Photoluminescent Colloidal Nanohelices Self-Assembled from CdSe

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 2794−2801

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Photoluminescent Colloidal Nanohelices Self-Assembled from CdSe Magic-Size Clusters via Nanoplatelets Yuanyuan Liu,† Nelson Rowell,‡ Maureen Willis,§ Meng Zhang,† Shanling Wang,∥ Hongsong Fan,⊥ Wen Huang,# Xiaoqin Chen,*,⊥ and Kui Yu*,†,⊥,@ †

Institute of Atomic and Molecular Physics, Sichuan University, 610065 Sichuan, P. R. China Metrology Research Centre, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada § School of Physical Science and Technology, Sichuan University, 610065 Sichuan, P. R. China ∥ Analytical & Testing Center, Sichuan University, 610065 Sichuan, P. R. China ⊥ Engineering Research Center in Biomaterials, Sichuan University, 610065 Sichuan, P. R. China # Laboratory of Ethnopharmacology, West China School of Medicine, Sichuan University, 610065 Sichuan, People’s Republic of China @ State Key Laboratory of Polymer Materials Engineering, Sichuan University, 610065 Sichuan, P. R. China

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‡

S Supporting Information *

ABSTRACT: Reports on photoluminescent colloidal semiconductor two-dimensional (2D) helical nanostructures with one-dimensional quantum confinement are relatively rare. Here, we discuss the formation of such photoluminescent nanostructures for CdSe. We show that when as-synthesized unpurified zero-dimensional (0D) CdSe magic-size clusters (MSCs) (passivated by carboxylate ligands with three-dimensional quantum confinement) are dispersed in a solvent (such as toluene or chloroform) containing hexadecylamine and then subjected to sonication, helical nanostructures are obtained, as observed by transmission electron microscopy. We demonstrate that the formation involves the self-assembly of 0D MSCs into 2D nanoplatelets, which act as intermediates. The CdSe MSCs and their self-assembled 2D nanostructures display almost identical static optical properties, namely, a sharp absorption doublet with peaks at 433 and 460 nm and a narrow emission peak at 465 nm; this is a subject for further study. This study introduces new methods for fabricating photoluminescent helical nanostructures via the self-assembly of photoluminescent MSCs.

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the reaction of a cadmium carboxylate salt {such as cadmium myristate [Cd(MA)2]} and Se in ODE; after nucleation and growth of CdSe QDs, an acetate salt such as M(OAc)2 (M = Cd or Zn) is added (such as at 195 °C). Intense purification is performed to remove the coproduced 0D QDs.18 The third method takes advantage of the addition of a small molecule such as acetic acid (CH3COOH, HOAc) or Zn(OAc)2 to the reaction mixture of Cd(MA)2 and Se in ODE at 120 °C, at which point the reaction remains in the prenucleation stage, which occurs prior to nucleation and growth of CdSe QDs.1,2 The small molecule addition promotes the evolution of MSCs and suppresses the presence of QDs, even at high temperatures such as 240 °C. Such production is described by a twopathway model proposed for the prenucleation stage, which involves two individual but linked pathways.19 Starting from the Cd and Se precursors, one pathway involves the formation of monomers and fragments that result in the nucleation and

wo-dimensional (2D) nanomaterials continue to attract an increasing amount of attention,1−13 due to their applicability in various technologies, such as in light-emitting diodes (LEDs),8−10 lasers,11,12 and displays.13 In particular, photoluminescent colloidal 2D semiconductor nanocrystals (NCs) with quantum confinement in the third dimension are interesting; among them, CdSe NCs are the most studied.8−13 They exhibit narrow photoemission and a single optical absorption doublet, such as peaks at ∼465 and ∼433/460 nm, respectively. The full width at half-maximum of the emission is ∼70 meV, a value similar to that of a single CdSe zerodimensional (0D) quantum dot (QD) with three-dimensional (3D) quantum confinement.14,15 Three methods, mainly, have been reported to engineer this type of colloidal CdSe 2D NC passivated by carboxylate ligands via post-treatment.1,2,16−18 The first method involves a reaction of cadmium acetate [Cd(OAc)2], a long-chain carboxylic acid [myristic acid (MA, C13H27COOH), for example], and selenium (Se) powder in 1-octadecene (ODE).16,17 The reaction can be tailored to avoid the coproduction of CdSe QDs, through the use of high Cd:acid and high Cd:Se feed molar ratios. The second method involves © 2019 American Chemical Society

