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Cite This: J. Phys. Chem. Lett. 2018, 9, 6356−6363

Effect of Small Molecule Additives in the Prenucleation Stage of Semiconductor CdSe Quantum Dots Yuanyuan Liu,† Maureen Willis,†,‡ Nelson Rowell,§ Wenzhi Luo,† Hongsong Fan,∥ Shuo Han,*,† and Kui Yu*,†,∥,⊥ †

Institute of Atomic and Molecular Physics, Sichuan University, 610065 Sichuan, P. R. China School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, U.K. § National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada ∥ Engineering Research Center in Biomaterials, Sichuan University, 610065 Sichuan, P. R. China ⊥ State Key Laboratory of Polymer Materials Engineering, Sichuan University, 610065 Sichuan, P. R. China

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S Supporting Information *

ABSTRACT: For the addition of small molecules in the prenucleation stage of colloidal CdSe conventional quantum dots (QDs), there is insufficient knowledge regarding what are advantageous circumstances. Here, we present a study about such addition. When CH3COOH or Zn(OOCCH3)2 is added in the prenucleation stage (at 120 °C) of a reaction consisting of cadmium myristate (Cd(OOC(CH2)12CH3)2, Cd(MA)2 made from CdO) and Se powder in 1-octadecene (ODE), CdSe magic-size clusters (MSCs) exhibiting a single sharp absorption doublet at 433/460 nm are synthesized in a singleensemble form (around 220 °C). We demonstrate that such small molecule addition suppresses the nucleation and growth of QDs and thus directs a competitive process to the formation of MSCs. The present study provides insight into the individual but linked pathways to forming CdSe QDs and MSCs and introduces new avenues to improve the production of MSCs through the addition of small molecules in the prenucleation stage.

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wavelengths almost, while that of QDs often continuously red shifts due to QD growth in size. Two distinct types of MSCs have been reported. One type exhibits one sharp absorption singlet;1,6−10,12−14 the other type displays one sharp absorption doublet.11,14−25 The two types are referred to as sMSCs and dMSCs, respectively.11 For dMSCs, CdTe dMSC-371 with one sharp absorption doublet at ∼350/371 nm was recently reported to evolve from IP samples of cadmium acetate (Cd(OOCCH3)2, Cd(OAc)2) and tri-n-octylphosphine telluride (TeTOP).11 The addition of small molecules, such as an acetate salt Cd(OAc)2 or Zn(OAc)2, was shown to produce CdSe nanocrystals (NCs) exhibiting one sharp absorption doublet from reactions with cadmium myristic (Cd(OOC(CH2)12CH3)2, Cd(MA)2 prepared from CdO) and Se powder in 1-octadecene (ODE).14,26,27 Critically, the small molecule was added at an elevated temperature such as 160 °C, at which point nucleation and growth of CdSe QDs had already taken place.14 Thus, the resulting product consisted of both CdSe QDs and dMSCs; to remove the QDs produced, tedious purification had to be performed, which also led to the selfassembly of the dMSCs to nanoplatelets with 1-dimensional

ery recently, a two pathway model has been proposed for the formation of colloidal semiconductor quantum dots (QDs).1 This model hypothesizes that there exist two individual but linked pathways during the prenucleation stage of QDs. The prenucleation stage, which occurs prior to nucleation and growth of QDs, is also termed as the induction period (IP).2 In one of the two proposed pathways, monomers and fragments formed in the prenucleation stage lead to nucleation and growth of QDs, as suggested by the conventional LaMer model of classical nucleation theory (CNT).3−5 In the other pathway, the self-assembly of cation (M = Cd and Zn) and anion (E = S, Se, and Te) precursors is postulated to result in the formation of magic-size clusters (MSCs) via structural transformations from their special precursor compounds formed in the prenucleation stage.6−11 The precursor compounds of MSCs are transparent in optical absorption. Importantly, the formation of one precursor compound from one assembled species (via dense phase reactions) occurs often before that of the monomer and fragment, while the growth of QDs is able to reduce the precursor compound via its fragmentation. MSCs and QDs differ from each other in many respects.12,13 For example, the optical absorption bandwidth of MSCs is narrower than that of QDs, due to the tighter size distribution of MSCs. The narrow size distribution is a direct result of structural stability of MSCs. Also, for samples extracted from a reaction batch, the absorption feature of MSCs is at constant © XXXX American Chemical Society

Received: September 30, 2018 Accepted: October 18, 2018

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DOI: 10.1021/acs.jpclett.8b03016 J. Phys. Chem. Lett. 2018, 9, 6356−6363

