Role of absorbing nanocrystal cores in soft photonic crystals: A

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Role of absorbing nanocrystal cores in soft photonic crystals: A spectroscopy and SANS study Astrid Rauh, Nico Carl, Ralf Schweins, and Matthias Karg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01595 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Role of absorbing nanocrystal cores in soft photonic crystals: A spectroscopy and SANS study Astrid Rauh1,2, Nico Carl2,3,4, Ralf Schweins4, Matthias Karg1,2* 1

Physical Chemistry I, Heinrich-Heine-University, 40225 Düsseldorf, Germany 2

3

Physical Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany

Department of Chemistry, University of Paderborn, 33098 Paderborn, Germany

4

Large Scale Structures Group, Institut Laue-Langevin, 38402 Grenoble, France

E-Mail: [email protected], Phone: +49 (0)211 81 12400, Fax: +49 (0)211 81 12179, Homepage: www.karg.hhu.de

Abstract Periodic superstructures of plasmonic nanoparticles have attracted significant interest since they can support coupled plasmonic modes making them interesting for plasmonic lasing, metamaterials and as light management structures in thin-film opto-electronic devices. We have recently shown that noble metal-hydrogel core-shell colloids allow for the fabrication of highly ordered 2-dimensional plasmonic lattices that show surface lattice resonances as the result of plasmonic/diffractive coupling [Adv. Optical Mater., DOI: 10.1002/adom.201600971]. In the present work we study the photonic properties and structure of 3-dimensional, crystalline superstructures of gold-hydrogel core-shell colloids and their pitted counterparts without gold cores. We use far-field extinction spectroscopy to investigate the optical response of these superstructures. Narrow Bragg peaks are measured, independent on the presence or absence of the gold cores. All crystals show a significant reduction in the low wavelength scattering. This leads to a significant enhancement of the plasmonic properties of the samples prepared from gold nanoparticle containing core-shell colloids. Plasmonic/diffractive coupling is not evident which we mostly attribute to the relatively small size of the gold cores limiting the effective coupling strength. Small angle neutron scattering is applied to study the crystal structure. Bragg peaks of several orders clearly assignable to an fcc arrangement of the particles are observed for all crystalline samples in a broad range of volume fractions. Our results indicate that the nanocrystal cores do not influence the overall crystallization behavior nor the crystal structure. 1 ACS Paragon Plus Environment

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These are important prerequisites for future studies on photonic materials built from core-shell particles, in particular, the development of new photonic materials from plasmonic nanocrystals.

Keywords soft colloidal crystals; core-shell nanoparticles; small angle neutron scattering; fcc lattice; localized surface plasmon resonance

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Introduction Plasmonic superlattices show fascinating collective optical properties that make them interesting for new material development in the fields of metamaterials1-2, optoelectronic devices3 and plasmonic lasing.4-7 Fundamentally relevant for the optical response of the superlattice is the single particle behavior that is governed by the localized surface plasmon resonance (LSPR). The LSPR of a plasmonic nanoparticle emerges due to collective electron oscillations driven by incident light. In a superlattice single particle LSPRs can interact leading to coupled resonances with a resonance frequency and linewidth that depend on the average inter-particle distance and the superlattice structure. For example, when arranged in close proximity, near-field coupling occurs if the active near-field zones of neighboring plasmonic nanostructures overlap resulting in coupled modes.8-9 In contrast resonance coupling at much larger inter-particle distances than required for pronounced near-field coupling can occur in arrays of plasmonic nanostructures with a periodic arrangement. Then diffractive and localized plasmonic resonances can interact.10 This interaction results in new resonance modes, so-called surface lattice resonances (SLRs) or lattice plasmons (LPs), with typically very narrow resonance widths as compared to the modes developed by near-field coupling.11 Hence, SLRs have longer plasmon lifetimes which makes them very interesting for nanophotonic applications, e.g. in lasing. The emergence of SLRs requires inter-particle distances on the order of the single particle LSPR. For the most commonly studied plasmonic nanostructures this means that inter-particle distances in the visible wavelength range are required. In case of 2dimensional (2D) arrays, this requirement has been met by using lithographic approaches12-13 and very recently by colloidal self-assembly.14 In the latter approach thick hydrogel shells were employed as semitransparent spacer material that controls the inter-particle spacing between plasmonic nanoparticle cores in 2D monolayers. While resonance coupling phenomena have been widely studied in the past using dimers, trimers and small clusters as well as larger 2D arrays of plasmonic particles, only a few works deal with 3-dimensional (3D) plasmonic superstructures. This is related to the challenging fabrication of defined superstructures with suitable and highly regular inter-particle distances and ideally macroscopic overall dimensions allowing the characterization by far-field spectroscopy.15-17 In this work we prepared macroscopically sized colloidal superlattices with lattice constants approaching the visible wavelength range. Soft hydrogel particles (microgels) were used as colloidal building blocks because of their capability to crystallize within a very broad range of 3 ACS Paragon Plus Environment

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volume fractions even exceeding the hard sphere limit of  = 0.74.18-20 Thus, the tuning of the lattice constant becomes possible over a broad range by using a single colloidal system. Furthermore, the introduction of plasmonic nanocrystal cores to these microgels, i.e., in the form of core-shell particles, offered access to plasmonic superlattices that are constituted of non-close packed, highly periodic arrangements of plasmonic nanocrystals. We focus on colloidal core-shell (CS) systems with gold cores of approximately 15 nm in diameter and chemically cross-linked hydrogel shells composed of poly-N-isopropylacrylamide (PNIPAM) with two different cross-linker contents determining the hydrogel softness. As non-plasmonic references we prepared pitted colloids by dissolving the gold cores without affecting the overall particle size, PNIPAM shell structure and the microgel crystallization behavior. This way we could systematically investigate the influence of the gold cores and the shell softness on the crystal structure and optical response of the superlattices. We could show that the optical spectra measured for crystals of the CS particles result from a superposition of the Bragg diffraction of the overall particle lattice and the absorption of the nanocrystal cores. Resonance coupling however was not observed. Analysis of SANS data from crystalline samples reveals fcc lattices with center-to-center nearest neighbor distances on the order of 250 nm agreeing well with the analysis of the Bragg diffraction measured by UV-vis spectroscopy of the same samples. Thanks to the powerful combination of different spectroscopic and microscopic as well as scattering techniques we could derive a clear correlation between the superlattice structure and the optical response.

Theoretical Background SANS profiles of the dilute samples. It has been shown that the form factor of polydisperse hard spheres is not leading to a successful description of the scattering profiles of microgel particles in the swollen state, especially in the low q range.21-22 This is related to the gradientlike cross-linking distribution with a decreasing cross-linking degree from the interior to the outside of the particles resulting from the faster consumption of the BIS molecules as compared to NIPAM during the polymerization.23-24 This inhomogeneous network structure deviates from the simple box profile valid for hard spheres. Stieger et al. developed a model accounting for this inhomogeneous network structure using a radial density profile with a gradually decaying segment density at the surface.


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Recently, Dulle et al. showed that SANS profiles recorded from core-shell microgels that were prepared by sequential semi-batch polymerizations could be well described by a model comprising of an inner homogeneous segment density (box profile) and an exponentially decaying shell at the surface. Due to the polymerization route employed, their core-shell particles contained a large inner volume of constant cross-linking and a lower cross-linked outer region with a thickness of only 15 - 40 nm.25 In the present work we show that this model can also be successfully applied to describe the SANS profiles of microgels with different crosslinker contents prepared by classical batch polymerizations, i.e., microgels that have a more inhomogeneous internal network structure. The overall scattered intensity I(q) for our dilute systems derived using this model can be described as follows: 𝐼(𝑞) = 𝑁 ∙ 𝐴2𝑒𝑥𝑝 (𝑞) + 𝐼𝑑𝑦𝑛 (𝑞) + 𝐼𝑏𝑐𝑘𝑔

(1)

In eq 1 N gives the number density of the particles, Aexp(q) provides the scattering amplitude which accounts for the radially decaying scattering length density profile, Idyn describes the dynamic component which is ascribed to internal fluctuations of the PNIPAM network of the particles and Ibckg considers incoherent and additional background contributions. Further descriptions of these contributions as well as a detailed derivation of Aexp(q) can be found in the Supporting Information.

