CdS-Rod Nanocrystals with Induced

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Optically Active CdSe-Dot/CdS-Rod Nanocrystals with Induced Chirality and Circularly Polarized Luminescence Jiaji Cheng,† Junjie Hao,‡ Haochen Liu,‡ Jiagen Li,§ Junzi Li,† Xi Zhu,§ Xiaodong Lin,† Kai Wang,*,‡ and Tingchao He*,† Downloaded via KAOHSIUNG MEDICAL UNIV on June 21, 2018 at 21:15:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

College of Physics and Energy, Shenzhen University, Shenzhen 518060, China Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China § School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen 518172, China ‡

S Supporting Information *

ABSTRACT: Ligand-induced chirality in semiconductor nanocrystals (NCs) has attracted attention because of the tunable optical properties of the NCs. Induced circular dichroism (CD) has been observed in CdX (X = S, Se, Te) NCs and their hybrids, but circularly polarized luminescence (CPL) in these fluorescent nanomaterials has been seldom reported. Herein, we describe the successful preparation of L- and D-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors and a maximum anisotropic factor (glum) of 4.66 × 10−4. The observed CD and CPL activities are sensitive to the relative absorption ratio of the CdS shell to the CdSe core, suggesting that the anisotropic g-factors in both CD and CPL increase to some extent for a smaller shell-to-core absorption ratio. In addition, the molar ratio of chiral cysteine to the DRs is investigated. Instead of enhancing the chiral interactions between the chiral molecules and DRs, an excess of cysteine molecules in aqueous solution inhibits both the CD and CPL activities. Such chiral and emissive NCs provide an ideal platform for the rational design of semiconductor nanomaterials with chiroptical properties. KEYWORDS: CdSe-dot/CdS-rod, ligand-induced chirality, circular dichorism, circularly polarized luminescence, excitonic interactions, orbital coupling theory

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chemical properties. To date, an extensive body of experiments and theoretical calculations have been reported toward discovering the origin and modulating the intensity of chirality with various parameters, including the surface ligand motif38,39 (thiolated or nonthiolated,40,41 number of stereocenters,42,43 and surface ligand conformation44 when bound to QDs) and chemistry of the QDs, namely, the size and shell thickness of the QDs.15,45 Ferry’s work,43 for instance, compared carboxylatebound and thiolate-bound chiral CdSe QDs and showed that chiral carboxylic acids can exhibit intense CD signals with anisotropic gCD factors46 of up to 7.0 × 10−4, where gCD = Δε/ε, and Δε is the absorptivity difference between left- and righthanded circularly polarized light. In addition, the authors noted that increasing the number of stereocenters in the ligand could further enhance the anisotropy factors of the chiral QDs. Gun’ko et al.45 showed the impact of the shell thickness on the CD and photoluminescence (PL) performances of CdSe/CdS QDs; they

hiral core−shell quantum nanocrystals (NCs), such as chiral CdSe/CdS and CdSe/ZnS with enhanced photoluminescence quantum yields (PLQYs) and environmental stabilities, have recently become an emerging topic of interest because of their promising applications in chiral recognition,1−3 stereoselective synthesis,4,5 biosensing,6 display devices,7−11 and so forth.12−14 Chiral core−shell quantum NCs also provide an ideal platform for exploring the mechanisms15−17 underlying the chirogenesis of excitonic circular dichroism (CD) and the possibility for controlling circularly polarized luminescence (CPL) through the extensive tunability of the sizedependent fluorescence properties as a result of quantum confinement effects.18,19 Three mechanisms for explaining the induction of chirality in chiral metal nanoparticles20,21 (NPs) are often extended to semiconductor NCs:22,23 (i) intrinsically chiral NCs with dislocations and defects,5,24−26 (ii) ligand-induced chiral surfaces in QDs27,28 or chiral interactions between chiral ligands and achiral QDs,15,29,30 and (iii) achiral QD-based chiral assemblies.31−37 Among these mechanisms, ligand exchange with chiral molecules produces tremendous advances in obtaining QDs with a uniform size distribution and providing diverse choices for designing the surface chemistry of QDs to impart © XXXX American Chemical Society

