption and PL spectra at 210 C. From 230
to 250 C, dMSC-460 increased in strength. When the
concentration of Sample A was as low as 2.5 mg∙mL-1 (Part c),
dMSC-393 decreased in strength from 150 to 190 C. From 190 to
230 C, Sample A had relatively featureless optical absorption and
PL spectra. At 250 C, a broad absorption peaking at ~607 nm
emerged, which indicates the presence of CdSe QDs.
The optical properties of Sample A in the three batches for Figure
2 became different mainly after 190 C. The optical properties have
been compared for the stages from 150 to 170 C (Figure S2-1),
from 190 to 210 C (Figure S2-2), and from 230 to 250 C (Figure S2-
3). The lower the concentration was, the broader the temperature
window was for featureless optical absorption, such as at 210 C
(Figures 1 and 2a), from 210 to 230 C (Figure 2b), and from 190 to
230 C (Figure 2c). The temperature ranges with featureless optical
absorption are symptomatic practically of complete dMSC-393 to
PC-393 transformation (Step 1). When the concentration of dMSC-
393 was relatively high (such as shown in Figure 1 and Part a of
Figure 2), the dMSC-393 to dMSC-460 transformation appeared to
dominate. When the concentration was relatively low (Figure 2 Part
Figure 1. Optical characteristics of the transformation from dMSC-393
to dMSC-460 when dMSC-393 (purified Sample A) was heated (without
ODE). Sample A was obtained from a reaction (190 C/5 min) of
Cd(MA)2 and Se in ODE, with the addition of HOAc within the induction
period (110 C). Sample A was purified three times with Tol, and the
resulting precipitate (dMSC-393 about 50 mg) was heated directly.
From 170 to 230 C (in temperature steps of 20 C with a 5 min hold
time at each step), four samples were extracted and dispersed in Tol
for the measurements of optical absorption (left y axis, solid traces) and
PL (right y axis, dashed traces normalized) (offset). At 210 C, dMSC-
393 almost disappeared from the spectrum, whereas dMSC-460
increased in strength from 210 to 230 C. Remarkably, Sample A
displayed almost featureless optical properties at 210 C, which
supports the PC-enabled pathway as depicted by Scheme 1 for the
apparent transformation from dMSC-393 to dMSC-460.
(purified Sample A) was heated directly without using ODE, four
samples were taken from 170 to 230 C (in temperature steps of 20
C after 5 min at each step). Each extracted sample was dispersed
in 3.0 mL Tol for the optical absorption and PL measurements.
When dMSC-393 (purified Sample A) was heated at 170 and 190
C, dMSC-393 remained, exhibiting an optical absorption doublet
peaking at ~373/393 nm and a PL peak at ~398 nm. At 210 C, this
sample transitioned to display a relatively featureless absorption,
compared to the other three temperatures. Thus, it is evident that
the dMSC-393 to PC-393 transformation was more or less complete
at this temperature (Scheme 1 Step 1). While Scheme 1 describes
our understanding of the transformation process, any reference to
a specific step in the following discussion refers to Scheme 1 utterly,
such as Step 1. At 230 C, a new absorption doublet appeared in
the spectrum at 435/462 nm with the corresponding PL peaking at
464 nm, which indicates the development of dMSC-460 (from PC-
460, Step 3). Accordingly, Sample A contained dMSC-393 from
room temperature to 190 C, but dMSC-460 at 230 C. The
3
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Figure 2. Optical absorption (solid traces) and PL (dashed traces, normalized) spectra when dMSC-393 (purified Sample A) was heated in ODE
with concentrations of 10.0 (a), 5.0 (b) and 2.5 (c) mg∙mL-1. For each batch, six samples were extracted between 150 and 250 C (using a similar
procedure to that used for Figure 1). For the optical measurements, 50 μL (a and b) or 200 μL (c) of each sample was dispersed in 3.0 mL of Tol.
