Four Types of CdTe Magic-Size Clusters from One Prenucleation

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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4345−4353

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Four Types of CdTe Magic-Size Clusters from One Prenucleation Stage Sample at Room Temperature Chaoran Luan,† Junbin Tang,‡ Nelson Rowell,§ Meng Zhang,‡ Wen Huang,∥ Hongsong Fan,*,† and Kui Yu*,†,‡,⊥ †

Engineering Research Center in Biomaterials, Sichuan University, Chengdu 610065, Sichuan, P. R. China Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, Sichuan, P. R. China § Metrology Research Centre, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada ∥ Laboratory of Ethnopharmacology, West China School of Medicine, Sichuan University, Chengdu 610065, Sichuan, P. R. China ⊥ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, P. R. China

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‡

S Supporting Information *

ABSTRACT: Four types of colloidal semiconductor CdTe magic-size clusters (MSCs), each of which is in a single-ensemble form, have been obtained at room temperature from a single induction period (IP) sample in dispersion. The induction period is the prenucleation stage that occurs prior to nucleation and growth of colloidal quantum dots (QDs). Three types display sharp optical absorption peaking at either 371, 417, or 448 nm, and the fourth type exhibits a sharp absorption doublet with peaks at 350 and 371 nm. These MSCs are respectively denoted as sMSC-371, sMSC-417, sMSC-448, and dMSC371. We show that the evolution of the various MSCs is affected by the nature of their dispersions. We hypothesize that the evolution of MSCs involves their precursor compounds (PCs), which are transparent in optical absorption. The present study explores new avenues for the exclusive synthesis of four types of CdTe MSCs (with each in a single-ensemble form) and provides an improved understanding for their formation.

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of a special type of precursor compounds (PCs) that leads to the evolution of MSCs via intramolecular reorganization.22−27 Similar to the monomers and fragments, the precursor compounds are transparent in optical absorption (compared to their corresponding MSCs).21−27 The precursor compounds and their corresponding MSCs might be respectively compared as amorphous and crystalline polymers. At the same time, a selective two-step approach has been developed for the production of single-ensemble MSCs free of QDs, such as for CdTe,21,22 CdS,23−25 CdSe,26 and ZnSe MSCs.27 In particular, for a reaction of Cd(OAc)2/OLA (made from cadmium acetate (Cd(OAc)2) and oleylamine (OLA)) and TeTOP (tri-n-octylphosphine telluride), the first step is to obtain a CdTe induction period sample from the reaction (which was heated at 130 °C for a period of 30 min). The second step is to engineer CdTe MSCs at room temperature from this CdTe induction period sample.21,22 The presence of the precursor compound in the first step (at 130 °C with the period of 30 min) was experimentally supported by 113Cd and 31 P nuclear magnetic resonance spectroscopy (NMR).21 For the second step at room temperature, two types of CdTe MSCs, dMSC-371, and sMSC-371, were detected to evolve

luster science has been focusing on narrowing the knowledge gap, regarding the evolution of fundamental properties between those of molecules and extended bulk materials.1−8 In this respect, there is a special type of clusters, which is colloidal semiconductor binary magic-size clusters (MSCs).9−27 They provide essential understanding for the variation of electronic and optical properties from molecules to colloidal semiconductor binary quantum dots (QDs). Semiconductor QDs have attracted intense interest for the last three decades,28−43 with present effort significantly made toward applying oriented research.40−43 Meanwhile, much less attention is paid to the MSCs, which are sometimes produced with QDs in conventional hot-injection and non-hot-injection approaches.35−37 Compared with a conventional QD ensemble, one MSC ensemble is usually characterized by a relatively narrow optical absorption band. The narrow bandwidth is related to the relatively small size variation of MSCs, which results from an unknown internal structure with particular stability.9,19 Also, the optical absorption band of one MSC ensemble usually has a constant peak position. Recently, a two-pathway model has been used to describe the very prenucleation stage, also called the induction period (IP), of colloidal semiconductor binary QDs.23 One pathway is associated with the conventional LaMer model of classical nucleation theory (CNT), which contains the formation of monomers (M) and fragments (F) to result in nucleation and growth of QDs.44,45 The other pathway involves the formation © XXXX American Chemical Society

Received: June 3, 2019 Accepted: July 15, 2019

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DOI: 10.1021/acs.jpclett.9b01601 J. Phys. Chem. Lett. 2019, 10, 4345−4353

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The Journal of Physical Chemistry Letters

Scheme 1. Schematic Drawing of Our Comprehension for the Changes (Shown in Figures 1 and 2) of the Four Types of CdTe MSCs, sMSC-371 (a), sMSC-417 (b), sMSC-448 (c), and dMSC-371 (d), Which Involve Their Corresponding Precursor Compounds (e)a