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The Journal of Physical Chemistry Letters quantum confinement.14,15 To the best of our knowledge, there has been no systematic study reported regarding the addition of the small molecules within the prenucleation stage of CdSe QDs. Here, we report a deliberate study about the addition of small molecules during the prenucleation stage of CdSe QDs. Small molecules, especially those containing the acetate group CH3COO−, are usually present in the synthesis of colloidal CdSe MSCs, which are particularly characterized by a single, relatively sharp absorption doublet, a property that is distinctly different from those of conventional CdSe QDs.14−17,26−28 Accordingly, Figure 1 addresses the reaction of Cd(OAc)2 and Se in ODE. In order to suppress the presence of QDs, the addition of HOAc in the prenucleation stage (at 120 °C) of the same reaction of Cd(OAc)2 and Se in ODE is investigated. To better control the presence of HOAc, the reactions of Cd(MA)2 (made from CdO) and Se in ODE are developed with the addition of HOAc or Zn(OAc)2 in the prenucleation stage (Figures 3−5). All the reactions have a fixed feed molar ratio of 4Cd to 1Se and a fixed Se concentration of 30 mmol/ kg in ODE. We show that when a reaction of Cd(MA)2 and Se in ODE is heated from room temperature and the addition of CH3COOH (HOAc) or Zn(OAc)2 is performed at 120 °C, the resulting product at 220 or 240 °C is single-ensemble CdSe dMSCs exhibiting a single sharp absorption doublet at 433/ 460 nm and without QDs. The ensemble is labeled as dMSC460. The small molecule addition suppresses the nucleation and growth of QDs and facilitates the production of dMSC460 in a single-ensemble form at elevated temperatures, as shown by Scheme 1. The present study introduces new Scheme 1. Schematic Drawing To Illustrate the Effect of the Addition of CH3COOH (HOAc) in the Prenucleation Stage (at 120 °C) of the Reaction of Cd(MA)2 and Se in ODEa

Figure 1. Optical absorption (dashed lines, left y axis) and emission spectra (solid lines, excited at 350 nm, right y axis) of the CdSe 220 °C/15 min samples (30 μL dispersed in 3.0 mL of toluene) from three reaction batches consisting of the same amounts of HMA, Cd(OAc)2, and Se powder. They were mixed at room temperature with the feed molar ratios of 8.8HMA (1.32 mmol) to 4Cd(OAc)2 (0.60 mmol) to 1Se (0.15 mmol) and a Se concentration of 30 mmol/kg in ODE. For the heating stage from room temperature to 120 °C and the 2 h stage at 120 °C, either a N2 atmosphere or vacuum is applied, as indicated. There appears to be a strong correlation between the vacuum and N2 conditions and the resulting products produced.

a

Without addition (top), a mixture of MSCs and QDs is obtained. With the addition (bottom), a single ensemble of dMSC-460 is obtained.

avenues to synthesize single-ensemble CdSe dMSCs upon the addition of small molecules in the prenucleation stage of CdSe QDs and brings deeper insight into the existence of two individual but linked pathways to the production of CdSe QDs and dMSCs. Figure 1 presents the optical properties of three samples extracted from three batches at 220 °C with a reaction period of 15 min at this temperature. The three batches address the same reactions of Cd(OAc)2, myristic acid (CH3(CH2)12COOH, HMA), and Se in ODE. The only difference is in the application of vacuum and/or N2 during the

two stages, when the temperature is increased from room temperature to 120 °C and when the temperature is kept at 120 °C for 2 h (Table S1). Afterward, the three batches are protected under a N2 atmosphere and the temperature is increased. Figure S1-1 shows evolution of the optical properties of the three batch samples; from each batch, there are 10 samples extracted during the temperature increase from 6357


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Figure 2. Evolution of the absorption (left) and emission (right, excited at 350 nm) spectra of the CdSe samples extracted (15 μL dispersed in 3.0 mL of toluene). The addition of HOAc (0.50 mmol) was performed under a N2 atmosphere at 120 °C after a vacuum was applied throughout the heating stage to 120 °C and the constant temperature stage at 120 °C. After the addition, the reaction is kept at 120 °C for 15 min. Afterward, the reaction temperature is increased and samples are taken, similar to those of Figure 1 batches (Figure S1-1) at (1) 140 °C/0 min, (2) 160 °C/0 min, (3) 160 °C/15 min, (4) 180 °C/0 min, (5) 180 °C/15 min, (6) 200 °C/0 min, (7) 200 °C/15 min, (8) 220 °C/0 min, (9) 220 °C/15 min, and (10) 240 °C/0 min. Interestingly, no QDs are produced and one single ensemble of CdSe dMSC-460 is obtained at 220 and 240 °C (samples 8− 10).

