Controlled Self-Assembly and Tuning of Large PbS Nanoparticle

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Controlled Self-Assembly and Tuning of Large PbS Nanoparticle Supercrystals Kaifu Bian, Ruipeng Li, and Hongyou Fan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02691 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Chemistry of Materials

Controlled Self-Assembly and Tuning of Large PbS Nanoparticle Supercrystals Kaifu Bian1, Ruipeng Li2, Hongyou Fan1,3,4,* 1

Sandia National Laboratories, Albuquerque, NM 87123; 2 NSLS II, Brookhaven National Laboratories, Upton, NY 11973 Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials, the University of New Mexico, Albuquerque, NM 87106; 4 Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States.

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ABSTRACT: Self-assembly of colloidal nanocrystals (NCs) into ordered superlattices (SLs) and supercrystals (SCs) enables new artificial NC solids for nanoelectronic and nanophotonic applications, which requires critical control on nucleation and growth conditions. Herein large SCs of PbS NCs up to ~100 µm size were synthesized by two controlled self-assembly methods from NC solutions. Both translational symmetry and orientational ordering of the nanocrystals in the SCs were readily tuned by excess oleic acid ligands and anti-solvents. Slow evaporation and counter-diffusion method of solvents resulted in the formation of single SCs with two different SLs from the same PbS NCs: a face-centered cubic SL with weak yet complex orientational order or a body-centered cubic SL with strong and uniform particle orientation, respectively. The translational ordering was mainly determined by effective shape of the NCs while the difference in orientational order was a result of the balance between ligand-ligand attraction and rotational entropy. The ease of the growth of large SC solids could lead to diverse NC systems and facilitates essential investigation of nanoparticle interactions and coupling based nanoelectronic and nanophotonic properties.

1. Introduction Self-assembly of colloidal nanocrystals (NCs) into ordered superlattices (SLs) and supercrystals (SCs) enables new artificial NC solids for nanoelectronic and nanophotonic applications, which requires critical control on nucleation and growth conditions.1-5 The SLs are considered a new class of condensed material which possesses not only the size-tunable features of the constituent NCs but also ordering-induced unique collective properties.6-11 The self-assembly process of NCs involves a variety of interactions including van der Waals forces between cores, ligands and solvents, steric repulsion and capillary forces, possible Coulombic and magnetic forces, etc.12 On the other hand, entropy also plays an important role to balance enthalpic terms to minimize free energy, especially at higher temperature.13-16 By mediating these interactions, the free energy landscape and thus structure of SLs can be delicately controlled.3,17-21 Large single domain SCs are desirable for their potential in applications as well as fundamental research serving as the key to understand complex processes such as miniband formation, optical coupling and pressureinduced transitions. 9-10,22-24 In this study, large SCs up to ~100 µm were grown from PbS NCs passivated by oleic acid (OA) ligand. The SC growth was driven by slowly oversaturating the NC solution system by either a solvent evaporation with excess ligand or a counter-diffusion with anti-solvent control. The SL and atomic structures of the SCs were characterized by synchrotron-based small and wide angle X-ray scatterings respectively.5 The SCs grown by the evaporation method displayed face-centered cubic or fcc symmetry with complex but relatively weak NC orientational preference. In contrast, the counter-diffusion approach resulted in a bcc SC with strong and uniform NC

orientation. Such difference in translational symmetry was attributed to either a spherical or octopod effective shape of the NCs during self-assembly depending on surface ligand coverage, while the orientational order resulted from the balance between ligand-ligand attraction and rotational entropy. It had been reported that the symmetry of PbS SL could be switched between fcc and bcc by aging in ambient and selection of solvents.17-18 Here we demonstrate that such control could be achieved in a more reproducible way of tuning the presence of excess ligands and anti-solvents. 2. Experimental Section Nanocrystal Synthesis. PbS NCs were synthesized in a single batch according to a literature method25. Briefly, 5 g of PbCl2 powder was mixed with 15 mL oleylamine in a 100 mL threeneck flask connected to a Schlenk line. The mixture was first degassed under vacuum for 1 hr while being stirred. Then the flask was filled with nitrogen gas and then heated up to 130 °C. To prepare sulfur precursor, in a nitrogen glove box, 0.33 M solution of elemental sulfur was prepared by mixing sulfur powder with oleylamine. The mixture was heated and stirred on a hotplate at 120 °C until the powder was completely dissolved and an amber solution was obtained. 3 mL of the sulfur solution was quickly injected into the PbCl2 solution with a syringe. The mixture turned black indicating the formation of PbS NCs. The reaction was quenched immediately after injection by immersing the flask in a cold water bath and adding 20 mL of chilled hexane. To purify the NCs, the raw product was first centrifuged to remove excess PbCl2. Then the NCs in supernant were precipitated by adding ethanol. The NCs were dissolved in 10 mL hexane. Then 40 mL oleic acid (OA) was added to precipitate the NCs. This hexane-oleic acid cleaning