Received: March 23, 2019 Accepted: May 7, 2019 Published: May 14, 2019 2794

DOI: 10.1021/acs.jpclett.9b00838 J. Phys. Chem. Lett. 2019, 10, 2794−2801

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The Journal of Physical Chemistry Letters growth of QDs, while the other pathway consists of the selfassembly of the two precursors followed by the formation of precursor compounds (PCs) that can transform into magicsize clusters (MSCs) by first-order reaction kinetics.19−25 For the first and third approaches, it has been demonstrated that the as-synthesized products are 0D CdSe MSCs passivated by carboxylate ligands.1,16 During purification when a protic solvent such as ethanol (EtOH) is used, self-assembly of the 0D MSCs takes place, which results in the formation of 2D NCs called nanoplatelets (NPLs). The crystal structure of the resulting self-assembled NPLs seemed to be hexagonal or cubic according to transmission electron microscopy (TEM) or Xray diffraction (XRD), respectively.1,16 When an aprotic agent such as acetonitrile (CH3CN) is used instead of EtOH, the 0D MSCs do not self-assemble but remain isolated.1,2 It is remarkable that these 0D CdSe MSCs and their self-assembled 2D NPLs exhibit similar optical properties, notably including a single absorption doublet peak at 433/460 nm and a sharp emission peak at 465 nm. It is helpful to point out that other colloidal 0D NCs and their assembled 2D NCs, such as perovskites,5 PbS,6 and CdTe,7 exhibit similar peak positions of optical absorption and/or emission. There have been several, but no definitive, models proposed, regarding the mechanism of formation of the photoluminescent colloidal 2D semiconductor NCs passivated by carboxylate ligands.1,18,26−29 Moreover, there is some uncertainty regarding the thickness of the 2D NPLs. The thickness has been variously reported to be 6,18 5,27 3,30 4,4 and 3.5 monolayers (MLs)31 for the 2D NPLs emitting at 465 nm with absorption doublet peaks at 433/460 nm. These reports are based on the lattice constant (0.608 nm) of zinc-blende CdSe defined as the thickness of 2 MLs. Morphologically, rectangular NPLs in a dispersion have been suggested to be curved like helices, and two are often attached together; this information was obtained by imaging silica-coated CdSe 2D NCs.4 In view of the somewhat contradictory results to date with respect to the formation mechanism and NPL thickness, it appears that research is at an early stage regarding the photoluminescent 2D semiconductor NCs exhibiting one absorption doublet. It is important to be able to controllably fabricate detached helical nanostructures and to image them directly, for example, via TEM. Here, we report the formation of photoluminescent CdSe 2D helical nanostructures that exhibit one sharp absorption doublet peak at 433/460 nm and a photoluminescence peak at 465 nm; they are imaged directly by TEM. We demonstrate that post-treatment can transform photoluminescent CdSe 0D MSCs into photoluminescent 2D helical nanostructures through the formation of 2D NPLs (Scheme 1). We show that when as-synthesized photoluminescent 0D CdSe MSCs (from the third approach mentioned above1,2) are dispersed in a solvent [such as toluene (Tol)] containing hexadecylamine (C16H33NH2, HDA) and undergo sonication, 2D helical nanostructures with similar static optical properties are formed (Figure 1). The combined use of HDA and sonication seems to facilitate the formation, with a broad experimental window for the amount of HDA and sonication period. We hypothesize that the CdSe 2D helical nanostructures are transformed from CdSe 2D NPLs that resulted from the self-assembly of CdSe 0D MSCs. Accordingly, we design and perform purification experiments that result in 2D NPLs (Figure 2) and 0D MSCs (Figure 3); 2D helical nanostructures are induced from the both types of purified samples, which are well dispersed in

