Aqueous-phase evolution of CdS MSC-360

Figure 1 presents the evolution of optical absorption spectra collected from one mixture in two solvents. The mixture contains CdCl₂ (2.0 mM), MPA (8.0 mM), KOH (20.0 mM), and TU (1.0 mM). One solvent is a mixture of BTA (1.5 mL) and water (H₂O, 1.5 mL) (Fig. 1a, b). The other solvent is H₂O (3.0 mL) (Fig. 1c, d). For the BTA-water solution, the mixture is first placed in 1.5 mL of water followed by the addition of 1.5 mL of BTA. See the “Methods” section for details regarding the preparation of the two solutions. The absorption spectra are collected after elapsed times of 0, 0.5, 1, 3, 6, and 12 h (Fig. 1a, c); at each time point, an aliquot (50 μL) is extracted and diluted in deionized water (3.0 mL) and absorption measurements are performed (Fig. 1b, d).

In the mixture of BTA and water (Fig. 1a), an absorption feature gradually develops to peak at ~356 nm, which indicates the presence of CdS MSC-360. At the beginning (0 h), the spectrum is quite featureless. At 3 h, a peak at 347 nm can be identified clearly. Afterwards, this peak increases in intensity considerately and red shifts slightly to 356 nm at the 12 h point. Simultaneously, the intensity of the absorption around 228 nm decreases (Fig. 1b). Supplementary Fig. 1 suggests such short wavelength absorption is due to CdCl₂, MPA, and TU together. Accordingly, the overall conversion yield is estimated to be about 60% after a period of 6 h (as illustrated by Supplementary Fig. 2). In water (Fig. 1c), the absorption spectra are featureless, which suggests that little reaction takes place. Also, the absorption at 228 nm remains constant (Fig. 1d), in agreement with that there is no consumption of the Cd and S sources.

Therefore, the evolution of CdS MSC-360 at room temperature occurs only when the mixture of CdCl₂, MPA, KOH, and TU is in the BTA and water environment (Fig. 1a), in which case we observe the consumption of the starting materials (Fig. 1b). When this mixture is placed in water without BTA, the starting materials are not consumed and the evolution of CdS MSC-360 does not take place (Fig. 1c, d). Figure 1a, c focuses on the possible development of MSCs, while Fig. 1b, d illustrates the likely consumption of the reactants. The synthesis conditions have been carefully optimized, such as the Cd to S feed molar ratio (Supplementary Fig. 3 with a constant Cd concentration of 2.0 mM). Clearly, the characteristic optical absorption peak of MSC-360 broadens gradually upon increasing the TU concentration. In this regard we use a 2CdCl₂ to 1TU feed molar ratio to synthesize CdS MSC-360.

Transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and thermogravimetric analysis (TGA) have been employed (Supplementary Fig. 4). Although the conventional characterization tools have some shortcomings with regard to providing precise structural and compositional information of colloidal semiconductor small-size QDs and MSCs^{20,25,26,27,28}, Supplementary Fig. 4 suggests that the aqueous-phase CdS MSCs are spherical with a diameter smaller than 3 nm, and have a similar structure as that of organic-phase CdS MSC-361⁴², with the ligand to inorganic core weight ratio of 20 to 80 (Supplementary Note 1). The Fourier transform infrared (FT-IR) spectrum of the purified CdS MSC-360 sample (Supplementary Fig. 5) indicates that the purified MSCs are passivated only by MPA molecules which act as surface ligands. Therefore, there is no evidence that these MSCs have any of the primary amine, BTA, associated with them.

Effect of amounts and nature of primary amines

For the formation of MSC-360 in the mixture of CdCl₂, MPA, KOH, and TU, Fig. 1 suggests that BTA plays an important role. In Fig. 2, we present the effects of the quantity of BTA (Fig. 2a) and of different primary amines (Fig. 2b). The absorption spectra are collected from the same mixture but in four solvents containing four different amounts of BTA (Fig. 2a) and in three solvents with three different primary amines (Fig. 2b). The mixture consists of CdCl₂ (2.0 mM), MPA (8.0 mM), KOH (20.0 mM), and TU (1.0 mM); after the mixture is placed into water, a primary amine is added to result in a final volume of 3.0 mL. In Fig. 2a, the added BTA is 0.5 (gray trace), 1.0 (green trace), 1.5 (blue trace), and 2.0 mL (magenta trace). In Fig. 2b, 1.5 mL of a primary amine of BTA (blue trace), propylamine (PrA, CH₃–(CH₂)₂–NH₂, lighter blue trace), and ethylamine (ETA, CH₃–CH₂–NH₂, lightest blue trace) is added. Supplementary Fig. 6 presents their chemical structures and 3D models. All the spectra are collected 24 h after the preparation of the solutions at room temperature. The temporal evolution of the absorption properties of these solutions are shown in Supplementary Figs. 7 (BTA), 8 (PrA), and 9 (ETA).

In Fig. 2a, the absorption spectrum of the solution having 2.0 mL BTA displays a sharp optical absorption peaking at about 354 nm, while that of the solution with 0.5 mL BTA exhibits a relatively broad absorption. Supplementary Fig. 7 demonstrates that CdS MSCs with a sharper absorption are produced at 24 h in a solvent with more BTA. This seems to be the case with PrA as well (Supplementary Fig. 8). When 1.5 mL of ETA is added instead of BTA or PrA, more CdS MSC-360 is present at 24 h (Fig. 2b). Solutions with amine volumes up to 1.5 mL are relatively transparent at all times, and MSC-360 appears gradually (Supplementary Figs. 7–9). Solutions with 2.0 mL of BTA or ETA appear milky at the start and become clear after around 12 or 24 h, respectively, and the evolution of MSC-360 is observed (Supplementary Figs. 7d and 9d). Because of the precipitation (with 2.0 mL of amine) and the evaporation of PrA (boiling point 48 °C) and ETA (boiling point 17 °C), the mixture of BTA (1.5 mL), and water (1.5 mL) is used in this study to synthesize CdS MSC-360.

