Diamond Annual Review 2019/20

88 89 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Soft CondensedMatter Group Beamline I22 A potential newmethod of producing high-quality supercrystals Related publication: Lehmkühler F., Schroer M. A., MarkmannV., Frenzel L., Möller J., Lange H., Grübel G. and Schulz F. Kinetics of pressure-induced nanocrystal superlattice formation. Phys. Chem. Chem. Phys. 21 , 21349-21354 (2019). DOI:10.1039/c9cp04658e Publication keywords: Nanocrystal self-assembly; High pressure; Kinetics N anoparticle (NP) supercrystals can demonstrate tunable and collective properties that are different from that of their component parts, with potential applications in areas such as optics and electronics. Although the formation of high-quality supercrystals is usually a slow and complicated process, recent research by a team of German researchers has shown that applying pressure can induce gold nanoparticles to form supercrystals. They investigated the self-assembly of gold nanocrystals with a special polymer surface coating, in suspension after performing a rapid pressure-jump. Using time-resolved small-angle X-ray scattering (SAXS), they were able to follow the formation of supercrystals and understand the assembly process. They performed these experiments on beamline I22, which provides an integrated high-pressure setup for SAXS experiments of soft matter and nanoscale systems, at pressures of up four kbar. The specific solubility properties of the polymer coatingmean that pressure can lead to the formation of high-quality supercrystals directly in aqueous suspension. This formation process is homogeneous, takes placewithinmilliseconds and can be tuned by the type of pressure-jump. By demonstrating the pressure-induced crystallisation of nanocrystals and its kinetics, this research provides a potential new method of producing high-quality supercrystals. These supercrystals have potential applications in many technical fields, including electronics and optoelectronics. Highly ordered supercrystals built from metallic nanocrystals (NC) have potential applications in optics, electronics, and sensor platforms because they can exhibit collective properties different from their component parts. In particular, assemblies of gold nanocrystals (AuNC) show collective plasmonic properties that may be utilised for future optical metamaterials 1 . The most popular route to obtain such supercrystals is the self-organisation from nanocrystals ina liquidsolvent. Here,thenanocrystalsaredispersedthroughout the solvent and as the solvent is loss due to evaporation, the nanocrystals will self-organise into a supercrystal 1 . However, the self-assembly of high-quality supercrystals is a very slow process and may take up to several hours or even days. Recently, the formation of supercrystals from AuNC with a coating based on polyethylene (PEG) in salt solutions was observed at pressures up to 4 kbar. The PEG forms a shell around the AuNC keeping the AuNC happy in its water environment. While salt leads to a reduced solubility of PEG in water (so-called salting out), pressure in the kbar range was reported to compress the PEG shell and thus fosters attractive interaction between the nanocrystals 2,3 . Experiments that searched throughout different pressures and salt types and concentrations produced a map (phase diagram), Fig. 1, that showed AuNC supercrystal formation is very sensitive to the salt type and concentration 4 . Different salt types include the common sodium and potassium salts, but also rubidium salts. Most importantly, this pressure-induced crystallisation happened within a few seconds and thus promises to be a fast alternative to the standard assembly methods. Experiments at high pressure demand a special pressure-sustainable sample environment, and beamline I22 offers a dedicated high-pressure sample environment that uses small diamond windows to resist the pressure and allow X-rays through into the sample. I22 is a small-angle X-ray scattering (SAXS) beamline that specialises in studying materials from the smallest, nano-scale, to larger micro-scale. I22 has been used in previous experiments on PEGylated AuNC 2-4 studying the structure of the dispersion up to 4 kbar. Furthermore, because the I22 sample environment was designed for performing pressure jumpswithin1/1000 th ofasecond,SAXSexperimentscanbeperformeddetailing the nanoscale changes during formation of the supercrystals. Gold nanocrystals coated with a PEG called PEGMUA ( a -methoxypoly(ethylene glycol)-o- (11-mercaptoundecanoate)) and dispersed in an aqueous salt solution with rubidium chloride were used. This salt shows the lowest crystallisation pressure (Fig. 1) and thus allows a broader range of