SANS patterns of crystalline samples. At high concentrations the inter-particle distances in a colloidal dispersion are significantly reduced and particle-particle interactions become important. Hence - in contrast to the dilute state - the spatial distribution of particles contributes to the scattered intensity and S(q)  1 is no longer valid. In particular for crystalline samples pronounced Bragg diffraction from lattice planes characterized by the Miller indices h, k and l is observed. The position of the first Bragg peak, 𝑞𝑚𝑎𝑥 , allows the calculation of the average distance between lattice planes 𝑑ℎ𝑘𝑙 :26

𝑑ℎ𝑘𝑙 = 𝑞

2𝜋

(2)

𝑚𝑎𝑥

For crystals with cubic symmetry dhkl is directly related to the lattice constant a: 5 ACS Paragon Plus Environment

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𝑎 = 𝑑ℎ𝑘𝑙 √ℎ2 + 𝑘 2 + 𝑙²

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(3)

For a face-centered-cubic (fcc) crystal with the (111) plane oriented parallel to the cell walls, i.e., perpendicular to the incoming neutron beam, 𝑞𝑚𝑎𝑥 allows the calculation of d111 and eq 3 simplifies to 𝑎 = 𝑑111 √3. In order to clearly assign a crystal structure to the measured samples, e.g., fcc, the positions and arrangement of Bragg peaks in the recorded 2D scattering patterns can be used. Furthermore the intensities and azimuthal (𝛿𝐺𝜓 ) and longitudinal peak width (𝛿𝐺𝑞 ) of the Bragg peaks allow to draw conclusions on the domain sizes and coherence lengths. Finite peak widths result from limited coherence as a result of the instrumental resolution (wavelength distribution, angular spread, sample aperture, finite detector resolution) and lattice imperfections (lattice defects, finite domain size). The longitudinal domain size, Dl, and the azimuthal domain size, D are connected to the respective peak width by:27 4

𝐷𝑙 = 𝛿

(4)

𝐺𝑞

𝐷𝜓 = 𝛿

4

(5)

𝐺𝜓

Furthermore, deviations of the particle positions from ideal lattice points can be accounted by the Debye-Waller factor G(q) that we consider by a Gaussian lattice point distribution function with a relative mean square displacement 𝜎 2 .27

Bragg diffraction measured by UV-vis absorbance spectroscopy. When the inter-plane distance of crystalline samples lies in the range of a few hundred nm, Bragg diffraction can be studied by UV-vis spectroscopy. The position of the Bragg peak measured in transmission, for example, directly depends on dhkl: 2 𝑚 ∙ 𝜆𝑑𝑖𝑓𝑓 = 2 ∙ 𝑑ℎ𝑘𝑙 ∙ √𝑛𝑐𝑟𝑦𝑠𝑡𝑎𝑙 − sin² 𝜃

(6)

Here m is the diffraction order (in our case m = 1), 𝜃 defines the angle between the incident beam and the normal to the (111) plane and ncrystal describes the average refractive index of the crystalline samples. 6 ACS Paragon Plus Environment

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In our case the (111) plane is oriented parallel to the walls of the sample cell and 𝜃 = 0°. Equation 6 simplifies to: 𝜆𝑑𝑖𝑓𝑓 = 2 ∙ 𝑑ℎ𝑘𝑙 ∙ 𝑛𝑐𝑟𝑦𝑠𝑡𝑎𝑙

(7)

Materials and Methods Chemicals. Gold(III)chloride trihydrate (HAuCl4; Sigma-Aldrich, ≥ 99.9 %), sodium citrate dihydrate (Sigma-Aldrich, ≥ 99 %), butenylamine hydrochloride (B-en-A; Sigma-Aldrich, 97 %), sodium dodecyl sulfate (SDS; Merck), N-Isopropylacrylamide (NIPAM; SigmaAldrich, 97 %), N,N‘-methylenebisacrylamide (BIS; Sigma-Aldrich, 99 %), potassium peroxodisulfate (PPS; Fluka, ≥ 99 %), hydrochloric acid (HCl; Bernd Kraft, 37 %), nitric acid (HNO3; Merck, 65 %) and deuterium oxide (D2O; Sigma-Aldrich, 99.9 % atom D) were used as received. Water was purified with a Milli-Q system (Millipore) resulting in a final resistivity of 18 MΩcm.

Synthesis of the core-shell nanoparticles. Gold-PNIPAM core-shell (CS) nanoparticles with two different nominal cross-linking densities (10 and 25 mol% referred to the monomer NIPAM) were synthesized by seeded precipitation polymerization following a previously reported method.28 In the following these particles are referred to as CS10 and CS25, where the number indicates the nominal cross-linker concentrations. For both syntheses 1.1697 g NIPAM and the respective amount of BIS were dissolved in 600 mL of water. The CS10 particles were prepared using 10 mol% BIS (0.231 g) while for the CS25 particles 25 mol% BIS (0.578 g) were added. 6.7 mL of a seed stock solution with an elemental gold concentration cAu(0) of 0.02525 mol/L were added to each reaction mixture prior to the initiation with 12 mg of PPS dissolved in 1 mL of water.

Dissolution of the gold nanocrystal cores. For the core dissolution the respective particle dispersion was diluted to a 0.25 wt% dispersion. 120 drops of aqua regia (7.5 mL H2O, 12 mL HCl, 4 mL HNO3) were added to 20 mL of a 0.25 wt% dispersion. The dissolution of the gold nanoparticles could be observed by naked eye, since the rose color that can be assigned to the plasmon resonance of the gold nanoparticle cores disappeared within 15 min. After shaking for 2 h, the dispersions were purified three times by centrifugation (8422 rcf, 1.5 h) and redispersed in water to return to a neutral pH. Further centrifugation (8422 rcf, 1.5 h) was performed 7 ACS Paragon Plus Environment

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whereas the particles were now redispersed in D2O. This procedure was repeated at least twice. In this manuscript particles whose gold nanoparticle core was dissolved are termed as CSd.

Preparation of crystalline samples. The dispersion of CS and CSd particles were concentrated in a similar manner using centrifugation in order to prepare crystalline samples of the same number density, i.e., volume fraction. This way CS and CSd dispersions between 6 and 11 wt% were prepared. Crystalline samples from these dispersions for analysis by spectroscopy and scattering were prepared in the following way: 40 µL of a dispersion were filled in a quartz glass cuvette with a pathlength of 100 µm. The sample was annealed at a temperature above the VPTT causing the particles to shrink. This shrinking significantly lowered the effective volume fraction and a transition to the fluid phase was observed for all concentrations under investigation. Subsequently, the sample was allowed to cool slowly to room temperature, so that the particles swelled again due to water uptake. The resulting rise in volume fraction led to recrystallization of the samples.

Transmission electron microscopy. Transmission electron microscopy was conducted with a Zeiss CEM 902 transmission electron microscope in bright-field mode. The respective instrument was operated at an acceleration voltage of 80 kV. The sample preparation proceeded as follows: A crystallizing dish was filled with a 0.05 mM aqueous solution of SDS. Subsequently, a concentrated ethanolic dispersion of the respective particles was assembled at the air-water interface. Then, the freely floating monolayer was transferred to a carbon-coated copper grid (200 mesh, Electron Microscopy Sciences). For the sample preparation of the gold nanoparticles, diluted aqueous dispersions were drop-casted on the carbon-coated copper grids. Before investigation of the particle-loaden grids, they were allowed to dry for several hours at room temperature.