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was repeated for four times. During this process, the oleylamine surface ligands were replaced by oleic acid. Finally, the product NCs were purified by hexane-ethanol again to remove excess oleic acid and then dispersed in toluene. Supercrystal Growth A. Evaporation Growth. In a 4 mL glass vial, 200 µL of 10 mg/mL toluene solution of PbS NCs was mixed with 20 µL liquid OA. The vial was sealed by hand-tightening the cap and then let stay undisturbed. The sealing was not perfect so that toluene could still slowly evaporate and leak into atmosphere. After 1-2 month, toluene completely evaporated. Pieces of PbS NC SCs were found immersed in the remaining OA. B. Counter-Diffusion Growth. In a 4 mL glass vial, 1 mL of 10 mg/mL toluene solution of PbS NCs was added. Then 2 mL of anhydrous isopropanol as anti-solvent was added carefully on top of the NC solution forming a clear interface. Then the vial was left to stay undisturbed, allowing the two phases to slowly diffuse into each other. After approximately one week, the mixture became a homogenous colorless liquid while PbS NC SCs were grown at both the inner wall and the bottom of the vial. X-ray Scatterings. Small and wide angle X-ray scattering data was collected at the B1 station of Cornell High Energy Synchrotron Source (CHESS) with a monochromatic beam of 25.514 keV, equivalent to 0.485946 angstrom. The beam was collimated down to 100 µm in diameter, large enough to cover an entire SC sample. The sample-detector distance was determined using a mixture of CeO2 and silver behenate powder. The scattering patterns were collected by a large-area Mar345 2-D detector. In a typical measurement, a piece of single SC was carefully picked and mounted onto a two-circle diffractometer which provides a rotation ability along the two axes of θ (parallel with the beam) and ϕ (perpendicular to the beam). First a high symmetry orientation of the SC was located by trial. The angle θ was first adjusted so that the ϕ axial of the diffractometer is aligned with one of the high symmetry directions of the SC, e.g. SL[100]. Then the SC was rotated around ϕ axial for 180° at an interval of 1° while SAXS and WAXS patterns were collected. The x-ray scattering data was integrated by the software Fit2D 26 to obtain the precise scattering peak positions which were then used to determine lattice parameters. Simulations of SAXS and WAXS patterns were performed by the commercial software Crystal Maker (version 9.2.2). The simulated patterns were compared with experiments to determine the fine structures of the NC SC including the SL symmetry and the orientation of the constituent NCs. Electron Microscopy. The size of PbS NCs used in this study was measured by transmission electron microscopy (TEM) using a JEOL 2010 TEM operated at 200 kV. At least 300 NCs in TEM images were counted to provide statistic of particle size. The shape and microscopic texture of the surface of the SCs was characterized by scanning electron microscopy using a Hitachi S-5200 SEM operated at 10-20 kV. Typical high symmetry SC planes, i.e. SL(100), SL(110) and SL(111) were identified. 3. Results and Discussions PbS NCs with surface passivation of oleic acid ligands were synthesized by a literature method.25 A representative TEM image of the NCs is shown as Figure 1a. The average diameter of the NCs was 6.7 nm with a standard deviation of 10%. PbS NCs of such size possess a truncated-cubic shape with six

PbS{100} and eight PbS{111} facets.5 As will be demonstrated in this paper, on one hand, the different facets of a NC building block introduced directional anisotropy during selfassembly. On the other hand, the truncation from the cubic shape rendered particles as semi-spheres, especially when full surface ligand coverage presents. Such NCs have been reported to form either face-centered cubic (fcc) or body-centered cubic (bcc) SLs (SLs), depending on the balance between interactions among solvent and ligand molecules.17-18 To obtain deeper insights into the self-assembly process, single SCs, i.e. macroscopic grains that contain only one domain of PbS NC SL, were grown. As an effort to investigate the role of ligands, two growth methods were studied: evaporation and counterdiffusion. Details of these methods are provided in the experimental section. In either approach, the crystallization processes of NC SCs were very slow in order to facilitate long range order and growth of large SCs (Figure 1b-c).17

Figure 1. Electron microscopy characterizations of PbS NCs and SCs. (a) TEM image of the PbS NCs. Top-right inset shows NC size distribution and bottom-left inset presents a schematic illustration of one NC that possesses six PbS{100} (gray) and eight PbS{111} (yellow) facets. (b) and (c): SEM images of SCs grown by evaporation and counter-diffusion methods respectively. Scale bars are 10 µm.