Received: January 5, 2018 Accepted: May 23, 2018 Published: May 23, 2018 A

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Figure 1. (A) Schematic illustration of the chiral Cys-CdSe/CdS DRs. The red particle represents the CdSe core; the yellow rod-like shell represents the CdS shell, and the purple ligands are the chiral cysteine molecules. (B) Evolution of the normalized absorption spectra (normalized at the band gap absorption) as a function of the ARSC (ARSC refers to the absorption ratio of the shell to the core of the DRs). The inset shows the magnified absorption spectra. (C) Corresponding TEM images of the chiral Cys-CdSe/CdS DRs with different ARSC values. All the TEM scale bars represent 20 nm.

insufficient theoretical background on this topic and the complex experimental control that is required over the synthesis. To date, only one example of chiral-ligand-induced CPL has been reported by Balaz,48 in which L- and D-cysteine induced modular CPL signals in achiral CdSe QDs, but the effects of properties, such as the size and surface ligands in the chiral CdSe QDs on the CPL signals, were not described. Therefore, an active dissymmetry factor in CPL with fine tunability has not yet been accomplished in chiral-ligand-induced quantum NCs, and more importantly, the question arises as to whether we can identify key factors for regulating these optical phenomena through feasible parameters. Herein, we present the preparation of optically active CdSedot/CdS-rod NCs, named CdSe/CdS DRs, with tunable absorption and luminescence properties (Figure 1A). To avoid complexity and redundancy, the absorption ratio of shell to core (ARSC), Ashell/Acore, is used to describe the geometry-dependent CD and CPL phenomena, in which the absorption spectra of the

asserted that the induced CD was inversely proportional to the CdS shell thickness, but the PL measurements showed a direct relationship with the shell thickness. For theoretical support, density functional theory (DFT)40,42 and time-dependent DFT (TD-DFT) methods44 have been widely employed to further elucidate the origin of the induced chiroptical response in chiralligand-conjugated QDs with respect to the conformation of the surface ligands and the intrinsic properties of the QDs. Recently, with the aim to obtain an enhanced CD intensity and dissymmetry factor, Tang’s group47 used aspect-ratio-tunable CdSe quantum rods (QRs) to increase the anisotropy. The researchers used the nondegenerate coupled oscillator (NDCO) model to clarify the coupling of the electric dipole transition moments in the chiral cysteine−CdSe QR system, showing that different transition polarizations are the key to the induction of chirality. However, CPL, a counterpart of CD in terms of emission, has been seldom reported in semiconductor NCs because of the low accessibility of the CPL instruments and the B

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ACS Nano Table 1. CD and CPL Anisotropy Factors of D-Cys-CdSe/CdS DRs ARSCa 1.42* 8.95 10.05 14.33 16.19 17.86

DCdSeb 3.52 3.70 4.84 4.75 4.84 3.33

aspect ratioc 1.0 3.3 4.5 4.0 7.8 3.0

gCD+/λCD (nm)d −4

1.15 × 10 /579.6 2.03 × 10−4/630.0 1.09 × 10−4/595.8 3.32 × 10−6/642.2 0.67 × 10−4/591.4 0.46 × 10−4/626.0

gCD−/λCD (nm)e −4

−0.58 × 10 /549.6 −1.14 × 10−4/598.8 −1.36 × 10−4/617.4 −1.43 × 10−4/615.6 −0.51 × 10−4/614.6 −0.22 × 10−4/601.8

|gCD+ − gCD−|/2f −4

0.86 × 10 1.59 × 10−4 1.23 × 10−4 0.72 × 10−4 0.59 × 10−4 0.34 × 10−4

QYg

glum/λCPL (nm)h

0.5% 54% 38.1% 47.4% 37.6% 39.4%

none 4.66 × 10−4/664.3 3.70 × 10−4/676.1 2.75 × 10−4/667.7 2.29 × 10−4/668.8 1.77 × 10−4/671.8

a ARSC determined from the normalized UV/vis absorption of chiral CdSe/CdS DRs at 450 nm and the band gap. bDiameter of the CdSe core determined from the absorption spectrum by Peng’s equation50 (nm). cAspect ratio of the DRs. e,dCD anisotropy gCD factors at the most intense positive and negative CD bands (nm). fMagnitude of the gCD factor, defined as |gCD+ − gCD−|/2. gPLQY of the DRs after ligand exchange. hCPL anisotropic glum at the most intense CPL wavelength (nm). *A control sample made of CdSe QDs without the CdS shell. Check Table S2 for information about L-Cys-CdSe/CdS DRs and Table S3 for other physical parameters of cysteine-capped CdSe/CdS DRs.