The four types of CdTe MSCs were obtained from one induction period sample (135 °C/10 min) at room temperature as monitored by optical absorption spectroscopy. The induction period sample (30 μL) was dispersed in 3.00 mL of four mixtures of Tol and OTA; the OTA volumes in the four mixtures were 1.00 (a), 0.50 (b), 0.20 (c), and 0.05 (d) mL. For the apparent MSC changes shown in Figures 1 and 2, which are displayed by dashed blue arrows, their corresponding precursor compounds are involved (as featured by solid line arrows). The evolution pathway hypothesized is labeled in sequence from (1) to (11). The present study does not address step 11 (indicated by a downward arrow in dashed gray). Steps 1/2, 4/5, 7/8, and 10/11 stand for the four transformations between the corresponding precursor compounds and MSCs. The precursor compound to MSC directions (represented by upward arrows in blue, steps 1, 4, 7, and 10) are relatively active, while the reverse directions (signified by downward arrows in gray, steps 2, 5, 8, and 11) are relatively passive. Steps 2, 5, and 8 are driven by steps 3/4, 6/7, and 9/10, respectively, while steps 3, 6, and 9 (denoted by solid gray arrows) may be the rate-determining steps for the apparent changes detected between corresponding two types of MSCs. a

from one induction period sample.21,22 The former displays one sharp absorption doublet peaking at 350/371 nm, while the latter exhibits one sharp absorption peaking at 371 nm. It

was concluded that, prior to the formation of CdTe dMSC-371 and CdTe sMSC-371, the CdTe precursor compounds (that was produced in the induction period sample) transformed 4346

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The Journal of Physical Chemistry Letters first, to the immediate precursor dIP-371 of CdTe dMSC-371 or to the immediate precursor sIP-371 of CdTe sMSC-371.22 In order to differentiate the immediate precursor (IP) from the induction period (IP), we now designate dIP-371 as PC-d371 and sIP-371 as PC-s371. Importantly, the CdTe precursor compounds ⇒ CdTe PC-d371 transformation took about 200 min.22 Afterward, the CdTe PC-d371 ⇒ dMSC-371 transformation featured a first-order unimolecular reaction kinetics behavior,22 similar to that when the CdS PC-311 ⇒ CdS MSC-311 transformation occurred.24 A mass spectrometry study showed that the CdS precursor and CdS MSC-311 had an almost identical mass.24 When CdS QDs grew in size, the reverse transformation of CdS MSC-311 ⇒ CdS precursor compounds was demonstrated.23 Moreover, a pair of CdS isomers, CdS MSC-311 and CdS MSC-322, was proposed.25 Clearly, there have been significant progresses made for CdS MSCs.20−22,46. For the various CdTe MSCs reported, sMSC371,21,22 sMSC-417,11,21 sMSC-448,11−13,15,21 and dMSC371,16,22 the evolution relationship of these CdTe MSCs has been scantly explored. Here, we report the evolution of the four types of CdTe MSCs, which exist in single-ensemble forms or in coexistence forms, from the induction period samples from reactions of Cd(OAc)2/OLA and TeTOP with a feed molar ratio of 4 to 1 and a TeTOP feed concentration of 44 mmol/kg. Scheme 1a− d presents the four types of CdTe MSCs in four singleensemble forms evolved from a single induction period sample. Scheme 1e shows the probable pathways hypothesized for the apparent changes in population of the four types of CdTe MSCs. The apparent decrease of sMSC-371 accompanied by the increase of sMSC-417 is illustrated in Figure 1, together with the apparent decrease of sMSC-417 accompanied by the increase of sMSC-448. Figure 2 deals with the development of dMSC-371 primarily with the decrease of sMSC-448. Figure 3 addresses the direct evolution of sMSC-448 from an induction period sample in methanol-containing dispersions, without the involvement of the other types of MSCs. The present study introduces a room-temperature approach to engineer these four types of CdTe MSCs, each of which is in a singleensemble form without the contamination of the other MSC types or QDs. Also, the present study suggests that these MSCs are a group of polymorphs, with the same CdTe core composition. The present study provides a step forward toward understanding the evolution of MSCs via their own precursor compounds (Scheme 1e). The optical absorption spectra shown in parts a−d of Scheme 1 were collected from a single induction period sample, which was obtained from the reaction of Cd(OAc)2/ OLA and TeTOP heated from room temperature to 135 °C and was held at that temperature for 10 min. Prior to the spectrum collection, an aliquot of 30 μL of the induction period sample was dispersed into four mixtures of 3.00 mL of toluene (Tol) and octylamine (OTA) at room temperature. When the mixture contained of 1.00 mL of OTA (a), the induction period sample dispersion displayed a sharp absorption peaking at 371 nm, indicating the presence of single-ensemble sMSC-371. When the OTA volume was reduced by 50% (b), a sharp absorption peaking at 417 nm was observed after 60 min, designating the presence of singleensemble sMSC-417. When the volume of the primary amine was further decreased to 0.20 mL (c), a single ensemble of sMSC-448 was apparent after 120 min. For the dispersion consisting of 0.05 mL of OTA (d), the presence of dMSC-371,