80 °C), HOAc is produced. By a side note, the melting point of Cd(OAc)2 is ∼255 °C; the apparent disappearance of Cd(OAc)2 at ∼80 °C for the three batches suggests that similar amounts of HOAc are produced around this temperature. Thus, prior to the increase of temperature from 120 °C under a N2 atmosphere, the amount of HOAc in a reaction depends on the use of vacuum and/or N2. From batch a to c, it is reasonable that the amount of HOAc decreased. At the end of the two stages, it seems that there remains enough HOAc in batches a and b to produce CdSe dMSCs only, while both CdSe dMSCs and QDs are produced in batch c due to the presence of little HOAc. The presence of HOAc seems to suppress the nucleation and growth of QDs (after the duration stage at 120 °C). To test this hypothesis, 0.50 mmol of HOAc is added into a batch. Before the addition, this batch is identical to Figure 1 batch c with a vacuum applied throughout the heating stage to 120 °C and the constant temperature stage at 120 °C. The addition is performed at the end of the two stages at 120 °C but under a N2 atmosphere. Figure 2 presents evolution of the optical properties of 10 samples extracted from the batch. Similar to those obtained from the Figure 1 batches, sampling is performed when the temperature is increased to 240 °C. Figure S2 shows the absorption and emission spectra of sample 9 (220 °C/15 min). Remarkably, no QDs but dMSCs are produced; furthermore, dMSC-460 is almost in a single-ensemble form at elevated temperatures (such as 240 °C). The evolution of dMSC-393 and dMSC-460 seems to be similar to that of Figure 1 batch a (Figure S1-1); dMSC-393 seems to develop slightly earlier than dMSC-460. For the batch associated with Figure 2, however, dMSC-460 becomes dominant at 220 °C (sample 8) and changes little when the temperature is increased to 240 °C (sample 10). For Figure 1 batch a, a mixture of both dMSC393 and dMSC-460 is observed to coexist at 220 °C (sample 8); when the temperature is increased to 240 °C, the relative population of the two ensembles is almost constant. For Figure 1 batch b, only dMSC-460 appears; when the temperature is increased from 140 to 240 °C, its population increases. It is probable that the different evolution of dMSC-393 and dMSC-

140 to 240 °C. Further details regarding the experiments can be found in the Supporting Information. It is noteworthy that a relatively large amount of HMA and a relatively large Se concentration are used to obtain the Figure 1 results, compared to literature reports about the similar reaction of Cd(OAc)2 and HMA and Se in ODE.14,16,17 The feed molar ratio of HMA to Cd(OAc)2 is 2.2 to 1.0 instead of, for example, 1.0 to 1.0. The Se concentration is 30 mmol/kg instead of 10 mmol/kg. Interestingly, the optical properties of the resulting CdSe products from the three batches are significantly different. As shown by Figure 1, only when N2 is used during the heating stage to 120 °C and the constant temperature stage at 120 °C (Batch a) are two CdSe dMSC ensembles produced. One exhibits one sharp absorption doublet at 371/392 nm and an emission peak at 399 nm, while the other displays one sharp absorption doublet at 432/459 nm and an emission peak at 463 nm. These MSCs are referred to as dMSC-393 and dMSC460, respectively. When vacuum is used during the heating stage and N2 is applied in the constant temperature stage at 120 °C (batch b), a CdSe dMSC-460 ensemble is detected, with an absorption doublet at 431/454 nm and an emission peak at 459 nm. Figure S1-2 shows that dMSC-460 is 0dimension with a size of ∼2 nm. When a vacuum is applied throughout the two stages (batch c), three emission peaks at 463, 503, and 567 nm are discernible. The relatively narrow emission peaking at 463 nm is associated with CdSe dMSC460 exhibiting an absorption doublet at 430/454 nm, while the relatively broad emission peaking at 567 nm is related to the CdSe QDs exhibiting a relatively broad absorption peaking at ∼550 nm. For the temperature increase stage to 120 °C from room temperature followed with the 2 h stage at 120 °C, Figure 1 demonstrates a clear correlation between the use of vacuum and/or N2 and the dMSCs and QDs produced at higher temperatures. Such dependence, which is related to synthetic reproducibility, can be attributed to the reaction of Cd(OAc)2 + 2HMA ⇒ Cd(MA)2 + 2HOAc; when the Cd source compound Cd(OAc)2 is dissolved in a reaction batch (around 6358


