Assembled Suprastructures of Inorganic Chiral Nanocrystals and

Apr 17, 2017 - Recent development of cinnabar HgS NCs that can possess precisely engineered chirality originating from both atomic lattice and morphol...
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Assembled Suprastructures of Inorganic Chiral Nanocrystals and Hierarchical Chirality Peng-peng Wang,† Shang-Jie Yu,†,‡ and Min Ouyang*,† †

Department of Physics and Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742, United States ‡ Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

NCs as building blocks.12 A thorough mechanistic study has been performed to elucidate corresponding assembly mechanisms for different suprastructures. Importantly, finite element method (FEM) simulations as well as optical measurements of collinear chain suprastructures built from different chiral NCs have clearly revealed the dependence of their chiroptical response on the electromagnetic coupling of building blocks. Our current work represents the first study of well-ordered inorganic chiral suprastructures constructed from tunable chiral building blocks. These emerging chiral suprastructures thus create new opportunities for both achieving large-scale metadevices with chiroptical functionalities and understanding chiral light−matter interactions at different length scales. Figure 1 illustrates two assembly routes employed in our study with assistance of metal ions, like Ag. We have started

ABSTRACT: Chiral organizations ubiquitously exist in biomaterials via hierarchical assembly of chiral molecules, but assembly of chiral inorganic nanocrystals (NCs) has been lacking. Recent development of cinnabar HgS NCs that can possess precisely engineered chirality originating from both atomic lattice and morphology offers an emerging class of inorganic building blocks to explore their hierarchical assembly. Two different forms of suprastructures, collinear chains and propellers, have been achieved with various chiral HgS NC building blocks via distinct assembly mechanisms. The chiroptical responses of suprastructures are further evaluated both experimentally and theoretically, and are found to uniquely depend on intrinsic chirality of building blocks and their coupling. Our study therefore opens up a gateway to new assembled inorganic suprastructures with desired chiroptical response for wide-ranging functionalities and applications by bottom-up modular approach.

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iven its prevalence in naturally occurring systems (from seashells and protein to DNA and other supramolecules), suprastructural chirality has aroused significant interest from fundamental science to technological applications, by combining self-assembly and molecular chirality in synthetic chemistry to induce and develop geometrical asymmetry through intermolecular interactions.1,2 Creation of chirality by means of suprastructures in inorganic systems is a rather new research realm,3−7 but can mimic interactions present in chiral entities found in nature and offer new test beds for understanding evolution of hierarchical chirality that might not be achievable in organic systems. Most works on chiral inorganic suprastructures so far have been limited to either achiral building blocks or ones having surface induced chirality with narrow scope of relevant chiroptical study and tunability.6−9 As compared with chiral entities in organics, recent development of inorganic nanocrystals (NCs) that can possess chirality originating from both crystallographic lattice and morphology can potentially serve as ideal nature-mimicking building blocks for constructing artificial chiral solids and chiroptical devices as well as understanding hierarchical chiral interactions at different length scales,10−12 but relevant study has been lacking. Herein, we report the formation of two well-defined chiral suprastructures, collinear chains and propellers, through “supramolecular” self-organization of different chiral α-HgS © 2017 American Chemical Society

Figure 1. Schematic illustration of Ag ions assisted assembly of chiral NCs for inorganic chiral “supramolecules”.

with α-HgS NCs with well-defined handedness of both lattice and morphology as building blocks,12 which are capped by oleylamine ligands and can be well-dispersed in toluene (Supporting Information, SI, Figure S1). A certain amount of Ag ions is then introduced to solution at the elevated temperature to induce assembly. The amount of Ag ions is found to be the key factor for the formation of suprastructures: for small trace of Ag ions, the α-HgS NCs intend to align collinearly end-to-end to form a straight chain (Route I). On the other hand, a large amount of Ag ions typically leads to a propeller-like suprastructure consisting of three or four NCs as blades (Route II). Figure 2 presents one example of Route I assembly by utilizing 25 nm long α-HgS nanoellipsoids (NEs) that possess chiral lattice but achiral ellipsoidal morphology as building Received: March 13, 2017 Published: April 17, 2017 6070