Figure 2. CD spectra and corresponding UV/vis absorption spectra and anisotropic gCD factors of the L- and D-Cys-CdSe/CdS DRs with different ARSC values in aqueous solution. CD spectra of the L- (A) and D-Cys-CdSe/CdS DRs (B) with different ARSC values. The insets show the magnified CD spectra from 500 to 700 nm. (C) Normalized UV/vis absorption spectra of the L-Cys-CdSe/CdS DRs with different ARSC values (the curves are normalized at the first exciton absorption peak). The inset is the magnified UV/vis spectrum. (D) Plot of the gCD factor magnitudes of the L- and D-Cys-CdSe/CdS DRs against the ARSC and their linear fittings. The black dots and line represent the L-Cys-CdSe/CdS DRs, whereas the red dots and line represent the D-Cys-CdSe/CdS DRs. The gCD+ and gCD− values are referenced in Table 1 and Table S2. For LCys-CdSe/CdS DRs, the corresponding wavelengths are 598.6, 619, 555.2, 612.2, and 602.6 nm for gCD+ and 627.8, 596.2, 598.6, 637, and 625.8 nm for gCD− when ARSC varied from 8.95 to 17.86. For D-Cys-CdSe/CdS DRs, the corresponding wavelengths are 630, 595.8, 642.2, 591.4, and 626 nm for gCD+ and 598.8, 617.4, 615.6, 614.6, and 601.8 nm for gCD− when ARSC varied from 8.95 to 17.86. All the samples have a concentration of 1× 10−6 M.

chiral CdSe/CdS DRs are normalized at 450 nm49 as the featured absorption for the CdS shell, and the band-edge absorption at longer wavelengths is then selected as the featured excitonic transition for the CdSe core (Figure 1B). Moreover, the effect of the chiral ligand concentration in the surrounding medium (i.e., the concentration of cysteine) is studied and found to be efficient

for modulating both CD and CPL performances of the superstructures.

RESULTS AND DISCUSSION For the purpose of activating the CD and CPL responses, CdSe/ CdS DRs were first synthesized in the organic phase and then C

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Figure 3. Illustration of interactions and transitions in a chiral molecule and a semiconductor NC system and its relationship with the CD activity in terms of the ARSC effect. The red vertical arrows and curved blue arrows represent excitation transitions and relaxations, respectively. The CdSe/CdS DR having strong exciton interactions (left) with the chiral cysteine molecules is depicted with a red CdSe core; the CdSe/CdS DR having weak exciton interactions (right) is depicted with a dark red CdSe core.

1.59 × 10−4 for D-Cys-CdSe/CdS DRs when the ARSC value decreases from 17.86 to 8.95. This indicates, to some extent, that chiral CdSe/CdS DRs with thinner shells and larger cores are more preferred for inducing CD activity. However, not only do the size factors of the DRs, such as the sizes of the CdS shell and the CdSe core, affect the CD response, but the aspect ratio could also have a large impact on the coupling interactions between the chiral ligand and the CdSe/CdS DRs. Although increasing the aspect ratio could enhance the transition of a linearly polarized exciton, this transition reaches saturation when the aspect ratio is approximately 4, as indicated by the NDCO model from Tang’s work.47 Moreover, too high of an aspect ratio may also decrease the effective number of cysteine molecules that couple with the DRs, leading to a decrease in the CD activity. This relationship is consistent with our observations for the samples with gCD factor magnitudes of 1.23 × 10−4 and 0.59 × 10−4 (Table 1), which have very similar CdSe core size and CdS shell thickness but different aspect ratio values. In addition, factors such as the presence of surface defects and the size uniformity of the CdSe/CdS DRs also play critical roles in determining the CD activity. Furthermore, like chiral-ligand-conjugated NPs and QDs, the orbital coupling theory48 and Coulombic dipole interactions23,51 are considered to be the possible chirality transfer mechanisms for the L/D-Cys-CdSe/CdS DRs, as illustrated in Figure 3. In the orbital coupling theory, the orbital wave functions of the chiral cysteine hybridize with the electronic states of the semiconductor DRs, thereby allowing chiral information to be transferred, whereas in the Coulombic dipole interactions, excitons in the DRs can be coupled to the molecular exciton in cysteine through the Coulomb interactions, which allows chirality transfer. As the postsynthetic ligand exchange method maintains a narrow DR size distribution and does not introduce obvious dissymmetric agglomerates,52 interactions and transitions in the chiral molecule−DR system should play a major role in excitonic CD induction in the chiral CdSe/CdS DRs system even though the slightly broadened fluorescence line shape (Table S1) may be attributed to surface defect effects. Further, DRs with smaller ARSC values should have larger absorption cross sections of the core, leading to stronger orbital and Coulomb couplings between the excited chiral ligands and the excited DRs, which results in strong CD activity. However, with respect to the complexity of the orbital and Coulomb couplings between the semiconductor

