Surfactant induced gelation of TEMPO-oxidized cellulose nanofibril dispersions probed using small angle neutron scattering.

In this work, we studied TEMPO-oxidized cellulose nanofibril (OCNF) suspensions in the presence of diverse surfactants. Using a combination of small angle neutron scattering (SANS) and rheology, we compared the physical properties of the suspensions with their structural behavior. Four surfactants were studied, all with the same hydrophobic tail length but different headgroups: hexaethylene glycol mono-n-dodecyl ether (C12EO6, nonionic), sodium dodecyl sulfate (SDS, anionic), cocamidopropyl betaine (CapB, zwitterionic), and dodecyltrimethylammonium bromide (DTAB, cationic). Contrast variation SANS studies using deuterated version of C12EO6 or SDS, or by varying the D2O/H2O ratio of the suspensions (with CapB), allowed focusing only on the structural properties of OCNFs or surfactant micelles. We showed that, in the concentration range studied, for C12EO6, although the nanofibrils are concentrated thanks to an excluded volume effect observed in SANS, the rheological properties of the suspensions are not affected. Addition of SDS or CapB induces gelation for surfactant concentrations superior to the critical micellar concentration (CMC). SANS results show that attractive interactions between OCNFs arise in the presence of these anionic or zwitterionic surfactants, hinting at depletion attraction as the main mechanism of gelation. Finally, addition of small amounts of DTAB (below the CMC) allows formation of a tough gel by adsorbing onto the OCNF surface.

Exploring Nanoscale Structure in Perovskite Precursor Solutions Using Neutron and Light Scattering.

Tailoring the solution chemistry of metal halide perovskites requires a detailed understanding of precursor aggregation and coordination. In this work, we use various scattering techniques, including dynamic light scattering (DLS), small angle neutron scattering (SANS), and spin-echo SANS (SESANS) to probe the nanostructures from 1 nm to 10 µm within two different lead-halide perovskite solution inks (MAPbI3 and a triple-cation mixed-halide perovskite). We find that DLS can misrepresent the size distribution of the colloidal dispersion and use SANS/SESANS to confirm that these perovskite solutions are mostly comprised of 1-2 nm-sized particles. We further conclude that if there are larger colloids present, their concentration must be <0.005% of the total dispersion volume. With SANS, we apply a simple fitting model for two component microemulsions (Teubner-Strey), demonstrating this as a potential method to investigate the structure, chemical composition, and colloidal stability of perovskite solutions, and we here show that MAPbI3 solutions age more drastically than triple cation solutions.

X-ray nano-tomography of complete scales from the ultra-white beetles Lepidiota stigma and Cyphochilus.

High resolution X-ray nano-tomography experiments are often limited to a few tens of micrometer size volumes due to detector size. It is possible, through the use of multiple overlapping tomography scans, to produce a large area scan which can encompass a sample in its entirety. Mounting and positioning regions to be scanned is highly challenging and normally requires focused ion beam approaches. In this work we have imaged intact beetle scale cells mounted on the tip of a needle using a micromanipulator stage. Here we show X-ray holotomography data for single ultra-white scales from the beetles Lepidiota stigma (L. stigma) and Cyphochilus which exhibit the most effective scattering of white light in the literature. The final thresholded matrices represent a scan area of 25 × 70 × 362.5 µm and 25 × 67.5 × 235µm while maintaining a pixel resolution of 25 nm. This tomographic approach allowed the internal structure of the scales to be captured completely intact and undistorted by the sectioning required for traditional microscopy techniques.

Mesoporous Silica Formation Mechanisms Probed Using Combined Spin-Echo Modulated Small-Angle Neutron Scattering (SEMSANS) and Small-Angle Neutron Scattering (SANS).

The initial formation stages of surfactant-templated silica thin films which grow at the air-water interface were studied using combined spin-echo modulated small-angle neutron scattering (SEMSANS) and small-angle neutron scattering (SANS). The films are formed from either a cationic surfactant or nonionic surfactant (C16EO8) in a dilute acidic solution by the addition of tetramethoxysilane. Previous work has suggested a two stage formation mechanism with mesostructured particle formation in the bulk solution driving film formation at the solution surface. From the SEMSANS data, it is possible to pinpoint accurately the time associated with the formation of large particles in solution that go on to form the film and to show their emergence is concomitant with the appearance of Bragg peaks in the SANS pattern, associated with the two-dimensional hexagonal order. The combination of SANS and SEMSANS allows a complete depiction of the steps of the synthesis that occur in the subphase.

Different agglomeration properties of PC61BM and PC71BM in photovoltaic inks - a spin-echo SANS study.

Fullerene derivatives are used in a wide range of applications including as electron acceptors in solution-processable organic photovoltaics. We report agglomeration of fullerene derivatives in optically opaque solutions of PC61BM and PC71BM, with concentrations ranging from 30 mg mL-1 up to 90 mg mL-1, in different solvents with relevance to organic photovoltaics, using a novel neutron scattering technique, Spin-Echo Small Angle Neutron Scattering (SESANS). From SESANS, agglomerates with correlation lengths larger than 1 µm are found in some PC61BM solutions, in contrast no agglomerates are seen in PC71BM solutions. These results clearly show that PC71BM is fundamentally more soluble than PC61BM in the solvents commonly used in photovoltaic inks and corroborating similar observations previously achieved using other experimental techniques. Computer models are presented to study the energetics of solution and agglomeration of both species, ascribing the difference to a kinetic effect probably related to the larger anisotropy of PC71BM. Also, this work showcases the power of SESANS to probe agglomerates of fullerene derivatives in completely opaque solutions for agglomerates of the order of one to several microns.