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Figure 3. Evolution of optical absorption (left) and emission (right, excited at 350 nm) of CdSe samples (15 μL dispersed in 3.0 mL of toluene) from three synthetic batches that have a feed molar ratio of 8.8HMA (1.32 mmol)−4CdO (0.60 mmol)−1Se (0.15 mmol) and a Se concentration of 30 mmol/kg in ODE. The three batches are evacuated during the temperature increase from room temperature to 120 °C and the 2 h duration at 120 °C. Afterward, a N2 atmosphere is applied. (a) No addition is performed. (b) 5.00 mmol of HOAc is added and the temperature is kept at 120 °C for 15 min. (c) The temperature is cooled to 80 °C for the addition, and the temperature is kept for 15 min after addition. After the first sample is taken at 120 °C/15 min (a and b) or 80 °C/15 min (c), the reaction temperature is increased and samples are extracted. For batches a and b, sampling is performed at (2) 140 °C/15 min, (3) 160 °C/15 min, (4) 180 °C/15 min, (5) 200 °C/15 min, (6) 220 °C/15 min, (7) 240 °C/ 0 min, and (8) 240 °C/15 min. For Batch c, samples are taken at (2) 100 °C/15 min, (3) 120 °C/15 min, (4) 140 °C/15 min, (5) 160 °C/15 min, (6) 180 °C/15 min, (7) 200 °C/15 min, (8) 220 °C/15 min, (9) 240 °C/0 min, and (10) 240 °C/15 min. Evidently, the addition of HOAc in the IP suppresses the nucleation and growth of QDs and enables the evolution of a single ensemble of CdSe dMSC-460 at an elevated temperature (220 °C, trace 6 for a and b while trace 8 for c).

would be improved. It is fairly straightforward that the addition of HOAc after vacuum (at the end of the two stages) allows much better control over the concentration of HOAc in reactions, such as seen by the Figure 2 batch compared with Figure 1 batches a and b. Toward this end, the synthesis with Cd(MA)2 as a Cd precursor (made from CdO) is designed; for the reactions studied (as shown by Figures 3−5 below), they have the same Cd to Se feed molar ratio of 4 to 1 and the Se

460 in the four batches is due to the different amounts of HOAc presented in these batches. From what we have seen, the presence of HOAc in the prenucleation stage plays apparently an important role both in the suppression of the nucleation and growth of QDs and in the formation of dMSCs. This observation leads to the hypothesis that if the amount of HOAc in the prenucleation stage is controlled, the reproducibility of the dMSC synthesis 6359


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Figure 4. Evolution of optical absorption (left) and emission (right, excited at 350 nm) of CdSe samples (15 μL dispersed in 3.0 mL of toluene) from two synthetic batches similar to those of Figure 3, with a feed molar ratio of 8.8HMA (1.32 mmol)−4CdO (0.60 mmol)−1Se (0.15 mmol) and a Se concentration of 30 mmol/kg in ODE. The addition of Zn(OAc)2 (0.13 mmol (a) or 0.25 mmol (b)) is performed at 120 °C. After 15 min, the first sample is taken at 120 °C/15, and the reaction temperature is increased with sampling at (2) 140 °C/15 min, (3) 160 °C/15 min, (4) 180 °C/15 min, (5) 200 °C/15 min, (6) 220 °C/15 min, (7) 240 °C/0 min, and (8) 240 °C/15 min. Clearly, with the addition of Zn(OAc)2 in the IP, the nucleation and growth of QDs is suppressed as well.

microscopy (TEM) and X-ray diffraction (XRD) study of the CdSe 220 °C/15 min sample. Similar to published results,14−17 the as-synthesized CdSe species are 0-dimensional with a diameter of ∼2 nm. Figure S3-2 presents photoluminescence quantum yield characterization. For batch c, again, no QDs are produced even at 240 °C, and a single ensemble of CdSe MSC-460 is also obtained at 220 °C (sample 8), obviously without the coproduction of QDs. Compared with batches a and b, there are two more samples extracted at 80 and 100 °C from this batch; they are samples 1 and 2, respectively. The evolution of dMSC-393 and dMSC-460 seems to take place at 160 °C (trace 5). At 220 °C (trace 8), dMSC-393 disappears, but the population of dMSC460 increases. Afterward, dMSC-460 changes little when the temperature is further increase to 240 °C. The growth patterns of the two dMSC ensembles are quite similar to those of batch b, suggesting high synthetic reproducibility. The suppression of the nucleation and growth of QDs is evidently achieved by the addition of HOAc in the prenucleation stage, which also facilitates the production of a single ensemble of CdSe dMSC-460 at an elevated temperature (220 °C). This ensemble exhibits one sharp emission at ∼465 nm with little broad trap emission at longer wavelengths. The broad temperature range for small molecule addition is an obvious advantage, regarding the improvement of synthetic reproducibility of the CdSe dMSCs.