As illustrated by Figure 2a, in the evaporation approach, briefly, toluene solution of PbS NCs was intentionally mixed with excess oleic acid to prevent a ligand detachment from NC surfaces and to serve as a high boiling point anti-solvent as well. The mixture was left undisturbed for months while toluene evaporated at a very slow rate. During this process, the NCs nucleated and crystallized slowly. Large SCs were found by even bare eyes at the bottom of the glass vial upon complete removal of toluene through evaporation. As the other method investigated, the counter-diffusion method (Figure 2b) had been reported to produce SCs of different NC species such as Au, PbS, CdS and CdSe.4,27 In this study, toluene solution of the same NC concentration was first placed in a glass vial. Then the anti-solvent, isopropanol, was carefully added on the top to form a clear interface. The vial was then sealed and kept


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Chemistry of Materials still to allow the two phases to diffuse into each other very slowly. The NCs in the toluene phase was thus destabilized by the increasing concentration of anti-solvent and precipitated to form SCs on the inner wall and bottom of the vial. A detailed study of the counter-diffusion crystallization process of NCs is reported elsewhere.28

in three selected SL projections: SL[100], SL[111] and SL[011]. Here, the SL crystal was aligned by SL[110] along the ϕ axis with SL[100] defined as ϕ=0o. Corresponding highresolution SEM images of the SC facets and their Fourier transformed patterns consistently confirmed the indexing of fcc symmetry.

Figure 2. Two methods of PbS NC SC growth: (a) Slow evaporation with excess oleic acid ligand resulted in fcc SL and (b) Slow diffusion between NC solution and anti-solvent resulted in bcc SL.

The shape and size of the product SCs were first examined by SEM. As shown by Figure 1b and 1c, SCs grown by either method reached sizes up to tens of micrometers, large enough to be manipulated under an optical microscope. As a noticeable feature in both cases, the NCs self-assembled into faceted SC grains. However, while the SCs grown by counterdiffusion showed relatively flat and smooth surfaces with stepped terrain, the surface of evaporation-grown SC grains was rough without sharply cleaved crystallographic planes. Such difference can be explained by the excess oleic acid ligands in the evaporation case. It was also observed that at the end of growth, the SCs produced by counter-diffusion tend to anchor on the inner wall or bottom of the glass vial while those grown by evaporation could move freely in the remaining oleic acid, indicating heterogeneous and homogenous crystallization respectively. This could be a result of that the excess ligands occupied the glass surface and hindered its attraction to SC for continuous nucleation. Without the glass surface as a flat template, rough and less faceted grains were obtained in the evaporation case. As another contribution, excess ligands in the system blurred the difference between high and low-energy SC facets by their passivation on the surfaces. Thus a well-faceted grain exposing low-energy surfaces was not necessarily preferred. To unveil the 3D structures of these SCs, they were examined by a recently developed synchrotron-based x-ray scattering characterization approach namely supercrystallography.29 Briefly, in a typical measurement a specimen SC was positioned so that a high symmetry projection was aligned with the ϕ axis of the diffractometer. Then the SC was rotated around ϕ while small and wide angle x-ray scattering (SAXS and WAXS) patterns were collected simultaneously to probe the SL symmetry and atomic structures, respectively. With such omnidirectional scattering data, the structures at different length scales in the SCs can be determined precisely. To interpret the x-ray scattering data sets, SAXS and WAXS patterns were simulated from model lattices to compare with experimental results at different ϕ. Figure 3a-f show scattering patterns (SAXS and WAXS) and related simulations, as well as the orientational arrangement of corresponding nanoparticles,

