A general approach to maximise information density in neutron reflectometry analysis

Neutron and X-ray reflectometry are powerful techniques facilitating the study of the structure of interfacial materials. The analysis of these techniques is ill-posed in nature requiring the application of a model-dependent methods. This can lead to the over- and under- analysis of experimental data, when too many or too few parameters are allowed to vary in the model. In this work, we outline a robust and generic framework for the determination of the set of free parameters that is capable of maximising the in-formation density of the model. This framework involves the determination of the Bayesian evidence for each permutation of free parameters; and is applied to a simple phospholipid monolayer. We believe this framework should become an important component in reflectometry data analysis, and hope others more regularly consider the relative evidence for their analytical models

The elevated Curie temperature and half-metallicity in the ferromagnetic semiconductor Lax_{x}Eu1−x_{1-x}O

Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotation ($\mu$SR), polarized neutron reflectivity (PNR), and density functional theory (DFT). The $\mu$SR data shows that the La$_{0.15}$Eu$_{0.85}$O is homogeneously magnetically ordered up to its elevated $T_{\rm C}$. It is concluded that bound magnetic polaron behavior does not explain the increase in $T_{\rm C}$ and an RKKY-like interaction is consistent with the $\mu$SR data. The estimation of the magnetic moment by DFT simulations concurs with the results obtained by PNR, showing a reduction of the magnetic moment per La$_{x}$Eu$_{1-x}$O for increasing lanthanum doping. This reduction of the magnetic moment is explained by the reduction of the number of Eu-4$f$ electrons present in all the magnetic interactions in EuO films. Finally, we show that an upwards shift of the Fermi energy with La or Gd doping gives rise to half-metallicity for doping levels as high as 3.2 %.Comment: 7 pages, 11 figure

Advice on describing Bayesian analysis of neutron and X-ray reflectometry

Driven by the availability of modern software and hardware, Bayesian analysis is becoming more popular in neutron and X-ray reflectometry analysis. The understandability and replicability of these analyses may be harmed by inconsistencies in how the probability distributions central to Bayesian methods are represented in the literature. Herein, we provide advice on how to report the results of Bayesian analysis as applied to neutron and X-ray reflectometry. This includes the clear reporting of initial starting conditions, the prior probabilities, and results of any analysis, and the posterior probabilities that are the Bayesian equivalent of the error bar, to enable replicability and improve understanding. We believe that this advice, grounded in our experience working in the field, will enable greater analytical reproducibility among the reflectometry community, as well as improve the quality and usability of results

Using Bayesian Model Selection to Advise Neutron Reflectometry Analysis from Langmuir-Blodgett Monolayers

The analysis of neutron and X-ray reflectometry data is important for the study of interfacial soft matter structures. However, there is still substantial discussion regarding the analytical modelsthat should be used to rationalise relflectometry data. In this work, we outline a robust and generic framework for the determination of the evidence for a particular model given experimental data, byapplying Bayesian logic. We apply this framework to the study of Langmuir-Blodgett monolayers by considering three possible analytical models from a recently published investigation [Campbell et al., J. Colloid Interface Sci, 2018, 531, 98]. From this, we can determine which model has the most evidence given the experimental data, and show the effect that different isotopic contrasts of neutron reflectometry will have on this. We believe that this general framework could become an important component of neutron and X-ray reflectometry data analysis, and hope others more regularly consider the relative evidence for their analytical models.<br /

Interactions between Asphaltenes and a Model Demulsifier in Bulk and at an Interface Studied by Small-Angle Neutron Scattering (SANS) and Neutron Reflectometry