DOI: 10.1021/jacs.7b02523 J. Am. Chem. Soc. 2017, 139, 6070−6073

Communication

Journal of the American Chemical Society

together confirm that the Ag ion is pivotal for triggering the self-orientation and assembly of collinear chains. We, however, emphasize that although metal ions have been employed to assist assembly processes of different NCs,14,15 our observed collinear chain suprastructures are different from prior reports of branched or random network assembly: there is no Ag remaining in the interconnection of the chains, and the assembled suprastructures are not randomly oriented. In our work, when the Ag ions are introduced to the solution of αHgS NCs, they can selectively attack HgS surface in the regions possessing low density of capping molecules (such as ends of NEs). Because the solubility product constant (Ksp) of Ag2S is in the similar range of that of HgS,16 Ag+ can limitedly exchange with Hg ions on the surface to form soluble Ag2S to etch and expose fresh α-HgS (003) surface for assembly (more discussions in the SI notes). The (003) facets of α-HgS are either Hg or S terminated as shown in Figure 2b, leading to polar surface with large dipole moments along the [001] axis. These large longitudinal dipole moments and fresh facets offer the driving force to orient and adjoin α-HgS NCs end-to-end along the [001] axis via oriented attachment process, resulting in seamlessly straight chain suprastructures. This proposed mechanism immediately suggests that the choice of Ag ions to allow similar Ksp of Ag2S and HgS should be critical in order to achieve clean and exceptional quality of the interconnection, making it different from prior reports. We have evaluated and compared two more different metal ions, Au+ and Cu+, that have substantially different Ksp values from that of α-HgS (the Ksp of Au2S (Cu2S) at room temperature is ∼20 (∼5) orders smaller (larger) than that of HgS).16,17 For Au ion in our process, it has led to the formation of Au NCs onto HgS NEs, whereas the addition of Cu ions results in no assembly (Figure S8). One immediate implication of our proposed assembly mechanism for collinear chains is the conceivable control of n in a chain suprastructure by simply maneuvering concentration of Ag ions within certain range (Figure S9). Importantly, the same approach is transferrable to assembling other chiral building blocks. Figure 3a shows similar collinear chain suprastructures by using a 60 nm long twisted triangular bipyramid α-HgS NCs as building blocks that possess not only chiral lattice but also chiral morphology.12 The HR-TEM characterization of their interconnections (Figure 3b) reveals that these twisted NCs are also self-oriented and adjoined along the same direction, [001]. The morphology of these chiral building blocks remains unchanged in the suprastructures, however their relative orientation (about the chain axis) in the suprastructure is currently unclear and will require more advanced characterization like tomography (see also the SI notes). Because the handedness of lattice and morphology of the α-HgS building blocks can be independently controlled,12 it should allow regulation of prevailing chirality of the resultant suprastructures. This can thus provide a new way to create novel chiral suprastructures with fascinating chiroptics. For example, Figure 3e,f shows two sets of collinear chain suprastructures with left- and right-handed morphology and different n, which are correspondingly obtained from the NCs with prevailing left- and right-handed chirality, respectively. Our observed formation of chain suprastructures from chiral NCs therefore provide an interesting analogue of metal-ion induced self-assembly of chiral supramolecules,18 but with a broad range of tunable inorganic building blocks. We have found that the collinear chain suprastructures in Figure 2 and 3 can only be achieved under triggering of low

Figure 2. (a) Typical LR-TEM image of collinear chains assembled from α-HgS NEs. Scale bar, 200 nm. (b) Typical TEM image of a single chain. Scale bar, 25 nm. Corresponding structural model is shown on the right. (c) Typical HR-TEM image of the interconnection between two NEs in a chain. Scale bar, 5 nm. (d) SP-EDX spectra recorded at different locations along a collinear chain as specified in (c), respectively. Spectra are vertically shifted for clarity. The brown, blue and yellow dot-dashed lines represent Hg, S and Ag peaks, respectively.

