Pressure-Enabled Synthesis of Hetero-Dimers and Hetero-Rods

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Pressure-Enabled Synthesis of Hetero-Dimers and Hetero-Rods through Intraparticle Coalescence and Interparticle Fusion of QD-Au Satellite Nanocrystals Hua Zhu, Yasutaka Nagaoka, Katie Hills-Kimball, Rui Tan, Long Yu, Yin Fang, Kelly Wang, Ruipeng Li, Zhongwu Wang, and Ou Chen J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Pressure‐Enabled Synthesis of Hetero‐Dimers and Hetero‐Rods  through Intraparticle Coalescence and Interparticle Fusion of QD‐Au  Satellite Nanocrystals  Hua Zhu,† Yasutaka Nagaoka,† Katie Hills-Kimball,† Rui Tan,† Long Yu,‡ Yin Fang,‡ Kelly Wang,† Ruipeng Li,§ Zhongwu Wang§ and Ou Chen*,† †Department

of Chemistry, Brown University, Providence, RI 02912 of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611 §Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14853 ‡Department

Supporting Information Placeholder

ABSTRACT: This report presents the fabrication and pressuredriven processing of hetero-structural nanocrystal superlattices (HNC-SLs) self-assembled from quantum-dot-Au (QD-Au) satellite-type HNCs. In situ small/wide angle X-ray scattering and electron microscopic measurements showed that the HNC-SLs underwent structural transformation at both atomic- and meso-scales during the pressure processing. Upon deviatoric stress-driven orientational migration, the intraparticle coalescence of Au satellites at QD surfaces transforms individual HNCs into hetero-dimers, whereas the interparticle fusion drives assembled HNCs into ordered hetero-rod arrays. These results demonstrate high-pressureprocessing as a clean and fast means for conversion of HNCs into novel hetero-materials that are difficult to achieve through conventional synthetic routes. Hetero-structural nanocrystals (HNCs), which simultaneously contain two components of distinct nature, are a subclass of NCs with unique properties.1 As a result, extensive efforts have been devoted to exploit a wide spectrum of promising examples.2 Given the dual nature inherited from individual constituents and their interactions, HNCs show great promise to exhibit both combined properties and synergistically enhanced functionalities.3 Moreover, HNCs offer the possibility for regiospecific functionalization on single-NCs with asymmetric surfaces, possibly leading to their superiority as building blocks for self-recognized and self-regulated superstructures in various dimensions.4 However, the synthesis of HNCs usually relies on anisotropic nucleation of guest materials on the host crystals, heavily involving chemical reactions and posttreatments.1d, 5 Thus, in situ pressure-processing emerges as a fast and clean mechanical method to fabricate novel nanomaterials without involvement of chemical reactions and post-purification processes. Recent studies reveal that the pressure-treatment of nanomaterials is able, not only to induce structural transformation at the atomic level, but also to regulate translational alignment at nano-, meso- and even macro-scales.6 Novel nanomaterials with unique structures and properties have been generated through a stress-driven ‘permanent-sintering’.6c, d, 7 However, to date this technique has only been used to study single-component nanomaterials. Here, we report the first study of the self-assembled superlattices (SLs) of quantum-dot-Au (QD-Au) satellite-type HNCs under

diamond anvil cell (DAC) enabled high pressure environments. After a full pressurization cycle of 0-15.8 GPa, both QD-Au zero-dimensional (0D) hetero-dimers and 1D hetero-rods were generated. Our study demonstrates that pressure driven processing can be employed as a novel and efficient means to fabricate HNCs, bypassing need for a conventional chemical synthesis.

Figure 1. Characterizations of the CdSe-CdS QDs and QD-Au HNCs. (a) UV-Vis absorption spectrum before (blue) and after (red) Au satellite growth. (b, c) Typical TEM images of the QDs (b) and QD-Au HNCs (c). (d) A HR-TEM image and (e) an integrated WAXS pattern of the HNCs showing the epitaxial growth between Au satellites and QD hosts. The x-ray scattering peak positions of bulk CdS and Au satellites are labeled in red and purple, respectively. An asterisk labels the gasket peak. Inset: 2D WAXS image. QD-Au satellite HNCs were synthesized through a modified procedure (See details in SI).2b The absorption spectrum shows enhanced absorbance of the QD-Au HNCs indicative of a plasmonic contribution from the Au satellites (Figure 1a). Transmission electron microscopy (TEM) measurements reveal the uniform sizes of CdSe-CdS QD hosts (9.7 ± 0.5 nm) and Au satellite NCs (1.6 ± 0.2 nm) (Figures 1b, c and S2, 3). High-resolution TEM (HR-TEM) images clearly show atomic fringes crossing the QD-Au interfaces (Figure 1d), demonstrating epitaxial growth. For example, Figure 1d shows that the (200) planes of Au satellites (d-spacing of 2.4 Å)