Dynamic and static light scattering. Angular dependent DLS and SLS measurements were performed with a standard goniometer setup (3D-LS) from LS instruments operated in 2D mode at 25.6°C. A HeNe laser with a wavelength of 632.8 nm and a maximum constant output power of 35 mW was used as light source. Two avalanche photodetectors in pseudocross-correlation mode detected the scattered light. The sample temperature was monitored by a Pt100 thermoelement and adjusted using a heat controlled decalin bath which was connected with a Julabo CF31 thermostat. In addition, the decalin bath acted as refractive index matching bath. Angular dependent SLS measurements were conducted between 20 and 150° in intervals of 5°. 8 ACS Paragon Plus Environment

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At each angle six measurements with an acquisition time of 30 s were conducted. In addition, angular dependent DLS measurements in the range from 20-150° in intervals of 10° were performed. Three intensity-time autocorrelation functions per angle with an acquisition time of 60 s were recorded. Temperature dependent measurements were performed using a standard goniometer setup by ALV (Langen, Germany) at a constant scattering angle of 60° between 20-50°C in steps of 2°C or smaller. The light source was a HeNe laser with a wavelength of 632.8 nm and a maximum constant output power of 35 mW. A temperature/matching toluene bath was used to adjust the temperature. Therefore, a Pt100 thermocouple was placed in the bath next to the sample allowing a temperature stability of ± 100 mK. The temperature was controlled using a Haake C25 thermostat with a Haake F6 control unit. Three autocorrelation functions were measured per temperature with an acquisition time of 60 s. Prior to each measurement the sample was equilibrated for at least 20 min. The analysis of the intensity-time autocorrelation functions obtained from angular und temperature dependent DLS measurements was conducted by inverse Laplace transformations (ILT) using the software After ALV version 1.0d by Dullware. The obtained mean relaxation rates allowed to calculate the diffusion coefficient DT for translational diffusion. Using the Stokes-Einstein equation (Rh=kBT/6πηDT with Rh being the hydrodynamic radius, kB indicating the Boltzmann constant, η denoting the viscosity of the solvent) Rh can be determined. The deviation of each radius at a respective temperature was calculated using error propagation of the standard deviation calculated for the relaxation rate. Generally the sample preparation occurred as follows: Freeze-dried material of a respective sample was dissolved in D2O to prepare the samples. Highly diluted samples were investigated to avoid multiple scattering. The samples were filtered using a PTFE filter (pore size: 5 µm) and filled into dust-free, cylindrical quartz cells.

UV-Vis absorbance spectroscopy. Spectra of dilute samples (0.03 wt%) were measured in quartz cells (Hellma Analytics, Germany) with a pathlength of 1 cm by an Agilent 8453 spectrophotometer or a Cary 5000 UV–vis–NIR spectrophotometer (Agilent Technologies). UV-vis absorbance spectra of crystalline samples in quartz glass cells with 0.1 mm lightpath (Hellma Analytics, Germany) were recorded by a Jasco V-630 (Science building, ILL Grenoble). All UV-vis measurements were conducted in a spectral range from 190 to 1100 nm at room temperature.


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Zeta(ζ)-potential determination. Electrophoretic mobility (μe) measurements were performed with a Zetasizer Nano ZS from Malvern Instruments. The instrument is equipped with temperature-jacket for the cuvette, a 4 mW HeNe laser (λ = 633 nm) and a detector at a scattering angle of 173°. Dilute aqueous dispersions were measured six times at 25°C without the addition of salt at neutral pH. Prior to the measurement the sample was equilibrated for two minutes. Using the Smoluchowski equation (ζ = μeη/ε0ε with η denoting the viscosity of the solvent and ε0ε representing the permittivity of the dispersion) the ζ-potential was calculated.

Small angle neutron scattering. SANS measurements were performed at the Institut LaueLangevin (ILL) in Grenoble (France) using the D11 instrument. The neutron wavelength was either 0.6 nm for measurements at sample-to-detector distances of 1.2, 8 and 28 m or 1 nm for a distance of 39 m. A 3He CERCA gas multi-detector with an area of 96 x 96 cm² and a pixel size of 0.75 x 0.75 cm² was used to collect 2D scattering patterns. For the dilute samples (0.5 wt%) data were collected at sample-to-detector distances of 1.2, 8, 28 and 39 m to cover a broad q range. The acquisition times were increased with increasing sample-to-detector distance from at least 480 s, 600 s, 900 s and 1200 s, respectively. The dilute dispersions were measured in rectangular quartz cells (Hellma Analytics Germany, light path 1 mm). The 2D detector images were radially averaged due to their isotropic character. The spectra recorded at different sampleto-detector distances were merged using the software Large Array Manipulation Program (LAMP) provided by the ILL. In addition, this software was used to correct the collected data for D2O and empty cell scattering of the sample. Subsequently, the merged scattering profiles were analyzed by the software SASfit by Kohlbrecher.29 For the concentrated, crystalline samples measurements at sample-to-detector distances of 28 and 39 m were conducted. These samples were measured in rectangular quartz cells with a pathlength of 100 µm (Hellma Analytics Germany). At a sample-to-detector distance of 28 m scattering profiles were recorded for at least 900 s and at 39 m for at least 1200 s. The lattice constant was determined from radially averaged scattering curves using the maximum of the first structure factor peak. The structure factor profiles were determined by division of the radially averaged, merged profiles measured at 28 and 39 m of the crystalline samples by the profiles of the respective dilute samples. Prior to the division, the profile of the dilute sample was shifted vertically by multiplication with a q-independent prefactor in order to have intensity overlap in the high qrange with the profile of the respective concentrated sample. 2D scattering patterns were analyzed using the software Scatter by Förster and Apostol.30


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Results and Discussion Particle characterization in the dilute regime. CS nanoparticles with absorbing nanocrystal cores (Au) and chemically cross-linked PNIPAM shells were synthesized following a previously reported seeded precipitation polymerization.28 TEM images of the respective bare gold nanoparticles prior to the encapsulation into the PNIPAM shells are shown in Figure S.1 in the Supporting Information. The polycrystalline gold nanoparticles have a spherical shape with an average diameter of 15.2 ± 2.2 nm. Figure 1A and B show representative TEM images of the CS10 and CS25 particles obtained from seeded precipitation polymerizations using two different nominal cross-linker contents (10 and 25 mol%). The CS structure can be clearly identified due to the difference in electron density of the gold cores with respect to the polymer shell. Analysis of several TEM images recorded from different positions on the TEM grids reveals that 99.6 % (CS10) and 98.3 % (CS25) of the CS particles contain single gold nanocrystal cores. Furthermore, comparing the TEM images in Figure 1A and B reveals two important differences: 1) The PNIPAM shell appears much darker for CS25 (B) and is hence more easily visible on the TEM grid as compared to the CS10 sample (A). This is related to the higher degree of cross-linking that enhances the electron contrast and leads to a more rigid shell structure that cannot collapse to the same extent on the TEM grids than for the lower crosslinked sample. 2) The gold cores appear much more off-center in the CS10 particles. Although this might result from sample drying and the high vacuum in the TEM chamber strongly collapsing the PNIPAM shells, we believe that this is also related to the larger mesh sizes and size distributions not entrapping the gold cores right in the center of the CS particles. This also means that the cores are not covalently bound to the shell and, upon swelling of the PNIPAM shells, can move to some extent until they get trapped in a small enough mesh of the shell. This is in agreement with our previous study on seeded precipitation polymerization for the synthesis of Au-PNIPAM CS particles where we have shown that gold cores are encapsulated by PNIPAM precipitating on the cores under bad solvent conditions during the polymerization.31 The only requirement for a successful encapsulation is a slightly hydrophobic surface functionalization of the gold cores, here realized by B-en-A. A linker that covalently bridges the shell with the cores is not required. A rather weak coordination between B-en-A and the polymer network is expected. In contrast the gold cores of the CS25 sample appear much more centered in the PNIPAM shell indicating that the shell network is stronger cross-linked in the central particle region. This entraps the gold cores more centrally and inhibits core diffusion within the PNIPAM shell after swelling. 11 ACS Paragon Plus Environment

