Victoria García Sakai

The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a devices built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis.

Segmental Dynamics in PMMA-Grafted Nanoparticle Composites

We have recently shown that silica nanoparticles grafted with polystyrene chains behave akin to block copolymers due to the “dislike” between the nanoparticles and the grafts. These decorated nanoparticles, thus, self-assemble into various morphologies, from well-dispersed nanoparticles to anisotropic superstructures, when they are placed in homopolystyrene matrices of different molecular masses. Here, we consider a slightly different case, where the grafted chains and thematrix (both PMMA) are strongly attracted to the silica nanoparticle surface. We then conjecture that these systems show phase mixing or demixing depending on the miscibility between the brush andmatrix chains (“autophobic dewetting”). At 15 mass % particle loading, composites created using the same grafted nanoparticle, but with two different matrices, yield well dispersed nanoparticles or nanoparticle “agglomerates”, respectively. Rheology experiments show that the composites display solid-like behavior only when the particles are aggregated. As deduced in previous work, this difference in behavior is attributed to the presence of percolating particle clusters in the agglomerated samples which allows for stress propagation through the system. Going further, we compare the local mobility of matrix and grafted segments of both composites using quasi-elastic neutron scattering experiments. For the liquid-like system, the mean square displacements of the grafted chains and matrix chains, the particle structuring and mechanical response are all unaffected by annealing time. In contrast, in the reinforced case, only the localmatrixmotion is unaffected by time. Since the particle clustering and solid-like mechanical reinforcement increase with increasing time, we conclude that mechanical reinforcement in polymer nanocomposites is purely based on the nanoparticles, with essentially no “interference” from the matrix. In conjunction with other results in the literature, we then surmise that mechanical reinforcement is caused by the bridging of particles by the grafted polymer layers and not due to the formation of “glassy” polymer layers on the nanoparticles.

Local polymer dynamics in polymer-C60 mixtures.

The complexity of intermolecular interactions in polymer-nanoparticle systems leads to spatial variations in structure and dynamics at both the meso- and nanoscale. Much of this behavior is manifested in properties such as the glass transition and the viscosity. Incoherent neutron scattering measurements of C60-polymer mixtures reveal that local polymer chain backbone motions in the glassy state are suppressed relative to those of the pure polymer. Moreover, the scattering spectrum of the melt suggests that the influence of C60 on polymer dynamics is limited to the vicinity of the particles at nanosecond time scales. A model is presented to reconcile these observations with the bulk dynamical properties exhibited by the mixtures.

Micro-focused X-ray diffraction characterization of high-quality [6,6]-phenyl-C61-butyric acid methyl ester single crystals without solvent impurities

We report the preparation of high-quality, solvent-free [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) large single crystals (size up to 0.5 mm) by slow drying of a chlorobenzene solution at room temperature. The monoclinic structure containing four PCBM molecules per unit cell was successfully solved (R-factor = 0.0512) via micro-focused X-ray diffraction and employed as a reliable experimental model for further molecular dynamics simulations. We find that the first peak of the simulated fullerene–fullerene radial distribution function is centred at 10.05 A, giving a nearest neighbour coordination number of 7.0. The work reported herein provides the structural basis for a fundamental understanding of charge transport in this important functional material that is particularly relevant to organic solar cells.

Partitioning of ethanol into lipid membranes and its effect on fluidity and permeability as seen by X-ray and neutron scattering

We present a combined neutron and X-ray scattering investigation to study the effect of ethanol on the molecular structure and dynamics of lipid membranes. 1,2-Dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) powder hydrated with a 5 wt% ethanol solution (corresponding to 2 mol% of ethanol) was used in this study. From high-resolution X-ray experiments the position and partitioning of the ethanol molecules in the phospholipid bilayers was determined in their gel and fluid phases. We find that the ethanol molecules reside in the head group region of the bilayers, with 1.6 ethanol molecules per lipid molecule in the gel phase and 1.2 ethanol molecules per lipid molecule in the fluid phase. We find evidence for enhanced permeability in both fluid and gel phases of the phospholipid bilayers in the presence of ethanol molecules. Elastic and quasi-elastic neutron scattering data, obtained using a neutron backscattering spectrometer, was used to study slow, nanosecond molecular dynamics on length scales corresponding to lipid diffusion, acyl chain dynamics and solvent dynamics. While the presence of ethanol molecules had no observable effect on these types of dynamics in the fluid (Lα) phase, the membranes appeared to have a higher degree of order in gel (Lβ) and ripple (Pβ′) phases. In particular, lipid diffusion was found to be slower by a factor of two in the more rigid gel phase when ethanol was present.