ligand-exchanged with L/D-cysteine to obtain L- or D-Cys-CdSe/ CdS DRs in the aqueous phase, as indicated in the Methods section. Typically, CdSe/CdS DRs with various ARSC values were prepared by finely tuning the sizes of the CdS shell and CdSe core, as observed by transmission electron microscopy (TEM) in Figure 1c, in which the diameter and aspect ratio were each measured for more than 100 NCs and are summarized in Table 1. The PL and PLQY are known to be highly important optical properties of the DRs, especially in relation to the CPL activity. Figure S1 and Table S1 show the PL spectra of the DCys-CdSe/CdS DRs and their PL emission peak positions together with their full widths at half-maximum (fwhm) before and after ligand exchange. Furthermore, the PLQY measurements show that a considerable QY (>37%, Table 1) is preserved in all the samples during ligand exchange from the organic phase to the aqueous phase due to the shell effect of CdS in the type I CdSe/CdS QDs. Electronic CD measurements were then performed to study the chiroptical response of the cysteine-functionalized CdSe/ CdS DRs. Indeed, the CD signals clearly had active response with multiple peaks and dips in the vicinity of the exciton absorption of the NCs, and a mirrored CD line shape was recorded when Land D-cysteine were separately used as surface ligands (Figure 2A,B). Additionally, the Cotton effect region and the exciton absorption could be modulated from approximately 400 to 500 nm and from 550 to 650 nm, respectively (Figure 2C), by changing the ARSC, revealing the geometry-dependent nature of the chiral properties of the DRs. To quantitatively analyze the CD responses in these experiments, their anisotropic gCD factors were calculated and are plotted in Figure S2. Figure 2D shows the highest gCD factor magnitude, |gCD− − gCD+|/2, which is defined as the average magnitude of the highest gCD factor generated by gCD+ and gCD− that occurs in the band-edge transition region as a function of the ARSC (see Table 1 for the summarized gCD factor values of the D-Cys-CdSe/CdS DRs and Table S2 for L-CysCdSe/CdS DRs). Apparently, close to the band-edge transition region, the highest gCD factor magnitude was obtained for the DR sample with ARSC = 8.95 (note that the bare spherical CdSe QD sample was not considered here), and indeed, a clear tendency that the smaller ARSC value of the chiral CdSe/CdS DRs the higher gCD factors can be achieved, typically from 0.49 × 10−4 to 1.48 × 10−4 for L-Cys-CdSe/CdS DRs and from 0.34 × 10−4 to D

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Figure 4. CPL spectra and corresponding anisotropic glum factors of the L- and D-Cys-CdSe/CdS DRs with different ARSC values in aqueous solution. (A) CPL spectra of the L- and D-Cys-CdSe/CdS DRs with different ARSC values. (B) Plot of the maximum glum factor values of the L- and D-Cys-CdSe/CdS DRs against ARSC and their linear fittings. The black dots and line represent the L-Cys-CdSe/CdS DRs, whereas the reds dots and line represent the D-Cys-CdSe/CdS DRs. [DRs] = 1× 10−6 M, λex = 400 nm.

Figure 5. CD spectra and corresponding UV/vis absorption spectra and anisotropic gCD factors of the L- and D-Cys-CdSe/CdS DRs with different cysteine/DRs molar ratios. CD spectra of L- (A) and D-Cys-CdSe/CdS DRs (B) with different cysteine/DRs molar ratios from 2 × 105 to 1 × 104. (C) Normalized UV/vis spectra of the L-Cys-CdSe/CdS DRs with different cysteine/DR molar ratios (the curves are normalized at the first exciton absorption peak). (D) Plot of the gCD factor magnitude of the L- and D-Cys-CdSe/CdS DRs against the cysteine/DRs molar ratio and their linear fittings. The black dots and line represent the L-Cys-CdSe/CdS DRs, whereas the reds dots and line represent the D-Cys-CdSe/CdS DRs. Note that the ARSC = 8.95 sample with a concentration of 1× 10−6 M was used in each run.

