Precursor Self‐Assembly Identified as a General Pathway for Colloidal Semiconductor Magic‐Size Clusters

Abstract Little is known about the formation pathway of colloidal semiconductor magic‐size clusters (MSCs). Here, the synthesis of the first single‐ensemble ZnSe MSCs, which exhibit a sharp optical absorption singlet peaking at 299 nm, is reported; their formation is independent of Zn and Se precursors used. It is proposed that the formation of MSCs starts with precursor self‐assembly followed by Zn and Se covalent bond formation to result in immediate precursors (IPs) which can transform into the MSCs. It is demonstrated that the IPs in cyclohexane appear transparent in optical absorption, and become visible as MSCs exhibiting one sharp optical absorption peak when a primary amine is added at room temperature. It is shown that when the preparation of the IP is controlled to be within the induction period, which occurs prior to nucleation and growth of conventional quantum dots (QDs), the resulting MSCs can be produced without the complication of the simultaneous coproduction of conventional QDs. The present study reveals the existence of precursor self‐assembly which leads to the formation of colloidal semiconductor MSCs and provides insights into a multistep nucleation process in cluster science.


Table of contents
Experimental S2 Table S1a.
Summary of literature reports on the synthesis of ZnSe clusters S6 Table S1b.

Figure S7
CdSe MSC-415 from two Cd precursors reactions with SeTOP S43 Note S1 On the second step and conventional characterization tools S44 S2 EXPERIMENTAL Zinc selenide (ZnSe) was used as a model system. The resulting clusters are denoted as MSC-299, based on the wavelength in nanometers of their absorption peak. ZnSe MSC-299 has not been reported before, to the best of our knowledge.

Chemicals and Stock Solution Preparation
Commercial chemicals, including zinc oxide (ZnO, 99.0%, Chengdu Kelong Chemical), zinc acetate (Zn(OAc) 2  To prepare a Zn(OA) 2 stock solution (with the total weight of 9.221 g and the concentration of Zn(OA) 2 of 0.545 mmol/mL), ZnO (0.492 g, 6.04 mmol), OA (3.729 g, 13.20 mmol) and ODE (5.000 g) were added into a 50 mL three-necked reaction flask at room temperature and the flask was evacuated for ~ 30 min, until no bubbles were observed.
Under a nitrogen (N 2 ) atmosphere, the mixture was then heated up to 120 C, and it was placed under vacuum at this temperature for two hours. Afterwards, under a nitrogen (N 2 ) atmosphere, the mixture was heated up to 290 C to form a clear solution, which was then cooled down to 120 C. This mixture was subjected to vacuum for two hours. The stock solution was stored in a glovebox at room temperature, which was also how the stock solutions described below were stored.
To prepare a SeTOP stock solution, Se powder (0.328 g, 4.15 mmol), TOP (3.390 g, 9.15 mmol) and ODE (0.437 g), were put in a 25 mL three-necked reaction flask at room temperature. The total weight of the reaction mixture was ~4.150 g with the Se concentration of 1 mmol/g). The mixture was stirred at 40 C for 20 min to achieve a clear solution under vacuum.
To prepare SePPh 2 H, Se powder (0.024 g, 0.30 mmol) and HPPh 2 (52 L, 0.30 mmol) were mixed and then heated at ~90 C for 1 hr in the glove box until all the Se powder disappeared completely. The SeHPPh 2 solid was opaque and white (a little yellow) in S3 appearance at room temperature.

Synthesis of ZnSe MSC-299 via a Two-step Approach
For the first step of a typical reaction of Zn(OA) 2  The presence and evolution of MSC-299 in the latter dispersion was monitored mainly by in situ UV-vis absorption spectroscopy.

Characterization
The UV-vis absorption spectra were collected between 270 nm and 500 nm with a Hitachi UH4150 spectrometer using intervals of 0.5 nm or 1 nm. The quartz cuvettes (3.5 mL standard QS cells with the light path of 10 mm) were purchased from Hellma Analytics.
Background measurements were performed with cyclohexane.
Electrospray ionization mass spectrometry (ESI-MS) was performed in the negative ion mode on Agilent 6210A HPLC-TOF/MS (at Chengdu Branch, Chinese Academy of Sciences).
Data analysis and instrument operation were performed with Agilent's "Mass Hunter" software. The first-step samples used were stored in dried toluene with a volume ratio of 1 to 1. For MS characterization, 1 L of the stored sample in toluene was further dispersed in 1.0 mL of toluene, with acetonitrile as the mobile phase.
In summary, Tri-n-octylphosphine selenide (SeTOP, Se=P(C 8 H 17 ) 3 ) or tri-n-butylphosphine selenide (SeTBP, Se=P(C 4 H 9 ) 3 ) was employed for the production of ZnSe RQDs, [49][50][51][52] with the reaction medium usually of 1-octadecene (ODE) not containing a primary amine and with zinc stearate (  oleic acid (OA) in ODE 30 L sample 3.0 mL CH (9)  oleic acid (OA) in ODE 30 L sample 2.0 mL CH -1.0 mL OTA (9)  and of the mixture (2.0 mL CH + 1.0 mL OTA, red traces). The induction period was below 220 C. ZnSe MSC-299 was detected in the c to f samples dispersed in the CH -OTA mixture, but not when dispersed in CH. By a side note, various clusters with magic numbers of atoms have been reported, and they embrace superb stability which has been attributed to their unique structures. [1][2][3][4][5] For example, C 60 and C 70 are acknowledged to be the two most stable clusters of fullerenes, consisting of the magic carbon number of 60 and 70, respectively. [1,2] Also, metal sodium clusters containing the magic atom number 8, 20, 40 or 58 are more abundant than other-number clusters. [4] Au 55 consisting of the magic number of 55 gold atoms with a closed-shell structure exhibits a supreme resistance to oxidation. [5] For the RQDs 380 nm S11 present study, that the final product is of the "magic-size" type is supported by reports in the literature. [6][7][8][9][10] For semiconductor MSCs, experimental evidence for their presence has been based on optical measurements in which sharp absorption peaks are present at persistent positions. [6][7][8] The full width at half maximum (FWHM) of absorption or emission peaks of one MSC ensemble is similar to that of a single quantum dot (QD) and smaller than that of a conventional QD ensemble. [9,10] S12 Figure           . .    Figure S5-1e. Summary of the cluster fragments detected in the m/z 800-1600 region for the 160 °C/15 min sample.    [1] a mixture of the Zn(OAc) 2 (0.2202 g, 1.20 mmol) and OLA (5.3 mL, oleylamine 70%, Aldrich) was placed in a 50 mL three-necked flask.