Figure 3. Structural characterization of PbS NC SCs grown by the evaporation method in three high-symmetry SL projections: SL[100] (a, d, g), SL[111] (b, e, h) and SL[110] (c, f, i), including SAXS (top row), WAXS (middle row) patterns and high resolution SEM images (bottom row). SAXS patterns present the indexing of fcc SL with insects illustrating the lattice orientation. The complex spotty features of WAXS patterns are reconstructed in three colors by three co-existing NCs in different orientations. Insets of SEM images present the FFT of corresponding images. Scale bars in SEM images are 20 nm. The SAXS patterns of evaporation-grown SC were indexed into a face-centered cubic (fcc) SL, which has a lattice parameter of a = 15.1 nm. The nearest neighboring distance between face center and corner in fcc lattice is 10.7 nm and the surfacesurface distance is 4.0 nm by subtraction of NP size. Wideangle scatterings from the atomic lattice of PbS NC cores (Figure 3d-f) displayed distinguishable spotty feature of single crystal rather than the powder-like rings, indicating that the NCs displayed not only a translational fcc symmetry but also preferential orientation(s). By carefully analyzing the WAXS patterns, it is found that the complexity of the patterns cannot be explained by a single uniform orientation of the NC cores. For example, it is impossible for a single PbS cubic rock salt lattice to generate the eight PbS(200)WAXS peaks in the SL[100] projection (Figure 3d). Instead, three different NC orientations must coexist to produce such complex WAXS patterns. Looking along the SL[100] direction of the fcc SL, the cartoons in the bottom subset of Figure 3d illustrate the three NC orientations. For visual convenience, NCs were drawn as cubes rather than their actual truncated shape. The



PbS NC marked by the green box has its PbS[100] pointing to the reader and its PbS[110] directions parallel to SL[100]. The NCs marked by yellow and red point their PbS[110] towards the reader with a 90° different in-plain orientation. The simulated scattering peaks correspond to the three groups of NCs are overlaid on the WAXS patterns with matching colors and Miller indices. Quantitative analysis of peak intensities showed that the ratio of the particles in the green, red and yellow groups is approximately 2:1:1. Based on these results, a detailed fcc SL model was constructed. This fcc model can be viewed as piled up by two alternating types of SL{100} layers: one is made of green NCs only (top and bottom layers as in Figure 2a) and another contains equal amount of yellow and red NCs which are arranged in a symmetric pattern (middle layer). It maximized the energetically and sterically favorable PbS{111}-{111} contacts between nearest neighbor NCs. Detailed analysis that justified such an fcc model can be found in another recent publication.5 It’s worth pointing out that significant complexity is added to the SL structure when NC orientations are taken into account. Even though this sample was confirmed to be a single SC in terms of translational symmetry by the sharp peaks in the SAXS patterns, it cannot be concluded that there is only one domain of NC orientations throughout the SC. Actually a SC can contain multiple orientation-related sub-domains but still produces the single crystal SAXS patterns. Therefore a combination of SAXS and WAXS techniques is necessary to unambiguously define structures in SCs.

In the same fashion, SCs grown by the counter-diffusion method was also analyzed by supercrystallography. Instead, SAXS patterns revealed a bcc SL with the lattice parameter of a = 9.67 nm. The nearest neighboring distance between cubic center and corner in bcc lattice is 8.4 nm and the surfacesurface distance is 1.7 nm by subtraction of NP size. Figure 4a-c show selected SAXS patterns in the three high-symmetry projections: SL[100], SL[111] and SL[011]. It can be seen that the bcc SC had a better long-range translational order than the fcc as indicated by sharper and higher order SAXS peaks. The corresponding high-resolution SEM images and FFT patterns (Figure 4g-i) further confirmed the indexing reliability of a bcc SL. By indexing the WAXS patterns (Figure 4d-f) collected simultaneously, it was found that NCs in the bcc SL also displayed preferential orientation but with less complexity than the fcc case. All the NCs shared one single orientation in which their cubic atomic lattice coincided with the bcc SL. The nearest neighbor NCs make contacts between PbS{111} facets along the SL[111] direction. Such bcc SL with single uniform NC orientation had been observed in PbS and PbSe NC thin films.17-18

Figure 5. Schematics showing the close-packing planes of (a) fcc, SL{111} and (b) bcc, SL {110} SLs grown by evaporation and diffusion methods respectively. The blue shades represent spherical or octopod effective shapes of NCs.

