Quantitative Identification of Basic Growth Channels for Formation of

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Quantitative Identification of Basic Growth Channels for Formation of Monodisperse Nanocrystals Jiongzhao Li, Huifeng Wang, Long Lin, Qun Fang, and Xiaogang Peng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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Journal of the American Chemical Society

Quantitative Identification of Basic Growth Channels for Formation of Monodisperse Nanocrystals

Jiongzhao Li†, Huifeng Wang†, Long Lin, Qun Fang*, and Xiaogang Peng* Center for Chemistry of Novel & High-Performance Materials, and Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China ---------------------------------------------------------------------------------------------------------------------------------------Abstract: Different mechanisms are proposed to account formation of monodisperse nanocrystals in literature, each of which is usually proposed to explain one set of experimental observations. Here, a general model based on mass conservation is developed to fully describe all possible channels, including free growth by direct incorporation of the monomers converted from the precursors, growth by dissolution of a portion of the regular nanocrystals in solution, and growth by dissolution of the clusters in solution. The new model provides convenient yet quantitative methods to determine the channel ratios at a given time. Experimentally, an automated micro-reactor system is developed and applied for synthesis of monodisperse CdS nanocrystals, which is coupled with liquid-phase FTIR and UV-Vis measurements to respectively determine precursor conversion and size/concentration of nanocrystals with high reproducibility (< 1%) and proper time resolution (< 1 second). Different from the most-accepted model for formation of monodisperse nanocrystals—a burst of nucleation followed by growth of all nuclei by direct incorporation of the monomers converted from the precursors (or “focusing of size distribution”), all three basic channels are found to co-exist during growth of monodisperse CdS nanocrystals. While the new theory and experimental methods are applied to study growth of monodisperse nanocrystals, it can be extended to offer a full kinetic picture for formation of colloidal nanocrystals.

Introduction Rapid progress on synthetic chemistry of high-quality nanocrystals in the past ~20 years is largely promoted by studies of their formation mechanism.

1-8

The unique size-dependent properties of colloidal nanocrystals are

convenient probes for understanding crystallization in general, given all crystallization starting from nanometer size regime. In literature, several growth mechanisms for high-quality nanocrystals are documented, including “focusing of size distribution”,

9-11

oriented

12-15

and non-oriented

16

attachment, “self-focusing of size

distribution” (including growth using either nanocrystals or clusters as intermediates),

17-21

etc. Though each of

these mechanisms is backed up by certain experimental results, it is not clear whether one of them could offer a 1

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full (or at least dominating) picture for synthesis of high quality nanocrystals. If these mechanisms would jointly exist as different reaction channels, to our knowledge, there is no report to identify the contribution of each channel.

“Focusing of size distribution”

9

is one of the most familiar mechanism for synthesis of monodisperse

nanocrystals in the field. This is not totally surprising because, different from other non-traditional mechanisms, “focusing of size distribution” is borrowed from conventional colloidal science proposed by Sugimoto.

22

This

conventional mechanism tells us that, with a sufficient amount of monomers converted from the precursors, size distribution of the nanocrystals will decrease because the diameter-growth rate of the nuclei formed within a short burst is anti-correlated with the size of the nuclei/nanocrystals. 2, 9 Evidently, this mechanism focuses on the growth channel by direct incorporation of the monomers converted from precursors and assumes concentration of nanocrystals in the solution as constant. However, while the size distribution of nanocrystals narrows down (or remains narrow), decrease of the concentration of nanocrystals in the growth process has been widely found in both commonly known model systems for “focusing of size distribution” –II-VI semiconductor nanocrystals 23-24

—and other types of nanocrystals.

25

While the concentration of nanocrystals in solution decreases, the

monomers from the dissolved nanocrystals grow onto the residual nanocrystals and open another reaction channel for the growth of nanocrystals, namely “self-focusing of size distribution” by dissolution of a portion of the regular nanocrystals in solution.

18, 25

Different from the traditional Ostwald ripening, the driving force of

“self-focusing of size distribution” by dissolution of regular nanocrystals is the monomer concentration gradient defined by the solubility difference between the closely adjacent nanocrystals. If both sufficient supply of monomers and high concentration of nanocrystals are in place, this mechanism would rapidly diminish the small 2


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nanocrystals in the population and result in narrow size distribution. Besides, either oriented 12-15 or non-oriented 16

attachment of the particles in solution will cause decrease of concentration of nanocrystals. It is observed that

particle attachment may also be entangled with other growth mechanisms. For instance, both monomer-addition growth and Ostwald ripening are found to accompany particle-attachment growth visually using in situ transmission electron microscope.

26

To make the situation even more complicated, clusters with the inorganic

composition similar to (or same as) the regular nanocrystals but smaller in size (usually < ~1.5 nm) are widely observed during synthesis of II-VI and III-V nanocrystals.19-21, 27-29 Due to their tiny sizes, stability of clusters is much lower than that of regular nanocrystals. This means that the clusters could readily be dissolved into monomers, which opens one more reaction channel for growth of the regular nanocrystals, which is “self-focusing of size distribution” by dissolution of the clusters in solution. Several groups have successfully used this process to synthesize colloidal nanocrystals.

17, 19-21

Evidently, both types of “focusing of size

distribution” involve indirect incorporation of monomers that are stored in the form of either clusters or regular nanocrystals, instead of direct incorporation of monomers into the lattice of the nanocrystal products from the precursors.