Scheme 1. Illustration of the Preparation of Photoluminescent CdSe 2D Helical Nanostructures via Post-Treatment Together with the Hypothesized ThreeStep Formation Pathwaya

a The first step is the formation of photoluminescent 0D MSCs exhibiting one sharp absorption doublet peak at 433/460 nm (A). The reaction of Cd(MA)2 and Se in ODE was carried out with the addition of HOAc at 120 °C followed by an increase in the temperature to 220 °C where it was held for 15 min. For the apparent MSC to nanohelix transformation (B), it is revealed with unpurified (Figure 1) and purified (Figure 3) samples. For the second step about the MSC to NPL transformation via the self-assembly of the MSCs (C), there are two methods designed [purified (Figure 2) and unpurified (Figure 4) samples]. For the third step about the NPL to nanohelix transformation (D), dispersion sonication works.

chloroform (CHCl3) containing HDA upon sonication. Importantly, for samples incubated for days (without purification after synthesis at room temperature), 2D NPLs are observed from HDA-free Tol dispersions without sonication, while nanohelices are seen with sonication (Figure 4). These experimental results are consistent with the formation pathway proposed for the CdSe helical nanostructures. It is important to note that the synthetic condition (such as 220 °C and 15 min), the amount (15 μL) of our assynthesized sample, and the volume (3.0 mL) of the dispersion used are the same for optical characterization and TEM and for purification. This study probes our understanding of the selfassembly process for semiconductor 0D MSCs, which results in the formation of 2D semiconductor NPLs and, ultimately, helical nanostructures. Our findings on the engineering of photoluminescent semiconductor 2D helical nanostructures via ready post-treatment of photoluminescent semiconductor 0D MSCs are promising for future applications and our understanding of the optical properties of these nanostructures.8−13,32−35 Figure 1 shows optical properties (a1 and b1) and typical TEM images (a2 and b2−b4) of our as-synthesized sample without purification, which was extracted from the reaction mixture of Cd(MA)2 and Se in ODE with HOAc added at a temperature of 120 °C, followed by a temperature increase to 220 °C, where the mixture was held for 15 min. For the sample (15 μL) dispersed in 3.0 mL of Tol (a) and in 3.0 mL of Tol with 0.010 g of HDA followed by a 5 min sonication (b), CdSe 0D MSCs (a2) and 2D helical nanostructures (b2−b4) were obtained. Notably, they exhibit similar optical properties (a1 2795


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Figure 1. Normalized optical properties (a1 and b1) of absorption (solid traces, left y axis) and emission (dashed traces, right y axis) and corresponding TEM images (a2 and b2−b4) collected from our as-synthesized samples (220 °C and 15 min) without purification. Two dispersions were prepared with 15 μL of the sample in 3.0 mL of Tol without (a) and with 0.010 g of HDA (b). Dispersion b was sonicated (Soni) for 5 min. The results are presented in parts a and b, respectively. Clearly, the 0D MSCs are synthesized (a) (as elucidated by part A of Scheme 1), and 2D nanohelices (b) are developed via post-treatment (as clarified by approach I of part B of Scheme 1). The 0D MSCs and 2D nanohelices display similar static optical absorption and emission.