Figure 4. Structural characterization of PbS NC SCs grown by the counter-diffusion method in three high-symmetry SL projections: SL[100] (a, d, g), SL[111] (b, e, h) and SL[011] (c, f, i), including SAXS (top row), WAXS (middle row) patterns and high resolution SEM images (bottom row). SAXS and WAXS patterns present the indexing of bcc SL and atomic structure of NCs respectively, with insets illustrating the corresponding orientations. Insets of SEM images present the FFT of corresponding images. Scale bars in SEM images are 10 nm.

To understand why the same NCs formed SCs of completely different structures, the interactions among ligand and solvent molecules were considered. Generally speaking it was the interplay between excess oleic acid and ligand-solvent interaction that tuned the structure in SCs. In the evaporation growth, as illustrated by Figure 5a, due to the presence of excess oleic acid molecules, the reversible detachment of surface ligands was minimized and the NC surface ligand coverage was always saturated. Because of the moderate size that is comparable to the length of OA molecule and the truncated-cubic shape of the PbS cores, the NCs were camouflaged by the ligands and thus interacted as spheres to form the closepacking fcc SL. The separation between nearest NCs in the fcc SL was found to be 3.9 nm, only slightly smaller than the thickness of two lamellar layers of pure OA molecules (4.0 nm).30 The large interparticle separation indicates little to no intercalation due to strong steric repulsion where full ligand coverage presented. That suggests the NC-NC interactions remain weak in the crystallization process, due to the energetically similar interactions between ligand-ligand and ligandsolvent interactions in well dispersed NCs in good solvent.31 Therefore the shape effect in determining the orientations of


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Chemistry of Materials NCs was not as pronounced as in the bcc case. The tendency to maximize rotational entropy was in balance with that to minimize the total free energy via the PbS{111}-{111} contacts, as reflected by the relatively powder-type WAXS patterns with a hint of smeared peaks (Fig. 3d-f). In contrast, larger NC (11.3 nm) which displayed stronger shape effect were reported to form similar fcc SL with stronger orientational order.5 In the counter-diffusion case, due to the abundance of antisolvent and the lack of replenishing OA, ligand detachment from NC surfaces could occur as preferential ligand loss from PbS{100} facets was expected due to lower binding energy 18,32 . The long duration of the diffusion process was sufficient to allow most of PbS{100} ligands to be removed and Pb{111} ligands partially lost. According to the FloryHuggins theory, the aliphatic OA chains prefer mixing with similar aliphatic chains rather than isopropanol or toluene molecules.33-34 On the other hand, the introduction of antisolvent also induces stronger ligand-ligand interactions comparing to ligand-solvent interactions. That suggests the formation of ligand bundle and stronger inter-particle interactions.35-37 Therefore the NCs in the diffusion growth presented a octopod effective shape with eight OA bundles on the eight PbS{111} facets hunting for another bundle in the solution (Figure 5b). When the NCs approached each other, their ligands intercalated completely in the PbS[111] directions and connecting NCs along SL[111] of the bcc SL. The full intercalation is evidenced by the small inter-particle separation of only 1.7 nm, nearly the length of only one OA molecule, between the nearest neighbors. In this case, the shape-driven enthalpy minimization via strong ligand-ligand van der Waals interaction dominated over rotational entropy to cause strong orientational ordering as represented by the sharp peaks in Figure 4d-f. In addition reduced ligand coverage resulted in a reduced solubility of the PbS NCs which further favored a bcc SL.16

H.F. conceived the idea. K.B. and R.L. performed the experiments and analyzed the data. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and Sandia’s Laboratory Directed Research & Development (LDRD) program. Research was carried out, in part, at the Center of Integrated Nanotechnology (CINT), a US Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

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4. Conclusion We show that large SCs of PbS NCs up to ~100 µm size were obtained. The structure of self-assembled NC superlattices can be tuned by excess oleic acid ligand molecules. Using two different growth methods, the same PbS NCs formed single SCs of two different superlattice structures: fcc superlattice with weak yet complex NC orientational order and a bcc superlattice with strong and uniform NC orientation. The translational order was mainly determined by effective shape of the NCs. With excess ligand, NCs appeared spherical and formed fcc SL. Without excess ligand, the octopod shape of NCs resulted in a bcc SL. The difference in orientational order was a result of the balance between ligand-ligand attraction and rotational entropy.

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AUTHOR INFORMATION Corresponding Author *Corresponding author: H.F ([email protected])

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Controlled Self-Assembly and Tuning of Large PbS Nanoparticle

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