Overall, the Sugimoto model—“focusing of size distribution”—is unlikely the only mechanism for formation of monodispersed nanocrystals and multiple reaction channels could co-exist in a synthetic system. Thus, a proper theoretical model should consider all possible reaction channels, instead of those dealing with single reaction channel. Experimentally, to fully uncover the growth pattern in synthesis of monodisperse nanocrystals, it is necessary to simultaneously determine both monomer and nanocrystal concentrations in addition to temporal evolution of size and structure of the nanocrystals. At present, experimental design often allows to only 3



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determine one of two concentrations. Furthermore, the incomplete data are usually with poor accuracy and reproducibility. Synthesis of high quality colloidal nanocrystals mostly uses high temperature (usually > 200 oC). To monitor such a reaction quantitatively, stabilization of reaction temperature caused by introduction of cold precursor solutions, manual sampling by taking aliquots with a syringe, and dilution of hot aliquots in a volatile cold solvent are difficult to be reproducible. Determination of nanocrystal concentration usually relies on absorption extinction coefficients of the nanocrystals, which are with quite high experimental uncertainties. 30

This work intends to explore a full picture for growth of monodisperse nanocrystals, instead of pre-assuming one specific channel. To do so, a new theoretical model based on mass conservation is developed for full description of growth of monodisperse nanocrystals, which considers all possible growth channels. Experimentally, a micro-reactor system yields CdS nanocrystals with the same quality as those synthesized in flasks by the same synthetic scheme.

27

The micro-reactor can simultaneously measure both precursor conversion by liquid-phase

FTIR and nanocrystal size/concentration by UV-Vis with great reproducibility, accuracy, and necessary time resolution. To make the measurements of size/concentration of CdS nanocrystals quantitatively reproducible, we have refined the process for determining the extinction coefficients of CdS nanocrystals, which yields accurate extinction coefficients per CdS nanocrystal (εNC) and extinction coefficients per CdS unit (εunit). 31

Theoretical section Monodisperse nanocrystals available at present are generally synthesized through conversion of precursors. Upon addition of the precursors into a closed system with a fixed volume, the system should follow mass 4


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conservation as follows. M =

NC +

cluster 1

M = P# − P 1′

Here, [M]total in Equation (1) is the total concentration of composition unit—CdS unit for CdS nanocrystals—converted from the precursors at a given time (Equation (1’)). The right side in Equation (1) tells us that, other than the unreacted precursors, [M]total is included in two types of species in the solution, namely, clusters and regular nanocrystals. Regular nanocrystals are those with a defined crystal structure, whose lattice parameters are the same as corresponding bulk crystals. Clusters here are broadly defined and include all species with the inorganic composition similar to (or same as) the regular nanocrystals but smaller in size and possibly without a fixed crystal structure. The size boundary between regular nanocrystals and clusters is ~1.5 nm. Those so-called magic-size clusters with close-shell geometry 32-33 are typically on the high end of the clusters defined here. Except magic-size clusters, most cluster species are difficult to be experimentally identified using conventional laboratory spectroscopy but their existence has been detected by synchrotron radiation technics

29

and mass spectroscopy. 21, 34 At a given moment, [P] is the concentration of the remaining precursors. [P]0 is the initial precursor concentration. [NC]i and ni are respectively the molar concentration and number of composition unit per nanocrystal for the ith type of regular nanocrystals. Similarly, [cluster]j and nj are respectively the molar concentration and number of composition unit per cluster for the &th type of clusters.

Equation (1’) accounts the composition units generated by precursor conversion (the left side) and consumption 5



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of the precursors (the right side). We assume that a monomer with single composition unit of the crystals is with extremely short life and converted to either clusters or regular nanocrystals instantaneously. If they do remain in the reaction solution, monomers can be included in the right side of Equation (1) as the smallest clusters.

The first sum in Equation (1) can be treated accurately by considering the size distribution function. It is necessary to do so if one intends to include the Ostwald-ripening stage—also known as “defocusing of size distribution” for synthesis of high-quality nanocrystals —in the kinetic analysis. Though the theoretical model presented here has the potential to include such a stage, the current work focuses on distinction between different growth channels for high-quality nanocrystals, whose size distribution can usually be sufficiently monodisperse. For the same reason, the exact structures of clusters are not in immediate concern. These considerations vender simplification of Equation (1) by replacing the sum over regular nanocrystals with an average size and the sum over clusters with a simple term. M =

NC +

cluster =

NC'( )*+ + M 2 ),

Here, [NC] and VNC are the concentration and average volume of the regular nanocrystals, respectively. Vm is the molar volume of regular nanocrystals, which is the same as the bulk crystal as commonly verified by x-ray diffraction, electron microscope, and electron diffraction. NA is the Avogadro constant. [M]cluster is the total concentration of composition unit in the form of clusters. Growth rate of nanocrystals can be represented by the total volume growth rate of all nanocrystals, i.e., [NC] dVNC/dt, which can be obtained by taking derivative of Equation (2) with respect to time and rearranging the results. 6


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NC

dNC ), dM d)*+ ), dM = − )*+ − 3 d/ dt '( d/ '( d/

Equation (3) reveals that the total volume growth rate ([NC] dVNC/dt) of regular nanocrystals in solution possesses three basic channels. The first channel is free growth by direct incorporation of the monomers converted from the precursors (the first term on the right), which is the root of “focusing of size distribution”. 9 The second channel is growth by incorporation of the monomers from dissolution of a portion of the regular nanocrystals in solution (the second term on the right), and the third channel accounts growth by incorporation of the monomers from dissolution of the clusters in solution (the third term on the right).

As discussed in Introduction, the last two channels are both associated with “self-focusing of size distribution”. In addition, if attachment of nanoparticles in the form of either clusters or regular nanocrystals occurs in a synthetic system, it shall also be reflected in the latter two channels. It should be noted that the negative sign for the second and third terms indicate that the total volume growth rate increases when concentration of regular nanocrystals and/or clusters decreases. For instance, if nucleation results in new regular nanocrystals, the second term would result in negative contribution to the total volume growth rate.