This article describes neutron reflectometry to probe the structure of the asphaltene layer adsorbed onto a hydrophilic silicon surface and the interactions between asphaltenes and a model demulsifier (pluronic). To start with, asphaltene nanoaggregation is studied in bulk by small-angle neutron scattering (SANS). In pure toluene, asphaltenes form nanoaggregates with sizes that are found to moderately increase when heptane is added. Then, the structure of the asphaltene layer onto a silicon surface is determined by neutron reflectometry. It is first shown that conclusive results on the structure of the adsorbed asphaltene layer can only be determined by varying the scattering length density of the solvent, i.e., by measuring reflectivity curves in various mixtures of D- and H-toluene and by simultaneously fitting all the data sets. The asphaltene layer can be successfully modeled using a single layer; a two-layer model always converges with the second layer having zero thickness. This layer has a thickness of 51 Å, with a low solvation close to the silicon surface (estimated ≈29%). The solvation increases with distance from the silicon oxide layer, reaching finally a value of 91%. Small-angle neutron scattering data indicate that the thickness is close to twice the radius of gyration of asphaltene nanoaggregates in solution. This result indicates that the extent of the asphaltene layer is linked to its self-associative properties in bulk. Finally, using the results obtained by contrast matching to constrain the fitting, the influence of a model demulsifier (pluronic PE 8100) on the asphaltene layer is investigated. In the presence of pluronic, the layer does not protrude as far into the bulk (thickness is reduced) and becomes rougher due to the partial incorporation of pluronic.submittedVersionacceptedVersio

Designing Selective Electrode Materials for Electroanalysis -New Tungsten Bronzes as Selective Potassium Hosts

Commercially available monoclinic tungsten oxide WO3(Pearson symbolmP32) is investigated electrochemically for its selectivity and sensitivity towards potassium ion detection. Electroreduction in aqueous potassium chloride solution shows two distinct phases, identified as tungsten bronzes KxWO3(phase 1: maximumx(1)approximate to 0.1 and phase 2: maximumx(2)approximate to 0.3). In situ synchrotron powder X-ray diffraction reveals the concomitant structural changes, and both phases are identified as new, so far unreported, perovskite-derived potassium bronzes (space groupP4/nmmandI4?3 m, respectively). In cyclic voltammetry, only the first insertion step is found to be reversible. Potentiostatic insertion and the subsequent voltammetric de-insertion show selectivity toward potassium against lithium and sodium in 0.1 M aqueous solutions. The sensitivity towards potassium is analysed in a concentration range between 1 mM and 100 mM. Two linear regions are found for the response, which we relate to two different rate-determining steps, depending on the concentration of potassium in solution

Electrolyte/dye/TiO2 interfacial structures of dye-sensitized solar cells revealed by in situ neutron reflectometry with contrast matching

The nature of an interfacial structure buried within a device assembly is often critical to its function. For example, the dye/TiO2 interfacial structure that comprises the working electrode of a dye-sensitized solar cell (DSC) governs its photovoltaic output. These structures have been determined outside of the DSC device, using ex situ characterization methods; yet, they really should be probed while held within a DSC since they are modulated by the device environment. Dye/TiO2 structures will be particularly influenced by a layer of electrolyte ions that lies above the dye self-assembly. We show that electrolyte/dye/TiO2 interfacial structures can be resolved using in situ neutron reflectometry with contrast matching. We find that electrolyte constituents ingress into the self-assembled monolayer of dye molecules that anchor onto TiO2. Some dye/TiO2 anchoring configurations are modulated by the formation of electrolyte/dye intermolecular interactions. These electrolyte-influencing structural changes will affect dye-regeneration and electron-injection DSC operational processes. This underpins the importance of this in situ structural determination of electrolyte/dye/TiO2 interfaces within representative DSC device environments

Correction: Efficient non-fullerene organic solar cells employing sequentially deposited donor–acceptor layers(vol 6, pg 18225, 2018)

Correction for Efficient non-fullerene organic solar cells employing sequentially deposited donor-acceptor layers by Jiangbin Zhang et al., J. Mater. Chem. A, 2018, 6, 18225-18233

Quantifying the computational capability of a nanomagnetic reservoir computing platform with emergent magnetisation dynamics

Devices based on arrays of interconnected magnetic nano-rings with emergent magnetization dynamics have recently been proposed for use in reservoir computing applications, but for them to be computationally useful it must be possible to optimise their dynamical responses. Here, we use a phenomenological model to demonstrate that such reservoirs can be optimised for classification tasks by tuning hyperparameters that control the scaling and input-rate of data into the system using rotating magnetic fields. We use task-independent metrics to assess the rings' computational capabilities at each set of these hyperparameters and show how these metrics correlate directly to performance in spoken and written digit recognition tasks. We then show that these metrics, and performance in tasks, can be further improved by expanding the reservoir's output to include multiple, concurrent measures of the ring arrays' magnetic states.ISSN:0957-4484ISSN:1361-652