Role of Absorbing Nanocrystal Cores in Soft Photonic Crystals: A Spectroscopy and SANS Study

Periodic superstructures of plasmonic nanoparticles have attracted significant interest because they can support coupled plasmonic modes, making them interesting for plasmonic lasing, metamaterials, and as light-management structures in thin-film optoelectronic 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 (Volk, K.; Fitzgerald, J. P. S.; Ruckdeschel, P.; Retsch, M.; König, T. A. F.; Karg, M. Reversible Tuning of Visible Wavelength Surface Lattice Resonances in Self-Assembled Hybrid Monolayers. <i>Adv. Optical Mater</i>. <b>2017</b>, <i>5</i>, 1600971, 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, independently of the presence or absence of the gold cores. All crystals show a significant reduction in 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 or the crystal structure. 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

Laser Flash Photolysis of Au-PNIPAM Core–Shell Nanoparticles: Dynamics of the Shell Response

Hydrophobic forces play a key role in the processes of collapse and reswelling of thermoresponsive polymers. However, little is known about the dynamics of these processes. Here, thermoresponsive poly­(<i>N</i>-isopropylacrylamide)-encapsulated gold nanoparticles (Au-PNIPAM) are heated via nanosecond laser flash photolysis. Photothermal heating via excitation of the localized surface plasmon resonance of the Au nanoparticle cores results in rapid PNIPAM shell collapse within the 10 ns pulse width of the laser. Remarkably, reswelling of the polymer shell takes place in less than 100 ns. A clear pump fluence threshold for the collapse of the PNIPAM shell is demonstrated, below which collapse is not observed. Reswelling takes longer at higher laser intensities

Atomic Layer Deposition of Silica on Carbon Nanotubes

Low-temperature ozone-assisted atomic layer deposition (ALD) of SiO₂ with four silane derivatives (3-aminopropyl)­triethoxysilane (APTES), bis­(diethylamino)­silane (BDEAS), diphenylaminosilane (DPAS), and triethylsilane on carbon nanotubes (CNTs) leads to the one step formation of SiO₂ nanotubes. In the process, CNTs act as templates and are removed during the ongoing deposition. From transmission electron microscopy images, the formation of a void between the CNTs surface and the SiO₂ coating was observed, indicating an unexpected removal of carbon from the CNTs. This gap grows as the number of ALD cycles is increased, eventually leading to SiO₂ nanotubes almost free of carbon. ATR-IR and EELS spectra proved the SiO₂ formation. Depending on the CNTs templates used in this process, different morphologies of one-dimensional SiO₂ nanostructures are obtained, including simple nanotubes, hollow wall nanotubes, tube-in-tube structures, and SiO₂ nanowires. The application of this process on vertically aligned CNTs (VACNTs) templates allows the formation of a perfect SiO₂ replica of the VACNTs. From experiments with different oxygen and silicon precursors, it is proposed that peroxides and oxygen-based radicals, which can be formed from the reaction of surface Si–H species with ozone, are the main reactive species leading to the unexpected etching of carbon from the CNTs during silica ALD

Stable in Bulk and Aggregating at the Interface: Comparing Core–Shell Nanoparticles in Suspension and at Fluid Interfaces

Colloidal particles are extensively used to assemble materials from bulk suspensions or after adsorption and confinement at fluid interfaces (e.g., oil-water interfaces). Interestingly, and often underestimated, optimizing interactions for bulk assembly may not lead to the same behavior at fluid interfaces. In this work, we compare model composite nanoparticles with a silica core coated with a poly-<i>N</i>-iso­propyl­acryl­amide hydrogel shell in bulk aqueous suspensions and after adsorption at an oil-water interface. Bulk properties are analyzed by confocal differential dynamic microscopy, a recently developed technique that allows one to simultaneously obtain structural and dynamical information up to high volume fractions. The results demonstrate excellent colloidal stability and the absence of aggregation in all cases. The behavior at the interface, investigated by a range of complementary approaches, is instead different. The same hydrogel shells that stabilize the particles in the bulk deform at the interface and induce attractive capillary interactions that lead to aggregation even at very low area fractions (surface coverage). Upon further compression of a particle-laden interface, a structural transition is observed where closely packed particle aggregates form. These findings emphasize the manifestation of different, and possibly unexpected, responses for sterically stabilized nanoparticles in the bulk and upon interfacial confinement

Patchy Wormlike Micelles with Tailored Functionality by Crystallization-Driven Self-Assembly: A Versatile Platform for Mesostructured Hybrid Materials

