Perspective of Chiral Colloidal Semiconductor Nanocrystals

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Perspective

Perspective of Chiral Colloidal Semiconductor Nanocrystals: Opportunity and Challenge Xiaoqing Gao, Bing Han, Xuekang Yang, and Zhiyong Tang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05973 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Perspective of Chiral Colloidal Semiconductor Nanocrystals: Opportunity and Challenge Xiaoqing Gao,†,‡ Bing Han,§ Xuekang Yang,† Zhiyong Tang*,† †CAS

Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China ‡College of Physics and Optoelectronic Engineering, Shenzhen University, Guangdong 518060, People’s Republic of China §North

China Power Electric University, Beijing 102206, People’s Republic of China

ABSTRACT: Chiral colloidal semiconductor nanocrystals (NCs) are an emerging type of chiral materials. These chiral NCs exhibit unique quantum confinement-determined optical activity and have aroused much interest in the multidisciplinary fields of chemistry, physics and biology. Herein, the state-of-the-art progresses of their rational synthesis, fundamental understanding and potential application are summarized. In addition, a personal view about the future development of chiral semiconductor NCs is offered.

As a new research field, many challenges are faced, particularly full understanding of the impact factors about the optical activity of chiral semiconductor NCs. Thus, the perspective provided here aims to help the researchers to quickly grasp the whole picture of these emerging materials. In following parts, we present the basic principle of CD and CPL, the progress of the experiment and the theory of the chiral colloidal semiconductor NCs, their possible applications and future developments.

1. Introduction Chirality means the ones that are not superimposable on their mirror images.1 It is an amazing phenomenon ubiquitous in nature and universe ranging from small molecules, to nanoscale proteins, sugars and DNA, to meter-scale eyes, flowers and animal shells, and to vast galaxy. Obviously, chirality has been always one of the most important research topics with its profound significance in the origin of life.2 A basic property of chiral materials is their optical activity. Actually, the earliest study of chirality dates to 1811 with observation of the optical activity in quartz,3 and then this subject achieved tremendous success in both experiment and theory.2a, 4 Notably, with recent flourishing of nanotechnology, chiral nanomaterials composed of noble metals or semiconductors are marching into this fascinating family,2b, 5 and they display the tunable optical activity in UV-visible wavelength as well as potential applications.2b, 5a, 6 Among those, the chiral ligands stabilized colloidal semiconductor nanocrystals (NCs) possess many special characteristics, i.e. the easily adjusted circularly polarized luminescence (CPL)7 and the promise in gene editing.6c In this perspective, we will focus our discussions on chiral colloidal semiconductor NCs. The optical activity of the semiconductor NCs originates from the surface chiral molecules. The related scheme is presented in Figure 1. Attachment of fluorescence-free chiral ligands on the surface of fluorescent semiconductor NCs would induce the strong optical activity of both absorption [detected by circular dichroism (CD) instrument] and emission (detected by CPL instrument) at the wavelength corresponding to the exciton transitions. Thanks to the optical features, a lot of innovative applications have been explored, such as biological labels,8 chiral analysis and detection,9 enantioselective separation,10 gene editing,6c spin electronic devices,11 and so on.

Figure 1. The scheme of optical activity of colloidal semiconductor NCs induced by chiral organic ligands. When chiral ligands with strong CD signal (a-b) but free of emission (c) are adsorbed on the surface of achiral semiconductor NCs (d-f), new optical activity with both absorption (h) and emission (i) appears at the characteristic wavelength corresponding to the exciton transitions in NCs (g). 2. Circular Dichroism and Circularly Polarized Luminescence Spectroscopies The basic property of chiral colloidal semiconductor NCs is their tunable optical activity involving with both absorption and emission. Typically, the related detection equipment is CD and CPL spectroscopy, respectively. Thus, a brief introduction about the principle of these two spectroscopies is given. It needs to be noted that the CPL investigation of the chiral colloidal semiconductor NCs is just at its beginning, and the related reports are rare, especially on control of CPL intensity.7d, 7e, 12 Here, the major discussions in this perspective involve with the CD study. 2.1 Circular Dichroism Spectroscopy

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CD consisting of equal left-hand circularly (LHC) and right-hand circularly (RHC) polarized light is an important instrument to characterize the optically active materials. When circularly polarized light passes through an absorbing optically active medium, the absorptivity to LHC and RHC polarized light differs ( 𝜀L ≠ 𝜀R). CD intensity is determined by the difference Δ𝜀 = 𝜀L ― 𝜀R .13 In detail, the vertical coordinate in CD spectrum is ellipticity θ (with the unit of mdegree), which is 𝜃 = 3.3 × 104 ∙ ∆𝜀 ∙ 𝐶 ∙ 𝑙