NCs and chiral molecules, it should be emphasized that the exact origin of the induced chirality and the detailed relationship between the intrinsic parameters and the chiroptical activity are still open for further investigation.

The activation of chiral emission from these chiral CdSe/CdS DRs can therefore be rationally envisioned after the successful observation of induced chirality in the CD measurements. Figure 4 illustrates the corresponding CPL studies of the chiral CdSe/ E

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Figure 6. CPL spectra and corresponding anisotropic glum factors of the L- and D-Cys-CdSe/CdS DRs with different cysteine/DRs molar ratios. (A) CPL spectra of the L- and D-Cys-CdSe/CdS DRs with different cysteine/DR molar ratios. (B) Plot of the maximum glum factor values of the Land D-Cys-CdSe/CdS DRs against the cysteine/DR molar ratio and their linear fittings. The black dots and line represent the L-Cys-CdSe/CdS DRs, whereas the reds dots and line represent the D-Cys-CdSe/CdS DRs. [DRs] = 1× 10−6 M, λex = 400 nm. Note that the ARSC = 8.95 sample with a concentration of 1× 10−6 M was used in each run. The monitored wavelength of glum for L-Cys-CdSe/CdS DRs is 646, 643, 658, 644, and 659 nm, and the monitored wavelength of glum for D-Cys-CdSe/CdS DRs is 678, 677, 664, 664, and 672 nm.

ranging from 4.3 ± 0.4 to 19.2 ± 1.9%.45 In the present work, as a comparison, the maximun gCD factor is 2.03 × 10−4 at 630 nm with an ARSC value of 8.95, CdS shell thickness of 0.8, and the PLQY of ∼50%, which is higher than that of CdSe/CdS QD system with the most similar CdS shell thickness of 0.85 nm (Table S4). In addition, the present work shows active CPL response although the glum (4.66 × 10−4) in such a system is 1 order of magnitude smaller than that of bare cysteine-stabilized CdSe QDs reported by Balaz,48 which is due to the fact that the induced CD signal rapidly decreases with increasing molecule− NP distance, as indicated by the equation: CD ∝ (aNP/R)3.23 Accordingly, although the CdS shell increases the separation between the chiral cysteine and DRs, which leads to a smaller glum value in CPL measurement, the presented anisotropic chiral CdSe/CdS DR system shows comparable chiroptical behavior in CD with high PLQY in aqueous solution, which may provide information on the relationship of induced CD, CPL, and PLQY and allow for potential studies on the underlying mechanism of induced chiroptical properties in chiral inorganic NCs. Additionally, to investigate the role of the chiral ligands present in the surrounding medium in the induced chirality of the CdSe/CdS DRs, the amount of chiral cysteine molecules that is used for the ligand exchange was varied. The sample with an ARSC of 8.95 was chosen, and the surface ligands were exchanged with different excess amounts of cysteine to form chiral CdSe/CdS DRs. CD signals with mirrored line shapes and UV/vis absorption were observed as expected for each sample with cysteine/DR molar ratios of approximately 200k, 100k, 50k, 20k, and 10k, as shown in Figure 5A−C (note that all excess cysteine molecules are preserved in aqueous solution for each sample without any purification). The observed Cotton effects and corresponding gCD factor values (see Figure S5) all indicate that lower amounts of excess cysteine produce more intense CD signals and higher gCD factor values. The maximum gCD factor value is plotted against the molar ratio of cysteine/DRs in Figure 5D, which shows that for a molar ratio of cysteine/DRs in the range of 105 to 104, the measured gCD factor in the vicinity of the band-edge absorption can increase from 0.96 × 10−4 to 1.55 × 10−4 for L-Cys-CdSe/CdS DRs and from 0.80 × 10−4 to 1.52 × 10−4 for D-Cys-CdSe/CdS DRs, giving an enhancement of approximately 1.5 to 2.0.