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Subsequently, half of the obtained core-shell material from each synthesis was treated with aqua regia to dissolve the gold nanoparticle cores. This way we could prepare pitted microgels (CSd10 and CSd25) without absorbing nanocrystal cores. In Figure 1C and D TEM images of these particles are shown. The images clearly show that the gold cores were successfully dissolved. This is even better visible in the magnified images of single microgel particles in the upper right corner of C and D. One should note that contrast and brightness for images C and D were adjusted more drastically than for images A and B. Again the higher cross-linked particles (CSd25) appear much more clearly (darker) in the TEM images due to the higher electron contrast of the more rigid PNIPAM shells. All TEM images in Figure 1 show that each type of particle crystallizes into 2D lattices with regular inter-particle spacings and domains of hexagonal order. This underlines the low degree of polydispersity. After the sample preparation and in particular under the high vacuum condition of the TEM, the PNIPAM shells collapse. Hence, the TEM images show non-close packed lattices.

Figure 1: TEM images of CS10 (A), CS25 (B), CSd10 (C) and CSd25 (D). In the upper right corners in C and D magnified images of single particles after the core dissolution are shown (scale bar: 100 nm).


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In order to study the potential influence of the core dissolution on the hydrogel network structure and the overall particle size, we analyzed the temperature dependent swelling behavior of the particles by DLS. The temperature dependent evolutions of the hydrodynamic radii (Figure S.2 A/B) and the de-swelling ratios α (Figure S.2 C/D) are shown in the Supporting Information. All samples show a clear volume phase transition behavior with larger particle radii at low temperatures, i.e., the particles are highly swollen with D2O, and decreasing radii with increasing temperature, i.e., the particles shrink due to release of D2O. In agreement with their softer character, lowly cross-linked samples (A) show a larger swelling capacity as compared to the more rigid, highly cross-linked particles (B). Comparing the CS and the CSd particles with the same cross-linking density, the temperature dependent evolution of Rh and α are very similar indicating that the network morphology is not affected by the relatively harsh etching conditions (aqua regia) used for the core dissolution. This is an important prerequisite for a comparison of the structural and optical properties of crystalline samples discussed later on. In the following we will focus on experiments that were performed in the swollen state of the PNIPAM shells (approximately 25°C). Angular dependent light scattering was used to determine the radius of gyration Rg from SLS at 25.6°C. Figure S.3 in the Supporting Information shows Guinier plots of the SLS data for all samples. A linear dependence of the data throughout the whole q-range is observed. The slopes of the linear fits to the data allow the determination of Rg (slope = - Rg2/3). Table S.1 in the Supporting Information lists the obtained values of Rg and the corresponding hydrodynamic radii Rh obtained from angular dependent DLS measurements at 25.6°C (see Figure S.4 in the Supporting Information). Both radii, Rg and Rh, are slightly smaller for the higher cross-linked samples (CS25 and CSd25). This indicates a more rigid internal network structure of the latter particles since the polymerizations for the differently cross-linked particle batches were performed with the same total concentration of the monomer NIPAM. Comparing the CS and the corresponding CSd particles for each cross-linker density, the values for Rg are identical within the experimental error. On average values of Rg of 83 nm and 79 nm and Rh of 123 nm and 114 nm were determined for the samples with low and high cross-linking, respectively. The ratios Rg/Rh listed in Table S.1 are very similar for all samples (0.67-0.70) with only slightly larger values for the higher cross-linked samples. Overall the values for our particles are smaller than the expected value of √3/5 = 0.775 for hard spheres. We attribute this to the inhomogeneous polymer network structure of the PNIPAM shells. The shell displays a decreasing cross-linker density 13 ACS Paragon Plus Environment

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from the center to the outside. This influences the hydrodynamic radius to a larger extent than the radius of gyration in agreement with previous studies in the literature.25 Table S.2 in the Supporting Information lists results from electrophoretic mobility (μe) measurements showing similar values for all samples (μe: 2.8-3.2 µmcm/Vs). This indicates that also the surface charge was not affected by the core dissolution. Hence, due to the similar charge and hydrodynamic particle size for each cross-linker density, we expect comparable interaction potentials for the particles with and without gold nanocrystal cores. Light scattering does not allow to get insights into the internal colloid structure. Furthermore, due to the relatively small size of all particles in this study, SLS does not resolve the form factor P(q) of the particles but only covers the Guinier region. Because of the much smaller wavelength accessible in neutron scattering experiments, SANS is well suited for the latter tasks. We measured SANS profiles of all particles in dilute dispersion (S(q)  1) in D2O. Figure 2 shows the radially-averaged scattering profiles measured at 25.6°C. All scattering profiles show the Guinier plateau at low q, i.e., the scattering intensities reach constant values. In the medium q-range all samples show two pronounced form factor minima while again plateaus are reached at high q. Already at first glance it appears that the evolution of all SANS profiles is very similar. In particular the direct comparison of the two scattering profiles (CS and CS d) for each cross-linker content reveals a perfect match between the profiles without any noticeable difference. Analysis of the data was performed with the software SASfit using the values of Rh from DLS as starting parameters. All profiles could be successfully fitted (red lines in Figure 2) by eq 1 taking into account for Gaussian polydispersity of the particle size. Figure S.5 in the Supporting Information shows the two main contributions included in the model of eq 1, the form factor dominating the low to medium q-range and the Ornstein-Zernike (OZ) contribution at medium to high q, separately. The residuals resulting from the fit with the full model (eq 1) included in Figure S.5 highlight the very good agreement between the fit and the measured SANS data. It is important to note that the gold nanocrystal cores of the CS10 and CS25 samples were not considered in our fitting model. This is reasonable since the volume fractions of the gold cores in the CS particles are less than 0.2 %. To illustrate that the scattering of the nanocrystal cores does indeed not contribute significantly to the measured scattering profiles, we simulated the theoretical scattering contribution in Figure S.6 shown in the Supporting Information. The intensities of the theoretical core contribution are much smaller than the measured intensities in the low to medium q-range, where the form factor and OrnsteinZernike contribution are analyzed. Table S.3 in the Supporting Information summarizes the 14 ACS Paragon Plus Environment

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parameters to fit the SANS profiles using eq 1. First of all, the correlation lengths ξ are very similar comparing the CS and CSd particles for each cross-linker density. As expected the values of ξ are smaller for the higher cross-linked particles (1.3 nm as compared to 1.9 nm). This points to a more rigid network structure. Normalizing the amplitude of the OZ contribution IL(0) to the particle number density N reveals that the dynamic fluctuations are less pronounced for the higher cross-linked samples. The values of IL(0)/N are approximately by a factor of four smaller for the CS25 and CSd25 particles (0.25 cm²) as compared to the CS10 and CSd10 particles (1.0 cm²). This agrees with previous scattering studies of similar particles.32 The form factor contribution of the fits to the data provides the radii of the homogeneous inner shell region, Rhom (CS10/CSd10:  39 nm; CS25/CSd25:  49 nm), and the thickness of the exponentially decaying outer shell region, R (CS10/CSd10:  65 nm; CS25/CSd25:  51 nm). Again the values for the CS and the corresponding CSd particles for each cross-linker density do not differ significantly manifesting that the presence and absence of the gold cores as well as the core dissolution by aqua regia did not change the internal shell morphology. Furthermore, we find that the inner homogeneous part is larger and the volume element with an exponentially decaying shell is smaller for the higher cross-linked samples compared to the samples with less cross-linking. This allows the conclusion that the overall PNIPAM network structure is more homogeneous for the higher cross-linked particles. This is in agreement with the findings of Varga et al. who investigated the structure of PNIPAM microgels by DLS and SLS.33 They found that lowly cross-linked microgels can be well described using a core-shell morphology with a shell of lower density. Conversely, highly cross-linked microgels displayed a more homogeneous structure. To summarize, the results from SLS and DLS as well as SANS of the dilute samples reveal that the network structure and swelling behavior of the PNIPAM shells are not affected by the core dissolution. In addition, the scattering data confirm that the higher cross-linked particles are more rigid and possess a more homogeneous network structure.