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make direct connection to the (1011) planes of the QD (d-spacing of 3.2 Å). The epitaxial growth was also confirmed by synchrotronbased wide angle X-ray scattering (WAXS) measurements (Figure 1e). Besides the typical scattering peaks of the QD wurtzite (WZ) phase, the WAXS pattern also shows three additional broad scattering features assigned to Au(111), Au(200) and Au(220) (Figure 1e and S4). The calculated d-spacings reveal large lattice expansions of ~18% for Au(111) and Au(200) planes, and 39% for Au(220) planes, consistent with HR-TEM observations (Figures 1d and S5). This lattice expansion mainly originates from large lattice mismatch between WZ CdS and cubic Au crystals and the small size of Au satellites. Although the experiments provide direct evidence for epitaxial growth, the mechanism appears complicated. For example, both Au(111) and Au(200) can grow epitaxially along the QD( 1011 ) direction, suggesting the existence of multiple growth models (Figures 1d and S5). HNC-SLs formed upon solvent evaporation (SI). The resultant HNC-SLs exhibited fcc symmetry with a TEM-determined lattice constant of ~16.4 nm (Figures 2a and S6). The selected-area electron diffraction (SAED) showed a spot-type texture, indicative of atomic orientational alignment of HNCs inside the SLs (Figure 2b and S7).8 Small angle X-ray scattering (SAXS) confirmed the fcc SL with a lattice parameter of 16.36 ± 0.06 nm (Figure 2c and Table S2). Given the QD size of 9.7 nm, the inter-QD distance was determined as ~1.9 nm, suggesting significant surface ligand intercalation (i.e., oleic acid or oleylamine) between neighboring HNCs.6d, 9 The ring-type SAXS pattern reveals the multi-domain nature of the SL (Figure 2c inset).

Figure 3. (a) The WAXS patterns collected during compression (black) and releasing (blue) processes. (b) WZ phase percentage of the QDs, and (c) d-spacing of Au(111) as a function of the pressure during compression (black) and releasing (blue) processes. metastable phase could be completely harvested at ambient conditions. However, the stabilizing mechanism remains unclear and is still under investigation. We also monitored the evolution of the Au(111) peak as a function of pressure. Upon compression from 0 to ~6 GPa, the Au(111) d-spacing decreased notably from 2.76 Å to 2.68 Å with dramatically reduced peak intensity (Figures 3a, c). When QDs started the phase transition at 6.1 GPa, the RS(200) peak emerged and overlapped with the Au(111) peak (Figures 3a, c). Interestingly, after the QD phase transition, a new peak with a d-spacing of 2.32 Å gradually arose which slightly red-shifted and became pronounced upon continuous compression (Figure 3a). This peak was also retained during the decompression process and blue-shifted to the position with a bulk Au(111) d-spacing of 2.35 Å after releasing pressure (Figures 3a, c). These observations indicated a release of the Au lattice stress from the QD/Au interface

Figure 2. (a) A representative TEM image of self-assembled HNC-SLs (Inset: corresponding FFT pattern) and (b) the corresponding ED pattern showing orientational atomic alignment. (c) The integrated SAXS pattern of the HNC-SLs inside a DAC. Inset shows a unit-cell model of the fcc superstructure and the 2D SAXS image. The fitted peaks (blue) and SAXS curve (red) show a perfect fcc structure of HNC-SLs. In situ high-pressure WAXS/SAXS experiments of the HNCSLs were performed.10 Upon compression, QDs exhibited a transition from WZ to rock salt (RS) structure within the pressure range of 6.1-8.5 GPa (Figures 3a, b and S8).11 Interestingly, the RS phase of the QD remained stable over the compression cycle, and was harvested at ambient conditions (Figures 3a, b and S9 and Table S3). While previous work has reported that the orthorhombic PbS metastable phase created under pressure could be partially harvested,12 our results provide an example in which a high-pressure