which gives rise to a sharp absorption doublet peaking at 350/ 371 nm, was detected after 240 min. For each of the four types of CdTe MSCs, it is possible to evolve into a single-ensemble form at room temperature from a unique induction period sample, without the contamination of the other type MSCs or QDs. It is clear that the particular type of CdTe MSCs that developed was considerably influenced by the amount of OTA in dispersion. For the four different volumes of OTA in the four Tol and OTA mixtures with a total volume of 3.00 mL each, a relatively large amount of OTA favors sMSC-371, while a relatively small amount promotes dMSC-371. Without OTA, the absorption of the induction period sample in 3.00 mL of Tol was transparent at wavelengths longer than 371 nm (Scheme S1a-1). A dispersion with an even larger amount of OTA, 1.50 mL, was examined, with another induction period sample that had been preheated at 130 °C for 30 min. The result is presented in Scheme S1a-2, which shows the in situ optical absorption spectra collected (up to 60 min) from 10, 30, 50, and 100 μL of the induction period sample in 3.00 mL of Tol and OTA. Single-ensemble sMSC371 evolved from each of the four 1.50 mL OTA-containing dispersions, without the presence of sMSC-417, and the population was apparently stable. The amount of sMSC-371 that developed depended roughly linearly on the amount of the induction period sample that was used. In the case of the dispersion with 1.00 mL of OTA for which the result is presented in Scheme 1a, sMSC-371 evolved to a maximum amount after 5 min dispersion (optical density (OD) = 1.12), and then decreased slightly up to 60 min (OD = 0.94) (Scheme S1a-3). The OD values were reduced by the OD value at 400 nm. Close examination of the spectra suggests the presence of weak absorption peaking at ∼415 nm, which is indicative of the evolution of sMSC-417. To understand the evolution more thoroughly, we examined the temporal evolution of the CdTe MSCs in the other three dispersions containing 0.50, 0.20, and 0.05 mL of OTA, for which the spectra collected at 60, 120, and 240 min are presented in parts b−d, respectively, of Scheme 1. The temporal evolution of optical absorption was in situ monitored by time-resolved optical absorption spectroscopy, shown in Figures 1 and 2. In particular, Figure 1a contains 13 optical absorption spectra, which were recorded every 5 min up to 60 min. After the induction period sample was dispersed in the mixture of 2.50 mL of Tol and 0.50 mL of OTA, sMSC-371 evolved, displaying a maximum OD value of 0.91 immediately after dispersion (at 0 min), together with a tiny amount of sMSC417 (OD = 0.02). The OD values were reduced by the OD value at 450 nm. Initially, the strength of sMSC-371 decreased monotonically and that of sMSC-417 increased. After 35 min, sMSC-371 essentially disappeared, while sMSC-417 was still present in a single-ensemble form (OD = 0.69 at 60 min). It is reasonable to estimate the extinction coefficient ratio of sMSC371 to sMSC-417 as 0.91/0.69 = 1.32. Interestingly, a welldefined isosbestic point was apparent at ∼384 nm, for which an expanded view (in the range 340−450 nm) is presented in part a of Figure S1. In Figure 1b, we follow the temporal evolution of MSCs in the dispersion with 13 optical absorption spectra that were recorded every 10 min up to 120 min. When the induction period sample (30 μL) was dispersed into the mixture of 2.80 mL of Tol and 0.20 mL of OTA, sMSC-417 evolved right after dispersion (at 0 min), together with a much weaker peak 4347

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The Journal of Physical Chemistry Letters associated with sMSC-448 (OD = 0.02). The amount of sMSC-417 reached the maximum after 5 min (OD = 0.49) and then decreased for the time up to 120 min (OD = 0.09). In the same period, sMSC-448 constantly increased from OD = 0.10 (at 5 min) to OD = 0.49 (at 120 min, with OD = 0.09 at 417 nm). After 5 min, both sMSC-417 and sMSC-448 appeared, and the OD of sMSC-417 is thus approximated by the apparent OD reduced by the OD contribution of sMSC-448, which is obtained as 0.49 − 0.10(0.09/0.49) = 0.47. Thus, the ratio of the extinction coefficient of sMSC-417 to sMSC-448 is estimated to be 0.47/0.49 = 0.96. The OD values were subtracted by the OD value at 470 nm. A well-defined isosbestic point at ∼431 nm was detected during the decrease in strength of sMSC-417 and the increase in strength of sMSC448. An expanded view for the range from 390 to 480 nm is presented in Figure S1b. It is noteworthy that in the first 5 min, both sMSC-417 and sMSC-448 increased. To validate the isosbestic point observed at ∼384 nm during the course of the decrease of sMSC-371 and the increase of sMSC-417 shown in Figure 1a, we studied another induction period sample (130 °C/30 min, 25 μL in a 3.00 mL mixture containing 0.50 mL of OTA). Figure S1a-1 shows the temporal evolution of optical absorption in situ monitored by timeresolved optical absorption spectroscopy up to 80 min. After 1 min of dispersion, sMSC-371 evolved and little sMSC-417 developed. At the 2 min mark, sMSC-371 increased further in strength and reached a maximum OD value of 0.68. Afterward, sMSC-371 decreased monotonically. Meanwhile, sMSC-417 increased and developed into a single-ensemble form during the time period from 70 to 80 min (OD = 0.53) after dispersion. Thus, the ratio of the extinction coefficients of sMSC-371 to sMSC-417 is approximately 0.68/0.53 = 1.28. An isosbestic point was apparent at ∼382 nm. Interestingly, in the first 2 min, both sMSC-371 and sMSC-417 increased. We varied the amount of the induction period sample in dispersion. In this respect, four different amounts of 10, 30, 50, and 100 μL of one induction period sample were dispersed in 3.00 mL of the same mixture of 2.50Tol−0.50OTA. As shown by Figure S1a-2, there were different rates for these four dispersions, regarding the decrease of sMSC-371 and the increase of sMSC-417. The larger the concentration of the induction period sample was in dispersion, the faster sMSC371 disappeared and sMSC-417 developed. To examine the isosbestic point observed at ∼431 nm during the decrease of sMSC-471 and the increase of sMSC448 (shown in Figure 1b), an induction period sample (135 °C/10 min, 30 μL) was dispersed in 3.00 mL of Tol and OTA mixtures containing the following amounts of OTA of 0.40 mL (Figure S1b-a), 0.30 mL (Figure S1b-b), and 0.10 mL (Figure S1b-c). When 0.40 mL of OTA was used (Figure S1b-a), sMSC-371 appeared at 0 min and then decreased, together with a generation of sMSC-417 during the first 30 min. In this period (a2), an isosbestic point at ∼383 nm was noticed. In the next 40 min (a3), the amount of sMSC-417 changed little. Afterward, during the period from 80 to 480 min (a4), sMSC448 evolved and sMSC-417 decreased, with an isosbestic point at ∼431 nm. At 480 min, sMSC-417 continued to be present. When a smaller amount of OTA (0.30 mL) was used (Figure S1b-b), the three types of sMSCs also evolved in sequence, but relatively rapidly compared to those shown in Figure S1b-a. Between 0 and 10 min (b2), the strength of sMSC-371 decreased and that of sMSC-417 increased. During the next period from 10 to 50 min (b3), the population of

