Symposium MT04— Advanced Materials Exploration with Neutrons

Conventional magnetic order spontaneously produces a static periodically modulated magnetization in the sample. It thus breaks rotational and time-reversal symmetries of the underlying spin Hamiltonian. A spin nematic is an exotic state that only breaks rotational but not time reversal symmetries. The spins remain disordered and fluctuating, but these fluctuations are spontaneously anisotropic. Theory predicts a spin nematic ground state for quantum spin systems with competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions in high magnetic fields. The simplest model is a FM-AFM S=1/2 square lattice. To date there seems to be only one family of materials that approximate this scenario, namely the layered vanadophosphate. Unfortunately, our understanding of these compounds has been severely limited by a lack of single crystal samples. This is particularly problematic for experiments in magnetic fields where any features or phase transitions are smeared out by the powder averaging of the material's anisotropic properties.

In my talk I will present breakthrough magnetic, thermodynamic, ESR, NMR and neutron scattering studies on single crystals of two vanadophosphate species, namely BaCdVO(PO₄)₂ and Pb₂VO(PO₄)₂. Neither turned out to be quite the proximate FM-AFM square lattice that it was advertised to be. Nevertheless, in both systems we find clear indications of "hidden" nematic order, just below saturation in high magnetic fields.

What are the experimental signatures of exotic Spin Liquid states? What is the best approach in solving the Hamiltonian of the systems with disordered ground states? What is the role of chemical disorder and how can it be addressed? What part can the external tuning parameters play in shedding light on the mystery of long-sought Quantum Spin Liquid state and other emergent phases of matter in real materials? These are some of the pressing questions intriguing the researchers for the past decades, creating a rich and diverse arena in the field of Quantum Materials. In this talk, I will discuss how chemical composition, external pressure, and application of magnetic field, can regulate the underlying electronic and magnetic interactions in frustrated quantum magnets, ultimately driving the ground state across the phase diagram, and providing key information. As examples, I will present our recent results for neutron scattering studies performed under extreme sample environments on 2D and 3D rare-earth based geometrically frustrated antiferromagnets.

The presentation will provide an overview of the DOE’s Office of Basic Energy Sciences (BES) Neutron scattering programs for the fundamental materials research. The programs support basic research on the unique interactions of neutrons with matter to achieve a fundamental understanding of the atomic, electronic, and magnetic structures and excitations of materials and their relationship to macroscopic properties. Major focus is on the transformative research that uniquely requires neutron scattering as a major tool and serves as a driver for the concomitant advancement of neutron scattering techniques and capabilities for materials research at the two BES world-leading neutron scattering facilities. Research opportunities identified by the recent BES workshops and roundtables will be discussed. The application process and opportunities for white papers will be overviewed, as well as the on-line resources for principal investigators.

For additional information, visit the BES Web page at: https://science.energy.gov/bes/

Spinel antiferromagnets have long been at the center of research into strong spin-lattice coupling and orbital effects. Among other properties, these materials frequently demonstrate concomitant magnetic and structural phase transitions, heightened magneto-elastic or dielectric response functions, and low-temperature multiferroism. There is very little agreement on the microscopic picture to be associated with these effects, but recent work has shown that mesoscale inhomogeneity can play a key role in raising the susceptibilities of complex materials to external perturbations.

In this talk, I will be discussing recent work at the University of Illinois which establishes the importance of mesoscale heterogeneity in determining bulk magnetic properties of spinel ferrimagnets Mn₃O₄ and MnV₂O₄. This will first include a review of Raman scattering, magnetic force microscopy and muon spin rotation data from our group, which reveal the existence of stripe-like magnetic domains in these materials that include ordered and disordered regions. The majority of the talk will then focus on our recent work performed using small angle neutron scattering (SANS), with which we observe Bragg signatures of the same domain wall patterning in the bulk with a typical length scale of ~100nm. Variation with temperature associate these domains with known low-temperature magnetostructural transitions, and measurements taken in applied field reveal a highly anisotropic response associated with domain wall motion. I will correlate key features of our SANS data with observations of non-equilibrium behavior in our magnetization data, and discuss the implications for the properties of other magnetic materials.

Research was supported by the National Science Foundation under Grant NSF DMR 1455264.

The quantum paraelectric behavior and strongly anharmonic lattice dynamics of SrTiO₃ have attracted interest for decades [1]. Reflecting the incipient ferroelectric instability near the quantum critical point and anharmonic couplings between acoustic and optic phonons, anomalous temperature-dependent phonon intensities were observed in multiple Brillouin zones (BZs) from inelastic neutron scattering (INS) experiments on SrTiO₃. The Hybrid Spectrometer (HYSPEC) and HB3 triple-axis spectrometer at Oak Ridge National Laboratory (ORNL) were used to track phonon intensities over a wide temperature range and for a large volume in reciprocal space. The S(Q,E) data reveal a strong softening of the zone-center transverse optic (TO) mode, congruent with ferroelectric (FE) incipient, and simultaneously a strongly anomalous evolution of the intensity of transverse acoustic (TA) modes, which decreases dramatically on cooling. The experimentally observed trends are confirmed and rationalized using ab initio molecular dynamics (AIMD) and anharmonically renormalized phonon methods [2], which achieve quantitative agreement with the INS experiments. By analyzing the simulated temperature-dependent force constants (FC) and eigenvectors, it is found that the structure factors |F|² of TA and TO modes change dramatically with temperature, as a direct consequence of the strong anharmonicity in this system. Moreover, we identify that the changes of Ti and O eigenvectors are responsible for these striking observations, which originates from Ti-O inter-atomic FC changes on cooling. These results explain the long-standing question of the origin of the phenomenon first observed by Yamada and Shirane [1]. Our results also systematically extend this observation to multiple BZs and different high-symmetry directions ( [1,0,0] and [1,1,0] ). These results establish how temperature-dependencies of phonon intensities beyond the harmonic picture can be quantitatively measured through INS mapping of S(Q, E) volumes, providing direct insights into the behavior of phonon eigenvectors in real space, and also show how first-principles simulations including anharmonic effects can reproduce and rationalize such anharmonic effects. These findings are also valuable to understand other perovskite materials with a variety of phase transitions, such as KTaO₃ and EuTiO₃ , and related ferroelectricity/paraelectricity and quantum effects [3-6].