Dynamics in Protein Powders on the Nanosecond–Picosecond Time Scale Are Dominated by Localized Motions

We present analysis of nanosecond-picosecond dynamics of Green Fluorescence Protein (GFP) using neutron scattering data obtained on three spectrometers. GFP has a β-barrel structure that differs significantly from the structure of other globular proteins and is thought to result in a more rigid local environment. Despite this difference, our analysis reveals that the dynamics of GFP are similar to dynamics of other globular proteins such as lysozyme and myoglobin. We suggest that the same general concept of protein dynamics may be applicable to all these proteins. The dynamics of dry protein are dominated by methyl group rotations, while hydration facilitates localized diffusion-like motions in the protein. The latter has an extremely broad relaxation spectrum. The nanosecond-picosecond dynamics of both dry and hydrated GFP are localized to distances of ∼1-3.5 Å, in contrast to the longer range diffusion of hydration water.

Conformational and segmental dynamics in lipid-based vesicles†

We have performed a detailed analysis of the conformational and segmental dynamics in lipid-based vesicles using quasielastic neutron scattering. Our data evidence the presence of dynamical heterogeneities: the hydrogens in the headgroups and in the initial part of the hydrophobic chains perform slow diffusive motions in a confined volume; those belonging to the end part of the chains show a faster confined diffusivity together with torsional isomerization transitions. A model with three hydrogen populations has been proposed to describe the main features of this complex dynamics. The addition of a charged polysaccharide component to the vesicles mostly slows the dynamics of the headgroups and of the upper part of the acyl chains without significantly affecting the isomerization dynamics.

Solvent effects on protein fast dynamics: implications for biopreservation

In the context of biopreservation, we study the influence of water, glycerol and trehalose on the ps–ns dynamics of lyzosyme using neutron scattering. Results indicate that the choice of bioprotectant depends on the storage temperature; glycerol is the most effective for low temperatures and trehalose for high temperatures.

Biodegradable dextran based microgels: a study on network associated water diffusion and enzymatic degradation

Sustained drug delivery represents a major challenge in nanomedicine. Solutions to the many requirements posed by this field are not easy to address using a unique delivery vehicle. In recent years, our goal has been to implement such requirements in a single device by manipulating the structural and functional features of “soft” biocompatible drug delivery platforms. In this paper we describe a set of biocompatible drug delivery materials, with controlled structure and dimension, which are both biodegradable in a time frame of interest and designed as drug vectors for therapeutic approaches. These microdevices were obtained using ultrasound assisted “water-in-water” emulsification. The resulting material was a spherical shaped microgel with controlled pore size and water content. The dynamic behaviour of water in these matrixes showed a remarkable supercooling effect, an effect which was more pronounced for those microgels with smaller mesh sizes. The biodegradability of the microgel was monitored by observing the enzymatic breakdown of the material both as a whole, i.e. by observing a large number of microgel particles, and by focussing on single particles. A complex degradation pattern was observed, with the particles first increasing their size followed by a complete structural demolition. The time required to fully degrade a microgel can be tuned by varying the relative enzyme content and/or the degree of crosslinking of the network.

Microscopic insights into ion gel dynamics using neutron spectroscopy

We have investigated the microscopic dynamics of ion gels consisting of a PMMA [poly(methyl methacrylate)] network and EMITFSI [1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] as the ionic liquid by means of quasi-elastic neutron scattering (QENS). These ion gels interestingly exhibit two glass transitions (Tgs) which drastically decrease as the ionic liquid content increases. QENS allows us to probe the dynamics of PMMA and the EMI [1-ethyl-3-methylimidazolium] cation separately, by selectively deuterating the individual components, and gain insight into the glassy properties of this system. A comprehensive analysis of the QENS spectra was performed, revealing a number of characteristic relaxations, including intramolecular ones, each of which was assigned. We found that the activation energy for PMMA diffusion decreases with increasing ionic liquid content, corresponding to the plasticization of the polymer. The ionic liquid showed two characteristic relaxations: a motion strongly coupled to the motion of PMMA which we argue to be the motion of part of the ionic liquid which is bound to the PMMA giving rise to a higher effective Tg and an ionic diffusion associated with ionic liquid molecules far from the polymer chains which behave nearly as free liquid, exhibiting a lower Tg.