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Figure 2: SANS profiles measured at 25.6°C (symbols) and respective fits (solid red line) obtained from SASfit analysis using eq 1. The error bars of the data points are in the range of the symbol size. For the sake of clarity the profiles of the CS particles were shifted vertically by multiplication of the intensities with a factor of 10.

Before turning to the photonic properties of crystalline samples at high volume fraction, we will first discuss the optical appearance of the particles in the dilute state. The photographs of the dilute samples (0.03 wt%) shown in Figure S.7 in the Supporting Information reveal that the successful dissolution of the absorbing nanocrystal cores is already visible by the naked eye. While the dispersions of the CS particles show a characteristic, reddish color due to the LSPR of the gold cores, the dispersions of the CSd particles appear colorless. The turbidity of all dispersions is related to scattering from the overall particles. Figure 3 shows absorbance spectra of the dilute dispersions measured at room temperature. The spectra of the CSd particles show a continuous increase in absorbance with decreasing wavelength. This increase is related to Rayleigh-Debye-Gans scattering. The spectra of the CS particles are as well influenced by this scattering contribution. In addition these spectra show peaks at approximately 522 nm. These peaks are related to the LSPR of the gold nanocrystal cores.28 Due to the small size of the cores, the LSPR peak is purely absorptive and the scattering cross-section is close to zero.34 Hence, the optical properties of the CS particles result from: 1) Scattering from the PNIPAM shell, and 2) Absorption from the gold nanocrystal cores.


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Figure 3: UV-vis absorbance spectra of dilute nanoparticle dispersions (0.03 wt%) measured at room temperature in D2O.

Spectra that were normalized to the absorbance at a wavelength of 400 nm in direct comparison to the spectrum of the gold nanoparticles prior to polymer encapsulation can be found in Figure S.7 in the Supporting Information. For further analysis of the spectra we determined the molar extinction coefficients ε using the Lambert-Beer law: 𝐸𝜆 = 𝜀 ∙ 𝑐 ∙ 𝑑

(8)

In this equation, E is the extinction at the wavelength , d is the pathlength of the sample in m and c the sample concentration in mol/m3. The concentrations c were determined using the weight concentrations of the samples (0.03 wt%) and the molar mass of the particles. Very crucial for this determination is the consideration of the residual water content in the freezedried particles that we analyzed by thermogravimetric analysis (TGA). We found that the CS10 particles contain 5.7 wt% and the CS25 particles 5.3 wt% residual water (see Supporting Information, Figure S.8). The molar mass of the particles was determined on the basis of the absorbance spectra using the extinction coefficient of gold at 400 nm as previously demonstrated by us for similar particles.28 The details of this calculation are discussed in the Supporting Information. For all particles ε was calculated using the extinction at the plasmon 17 ACS Paragon Plus Environment

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maximum of the respective CS particles (522 nm for CS10 and CSd10, and 521 nm for CS25 and CSd25). Table 1 summarizes the results from these calculations. Due to the core absorption ε is approximately three times higher for the lowly cross-linked and nearly two times higher for the highly cross-linked CS particles when compared to ε of the respective pitted counterparts. In order to understand the differences in ε for the two cross-linker densities, we need to quantify the ratio of the scattering contribution of the PNIPAM shell to the absorbing contribution from the gold nanocrystal cores. We do this by calculating the ratio :28

𝛾=

𝐴𝑏𝑠 (𝜆𝑚𝑎𝑥 ) 𝐴𝑏𝑠(400 𝑛𝑚) 𝐴𝑏𝑠𝐴𝑢 (𝜆𝑚𝑎𝑥 ) 𝐴𝑏𝑠𝐴𝑢 (400 𝑛𝑚)

(9)

Here the absorbance of the CS particles at the plasmon peak (Abs(max)) is related to the absorbance at 400 nm (Abs(400 nm)), i.e., at a wavelength where the scattering contribution from the PNIPAM shell dominates the extinction. Relating the latter ratio to the respective absorbance ratio of the pure gold nanocrystal cores as measured from dilute aqueous dispersion, AbsAu,  provides a convenient means to characterize the plasmonic character of the CS particles. The smaller the values of γ the stronger is the scattering contribution and consequently the less pronounced are the plasmonic properties. In contrast, for particles that do not show a significant scattering contribution  will approach unity. For reasons of comparison we also calculated γ for the CSd particles in order to estimate the lower limit of  for purely scattering particles. In this case the absorbances at 521/522 nm, corresponding to the plasmon peak maxima of the CS nanoparticles, were used to calculate γ. For both pitted particles values of γ = 0.31 were obtained indicating that the wavelength dependent scattering is comparable. This is reasonable as these particles have very similar hydrodynamic dimensions. The respective values for the CS particles are significantly higher which is due to the plasmonic contribution of the cores. The lower cross-linked sample shows a higher value (γ = 0.54) pointing to a smaller scattering cross-section of the particles as compared to the CS25 particles. The latter scatter stronger because of the higher cross-linking content increasing the effective average refractive index of the PNIPAM shell.28


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Table 1: The position of the plasmon resonance λmax, the molar extinction coefficient ε at λmax and the ratio γ obtained from analysis of the UV-vis absorbance spectra. λmax

ε(λmax)

[nm]

[m²/mol]

CS10

522

1.86 ∙ 107

0.54

CSd10

-

0.64 ∙ 107 a

0.31 b

CS25

521

2.67 ∙ 107

0.42

CSd25

-

1.49 ∙ 107 a

0.31 b

Sample

γ

a

These values were calculated using the respective absorbances at 522 nm (CSd10) or 521 nm (CSd25), respectively. b These values were calculated using the respective absorbances at 400 nm and 522 nm (CSd10) / 521 nm (CSd25), respectively.

Crystal structure and photonic properties in the concentrated regime. With the aim to prepare samples at volume fractions above the crystallization threshold, we dispersed our freeze-dried colloids at high weight fractions (6-11 wt%) in heavy water. These samples were then filled into quartz cells with pathlengths of 100 m. Immediately after filling the cells, strong iridescence indicated the successful crystallization for all colloidal systems. Applying several annealing steps (heating above the VPTT and slow cooling to room temperature), the iridescence became even stronger due to the reduction of crystal defects and an overall higher crystal fraction. In order to compare the photonic properties of the CS and the CSd particles, we prepared crystalline samples at comparable particle volume fractions as probed by Bragg diffraction in the visible wavelength range. The diffraction properties and the connection to the volume fraction will be discussed later on in this section. For each cross-linker content, we prepared samples at a low and a high volume fractions aiming at close spectral overlap of the Bragg diffraction with the plasmon resonance of the CS particles. In the case of 2D lattices of plasmonic particles this overlap has been shown to lead to pronounced plasmonic/diffractive coupling.13-14 Figure 4 shows absorbance spectra of the crystalline samples recorded at room temperature (A: CS10 and CSd10; B: CS25 and CSd25). The red spectra correspond to the samples prepared at lower and the black spectra at higher volume fraction.