and b1). Additional TEM images for dispersion b (HDAcontaining Tol) are shown in Figure S1-1. Figure 1a1 presents the normalized absorption (solid trace) and emission (dashed trace, excited at 350 nm) spectra in dispersion a (Tol). The absorption displays a sharp doublet at 432/459 nm, while the emission shows a narrow peak at 465 nm. These wavelength values are consistent with those previously reported, which were synthesized with the first16,17 and second methods.18 An important fact is that when the as-synthesized sample is dispersed in Tol only, the product retains a 0D nature, apparently dotlike with a size of ∼2 nm (Figure 1a2). This diameter is in agreement with the previously reported values, for which the synthesis was the first method mentioned above.1,16,17 Self-assembly of amine molecules induced by sonication was reported to produce supramolecular gels.36 It is reasonable that sonication increases the likelihood of interaction and provides energy.37,38 Also, the self-assembly of 0D MSCs into 2D NCs has been illustrated, for as-synthesized samples from the reaction of CdCl2 and octylammonium selenocarbamate in octylamine (C8H17NH2, OTA).1 After the as-synthesized 0D MSCs passivated by amine ligands are dispersed in a conventional solvent such as CHCl3 containing HDA and subjected to sonication, they self-assemble into 2D NCs, which are called nanoribbons.1,39,40 The combined use of HDA in a dispersion and a period of sonication has been demonstrated to favor the self-assembly of these 0D MSCs passivated by amine ligands into 2D NCs. Both the amine ligand-passivated 0D MSCs and their assembled 2D NCs exhibit similar optical properties, including one absorption doublet peak at 426/453 nm and one emission peak at 457 nm, similar to those passivated by carboxylate ligands.1 Accordingly, the morphological effect of HDA and sonication was explored for our reaction product as presented in part a of Figure 1. HDA (0.010 g) was added to 3.0 mL of Tol prior to dispersion of the as-synthesized sample (15 μL); the resulting Tol-HDA dispersion was subjected to sonication

(in this case 5 min). These procedures were performed before the optical measurements and TEM; the result of dispersion b (HDA-containing Tol) is presented in part b of Figure 1. Figure 1b1 shows the optical properties of dispersion b (HDAcontaining Tol), with an absorption doublet at 434/461 nm and an emission peak at 465 nm. The optical properties are similar to those shown in Figure 1a1 for dispersion a (Tol); the position difference (of 2 nm) of the absorption doublet compared to that shown in Figure 1a1 may be due to the different dispersion environments.25 Panels b2−b4 of Figure 1 demonstrate that 2D helical nanostructures have been fabricated. One would expect a difference between the static optical properties of the 0D MSCs and the 2D helical nanostructures if a change in the electronic structure had occurred. This appears not to be the case here. Moreover, the similarity in the optical properties suggests that for the formation of the helical nanostructures, the intermediate might be 2D NPLs with similar optical properties,1 as also demonstrated by Figure 2. Indeed, via TEM, a small amount of 2D NPLs is observed occasionally (as shown by Figure S11), although helices are the predominant morphology. Both right-handed and left-handed helices are seen. Figure 1b4 shows the associated parameters of a typical right-handed helix. Controlling the formation of right-handed versus left-handed helices is a promising avenue for further study. Following a study of gold nanohelices, we evaluated our CdSe nanohelices.41 The helical pitch (P), the repeating helical unit, is estimated to be 122 ± 1 nm, and the helical width (W) is 51 ± 1 nm. The diameter (D) of the nanostructure is 27 ± 1 nm. Evidently, the post-treatment with sonication and the addition of HDA in a dispersion play important roles in the generation of the 2D helical nanostructures after the synthesis of 0D MSCs. Accordingly, the effect of sonication on the conversion process is explored by keeping the amount of HDA constant (0.010 g) in 3.0 mL of Tol; the results are presented in Figure S1-2 with the sonication period controlled in the range of 0−120 min. Overall, these dispersions display similar 2796


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Figure 2. Normalized optical properties (a1 and b1) of absorption (solid traces, left y axis) and emission (dashed traces, right y axis) and corresponding TEM images (a2, a3, b2, and b3) obtained from two dispersions made with purified CdSe samples (15 μL each prior to purification). Dispersion a was prepared with 3.0 mL of Tol without sonication, and dispersion b was prepared with 3.0 mL of CHCl3 containing 0.010 g of HDA followed by a 30 min sonication (Soni). The results are presented in the top (a) and bottom (b) panels, respectively. Evidently, the purification with Tol and EtOH results in 2D NPLs (a2 and a3). After the purified sample dispersion is sonicated, the helical nanostructures are formed (b2 and b3) from the 2D NPLs.