1. Y. Yamada, and Gen Shirane. "Neutron scattering and nature of the soft optical phonon in SrTiO₃ ." Journal of the Physical Society of Japan 26.2 (1969): 396-403.
2. O. Hellman, I. A. Abrikosov, and S. I. Simak. "Lattice dynamics of anharmonic solids from first principles." Physical Review B 84.18 (2011): 180301.
3. S. E. Rowley et al. "Ferroelectric quantum criticality." Nature Physics 10.5 (2014): 367.
4. Taniguchi, Hiroki, Mitsuru Itoh, and Toshirou Yagi. "Ideal soft mode-type quantum phase transition and phase coexistence at quantum critical point in O¹⁸ -exchanged SrTiO₃ ." Physical Review Letters 99.1 (2007): 017602.
5. L. Foussadier, M. D. Fontana, and W. Kress. "Phonon dispersion curves in dilute KTN crystals." Journal of Physics: Condensed Matter 8.9 (1996): 1135.
6. J. L. Bettis, et al. "Lattice dynamical analogies and differences between SrTiO₃ and EuTiO₃ revealed by phonon-dispersion relations and double-well potentials." Physical Review B 84.18 (2011): 184114.

Phonons are the main source of total entropy, thermal expansion, and thermophysical properties of most materials, so it is essential to know the temperature-dependent lattice dynamics. We measured phonon dispersions in silicon by inelastic neutron scattering at elevated temperatures. We determined the thermal shifts of phonon frequencies and broadenings of phonon linewidths throughout the Brillouin zone from these experiments. Large phonon anharmonicities manifested through shifts and broadenings go beyond the quasiharmonic model even at low temperatures. Although the quasiharmonic model (harmonic phonons renormalized by volume) predicts the experimental thermal expansion of silicon well, it does not correctly depict the temperature-dependent lattice dynamics. The necessity to include anharmonicities and nuclear quantum effects to describe the lattice dynamics throughout the temperature range was verified by state-of-the-art ab-initio calculations using the stochastically initialized temperature-dependent effective potential method. The quasiharmonic model correctly predicts the macroscopic values due to a cancelation of phonon shifts, but its validity should not be assumed because it predicts average quantities. The pure anharmoncity included ab-initio calculated phonon lifetime changes and thermal conductivity correctly predict the experimentally determined lifetimes and thermal conductivity as well as previously reported values.

Magnetism is a fascinating phenomenon: it is rooted in relativistic quantum mechanics and yet an integral component of the technologies we use every day. In magnetic insulators, where atomic-scale magnetic dipoles carried by electrons are closely bound to a crystal lattice, novel phases of matter with no classical analogues are possible. Chief among these phases are spin-liquids, in which strong fluctuations of magnetic dipoles preclude conventional magnetic order even for temperatures low compared to the average interaction between spins. Such exotic magnetic matter is of great fundamental interest because it features a wealth of coherence and entanglement phenomena – the hallmarks of the quantum world – and is often amenable to theoretical and computational predictions. In this talk, I will present experimental research that brings together materials chemistry, neutron scattering and computer modeling to understand the magnetic excitations in a range of frustrated oxide compounds with triangular, kagome and pyrochlore lattice structures. My talk will emphasize the importance of neutron scattering instrumentation to probe complex materials behavior in which chemical disorder, geometrical frustration and quantum fluctuations interplay to stabilize – or destroy – spin-liquid physics. Supported by the U.S. Department of Energy under award DE-SC-0018660 and the National Science Foundation under award NSF-DMR-1750186.

Molecular nanomagnets (MNMs) are clusters made of a finite number of magnetic ions coupled by a strong exchange interaction within the clusters and with a negligible magnetic interaction between adjacent clusters in the crystal lattice. They are promising systems for technological applications in the fields of high-density magnetic memory devices, quantum information processing and spintronics. They are also model systems to study the fundaments of quantum mechanics as they display quantum mechanics effects at the macroscopic level. The advances in the chemical engineering of these molecules have allowed the synthesis of tailor-made systems displaying several interesting quantum phenomena and to improve their properties to bring them closer to technological applications. Neutron scattering techniques have been intensively and successfully used to study the microscopic properties of molecular magnets and have enabled to reveal the signatures of their quantum behaviour.
I will show how advanced neutron scattering experiments have been pivotal for the understanding of the magnetic properties and quantum behaviour of a selection of molecular magnets model systems. The new generation of neutron instruments equipped with position sensitive detectors together with the availability of large single crystals has allowed us to reveal the microscopic details of prototypical MNMs unambiguously characterising their Spin Hamiltonian [1,2], to reveal finite size effects on the magnetic properties of linear antiferromagnetic chains [3] and the entanglement between complex spin systems [4].

[1] M. L. Baker, T. Guidi et al., Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering. Nature Physics, vol. 8, p 906 (2012).
[2] A. Chiesa, T. Guidi, et al., Magnetic Exchange Interactions in the Molecular Nanomagnet Mn₁₂, Phys. Rev. Lett. 119, 217202 (2017). [3] T. Guidi, et al, Direct observation of finite size effects in chains of antiferromagnetically coupled spins, Nat. Comm. 6, 7061 (2015)
[4] E Garlatti, T Guidi et al, Portraying entanglement between molecular qubits with four-dimensional inelastic neutron scattering, Nat. Comm. 8, 14543 (2017).