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Figure 4: UV-vis absorbance spectra of the crystalline samples of particles with 10 mol% cross-linker content in A and with 25 mol% cross-linker content in B for low (spectra in red) and high volume

fractions (spectra in black). Solid lines correspond to spectra of the CS particles and dotted lines are spectra of the CSd particles.

All samples show pronounced diffraction peaks with peak widths (full width at half maximum, FWHM) in the order of 7-14 nm (see Table 2) indicating that highly ordered crystals have formed. Samples prepared at higher volume fraction reveal a Bragg peak at lower wavelength attributed to smaller lattice constants as compared to the samples at lower volume fraction. In fact, the position of the Bragg peak diff allows the determination of these volume fractions (, a quantity that is typically rather hard to estimate precisely for soft particles at high volume fractions.21 To do so we first calculate dhkl using eq 7 by assuming that all samples crystallize in fcc lattices. We will later show that the assumption of fcc lattices is justified by analyzing SANS patterns measured from the same samples. Due to the flat walls of the quartz cells used as sample containers, we expect the (111) plane to be oriented parallel to the wall so that θ = 0°. The Miller indices for this orientation are h = k = l = 1. Since the position of the Bragg peak can be determined with a very low error (diff = +/- 1 nm, i.e., a relative error of < 0.3 %) the only defective parameter influencing the calculation of d111 is ncrystal. For our samples we assume that ncrystal lies within the refractive index of D2O (n = 1.33) and the refractive index of the highly swollen particles (n = 1.36 assuming 85 vol% D2O in the swollen PNIPAM shells). A refractive index of 1.53 as determined from measurements with a refractometer was used for the cross-linked PNIPAM (see Figure S.10 in the Supporting Information). Hence the expected refractive index of the crystalline samples is on average 𝑛𝑐𝑟𝑦𝑠𝑡𝑎𝑙 = 1.345 +/- 0.015.


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Using eq 3 the lattice constant a can be determined. The unit cell of an fcc lattice contains 8 × 1 8

= 1 particles at the corners and 6 ×

1 2

= 3 particles at the planes. Hence the volume 

fraction can be calculated using the sphere radius R by:

ϕ=

4 3 𝑎3

(3+1) 𝜋𝑅 3

(10)

The most reliable radius for this calculation is Rh that we determined from angular dependent DLS, although this will slightly overestimate the particle dimensions and hence . Table 2 summarizes the results from the spectral analysis of all crystalline samples. Table 2: Position of the diffraction peak λdiff, FWHM of the Bragg peaks, lattice plane distance d111, lattice constant a, the volume fraction 𝜙𝑈𝑉−𝑣𝑖𝑠 and the ratio γ of the different crystalline samples. Volume fraction

low

high

𝝓𝑼𝑽−𝒗𝒊𝒔a

γ

353 +/- 4

0.70 +/- 0.02

1.0

204 +/- 2

353 +/- 4

0.72 +/- 0.02

-

10.7

217 +/- 2

377 +/- 4

0.43 +/- 0.01

0.85

585

10.6

217 +/- 2

377 +/- 4

0.49 +/- 0.02

-

CS10

480

10.2

178 +/- 2

309 +/- 3

1.04 +/- 0.03

1.05

CSd10

487

10.1

181 +/- 2

314 +/- 3

1.02 +/- 0.03

-

CS25

536

8.9

199 +/- 2

345 +/- 4

0.56 +/- 0.02

1.10

CSd25

551

7.2

205 +/- 2

355 +/- 4

0.59 +/- 0.02

-

λdiff

FWHM

d111

a

[nm]

[nm]

[nm]

[nm]

CS10

548

13.9

204 +/- 2

CSd10

548

10.4

CS25

585

CSd25

Sample

a

Volume fraction calculated using the lattice constant a, fcc lattices (four particles per unit cell with a volume a3) and Rh as the particle radius.

The calculated volume fractions span a large range of 0.43 – 1.04 where crystalline samples were observed in agreement to a previous work with similar particles.35 Values higher than the hard sphere limit () and even higher than unity are possible due to the soft character of the particles allowing deformation and interpenetration at high concentrations.36 Nevertheless we would like to highlight again that, because of the use of the hydrodynamic particle radii for calculation of , the values in Table 2 will be slightly overestimated. We indeed managed to prepare samples at very similar  for the CS and the corresponding CSd particles at both volume fractions, low and high. Due to the similar surface charge and hydrodynamic radii the crystallization behavior of the particles with and without absorbing gold cores is nearly identical, i.e. the Bragg peak position and widths are comparable for each pair of particles at 21 ACS Paragon Plus Environment

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similar . The nearest neighbor distances dc-c of the nanocrystal cores in the CS particle superlattices can be calculated directly using the lattice constants a: 𝑑𝑐−𝑐 =

𝑎

(11)

√2

The calculated values lie in the range of dc-c = 218 – 267 nm. Looking more closely at the absorbance spectra in Figure 4, we observe a plasmon resonance with a maximum at a wavelength of approximately 522 nm for all CS samples similar to the spectra in the dilute regime (Figure 3). Interestingly, the plasmon peak is more pronounced in the crystal spectra and a significantly decreased scattering contribution at lower wavelength is found. This is also the case for the spectra of the crystals prepared from the CSd particles where the plasmonic contribution is missing. In the latter case the absorbance at wavelength lower than the Bragg peak is close to zero until wavelengths of approximately 350 nm. Apart from the Bragg peak, the samples are nearly transparent in the whole visible wavelength range. The strong interference causing the Bragg diffraction is dominating the spectra and the single particle scattering is suppressed. Furthermore, the low refractive index of the highly solvent swollen particles and their soft network structure result in an overall rather homogeneous medium at high volume fractions. The PNIPAM shells can come into close contact, can deform and fill the available space much more efficient than hard spheres. The more it comes as a surprise that we observe strong Bragg peaks for all samples. The reason for this lies in the inhomogeneous network structure of the PNIPAM shells that show a higher degree of cross-linking and hence a higher refractive index in the inner shell region as compared to the decaying density profile towards the outside. This gradual increase in refractive index from the outside to the inside of the particles leads to the required periodicity in refractive index of the overall crystalline samples. This also implies that microgels with a homogeneous cross-linker distribution would not show pronounced Bragg diffraction at comparable volume fractions. To gain more insight on the role of the plasmonic core absorption to the spectra of the colloidal crystals of the CS particles, we analyzed whether the solid line spectra in Figure 4 can be described by a simple superposition of spectra of the CSd colloidal crystals at comparable volume fraction (Bragg peak only) and the spectrum of the initial gold nanoparticle cores in dilute dispersion (core spectrum only). Figure S.11 in the Supporting Information shows that the spectra are indeed well described by these two individual contributions. Only slight deviations in the Bragg peak position and the plasmon peak position are found. The latter differences are caused by the slightly higher refractive index environment in the CS particles 22 ACS Paragon Plus Environment