Figure 1. Time-resolved in situ optical absorption spectroscopy exploring the evolution of single-ensemble sMSC-417 and sMSC-448 in the dispersions presented in Scheme 1b,c, respectively. The 13 spectra presented were recorded every 5 min up to 60 min (a) and every 10 min up to 120 min (b). (a) The monotonic decrease of sMSC-371 is synchronized with the monotonic increase of sMSC417, with an isosbestic point at 384 nm detected. After 35 min, sMSC417 exists in a single-ensemble form. (b) The evolution of sMSC-448 takes place over 120 min along with a parallel decrease of sMSC-417 (after 5 min); an isosbestic point at 431 nm is apparent. Singleensemble sMSC-448 appeared during the period from 110 to 120 min.

sMSC-417 changed little. Later, from 60 to 480 min (b4), the apparent strength of sMSC-417 decreased and that of sMSC448 increased. At 480 min, only a small amount of sMSC-417 remained. When an even lower amount (0.10 mL) of OTA was used (Figure S1b-c), the three types of sMSCs developed consecutively and much faster. During the period from 35 to 60 min (c2), single-ensemble sMSC-448 evolved without the presence of sMSC-371 and sMSC-417, and its strength changed little. We have illustrated that the three types of CdTe sMSCs can be generated from one induction period sample in the 3.00 mL mixtures of Tol and OTA composed of OTA volumes from 0.10 to 0.40 mL (Figures 1b and S1b), 0.50 mL (Figures 1a and S1a), and 1.00 mL (Schemes 1a and S1a-3). Single ensembles of sMSC-371 (without the presence of the other two types of sMSCs) seem to be favored by a volume of 1.50 mL of OTA (Scheme S1a-2). With the volume of 0.50 mL of OTA (Figure 1a), the decrease of sMSC-371 and the increase 4348

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Figure 2. Time-resolved in situ optical absorption spectroscopy investigating the development of dMSC-371 in the dispersion presented in Scheme 1d. There are 49 spectra presented in part a which were collected every 5 min in the period from 0 to 240 min. The spectra collected in the periods of 0 to 25 min (every 5 min), 30 to 230 min (with a 10 min interval to 70 min and a 30 min interval from 70 to 220 min, and then a 10 min interval to 230 min), and 235 to 240 min are presented in parts b−d, respectively. Interestingly, during the period from 5 to 25 min, the strength of sMSC448 changed little, while the evolution of dMSC-371 started around 20 min.

min, sMSC-417 disappeared, while the strength of sMSC-448 increased (OD = 0.31) and changed little until 25 min. Around 20 min, dMSC-371 started evolving. During the period from 30 to 230 min (c), the amount of dMSC-371 increased monotonically, while the amount of sMSC-448 decreased noticeably. The spectra collected at 235 and 240 min are quite similar (with OD = 0.49 at 371 nm) (d); dMSC-371 seemed to develop into a nearly single-ensemble form. The OD values have been reduced by the OD value at 470 nm. Apparently, the evolution of dMSC-371 occurred after that of sMSC-371, sMSC-417, and sMSC-448. The existence of a relatively stable period from 5 to 25 min for sMSC-448 is worthy of notice. Figure S2-1 shows the results with the use of 0.075 mL of OTA, the amount of which is smaller than that used for Figure S1b-c (without the evolution of dMSC-371), but larger than that used for Figure 2 (with the evolution of dMSC-371). The evolution of dMSC-371 seems to start at a later stage (around 120 min) than that (about 20 min) shown in Figure 2. At 0 min (b), the three types of sMSCs were detected; up to 40 min (b), the strength of both sMSC-371 and sMSC-417 decreased, while that of sMSC-448 increased. During the 70 min period from 45 to 115 min (c), the strength of sMSC-448 changed little; this stable period for sMSC-448 is noteworthy. Afterward