It is noteworthy that BTA has been claimed to assist the decomposition of a TU derivative in N,N-dimethylformamide (DMF) during the nucleation and growth of PbS nanocrystals⁴³. In the absence of CdCl₂, Supplementary Fig. 10 suggests that TU remains stable at room temperature in a mixture of BTA (1.5 mL) and water (1.5 mL). In this regard the room-temperature decomposition of TU requires both BTA and the Cd precursor. Moreover, only primary amines are found to facilitate the room-temperature evolution of CdS MSC-360. For example, when a secondary amine diethylamine is used (Supplementary Fig. 11), no CdS MSC-360 is observed.

CdCl₂ concentration effect upon aggregation

To intensify the production of CdS MSC-360, we increase the concentration of the solution, the result of which is shown in Fig. 1a. Figure 3 presents the absorption spectra of four solutions with four different concentrations after 24 h of preparation (a), and a summary of time-dependent absorbance of CdS MSC-360 (at its lowest energy transition peak position) in these solutions for times up to 24 h (b). Mixtures with the same feed molar ratio as that used in Fig. 1a are placed in the same solvent of BTA (1.5 mL) and water (1.5 mL), but with resulting CdCl₂ concentrations of 2.0 (magenta trace (a) and circular symbols (b)), 4.0 (blue trace (a) and square symbols (b)), 6.0 (green trace (a) and triangular symbols (b)), and 10.0 mM (gray trace (a) and diamond symbols (b)). The solutions are kept in cuvettes for the absorption measurements after different time durations.

The solution with 2.0 mM CdCl₂ effectively produces CdS MSC-360, with the absorbance of CdS MSC-360 increasing gradually to about 0.42 after 24 h. When the concentration of CdCl₂ is increased to 4.0 mM, the absorbance of MSC-360 at 24 h is reduced to 0.31. With 6.0 mM of CdCl₂, the strength of MSC-360 at 24 h is further reduced to 0.25. When the concentration of CdCl₂ is increased to 10.0 mM, an evolution of MSC-360 does not occur. Accordingly, an increase in the solution concentration suppresses the production of CdS MSC-360.

For the two solutions of 2CdCl₂-8MPA-20KOH-1TU in the BTA (1.5 mL) and water (1.5 mL) solvent with the CdCl₂ concentrations of 5.0 and 10.0 mM, dynamic light scattering (DLS) (Supplementary Fig. 12) indicates large aggregates are present with hydrodynamic diameters (D_h) of 272 ± 4 and 379 ± 3 nm, respectively. The higher the solution concentration is, the larger the aggregates are. We would like to point out that these aggregates are much larger than those which are resulted from the self-assembly of Cd and S precursors in the organic-phase reactions^18,19,20,28. The formation of such large aggregates has also been validated by TEM (Supplementary Fig. 13).

To investigate why the aggregation occurs, we use the fluorescence spectroscopy of pyrene. This molecule has been used as a solvatochromic probe for aqueous solutions to detect the onset of self-assembly and/or aggregation, which can result in moieties including micelles and nanofibers^{44,45,46,47,48}. Pyrene has high affinity for nonpolar environments, such that when pyrene moves into a relatively more hydrophobic environment, the fluorescence intensity ratio of its third (I₃) to first (I₁) peaks increases. Supplementary Fig. 14 presents the I₃/I₁ ratios of pyrene for two types of mixtures, which are 2CdCl₂-8MPA-20KOH (a) and 2CdCl₂-8MPA-20KOH-1TU (b). The mixtures are placed in 3.0 mL of water and in the BTA (1.5 mL) plus water (1.5 mL) solvent, with different CdCl₂ concentrations ranging from 0.5 to 80.0 mM. When the CdCl₂ concentration is higher than 4.0 mM, the I₃/I₁ ratio increases significantly for the two types of mixtures without or with TU. From these observations, the critical aggregation concentration (CAC) of CdCl₂ can be estimated to be ~4.0 mM. The term “Cd–MPA complex” was used for the aqueous-phase approach to CdTe QDs with a reaction of CdCl₂, MPA, NaOH, and Te precursor in water^34,36.

Accordingly, we hypothesize that the “Cd–MPA complex” forms in our 2CdCl₂-8MPA-20KOH solutions and aggregates when the CdCl₂ concentration is higher than the CAC. In our 2CdCl₂-8MPA-20KOH-1TU solutions, when the CdCl₂ concentration is lower than CAC, both the Cd–MPA complex and TU are present in the BTA-containing aqueous environment, and CdS MSC-360 evolves readily (as presented in Fig. 1a). When the CdCl₂ concentration is higher than the CAC, it is the Cd–MPA complex that dominates the aggregation observed. The resulting aggregates effectively separate a majority of the Cd–MPA complex from the BTA molecule which stays in the solution phase (outside of the aggregates). Consequently, the production of CdS MSC-360 is suppressed almost completely when the CdCl₂ concentration is as high as 10.0 mM (Fig. 3 gray trace and diamond symbols). Thus, a relatively low CdCl₂ concentration of 2.0 mM is used in this study to synthesize CdS MSC-360 with a constant Cd to MPA feed molar ratio of 1 to 4. In the present work we also discovered that when the ratio was 1 to 1, precipitation took place, and when the ratio was 1 to 2, the CAC became larger at about 20.0 mM.