Ligand Shell Structure in Lead Sulfide-Oleic Acid Colloidal Quantum Dots Revealed by Small-Angle Scattering.

Nanocrystal quantum dots are generally coated with an organic ligand layer. These layers are a necessary consequence of their chemical synthesis, and in addition they play a key role in controlling the optical and electronic properties of the system. Here we describe a method for quantitative measurement of the ligand layer in 3 nm diameter lead sulfide-oleic acid quantum dots. Complementary small-angle X-ray and neutron scattering (SAXS and SANS) studies give a complete and quantitative picture of the nanoparticle structure. We find greater-than-monolayer coverage of oleic acid and a significant proportion of ligand remaining in solution, and we demonstrate reversible thermal cycling of the oleic acid coverage. We outline the effectiveness of simple purification procedures with applications in preparing dots for efficient ligand exchange. Our method is transferrable to a wide range of colloidal nanocrystals and ligand chemistries, providing the quantitative means to enable the rational design of ligand-exchange procedures.

Wing scale ultrastructure underlying convergent and divergent iridescent colours in mimetic Heliconius butterflies.

Iridescence is an optical phenomenon whereby colour changes with the illumination and viewing angle. It can be produced by thin film interference or diffraction. Iridescent optical structures are fairly common in nature, but relatively little is known about their production or evolution. Here we describe the structures responsible for producing blue-green iridescent colour in Heliconius butterflies. Overall the wing scale structures of iridescent and non-iridescent Heliconius species are very similar, both having longitudinal ridges joined by cross-ribs. However, iridescent scales have ridges composed of layered lamellae, which act as multilayer reflectors. Differences in brightness between species can be explained by the extent of overlap of the lamellae and their curvature as well as the density of ridges on the scale. Heliconius are well known for their Müllerian mimicry. We find that iridescent structural colour is not closely matched between co-mimetic species. Differences appear less pronounced in models of Heliconius vision than models of avian vision, suggesting that they are not driven by selection to avoid heterospecific courtship by co-mimics. Ridge profiles appear to evolve relatively slowly, being similar between closely related taxa, while ridge density evolves faster and is similar between distantly related co-mimics.

Does 1,8-diiodooctane affect the aggregation state of PC71BM in solution?

1,8-Diiodooctane (DIO) is an additive used in the processing of organic photovoltaics and has previously been reported, on the basis of small-angle X-ray scattering (SAXS) measurements, to deflocculate nano-aggregates of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene. We have critically re-examined this finding in a series of scattering measurements using both X-rays and neutrons. With SAXS, we find that the form of the background solvent scattering is influenced by the presence of DIO, that there is substantial attenuation of the X-rays by the background solvent and that there appears to be beam-induced aggregation. All three factors call into question the suitability of SAXS for measurements on these samples. By contrast, small-angle neutron scattering (SANS) measurements, performed at concentrations of 15 mg ml-1 up to and including 40 mg ml-1, show no difference in the aggregation state for PC71BM in chlorobenzene with and without 3% DIO; we find PC71BM to be molecularly dissolved in all solvent cases. In situ film thinning measurements of spin-coated PC71BM solution with the DIO additive dry much slower. Optical imaging shows that the fullerene films possess enhanced molecular mobility in the presence of DIO and it is this which, we conclude, improves the nanomorphology and consequently solar cell performance. We propose that any compatible high boiling solvent would be expected to show the same behaviour.

Control of the Porous Structure of Polystyrene Particles Obtained by Nonsolvent Induced Phase Separation.

Porous polystyrene microspheres were produced by a process of nonsolvent induced phase separation (NIPS) from ternary polymer-solvent-nonsolvent (polystyrene-toluene-ethanol) systems and characterized by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) techniques. This study provides evidence for a link between the structural morphology of the porous polystyrene particles and the polystyrene concentration in the initial solutions. A reciprocal relationship between pore diameter and polymer concentration was observed for the systems with the polymer amount below the critical chain overlap concentration, C*. Above C*, this relationship breaks down. The reciprocal relationship between porosity and polymer concentration can be used to facilitate the fine control of the void size. We demonstrate that the observed reciprocal relationship between pore diameter and polymer concentration correlates well with the relative amount of nonsolvent present in the system at the onset of the phase separation process. The pore size can be reduced and, consequently, the pore surface area can be increased either by reducing the polymer concentration in the initial solution or by decreasing the polymer molecular weight in the sample composition.

Spatially modulated structural colour in bird feathers.

Eurasian Jay (Garrulus glandarius) feathers display periodic variations in the reflected colour from white through light blue, dark blue and black. We find the structures responsible for the colour are continuous in their size and spatially controlled by the degree of spinodal phase separation in the corresponding region of the feather barb. Blue structures have a well-defined broadband ultra-violet (UV) to blue wavelength distribution; the corresponding nanostructure has characteristic spinodal morphology with a lengthscale of order 150 nm. White regions have a larger 200 nm nanostructure, consistent with a spinodal process that has coarsened further, yielding broader wavelength white reflectance. Our analysis shows that nanostructure in single bird feather barbs can be varied continuously by controlling the time the keratin network is allowed to phase separate before mobility in the system is arrested. Dynamic scaling analysis of the single barb scattering data implies that the phase separation arrest mechanism is rapid and also distinct from the spinodal phase separation mechanism i.e. it is not gelation or intermolecular re-association. Any growing lengthscale using this spinodal phase separation approach must first traverse the UV and blue wavelength regions, growing the structure by coarsening, resulting in a broad distribution of domain sizes.