of sMSC-417 lead to single-ensemble sMSC-417 in about 60 min (Scheme 1b). With the volume of 0.20 mL of OTA (Figure 1b), the decrease of sMSC-417 and the increase of sMSC-448 result in single-ensemble sMSC-448 in about 120 min (Scheme 1c). Upon the use of 0.40 (Figure S1b-a) or 0.30 mL (Figure S1b-b) of OTA, the two changes of sMSC-371 and sMSC-417 and of sMSC-417 and sMSC-448 occur apparently in sequence. There is a period between the two changes from 35 to 75 min (Figure S1b-a) or from 10 to 50 min (Figure S1b-b) that the strength of sMSC-417 changed little. This relatively stable stage for sMSC-417 is noteworthy. A further reduction of the amount of OTA to 0.05 mL was investigated. Figure 2 shows the temporal evolution of the CdTe MSCs at room temperature after the induction period sample (135 °C/10 min, 30 μL) was dispersed in the mixture with such an amount of OTA. After 240 min, the spectrum collected from this dispersion is presented in Scheme 1d. There are 49 spectra collected for the elapsed time up to 240 min (a). The spectra collected between 0 and 25 min (every 5 min), 30 and 230 min (with a 10 min interval to 70 min, a 30 min interval from 70 to 220 min, and a 10 min interval to 230 min), and at 235 and 240 min, are presented in parts b−d, respectively. At 0 min (b), a small amount of sMSC-417 evolved, together with a noticeable amount of sMSC-448. At 5 4349

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The Journal of Physical Chemistry Letters from 120 to 480 min (d), sMSC-448 and dMSC-371 coexisted, with a decrease of the former and an increase of the latter. To study further the evolution of dMSC-371, we explored one induction period sample (130 °C/30 min) with four different amounts of 10, 30, 50, and 100 μL in a mixture of 2.90 mL of Tol and 0.10 mL of OTA. During the dispersion period of 60 min, as shown in Figure S2−2, dMSC-371 was not seen for dispersion a with 10 μL of the induction period sample. For dispersions b and c with 30 and 50 μL of the induction period sample, respectively, an evolution of dMSC371 was detected, together with sMSC-371, sMSC-417, and sMSC-448. Interestingly, for dispersion b, sMSC-371 and sMSC-417 were present at 0 min but in quite small amounts; even after their complete disappearance, the strength of sMSC448 kept increasing. For dispersion d with 100 μL of the induction period sample, an evolution of dMSC-371 occurred, but without the presence of the other three types of sMSCs. Another primary amine, butylamine (BTA), was used to substitute OTA. As shown in Figure S2-3 with the BTA amount of 0.20 mL, the evolution of dMSC-371 from the induction period sample (130 °C/30 min, 25 μL) into a singleensemble form occurred in the period from 30 to 40 min (d). In the first 8 min (b), a decrease of sMSC-371 and sMSC-417 together with an increase of sMSC-448 and dMSC-371 occurred. During the period of 9−20 min (c), the strength of sMSC-448 decreased, while that of dMSC-371 increased. Careful observation suggests that the amount that sMSC-448 decreased was small, while the amount that dMSC-371 increased was large. When a larger amount of BTA of 0.30 mL was used (Figure S2-4), the evolution of dMSC-371 from the induction period sample (130 °C/30 min, 25 μL) into a single-ensemble form took place around 50 min and changed little up to 220 min. During the course of the evolution of dMSC-371, sMSC-371 and sMSC-417 were present and sMSC-448 was absent. Next, we discuss replacing the primary amine in dispersion with an alcohol. Figure 3 shows the temporal evolution of optical absorption after one induction period sample (130 °C/ 30 min, 25 μL) was dispersed in two mixtures of Tol and methanol at room temperature. With a total volume of 2.975 mL, one dispersion contained 0.010 mL of methanol (a); the other dispersion contained 0.020 mL of methanol (b). There are seven spectra presented in Figure 3a, which were recorded at 1, 3, 5, 10, 20, 40, and 60 min after dispersion. At 1 min, there is no signal for any MSCs. Afterward, an increase of sMSC-448 was observed up to 60 min (OD = 0.09), in the absence of the other three types of MSCs. Figure 3b contains five absorption spectra, which were recorded at 1, 2, 5, 20, and 60 min after dispersion. At 1 min, a considerable amount of sMSC-448 (OD = 0.24) evolved. Afterward, more sMSC-448 developed up to 60 min (OD = 0.27), still without the presence of the other three types of MSCs. The OD values were reduced by the OD value at 480 nm. Interestingly, the production of sMSC-448 from the induction period sample in the two mixtures of Tol and methanol does not seem to include the other three types of CdTe MSCs. Figure S3-1 summarizes the time-dependent absorbance at 448 nm for Figure 3. Evidently, the more methanol used, the more sMSC-448 developed, with a higher production rate (for the precursor compound to sMSC-448 transformation). Importantly, Figure 3 suggests that there may be equilibria existing among sMSC-448 and its corresponding precursor compound, which is labeled as PC-s448, and the