CdS DRs with L/D-cysteine in aqueous solutions at room temperature. Indeed, active CPL signals with opposite line shapes resulting from the two chiral enantiomers are recorded within the PL-active region (Figure S1), suggesting the induction of chirality in the PL behavior of the chiral DRs. To compare the CPL activity, the luminescence dissymmetry ratio was defined as follows:53 glum = 2(IL − IR )/(IL + IR )

where IL and IR, which are the intensities of the left and right CPL, respectively, are employed to quantitatively analyze the ability to induce CPL in the chiral CdSe/CdS DR systems. Figure S3 and Figure 4B illustrate the evolution of the glum value with increasing wavelength and their maxima for the different ARSC values, respectively. The fitted lines in Figure 4B reveal an obvious relationship between ARSC and |glum|, in which lower ARSC values correspond to higher |glum|values, which is consistent with the CD measurements above. In addition, the chiral CdSe/CdS DRs generally exhibit a glum of ∼10−4 with a maximum value of 4.66 × 10−4. Note that the bare CdSe QDs with the lowest ARSC value of 1.42 show a silent CPL spectrum in our measurements because their PL is quenched during the ligand exchange reaction into the aqueous phase (Figure S4 and Table 1). Additionally, although the exact mechanism for the induction of CPL still needs to be determined, the chiral interactions between the cysteine molecules and the DRs are asserted to be key in producing CPL, similar to their role in producing the CD response. With a purpose to evaluate the impact and significance of the presented data, we also compared our chiral DRs with some existing similar systems. As mentioned in the introduction part, the chiral activity of CdSe QDs capped by cysteine molecules has previously been reported with a gCD factor up to 2.1 × 10−4 at 588 nm for CdSe particles with 4.4 nm in diameter48 and CdSe quantum rods with a gCD factor up to 4.8 × 10−4 at 571 nm with an aspect ratio of 2.7.47 Moreover, when the CdSe QDs are coated with a CdS shell, the induced CD was inversely proportional to the CdS shell thickness with a gCD factor of about 1.2 × 10−4 at 583 nm when the thickness is 0.85, and the PLQY showed a direct relationship with the CdS shell thickness F

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ACS Nano The corresponding CPL spectra of these samples are displayed in Figure 6 for the opposite enantiomers. As expected, opposite line shapes are observed for the CPL spectra within the PL-active region. The evolution of the glum values with increasing wavelength and the maximum values for the different Cys/DR molar ratios are shown in Figure S6 and Figure 6B. The glum factors for L- and D-Cys-CdSe/CdS DRs are in the range of −1.8 × 10−4 to −3.4 × 10−4 and 2.5 × 10−4 to 3.9 × 10−4, respectively, and generally, the highest |glum| is observed for the sample with the lowest excess amount of chiral cysteine molecules in the medium, which is consistent with the above observations from the CD measurements. With these findings, we assume that rather than increasing the possibility for coupling with the chiral CdSe/CdS DRs, an excess of chiral molecules in the solution will inhibit both the molecule−DR exciton coupling and CPL in the chiral CdSe/CdS DRs, which is often underestimated in the benchtop synthesis of chiral inorganic NCs but provides an alternative method for adjusting the induced chirality for potential applications.54,55 To probe the chirogenesis of cysteine capped DRs systems, theoretical simulations of the CD spectra were performed using the DFT method (details are in the simulation method part). Cd10Se6S14 nanoclusters were used as the prototype for the DR with noting that the simulated absorption and CD responses are comparable with the experimental measurements in conditions where the prototype is small enough to keep the computation feasible, and the intrinsic CD activity of the bare Cd10Se6S14 nanocluster is negligible at the characteristic absorption region of the cysteine-capped Cd10Se6S14 complexes. Then, L-Cys(Cd10Se6S14) and D-Cys-(Cd10Se6S14) complexes are constructed as shown in Figure 7, noting that the models with the lowest energy are preferred.

Figure 8. Calculated CD (top) and UV−vis (bottom) spectra for LCys-(Cd10Se6S14) and D-Cys-(Cd10Se6S14) complexes. Insets are the magnified UV−vis spectra at 400−500 and 500−700 nm. The absorption peaks around 450 and 575 nm (highlighted by the dashed line) are correlated with the CD-active region on top.

induction of chirality between the L(D)-Cys and the quantum dots. On the other hand, because the cysteine-capped Cd10Se6S14 complexes are based on the ground state construction, it rules out the possibility that the simulated CD responses result from the surface distortion of the nanoclusters caused by the cysteine molecule. However, it is necessary to emphasize that in benchtop ligand exchange experiments, the induction of cysteine could induce surface defects, as confirmed by the decrease of PLQY (Table 1), which inevitably produces dissymmetric surface distortions. In addition, with only one cysteine molecule bound to the Cd10Se6S14 surface, the calculated CD activities exclude the cooperative ligand effect, whereas it happens in reality, and this may explain the difference of the line shapes at the band gap between the calculated and experimental CD signals. It also should be emphasized that the DFT method is limited to models made by a large number of atoms and anisotropic shapes such as the rod-like shape for the DRs, but one can still envisage more advanced simulation methods to overcome these obstacles in the near future.