During the temperature increase, the signal for dMSC-393 disappeared at 210 C (a and b, similar to Figure 1) or at 190 C (c). For the largest
concentration (a), dMSC-460 started to evolve at 210 C (similar to the behavior in Figure 1). For the medium concentration (b), dMSC-460
appeared at 230 C. For the lowest concentration (c), QDs nucleated and grew at temperatures above 230 C. A featureless optical absorption
was observed at 210 C (a and b) or between 190 and 210 C (c). At sufficiently high concentrations (a and b), the fragmentation of PC-393
enabled the dMSC-393 to dMSC-460 transformation (Scheme 1). At low concentrations (c), the fragmentation of PC-393 facilitated the
nucleation and growth of conventional QDs (Scheme S1).[1,2]
c), the dMSC-393 to QD transformation occurred readily. We
therefore propose that during the increase of the temperature,
dMSC-393 transferred to PC-393 below 210 C (via Step 1). At
sufficiently high concentrations (Parts a and b in Figure 2 as well as
Figure 1), some PC-393 fragmented into monomers, which
transformed the remaining PC-393 to PC-460 at ~210 C (via Step
2). Thus, dMSC-460 evolved (via Step 3). When the concentration
of dMSC-393 was low (Part c in Figure 2), almost all PC-393
fragmented, feeding the nucleation and growth of QDs observed at
temperatures higher than 230 C. The MSC to PC to monomer to
QD pathway (as shown in Scheme S1) has been illustrated
elsewhere.[1-3]
To assess the hypothesis of the PC-enabled transformation
shown in Scheme 1, we carried out reactions using a 4Cd to 1Se
feed molar ratio with eight Se feed concentrations ranging from 1.9
to 75.0 mmol∙kg-1 as shown in Figures 3 and S3-1. For the study in
which we started with dMSC-393 (purified Sample A as presented
in Figures 1 and 2), PC-393 was generated as a result of heating (as
described by Step 1), which caused monomers to be produced via
the fragmentation of PC-393. However, for reaction mixtures of
Cd(MA)2 and Se in ODE (with the addition of HOAc in the induction
period at 120 C),[16,38] PC-393 and monomers will form prior to
dMSC-393.[16,38] In this case, the PC-393 to dMSC-393
transformation (Step 4) competes with the PC-393 to PC-460
transformation (Step 2). The relative evolution of dMSC-393 and
dMSC-460 is expected to be different for reaction mixtures of
Cd(MA)2 and Se (Figures 3 and S3-1), compared to that for dMSC-
393 (purified Sample A shown in Figures 1 and 2).
1. The addition of HOAc was performed at 110 C. From 170 to 250
C in 20 C steps, six samples were taken. For the optical
measurements, a volume of 10 μL (Batch a) or 50 μL (Batch b) from
each extracted sample was dispersed in 3.0 mL of Tol. For the
reaction mixture with a relatively high Se concentration of 37.5
mmol∙kg-1 (Part a of Figure 3), dMSC-393 seemed to evolve at 170
C and increased in strength at 210 C. At 230 C, dMSC-393
decreased, while dMSC-460 developed. At 250 C, dMSC-393
disappeared completely (according to PL), while dMSC-460
increased in strength. The coexistence of dMSC-393 and dMSC-460
at 230 C is noteworthy, and the evolution of dMSC-460 seems to
be at the expense of dMSC-393. This effect has also been observed
in the reaction mixture of cadmium stearate (Cd(St)2) and Se (Figure
S3-2). For the reaction mixture with
a relatively low Se
concentration of 7.5 mmol∙kg-1 (Part b of Figure 3), dMSC-393 did
not appear to evolve, while dMSC-460 did at a relatively low
temperature of 190 C. Afterwards, dMSC-460 kept increasing in
strength (up to the maximum monitored temperature of 250 C).
The evolution of dMSC-460 without dMSC-393 in Batch b is
noteworthy; the absence of dMSC-393 is consistent with the
concept that dMSC-460 directly transforms from PC-460 (Step 3 of
Scheme 1) and not from dMSC-393.
The PC-enabled transformation pathway depicted in Scheme 1
provides a basic understanding of the various relationships
observed for the evolution of dMSC-393 and dMSC-460 in the
reaction mixtures shown in Figures 3 and S3-1. We argue that for
the reaction mixture of Figure 3 Batch a, the direct formation of PC-
393 and the PC-393 to dMSC-393 transformation (Step 4) both
started around 170 C. At 230 C, the PC-393 to PC-460
transformation (Step 2) and PC-460 to dMSC-460 transformation
(Step 3) took over. For the reaction mixture of Figure 3 Batch b, Step
In Figure 3 we present the evolution of the optical properties of
samples extracted from two reaction mixtures of Cd(MA)2 and Se
in ODE with the Se concentrations of 37.5 (a) and 7.5 (b) mmol∙kg-
4
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PC-393 present in the reaction mixtures, the PC-393 to PC-460
transformation (Step 2) was relatively likely. Subsequently, a
continuous increase of dMSC-460 was observed with increasing
temperature (in the presence or absence of dMSC-393 depending
on the Se feed concentration).