(1)

where 𝐶 is the solute concentration and 𝑙 is the path length. As for chiral NCs, anisotropic g-factor is a key parameter to evaluate the optical activity due to its readily availability by comparing CD spectrum with absorption spectrum. The equation is as follow:5f, 14 𝑔=

𝜃 3.3 × 104 × 𝐴

(2)

here 𝐴 is the intensity in the absorption spectrum at the same solute concentration used to measure the CD spectrum. Obviously, gfactor is independent with NCs’ concentration in solution. 2.2 Circularly Polarized Luminescence Spectroscopy Similarly, CPL spectroscopy is employed to measure the different emission intensity of LHC and RHC polarized light by chiral luminescent materials.15 The detected physical quantity is the emission dissymmetry factor 𝑔𝑙𝑢𝑚. At a certain emission wavelength, 𝑔𝑙𝑢𝑚 is defined by: 𝑔𝑙𝑢𝑚 =

2(𝐼𝐿 ― 𝐼𝑅)

(𝐼𝐿 + 𝐼𝑅)

(3)

where 𝐼𝐿 and 𝐼𝑅 are the intensity of LHC and RHC polarized components of the emitted radiation of chiral materials, respectively. 3. Experimental Manipulation on Optical Activity Early in 2000, when semiconductor colloidal NCs with sizedependent optical properties were intensively studied, the scientists tried to combine chiral amino acids on their surface to explore the biological application.8 Until 2007, Gun’ko and coworkers reported the first work about the optical activity of D-/L-penicillamine capped CdS NCs.16 Since then, chiral colloidal semiconductor NCs have achieved fast development thanks to the efforts of many research groups. In this section, the main experimental progress is categorized into three aspects: preparation, structure adjustment and composition control of chiral NCs.

3.2 Structure Adjustment The structure adjustment of semiconductor NCs includes alternation of size, shape, crystal sturcture, core/shell structure and the superstructure, etc. 1) Size. Our group firstly reported the size-dependent optical activity of chiral semiconductor NCs,18d and their CD peaks showed a clear red-shift with size increasing. In addition, Markovich’s group found that with size increasing, the CD intensity of CdS decreased;18a however, Balaz’s group pointed out that the CD intensity was independent with the size of CdSe NCs.12b Such disagreement likely results from the fact that the CD intensity varies with its peak width. Because of the weak CD signals and the ununified peak width, the exact relationship between NCs’ size and CD intensity remains unclear. 2) Shape and Crystal Structure. Our group reported that keeping the same crystal structure of wurtzite (WZ), with the shape change from spheric NCs to one-dimeonsional nanorods (1D NRs) and then to two-dimenisonal nanoplatelets (2D NPLs), the CD intensity increased exponentially (Figure 2a-b).20g, 20h Furthermore, as for CdSe NRs, the CD peak red shifted and reached to a certain value with length increasing, while the CD intensity showed a similar tendency due to the continuous electron state along the length (Figure 2a).20g Interestingly enough, the CD spectra were obviously different in both CD intensity and the lineshape between WZ and zincblende (ZB) CdSe NPLs (Figure 2b-c).20h The above results demonstrate that symmetry of the shape and crystal structure is the key to affect the optical activity of chiral semiconductor NCs. 3) Core--Shell Structure. The increased thickness of the shell was explored to decrease the g-factor of chiral cysteine stabilized core-shell structured CdSe/CdS NCs (Figure 2c).5l Similar phenomenon was observed in chiral cysteine stabilized CdSedot/CdS-rod structures, and their g-factor was decreased with the increased absorption of the shell.7d The CdSe/CdS core/shell NPLs also displayed the decreased optical activity compared with pure CdSe NPLs.20i 4) Superstructure. Generally, preparation of the chiral semiconductor NCs via ligand exchange with chiral molecules would dramatically decrease their luminescence. Recently, Liu’s group proposed a method with the supramolecular self-assembly of semiconductors (CdSe/ZnS, CdS/ZnS, and ZnSe/ZnS core/shell structures) or perovskite NCs (CsPbX3, X = Cl, Br, and I) with chiral gelators.21-22 This strategy greatly improves the CPL property of system (Figure 2e-f). Ouyang’s group also reported the superstructured δ-HgS NCs with collinear chains and propellers.23