Figure 3. Time-resolved in situ optical absorption spectroscopy studying the direct development of sMSC-448 from one IP sample (130 °C/30 min). The sample (25 μL) was dispersed in the 2.975 mL mixture of Tol and methanol, with 0.010 (a) and 0.020 mL (b) of methanol. There are seven spectra presented in part a, which were recorded after 1, 3, 5, 10, 20, 40, and 60 min of dispersion. There are five spectra presented in part b, which were recorded after 1, 2, 5, 20, and 60 min of dispersion. Interestingly, the evolution of sMSC-448 into a single-ensemble form from the IP sample does not involve the presence of sMSC-371 and sMSC-417. With a slower evolution speed, the amount of sMSC-448 obtained during the period up to 60 min in dispersion a is smaller than that in dispersion b. The equilibrium of PC ⇔ sMSC-448 shifts more to the left for dispersion a than it does for dispersion b.

precursor compound produced in the induction period sample. When more methanol is used, the equilibria of precursor compounds ⇔ PC-s448 ⇔ sMSC-448 is weighted more to the right. In addition to methanol, we studied ethanol, hexanol, and decanol (with 0.02 mL used in dispersions), as shown by Figures S3-2 and S3-3. In these cases as well, the evolution of sMSC-448 did not involve the other three types of CdTe MSCs as well. Furthermore, the shorter alkyl chain of an alcohol used, the more sMSC-448 developed (up to 60 min as monitored). Thus, the above equilibria should be weighted also more to the right. To validate the presence of the equilibria of precursor compounds ⇔ PC-s448 ⇔ sMSC-448 proposed for the alcohol-containing dispersions, we added OTA into a methanol-containing dispersion after the evolution of sMSC-448 (Figure S3-4). The evolution of sMSC-371 and 4350

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Letter

The Journal of Physical Chemistry Letters

sMSC-417 was driven to the left (step 5) by PC-s417 ⇒ PCs447 ⇒ sMSC-448 transformations. The PC-s417 ⇒ PC-s448 process (step 6) is the rate-determining step, being relatively slow. This slow process is related to a relatively stable period for sMSC-417 (Scheme S1-1), after the disappearance of sMSC-371 and prior to the significant increase of sMSC-448 (Figures S1b-a and S1b-b). For the evolution of dMSC-371 together with the presence of sMSC-371, sMSC-417, and sMSC-448 observed after an induction period sample was dispersed in a mixture of 2.925 mL of Tol and 0.075 mL of OTA (Figure S2−1), we offer a similar interpretation. The prompt production of PC-s371, PCs417, and PC-s448 led to the presence of sMSC-371, sMSC417, and sMSC-448 at 0 min, via steps 1, 4, and 7, respectively. Afterward up to 60 min, the continuous evolution of sMSC448 from PC-s448 (step 7) resulted in the disappearance of sMSC-371 and sMSC-417, via steps 2 and 5, respectively, together with PC-s371 ⇒ PC-s417 ⇒ PC-s448 of respective steps 3 and 6. During the period of 120−480 min, the equilibrium of PC-s448 ⇔ sMSC-448 was driven to the left (step 8) by the PC-s448 ⇒ PC-d371 ⇒ dMSC-371 transformations. The PC-s448 ⇒ PC-d371 process (step 9) is the rate-determining step, being relatively slow. This slow process is related to a relatively stable period from 45 to 115 min for sMSC-448. Also, this argument holds for the relatively stable period from 5 to 25 min for sMSC-448 (Scheme S1-2), during the evolution of dMSC-371 in the mixture of 2.95 mL of Tol and 0.05 mL of OTA, for which the data is shown in Figure 2. In conclusion, we have demonstrated that from a single induction period sample (30 μL), four types of CdTe MSCs, sMSC-371, sMSC-417, sMSC-448, and dMSC-371, evolve into a single-ensemble form (Scheme 1a−d). For a 3.00 mL mixture of Tol and OTA, the MSCs are respectively facilitated with the OTA amount of about 1.5 (Scheme S1a-3), 0.5 (Figure 1a), 0.2 (Figure 1b), and 0.05 mL (Figure 2). In a methanol-containing dispersion, sMSC-448 evolved without the presence of the other types of MSCs (Figure 3). We therefore propose that the evolution of the MSCs, including the apparent changes among them, involves their corresponding precursor compounds, as shown by Scheme 1e. Similar to the isomeric pair of CdS MSC-311 and CdS MSC-322,25 the four types of CdTe MSCs and their corresponding precursor compounds may be a group of polymorphs with the same CdTe core composition. Our proposed model (Scheme 1e) explains well our experimental observations, such as a relatively stable period for sMSC-417 occurring after the disappearance of sMSC-371 and prior to the significant development of sMSC-448 (Figures S1b-a and S1b-b) (Scheme S1-1), a relatively stable period for sMSC-448 appearing after the disappearance of sMSC-371 and/or sMSC-417 and prior to the significant development of dMSC-371 (Figures 2 and S21) (Scheme S1-2), and a significant amount of sMSC-448 evolving after the disappearance of sMSC-371 and sMSC-417 (Figure S2-2b) (Scheme S1-3). The exact structures of the four types of MSCs will be a subject for further study, together with the role of the small molecules used in dispersion, which includes amine and alcohol. It is unclear at present how to determine these morphologies with conventional characterization tools; such an effort will require both theoretical and experimental contributions from the community. The present study brings a more in-depth understanding of the formation of the binary CdTe MSCs. The discussion on the apparent