A key parameter for understanding nanocrystal growth is growth channel ratios and their temporal evolution, which is the main target for the current work. For any specific channel, the channel ratio is defined as its contribution to the total volume growth rate at a given moment. Thus, to obtain the channel ratios, both sides of Equation (3) are divided by the total volume growth rate ([NC] dVNC/dt) and the channel ratio of each term is given as below. 7



), dM dNC ), dM )*+ '( d/ d/ d/ − '( 1= − d)*+ d)*+ d)*+ NC NC NC d/ d/ d/

Page 8 of 33

= 123 + 145 + 16789:;< 4

), dM 5 '( NCd)*+ d?*+ 145 = − 6 NCdln)*+ ), dM 16789:;< = − 7 '( NCd)*+ 123 =

Here, three channel ratios are defined, namely, RFG for free growth by direct incorporation of the monomers converted from the precursors, RNC for growth by incorporation of the monomers from dissolution of a portion of the regular nanocrystals in solution, and Rcluster for growth by incorporation of the monomers from dissolution of the clusters in solution. All parameters needed for determining three channel ratios are either known constants or experimentally measurable. The average volume (VNC) of monodisperse semiconductor nanocrystals in quantum-confinement size regime could be measured by their first exciton absorption peak in UV-Vis spectrum. The corresponding peak absorbance gives the concentration of nanocrystals ([NC]), given their molar extinction coefficients being determined accurately. The total concentration of composition unit converted from the precursors ([M]total) could be measured by spectroscopy, such as infrared (IR) and nuclear magnetic resonance (NMR). With known [NC] of the nanocrystals with a given size and [M]total at a given time, the total

concentration

of

composition

unit

accumulated in clusters ([M]cluster) could be indirectly determined by the difference between

8

Figure 1 Schematic illustration of three basic reaction channels and their calculation approaches.


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[M]total and the total concentration of composition unit accumulated in the nanocrystals (NC'( )*+ /), ).

Figure 1 illustrates complete picture of the model described above, i.e., the growth rate of the total volume of nanocrystals in solution being fully described by three basic channels (top panel). The derivative in Equations (5-7) could be acquired by the slope of a specific plot (Figure 1, bottom panel), each of which has its unique xand y-axis. Results below shall demonstrate that, with a newly developed micro-reactor system, all experimental variables needed for determining the channel ratios defined above can be reproducibly and quantitatively obtained.

Experimental results and discussion: Synthetic system and characterization methods. The kinetic model described above indicates that, in order to precisely distinguish three basic channels, it is necessary to quantitatively determine the size/concentration of monodisperse nanocrystals and the conversion of precursors with proper time resolution. To this regard, synthesis of zinc-blende CdS nanocrystals with cadmium oleate and elemental sulfur as precursors in non-coordinating solvent (octadecene)

27

is an ideal model system for its simplicity. It is well-known that this

synthetic system offers well-defined growth stage with monodisperse CdS nanocrystals needed for the current study.

Recent development on synthesis and purification of cadmium chalcogenide nanocrystals allows to re-determine both sizing curve of UV-Vis absorption peak and extinction coefficients with high accuracy. 9


31

Different from


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large uncertainties (~20%) for the extinction coefficients reported previously,

30

the experimental uncertainty of

the newly-measured extinction coefficients per composition unit and per particle is small ( 640s) shows gradual broadening of the size distribution and should be the “defocusing of size distribution” proposed in literature. 9 According to the design of experiments, these two stages are not the focus of the current study.

14


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The second stage in Figure 3a (11s ≤ time ≤ 640s) features appearance of well-defined absorption peaks in each spectrum and narrow spectral linewidth. Quantitatively, the half-width-at-half-maximum at the long-wavelength side of the first absorption peak is less than 14 nm within this stage. As reaction proceeds in this stage, the half-width-at-half-maximum at the long-wavelength side of the first absorption peak evidently becomes narrower (Figure 3a), indicating narrowing of size distribution. Given the narrow size distribution, the VNC can be calculated accurately in this stage, which is well suited for studying nanocrystal growth using the simplified version of the theoretical model described above. Thus, this stage represents the time window for the current study.

The temporal evolution of UV-Vis for all reactions reveals some important facts within the time window for study. The evolution patterns are always smooth and continuous, and all absorption features are either retained or improved in this growth stage (also see high-time resolution spectra in Figure S6, Supporting Information). These facts are consistent with non-existence of particle attachment of regular nanocrystals. In literature, particle attachment is commonly reported to accompany non-continuous changes in size/shape and size/shape distribution.

12-16, 40

Mechanistic studies on particle attachment have indicated that insufficient ligands

passivation is a likely pre-requisite.

13, 15

In the current system, the ligands—cadmium oleate—are strong and

always in large excess, especially considering the conversion yield of elemental sulfur being below two-thirds (see below). However, it should be pointed out that, if attachment of clusters onto nanocrystals occurred, the UV-Vis measurements could not distinguish it from the third channel in Figure 1. As a result, attachment of clusters onto regular nanocrystals—if it exists—would be included in the third channel in Figure 1.