To examine further our hypothesis of the PC-enabled
transformation shown in Scheme 1, we carried out also
synchrotron-based SAXS. With excellent statistics as well as simple
and non-damaging sample preparation, SAXS has been applied to
investigate various semiconductor nanostructures, including
QDs,[46-48] MSCs,[9,13,19] and NPLs.[49,50] Part a of Figure 4 presents
the SAXS profiles collected from purified Samples A (blue circular
symbols) and B (red square symbols) at room temperature. Three
Bragg peaks are observed for both purified samples. Also, Part a
contains the corresponding traces for the data fitted assuming two
unified Guinier-Porod levels (Level 1 (L1) and Level 2 (L2)) and three
Lorentz peaks. See Experimental methods in Supplementary
Information for the method used for the data fitting. The fitted
SAXS data (the vertical values of Part a) are mathematically
separated and presented in Parts b and c of Figure 4, with blue for
purified Sample A and red for purified Sample B. Part b (the form
factor) shows the L1 and L2 signals, and Part c (the structure factor)
the three Lorentz peak signals. The scale of the y axis of Part b is
about 100 times larger than that of Part c.
Figure 3. Evolution of optical absorption (solid traces) and PL (dashed
traces, normalized) spectra for samples from two reactions with Se feed
concentrations of 37.5 (a) and 7.5 (b) mmol∙kg-1. The reactions in ODE
had a feed molar ratio of 4Cd(MA)2 to 1Se, with the HOAc addition at
110 C prior to the nucleation and growth of QDs. Six samples were
extracted from 170 to 250 C as indicated. For the optical
measurements, a volume of 10 (a) and 50 (b) μL of each sample was
dispersed in 3.0 mL of Tol. For Batch a, dMSC-393 evolved at 170 C and
increased at 210 C, while dMSC-460 evolved at 230 C and increased
at 250 C. For Batch b, only dMSC-460 evolved at 190 C and increased
at 250 C. The evolution of dMSC-393 competed with that of dMSC-460
for Batch a, while dMSC-460 evolved with the absence of dMSC-393 for
Batch b. With the PC-enabled transformation pathway (Scheme 1)
providing a basic understanding of the experimental observations, it is
reasonable that the Se feed concentration affects the evolution
relationships. The lower the Se feed concentration, the smaller the
amount of PC-393 produced. Relatively large and small amounts of PC-
393 favor Steps 4 and 2, respectively; hence, relatively high and low Se
feed concentrations facilitate the evolution of dMSC-393 and dMSC-
460, respectively.
The SAXS result shown in Part a is consistent with the TEM and
4 was inhibited due to a low concentration of PC-393, while Step 2
occurred at a relatively low temperature of 190 C, together with
Step 3. From 190 C to higher temperatures (such as 250 C), Steps
2 and 3 continued and dMSC-460 increased in strength.
Thus, for the reaction mixtures of Cd(MA)2 and Se in ODE, the
development of dMSC-393 from PC-393 via Step 4 competes with
that of dMSC-460 via Steps 2 and 3. This competition (between
Steps 4 and 2) can be tuned by the Se feed concentration, which
strongly influences the amount of PC-393. For example (Figures S3
and S3-1), when the Se feed concentration was 37.5, 30.0, and 15.0
mmol∙kg-1, both dMSC-393 and dMSC-460 were observed at 230,
210, and 190 C, respectively. Figure S3-3 shows the optical
properties of the samples extracted at 210 C from the eight
batches shown in Figures 3 and S3-1. When the Se concentration
was deceased to 7.5 and 3.8 mmolkg-1, dMSC-460 started to evolve
at 190 C without dMSC-393 being present. Thus, Step 4 is favored
by a relatively high Se feed concentration (such as above 15.0
mmol∙kg-1), while Step 2 by a relatively low Se feed concentration
(such as 7.5 mmol∙kg-1).