sMSC-417 is in agreement with the existence of the precursor compound still after the evolution of sMSC-448 in the methanol-containing dispersion (dispersion a shown in Figure S3-4). By a side note, the alcohol-facilitated development of CdS MSC-311 from precursor compounds in an induction period sample and from CdS MSC-322 containing dispersions has been documented to feature a first-order reaction kinetics behavior.21,22 To gain insight into the evolution pathway of the various CdTe MSCs, we now consolidate some of the important experimental observations discussed above. Right after one induction period sample is dispersed (at 0 min), the direct evolution to single ensembles of sMSC-371 (Scheme 1a), sMSC-417 (Figure 1b), sMSC-448 (Figure 3), and dMSC-371 (Figure S2-2d) without the apparent involvement of the other types of CdTe MSCs has been observed. Also, after an induction period sample is dispersed, the simultaneous evolution of sMSC-371 and sMSC-417 in the first 2 min (Figure S1a-1), and that of sMSC-417 and sMSC-448 in the first 5 min (Figure 1b) have been seen, together with sMSC371, sMSC-417, and sMSC-448 at 0 min (Figure S2-1). Furthermore, after an induction period sample is dispersed, there exists a relatively stable period for sMSC-417, after the disappearance of sMSC-371 and prior to the development of a significant amount of sMSC-448 (Figures S1b-a and S1b-b). Also, a relatively stable stage is seen for sMSC-448, after the disappearance of sMSC-371 and/or sMSC-417 and prior to the obvious evolution of dMSC-371 (Figures 2 and S2-1). Moreover, a significant amount of sMSC-448 evolved after the disappearance of sMSC-371 and sMSC-417 (Figure S2-2), and a considerable amount of dMSC-371 developed during the decrease of a small amount of sMSC-448 (Figure S2-3). For these experimental observations, direct transformations among the MSCs do not seem to be taking place. Therefore, we propose that the corresponding precursor compounds are involved for the evolution of the MSCs, which includes the apparent changes among them, as shown by Scheme 1e. Similar to the discussion addressed around Scheme 1a−d, it seems reasonable that these binary MSCs together with their corresponding precursor compounds are a group of polymorphs, with an essentially identical CdTe core composition. By a side note, the pathway proposed (Scheme 1e) is consistent with the pathway suggested for the formation of alloy CdTeSe MSC-399 at room temperature.47 For the apparent change of sMSC-371 and sMSC-417 detected with an isosbestic point at ∼384 nm (as shown in Figure 1a for the dispersion of 2.50 mL of Tol and 0.50 mL of OTA), we argue that it is related to the prompt production of PC-s371, which quickly evolved to sMSC-371 (step 1). Then, the equilibrium of PC-s371 ⇔ sMSC-371 was driven to the left (step 2) by PC-s371 ⇒ PC-s417 ⇒ sMSC-417 transformations. The PC-s371 ⇒ PC-s417 process (step 3) is the rate-determining step, being relatively slow. This explanation agrees with the experimental observation that in the first 2 min (Figure S1a-1), both sMSC-371 and sMSC417 increased (from their PC-s371 and PC-s417, respectively). For the apparent change of sMSC-417 and sMSC-448 observed with an isosbestic point at ∼431 nm (as presented in Figure 1b for the dispersion of 2.80 mL of Tol and 0.20 mL of OTA), we present a similar comprehension. The prompt production of PC-s417 and PC-s448 resulted in the presence of sMSC-417 and sMSC-448 in the first 5 min, via steps 4 and 7, respectively. Afterward, the equilibrium of PC-s417 ⇔ 4351