The defect structure in the near surface region of magnetite (Fe₃O₄) is particularly important for a better understanding of the material’s electronic properties and catalytic activity. Hence, the mobility of Fe in Fe₃O₄ is a key ingredient towards a better understanding of near surface processes. Recent scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) studies of the (√2×√2)R45° reconstructed (001) surface suggested a subsurface vacancy stabilisation model for this surface, later proved by surface x-ray diffraction (SXRD) [1,2]. The surface reconstruction is reported to get lifted by formic acid adsorption at room temperature [3]. Low energy electron microscopy (LEEM) experiments under oxidising conditions showed a regrowth process of Fe₃O₄-layers on (001) surfaces [4]. These observations indicate an interesting interplay between cation vacancy formation and diffusion processes. The role of near surface cations was investigated by neutron scattering using ⁵⁷Fe as a marker. A 25 nm thick ⁵⁷Fe₃O₄ marker layer was homoepitaxially grown on top of a (001) polished natural Fe₃O₄ single crystal substrate by reactive molecular beam epitaxy (MBE). The interdiffusion of iron ions across the film-substrate interface was followed by neutron reflectivity (NR) [5]. Due to the lower scattering length of ⁵⁷Fe₃O₄ compared to natural Fe₃O₄, Kiessing fringes were observed by NR. Comparison with NR simulations assuming a perfectly smooth interface between the film and the substrate showed that a notable intermixing of the iron isotopes already occurred during the growth of the marker layer at only 420 K. The diffusion process was followed by successive annealing steps in ultra-high vacuum (UHV) and the subsequent acquisition of NR curves. In the course of the annealing, the Kiessing fringes faded out due to interfacial diffusion. The diffusion process is already observed to take place in the temperature range of 500 K-600 K over the probed depth of several tens of nanometers, which shows that substantial mass transport happens in the near-surface region at much lower temperatures as previously observed by bulk diffusion experiments [6]. Diffusion lengths and the respective diffusion constants obtained from fitted scattering length density (SLD) profiles will be compared to the results of earlier studies on bulk Fe₃O₄ at 800 K-1600 K [6,7]. The NR results are complemented by time of flight secondary ion mass spectroscopy (TOF-SIMS) data and x-ray data characterising the structural changes of the samples during the diffusion experiments.

[1] Bliem, R. et al., Science 346, 1215 (2014)
[2] Arndt, B. et al., Surf. Sci. 653, 76 (2016)
[3] Gamba, O., Arndt, B. et al., 2019, submitted
[4] Nie, S. et al., J. Am. Chem. Soc. 135, 10091 ( 2013)
[5] Schmidt, H. et al., Adv. Eng. Mat. 11, 466 (2009)
[6] Atkinson, A. et al., Journal of materials science 18, 2371 (1983)
[7] Dieckmann, R. et al., Ber. Bunsenges. Phys. Chem. 81, 344 (1977)

Relaxor ferroelectrics are of great importance in many modern technologies, including high energy density capacitors, electrocaloric cooling and energy harvesting. Due to environmental concerns regarding the current Pb-based relaxors, it is necessary to develop new Pb-free alternatives. Characterization of composition-structure-property relationships is a prerequisite first step towards rational and efficient design of new materials. Nevertheless, structural understanding of relaxor ferroelectrics is challenging because of their highly disordered nature. For the new Pb-free relaxors, the polar atomic displacements are correlated over only nanometer length scales and THz frequencies, although the details of such nanoscale correlations are yet to be fully resolved. In this talk, I will present our recent findings on the nanoscale orderings of polar atomic displacements in the newly designed Pb-free relaxors, which are measured using pair distribution function analysis of neutron and X-ray total scattering patterns. In addition, I will show that while conventional pair distribution function study offer insights into the instantaneous snapshots of local atomic correlations, they do not inform about the timescales over which such correlations become stable. I will demonstrate the application of the dynamic pair distribution function (DyPDF) method to elucidate the timescales over which such nanoscale atomic orderings become stable in some Pb-free relaxor systems. The implication of nanoscale atomic ordering and dynamics towards the dielectric, ferroelectric and electrothermal properties of Pb-free relaxors will be discussed.

In this talk, I will present our recent results exploring the magnetic ground state and field-induced phase behavior in the triangular lattice compound NaYbO₂. The triangular lattice decorated with J_eff=1/2 moments has enjoyed renewed interest due to recent reports of quantum spin liquid formation in YbMgGaO₄ and related materials. One ambiguity however remains the role played by structural/exchange disorder in generating this behavior, which motivates the exploration of alternate lattice types hosting the same equilaterial triangles of J_eff=1/2 moments. Here we establish NaYbO₂ as one such alternative with an ideal R-3m structure and minimal lattice disorder. Yb moments experience an enhanced exchange field due to smaller Yb-Yb distances, yet they fail to order at temperatures as low as 20 mK and demonstrate a frustration parameter f>400. The resulting quantum disordered state can be quenched via the application of a magnetic field that drives the formation of a fluctuation stabilized up-up-down spin state. Interlayer frustration of the native, zero-field 120 deg antiferromagnetic order on the triangular lattice is proposed as a critical ingerdient for realizing this phase behavior. If time permits, recent results on related materials will also be presented.

This work is supported by DOE, Office of Science, Basic Energy Sciences under Award DE-SC0017752.

Long carrier lifetime is what makes hybrid organic-inorganic perovskites high performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photo-excited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.