Figure 7. Optimized geometries of (A) L-Cys-(Cd10Se6S14) and (B) DCys-(Cd10Se6S14) nanoclusters.

Figure 8 shows the simulated UV−vis and CD spectra of LCys- and D-Cys-(Cd10Se6S14) complexes. As expected, the UV− vis absorption spectra of the chiral complexes are blue-shifted due to small size of the prototype, but their line shapes are in line with the experimental measurements. Mirrored line shapes are independently presented for L- and D-Cys-capped Cd10Se6S14 complexes and correlated to the absorption region in the UV−vis spectra, indicating that the observed CD signals are induced by the interaction with the chiral cysteine molecules. Moreover, the highest occupied molecular orbitals (HOMO, HOMO−1, and HOMO−2) of the Cd10Se6S14 nanocluster and chiral cysteine are delocalized (Figure S8), suggesting that the wave function of the cysteine molecule can easily overlap with that of the Cd10Se6S14 nanoclusters; this electronic state coupling suggests the

CONCLUSION In summary, cysteine-induced optically active CdSe/CdS DRs were synthesized via a postsynthetic ligand exchange method. Their CD and CPL activities were first observed and found to be sensitive to not only their intrinsic geometry properties but also their surrounding chiral medium (i.e., the concentration of chiral cysteine). The study of the ARSC of the chiral CdSe/CdS DRs revealed that, to some extent, CD and CPL prefer smaller ARSC values. The CD and CPL activities could be further modulated via many geometry-dependent parameters, and the origin of G

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mL, [Cys] = 0.1 M). The pH of the resulting solution was adjusted to 12.0 with TMAH. A solution of CdSe/CdS DRs or CdSe QDs in nhexane (5 mL, 1× 10−6 M) was added to the cysteine solution, and the reaction mixture was deoxygenated and stirred at room temperature under nitrogen in the absence of light for 24 h. The reaction mixture was left to stand for 1 h to allow the phases to separate. The bottom aqueous layer was removed with a syringe, and the Cys-CdSe QDs or Cys-CdSe/ CdS DRs were purified by precipitation with ethanol/DI water (4:1, two times). The purified Cys-QDs and Cys-DRs were dissolved in DI H2O and stored at room temperature in the dark. Simulation Method. Gaussian 0958 software is used for all chemical calculations. Ground state geometries were optimized at the DFT. UV and CD spectra were calculated with TD-DFT. B3LYP59 and LanL2DZ60−62 basis sets were used for all the elements in the calculations. The small CdS or CdSe clusters usually are in the form of a tetrahedron;63,64 herein, we constructed a tetrahedron-like Cd10Se6S14 cluster to mimic the interactions between the chiral light and the quantum dots. The full chemical formula for the Cd10Se6S14 cluster is H16Cd10Se6S14, we take the charge state of H, Cd, S, and Se as 1+, 2+, 2−, and 2−. The net charge for the Cd10Se6S14 is −4 in our calculation. Structural and Optical Characterization. The UV/vis absorption spectrum of each sample was measured using a TU-1901 double-beam UV/vis spectrophotometer (Beijing Purkine General Instrument Co. Ltd., China), and the PL spectra were recorded on a fluoroSENS spectrophotometer (Gilden Photonics). The absolute PLQYs of the QD and DR solutions were measured using an Ocean Optics FOIS-1 integrating sphere coupled with a QE65 Pro spectrometer. CD measurements were conducted on a JASCO J-1500 CD spectrometer. The scan rate was 20 nm/min. All CD experiments were carried out in Milli-Q water with a quartz cuvette (0.1 cm path length, Hellma). CPL measurements were performed on a JASCO CPL-300 spectrometer in Milli-Q water with a quartz cuvette (0.1 cm path length, Hellma) with an excitation wavelength of 400 nm. All optical measurements were performed at room temperature under ambient conditions. TEM images were collected using a Tecnai F30 microscope.

induced chirality was elucidated to mainly involve the orbital hybridization between the chiral ligands and the CdSe/CdS DRs. Moreover, an excess of chiral cysteine in the aqueous solution inhibited both the CD and CPL behaviors of the superstructures, which is often neglected by researchers during the bottom-up synthesis. Although the complex relationship between the geometrical properties and the induced chiroptical activities in core−shell semiconductor NCs has not yet been fully explored, this work allows the potential applications of chiral semiconductor NCs in chiral synthesis and recognition, optical devices, and multifunctional material design to be envisioned.

