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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00112 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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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†,* †

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 *Address correspondence to [email protected], [email protected]

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

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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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Chiral 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 devices7-11 and so forth.12-14 Chiral core-shell quantum NCs also provide both 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 size-dependent 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 chemical properties. To date, an extensive body of experiments and theoretical calculations have been reported towards discovering the origin and modulating the intensity of chirality with various parameters, including the surface ligand motif38,39 (thiolated or non-thiolated,40,41 number of stereocenters42,43 and surface ligand conformation44 when bound to QDs) and chemistry of the QDs, namely, the size and shell thickness of the QDs.15,45 V.E. 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 = ∆ε/ε, and ∆ε is the absorptivity difference between left- and right-handed circularly polarized light. In addition,


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the authors noted that increasing the number of stereocenters in the ligand could further enhance the anisotropy factors of the chiral QDs. Y. K. Gun’ko et al.45 showed the impact of the shell thickness on the CD and photoluminescence (PL) performances of CdSe/CdS QDs; they 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, Z. Tang’s group47 used aspect ratio-tunable CdSe quantum rods (QRs) to increase the anisotropy. The researchers used the non-degenerate coupled oscillator (NDCO) model to clarify the coupling of the electric dipole transition moments in the chiral cysteineCdSe QRs 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 not only the low accessibility of the CPL instruments but also the 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 M. 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, to obtain 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.


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Herein, we present the preparation of optically active CdSe-dot/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 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 the 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 ligand-exchanged with L/D-cysteine to obtain L- or D-CysCdSe/CdS DRs in the aqueous phase, as indicated in the experimental section. Typically, CdSe/CdS DRs with various ARSC values were prepared via 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 D-Cys-CdSe/CdS DRs and their PL emission peak positions together with their full-widths at half-maximum (FWHMs) 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.


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Table 1. CD and CPL Anisotropy Factors of D-Cys-CdSe/CdS DRs.

a

Aspect Ratioc

gCD+/λCD(nm)d

gCD-/λCD(nm)e

|gCD+ -gCD-|/2f

3.52

1.0

1.15×10-4/579.6

-0.58×10-4/549.6

0.86×10-4

0.5%

None

8.95

3.70

3.3

2.03×10-4/630.0

-1.14×10-4/598.8

1.59×10-4

54%

4.66×10-4/664.3

10.05

4.84

4.5

1.09×10-4/595.8

-1.36×10-4/617.4

1.23×10-4

38.1%

3.70×10-4/676.1

14.33

4.75

4.0

3.32×10-6/642.2

-1.43×10-4/615.6

0.72×10-4

47.4%

2.75×10-4/667.7

16.19

4.84

7.8

0.67×10-4/591.4

-0.51×10-4/614.6

0.59×10-4

37.6%

2.29×10-4/668.8

17.86

3.33

3.0

0.46×10-4/626.0

-0.22×10-4/601.8

0.34×10-4

39.4%

1.77×10-4/671.8

ARSCa

DCdSeb

1.42*

QYg

glum/λCPL(nm)h

ARSC determined from the normalized UV/Vis absorption of chiral CdSe/CdS DRs at 450 nm

and the band gap.

b

Diameter of the CdSe core determined from the absorption spectrum by

Peng’s equation50 (nm). c Aspect ratio of the DRs.

d, e

CD anisotropy gCD-factors at the most

intense positive and negative CD bands (nm). f Magnitude of the gCD-factor, defined as |gCD+ gCD-|/2. g PLQY of the DRs after ligand exchange. h CPL 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. 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 L- and D-cysteine were separately used as surface ligands (Figure 2a and b). Additionally, the Cotton effect region and the exciton absorption could be modulated


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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 LCys-CdSe/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 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 1.59 × 10-4 for D-CysCdSe/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 also, the aspect ratio could 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


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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, while in the Coulombic dipole interactions, excitons in the DRs can be coupled to the molecular exciton in cysteine through the Coulomb interactions which allows as well chirality transfer. Since 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-DRs systemshould play a major role in excitonic CD induction in the chiral CdSe/CdS DRs system even though the slightly broadened fluorescence lineshape (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 couplingsbetween the semiconductor NCs and chiral molecules, it should be stressed 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/CdS DRs


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with L/D-cysteine in aqueous solutions at room temperature. Indeed, active CPL signals with opposite lineshapes 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 DRs 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


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CdSe quantum rods with a gCD-factor up to 4.8×10−4 at 571 nm with an aspect radio of 2.7.47 Moreover, when the CdSe QDs are coated with CdS shell, the induced CD was inversely proportional to the CdS shell thickness with a gCD-factor 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 ranging from 4.3 ± 0.4 to 19.2 ± 1.9%.45 In the present work, as a comparison, the maximun gCDfactor 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 QDs system with a most similar CdS shell thickness of 0.85 nm (Table S4). Besides, the present work shows active CPL response although the glum (4.66 × 10-4) in such system is one order of magnitude smaller than the work 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 the molecule-NP distance, as indicated by the equation:

CD ∞ ( aNP / R )

3

.23 Accordingly, in spite that the CdS shell increases the separation

between the chiral cysteine and DRs which leads to smaller glum value in CPL measurement, the presented anisotropic chiral CdSe/CdS DRs 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 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 lineshapes and UV/Vis absorption were observed as expected for each sample with cysteine/DR molar ratios of approximately 200k, 100k, 50k, 20k