Anharmonic phonon-phonon interactions are critical to rationalize and design both ferroelectrics and thermoelectrics, in which strong anharmonicity may be found near lattice instabilities. Large deviations from harmonic potential energy surfaces, as in the Landau double-well picture, give rise to interesting phonon properties characterized by extensive interactions between phonon quasiparticles and pronounced renormalization effects. Similarly, in superionic crystals, where ionic mobilities are comparable to those of liquids, the atomic dynamics are strongly anomalous but remain debated. A central question is whether phonon quasiparticles -which conduct heat in regular solids- can survive in the superionic state, where a large fraction of the system exhibits liquid-like behavior. I will present results from inelastic neutron scattering (INS) measurements of lattice dynamics in strongly anharmonic materials. Combined with large-scale first-principles simulations, such as ab-initio molecular dynamics, these INS experiments enable a detailed understanding of how peculiar atomic dynamics give rise to useful material properties. In thermoelectrics PbTe and SnSe near soft-mode transitions, large thermal conductivity suppressions are achieved by anharmonic scattering [1,2]. In superionics CuCrSe2 [3] and Na/Li solid-state electrolytes, our studies show how anharmonic phonon dynamics are at the origin of low thermal conductivity and superionicity. Yet, long-wavelength acoustic phonons capable of heat conduction remain in the superionic regime. Our studies of lattice dynamics and diffusion will help rationalize the emergence of ultralow thermal conductivity for thermoelectrics and facilitate the design of high-performance solid-state electrolytes for next-generation batteries.

[1] O. Delaire, J. Ma, K. Marty, A. May, M. McGuire, M.-H. Du, D. Singh, A. Podlesnyak, G. Ehlers, M. Lumsden, and B. Sales, “Giant Anharmonic Phonon Scattering in PbTe”, Nature Materials 10, 614 (2011).
[2] C. Li, J. Hong, A. May, D. Bansal, J. Ma, T. Hong, S. Chi, G. Ehlers, and O. Delaire^#, “Orbitally-driven giant phonon anharmonicity in SnSe”, Nature Physics 11, 1063 (2015).
[3] J. Niedziela*, D. Bansal*, A. May, J. Ding, T. Lanigan-Atkins, G. Ehlers, D. Abernathy, A. Said and O. Delaire^#, “Selective Breakdown of Phonon Quasiparticles across Superionic Transition in CuCrSe₂”, Nature Physics 15, 73 (2019).

The plasticity of Zr-Cu-Al bulk metallic glasses (BMGs) can be significantly improved by introducing nanoscale heterogeneous structure with the addition of Fe. However, the crystallization pathways of the phase-separated Zr-Cu-Fe-Al BMG is still unclear. The neutron and synchrotron pair distribution function (PDF) analysis illustrates that the phase-separated Zr-Cu-Fe-Al BMG behaves two-step crystallization behavior. Compared with the homogenous Zr-Cu-Al BMG with a single crystalline product tetragonal Zr₂Cu, the crystalline products of the phase-separated BMG are more complicated. Upon heating the phase-separated BMGs above the crystallization temperatures, the first crystalline products of the phase separated BMG, i.e., the cubic Zr₂Cu and Zr₂Fe with Fd-3mS space group, form firstly, and then transform to simpler tetragonal Zr₂Cu phase with I4/mmm space group and orthorhombic Zr₃Fe phase with Cmcm space group. Our experiments, by taking advantage of the phase contrast difference of the neutron and synchrotron PDF, reveal that there is a specific sequence of the appearance of the copper-rich and iron-rich crystalline phases during each stage. Our study would shed light on developing BMGs of a controllable heterogeneous structure with potentially tunable mechanical properties.

Diffraction techniques provide access to dynamical structure factors, which contain detailed information about dynamic processes in materials. The extraction and decoding of this information is, however, non trivial and can greatly benefit from atomic scale simulations. To this end, we here present a flexible and powerful tool that enables the calculation of static and dynamical structure factors from trajectories from molecular dynamics (MD) simulations. The DYNASOR software can readily handle input from several major open source MD packages and thanks to its C/Python structure can be readily extended to support additional software. DYNASOR is hosted as open source on gitlab [1] and can easily be installed via PIP. In addition to dynamical structure factors the code enables computation of current correlation functions and separating contributions in the form of partial correlation functions. The performance and potential of DYNASOR is demonstrated for both solids and liquids, in particular focusing on the possibility to extract the full temperature dependence of both phonon frequencies and lifetimes.

Variable temperature neutron diffraction studies of certain molecular organic framework (MOF) materials reveal that different types of H₂ can be coordinated based on the temperature of first exposure. Both neutron powder diffraction (NPD) and inelastic neutron scattering (INS) have been implemented to best describe this phenomenon. Rietveld refinements of the NPD data provides crystallographic evidence of the adsorbed H₂ locations, and INS provides descriptions of the dynamics of the H₂. Temperature dependent data indicates that a pseudo-chemisorbed state can transform into a phisisorbed state upon warming. Furthermore, certain materials studied display NTE that can be modulated based on the different types of H₂ adsorbed.

Concurrent neutron Compton scattering and neutron diffraction experiemetns at temperatures between 70 K and 300 K have been performed on proton conducting hydrated BaZr_0.7Ce_0.2Y_0.1O_3-δ (BZCY72) fabricated by spark plasma sintering. A combined neutron data analysis, augmented with density functional theory modelling of lattice dynamics, has enabled, for the first time in this technologically important family of proton conducting perovskite oxides, a mass-selective view of the cobmined thermal and nuclear quantum effect on local effective binding. The emerging picture is the one of increased anharmonic character of the lattice dynamics and local binding of the framework atoms above the orthorhombic to rhombohedral phase transition at 85 K, whereby a subtle interplay between mode hardening and softening with increased temperature tunes the effective local binding of nuclear species. Importantly, these anharmonic effects seem to be most pronounced in the case of oxygen and cerium. For the latter, the obtained results may shed more light on the origins of high sensitivity of the mixed conductivity observed in BaZr_0.7Ce_0.2Y_0.1O_3-δ. with increased Ce doping level. For the former, together with the neutron data on protons, our analysis strongly suggests the existence of a single type of trapped protons and trap proton sites in the room-temperature non-conducting phase of BZCY72. The protons at room temperature possess insufficiently high kinetic energy to overcome the local barrier for long-range diffusion but enough to perform transfer between different trap sites. A plausible explanation of the origins of the onset of ionic conduction would then involve distinct proton types, the ones trapped around the edges of YO6 octahedra, and the remainder formed of protons located near the ZrO6 and CeO6 octahedral edges, the sites that would favour mobile protons. The apparent proton conductivity would then result from a subtle interplay between the population size and mobility of the trapped and free proton fraction as a function of temperature.

