Ordered Structure Rearrangements in Heated Gold Nanocrystal

Oct 16, 2013 - Yixuan Yu , Brian W. Goodfellow , Michael R. Rasch , Christian Bosoy , Detlef-M. ... Crystal Structures of the Hard Core Square Shoulde...
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Letter pubs.acs.org/NanoLett

Ordered Structure Rearrangements in Heated Gold Nanocrystal Superlattices Brian W. Goodfellow,† Michael R. Rasch,† Colin M. Hessel,† Reken N. Patel,† Detlef-M. Smilgies,‡ and Brian A. Korgel*,† †

Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712-1062, United States ‡ Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: Small-angle X-ray scattering (SAXS) data reveal that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a series of ordered structure transitions at elevated temperature. An example is presented of a body-centered cubic superlattice that evolves into a hexagonal close-packed structure, followed by the formation of binary simple cubic AB13 and hexagonal AB5 superlattices. Ultimately the superlattice decomposes at high temperature to bicontinuous domains of coalesced Au and intervening hydrocarbon. Transmission electron microscopy revealed that the ordered structure transformations result from partial ligand desorption and controlled Au nanocrystal growth during heating, which forces changes in superlattice symmetry. These observations suggest some similarity between organic ligand-coated nanocrystals and microphase-segregated diblock copolymers, where thermally induced nanophase-segregation of Au and organic ligand influences the ordered arrangements in the superlattice. KEYWORDS: Nanocrystals, superlattices, phase transitions, colloids, self-assembly

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superlattices with an example of dodecanethiol-coated Au nanocrystals displaying four different superlattice structures as the temperature was increased. These transitions, however, are irreversible as heating permanently changes the nanocrystal size and ligand coverage. Ordered assemblies of ligand-stabilized nanocrystals, or superlattices, are interesting because of their unique optical and electronic properties. Nanocrystals in the quantum size regime, that is, less than about 10 nm in diameter, can be produced with uniform size distributions, exhibiting physical properties that are fundamentally different from their bulk counterparts and can be tuned synthetically by manipulating the particle size.7 In the assemblies, collective electronic coupling between neighboring nanocrystals can give rise to new physical properties that depend on their superlattice structure.8 These self-assembled materials have therefore been called “artificial solids” or “metamaterials” since nanocrystals instead of atoms serve as building blocks. These superlattices can also exhibit significant structural polymorphism. Colloidal crystals of hard-sphere particles (like opals) tend to form close-packed lattices like fcc structures.9,10 The soft

aterials respond to changes in environmental conditions, like temperature, pressure, mechanical force, electric fields, and so forth, by changing structure. Relatively minor environmental differences may lead to subtle effects, like an expansion or contraction without a change in packing symmetry or geometrical arrangement of the atoms. More extreme environmental variations can weaken bonds and lead to a phase transition. Phase transitions of solids can involve a change in state, for example, to a liquid or a gas, or from one crystal structure to another. Most solids exhibit structural polymorphism and solid−solid phase transitions. For example, graphite and diamond are well-known polymorphs of carbon; iron can exist with body-centered cubic (bcc) or face-centered cubic (fcc) crystal structure depending on temperature and pressure. Soft molecular materials, like amphiphilic diblock copolymers and surfactants, assemble into different ordered structures depending on conditions, ranging from micellar cubic, to hexagonal (cylindrical), to bicontinuous cubic, to lamellar structures, to name a few.1,2 Dense colloidal dispersions of charge-stabilized particles and spherical block copolymer micelles can also exhibit structural polymorphism, forming either fcc and bcc lattices depending on temperature and solvent parameters.3,4 Recently, organic ligand-stabilized nanocrystal superlattices have been observed to undergo structural transitions by solvent vapor annealing.5,6 Here, we report that heating can also change the structure of nanocrystal © 2013 American Chemical Society

Received: September 16, 2013 Revised: October 14, 2013 Published: October 16, 2013 5710

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Figure 1. (A) Contour plot of the radially integrated scattering intensity as a function of q (q = (qx2 + qz2)1/2) and temperature during in situ transmission SAXS of a nanocrystal superlattice of sub-2 nm dodecanethiol-capped Au nanocrystals heated from room temperature to 213 °C (see Supporting Information Figure S11 for an enlargement of the data at 155−185 °C and Supporting Information Movie S1 for the 2D scattering profiles). (B) Indexing of the simulated diffraction peak positions and schematics of the unit cells of the observed structures. The bcc superlattice has an initial lattice constant of a = 3.72 nm, corresponding to a nearest neighbor separation of 3.22 nm. The hcp superlattice has initial lattice constants of a = 6.32 nm and c = 10.3 nm (c/a = (8/3)1/2).