15



Page 16 of 33

Co-existence of multiple reaction channels. For the reaction in Figure 3a, temporal evolution of the precursor conversion (or yield of composition units, [M]total) determined by liquid-phase FTIR spectra is illustrated in Figure 3b (the red dots) along with those for the other reactions in Group III defined in Table 1. For the reactions in the other four groups, temporal evolution of the [M]total is summarized in Figure S7 (Supporting Information). Consistent with literature, 35 conversion of the precursors for this specific synthetic system is relatively slow and lasts for almost entire duration of the synthesis. In general, [M]total usually follows a smooth increase and gradually reaches a plateau (Figure 3b and S7 (Supporting Information)). The plateau of [M]total is the final conversion yield for a given reaction, which is topped at about two-thirds of the concentration of the limiting precursor—elemental sulfur—for all reactions studied. Again, this is consistent with the previous report. 35

By correlation with the UV-Vis measurements, we can illustrate the three stages in the plots of temporal evolution of [M]total (see Figure 3b for example), namely, the initial nucleation stage before the red short line, the “defocusing of size distribution” stage after the black short line, and the growth stage with narrow size distribution—the time window between the red and black lines for each reaction. Overall, given the x-axis in logarithm scale in Figures 3b and S7 (Supporting Information), the time window for the growth stage in this well-developed system is quite large. Figures 3b and S7 (Supporting Information) reveal that, in the duration of the targeted time window, conversion of precursors to composition units is always in place. This implies existence of the first channel defined in Equation (4), namely, free growth by direct incorporation of the monomers converted from the precursors. Given the size distribution being nearly monodisperse in the time window, one would conclude that the commonly known “focusing of size distribution” indeed contributes to the growth of mondisperse CdS nanocrystals. 16


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Figure 3c shows temporal evolution of the concentration of nanocrystals ([NC]) for the same reactions in Figure 3b (see Figure S8 (Supporting Information) for the other groups of reactions), which is determined by the UV-Vis spectrum of the resulting nanocrystals at a given time and the newly determined extinction coefficients of CdS nanocrystals.

31

For clarity, data points in Figures 3c and S8 (Supporting Information) only cover the

growth stage identified in the above paragraph. For all reaction conditions in Figure 3c and the other ones in Figure S8 (Supporting Information), the [NC] evidently goes through a considerable decrease during the growth stage and eventually reaches a plateau. This means that, under most reaction conditions, growth by dissolution of regular nanocrystals–the second reaction channel in Equation (4)—contributes to the growth of monodisperse CdS nanocrystals. With the narrow size distribution in the stage, “self-focusing of size distribution” through dissolution of regular nanocrystals certainly plays a significant role for growth of the monodisperse CdS nanocrystals for most reactions. According to literature, 2 “self-focusing of size distribution” through dissolution of regular nanocrystals should occur readily at a high nanocrystal concentration and with small sizes of the nanocrystals, which is consistent with the rapid decrease of nanocrystal concentration in the early stage (Figure 3c and S8 (Supporting Information)).

Existence of the third channel in Equation (4) for growth of monodisperse CdS nanocrystals—growth by consumption of the clusters in the solution—is confirmed by two sets of evidences. The first set of evidences are shown in Figure 3d for the reactions in Group III, namely the temporal evolution of the [M]NC/[M]total ratio. Here, [M]NC is the total concentration of composition units incorporated in the regular nanocrystals, which can be accurately determined by the extinction coefficients per CdS unit.

31

17


Figures 3d and Figure S9 (Supporting


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Information) show that the [M]NC/[M]total ratio is significantly below one in the early period of the growth window and gradually increases to near unity. These results indicate that a large portion of composition units are stored in the form of clusters in the initial period of the growth window, which are gradually converted into the regular nanocrystals. The second set of evidences for the third channel are observation of gradual disappearance of the “magic size clusters”. For some reactions, “magic size clusters”

27, 41

are observed by UV-Vis spectra,

which appear in the very early period of the growth window and gradually disappear (see Figure 3a for example). Figure S10 (Supporting Information) shows some extreme cases, in which the UV-Vis spectra are dominated by the “magic sized clusters” after a significant amount of composition units are formed.

Quantitative determination of the channel ratios during the growth of monodisperse nanocrystals. Results in Figure 3 and discussions in the above sub-section confirm co-existence of three basic channels. This sub-section shall illustrate the process to quantitatively determine the channel ratio for each one. Equations (5-7) and Figure 1 reveal that the channel ratios can be determined by the slope of a specific plot. Figure 4 and Figure S11 (Supporting Information) illustrate the experimental results with fitting functions for the [M]total-VNC and [NC]-ln(VNC) plots for all groups of reactions in Table 1. These two sets of plots yield channel ratios for two of three channels, namely free growth by direct incorporation of the monomers converted from the precursors and growth by consumption of a portion of the regular nanocrystals in solution. For the third channel associated with growth by consumption of the clusters in solution, one could obtain the channel ratio by either the [M]cluster-VNC plot (Figure S12) or one minus the other two channel ratios (Equation (4)). Two methods yield similar results when the [M]cluster is sufficiently high but the errors of the [M]cluster-VNC plot becomes very large when the [M]cluster approaches zero (Figure S12, Supporting Information). In the late period of the growth window, the 18


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[M]cluster is approaching zero. Although the measurements of both [M]total and [M]NC are with sufficient accuracy (less than 5% relative standard deviation), a small percentage of error for a large [M]total (and/or [M]NC) (Figure 3 and Figure S7, Supporting Information) would cause large relative error for a very small value of [M]cluster in the late period of the growth window. For this reason, we shall obtain RCluster by Equation (4), given both RFG and RNC

can

be

accurately

determined.

In Figure 4 and Figure S11 (Supporting Information), all experimental data are well fitted Figure 4. In the growth time window, fitting curves of [M]total-VNC and [NC]-ln(VNC) with different ratio of cadmium to sulfur (left) and concentration of oleic acid (right). The errors are not shown in Figure 4, which are the same with the corresponding ones in Figure 3. The solid lines are the corresponding exponential fitting functions.

with

function experimental

an

exponential

within errors.

the These

high-accuracy fitting functions enable calculation of three

channel ratios accurately using Equations (4-6). Evidently, such accuracy is ensured by the high time and concentration precisions of the automated micro-reactor system in Figure 2.

Temporal evolution of the channel ratios. The temporal evolution of the channel ratios for all reactions listed in Table 1 is readily obtained by applying Equations (4-6) to the experimental results in Figures 4 and S11 (Supporting Information). Figure 5 demonstrates the results for three representative reactions for Group I (left 19



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panel) and Group II (right panel), and the results for all other reactions in five groups are included in Figures S13-S17 (Supporting Information).