MSC
Following the same procedure as used for the Zn(OA) 2 preparation, the mixture was heated to 80 C under a N 2 atmosphere, then evacuated and backfilled with N 2 gas. This procedure was repeated three times. Next, under a N 2 atmosphere, the mixture was heated up to 120 C, then held under vacuum for 2 hrs, followed by cooling under a N 2 atmosphere to 80 C. A mixture of SeTOP (330 L, 0.30 mmol) and HPPh 2 (52 L, 0.30 mmol) and OLA (0.5 mL) was added to achieve a total weight for the reaction mixture of 5.000 g. The reaction flask was evacuated and then backfilled with N 2 gas, which was repeated three times. Under a N 2 atmosphere, the reaction mixture was heated from 80 C to 200 C with a 20 C interval; the samples were removed after 15 min at (1) 80 C, (2) 100 C, (3) 120 C, (4) 140 C, (5) 160 C,  Wavelength (nm) 4Zn(OAc) 2 /OLA-1SeTOP-1HPPh 2 60 mmol/Kg, 15 L sample 3.0 mL CH (9) (1). We believe that it is this H (labeled as 1 bond to N) that interacts with Zn and plays a role in the IP-299 to MSC-299 structural transformation. The assignment follows close after that for Supporting Information Figure 12. [1] The peak at ~ 2.1 ppm can be attributed to d 8 -toluene. [2] [1] M. Liu   (XRD) patterns (right) of (1) the zinc-blende ZnSe bulk, [1] (2) ZnSe RQDs, [1] ( the precipitate from first purification was dissolved in a mixture of 0.5 mL of Tol and 0.8 mL of acetonitrile followed by the centrifugation (8000 r/min, 3 min). It is noteworthy that the conventional characterization tools (including TEM [2,3] and XRD here) are not able to provide the accurate size and structure for the ZnSe MSCs studied. Although the formula and stoichiometry is an important piece of information, cluster stoichiometry seems to be related to the structure of a cluster. A stoichiometric composition has been proposed for a 20 nm   1 and on optical measurements of the present system (bottom panel). It is of help to point out that, in addition to noble metal, [1] the two-step nucleation model has been explored for calcium phosphate and carbonate. [2][3][4][5][6][7][8] On the way to the formation of nuclei from a homogenous solution, the non-classical nucleation theory, namely the two-step nucleation theory suggests that density and structure is decoupled, with an increase in density followed by the development of crystallinity. [1][2][3][4][5][6][7][8] However, these systems did not provide experimental evidence on the second step for the formation of nuclei. The present work on the structural transformation of IP-299 to MSC-299 does provide the very experimental evidence for the second step. MSC-299 is a nucleus without further growth. And the insight gained on the self-assembly process leading to IP-299 embraces the advance of MNT for the first step.  Accumulated experimental evidence has suggested that the formation of MSCs is surface-related; [1][2][3][4][5] for this reason, we tested a short-chain primary amine, butyl amine (BTA), to replace OTA. Figure S6-3 shows the presence of MSC-299 from the mixture of CH and BTA and not from the mixture of CH and di-BTA. The proton bonded to the N atom in the primary amine is believed to play a critical role in the transformation of IP-299  MSC-299 (see our 1 H NMR study by Figure S6-4), similar to that for CdTe MSC-371. [1] However, we have no full understanding of the role of a primary amine at the molecular level, and the precise mechanism remains an open question.
At the same time, the structural characterization of MSCs has not been unambiguous.
For example, in a similar system CdSe MSCs exhibiting absorption peaks at 415 nm [6] or 408 nm, [7] there is no consensus in the literature regarding the structure and stoichiometry composition; two different structures were reported, a core cage structure with 1Cd-to-1Se stoichiometry [6] and a tetrahedral structure with off 1Cd-to-1Se stoichiometry. [7] Transmission electron microscopy (TEM) does not provide an accurate measurement of the diameters of small-size NCs, [8,9] including MSCs; the underlying reasons may be related to changes occurring in the clusters, such as during TEM sample preparation. [10] Figure S6-5a shows the TEM images and X-ray diffraction (XRD) of MSC-299. Structural characterization is further inhibited by the limited stability of MSCs, which is affected by several factors (including the nature and amount of primary amines used) and the interplay between them ( Figures S6-5b and S6-6). Thus, no reliable structural information can be obtained using XRD.
Even if our samples had been stable (under the experimental condition for XRD), the use of "phase" to describe MSC structures is not entirely appropriate, due to the very small size of the clusters. [2,12] MS characterizations also provide limited information for MSCs (regarding their surface ligands and core compositions). [1,2,6,9,13,14] Given the unsuitability of MSCs for such characterization tools, [1,2,6,8,9,13,14] an uncertainty remains regarding the structure and composition of MSCs. Hence, we see there is a need to revisit relevant characterizations and to develop suitable characterization tools as well as sophisticated theoretical models for the