3.1 Preparation Until recently, the synthetic methods involved with construction of chiral colloidal semiconductor NCs are divided into two types: 1) Direct synthesis of colloidal semiconductor NCs with chiral molecules as surface ligands. This method includes three different treatments: microwave induced heating;16-17 aqueous synthetic method;6c, 7e, 17-18 and microwave irradiation.19 2) Post-modification of colloidal semiconductor NCs with chiral molecules. This method involves two different treatments: ligand exchange with chiral molecules5l, 7d, 9, 12b, 18d, 20 and the supramolecular co-assembly of NCs with the chiral gelators.12a, 21 Moreover, the summary of the synthesis methods is also presented in Table S1.


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Figure 2. Structure effect on optical activity. (a) CD spectra of NCs with transformation from spheres to 1D NRs. (Reproduced from reference 20g with permission from American Chemical Society.) (b) CD spectra of WZ CdSe NPLs with chiral cysteine as stabilizers. (c) CD spectra of ZB CdSe NPLs with chiral cysteine as stabilizers. (Reproduced from reference 20h with permission from American Chemical Society.) (d) Shell-dependent g-factor and photoluminescence quantum yield of CdSe/CdS core/shell NCs. (Reproduced from reference 5l with permission from American Chemical Society) (e) Photograph of various CdSe/ZnS NC doped co-gels in EtOH/H2O under UV light. (f) Mirror-image CPL spectra of corresponding co-gels. (Reproduced from reference 12a with permission from John Wiley and Sons.) 3.3 Composition Control Except for the structures, many investigations have been performed to regulate the NC composition including organic ligands and inorganic semiconductors. 1) Ligands. It is reported that carboxylic acids, such as chiral malic and tartaric acids with the increased number of stereo centers, might enhance the CD intensity of chiral NCs compared with the amino acid stabilizers.20c, 20d Moreover, modulation in the bidentate ligand coordination with thiolate-carboxylate (S-COO) or thiolateacetylcarbonyl (S-AC) would give rise to the CD inversion of Hg2S NCs.18f 2) Semiconductors. In the early stage, the main investigated objects are well-developed II-VI semiconductors, such as CdS, CdSe and CdTe. Recently, many new types of nanomaterials are introduced, e.g. δ-HgS NCs with intrinsic chirality,18e, 24 chiromagnetic Co3O4 nanoparticles (NPs) (Figure 3a-c),25 chiral perovskites (Figure 3d-f),26 carbon nanodots,5h, 19b MoS2 nanosheets18h and molybdenum oxides NPs.18j The extension of composition in semiconductors greatly promotes the potential application of chiral nanomaterials.

Figure 3. Examples of optical activity of different semiconductor NCs. (a) Photographs of light transmitting through chiral Co3O4 NCs, with rotation of the linear analyzer. (b) Transmission electron microscope image of L-Cys-capped Co3O4 NCs. (c) The g-factor of chiral Co3O4 NPs. (Reproduced from reference 18i with permission from American Association for the Advancement of Science.) (d) The scheme of the structures of reduced-dimensional chiral perovskites (RDCPs) with different inorganic layers. (e) CD spectra of chiral ligands and a racemic mixture in dimethylformamide. (f) Thin-film CD spectra of RDCPs prepared from the above chiral ligands. (Reproduced from reference 26 with permission from Springer Nature.) 4. Theoretic Models on Optical Activity. Many theories have been established, especially aiming to understand the optical activity of small organic molecules and large biomolecules.4a, 14, 27 By far, the chiral theory based on quantum electrodynamics of molecules is almost complete. Analysis methods related to CD spectra of chiral molecules can be categorized into below three types:28 1) Empirical: a qualitative analysis of the change in the CD spectrum of the chiral molecules based on the experience. 2) Ab initio: the calculation using complete molecular wave functions directly from Rosenfeld equation: 𝑅 = Im(𝝁 ∙ 𝒎)