post-treatment of the reaction product. The formation pathway of the CdSe helical nanostructures could be seen to be similar to that of crystalline indium sulfide nanocoils, which were synthesized with indium acetate [In(OAc)3] and sulfur powder in a mixture of octylamine (OTA, C8H17NH2) and EtOH.42 The indium sulfide nanocoils were formed by spontaneous selfcoiling of 2D ultrathin nanoribbons.42,43 On the basis of our experimental data shown above, it seems reasonable that the formation of the helical nanostructures occurs with the 2D NPLs as the intermediates. Thus, we decided to fabricate 2D NPLs via purification and then to prepare 2D helical nanostructures (Figure 2). Figure 2 demonstrates that 2D helical nanostructures are obtained from 2D NPLs. To fabricate 2D NPLs via purification (post-treatment, approach I of part C of Scheme 1), 15 μL of one as-synthesized sample was mixed with 3.0 mL of the mixture of Tol (2.0 mL) and EtOH (1.0 mL). Centrifugation was performed. After three purifications, the precipitate was dispersed in 3.0 mL of Tol without sonication. The resulting dispersion is in dispersion a (Tol), the result of which is presented in panel a of Figure 2. Clearly, the absorption and emission spectra (Figure 2a1) remain similar to those collected from the as-synthesized sample (15 μL) prior to purification (Figure 1a1), and 2D NPLs (parts a2 and a3 of Figure 2) are obtained after the purification, as addressed elsewhere.1 To fabricate the helical nanostructures, the purified sample was dispersed in 3.0 mL of CHCl3 containing 0.010 g of HDA followed by a 30 min sonication. The resulting dispersion is dispersion b (HDA-containing CHCl3), the result of which is presented in panel b of Figure 2. While, again, the optical behavior (Figure 2b1) does not change greatly, the presence of 2D helical structures can be readily observed via TEM (parts b2 and b3 of Figure 2). Of note is the fact that the helical associated parameters (as illustrated in Figure 2b2) are similar to those depicted by Figure 1b4. We repeated this experiment involving the purification, starting from synthesis. Following the presentation format of Figure 2, the results of optical

optical properties, with regard to the peak positions of absorption and emission. Without sonication, a mixture of 2D NPLs and 0D MSCs is observed; it seems that the 0D MSCs can self-assemble into 2D NPLs in a HDA-containing solvent in the absence of sonication. Meanwhile, with sonication, helical-shaped nanostructures are formed. Meanwhile, the duration of the sonication does not seem to have a significant influence on the outcome, as we see by comparing the results of the 5 and 120 min sonications. Thus, there is a large experimental window for the sonication process to facilitate the formation of the helical nanostructure. Furthermore, the formation of the helical nanostructure seems to occur via the 2D well-like nanostructure (as an intermediate state), which results from the self-assembly of the 0D CdSe MSC during post-treatment. Figure S1-3 addresses the effect of the amount of HDA in the dispersion on the formation of the CdSe helical nanostructures, showing the optical absorption and emission spectra and TEM images collected from three dispersions. To prepare the dispersions, the as-synthesized sample (15 μL) was dispersed in 3.0 mL of Tol containing 0.005 (a), 0.020 (b), and 0.040 g (c) of HDA. Sonication was performed for the same period of time (30 min) for the three dispersions. The three dispersions exhibit similar optical properties, and 2D nanohelices are observed. Again, there is a large experimental window for the amount of HDA required to facilitate the formation of the helical nanostructure, under the experimental conditions we explored. For TEM, we point out that it is better to use a relatively small amount of HDA in the dispersion, because HDA does not evaporate and thus leads to degradation of the TEM observation. In the development of the CdSe helical nanostructures from an as-synthesized sample of 15 μL in 3.0 mL of Tol, the use of 0.010 g of HDA and sonication (such as for 5 or 30 min) appears to be preferable. Evidently, our helical nanostructures are not formed directly in the reaction of Cd(MA)2 and Se in ODE at 220 °C for 15 min (after the addition of HOAc at 120 °C) but during the 2797