DOI: 10.1021/acs.jpclett.9b01601 J. Phys. Chem. Lett. 2019, 10, 4345−4353

Letter

The Journal of Physical Chemistry Letters

(9) Kasuya, A.; Sivamohan, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R. V.; Sundararajan, V.; Kawazoe, Y. Ultra-Stable Nanoparticles of CdSe Revealed from Mass Spectrometry. Nat. Mater. 2004, 3, 99−102. (10) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. Sequential Growth of Magic-Size CdSe Nanocrystals. Adv. Mater. 2007, 19, 548−552. (11) Wuister, S. F.; van Driel, F.; Meijerink, A. Luminescence and Growth of CdTe Quantum Dots and Clusters. Phys. Chem. Chem. Phys. 2003, 5, 1253−1258. (12) Dagtepe, P.; Chikan, V.; Jasinski, J.; Leppert, V. J. Quantized Growth of CdTe Quantum Dots; Observation of Magic-Sized CdTe Quantum Dots. J. Phys. Chem. C 2007, 111, 14977−14983. (13) Dagtepe, P.; Chikan, V. Effect of Cd/Te Ratio on the Formation of CdTe Magic-Sized Quantum Dots during Aggregation. J. Phys. Chem. A 2008, 112, 9304−9311. (14) Wang, R.; Ouyang, J.; Nikolaus, S.; Brestaz, L.; Zaman, M. B.; Wu, X.; Leek, D.; Ratcliffe, C. I.; Yu, K. Single-Sized Colloidal CdTe Nanocrystals with Strong Bandgap Photoluminescence. Chem. Commun. 2009, 8, 962−964. (15) Dukes, A. D.; McBride, J. R.; Rosenthal, S. J. Synthesis of Magic-Sized CdSe and CdTe Nanocrystals with Diisooctylphosphinic Acid. Chem. Mater. 2010, 22, 6402−6408. (16) Wang, Y.; Zhou, Y.; Zhang, Y.; Buhro, W. E. Magic-Size II−VI Nanoclusters as Synthons for Flat Colloidal Nanocrystals. Inorg. Chem. 2015, 54, 1165−1177. (17) Cossairt, B. M.; Owen, J. S. CdSe Clusters: At the Interface of Small Molecules and Quantum Dots. Chem. Mater. 2011, 23, 3114− 3119. (18) Cossairt, B. M.; Juhas, P.; Billinge, S.; Owen, J. S. Tuning the Surface Structure and Optical Properties of CdSe Clusters Using Coordination Chemistry. J. Phys. Chem. Lett. 2011, 2, 3075−3080. (19) Beecher, A. N.; Yang, X.; Palmer, J. H.; LaGrassa, A. L.; Juhas, P.; Billinge, S. J. L.; Owen, J. S. Atomic Structures and Gram Scale Synthesis of Three Tetrahedral Quantum Dots. J. Am. Chem. Soc. 2014, 136, 10645−10653. (20) Kurihara, T.; Noda, Y.; Takegoshi, K. Quantitative Solid-State NMR Study on Ligand−Surface Interaction in Cysteine-Capped CdSe Magic-Sized Clusters. J. Phys. Chem. Lett. 2017, 8, 2555−2559. (21) Liu, M.; Wang, K.; Wang, L.; Han, S.; Fan, H.; Rowell, N.; Ripmeester, J. A.; Renoud, R.; Bian, F.; Zeng, J.; Yu, K. Probing Intermediates of the Induction Period Prior to Nucleation and Growth of Semiconductor Quantum Dots. Nat. Commun. 2017, 8, 15467. (22) Luan, C.; Gökçinar, Ö . Ö .; Rowell, N.; Kreouzis, T.; Han, S.; Zhang, M.; Fan, H.; Yu, K. Evolution of Two Types of CdTe MagicSize Clusters from A Single Induction Period Sample. J. Phys. Chem. Lett. 2018, 9, 5288−5295. (23) Zhang, J.; Hao, X.; Rowell, N.; Kreouzis, T.; Han, S.; Fan, H.; Zhang, C.; Hu, C.; Zhang, M.; Yu, K. Individual Pathways in the Formation of Magic-Size Clusters and Conventional Quantum Dots. J. Phys. Chem. Lett. 2018, 9, 3660−3666. (24) Zhu, T.; Zhang, B.; Zhang, J.; Lu, J.; Fan, H.; Rowell, N.; Ripmeester, J. A.; Han, S.; Yu, K. Two-Step Nucleation of CdS MagicSize Nanocluster MSC-311. Chem. Mater. 2017, 29, 5727−5735. (25) Zhang, B.; Zhu, T.; Ou, M.; Rowell, N.; Fan, H.; Han, J.; Tan, L.; Dove, M. T.; Ren, Y.; Zuo, X.; Han, S.; Zeng, J.; Yu, K. ThermallyInduced Reversible Structural Isomerization in Colloidal Semiconductor CdS Magic-Size Clusters. Nat. Commun. 2018, 9, 2499. (26) Zhu, D.; Hui, J.; Rowell, N.; Liu, Y.; Chen, Q. Y.; Steegemans, T.; Fan, H.; Zhang, M.; Yu, K. Interpreting the Ultraviolet Absorption in the Spectrum of 415 nm-Bandgap CdSe Magic-Size Clusters. J. Phys. Chem. Lett. 2018, 9, 2818−2824. (27) Wang, L.; Hui, J.; Tang, J.; Zhang, B.; Zhu, T.; Rowell, N.; Fan, H.; Han, S.; Yu, K. Precursor Self-Assembly Identified as a General

changes among the various MSCs via the corresponding precursor compounds (Scheme 1e) could assist understanding the published results on changes among the other binary MSCs of CdS and CdSe, including those with isosbestic points.19,25,26

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01601. Experimental details including synthesis and characterization with optical absorption spectroscopy (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Meng Zhang: 0000-0002-2852-2527 Wen Huang: 0000-0002-9772-9492 Hongsong Fan: 0000-0003-3812-9208 Kui Yu: 0000-0003-0349-2680 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.Y. is grateful to National Natural Science Foundation of China (NSFC) 21773162 and 21573155, the State Key Laboratory of Polymer Materials Engineering of Sichuan University (Grant No. sklpme2018-2-08), and the Open Project of Key State Laboratory for Supramolecular Structures and Materials of Jilin University for SKLSSM 201935. H.F. and W.H. appreciate the National Major Scientific and Technological Special Project for “Significant New Drugs Development” (2018ZX09201009-005-004 and 2018ZX09201009005-001).

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