(4)

where, R is the CD intensity (or CD strength, rotatory strength), ‘Im’ denotes ‘imaginary part’, 𝝁 is the electric dipole transition moment for the transition from the final to the initial state, 𝒎 is the magnetic dipole transition moment for the reverse transition, and the boldface represents vector. 3) Chromophoric: a molecule is divided into separate chromophores and the corresponding calculation is performed. The development of CD spectrum analysis on chiral semiconductor NCs makes the same journey with chiral molecules. At first, the scientists just described change of the obtained CD spectrum of chiral ligand stabilized NCs, the emerging CD signals at the characteristic absorption peaks, and so on, similarly to the empirical analysis method.16-18 Subsequently, many groups offered



deeper understanding of generation and operation of the optical activity of this system. Up to now, the theoretic models introduced into chiral semiconductor NCs can be divided into four types: 1) discrete dipole approximation (DDA); 2) time-dependent density functional theory (TDDFT); 3) density functional theory (DFT) and ab initio molecular dynamics (AIMD); 4) non-degenerate coupledoscillator model (NDCO). 4.1 Discrete Dipole Approximation DDA is a method for computing scattering and absorption of radiation by particles of arbitrary shape and by periodic structures. In 2011, our group introduced this method to explain the excitonic transition in CD spectrum of chiral semiconductor NCs.18d In this model, the chiral molecule, which is considered with the polarizabilities 𝑎𝐿1 and 𝑎𝑅1 (for left and right-polarized light), is coupled with an achiral NC with polarizability 𝑎2. The interaction between chiral molecule and NC leads to the dipoles pj, j = 1, 2. Thus, the effective polarizability for a NC is a mixture of bare polarizabilities (𝑎2, 𝑎𝐿/𝑅 1 ) for both NC and chiral molecule. As the polarizability of a chiral molecule depends on the polarization of light, the effective polarizability of a NC is also polarizationdependent owing to the coupling to the chiral molecule, which results in the CD signal for NC observed in the experiment. 4.2 Time-Dependent Density Functional Theory TDDFT is a very useful method to offer all types of optical information of molecules or even clusters. Balaz and coworkers firstly introduced TDDFT to treat chiral semiconductor NCs.12b Noteworthily, the abundant atoms in the chiral NCs and two different structural units (semiconductor NCs and chiral molecules) make the TDDFT treatment hard to fit the real condition. Hence, they simplified the model in which a chiral biomolecule (L- or Dcysteine) was connected with a nanocluster containing tens of atoms (Figure 4a). Then, some constructive conclusions were obtained, for instance, attachment of L- or D-cysteine on the surface of model (CdSe)13 nanoclusters induced the measurable opposite CD signals at the excitonic band of nanoclusters. They also pointed out that the induced chirality was consistent with the hybridization of highest occupied molecular orbitals (HOMOs) of CdSe with those of the chiral ligands.12b The TDDFT theory was further used to understand the CD spectra of semiconductor NCs combined with different amino acid. As an example, the CD signs of N-acetyl-L-cysteine-CdSe NCs and L-homocysteine-CdSe NCs were inversed, and DFT and TDDFT analyses supported the proposal that inversion of chirality originated from different binding arrangements of N-acetyl-L-cysteine and L-homocysteineCdSe on the NC surfaces.20b Although TDDFT is a reliable method, the theoretical calculation can only deal with the nanoclusters containing tens of atoms. The excessive atoms in NCs impede the CD analysis by TDDFT method. Obviously, current TDDFT method couldn’t elucidate the structure effect of chiral semiconductor NCs, such as shape, size, core/shell, and so on.

Figure 4. Theoretical models introduced in the system of chiral semiconductor NCs with concern of the chiral surface ligands: (a) TDDFT model of chiral cysteine stabilized CdSe clusters. (Reproduced from reference 12b with permission from American Chemical Society.) (b) DFT model to treat CdTe NCs. (Reproduced from reference 18c with permission from American Chemical Society.) (c) AIMD model to treat Co3O4 NPs. (Reproduced from reference 18i with permission from American Association for the Advancement of Science.) (d) Scheme of NDCO model. (Reproduced from reference 20h with permission from American Chemical Society.) 4.3 Density Functional Theory and ab initio Molecular Dynamics DFT enables exploring the stable states of molecules and the local condition on the interface between organic ligands and semiconductor NCs. With the help of DFT, much useful information was obtained, for instance, the analysis of the cysteine stabilized CdTe NCs with help of DFT by the software package Spartan O4 demonstrated generation of new chiral Cd centers on the surface of CdTe (Figure 4b).18c Kotov’s group used DFT calculation to indicate covalent attachment of L/D-cysteine moieties to the edges of graphene quantum dots (GQDs), giving rise to their helical buckling owing to chiral interactions at the “crowded” edges.18g They also introduced AIMD to obtain the atomic geometry of chiral Co3O4 NPs.18i Nevertheless, the surface distortion models could not exactly explain the detected optical activity at the characteristic absorption of semiconductor NCs. 4.4 Non-Degenerate Coupled-Oscillator Model NDCO model is one of the chromophoric analysis method arising from the quantum electronic dynamics. In NDCO model only the interactions of the electronic transition moments between different chromophores are considered under the electric dipole approximation. The magnetic transition moment 𝒎 in eq 4 comes from the relative locations between different chromophores. As a result, in NDCO model, the CD peaks originate from coupling of electric dipole transition moments in different chromophores,28 and the negative or positive sign of CD peaks is determined by the location of two coupled chromophores (Figure 4d). Our group demonstrated that the whole system of chiral molecule stabilized semiconductor NC can be treated as a huge artificial chiral “molecule”, which had no symmetry plane and center owing to the “chiral surface”. In another word, chiral molecule on the surface of NC broke its intrinsic symmetry and endowed the hybrid optical activity. Therefore, NC, transition metal-ligand bond and chiral molecule were considered as three chromophores in this huge chiral “molecule”, respectively. The asymmetric carbon center in organic molecule produced a chiral geometry between the dipoles. Armed with this assumption, the