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as compared to the neat gold particles in aqueous dispersion causing a small redshift of the plasmon peak. The deviation of the Bragg peaks results from the slightly different volume fractions of the CS and the respective CSd colloidal crystals. To quantify the plasmonic contribution to the colloidal crystal spectra of the CS particles we again calculated the ratio γ as previously demonstrated for the particles in dilute dispersion (see eq 9). The results are summarized in Table 2. For the CS particles γ reaches values close to unity, much higher as compared to γ from dilute spectra (see Table 1). This again underlines the strong reduction in low wavelength scattering as a theoretical value of γ = 1 is attributed to the absence of any scattering. The plasmonic properties of the samples are significantly enhanced. This is neat because it means that our material combines the plasmon absorption of nanocrystal cores with a transparent but diffractive matrix with sharp diffraction peaks that can be nicely tuned in position by changing the particle concentration. This will be very important for future studies on plasmonic/diffractive coupling in 3D superstructures of plasmonic nanocrystals. In our case we did not see any indication of plasmonic near-field nor plasmonic/diffractive coupling. While the absence of near-field coupling can be exclusively related to the small size of the nanocrystal cores in relation to the large center-to-center distances, the absence of plasmonic/diffractive coupling may also be related to positional deviations in the core positions. The latter would significantly reduce the overall sample periodicity and hamper an efficient plasmonic/diffractive coupling. To circumvent these issues in future studies, the encapsulation of larger cores, i.e., cores with a larger extinction crosssection, in highly cross-linked hydrogel shells might lead to pronounced 3D coupling. Extinction spectroscopy can only be used to determine the position of the Bragg peak, the crystal structure however is not accessible. Therefore, we performed SANS measurements on the same samples at 25.6°C. The measurements were performed in the radial beam direction which means that the neutron beam was parallel to the [111] direction of the crystal. In Figure 5A a crystal lattice of an fcc crystal is exemplarily shown. The different lattice planes parallel to the (111) plane are indicated by different colors representing the A, B and C layers of the colloidal crystal. Figure 5B shows a theoretical map of the diffraction peaks from possible, allowed combinations of Miller indices (h, k, l: all even integers and h, k, l: all uneven integers). In Figure 5C we show an example of an experimentally determined SANS scattering pattern (sample CSd10) measured at a sample-to-detector distance of 28 m. Bragg peaks of several orders with a sixfold symmetry can be identified demonstrating the high crystalline order of the sample. Figure 5D shows the same scattering pattern with an indexing of the Bragg peaks (other 23 ACS Paragon Plus Environment

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examples are illustrated in Figure S.12 in the Supporting Information). The different colors of the circles used to highlight individual peaks correspond to the Miller indices in the map shown in Figure 5B with the same color code. Slight deviations in the circle and the Bragg peak positions are related to small deviations of the experimentally obtained peaks from a perfect hexagon that has been used to arrange the circles for each diffraction order, i.e., the green, purple, yellow and pink circles lie all on perfect hexagons. In the following we will use the relative position of the Bragg peaks to determine the size and shape of the unit cell and their relative intensities to access the position of the colloids within the unit cell. For this we use the software Scatter to simulate the experimentally obtained 2D scattering patterns.

Figure 5: A. Schematic depiction of an fcc crystal lattice with the A, B and C planes highlighted with grey, blue and red spheres. In the SANS experiments the neutron beam was parallel to the [111] direction. An fcc unit cell is exemplarily marked in green. For the sake of clarity a depiction of the (111) plane in a schematic unit cell is shown in the right corner at the bottom. B. Theoretically available Miller indices in an fcc lattice. C. Experimentally obtained 2D SANS scattering pattern of a representative


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crystalline sample. D. Indexing of the Bragg peaks using the same color code as in 5B. The indicated circles are arranged on perfect hexagons.

In Figure 6 the measured scattering patterns for CS10 and CSd10 are illustrated at low (upper row, UV-vis = 0.70 and 0.72, respectively) and high (bottom row, UV-vis = 1.04 and 1.02, respectively) volume fraction measured at a sample-to-detector distance of 28 m, covering a qrange of 0.02 – 0.19 nm-1. In Figure 7 the respective scattering patterns for CS25 and CSd25 are depicted (upper row: UV-vis = 0.43 and 0.49, respectively; bottom row: UV-vis = 0.56 and 0.59, respectively). The left half of each scattering pattern shows the experimental data while the right half of each image shows the simulation from Scatter. Corresponding scattering patterns measured at 39 m sample-to-detector distance along with the results from simulation using Scatter are provided in the Supporting Information (Figures S.13 and S.14). All samples show pronounced Bragg peaks of several orders indicating long-range order. A characteristic sixfold symmetry of the crystal lattice can be identified for all scattering patterns. The peak positions and peak spread from the simulation using fcc lattices (𝐹𝑚3̅𝑚) are in very good agreement with the experimental results. Slight deviations however are due to the rather large wavelength distribution ( = 9%) and the geometric smearing in the SANS experiment. Apart from allowed reflections like the , the and the reflections, we also observe crystallographically forbidden, secondary Bragg peaks such as the (labeled in green in Figure 5B/D) or the reflections (see Supporting Information, Figure S.12). This is often the case for soft matter crystals since their peak widths are usually large, i.e. they have small longitudinal and transverse coherence lengths.37 For the simulation of the scattering patterns from colloidal crystals of the lower cross-linked samples, CS10 and CSd10, an inhomogeneous core-shell model led to the best simulation of the experimental data. Similar to our model used for the description of SANS profiles measured in the dilute state, the latter model accounts for the decreasing cross-linking density in the outer shell as compared to a more homogeneously cross-linked volume in the inner PNIPAM shell region. In contrast, the scattering patterns of the CS25 and CSd25 particles could be well described by a simple hard sphere form factor. Table 3 lists the results from the Scatter simulation. The total particle radii Rtotal used in the simulation are close to the ones determined by SANS from the dilute samples (see Table S.3 in the SI). However, we obtained slightly smaller radii for the lower cross-linked samples at high volume fractions ( UV-vis = 1.04 for CS10 and UV-vis = 1.02 for CSd10). This may be attributed to either particle deformation or shrinkage of the softer particles at higher packing fractions. However, due to the pronounced 25 ACS Paragon Plus Environment

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structure factor S(q) the particle size cannot be determined reliably. To allow the determination of size and shape even at high particle densities, for example, the zero-average-contrast (ZAC) method would be a well-suited technique as recently demonstrated by Mohanty et al. for dense microgel suspensions.36 The lattice constants a determined from the lattice analysis of the SANS data are in good agreement to the values previously obtained from analysis of the Bragg peaks measured by absorbance spectroscopy. The domain sizes (𝐷𝑙 and 𝐷𝜓 ) are surprisingly small with values ranging between 375 and 450 nm, i.e., only of the order of the particle diameter. Yet, this is typical for soft crystals. There are several aspects leading to a limited coherence and therefore to small values of 𝐷𝑙 and 𝐷𝜓 as described in the theory part. The displacement of the particles from the ideal lattice points and a certain polydispersity (~10%) are a strong indication for lattice imperfections. Apart from that instrumental limitations play an important role which is supported by the SANS patterns measured at a sample-to-detector distance of 39 m which are shown in Figures S.13 and S.14 in the Supporting Information. Here, the calculations lead to larger domain sizes. This clearly shows that limited transverse and longitudinal coherence lengths owing to instrumental limitations are affecting the measurements. Comparing the crystals of samples with different cross-linker content at similar volume fractions (CS10 at UV-vis = 0.70 compared to CS25 at UV-vis = 0.56) shows similar domain sizes and a similar order of Bragg peaks indicating that the crystallinity is comparable. However, the displacement from the ideal lattice points 𝜎 is larger in the case of the higher cross-linked sample suggesting the defect toleration is lower for these more rigid building blocks. Table 3: Lattice constant a, the longitudinal 𝐷𝑙 as well as the azimuthal 𝐷𝜓 domain size, the overall radius Rtotal, the radius of the core Rc, α describing the algebraic density decay in the shell, the contrast ratio ρ and the standard deviation 𝜎 from the ideal lattice points as obtained by simulating the experimental data measured at a sample-to-detector distance of 28 m. Sample

ϕUV-vis

a

𝑫𝒍 [nm] 𝑫𝝍 [nm] Rtotal [nm] Rc [nm]

α

ρ

𝝈 [nm]

[nm] CS10

0.70 +/- 0.02

335

450

450

95

30

1.2

0.95

5

CSd10

0.72 +/- 0.02

335

450

450

95

30

1.25 0.95

5

CS25

0.43 +/- 0.01

340

375

375

99

-

-

-

25

CSd25

0.49 +/- 0.02

340

375

375

100

-

-

-

25

CS10

1.04 +/- 0.03

290

450

450

85

30

1.25 0.95

10

CSd10

1.02 +/- 0.03

295

450

450

85

30

1.25 0.95

5


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CS25

0.56 +/- 0.02

320

400

400

99

-

-

-

20

CSd25

0.59 +/- 0.02

325

400

400

100

-

-

-

15

Figure 6: Experimentally received scattering patterns (left half) and calculated scattering pattern (right half) in the radial direction [111] for the two different regimes of volume fraction of CS10 (A and C) and CSd10 (B and D) measured at a sample-to-detector distance of 28 m. A and B show results for the low and C and D for the high volume fractions.