concentration of 30 mmol/kg in ODE. Similar to what is done in conjunction with Figure 1 batch c, a vacuum is applied throughout the two stages. Afterward, the addition of HOAc or Zn(OAc)2 is performed in the prenucleation stage under a N2 atmosphere, and the temperature is kept for 15 min before it is increased to 240 °C. Again, further details regarding the experiments can be found in the Supporting Information. Figure 3 presents evolution of optical absorption (left) and emission (right) of samples taken from three batches with Cd(MA)2 as a Cd precursor. For batch a, there is no HOAc addition, and the result is presented as a background for batches b and c. The addition of HOAc (5.00 mmol) is performed for batch b at 120 °C and for batch c at 80 °C. For batch a, the nucleation and growth of QDs appears to occur around 140 °C (trace 2). During the temperature increase to 240 °C, the QDs grow in size (trace 8), exhibiting broad absorption and emission peaking at ∼590 and ∼610 nm, respectively. Meanwhile, it seems that at 160 °C (trace 3), a small amount of CdSe MSC-460 appears and its amount changes little afterward. For batch b, interestingly, no QDs are produced even at 240 °C. The evolution of dMSC-393 and dMSC-460 takes place at 180 °C (trace 4). At 220 °C (trace 6), the former ensemble disappears, and the population of the latter increases. When the temperature is further increased to 240 °C from 220 °C, dMSC-460 changes little and seems to be quite stable thermally. Figure S3-1 shows our transmission electron 6360


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Figure 5. Evolution of absorption (left) and emission (right, excited at 350 nm) of the CdSe samples (15 μL dispersed in 3.0 mL of toluene) extracted from two reactions, which are similar to Figure 3 batch b, but with the addition of HOAc of 0.50 (a) and 10.00 mmol (b) added at 120 °C. The sampling is performed at (1) 120 °C/15, (2) 140 °C/15 min, (3) 160 °C/15 min, (4) 180 °C/15 min, (5) 200 °C/15 min, (6) 220 °C/15 min, (7) 240 °C/0 min, (8) 240 °C/15 min, and (9) 240 °C/30 min. It seems that the optimal amount HOAc added in the IP is as broad as from 0.50 to 10.00 mmol, which produces CdSe dMSC-460 in a single ensemble form.

temperatures due to the presence of another cation in a reaction). For the approach developed with good synthetic reproducibility upon the addition of HOAc, Figure 5 presents further that the amount (0.50−10.00 mmol) of HOAc added at 120 °C does not play a critical role on the production of the single ensemble of CdSe dMSC-460 at 220 °C. Figure 5 shows evolution of the optical absorption (left) and emission (right) of the samples from two reaction batches. The two batches are similar to Figure 3 batch b, with the addition of HOAc performed at 120 °C but with different amounts of (a) 0.50 and (b) 10.00 mmol. Figure S5-1 presents evolution of the optical properties of the samples from a batch with the addition of HOAc of 0.25 mmol. Regarding the synthetic reproducibility of CdSe MSC-460 in a single ensemble form, which exhibits one sharp emission at 465 nm without broad trap emission, the optimal amount range of HOAc seems to be quite broad, which can span from 0.50 to 10.00 mmol. The present study explores a nonphosphorus containing approach to CdSe MSC-460 with a sharp emission peaking at 465 nm and with little broad trap emission at longer wavelengths. Intriguingly, when SeTOP is used as a Se precursor, as shown by Figure S5-2, the addition of HOAc (0.5−5.00 mmol) suppresses the nucleation and growth of QDs, and a single ensemble of CdSe dMSC-460 is produced at 220 °C also. However, this ensemble exhibits significant trap emission at wavelengths longer than 465 nm. The use of SeTOP as a Se precursor seems to affect the emission properties of resulting CdSe dMSCs; this is similar to that reported previously, where Cd(OAc)2 was used together with