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Figure 3. Normalized absorption (dashed traces, left y axis) and emission (solid traces, right y axis) spectra (a1 and b1) and corresponding TEM images (a2, a3, b2, and b3) of two dispersions made from two purified samples (15 μL each prior to purification). Dispersion a was prepared with 3.0 mL of Tol without sonication, and dispersion b was prepared with 3.0 mL of CHCl3 containing 0.010 g of HDA followed by a 30 min sonication (Soni). The results are presented in panels a (top) and b (bottom), respectively. Unambiguously, the purification with Tol and CH3CN keeps 0D MSCs intact (a2 and a3); after the purified samples are dispersed in a HDA-containing solvent and sonicated, helical nanostructures are formed (b2 and b3). It is reasonable to propose that the helical nanostructures resulted from the self-assembly of the 0D MSCs via 2D NPLs as the intermediate.

spectra and TEM images are presented in Figure S2-1. Again, 2D helical nanostructures are derived from 2D NPLs, which are developed during the post-treatment with the mixture of Tol and EtOH. It is useful to point out that for the NPL to helical nanostructure transformation, sonication alone works (part D in Scheme 1), while a purified sample disperses well in 3.0 mL of CHCl3 containing HDA (0.010 g). Importantly, when HDA was substituted with other primary amines, OTA, or oleylamine (OLA, C18H36NH2), a purified sample did not disperse well. Figure S2-2 shows the optical properties and TEM images of two purified samples in CHCl3 containing OTA (a) and OLA (b). Furthermore, Figure S2-3 shows the optical properties and TEM images of one purified sample in Tol with a 30 min sonication; again, 15 μL of the sample was used prior to purification with the Tol/EtOH mixture. Evidently, helical nanostructures can also be prepared in HDA-free Tol. As a side note, the use of CHCl3 and HDA instead of Tol and HDA is due to a better dispersity found for the purified sample (Figure S2-4). These results are strong evidence that the helical nanostructures are produced in a three-step pathway, as illustrated by Scheme 1, which involves the production of 0D MSCs (A), 2D well-like nanostructures formed as the intermediate via the self-assembly of 0D MSCs (C), followed by the transformation of the intermediate to the nanohelices (D). To further validate the proposed hypothesis (as illustrated by part B in Scheme 1), we purified our as-synthesized sample to fabricate 0D CdSe MSCs purposely (Figure 3). The purification was carried with an aprotic agent (CH3CN) replacing EtOH.1 A mixture of Tol (2.0 mL) and CH3CN (1.0 mL) was employed to purify 15 μL of one as-synthesized sample. Centrifugation was performed. After three repeats of the purification process, two dispersions were prepared. For dispersion a, one purified sample was dispersed in 3.0 mL of Tol without sonication. For dispersion b, the other purified

sample was dispersed in 3.0 mL of CHCl3 containing 0.010 g of HDA followed by a 30 min sonication. The absorption and emission spectra and TEM images of the two dispersions are presented in panels a and b of Figure 3, respectively. Amazingly, the helical nanostructures (b2 and b3) are also obtained from the purified 0D MSCs (a2 and a3). The purification experiment, which keeps 0D MSCs intact, was repeated, as shown by Figure S3. Figure 3 supports the finding that, in this study, the helical nanostructures are unambiguously the product of the self-assembly of 0D MSCs (with 2D well-like nanostructures as intermediates), as illustrated by approach II of part B of Scheme 1. It is reasonable that incubation of as-synthesized samples (prior to purification) is another means for the formation of 2D NPLs. Figure 4 shows typical TEM images of such a sample with a 38 day incubation. Two dispersions were prepared, each with 15 μL of the sample in 3.0 mL of Tol without (a) and with a 30 min sonication (b). Their results are shown in parts a and b of Figure 4, respectively. For dispersion a without sonication, the coexistence of 2D NPLs and 0D MSCs is observed (part a of Figure 4). It is noteworthy that the presence of 2D NPLs during dispersion incubation was documented,1 which is similar to the result shown in part a of Figure 4. The fact that sample incubation promotes the development of 2D NPLs from as-synthesized samples is further evident in Figure S4-1. For dispersion b with sonication, 2D helical nanostructures are detected (part b of Figure 4). It is noteworthy that HDA is not required for the formation of the helical nanostructures from incubated samples. The corresponding optical properties of the two dispersions can be found in Figure S4-2. Clearly, Figure 4 is in line with the hypothesis proposed about the formation of the helical nanostructures via 2D NPLs (Scheme 1). In conclusion, we have synthesized photoluminescent colloidal CdSe helical nanostructures, which emit at 465 nm 2798