The temporal evolution patterns of three channel ratios in Figures 5 and S13-S17 can be quite complex, which is due to multiple-parameter effects. Figures 3 and S7-S9 summarize results of the corresponding single-variable measurements, which are all easy to understand. Take one of relatively complex groups, namely Group II, for example. As the concentration of free acid increases with the other parameters fixed for the reactions in Group II, the conversion of precursors at a given reaction time decreases monotonically (Figure S7, Supporting Information), a strong anti-correlation between HOl concentration and nanocrystal concentration is observed (Figure S8, Supporting Information)), and the incorporation of composition units in regular nanocrystals decreases steadily (Figure S9, Supporting Information).

20


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Group I—variable Cd ： S ratio. In Figure 5 (left) and Figure S13 (Supporting Information), as the cadmium to sulfur ratio increases from Cd1S1 to Cd5S1 with other reaction parameters being fixed, RFG (the channel ratio of free growth by direct incorporation of the monomers converted from the precursors) gradually decreases from ~50% to practically 0%. Simultaneously, Rcluster (the channel ratio of growth by consumption of the clusters in solution) increases from the least contributor to the largest one while RNC (the channel ratio for growth by consumption of the nanocrystals) remains ~40% for all reactions. Because this group of reaction is equivalent to varying concentration of the cadmium precursor by fixing all other reaction parameters, all these trends are believed to be reasonable. It was reported that, for typical synthetic systems for II-VI and III-V semiconductor nanocrystals, the active anionic precursors (such as H2S,

35, 42

H2Se,

,

43-44

tris(trimethylsily)phosphine/H3P, 45-48

and

tris(trimethylsily)arsine/H3As 49-50

Figure 5 Temporal evolution of three channel ratios with experimental errors for three representative reactions in Group I (left) and Group II (right). The black lines, blue lines, and red lines connecting experimental data are trend lines added to guide the eyes for the channel ratios of free-growth by direct incorporation of the monomers converted from the precursors, growth by consumption of the regular nanocrystals, and growth by consumption of the clusters, respectively.

19,

) are extremely reactive towards the common cationic precursors, namely metal carboxylates. By

systematically increasing the concentration of cadmium oleate from 15 μmol/g (Cd:S = 1:1) to 65 μmol/g (Cd:S 21



Page 22 of 33

= 5:1) and fixing other reaction parameters, reaction between the gradually activated S precursors 35 and a large excess of cadmium oleate would be accelerated rapidly. As a result, formation of clusters becomes more and more dominating and free growth channel by direct incorporation of the monomers converted from the precursors would become less and less important. As a result, for growth of monodisperse CdS nanocrystals during the growth window, all monomers available would be more and more from the stored composition units in the form of clusters.

Figure 5 (top left) shows that, when Cd : S equals to 5 : 1, nearly all activated sulfur precursors are reacted with cadmium oleate to form clusters and free growth by direct incorporation of monomers converted from the precursors is barely playing a role in the growth window. To support this conclusion, results in Figure S10 (Supporting Information) confirm that, after the reaction proceeds for ~40 s, the system is still dominated by clusters and regular nanocrystals are barely observable. In other words, an excessively high concentration of cadmium oleate would efficiently store CdS units in the form of clusters. Such phenomenon has been commonly observed for synthesis of III-V semiconductor nanocrystals,

19, 21, 45-50

but it has not attracted much attention in

synthesis of II-V semiconductor nanocrystals likely because the other two channels are usually effective. As a result, the synthetic system of II-V semiconductor nanocrystals would not be fixed at the cluster-only stage for a long period time.

Group II—variable free acid concentration. While RNC remains nearly constant in Group I, Group II demonstrates large differences in RNC. Given the Cd : S ratio being fixed at 2 : 1 for the reactions in Group II, Rcluster remains visible but not dominating and roughly constant (~20%). This is consistent with the results for 22


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Group I, that formation of the clusters largely depends on the Cd : S ratio. Conversely, as the free acid concentration increases from HOl2X to HOl20X, RNC and RFG change drastically in the growth stage, and their temporal evolution trends for a given reaction also changes (Figures 5 (right) and S14 (Supporting Information)). In principle, high concentration of oleic acid promotes dissolution of regular nanocrystals back into precursors, i.e., the reverse reaction of formation of CdS units, which impacts both competitive channels. For free growth by direct incorporation of the monomers converted from precursors, an increased concentration of free acid decreases the conversion of precursors by offering a pathway to regenerate cadmium oleate and H2S at a fixed reaction time (Figure S7, Supporting Information). For growth by consumption of regular nanocrystals, persistently high concentration of free fatty acids would greatly reduce the initial concentration of regular nanocrystals at the beginning of the growth time window (Figure S8, Supporting Information), which would make this specific channel be difficult to last long.

Group III-varying sulfur precursor concentration. Figure S15 (Supporting Information) illustrates evolution of the growth channel ratios for the reactions in Group III, i.e., reactions with varying sulfur precursor concentration and fixing other reaction parameters. As the concentration of S precursor increases, contributions of different growth channels approximately follow an opposite trend observed by increasing the concentration of Cd precursors in Group I. This is expected because increasing S precursor concentration is equivalent to decreasing Cd concentration. However, it should be noted that, the free acid concentration is fixed at a quite high level for this group of reactions. As a result, the temporal evolution pattern of the channel ratios in this Group is also affected by the effects observed for the reactions in Group II with a high concentration of free acids.

23



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Groups VI-varying concentrations of all reactants. As shown in Figures S8, S9, and S16 (Supporting Information), by increasing the concentrations of all reactants and fixing their ratios, the reactions in Group VI combine the effects discussed in Group I and Group II. Results in literature 35 and shown above confirm that the S precursors are activated gradually in the current reaction system. Consequently, increasing concentrations of all reactants is equivalent to increasing the concentrations of the cadmium precursors and free acids for the gradually activated S species.