Figure 4. Superstructural evolution of the QD-Au HNC-SLs under high pressure. (a) In situ SAXS patterns during compression (black) and decompression (blue) processes. (b) Integrated SAXS pattern of the pressurized HNC-SLs at ambient conditions. Inset: 2D SAXS image. The fitted peaks shown in blue present (0-L) series (L=1-4) of the lamellar structure. (c) The inter-NC distance during compression (black) and decompression (blue) processes.

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tension. We attribute this lattice stress release to a combined effect of pressure-induced epitaxial Au-S bond breaking and structuraltransition-driven atomic movement at QD surfaces (i.e., WZ to RS). The pressure-induced SL transformation at the meso-scale was explored by monitoring in situ SAXS patterns (Figure 4). When the pressure was increased from ambient to ~9 GPa, the fcc SL remained with all peaks shifted to larger 2θ angles and the SL(111) d-spacing reduced from 11.6 nm to 11.1 nm (Figure 4c). Further compression caused a SL transformation to a lamellar superstructure (Figures 4a and S10). Meanwhile, a reversed shift of inter-NC spacing was observed (Figure 4c). At peak pressure of 15.8 GPa, the SL was entirely transformed and the lamellar structure retained after releasing to ambient pressure (Figure 4b, Table S5). During the pressure releasing process, the inter-layer distance of the lamellar SL monotonically increased to 12.6 nm. This irreversible superstructural evolution indicated migration and slight detachment of surface ligands and a morphological transformation of individual HNCs.7

Figure 5. TEM images of the harvested sample from high pressure. (a) A representative TEM image showing the preserved lamellar superstructure and the corresponding FFT image (Inset). (b) An ED pattern showing a typical RS structure. (c) A representative TEM image showing hetero-dimers (d-f) HR-TEM images and FFT images of the QD hosts (Inset) of the hetero-dimers showing the preservation of RS phase in QDs and the poly-crystallinity of Au dots. (d, e); (f) A HRTEM image showing the coexistence of the WZ and RS phases inside a QD. (g-i) TEM images of hetero-rods at various magnifications. Inset of (g) shows the length distribution histogram of the hetero-rods. TEM imaging provided additional evidence for the formation of the lamellar SL and its subsequent preservation (Figure 5a). The fast-Fourier transform (FFT) pattern clearly showed 1D lamellar periodicity (Figure 5a inset). The isotropic ring-type SAED pattern can be assigned to the QD RS and Au diffraction patterns (Figure 5b). The disappearance of the atomic alignment indicated that the HNCs underwent a pressure-driven process of NC movement and rotation within the SLs.6a,b TEM images of the disassembled samples confirmed the morphological transformations of the initial QD-Au satellite HNCs. In general, two new types of QD-Au HNCs were obtained: 1) hetero-dimers (Figures 5c-f); and 2) hetero-rods