blocks (Figure S2). Figure 2a shows a typical low-resolution transmission electron microscopy (LR-TEM) image after assembly, highlighting the directness and uniformity of the assembled suprastructures with more than 95% assembly yield. A close inspection of one single chain consisting of six repeating units (n = 6) is provided in Figure 2b, indicating that the α-HgS NEs are arranged end-to-end in a highly collinear manner. Importantly, we have observed no spatial separation between adjacent NEs, with a high resolution TEM (HR-TEM) image of their interconnection presented in Figure 2c (more images in Figure S3), in which fusion of two NEs with seamless extension of atomic lattice fringes can be identified and the lattice plane perpendicular to the axis of the chain is assigned to be (003) of the α-HgS. Fourier transform analysis of atomic lattices at different locations along the interconnection (Figure S3) reveals no distortion of lattice along the chain. All these evidence confirm that two neighboring NEs are self-oriented and fused along the same crystallographic direction, i.e. the caxis of cinnabar crystal. Single point energy dispersive X-ray spectroscopy (SP-EDX) measurement along the axis of the chain (Figure 2d) confirms that there is no detectable Ag species in the interconnection region, which is also in consistent with XRD characterizations (Figure S4). To elucidate underlying assembly mechanism and to understand the roles of Ag ions in the formation of collinear chains, a few control experiments have been performed. First, no assembly can be induced by addition of pure alcohol solvent, which can safely exclude effect of polar solvent (Figure S5).13 Second, we have evaluated possible effect of NO3− anion by substituting AgNO3 with other nitrates, including K+, Na+ and Pb2+ nitrates under otherwise the same assembly condition, and no suprastructure is found (Figure S6). This suggests that it is the Ag cation instead of NO3− anion that plays a key role in the assembly. Third, we have utilized F3COOAg as an alternative Ag source, and similar chain suprastructures can be obtained with lower yield (Figure S7). All these control experiments 6071

DOI: 10.1021/jacs.7b02523 J. Am. Chem. Soc. 2017, 139, 6070−6073

Communication

Journal of the American Chemical Society

shows propeller-like suprastructures with three- or four-blades, in which twisted NCs are cross-linked and coalesced with a spherical domain feature in the center, suggesting a different assembly mechanism from that of collinear chains. The SPEDX measurements (Figure 4d) reveal that the composition of three α-HgS twisted blades remains unchanged, but the central adjoining domain is an alloy of Hg0.64Ag0.36S. In contrast to metal-ion assisted oriented assembly mechanism for the formation of collinear chains (Figures 2 and 3), when the amount of Ag ions is increased a eutectic mixture of Ag2S and HgS can be formed by partial ionic exchange process at the end of NCs, due to the comparable Ksp values of Ag2S and HgS.16,19 The eutectic mixture can lead to deformation and recrystallization at the end of NCs, thus activating and promoting linkage of NCs under steric hindrance. This nanowelding process is different from previous observations,20 in which formation of pure metal nanoparticles acted as the connectors. We have employed an electromagnetic model with FEM simulation to evaluate assembly effect and hierarchical chirality in a collinear chain. Figure 5a compares computed chiroptical Figure 3. (a) Typical LR-TEM image of collinear chain suprastructures assembled from twisted bipyramid α-HgS NCs. Scale bar, 500 nm. (b) Typical HR-TEM image of the interconnection in a chain. Scale bar, 5 nm. (c, d) Models of a chain assembled from left- and right-handed NCs, respectively. (e1−e4, f1−f4) TEM images of lefthanded and right-handed chains with different n, respectively. Scale bar, 20 nm.

trace of Ag ions. When a large amount of Ag ions is added instead, a new type of assembled suprastructures can appear (Route II in Figure 1). By using again 60 nm twisted α-HgS NCs as exemplary building blocks, Figure 4 (and Figure S10)

Figure 5. (a) Evolution of chiroptical response in assembled α-HgS chain suprastructures with two different building blocks. Curves are vertically offset for clarity with zero line marked for comparison. (b, c) Experimental CD spectra of building blocks (dashed line) and their assembled chains (solid line) for the NE and twisted bipyramid building blocks, respectively.