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Journal of the American Chemical Society rich optical activity of chiral molecules stabilized NCs was well explained by the NDCO model.20g, 28-29 In detail, the CD signs of NCs and the metal-ligand bonds on their surfaces were attributed to the transition coupling between either of them with chiral molecules. Meanwhile, the observed CD peak of chiral molecule was the superposition of all the CD signals including its intrinsic CD, the transition coupling between molecule and NC, and the transition coupling between molecule and metal-ligand bond. NDCO model is a simplified one that visualizes understanding of the chiral NCs. We expect that this model might give the complete information about the optical activity of various types of chiral NCs by appropriate correction to the electronic transition energy in NC and its electronic dipole moment. Back to the origin of the chirality of semiconductor NCs, according to the eq 4, one knows that the chirality comes from the nonzero value of the dot produced between electric dipole transition moment 𝝁 and magnetic dipole transition moment 𝒎. In another word, the bigger of 𝝁 and 𝒎 as well as the smaller of the angle between them, the higher optical activity materials will have. Also, according to the NDCO model, R of chromophore A is represented as:28 𝑅(|00〉→|10〉) =

― 𝜀A𝜀C𝑉1c

∑ℏ(𝜀 C

2 C

01 (𝝁10 C × 𝝁A ∙ 𝒓AC)

― 𝜀2A)

(5)

where, the ground and excited states of chromophore A are noted as 0 and 1, and the ground and excited states of chromophore C are noted as 0 and c, ℏ is reduced Planck constant, 𝜀A(C) is the transition energy of A (C), 𝝁A(C) is the electric dipole transition moment of A (C), 𝒓AC is the distance vector from A to C, Σc means all the C around A. 𝑉1c comes from the dipole-dipole interaction. Since the origin of the optic activity of the chiral ligand stabilized semiconductor NCs comes from the dipole-dipole coupling between the surface ligands and the NCs, the key influenced factors based on the eq 5 would be 𝝁 of both the surface ligands and the semiconductors, the relative locations between them, and their relative transition energy. Altogether, we can conclude that the chirality of the colloidal semiconductor NCs is codetermined by many parameters including the semiconductor cores, surface ligands, configuration of ligands on the surface of host lattices, and even surface defects that may affect the relative transition energy. 5. Applications The chiral colloidal semiconductor NCs have exhibited many potential applications in the fields of biology, chemistry and physics. In biology, they have been attempted to chiral drug and amino acid detection, in vivo targeted imaging, cytotoxicity mediation, and gene editing.7c, 8-9, 30 In chemistry, their potentials have been shown in asymmetric catalysis, enantiomeric detection and separation,10, 30b, 31 In physics, they have demonstrated the efficient spin selectivity in electrons that sheds light on the application in spintronics.5k, 11