Furthermore, the reaction mixtures (shown in Figures 3 and S3-
1) did not have a temperature interval of featureless optical
absorption, such as that observed for purified Sample A (as shown
in Figures 1 and 2) between 190 and 210 C. We attribute this
difference to the different generation of PC-393 and monomers,
which are direct in the reaction mixtures (before dMSC-393 could
form) and indirect in purified Sample A (via Step 1 and the
fragmentation of PC-393, respectively). With both monomers and
Figure 4. SAXS investigation of dMSC-393 (purified Sample A) and
dMSC-460 (purified Sample B). The two samples were kept for 7
days before the measurements. Part a contains the original SAXS
profiles which were collected at room temperature, with blue
circles for Sample A and red squares for Sample B. The blue and red
traces in Part a represent the corresponding fitted data which are
separated presented in Parts b and c as well. In Part b, the
scattering signals Level 1 (L1) suggest that dMSC-393 and dMSC-
460 are disc-shaped and have the respective radii of gyration (Rg)
of 0.92 and 1.14 nm. The scattering signals Level 2 (L2) indicate that
dMSC-393 and dMSC-460 aggregated.[9] In Part c, the three Lorentz
peaks, q1, q2, and q3, have the peak position ratio of 1 : 2 : 3,
signifying the presence of multilayered nanostructures in both
samples with the same layered spacing of 2π/q1.[49,51]
5
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10.1002/anie.202104986
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RESEARCH ARTICLE
XRD study shown in Parts a and b of Figures S1-2 and S4-2 for
Samples A and B purified with Tol, respectively. We note that
purified Sample A used for Figures 1, 2, and 4 was from the same
synthetic batch. In the same presentation format as Figures S1-1
and S1-2 for Sample A, Figure S4-1 shows the optical properties of
Sample B (as-synthesized and purified). The corresponding TEM
and XRD results are shown in Figure S4-2. Combined with the TEM
and XRD results, we attribute the two types of scattering signals
shown in Parts b and c to 0D and 2D species, respectively. Each of
the two samples contained a mixture of 0D and 2D species, with
the latter species in a relatively small amount. See Scheme S2 for
the possible connection between these species.
The 0D species related scattering signals shown in Part b suggest
that dMSC-393 and dMSC-460 respectively have the radius of
gyration (Rg) of 0.92 and 1.14 nm, and they have a similar exponent
of the SAXS form factor of 2.1. Thus, dMSC-393 is smaller than
dMSC-460, both of which are disc-shaped. See Figure S4-3 for our
size estimation for dMSC-393 and dMSC-460. The effect of Cd to Se
feed molar ratios on the evolution of dMSC-393 and dMSC-460
(Figures S4-4 and S4-5) shows that relatively high and low Cd to Se
feed molar ratios favor dMSC-393 and dMSC-460, respectively. This
is in agreement with the SAXS result that dMSC-393 is smaller than
dMSC-460.
For the 2D species related scattering signals shown in Part b,
these two sets of peaks appear at the same positions of 1.59 (q1),
3.16 (q2), and 4.77 nm-1 (q3). The apparent ratios of the peak
positions of q1 to q2 to q3 are about 1 to 2 to 3. Such a result
suggests the presence of layered structures in both samples. The
two layered structures have a similar interlayered spacing, which is
estimated to be about 3.95 nm from the calculation of 2π/q1.[49,51]
See Figure S4-6 for further explanation. It is necessary to point out
that purified Samples A and B were kept for 7 days before
performing the SAXS measurements; thus, the self-assembly of
dMSC-393 and dMSC-460 had proceeded during the storage period
to a certain degree,[16] with the majority of the 0D dMSCs remaining.
transformation (Step 1) resulted in nearly featureless optical
properties at 210 C (Figures 1 and 2). When the concentration of
dMSC-393 was high enough (Figure 1 and Parts a and b of Figure 2),
the fragmentation of PC-393 to monomers accounted for the
subsequent evolution of dMSC-460 (via Steps 2 and 3). On the other
hand, when the concentration of dMSC-393 was low enough (Part
c of Figure 2), the fragmentation of PC-393 to monomers resulted
in the generation of QDs. For reaction mixtures of Cd(MA)2 and Se
in ODE (Figure 3), the evolution of dMSC-393 was competitive to
that of dMSC-460 (Batch a with a relatively high Se feed
concentration), while the latter developed without the presence of
the former (Batch b with a relatively low Se feed concentration).