Figure 4 presents evolution of the optical absorption (left) and emission (right) properties of two batch samples, with the addition of Zn(OAc)2. The two batches are the same as Figure 3 batch a; at the end of the two stages at 120 °C, a N2 atmosphere is applied and an addition of 0.13 mmol (a) or 0.25 mmol (b) is performed. After another 15 min at 120 °C, the first sample is taken. Then, the reaction temperature is increased with samples extracted, similar to Figures 3a and 3b, at (2) 140 °C/15 min, (3) 160 °C/15 min, (4) 180 °C/15 min, (5) 200 °C/15 min, (6) 220 °C/15 min, (7) 240 °C/0 min, and (8) 240 °C/15 min. Obviously, there are no QDs produced, and CdSe dMSC-393 does not seem to evolve either. The evolution of CdSe dMSC-460 occurs at 160 °C (trace 3) for batch a and at 140 °C (trace 2) for batch b. The population of dMSC-460 keeps increasing when the temperature is increased to 240 °C. Figure S4 presents the optical properties of another batch samples, with the same addition of Zn(OAc)2 but with a larger amount of 2.50 mmol. Similar to the HOAc addition in the prenucleation stage, the addition of Zn(OAc)2 in the prenucleation stage also articulately suppresses the nucleation and growth of CdSe QDs and enables the development of CdSe dMSCs. For the production of CdSe dMSCs without the presence of QDs, the addition of HOAc or an acetate salt in the prenucleation stage is a practical approach. Moreover, for the production of a single ensemble of CdSe dMSC-460 at an elevated temperature (220 °C) to exhibit one emission peak at 465 nm with little broad trap emission, the use of HOAc seems to be more efficient than that of Zn(OAc)2. At the same time, the use of Zn(OAc)2 might introduce complication (especially at high 6361


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No. sklpme2018-2-08), and Open Project of Key State Laboratory for Supramolecular Structures and Materials of Jilin University for SKLSSM 201830. S.H. thanks Sichuan University for postdoctoral fellowship financial support of 2017SCU12012. We thank Dr. Shanling Wang (Analytical & Testing Center, Sichuan University) for TEM and Prof. J. You for his quantum yield measurements.

HMA to react with SeTOP, but without the addition of HOAc.14 In conclusion, we have demonstrated that the presence of molecules containing the CH3COO− group in the prenucleation stage of CdSe QDs is able to effectively suppress the nucleation and growth of QDs. We have explored systematically the addition of HOAc and Zn(OAc)2 in the prenucleation stage (at 120 °C) of CdSe QDs, with Cd(MA)2 as a Cd precursor (made from CdO) to react with Se powder in ODE. The Cd to Se feed molar ratio is fixed at 4 to 1 and the Se concentration is 30 mmol/kg. Without the addition, the nucleation and growth of QDs seems to occur around 140 °C. With the addition, the nucleation and growth of QDs does not take place, even at 240 °C. Moreover, CdSe dMSC-460 evolves at 220 °C as a single ensemble exhibiting a sharp emission peak at ∼465 nm without broad trap emission at longer wavelengths. Importantly, the temperature at which the addition is performed and the added amount do not seem to be critical, suggesting that the approach has good synthetic reproducibility. Experimentally, the addition of small molecules in the prenucleation stage suppresses the nucleation and growth of QDs and enables the formation of CdSe dMSCs. We hypothesize that the added molecule interacts with the monomer, fragment, and precursor compound produced in the prenucleation stage,1,29 as shown by Scheme 1. The present study brings insights into the two pathway model to CdSe QDs and dMSCs;1,8,11 via the addition of small molecules within the prenucleation stage, only dMSCs are synthesized in a single ensemble form without the coproduction of QDs. The findings should assist the application development based on the species characterized by similar sharp absorption doublets.30−33 We also anticipate that the present results will encourage a reexamination of the published results,14−17,26−28,34−37 and will motivate theoretical efforts to narrow the knowledge gap regarding the structure−property relationship of sMSCs and dMSCs.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b03016. Experimental details including synthesis and characterization with optical absorption, emission, TEM, and XRD (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S.H. E-mail: [email protected]. *K.Y. E-mail: [email protected]. ORCID

Hongsong Fan: 0000-0003-3812-9208 Shuo Han: 0000-0003-0880-1833 Kui Yu: 0000-0003-0349-2680 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS K.Y. thanks National Natural Science Foundation of China (NSFC) 21573155 and 21773162, the State Key Laboratory of Polymer Materials Engineering of Sichuan University (Grant 6362


Letter

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