Figure 5. (a) Scheme of chiral CdTe-based specific DNA cleavage under CPL irradiation. (Reproduced from reference 6c with permission from Springer Nature.) (b) Scheme of tunnel junction under investigation in the magnetic conductive probe atomic force microscopy experiments and its corresponding energy diagram. (Reproduced from reference 11 with permission from American Chemical Society.) Since the details about the possible applications of chiral semiconductor NCs were summarized in some excellent reviews in 2016,5h, 32 42 we only present the representative examples during past three years: 1) Biology. Kuang’s group suggested many valuable biological applications of chiral semiconductor NCs, such as site-selective photoinduced cleavage and profiling of DNA by chiral cysteinemodified CdTe NCs (Figure 5a)6c, ameliorating Parkinson’s disease by chiral molecule-mediated porous CuxO NC clusters,18l and so on. 2) Chemistry. Kuang’s group demonstrated that the chiral Cu218k xS NCs had the potential in protein catalysis and profiling. 3) Physics. Waldeck’s group highlighted that the chiral imprinted CdSe NCs might act as the spin selective filters for charge transport. Here, the spin filtering properties of chiral NCs were investigated by magnetic conductive-probe atomic force microscopy (mCP-AFM, Figure 5b) and magnetoresistance measurements.11 6. Challenge and Opportunity The study of chiral colloidal semiconductor NCs is just at the infant stage that is full of challenges and opportunities. The combination of micro materials (chiral small molecules) and mesoscopic materials (achiral nanomaterials with quantum confinementdependent energy transition) greatly extends both fields of chirality and colloidal NCs. This emerging subject only experiences ten years’ development, and its multidisciplinary nature (chiral synthesis, semiconductor optical, and quantum mechanics) not only makes the strict demands on the scientists’ knowledge but also provides many possible applications in various fields. Based on the recent progresses, a personal view about the future development on optical activity manipulation, theoretic model, and application of chiral colloidal semiconductor NCs is outlined as follows: 1) Optical Activity Manipulation. One amazing property of colloidal semiconductor NCs is their adjustable photoluminescence (PL). Accordingly, chiral semiconductor NCs are anticipated to possess the excellent CPL features, which are the key in the nextgeneration display and optical storage devices. However, the



typical synthesis of chiral semiconductor NCs needs ligand exchange with chiral organic molecules, leading to increase of the surface defects and decrease of the fluorescence quantum yield. Interestingly, recent reports on the supramolecular self-assembly of NCs with chiral gelators demonstrate moderate CPL intensity.21-22 Nevertheless, to find novel strategies for further enhancing the CPL signs of chiral semiconductor NCs is imperative. It is worth mentioning that the successful synthesis of the gold NCs with chiral crystal structure33 gives hope to prepare the colloidal semiconductor NCs with chiral crystal structures and giant CPL. To quantitatively measure the enantiomeric purity of the chiral colloidal semiconductor nanocrystals is necessary for both fundamental study and practical application. Nevertheless, it is still a big challenge. Although there are authentic methods to determine the enantiomeric purity of chiral molecules, the method to determine that of the chiral semiconductor NCs is lack. 2) Theoretic model. The four types of theoretic calculations have been developed to understand the optical activity of the chiral colloidal semiconductor NCs. Unfortunately, each has its limitation. DDA could not introduce the electronic structures of ligand and NC. TDDFT is hard to treat the NC with thousands of atoms and offer the right CD spectrum. DFT and AIMD are time-independent and might not solve the problem of the interaction between radiation and molecules, or among the molecules. NDCO fails to treat the system containing chromophores very near each other. Thus, the improved theoretical models to elucidate the optical activity of chiral colloidal semiconductor NCs as well as predict the new chiral materials with exceptional properties are eager to be established. 3) Application. There are plenty of rooms for future applications of chiral semiconductor NCs. In chemistry, two dimensional chiral NCs have the potentials of constructing chiral films for asymmetrical (photo)catalysis and chiral separation; while in physics, the chiral semiconductor NCs are ideal candidates in spin-photovoltaic nano-devices.34 Moreover, the chiral semiconductor NCs with high biocompatibility and strong CPL would be used as the high-resolution markers for biomedical research, and the ones with the optical activity in the near-infrared region is highly desirable for in vivo imaging and therapy. Furthermore, the chiral NCs are expected to be used in chiral recognition, which is very important for design of new probes for sensing nanodrugs. In summary, the foundation of chiral colloidal semiconductor NCs has been laid in the past ten years, and we highly expect their explosive development in next decades.

ASSOCIATED CONTENT Supporting Information. Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledge financial support from National Key Basic Research Program of China (2016YFA0200700, Z.Y.T.), National Natural Science Foundation of China (Y8091111JJ, Z.Y.T.; 21805188, X.Q.G.), Frontier Science Key Project of Chinese Academy of Sciences (QYZDJ-SSW-SLH038, Z.Y.T.), K.C.Wong Education Foundation (Z.Y.T.). REFERENCES

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Journal of the American Chemical Society Table of Contents (TOC)


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Perspective of Chiral Colloidal Semiconductor Nanocrystals

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