material,20−22 or in some cases, a change in superlattice composition.23 Heating induces ligand desorption, which can lead to significant changes in nanocrystal size and a loss of structural integrity. Here, we report that thermally induced changes in the size of dodecanethiol-coated Au nanocrystals can actually be quite controlled and lead to transitions between different ordered superlattice structures. Figure 1 shows SAXS data for a superlattice of dodecanethiol-capped 1.8 nm diameter Au nanocrystals heated from room temperature to 213 °C. (See Supporting Information for Experimental Details and accompanying Supporting Information Movie S1.) A series of ordered structural transitions take place before the superlattice finally decomposes. These changes in superlattice structure were also directly observed by transmission electron microscopy (TEM), as shown in Figure 2. The changes in superlattice structure appear to be induced by ripening of the nanocrystal size, which are then accommodated by discrete changes in superlattice structure. In Figure 1, the initial bcc superlattice of Au nanocrystals exhibits a slight thermal expansion consistent with the thermal expansion of the ligands, as observed for heated dodecanethiolcoated silver nanocrystal superlattices.20 At 125 °C, extra Bragg rings appear in the diffraction patterns, which index to an hcp superlattice structure. The nearest neighbor spacing in the hcp superlattice (6.32 nm) is significantly larger than in the initial bcc superlattice (3.22 nm), corresponding to the increased size

organic ligand coating of nanocrystals, however, is deformable and occupies a significant volume of the superlattice, up to 90% in the case of dodecanethiol-coated sub-2 nm diameter Au nanocrystals. The self-assembly of ligand-stabilized nanocrystals into superlattices can be more or less understood in terms of a close-packing of spheres composed of a rigid inorganic core with a chemisorbed layer of flexible, yet incompressible, ligands,11,12 but the ligand shell can also respond significantly to environmental factors and induce changes in superlattice structure. For example, packing polymorphism has been observed in binary nanocrystal superlattices formed with similarly sized large and small nanocrystals under different deposition conditions, which might be related in part to capping ligands and how they respond to different formation conditions.13−16 Swelling of ligands (i.e., oleic acid) on PbSe and PbS nanocrystal by exposure to solvent vapors can reversibly change superlattice structure from bcc to fcc and body-centered tetragonal (bct).5 Dunphy et al.17 showed that subtle differences in surfactant composition can lead to changes in gold nanocrystal superlattice symmetry from fcc to bcc. The ligands should also be sensitive to changes in temperature. Thermally driven transitions between ordered and disordered structures in nanocrystal superlattices have been anticipated by theory and computer simulations18,19 yet not observed experimentally. Heated superlattices have only shown coalescence of inorganic material within a matrix of organic 5711

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Figure 2. (A,D,G,J) TEM images and fast Fourier transforms (FFTs), (B,E,H,K) model representations, and (C,F,I,L) simplified ligand domains for observed nanocrystal superlattice structures: (A−C) bcc, (D−F) hcp, (G−I), binary sc ico-AB13, and (J−L) binary hex AB5. The simplified ligand domains mimic microphase-separated morphologies for amphiphilic organic molecules: (C) bcc (spherical), (F) hexagonal (cylindrical), (I) cubic bicontinuous (plumber’s nightmare), and (L) inverse hexagonal. (M) TEM image of a nanocrystal superlattice that has been heated above 200 °C. Spinodal decomposition of the superlattice occurs as the ligand completely desorbs from the nanocrystal surface.

expansion of the ligands. At 158 °C, just before the bcc

observed by TEM. Subtracting the intervening ligand from the interparticle spacing, the Au core diameters in the bcc superlattice are 1.8 and 4.0 nm in the hcp superlattice (see TEM in Figure 2D). The bcc and hcp superlattice structures coexist until reaching 160 °C and the hcp structure also expands with increasing temperature due to the thermal

diffraction peaks disappeared, an additional diffraction peak emerged at q = 0.73 nm−1 that can be explained as the introduction of periodic (001)hcp stacking faults in the hcp superlattice (see Supporting Information Figure S12). 5712

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Figure 3. (A-D) TEM images of a single layer of nanocrystals heated to the indicated temperatures and the corresponding nanocrystal core size distributions measured from TEM images. (E) Porod plots of solution SAXS data with simulated intensity profiles (solid black lines), (F) in situ GIWAXS, and (G) absorbance spectra of Au nanocrystals after heating to the indicated temperature and being redispersed in toluene. The data show that the growth of the Au nanocrystal cores begins around 125 °C, which corresponds to the appearance of the hcp superlattice structure as shown in Figure 1.

The hcp structure then disappeared at 167 °C and a new superlattice structure emerged. On the basis of analysis of the SAXS data and TEM images (Figure 2G), this superlattice structure is a binary nanocrystal superlattice (BSL) with icosahedral AB13 (ico-AB13) structure. The ico-AB13 structure is a well-known BSL structure24 with simple cubic (sc) symmetry (isostructural with NaZn13 (PDF #01-071-9884), space group 226, Wyckoff sequence: iba) and a large cubic unit cell (a = 16 nm). One distinguishing feature of the ico-AB13 structure in the SAXS data is the strong (531) diffraction peak at q = 2.3 nm−1.10 In the TEM (Figure 2G) images, the simple cubic NaZn13-type BSL is oriented with (100) superlattice planes parallel to the substrate, but GISAXS shows no preferential superlattice orientation and the loss of diffraction spots and appearance of diffraction rings indicates that the range of order is significantly less than the bcc and hcp superlattices (Supporting Information Figure S3). It is rather unexpected that the Au nanocrystals ripen into two different specific sizes of approximately 2 and 4 nm in diameter. A few of the GISAXS peaks do not index directly to a simple cubic icoAB13 structure (compare Figure 1A,B), but can be explained by insertions of larger nanocrystals for the smaller ones in the centers of the B13 clusters, which transforms the superlattice symmetry from sc to fcc and bcc (Supporting Information Figures S13, and S14). The superlattice structure changed again at 177 °C to a hexagonal structure, which was then overshadowed by a structurally similar yet slightly more compact hexagonal superlattice when the temperature reached 181 °C. These two superlattice structures are also BSLs, but with AB5 structure