Doping of gallium nitride (GaN) has become an important topic in the field of wide bandgap semiconductors. We have recently introduced a method for uniform doping of GaN using neutron transmutation doping (NTD). In NTD, gallium inside GaN is transformed into germanium through the use of neutron irradiation. Because of radioactivity after irradiation, measurement of doping concentration has been very challenging. Thus, we employed gamma-ray spectroscopy to accurately measure concentrations of the germanium doping in NTD GaN. This method relies on the gamma-rays outputted by decaying Ga-72 atoms before they beta decay into Ge-72. However, due to a difference in half life, the Ga-70 activity cannot be safely measured this way. An estimate of Ga-70 activity was calculated based off the theoretical amount of Ga-72 and the measured amount of Ga-72. We have confirmed the accuracy of the measurement by comparing data from inductively coupled mass spectroscopy (ICP-MS) and secondary ion mass spectroscopy (SIMS). We have successfully confirmed that our gamma-ray spectroscopy measurements turn out to be a highly accurate method for doping concentration measurement of NTD GaN.

Melanin is a common and naturally occurring pigment that plays a number of important roles in the human body. Due to its biocompatibility and proton-conducting properties, melanin is a material of great interest in the field of electroceuticals; electronic devices that can interface with biological systems. Development of such devices would pave the way for personalised healthcare and for advanced health informatics, which was listed among the grand challenges for the 21^st century by the National Academy of Engineering in the USA. However, there is still much that is unknown about melanin and it remains an area of active research in particular for its conducting properties and associated proton dynamics in the solid state.
Inelastic neutron scattering (INS) is a technique that is uniquely suited to investigating proton dynamics within materials, as the vibrational spectra obtained from INS are particularly sensitive to modes involving hydrogen. Unlike optical methods such as Raman and IR spectroscopies, there are no selection rules for INS and so all modes are allowed. Additionally, as melanin lacks long-range order, it is not amenable to study by diffraction techniques and INS can indirectly offer structural information by virtue of comparison with theoretical spectra produced from calculations.
Peak intensities in INS can be wholly described by the neutron scattering cross-sections and absolute quantities of the atoms involved. This means that predicting INS spectra is often a much more reliable process than for other techniques. For small, isolated molecular species or materials with well-characterised crystal structures, agreement between predicted and experimental spectra can be very good, thus leading to full assignment of the features in the spectra. However, for larger and less well-characterised macromolecular materials such as melanin, performing the theoretical calculations necessary to predict the associated INS spectrum is non-trivial and in most cases the computational cost is prohibitively expensive.
We present our recent work towards a method by which the experimental INS spectrum of a melanin sample can be analysed by the combination of several short and simple first-principles calculations to gain insight into the system for a reasonable computational cost.
In the first stage we perform density functional theory (DFT) calculations for a range of expected monomeric moieties, which are used to obtain vibrational frequencies and atomic displacements for each of their vibrational modes. These calculations can then be interpreted by AbINS, a module within Mantid which was developed to generate predicted INS spectra from such calculation outputs. Lastly, we create a linear combination of these “monomer spectra” in order to replicate the experimentally measured spectrum of melanin itself. From here, the main features of the spectrum can be identified and characterised, leading to insight into its structure.

Abstract: Neutron radiography is a non-destructive non-invasive technique capable of characterizing materials in-situ. At reactor facilities, this method is capable of measuring defects such as porosity or cracks in advanced alloys and engineering components, water movement in geomaterials and plant roots, lithium transport in functioning batteries, etc. At these facilities, novel techniques such as grating interferometry have recently been developed as a mean to change contrast and to increase sensitivity to smaller features. Spatial resolutions of tens of µm can be achieved over a field-of-view of several cm². Moreover, in the past decade, the development of time-of-flight (TOF) imaging capabilities at spallation neutron sources have enabled the characterization of microstructures and isotope mapping using Bragg edge and resonance imaging, respectively. Spalled neutrons offer novel imaging contrast mechanisms in both the thermal/cold and epithermal range that are not easily attainable at reactors. This presentation aims at giving an overview of neutron radiography and computed tomography techniques at both reactor and spallation sources, followed by selected research examples such as lithium transport in batteries, hydrogen content in engineering materials, 3D mapping of additively manufactured (AM) components, microstructure evolution in AM samples as a function on temperature and stress, isotope mapping in nuclear materials, etc. A brief overview of the VENUS project, a TOF imaging beamline in construction at the Spallation Neutron Source of Oak Ridge National Laboratory, will also be given.

Acknowledgments:
This research was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE. This research used resources at the Spallation Neutron Source and the High Flux Isotope Reactor, U.S. Department of Energy (DOE) Office of Science User Facilities operated by the Oak Ridge National Laboratory. Research at the Manufacturing Demonstration Facility (MDF) was sponsored by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
Notice of Copyrights: This abstract has been authored by UT-Battelle, LLC, under Contract No. DE AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Imaging techniques based on neutron beams are rapidly developing and have become versatile non-destructive analysing tools in many research fields. Due to their intrinsic properties, neutrons differ strongly from electrons, protons or X-rays in terms of their interaction with matter: they penetrate deeply into most common metallic materials while they have a high sensitivity to light elements such as hydrogen, hydrogenous substances or lithium. This makes neutrons perfectly suited probes for research on materials that are used for energy storage and conversion, e.g. batteries, hydrogen storage, fuel cells, etc. Moreover, their wave properties can be exploited to perform diffraction, phase-contrast and dark-field imaging experiments. Their magnetic moment allows for resolving magnetic properties in bulk samples. This presentation will focus on recent applications of neutron imaging techniques in both materials research and fundamental science illustrated by examples selected from different areas.