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Figure 4. Typical TEM images of one reaction product that experienced a 38 day incubation after synthesis. Two dispersions were prepared, each with 15 μL of the sample in 3.0 mL of Tol, without sonication (a1 and a2) and with sonication for 30 min (b1 and b2). Clearly, 2D NPLs developed during the incubation, while 2D helical nanostructures evolved with sonication. For the NPL to helical nanostructure transformation, sonication alone works well (as clarified by part D in Scheme 1). Evidently, our hypothesis about the formation of the helical nanostructures via 2D NPLs explains these observations well.

photoluminescent semiconductor 2D helical nanostructures. The proposed formation pathway might well be helpful to other inorganic nanohelices, nanocoils, nanoribbons, and single-walled nanotubes.41−48

and display one sharp absorption doublet peak at 433/460 nm. We propose that there is a three-step pathway for the formation of the helical nanostructures (Scheme 1), which accurately describes our synthetic approach incorporating posttreatment. The first step is the formation of 0D MSCs (A), and the second step is the self-assembly into 2D NPLs (C), which transform into the nanohelices in the last step (D). The combined use of HDA in dispersion and dispersion sonication facilitates the second and third steps of as-synthesized samples (Figure 1). For one purified sample in the presence of 2D NPLs (Figure 2) or undisturbed 0D MSCs (Figure 3), 2D helical nanostructures are fabricated also. After a sample without purification is incubated for a substantial period of time such as ∼38 days (Figure 4), 2D NPLs or 2D helical nanostructures are observed from HDA-free Tol dispersions without or with sonication, respectively. On the basis of these experimental observations, we propose that for the formation of the helical nanostructures, the driving force is the selfassembly of the 0D MSCs via 2D NPLs. How exactly surface tension might affect the self-assembly of 0D MSCs into 2D NCs is a subject for future study.41,42 Understanding the optical properties of these nanostructures with quantum confinement in the various dimensions needs attention from the community. Our preliminary result about PL intensity versus detection angle is shown in Figure S5. These preliminary emission results indicate that the 0D MSCs, 2D NPLs, and 2D nanohelices have different structural anisotropies. The polarized PL observed, together with the comparison of PL lifetime and quantum yield (QY) (along the self-assembly), is a subject for further study. This study introduces novel post-treatment to facilitate the formation of photoluminescent CdSe 2D helical nanostructures (from assynthesized samples without purification), which lays the foundation for post-treatment to practically synthesize other

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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.9b00838. Experimental details of synthesis and characterization together with additional optical property and TEM results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Nelson Rowell: 0000-0001-7616-9396 Hongsong Fan: 0000-0003-3812-9208 Wen Huang: 0000-0002-9772-9492 Kui Yu: 0000-0003-0349-2680 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.Y. thanks the National Natural Science Foundation of China (NSFC, 21773162 and 21573155), the State Key Laboratory of Polymer Materials Engineering of Sichuan University (Grant sklpme2018-2-08), the Open Project of Key State Laboratory for Supramolecular Structures and Materials of Jilin University 2799


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(SKLSSM 201830), and the Fundamental Research Funds for the Central Universities. H.F. and W.H. are grateful for the National Major Scientific and Technological Special Project for “Significant New Drugs Development” (2018ZX09201009005-004 and 2018ZX09201009-005-001). For TEM, the authors are grateful to the Analytical & Testing Center of Sichuan University.

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