pressure to be explored during super- crystillisation. The samples were set on a pressure of 2.7 kbar, just below the crystallisation pressure. I22 is a giant X-ray camera, and by collecting a series of exposures after the pressure jump, changes at the nanoscale can be followed like a movie. More importantly, as the nanocrystals form a supercrystal, the SAXS movie frames will show a new intense spot called a Bragg reflection (Fig. 2). The data in Figure 2 shows onset and evolution of these Bragg reflections after the pressure jump, indicating a fast growth of AuNC supercrystals. The behavior of the AuNC after the pressure jumpwas studied quantitatively by extracting the brightness of the supercrystal Bragg reflection from each of the SAXS movie frames (Fig. 3). For all pressure jumps studied, the supercrystal Bragg reflection forms rapidly. The time scale of this process depends on the depth of the pressure jump, i.e. the larger the jump, the faster the growth. Characteristic timescales between few 10s of milliseconds and 10 seconds have been found. In contrast to this crystallisation speed, the melting of the AuNC supercrystals studied by sudden pressure releases has approximately been one order of magnitude slower. Furthermore, the final brightness values of each supercrystal Bragg reflection as well as the position on the movie frame and width of the Bragg reflection itself shows a strong dependence on the pressure jump. In particular, brightness and position are larger and the width smaller for larger pressure jumps. This indicates first an increasing crystalline quality at larger pressure jumps. Second, fine-tuning of next-neighbour distances in the superlattice becomes possible within fractions of the nanocrystal size. The results demonstrate that pressure variation on nanocrystal systems may help to study and understand phase transitions beyond temperature variations and allow for extremely fast formation of plasmonic supercrystals. Recently, high-pressure SAXS has been extended to supercrystal formation from non- isotropic particles 5 . Fixation of the nanocrystal assembly will enable the preparation of high- quality superstructure with desired properties from concentrated nanocrystal dispersions by ‘freezing’ the assembly at a certain selection of the width of pressure jump and time. References: 1. Boles M. A. et al. Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials. Chem. Rev. 116 , 11220-11289 (2016). DOI: 10.1021/acs.chemrev.6b00196 2. Schroer M. A. et al. Tuning the Interaction of Nanoparticles from Repulsive to Attractive by Pressure. J. Phys. Chem. C 120 , 19856-19861 (2016). DOI: 10.1021/acs.jpcc.6b06847 3. Schulz F. et al. Structure and Stability of PEG- and Mixed PEG-Layer- Coated Nanoparticles at High Particle Concentrations Studied In Situ by Small-Angle X-Ray Scattering. Part. Part. Syst. Char . 35 , 1700319 (2018). DOI: 10.1002/ppsc.201700319 4. Schroer M. A. et al. Pressure-Stimulated Supercrystals Formation in Nanoparticle Suspensions. Phys. Chem. Lett. 9 , 4720-4724 (2018). DOI: 10.1021/acs.jpclett.8b02145 5. Schroer M. A. et al. Supercrystal Formation of Gold Nanorods by High Pressure Stimulation. J. Phys. Chem. C 123 , 29994-30000 (2019). DOI: 10.1021/acs.jpcc.9b08173 Funding acknowledgement: This work is supported by the Cluster of Excellence ‘Advanced Imaging of Matter’of the Deutsche Forschungsgemeinschaft (DFG) – EXC 2056 – project ID 390715994. Further funding is acknowledged by the DFG (grant no. SCHU 3019/2-1) and Röntgen-Ångström cluster project TT-SAS (Bundesministerium für Bildung und Forschung project No. 05K16YEA) Corresponding authors: Dr Felix Lehmkühler, Deutsches Elektronen-Synchrotron DESY, felix. lehmkuehler@desy.de and Dr Martin A. Schroer, European Molecular Biology Laboratory EMBL c/o DESY, martin.schroer@embl-hamburg.de Figure 1: Schematic pressure-salt concentration phase diagram of PEGylated AuNC at the studied concentration of about 1 vol. %. The initial pressure used for the pressure jumps is marked by a blue circle at 2 mol/l RbCl, the final pressures are given by green circles. Figure 2: SAXS patterns. (a) Parts of 2D patterns of the dispersion before (initial) and after (final) the pressure jump. The initial data resembles the form factor of spherical particles while the final data shows a rich collection of Bragg reflections matching close-packed supercrystal structure. (b) SAXS curves before (initial) and at different times after the pressure jump as indicated. Figure 3: Structure factor peak value (i.e. brightness of the Bragg reflections) as function of time. The dashed line marks the pressure jump at 0 s. Solid lines represent an exponential growth model.

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