Light water nuclear reactor (LWR) technology and the development of advanced nuclear reactors depend on materials innovation and mitigating deleterious environmental effects. This talk will focus on the utility of neutron scattering techniques to interrogate LWR structural materials. Emphasis will be placed on the study of hydrogen behavior in LWR cladding. We use the newest generation of neutron sources and instruments to study hydrogen at low concentration; novel experiment results will be presented that demonstrate the utility of neutron scattering as applied to nuclear materials.

Advanced fuel materials for nuclear energy systems are being investigated using neutron imaging at both the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). Research efforts have specifically focused on understanding how production processes impact elemental distributions within the tristructural-isotropic (TRISO) fuel kernels. Conventional characterization techniques (i.e. scanning electron microscopy with energy dispersive spectroscopy, optical microscopy, etc.) often require significant and destructive sample preparations, are greatly limited by the penetration depth, and thus are unable to extract bulk information. In our work, attenuation-based neutron computed tomography at CG-1D beamline at HFIR was performed on TRISO fuel kernels to evaluate the presence of carbon agglomerates in 3 dimensions. Additionally, at the SNAP beamline at SNS, neutron resonance computed tomography was performed to map the isotopic content of the kernels contain uranium and gadolinium. Neutrons with energies higher than 1 eV are absorbed by the nucleus of an atom. Absorption as a function of neutron energy can be mapped from the radiographs and display isotope-specific peaks that can be fitted to quantify the amount of isotope present in the material in 3 dimensions. This presentation presents the characterization of nuclear fuel materials using both the attenuation-based and resonance computed tomography techniques. These two techniques can be utilized in unison to provide unique insight on the material’s manufacturing processes.

Acknowledgments
This research was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE. This research used resources at the Spallation Neutron Source and the High Flux Isotope Reactor, U.S. Department of Energy (DOE) Office of Science User Facilities operated by the Oak Ridge National Laboratory.

Notice of Copyright
This abstract has been authored by UT-Battelle, LLC, under Contract No. DE AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Polydimethylsiloxane (PDMS) or silicone, is widely used in both industrial applications and academic research area due to its low cost, easy manufacturability, backbone flexibility, low surface energy, and chemical and thermal stability. PDMS elastomers are typically prepared by a hydrosilyation reaction between the hydride groups and the vinyl groups in PDMS. Alternatively, it has been used to synthesize foams vulcanized at room temperature for cushioning applications by reacting a hydride-functional PDMS with a hydroxyl-terminated PDMS at room temperature.^1-3
There is an increased demand in aerospace, nuclear reactors, and other neutron-producing sources for neutron shielding materials to mitigate ionizing radiation damage.^4-5 The attenuation from neutrons radiation in space and nuclear applications can be performed using isotopically-enriched boron (¹⁰B) because of its large neutron cross-section. ¹⁰B has previously been successfully incorporated in a PDMS and other polymer matrixes for these types of applications.^{3, 6} However, compliant PDMS composites with high concentrations of ¹⁰B (≥ 70 wt%) have not been proposed.
The Energy-Resolved Neutron Imaging (ERNI) Flight Path at the Los Alamos Neutron Science Center (LANSCE) allows to probe boron-containing materials and obtain an energy resolved transmission spectra on a pixel-by-pixel bases. This allows 3D reconstructions via Computed Tomography (CT) of the samples and verifies the filler distribution.
In this work, we study the neutron attenuation and boron distribution of highly-filled boron-containing PDMS composites (50-70% by weight) using the capabilities at LANSCE and CT. Additionally, the chemical, thermal and mechanical properties will be studied using a wide range of experimental techniques are used including Fourier transform infrared spectroscopy, mass spectroscopy, differential scanning calorimetry, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and mechanical testing. The resistance of the composite materials to solvents will be investigated through solvent swelling experiments and exposure to high humidity. The presented work lays the foundation for highly-filled composite polymer foams to be considered for space and nuclear applications.

References
(1) Labouriau, A.; Robison, T.; Geller, D.; Cady, C.; Pacheco, A.; Stull, J.; Dumont, J. H. Coupled aging effects in nanofiber-reinforced siloxane foams. Polymer Degradation and Stability 2018.
(2) Labouriau, A.; Cox, J. D.; Schoonover, J. R.; Patterson, B. M.; Havrilla, G. J.; Stephens, T.; Taylor, D. Mössbauer, NMR and ATR-FTIR spectroscopic investigation of degradation in RTV siloxane foams. Polymer degradation and stability 2007, 92 (3), 414-424.
(3) Labouriau, A.; Robison, T.; Shonrock, C.; Simmonds, S.; Cox, B.; Pacheco, A.; Cady, C. Boron filled siloxane polymers for radiation shielding. Radiation Physics and Chemistry 2018, 144, 288-294.
(4) Thibeault, S. A.; Kang, J. H.; Sauti, G.; Park, C.; Fay, C. C.; King, G. C. Nanomaterials for radiation shielding. Mrs Bulletin 2015, 40 (10), 836-841.
(5) Schaeffer, N. M. Reactor shielding for nuclear engineers; Radiation Research Associates, Inc., Fort Worth, Tex.(USA): 1973.
(6) Harrison, C.; Weaver, S.; Bertelsen, C.; Burgett, E.; Hertel, N.; Grulke, E. Polyethylene/boron nitride composites for space radiation shielding. Journal of applied polymer science 2008, 109 (4), 2529-2538.