METHODS Chemicals. Tri-n-octylphosphine oxide (TOPO, 99%), tri-noctylphosphine (TOP, 97%), tributylphosphine (TBP, 97%), noctadecylphosphonic acid (ODPA, 97%), n-tetradecylphosphonic acid (TDPA, 97%), and n-hexylphosphonic acid (HPA, 97%) were purchased from Strem Chemicals. Cadmium oxide (CdO, 99.99%), sulfur (S, 99.98%), and selenium (Se, 99.99%) were purchased from Sigma-Aldrich. L-Cysteine hydrochloride monohydrate (L-Cys, 99%), Dcysteine (D-Cys, 99%), and tetramethylammonium hydroxide pentahydrate (TMAH, 97%) were purchased from Aladdin. Pure water was purchased from Wahaha, China. All chemicals were used as received without further purification. Synthesis of the CdSe Core. The synthetic procedure was based on the procedure reported in the literature.56,57 Typically, TOPO (1.5 g), ODPA (0.140 g), and CdO (0.030 g) were mixed in a 50 mL flask, heated to 150 °C, and alternately exposed to vacuum and argon at least five times until CdO was a brown solid and the rest of the reagents were a colorless liquid. Then, to dissolve the CdO, the solution was heated to greater than 300 °C under argon until it became optically clear and colorless, which indicated that the reaction between CdO and ODPA was complete. Then, the temperature was increased to 370 °C, and 1.5 mL of TOP was injected into the flask, which caused the temperature to naturally decrease to 300 °C. Then, a Se/TOP solution (0.4 mL, 1 mol/ L) was injected at 380 °C and reacted for several minutes. The size of the CdSe core could be adjusted by altering the temperature, reaction time, and type of phosphonic acid (ODPA or TDPA). Purification and Acidification the CdSe−TBP Core. The reaction mixture was cooled to 70−80 °C, and 4 mL of ethanol and a certain amount of TBP were added into the above solution, which was centrifuged at 10000 rpm for 3 min. Then, the precipitate was dissolved in a small amount of toluene, and the supernatant was discarded. After centrifugation again, the precipitate was dissolved in TBP. Unlike other similar synthetic procedures for producing DRs, TBP was used instead of TOP because of its relatively high activity, which allowed the shell to grow better after acidification, and a higher QY could be obtained. Synthesis of the CdSe/CdS DRs. In a typical synthesis of CdSe/ CdS nanorods via seeded growth, CdO (30 mg) was mixed in a 50 mL flask together with TOPO (1 g), ODPA (100 mg), and HPA (30 mg). After the flask was alternately exposed to vacuum and argon at least five times at 150 °C, the resulting solution was heated to 300 °C to make the solution become a completely transparent liquid without any solids. Then, the temperature was increased to 350 °C and a mixed solution of S/TOP (0.5 mL, 2.5 mol/L) and the above CdSe−TBP solution (100 μL) were injected into the flask, which caused the temperature to naturally decrease to 300 °C. After injection, the temperature dropped to 270−300 °C and then recovered to the preinjection temperature within 2 min. The CdSe/CdS DRs were allowed to grow for approximately 8 min after the injection. Finally, the reaction mixture was cooled to room temperature, and an extraction procedure was used to separate the NCs from the side products and unreacted precursors. CdSe/CdS DRs with different ARSC values were synthesized by adjusting the size of the core and the proportion of the shell precursor. Ligand Exchange of the CdSe/CdS DRs with Chiral Cysteine Molecules. The cysteine ligand exchange reaction was carried out using the previously reported method48 with some modifications. Cysteine hydrochloride monohydrate (87.82 mg) was dissolved in DI water (5

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00112. Additional data and figures (PDF)

AUTHOR INFORMATION Corresponding Authors

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

Jiaji Cheng: 0000-0002-2663-7881 Kai Wang: 0000-0003-0443-6955 Author Contributions

J.C. and J.H. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Nos. 11404219, 61674074, and 51402148), the Shenzhen Innovation Project (Nos. JCYJ2015032414171163, JCYJ20170302142433007, JCYJ20160301113356947, KC2014JSQN0011A, and JCYJ20160301113537474), Guangdong Distinguished Young Scholar of Natural Science Foundation (No. 2017B030306010) and the Tianjin New Materials Science and Technology Major Project (16ZXCLGX00040). H

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