Group V-varying reaction temperature. The temperature range between 220oC and 270oC for the reactions in Group V basically covers the practical reaction temperatures for the synthetic system. The results in Figure S17 (Supporting Information) shows that the channel ratios hardly change with increase of reaction temperature in the range between 220oC and 270oC, indicating the competition between three basic channels is barely affected by the reaction temperature. The slight differences among the five reactions in the group should be a result of the strong temperature dependence of the reaction rates (Figure S7, Supporting Information).

Implication of reaction mechanisms for formation of CdS nanocrystals. Though focus of this work is on distinguishing contributions of three basic reaction channels in growth of monodisperse nanocrystals, the quantitative and systematic results shown above may also be applied for studying other aspects of formation mechanisms of nanocrystals. Below, we shall try to briefly explore chemical reaction mechanisms involved in this specific reaction system. Though contribution ratios of different channels are important factor for understanding synthesis of high-quality nanocrystals, it is impossible to fully describe a crystallization system without identifying the chemical reaction mechanisms. 24


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Previous report 35 revealed that, in this synthetic system, elemental S would slowly react with octadecene under elevated temperatures to form activated S precursors (H2S and related species), which would in turn rapidly react with cadmium carboxylates in the solution to generate CdS units and fatty acids. Results in this work show that the slow activation may last for the entire growth window (Figure 3b), and it is strongly temperature dependent (Figure S7, Supporting Information).

It should be noted that the reaction between the activated S precursors and cadmium carboxylates have two major reaction pathways. When the cadmium carboxylates are dissolved in solution, the resulting CdS units would form clusters, which is the basis of the third growth channel—growth by consumption of the clusters. If the cadmium carboxylates are adsorbed on the surface of nanocrystals as ligands, the resulting CdS units would be incorporated onto the nanocrystals, which should be associated with free growth by direct incorporation of the monomers converted from the precursors. In the synthesis of III-V semiconductor nanocrystals, formation of clusters has been proven to be dominating in most cases. 19, 21, 45-50 Results here reveal that, by increasing the Cd : S precursor ratio, formation of clusters can also be dominating for synthesis of CdS nanocrystals—only clusters being detectable

with [M]total > 50% of the final value (Figure S10, Supporting Information). Presumably, the

main difference between II-VI and III-V synthesis is not formation of clusters. Instead, it is reverse reaction of formation of the composition units.

The reverse reaction, or decomposition of CdS composition units to H2S and cadmium carboxylates by free fatty acids, plays a key role in synthesizing monodisperse CdS nanocrystals. Without this reaction, two of three basic 25



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channels –“self-focusing of size distribution” through dissolution of either regular nanocrystals or clusters—would be impossible. It is this reaction that yields monodisperse CdS nanocrystals with increased sizes by increasing the concentration of free fatty acids. 27 This reverse reaction should be significantly slower than the forward reaction, i.e., formation of CdS units by the reaction between H2S and cadmium carboxylates. Otherwise, neither CdS clusters nor CdS nanocrystals would exist in solution.

This suggests that, for the recently developed layer-by-layer growth of monodisperse CdSe and CdSe/CdS core/shell nanocrystals with a large excess of cadmium carboxylates in the solution, the active anionic elements—H2Se

51

or H2S

52

—are temporally stored in the form of the corresponding clusters and gradually

released through the reverse reaction. This should result in well-controlled growth process for formation of semiconductor nanocrystals with nearly ideal optical properties. 51-52 For the current system, Figures 5 (top left), S7 (Supporting Information), and S10 (Supporting Information) demonstrate such a synthesis, in which growth of the CdS nanocrystals nearly always proceeds by consumption of the stored CdS units in either regular nanocrystals or clusters.

It should be pointed out that, to make the formation-decomposition cycle work, there is another prerequisite in addition to proper reaction rates (slow decomposition/rapid formation). The anionic molecules from the decomposition reaction are usually gaseous species, such as H2S, H2Se, H3P, H3As, etc., which should be reasonably stable in the reaction solution for the formation-decomposition cycle to work efficiently. Evidently, most reactions studied here are approaching the up-limit yield of the limiting precursor (elemental S).

35

The

final yield of composition units for four reactions in Groups III and IV is found to be proportional to the initial 26


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concentrations of the S precursors (Figures 3b and S7 (Supporting Information)), and all reactions reached nearly identical final yield in Groups I and V with the same concentration of the S precursors. For the reactions in Group II, the yield within the growth time window decreases with the increase of concentration of free fatty acids, which could be a result of shift of the formation-decomposition equilibrium towards the precursors by increasing the concentration of free acids. All these results suggest that H2S formed by the decomposition of either clusters or regular nanocrystals could be well preserved in the reaction system. Conversely, when fatty acids are in excess, InP nanocrystals would be completely decomposed and H3P is escaped with the Ar flow that is necessary for synthesis under elevated temperatures. 46

Figures 3d and S9 (Supporting Information) reveal that, though it takes different amounts of time and goes through various channel patterns, all CdS units would eventually be incorporated into the regular nanocrystals within the growth time window. This means that both formation and decomposition reactions might be size dependent, at least with a certain degree of selective decomposition of the clusters. While the exact selective law for formation and decomposition is not known at present, it should have played a critical role in synthesizing monodisperse nanocrystals.