Neutron imaging has seen rapid development in new methods. These methods exploit neutron diffraction, refraction, and magnetic precession, to generate novel image contrasts. Dark field image acquired over a broad range of autocorrelation lengths (nm to µm) provides a quantitative measure of the porosity in the sample and in 3 dimensions. By acquiring images as a function of wavelength, and determining the wavelength at which a particular lattice plane no longer contributes to scattering (so-called Bragg edges), one can infer crystal phase. If the scan is made with sufficiently fine wavelength resolution, one can measure the average strain along the beam direction. Another recent advance comes from combining neutron and X-ray tomography (NeXT) in a simultaneous fashion. NeXT provides researchers with a unique tool to study complex structures, as the bivariate histogram of attenuation values makes the problem of segmentation slightly easier. Because the two probes interact differently for a broad range of materials that are difficult to separate with one probe alone. As an example, in fluid flow in geological specimens, water and void space can be difficult to distinguish in an X-ray tomogram, while the matrix and void space can be difficult to distinguish in neutron tomograms. By combining both views simultaneously, the problem of segmenting flow is straightforward. This presentation will discuss dark field imaging in geology and additive manufacturing, the Bragg edge imaging system at NIST using the analysis of austenite vs martensite in TRIP steel, as well as introduce the NIST-NeXT system highlighting the analysis of concrete and shale rocks.

There has been growing interest in wrought magnesium alloys (most commonly, AZ31) for strong, lightweight structural components in cars, aircraft, and electronics. These components are subjected to non-linear loading conditions during cold-forming and use, which is known to have significant effects on the mechanical properties of metals. It is therefore important to study the microstructural evolution during strain path changes in these alloys to understand their macroscopic response under such conditions.

In situ neutron diffraction has been used extensively to investigate the evolution of intergranular strain during loading to provide insight on the relative behavior of different magnesium alloy deformation modes during uniaxial tension/compression tests. In this work, we report on the results from in situ neutron diffraction experiments combined with in situ acoustic emission of strain path changes of cold-rolled Mg AZ31B sheets. It is found that, while dislocation slip prevails as the main deformation mechanism, the twinning activity strongly depends on the nature of the strain path change. Post mortem EBSD is also presented to characterize the twin variant selection, which depends heavily on the strain path change angle as well.

This research is performed within the ERC Advanced Grant MULTIAX (339245).

Complex hydrides have been found to exhibit variety of functionalities that can be utilized for many applications. One of the important applications of hydrides is for energy storage such as hydrogen storage due to the high hydrogen capacities of the light weight complex hydrides and superionic battery application due to the fast ionic mobility in some small ion-containing hydride systems. In this talk, I will present some of our recent work on rational development of novel complex hydrides, e.g. inorganic-organic combined complex hydride systems for hydrogen storage, including metal guanidinates, borohydride guanidinium complexes, and novel salts comprised of alkali cations and large polyhedral boron-hydrogen-based anions for superionic conduction. I will focus on the structural studies of these materials with aspects of crystal structure determination of novel hydrides using the combined x-ray/neutron powder diffraction and neutron vibrational spectroscopy. The rich information obtained from the structural and dynamic analysis and their implications for hydrogen storage and ionic conductivity will be discussed.

Adsorption of molecules in functionalized and high surface area metal-organic frameworks (MOFs) is of emergent technological importance in a multitude of areas ranging from chemical separations to energy storage. We have been studying the properties of MOFs for storage and separations of industrially important small molecules such as hydrogen, oxygen, carbon dioxide, noble gases, and short chain organics. Besides the geometrical and porosity control available in MOF chemistry, the properties of the frameworks can be tweaked to elevate electrostatic interactions by exposing open metal cation sites or functionalizing ligands. Here, we discuss the information accessible from neutron scattering experiments on a selection of nominally rigid MOFs. The results illustrate the power, and limitations, of diffraction and spectroscopy in elucidating the governing characteristics of these material properties and the interactions with the guest molecules.

Organic semiconductors (OSCs) provide the unprecedented ability to tailor properties like electronic band gap, mechanical flexibility, processability, and biocompatibility. Recent theories suggest that low frequency dynamic intra- and intermolecular motions are critical to determining localization of the charge carrier, and thus, control the hole mobility. We used inelastic neutron scattering (INS) to probe thermal disorder directly by measuring the high resolution phonon spectrum in multiple small molecule OSCs. We achieved near perfect agreement between the INS spectra of OSC crystals and first principle electronic simulations. This simulation is used to generate a set of electron-phonon coupling parameters, which are used to compute hole mobility using transient localization theory. The charge mobility, calculated from first principles, is in excellent quantitative agreement with macroscopic measurements.

We note however, that most OSCs are not extended crystals. Instead most OSCs are a mixture of multi-crystalline and amorphous domains. The electronic simulation method used in our first study (plane-wave DFT) is much too computationally expensive to include chemical, structural, or even orientational disorder. So even though it is possible to measure a low energy phonon spectrum using INS in a polymeric or multi-crystalline sample, it was until now not possible to model the spectrum. We developed two multi-scale modeling techniques to model phonons in disordered samples and allow us to quantitatively validate the models using INS spectra. For modeling disordered crystals we developed the use of density functional tight binding to model phonons and INS spectra. For fully amorphous samples we developed the use of molecular dynamics to model INS spectra in amorphous small molecules and polymers. This presentation will detail the use of these modeling techniques.

The Neutron Imaging and Applied materials Group at the Paul Scherrer Institut (PSI) in Switzerland has a long tradition of supporting industry and industrial R&D with neutron imaging and diffraction investigations. Often results have an impact on advancing and improving manufacturing. In particular also the development of metal additive manufacturing (AM) can profit from the ability of neutrons to penetrate deep into and to hence non-destructively assess bulk metal materials.

Conventional neutron imaging is applied at the imaging beamlines NEUTRA and ICON to provide insight into produced porosities and geometric integrity of required structures inside a built part. The results are to validate predictions and build strategies and to guide e.g. sensor developments for in-situ AM process control. While neutron imaging at PSI provide spatial resolutions down to below 5 micrometers, with novel advanced methods structural variations on sub-micrometer length scales can be resolved.