(Figures 5g-i). Figures 5d and 5e display typical HR-TEM images of the hetero-dimers with characteristic orthogonal lattice fringes of single domain QDs and irregular polycrystalline Au islands. A low population (~8%) of single-crystalline Au islands was also observed (Figure 5f). The measured lattice distances of 3.2 Å and 2.0 Å for the QD and 2.3 Å for the Au were assigned to the RS(111), RS(220) and Au(111), respectively (Figures 5d-e and Table S3). These observations were consistent with WAXS datasets collected at ambient conditions (Figures 3a and S9). It is noteworthy that the Au islands on the QD surfaces are larger than the original Au satellites (5.3 nm vs 1.6 nm, Figure S11). Interestingly, a low population (< 1%) of the hetero-dimer HNCs with a mixture of RS and WZ phases was observed (Figures 5f and S12), indicative of the instability of the RS structure under electron beam radiation. The observed atomic structure of the WZ core surrounded by the RS shell suggests that the electron-radiation-induced RS-to-WZ phase transition is most likely followed by a radial expansion starting from the center of the QDs. Figures 5g-h show the TEM images of QD-Au hetero-rods, 227 ± 56 nm in length and 8.9 ± 0.7 nm in diameter (Figures 5g and S13). A ~ 8% reduction in diameter (8.9 nm vs 9.7 nm) was in line with a ~ 18% unit-cell volumetric shrinkage from the WZ to RS phase. The quasi-spherical Au islands (~3.0 nm) are randomly located on the hetero-rods (Figures 5g, h and S14). HR-TEM characterizations unambiguously revealed polycrystalline individual hetero-rods formed through direct attachment of individual QDs (Figures 5i and S15). Analyses of the WAXS, SAXS and TEM datasets allow a reconstruction of the pressure-driven hetero-structural formation pathway (Scheme 1). When the applied pressure is less than 6.1 GPa (before the QD phase transition), the HNC-SLs are compressed hydrostatically, thus, the SLs shrink isotropically and reversibly (Figure S16).12-13 When pressure falls into the QD phase transition region (i.e., 6.1-8.5 GPa), the Au-S epitaxial bonds at QD/Au interfaces break down due to pressure-induced stress and phase-transition-initiated surface atomic movements. This results in Au satellites only physically attached on QD surfaces without covalent bonding. Additional control experiments with compression of samples to a peak pressure of 8.5 GPa (immediately after the QD phase transition) showed that, while the RS structure was preserved at ambient conditions, no significant HNC morphological change was detected (Figure S17). Upon increasing pressure from 8.5 GPa to 15.8 GPa, an enhanced deviatoric stress drives the loosely attached Au satellites to migrate and coalesce on QD surfaces.14 This is also evidenced by a gradual emergence of the new Au(111) peak at the bulk d-spacing position (Figures 3a, c). In this pressure region, the two reaction pathways are developed due to

Scheme 1. Schematic demonstration of the proposed pressuresintering process. different translational alignments in the fcc SL relative to the uniaxial compression direction. When the SL[110] orientation is aligned with the compression axis, permanent-sintering occurs and accordingly, the 1D hetero-rods start to form (Scheme 1i). This is due to the smallest inter-NC distance with exact HNC center-tocenter 1D alignment along the SL[110] direction (Scheme 1i).7a However, given the multi-domains of HNC-SLs inside the DAC

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(Figure 2c, inset), the hetero-dimers can be formed in other SL domains due to HNC center mismatch along the compression axis and continuous Au satellite migration and coalescence (Scheme 1ii). Intermediate states (hetero-multimers and short hetero-rods) were captured when exposing HNC-SLs to 11.0 GPa (Figure S17). Moreover, no hetero-structural change was observed in the control experiment with loading of pressure media (i.e., silicone oil, Figure S18 and S19). This proved the requirement of anisotropic deviatoric stress and direct touching between HNCs, in line with our proposed mechanism (Scheme 1). In summary, we report, the pressure-enabled synthesis of QDAu hetero-dimers and hetero-rods. Uniform QD-Au satellite-type HNCs were first synthesized through an epitaxial growth of Au satellites on QD surfaces at the expense of large Au lattice expansion. The obtained HNCs were self-organized into a fcc SL. In the compression cycle of 0 – 15.8 GPa, the fcc SL transformed into lamellar structures. Additionally, an interfacial Au-S bond-breaking process coincided with the QDs phase transition from WZ to RS (6.1-8.5 GPa). This process was followed by the formation of hetero-dimers and hetero-rods through an intraparticle coalescence of Au satellites and interparticle fusion of QD hosts induced by deviatoric stress under high pressure (8.5-15.8 GPa). This study not only provides fundamental understanding of the pressure-driven HNC-SL transformations at atomic- and meso-scales, but also sheds light on the rational design of hetero-structural nanomaterial production through a clean and fast stress-driven nanofabrication technique.

ASSOCIATED CONTENT   Supporting Information  The Supporting Information is available free of charge on the ACS Publications website. Experimental methods and supporting figures (PDF)

AUTHOR INFORMATION  Corresponding Author  *[email protected]  Notes  The authors declare no competing financial interests. 

ACKNOWLEDGMENT   O. C. acknowledges the support from the Brown University startup fund and the Salomon award fund. O. C. also thanks the UAC grant from the Xerox foundation. CHESS was supported by the NSF award DMR-1332208. The TEM measurements were performed at the Electron Microscopy Facility in the IMNI at the Brown University.

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