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Figure 7: Experimentally received scattering patterns (left half) and calculated scattering pattern (right half) in the radial direction [111] for the two different regimes of volume fraction of CS25 (A and C) and CSd25 (B and D) measured at a sample-to-detector distance of 28 m. A and B show results for the low and C and D for the high volume fractions.

Knowing that all colloidal crystals have an fcc structure, we can now use the first maximum of S(q) to estimate the volume fraction from SANS measurements. S(q) was calculated by dividing the radially averaged scattering data of a concentrated sample (I(q)concentrated) by the respective data from the dilute state (I(q)dilute) assuming that the particle form factor, i.e., size and shape of the particles is nearly independent on concentration. The respective S(q) data for all samples are shown in the Supporting Information in Figure S.15. The determination of the first maximum of S(q), qmax, allows the calculation of d111 and consequently the lattice constants a according to eqs 2 and 3. Table 4 lists the results of this analysis and the corresponding volume fractions SANS obtained using eq 10. Taking into account the errors of the obtained values from SANS and the respective volume fractions from UV-vis absorbance spectroscopy we find a good match, although the values from SANS are in general larger for every sample studied. 28 ACS Paragon Plus Environment

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This discrepancy can be attributed to the determination of qmax from SANS that is a significantly defective quantity: On the one hand the calculation of S(q) is based on the assumption that the form factor of the particles is the same in the crystalline and the dilute samples. Consequently, potential deformation of the particles at high volume fractions is not taken into account. This will influence the position of qmax determined from S(q). On the other hand, S(q) has a relatively broad maximum because of the different instrumental parameters determining the resolution of the SANS measurements. The latter aspect becomes immediately clear when comparing the narrow Bragg peaks from UV-vis absorbance spectroscopy (Figure 4) to the broad peaks from SANS (Figures 6 and 7). Table 4: Position of the first maximum qmax of S(q), the average distance between lattice planes d111, the lattice constant a and the volume fraction ϕ obtained from analysis of SANS and UV-vis absorbance spectra. Sample

ϕUV-visa

qmax [nm-1]

d111 [nm]

a [nm]

ϕSANSa

CS10

0.70 +/- 0.02

0.0311 +/- 0.0006

202 +/- 4

350 +/- 7

0.72 +/-0.04

CSd10

0.72 +/- 0.02

0.0311 +/- 0.0006

202 +/- 4

350 +/- 7

0.73 +/- 0.04

CS25

0.43 +/- 0.01

0.0308 +/- 0.0006

204 +/- 4

353 +/- 7

0.53 +/- 0.03

CSd25

0.49 +/- 0.02

0.0288 +/- 0.0006

218 +/- 5

379 +/- 8

0.49 +/- 0.03

CS10

1.04 +/- 0.03

0.0366 +/- 0.0006

172 +/- 3

297 +/- 5

1.17 +/- 0.06

CSd10

1.02 +/- 0.03

0.0358 +/- 0.0006

176 +/- 3

304 +/- 5

1.12 +/- 0.06

CS25

0.56 +/- 0.02

0.0326 +/- 0.0006

193 +/- 4

334 +/- 6

0.62 +/- 0.03

CSd25

0.59 +/- 0.02

0.0316 +/- 0.0006

199 +/- 4

344 +/- 7

0.64 +/- 0.04

a

Volume fraction calculated using the lattice constant a, fcc lattices (four particles per unit cell with a volume a3) and Rh as the particle radius.

Conclusions In this work we investigated the crystal structure and the photonic behavior of soft colloidal crystals prepared from microgels with and without absorbing gold nanocrystal cores. We focussed on thermoresponsive PNIPAM microgels with two different softnesses prepared using different amounts of the chemical cross-linker N,N’-methylenebisacrylamide. For a direct comparison of microgels with and without gold cores without noticeable difference in the overall colloid size and network structure, we prepared pitted particles by dissolving the gold cores of gold-PNIPAM core-shell particles. The successful dissolution of the cores was manifested by microscopic (TEM) and spectroscopic analysis. DLS revealed that the 29 ACS Paragon Plus Environment

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dissolution of the cores did not affect the overall particle size nor the microgel swelling behavior in a significant manner. SANS measurements in the swollen state confirmed that also the internal network structure is not affected by the core dissolution. In fact the SANS profiles for particles with and without gold cores were nearly identical for each cross-linking degree. Macroscopic crystalline samples were achieved by filling samples at high volume fractions into quartz cells with 0.1 mm pathlength. Annealing of the samples resulted in narrow Bragg peaks in the visible wavelength range with peak widths on the order of 10 nm as probed by far-field UV-Vis extinction spectroscopy. We showed that the visible light scattering contribution from the colloids at lower wavelengths was significantly reduced in the crystalline samples. This reduction distinctly enhanced the plasmonic properties in case of the gold-PNIPAM core-shell systems. Analysis of the spectra revealed that the spectra of the latter particles can be well described by a superposition of the spectra resulting from pitted particles’ colloidal crystals at very similar volume fractions and the neat gold nanocrystal core spectrum recorded from dilute aqueous dispersion. This indicates that the gold cores only have an absorbing contribution to the spectra without occurrence of any interaction such as near-field plasmonic or plasmonic/diffractive coupling. Finally SANS spectra recorded from the crystalline samples revealed pronounced Bragg peaks of several orders. All 2D scattering patterns can be well described by simulation using fcc arrangement of the particles. The accessible lattice constants match nicely the ones obtained from extinction spectroscopy. In future studies 3D plasmonic/diffractive coupling in similar systems may become possible by using plasmonic cores with larger extinction cross-section as accessible by postmodification of the gold cores or the direct encapsulation of larger gold or silver cores. This will be an important step for the development of new photonic materials from soft colloidal building blocks that allow actuation by temperature. Larger metal cores would also significantly enhance the contrast for experiments with small angle X-ray scattering (SAXS) and ultra-small angle X-ray scattering (USAXS) which allow a much more precise determination of the crystal domain sizes due to a much better instrumental resolution.

Acknowledgements MK acknowledges financial support from the German Research Foundation (DFG) through the Emmy Noether-Programme (KA 3880/1). AR was supported by the Elite Network Bavaria in the framework of the Elite Study Program “Macromolecular Science”. The authors thank the ILL in Grenoble for granting SANS beamtime. D. Heß (ILL, Grenoble) is kindly acknowledged 30 ACS Paragon Plus Environment

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for the instruction to the labs in the Science building at the ILL during the beamtime as well as for the possibility to use the PSCM labs at the ILL. Further, we thank T. Honold and V. Leffler for conducting the TEM measurements.

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36. Mohanty, P. S.; Nöjd, S.; van Gruijthuijsen, K.; Crassous, J. J.; Obiols-Rabasa, M.; Schweins, R.; Stradner, A.; Schurtenberger, P., Interpenetration of polymeric microgels at ultrahigh densities. Sci Rep 2017, 7 (1), 1487. 37. Förster, S.; Timmann, A.; Schellbach, C.; Frömsdorf, A.; Kornowski, A.; Weller, H.; Roth, S. V.; Lindner, P., Order causes secondary Bragg peaks in soft materials. Nature Materials 2007, 6 (11), 888-893.


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Role of absorbing nanocrystal cores in soft photonic crystals: A

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