We argue that the Step 4 (PC-393 to dMSC-393 transformation)
competes with Steps 2 and 3 (the PC-393 to PC-460 and dMSC-460
transformation). Furthermore, Steps 4 and 2 are respectively
facilitated, when the amount of PC-393 is relatively high and low.
Thus, relatively high Se feed concentrations favor dMSC-393, while
low concentrations give rise to dMSC-460. We have carried out a
preliminary density function theory (DFT) study, as shown by
Figures S5-1 to S5-3 (which is based on XRD shown in Part b of
Figure S1-2), to assist our understanding of the transformation
pathway. More sophisticated theoretical models are needed to
understand better the thermodynamics and kinetics of the clusters
and their PC-enabled formation. It is worth noting that an
understanding of these PL nano-species and their formation will aid
their potential applications such as in optoelectronic displays and
solid-state lighting.[52-55] We anticipate that the PC-enabled
pathway (Scheme 1) has general applicability for transformations
such as from PL CdSe dMSC-460 to dMSC-513,[36] as well as from PL
CdTe dMSC-427 to dMSC-495 to dMSC-550 to dMSC-600.[31] The
present findings bring sophisticated insight into the pathway of the
MSC evolution and transformation, the latter of which experiences
reactions of monomer substitution[1] or addition that transform
two MSC counterpart PCs.
Acknowledgements
Conclusion
K. Y. thanks the National Natural Science Foundation of China
(NSFC) 21773162, the Fundamental Research Funds for the Central
Universities, the Applied Basic Research Programs of Science and
Technology Department of Sichuan Province 2020YJ0326, the State
Key Laboratory of Polymer Materials Engineering of Sichuan
University respectively for Grant No. sklpme2020-2-09, and the
Open Project of Key State Laboratory for Supramolecular Structures
and Materials of Jilin University for SKLSSM 2021030. We thank
Prof Jianyi Ma for useful discussions on DFT, and the Analytical &
Testing Center of Sichuan University for characterization including
TEM and XRD.
We have proposed a PC-enabled pathway (as depicted in Scheme
1) that is responsible for the transformation from one PL CdSe
species to another, based on the present experimental data and
data analysis. The two species display sharp optical absorption
doublets at 373/393 and 433/463 nm, with PL peaking around 399
and 466 nm, respectively. We performed reactions of Cd(MA)2 and
Se in ODE, with HOAc addition in the induction period at 110 C
prior to nucleation and growth of CdSe QDs. Two samples, A and B,
were obtained respectively at 190 and 230 C. After being purified
with Tol, the two samples still respectively contained mainly 0D
dMSC-393 and 0D dMSC-460, as suggested by TEM, XRD, and
synchrotron-based SAXS. Furthermore, SAXS indicated that the
radius of gyration (Rg) was 0.92 and 1.14 nm for dMSC-393 and
dMSC-460, respectively, and they were disc-shaped with the
exponent of the SAXS form factor close to 2 (Figure 4). Thus, dMSC-
393 is smaller than dMSC-460, and they can be selectively
synthesized with relatively high and low Cd to Se feed molar ratios.
With the monomer addition (Step 2) that allows smaller dMSC-393
to transform into larger dMSC-460, the PC-enabled pathway
(Scheme 1) provides a unique framework that satisfactorily
elucidates various experimental observations. When dMSC-393
(purified Sample A) was heated, the dMSC-393 to PC-393
Keywords: magic-size clusters (MSCs)  semiconductor quantum dots
(QDs) ∙cadmium selenide (CdSe)  transformation pathway ∙colloidal
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Entry for the Table of Contents
Magic-size clusters (MSCs) have their own unique precursor compounds (PCs). A PC-enabled pathway has been proposed for the
transformation from photoluminescent (PL) CdSe displaying one absorption doublet at 373/393 (dMSC-393) to at 433/460 (dMSC-460).
This pathway contains three key steps with monomer addition (Step 2) transferring PC-393 to PC-460.
8
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