Conclusion In conclusion, different from most adopted Sugimoto model

22

for understanding growth of monodisperse

colloidal particles (or “focusing of size distribution” for colloidal nanocrystals 9), storage of composition units in the form of either regular nanocrystals or clusters for latter growth is proven to be common for growth of 27



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monodisperse CdS nanocrystals. By focusing on the growth stage of monodisperse nanocrystals, three commonly known growth mechanisms—“focusing of size distribution”, “self-focusing of size distribution” by dissolution of the regular nanocrystals, and “self-focusing of size distribution” by dissolution of the clusters—are all found to act as an effective reaction channel for formation of monodisperse CdS nanocrystals. Contribution of each channel varies by changing the reaction conditions and none of them is dominating under commonly applied reaction conditions. Quantitative identification of these basic channels relies on both theory and experiments. Theoretically, a general model based on mass conservation can include all key variables in growth of high-quality nanocrystals and offer simple yet quantitative methods to identify contribution of each possible channel. Experimentally, a micro-reactor system allows quantitative, time-resolved, reproducible, yet convenient determination of key experimental parameters, especially the precursor conversion and size/concentration of regular nanocrystals during the growth stage. If experimental techniques allow to determine size distribution of a system, the theoretical and experimental methods reported here can be extended to nanocrystal systems without monodisperse size distribution. This work and future efforts along this direction shall open a new door for understanding formation of monodisperse nanocrystals, designing synthetic chemistry for colloidal nanocrystals, and establishing a general framework for crystallization in general.

Experimental section Chemicals. Oleic acid (OA, 99%), Sulfur powder (sublimed, 100 mesh, 99.5%), 1-octadecene (ODE, 90%), n-dodecane (99%), Cadmium nitrate tetrahydrate (99+%), and tetramethylammonium hydroxide (98%) were purchased from Alfa-Aesar. All chemicals were used directly without any further purification. 28


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Synthesis of Cadmium oleate (Cd(Ol)2). Cadmium nitrate tetrahydrate (10 mmol) was dissolved in methanol (20 mL) in a 50 mL centrifuge tube. In another 50 mL centrifuge tube, Oleic acid (20 mmol) and tetramethylammonium hydroxide (20 mmol) were dissolved in 20 mL methanol. The former solution was swiftly added into the later solution and the mixture was vigorously stirred for 30 min. After centrifuged at 4000 r/m for 5 min, the mixture was separated into two liquid phases. The upper liquid phase was removed quickly. To remove the non-reacted reactants, another 20 mL methanol was added, followed by stirring and centrifuging as above. This purification procedure was repeated three times. The final solution was dissolved in 40 mL hexane and the solvent was removed by a rotary evaporator. And the remaining white solid was dried under vacuum at room temperature for three days before using.

Synthesis of CdS nanocrystals in the sealed capillary. The reaction parameters were adopted from literature except a great reduction of the volume of the reaction solution, and the procedure was modified to match the micro-reactor system. The sulfur solution was prepared by dissolving sulfur powder (1 mmol) in ODE (15 mL) by sonication for 10 min. In a typical synthesis, Cd(Ol)2 (0.1 mmol), oleic acid (0.8 mmol) and ODE (3 mL) were loaded into a 25 ml vial. The mixture was heated to form a clear solution and the sulfur solution (0.05 mmol) was added at room temperature. After stirring, the vial was sealed and stored at 35 oC. The solution was drawn into a capillary (commercial capillary with a length of 100 mm, inner diameter of 1 mm and outer diameter of 1.5 mm) by the capillarity until two thirds of the capillary was filled by the solution (about 45 µL). Both ends of the capillary were sealed by melting the glass near the ends carefully by a blowtorch. The sealed capillary was placed on a sample holder fixed on the x-y-z translation stage. The heating time was set in the 29



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control program of the translation stage. After running the control program, the sealed capillary was automatically moved to a heater with a V-groove for loading the capillary. When the heating time reached the preset value, the capillary was moved away from the heater and rapidly cooled down to room temperature. The inside reaction solution was taken out by breaking the ends of the capillary. Without any dilution, aliquots of this reaction solution were loaded in a CaF2 colorimetric ware with short optical path length (500 µm for UV-Vis spectrum, 340 µm for FTIR spectrum). The same operation was repeated for a new capillary with different heating time to trace the reaction process.

Translation stage. The home-made x-y-z translation stage is composed of three individual linear actuators in the three orthogonal directions. Each linear actuator has a stepper motor (CVK series, Oriental Motor, Tokyo, Japan) and a caged ball linear guide actuator (SKR series, THK, Tokyo, Japan). The stage is controlled by a program written in Labview through a PCI-8144 motion controller (ADLINK, Shanghai, China) with a position resolution of 1 µm.

Measurements and Characterization. The UV-Vis spectra were taken on an Ocean Optics USB2000+ fiber-optic spectrometer. The FTIR spectra were taken on a Nicolet 380 spectrometer. TEM images were taken on a Hitachi 7700 transmission electron microscope with an acceleration voltage of 80 kV using copper grids (400-mesh) coated with pure carbon support film.

Supporting Information: Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. 30


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Author information: Corresponding Authors X. P. ([email protected]) or Q. F. ([email protected]) Author Contributions †These authors contributed equally to this work.

Notes: The authors declare no competing financial interest.

Acknowledgements: This work was supported by the National Program on Key Research and Development Project (2016YFB0401600) and the National Natural Science Foundation of China (Grant 21435004). Science and Technology Planning Project of Guangdong Province, CHINA (Grant 2015B090913001).