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and 10k, as shown in Figure 5a to c (note that all the excess of cysteine molecules is 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 gCDfactor 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. The corresponding CPL spectra of these samples are displayed in Figure 6 for the opposite enantiomers. As expected, opposite lineshapes are observed for the CPL spectra within the PLactive region. The evolution of the glum values with increasing wavelength and the maximum values for the different Cys/DRs 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


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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 condition that the prototype is small enough to keep the computation feasible and the intrinsic CD activity of 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 with noting that the models with the lowest energy are preferred. Figure 8 shows the simulated UV-Vis and CD spectra of L-Cys- 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 lineshapes are in line with the experimental measurements. Mirrored lineshapes 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 (Cd10Se6S14) nanocluster and chiral cysteine are both delocalized (Figure S8) suggesting that the wave function of cysteine molecule can well overlap with that of the (Cd10Se6S14) nanoclusters , this electronic states couplings suggests the induction of chirality between the L(D) Cys and the quantum dots. On the other hand, since the cysteine capped (Cd10Se6S14) complexes are based on the ground states 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 stress that in benchtop ligand exchange experiments, the induction of cysteine could


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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 bandgap between the calculated and experimental CD signals. It deserves as well to emphasize the DFT method is limited to models made by 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. 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 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 relation 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.


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METHODS Chemicals Tri-n-octylphosphine oxide (TOPO, 99%), tri-n-octylphosphine (TOP, 97%), tributylphosphine (TBP, 97%), n-octadecylphosphonic 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%), D-cysteine (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 5 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 above 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


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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 minutes. Then, the precipitate was dissolved in a small amount of toluene while the supernatant was discarded. After centrifugation again, the precipitate was dissolved in TBP. Unlike other similar synthetic procedures for producing DRs, TBP was used to 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 alternately exposing the flask to vacuum and argon at least 5 times at 150 °C, the resulting solution was heated to 300 °C to make the solution become 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 pre-injection temperature within two minutes. The CdSe/CdS DRs were allowed to grow for approximately 8 minutes 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


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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 mL, [Cys] = 0.1 M). The pH of the resulting solution was adjusted to 12.0 with tetramethylammonium hydroxide pentahydrate (TMAH). A solution of CdSe/CdS DRs or CdSe QDs in n-hexane (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, 2 times). The purified Cys-QDs and CysDRs were dissolved in DI H2O and stored at room temperature in the dark. Simulation Method: Gausian 0958 software is used for all chemical calculations. Ground state geometries were optimized at the DFT. UV and CD spectrum were calculated at the time-dependent DFT (TDDFT). B3LYP59 and LanL2DZ60-62 basis set were used for all the elements in the calculations respectively. The small CdS or CdSe clusters usually are in the form of tetrahedron63,64, herein we construct 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 are 1+, 2+, 2- and 2- respectively. 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


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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. ASSOCIATED CONTENT Supporting Information: Additional data and figures are included in the Supplementary Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: Tingchao He: [email protected] Kai Wang: [email protected] Author Contributions J. Cheng and J. Hao contributed equally to this work. ACKNOWLEDGMENT


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The work was supported by The National Key Research and Development Program of China administered by the Ministry of Science and Technology of China (No. 2016YFB0401702), the National Natural Science Foundation of China (Nos. 11404219，61674074 and 51402148), the Shenzhen

Peacock

Team

JCYJ2015032414171163 KC2014JSQN0011A,

and

Project,

the

Shenzhen

JCYJ20170302142433007,

JCYJ20150630145302223

and

Innovation

Project

(Nos.

JCYJ20160301113356947,

JCYJ20160301113537474),

the

Guangdong High Tech Project (Nos. 2014A010105005 and 2014TQ01C494), and the Tianjin New Materials Science and Technology Major Project (16ZXCLGX00040).


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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 insert 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.


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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 inserts show the magnified CD spectra from 500-700 nm. (c) Normalized UV/Vis absorption spectra of the LCys-CdSe/CdS DRs with different ARSC values (the curves are normalized at the first exciton absorption peak). The insert 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, while 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 L-Cys-CdSe/CdS DRs, the corresponding wavelength are 598.6, 619, 555.2, 612.2 and


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602.6 nm for gCD+, and 627.8, 596.2, 598.6, 637 and 625.8 nm for gCD- respectively when ARSC varied from 8.95 to 17.86. And for D-Cys-CdSe/CdS DRs, the corresponding wavelength 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- respectively when ARSC varied from 8.95 to 17.86. All the samples have a concentration of 1× 10-6 M.

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.


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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-CysCdSe/CdS DRs with different ARSC values. b. Plot of the maximum glum-factor values of the Land 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-CysCdSe/CdS DRs. [DRs] = 1× 10-6 M, λex = 400 nm.


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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/DRs 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.


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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/DRs molar ratios. b. Plot of the maximum glum-factor values 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. [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 respectively; and the monitored wavelength of glum for D-Cys-CdSe/CdS DRs is 678, 677, 664, 664, and 672 nm respectively.


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


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Figure 8. Calculated CD (Top) and UV-Vis (Bottom) spectra for L-Cys-(Cd10Se6S14) and DCys-(Cd10Se6S14) complexes. Inserted are the magnified UV-Vis spectra of at 400-500 nm and 500-700 nm respectively. The absorption peak at around 450 nm and 575 nm (highlighted by the dash line) are correlated with the CD active region on top.


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CdS-Rod Nanocrystals with Induced

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