responses of a chain suprastructure consisting of two NEs (Figure 2), a chain suprastructure consisting of two left-twisted bipyramids (Figure 3) and their corresponding building blocks. For an isolated α-HgS NC, its circular dichroism (CD) peak around 540 nm (highlighted by green arrow in Figure 5a) is attributed to the atomic scale chirality, whereas the CD feature under 500 nm (light-yellow shaded regime) is mainly determined by the interplay between chiral morphology and chiral lattice.12 The computed CD of the two-unit NE suprastructure shows essentially no dramatic difference from the sum CD signal of two individual NEs. However, the suprastructure assembled from NCs with chiral morphology manifests dramatic modification of their chiroptical response:

Figure 4. (a) Typical LR-TEM image of the propeller structures assembled from twisted bipyramid NCs. Scale bar, 100 nm. (b, c) Typical TEM image and model of a propeller, respectively. The three blades and central domain are pure HgS and alloyed Hg0.64Ag0.36S, respectively. (d) Comparison of SP-EDX spectra recorded at different locations specified in panel b: purple and orange circles are in the center and blade of a propeller, respectively. Spectra are vertically shifted for clarity. The brown, blue and yellow dot-dashed lines represent Hg, S and Ag peaks, respectively. 6072

DOI: 10.1021/jacs.7b02523 J. Am. Chem. Soc. 2017, 139, 6070−6073

Journal of the American Chemical Society

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ACKNOWLEDGMENTS We thank Dr. Kefeng Wang for assistance of XRD characterization. Work is supported by ONR award (N000141410328) and NSF award (DMR1608720). We also acknowledge facility support from Maryland Nanocenter and its AIMLab for sample characterizations.

By comparing with that of twisted building blocks, the crystallographic CD peak of their assembled chains around 540 nm remains almost unchanged, due to again oriented attachment, but the hierarchical CD in the shaded regime is significantly modified, which can be understood by the additional morphology-related chiral periodicity introduced by the assembly. Our theoretical study clearly reveals the existence of hierarchical chirality in the assembled suprastructures as well as its overall evolution and dependence on chiral building blocks. Furthermore, we have performed CD measurements of the suprastructures in Figures 2 and 3, and compared with their corresponding building blocks in Figure 5b,c, respectively. Corresponding extinction spectra are presented in Figure S11. Our experimental results have addressed the essence of hierarchical chirality predicted in Figure 5a: (1) For suprastructures assembled from NCs with either achiral or chiral morphology, their CD peaks around 540 nm remain the same as those of their building blocks; (2) For the suprastructures assembled from twisted bipyramid NCs, their CD feature in the shaded wavelength regime shows significant modification as compared with that of the building blocks, and qualitatively follows the prediction. More discussions about the comparison between experimental and simulated CD are provided in the SI notes. In conclusion, we have developed bottom-up assembly approaches to create well-defined suprastructures from an emerging class of inorganic chiral NC building blocks. Two different types of ordered suprastructures, collinear chain and propeller-like structure, have been achieved with thorough structural characterizations. Related mechanistic studies suggest different assembly mechanisms from the literature14,15,20 and provide insight into dynamic control of bottom-up selfassembly process that can be also applicable for other systems. Importantly, these inorganic chiral suprastructures can mimic supramolecular assembly of chiral biomolecules with manifestation of hierarchical chirality in both theoretical simulation and experimental chiroptical measurements. As compared with earlier work on assembly of achiral NCs for chiroptical studies,6 chiral inorganic NCs can possess unique tunability of cooperative chirality at different length scale by tailoring both crystallographic and morphological handedness. Therefore, in the future by combining with dynamic assembly control of such as relative orientation of building blocks in a suprastructure (Figure S12, and the SI notes), it should offer a unique platform for understanding evolution of hierarchical chirality and technological applications with desired chiroptical response based on assembly that otherwise cannot be achieved.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02523. Synthesis and simulation details, additional notes, TEM images, XRD, CD and other characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Min Ouyang: 0000-0002-1721-1571 Notes

The authors declare no competing financial interest. 6073

DOI: 10.1021/jacs.7b02523 J. Am. Chem. Soc. 2017, 139, 6070−6073