References: (1) Murray, C. B. ; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545-610. (2) Peng, X. Nano Res. 2009, 2, 425-447. (3) García-Rodríguez, R.; Hendricks, M. P.; Cossairt, B. M.; Liu, H.; Owen, J. S. Chem. Mater. 2013, 25, 1233-1249. (4) Wang, F.; Richards, V. N.; Shields, S. P.; Buhro, W. E. Chem. Mater. 2014, 26, 5-21. (5) Talapin, D. V.; Shevchenko, E. V. Chem. Rev. 2016, 116, 10343-10345. (6) Tamang, S.; Lincheneau, C.; Hermans, Y.; Jeong, S.; Reiss, P. Chem. Mater. 2016, 28, 2491-2506. (7) De Trizio, L.; Manna, L. Chem. Rev. 2016, 116, 10852-10887. (8) Nasilowski, M.; Mahler, B.; Lhuillier, E.; Ithurria, S.; Dubertret, B. Chem. Rev. 2016, 116, 10934-10982. (9) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. 31



Page 32 of 33

(10) Talapin, D. V.; Rogach, A. L.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 12278-12285. (11) Hendricks, M. P.; Campos, M. P.; Cleveland, G. T.; Jen-La Plante, I.; Owen, J. S. Science 2015, 348, 1226-1230. (12) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem. Int. Ed. 2002, 41, 1188-1191. (13) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240. (14) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140-7147. (15) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Kim, M.; Peng, X. J. Am. Chem. Soc. 2006, 128, 10310-10319. (16) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. J. Am. Chem. Soc. 2007, 129, 13939-13948. (17) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576-1584. (18) Thessing, J.; Qian, J.; Chen, H.; Pradhan, N.; Peng, X. J. Am. Chem. Soc. 2007, 129, 2736-2737. (19) Xie, R.; Peng, X. Angew. Chem. Int. Ed. 2008, 47, 7677-7680. (20) Gary, D. C.; Terban, M. W.; Billinge, S. J. L.; Cossairt, B. M. Chem. Mater. 2015, 27, 1432-1441. (21) Xie, L.; Shen, Y.; Franke, D.; Sebastián, V.; Bawendi, M. G.; Jensen, K. F. J. Am. Chem. Soc. 2016, 138, 13469-13472. (22) Sugimoto, T. Adv. Colloid Interface Sci. 1987, 28, 65-108. (23) Qu, L.; Yu, W. W.; Peng, X. Nano Lett. 2004, 4, 465-469. (24) Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C. Angew. Chem. Int. Ed. 2005, 44, 6712-6715. (25) Chen, Y.; Johnson, E.; Peng, X. J. Am. Chem. Soc. 2007, 129, 10937-10947. (26) Zheng, H.; Smith, R. K.; Jun, Y.-w.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P. Science 2009, 324, 1309-1312. (27) Yu, W. W.; Peng, X. Angew. Chem. Int. Ed. 2002, 41, 2368-2371. (28) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343-3353. (29) Abécassis, B.; Bouet, C.; Garnero, C.; Constantin, D.; Lequeux, N.; Ithurria, S.; Dubertret, B.; Pauw, B. R.; Pontoni, D. Nano Lett. 2015, 15, 2620-2626. (30) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854-2860.

(31) Li, J.; Chen, J.; Shen, Y.; Peng, X. Nano Res. 2018, (in press) Doi:10.1007/s12274-018-1981-4. (32) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259 , 1426-1428. (33) Vossmeyer, T.; Reck, G.; Katsikas, L.; Haupt, E. T. K.; Schulz, B.; Weller, H. Science 1995, 267, 1476-1479. (34) 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. Nat. Mater. 2004, 3, 99. (35) Li, Z.; Ji, Y.; Xie, R.; Grisham, S. Y.; Peng, X. J. Am. Chem. Soc. 2011, 133, 17248-17256. (36) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199-201. (37) Chan, E. M.; Alivisatos, A. P.; Mathies, R. A. J. Am. Chem. Soc. 2005, 127, 13854-13861. (38) Bannock, J. H.; Krishnadasan, S. H.; Nightingale, A. M.; Yau, C. P.; Khaw, K.; Burkitt, D.; Halls, J. J. M.; Heeney, M.; de Mello, J. C. Adv. Funct. Mater. 2013, 23, 2123-2129. (39) Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem. Int. Ed. 2005, 44, 3466-3470. (40) Chen, Y.; Chen, D.; Li, Z.; Peng, X. J. Am. Chem. Soc. 2017, 139, 10009-10019. (41) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665-7673. (42) Thomson, J. W.; Nagashima, K.; Macdonald, P. M.; Ozin, G. A. J. Am. Chem. Soc. 2011, 133, 5036-5041. (43) Deng, Z.; Cao, L.; Tang, F.; Zou, B. J. Phys. Chem. B 2005, 109, 16671-16675. (44) Pu, C.; Zhou, J.; Lai, R.; Niu, Y.; Nan, W.; Peng, X. Nano Res. 2013, 6, 652-670. 32


Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Xie, R.; Li, Z.; Peng, X. J. Am. Chem. Soc. 2009, 131, 15457-15466. (46) Li, Y.; Pu, C.; Peng, X. Nano Res. 2017, 10, 941-958. (47) Guzelian, A. A.; Katari, J. E. B.; Kadavanich, A. V.; Banin, U.; Hamad, K.; Juban, E.; Alivisatos, A. P.; Wolters, R. H.; Arnold, C. C.; Heath, J. R. J. Phys. Chem. 1996, 100, 7212-7219. (48) Battaglia, D.; Peng, X. Nano Lett. 2002, 2, 1027-1030. (49) Harris, D. K.; Bawendi, M. G. J. Am. Chem. Soc. 2012, 134, 20211-20213. (50) Franke, D.; Harris, D. K.; Xie, L.; Jensen, K. F.; Bawendi, M. G. Angew. Chem. Int. Ed. 2015, 54, 14299-14303. (51) Zhou, J.; Pu, C.; Jiao, T.; Hou, X.; Peng, X. J. Am. Chem. Soc. 2016, 138, 6475-6483. (52) Zhou, J.; Zhu, M.; Meng, R.; Qin, H.; Peng, X. J. Am. Chem. Soc. 2017, 139, 16556-16567.

TOC Figure:

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Quantitative Identification of Basic Growth Channels for Formation of

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