(isostructural with CaCu5; space group 191; Wyckoff sequence: gca). The TEM (Figure 2J) samples had their (001)SL planes oriented parallel to the substrate and GISAXS indicated several possible orientations (Supporting Information Figures S4 and S5). At 208 °C, the superlattice structure collapsed. The scattering signal shifted rapidly to low q with increasing temperature, characteristic of spinodal decomposition and phase separation between Au and the organic ligands. Figure 2M shows a TEM image of the structure, clearly showing the bicontinuous network of coalesced Au and organic. The ripening of the Au nanocrystal cores is accompanied by the partial desorption of the ligand shell. The superlattice transitions derive from the changes in nanocrystal size and the tendency toward Au-organic phase segregation. Figure 3 shows TEM, SAXS, grazing-incidence wide-angle X-ray scattering (GIWAXS), and optical absorbance spectra of the Au nanocrystals after they were heated to various temperatures, all of which confirm that the Au nanocrystals increase in size as they are heated, that is, the scattering peaks in the Porod plot shift to lower q, the XRD peaks sharpen and the plasmon peak in the absorbance peak becomes more prominent. Shimizu et al.25 have also observed similar controlled ripening of alkanethiol-coated Au nanocrystals during heating. The transitions to different superlattice structure occur from these changes in nanocrystal size, along with the significant changes in the amount of free ligand in the superlattice. The superlattice structures are minimizing the total interfacial area between Au and organic as illustrated in Figure 2, similar to microphasesegregating amphiphilic diblock copolymers.1,26 For instance, columns of ligand create a 2D hexagonal array in an hcp 5713

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and NIH/NIGMS via NSF award DMR-0936384. B.W.G. acknowledges financial support under the NSF IGERT program DGE-0549417. C.M.H. acknowledges financial support from the NSERC of Canada. We also thank Marleen Kamperman, Marvin Paik, and the Ober group for the use of their heating stage for the in situ heating experiments.

superlattice (Figure 2F) like the cylindrical (or hexagonal) morphology of diblock copolymer melts. The ligand network in the simple cubic NaZn13-type superlattice is similar to the (triply periodic) cubic bicontinuous “plumber’s nightmare.”27 The three domains correspond to the large nanocrystals, the small nanocrystals, and the ligand (Figure 2I). In the hexagonal CaCu5-type superlattice (Figure 2J), the ligands have a 2D hexagonal arrangement of hollow tubes extending up from the substrate (Figure 2L). While this topology is closely related to inverse hexagonal morphology for diblock copolymers, the simple hexagonal nanocrystal superlattice28 is the direct analog of the inverse hexagonal morphology of diblock copolymers. These experiments reveal a conceptual link between the selfassembly of nanocrystal superlattices and microphase-separated amphiphilic molecules like diblock copolymers. The selfassembly of soft materials is well understood based on microphase-separation and molecular packing constraints29 and there appear to be similar phenomena occurring in the ligand-stabilized nanocrystal superlattices studied here. The superlattices undergo a range of structural transitions with subtle changes in nanocrystal size and ligand content, transforming from a bcc superlattice to an hcp lattice, then to binary superlattices with sc symmetry (isostructural with NaZn13) and hexagonal (hex) structure (isostructural with CaCu5). Although these order−order structural transitions are not reversible, they derive from changes in nanocrystal size and capping ligand coverage leading to ordered nanophaseseparation. Indeed, there are analogs of some of the most complex nanocrystal superlattice structures30 in diblock copolymers,31 as illustrated by the structural representations of the organic ligands in the superlattices in Figure 2C,F,I,L. There should be opportunity to leverage the extensive theoretical understanding of microphase-separated organic systems to enable greater understanding of nanocrystal selfassembly. Additionally, nanocrystals can offer a unique fundamental test bed for exploring the self-organization of microphase-separated systems, since nanocrystals assemble on rapid time scales and several parameters like interfacial curvature and the “block” volume fraction can be independently controlled.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, additional GISAXS data and TEM images, Movie S1 showing GISAXS pattern evolution during heating. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1-512-471-5633. Fax: +1-512-471-7060. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christian Bosoy, Yixuan Yu, and Julián Villarreal for assistance in collecting SAXS data and Andrew Heitsch, Vahid Akhavan, and Vincent Holmberg for insightful discussions. This work was supported in part by funding from the Robert A. Welch Foundation (Grant F-1464) and the National Science Foundation (DMR-0807065). CHESS is supported by the NSF 5714

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