One-dimensional patchy nanostructures are interesting materials due to their excellent interfacial activity and their potential use as carrier for functional nanoparticles. Up to now only wormlike crystalline-core micelles (wCCMs) with a nonfunctional patchy PS/PMMA corona were accessible using crystallization-driven self-assembly (CDSA) of polystyrene-<i>block</i>-polyethylene-<i>block</i>-poly­(methyl methacrylate) (SEM) triblock terpolymers. Here, we present a facile approach toward functional, patchy wCCMs, bearing tertiary amino groups in one of the surface patches. The corona forming PMMA block of a SEM triblock terpolymer was functionalized by amidation with different <i>N</i>,<i>N</i>-dialkyl­ethylene­diamines in a polymer analogous fashion. The CDSA of the functionalized triblock terpolymers in THF was found to strongly depend on the polarity/solubility of the amidated PMMA block. The lower the polarity of the amidated PMMA block (increased solubility), the higher is the accessible degree of functionalization upon which defined, well-dispersed wCCMs are formed. Interestingly, also the structure of the patchy corona can be tuned by the composition/chemistry of the functional patch, giving rise to spherical patches for R = methyl, ethyl and rectangular patches for R = isopropyl. Patchy wCCMs were successfully used as template for the selective incorporation of Au nanoparticles within the amidated corona patches, showing their potential as versatile platform for the construction of functional, mesostructured hybrid materials

How Hollow Are Thermoresponsive Hollow Nanogels?

A main challenge in colloid science is the development of smart delivery systems that store and protect actives from degradation and allow release in response to an external stimulus like temperature. Hollow nanogel capsules made of temperature-sensitive polymers are particularly promising materials. The stimuli-sensitive void size, shell thickness, and permeability determine cargo storage and its release behavior. Thus, determination and control of these morphological parameters are of outmost relevance for the design of new, functional drug delivery vehicles. Here we investigate quantitatively void size and shell thickness of hollow nanogels at different states of swelling by means of small-angle neutron scattering (SANS) employing contrast variation. We demonstrate the structure-sensitivity dilemma: hollow nanogels with a slightly cross-linked shell reveal distinct temperature sensitivity but possess nearly no void (14% of the initial core volume) and are thus hardly “hollow”. Nanogels with a stiff shell are indeed hollow (albeit with smaller void as compared to the core size of the template) but less temperature sensitive

Distance and Wavelength Dependent Quenching of Molecular Fluorescence by Au@SiO₂ Core–Shell Nanoparticles

Gold nanoparticles and nearby fluorophores interact <i>via</i> electromagnetic coupling upon light excitation. We determine the distance and wavelength dependence of this coupling theoretically and experimentally <i>via</i> steady-state and time-resolved fluorescence spectroscopy. For the first time, the fluorescence quenching of four different dye molecules, which absorb light at different wavelengths across the visible spectrum and into the near-infrared, is studied using a rigid silica shell as a spacer. A comprehensive experimental determination of the distance dependence from complete quenching to no coupling is carried out by a systematic variation of the silica shell thickness. Electrodynamic theory predicts the observed quenching quantitatively in terms of energy transfer from the molecular emitter to the gold nanoparticle. The plasmonic field enhancement in the vicinity of the 13 nm gold nanoparticles is calculated as a function of distance and excitation wavelength and is included in all calculations. Relative radiative and energy transfer rates are determined experimentally and are in good agreement with calculated rates. We demonstrate and quantify the severe effect of dye–dye interactions on the fluorescence properties of dyes attached to the surface of a silica nanoparticle in control experiments. This allows us to determine the experimental conditions, under which dye–dye interactions do not affect the experimental results

Magnetic and Electric Resonances in Particle-to-Film-Coupled Functional Nanostructures

We investigate the plasmonic coupling of metallic nanoparticles with continuous metal films by studying the effect of the particle-to-film distance, cavity geometry, and particle size. To efficiently screen these parameters, we fabricated a particle-to-film-coupled functional nanostructure for which the particle size and distance vary. We use gold-core/poly­(<i>N</i>-isopropylacrylamide)-shell nanoparticles to self-assemble a monolayer of well-separated plasmonic particles, introduce a gradient in the nanoparticle size by an overgrowth process, and finally add a coupling metal film by evaporation. These assemblies are characterized using surface probing and optical methods to show localized magnetic and electric field enhancement. The results are in agreement with finite-difference time-domain modeling methods and calculations of the effective permeability and permittivity. Finally, we provide a proof of concept for dynamic tuning of the cavity size by swelling of the hydrogel layer. Thus, the tunability of the coupled resonance and the macroscopic self-assembly technique provides access to a cost-efficient library for magnetic and electric resonances

Aligned Linear Arrays of Crystalline Nanoparticles

Fabrication of one-dimensional arrays of crystalline nanoparticles with tunable particle size and spacing (down to 20 nm) is demonstrated. The individual nanocrystals are pentagonal prisms, and the arrays are up to 11 μm in length, with some arrays containing >50 nanocrystals. Precise particle morphology and interparticle spacing can be maintained down the array. The far-field scattering spectra of the arrays show the near-fields of the nanocrystals are coupled. The method is fast and produces precise, well-defined, coupled plasmonic arrays with optical properties that match well to theory

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances (LSPRs) of gold nanoparticles. The arrangement of such nanoparticles into periodic lattices gives rise to spectrally sharp and tunable surface lattice resonances (SLRs). Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the SLRs become optically active, i.e. demonstrating handedness-dependent excitation. Numerical simulations where the lattice parameters are systematically varied show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of molecular and plasmonic resonances can rationalize this chirality transfer