The fact that in additive manufacturing the material is established simultaneously together with the geometry of a specific part, enables to tune the material properties with respect to loading states it undergoes during service. At POLDI, a neutron diffractometer for engineering materials research, the mechanical behavior of TRIP and TWIP steels is investigated, in order to determine the crystallographic textures to be locally produced by AM for either promoting or suppressing the TWIP or TRIP effects with respect to the applied loading.

While neutron diffraction is routinely applied to study local residual stresses generated during the build process, respectively their relief during post build treatments, also advanced neutron imaging techniques provide sensitivity to local crystallographic features such as microstructure and residual stresses. For example, the residual stress variations in the critical surface regions achieved through laser shock peening (LSP), displaying a strong gradient towards the bulk reaching to depths of millimeters, have been shown to be best probed by Bragg edge imaging. The method provides sufficient spatial and crystallographic resolution while being able to provide large area views of treated specimen.

A number of recent examples of neutron measurements in the service of advanced manufacturing technology shall be presented and discussed.

In solids atomic dynamics is described in terms of phonons and diffusion. In liquids, however, the time-scales of the two are similar, of the order of ps, and consequently phonons are strongly damped and diffusive steps are dynamically correlated. To describe such dynamics the conventional approach in the momentum (Q) and energy (E) space in terms of the dynamic structure factor, S(Q, E), is grossly inadequate. Instead we propose to describe the local dynamics in real space and time, using the Van Hove function (VHF), G(r, t). We demonstrated recently that the VHF can be determined by inelastic x-ray or neutron scattering (IXS or INS) experiments as the double-Fourier-transform of S(Q, E). To calculate the Fourier-transformation accurately S(Q, E) has to be determined over wide ranges of Q and E. For a long time inelastic scattering measurements have been time-consuming, and to obtain S(Q, E) over wide ranges of Q and E has been impractical. However, the advent of pulsed neutron sources with two-dimensional detectors greatly increased the data collection rate, making it realistic to obtain the VHF in reasonable time, typically 4 to 6 hrs. The VHF of water determined by IXS and that of molten metallic alloy liquids by INS enable us to visualize correlated local atomic dynamics in liquids in real space and time. The lifetime of the local environment determined by the VHF explains quantitatively the origin of viscosity. The time-scale of VHF is dictated by the energy resolution of the measurement. However, energy resolution and the range of Q space are coupled. Therefore we use several incident energies and combine the data to obtain S(Q, E) with high resolution. We plan to apply this approach to the study of local dynamics of liquids with important applications, such as aqueous solution of salts and liquid electrolytes for energy storage and as insulator in field-effect-transistors. The knowledge garnered shall contribute to the bottom-up design of better functional liquids.

It is widely recognized in catalysis, fuel cell and battery chemistry, bio- and geochemical processes, and a host of additional functional materials areas that unique properties and characteristics are governed by intricate structural-chemical relationships. Uncovering the identity and role of locally ordered motifs, including those of surface species and interfaces, remains a challenge because experimental tools to observe materials at atomic length-scales, in relevant operating conditions, or within sufficiently fine time scales are limited. We present our efforts to apply and extend neutron total scattering and related probes towards capturing the interplay of crystal chemistry and functionality in nano- and nanostructured materials. Examples include: (1) exploration of internal dipole-dipole ordering in ferroelectric nanocrystals, demonstrating the enhancing effects of cubic particle shape and polar surface termination; (2) investigation of layered manganese oxide structures, where interlayer water molecules, hydrogen bonding, and the nature of vacancies/intercalants strongly impact electrochemically active variants; and (3) demonstrated abilities to probe the structure and dynamics of gas-solid interfaces in catalytic materials, where the signatures of interfacial species are enhanced through the use of neutron isotope contrast techniques. These examples improve understanding of technologically significant materials and highlight a broader theme of our research aimed at extracting crystal structure models from experimental data with the detail needed to guide and validate modern nanoscale theories, and design new and improved functional materials. Current challenges and future opportunities in this arena will be discussed.

High performance Li-ion batteries manufactured with bio-based and renewable materials are a necessity in a market focused on efficiency, sustainability and cleaner energy. Previous studies have suggested a solution through lignin, a low-cost, renewable bio-feedstock with a high carbon content. Processing and pyrolysis of the lignin produces a graphitic composite composed of nanoscale crystallite spheres dispersed in an amorphous matrix and have shown success in uses as high-performance anodes in Li-ion batteries. These bio-based composite structures have proven to have superior charge capacity, high reversible capacity, low irreversible capacity loss, and high cycle life when compared to traditional intercalated Li-ion anodes. Understanding the structure of the bio-based anode is critical to explaining the exceptional anode properties. The local atomic environment is often characterized via a radial distribution function calculated from neutron and x-ray scattering experiments, but when studying complex materials, interpretation of the RDF can present a significant challenge due to the nature of scattering from the amorphous phase. Traditionally, large scale molecular dynamics (MD) simulations are used to form a hypothetical structure and generate a corresponding RDF to be compared to experiment. Alterations to the material’s structure and constituent particle size in MD simulations is an expensive and time-consuming process. A significantly more efficient approach for the interpretation of RDFs of complex materials is the Hierarchical Decomposition of the RDF (HDRDF), in which the total RDF is decomposed into components, some of which are modeled with static atomic structures and others of which are modeled as continuous mesoscale objects. The HDRDF software generates a real-space model of the complex nanomaterial and correlates the RDF to a set of material descriptors, such as crystalline domain size, crystalline volume fraction, total density, etc. and compares its RDF to experiment. The HDRDF approach has been shown to use roughly one million times less computational resources compared to MD simulation. We report progress on the development of a second generation of the HDRDF software, which extends the capability to arbitrary domain geometries and we apply the HDRDF technique to the interpretation of neutron and x-ray scattering of lignin-based nanostructured carbon composites. This work implements HDRDF to quantitatively determine how choice of lignin feedstock and pyrolysis temperature impacts the nano- and mesoscale structures of the resulting graphitic composites. Initial results have shown crystallite radius increases and crystalline volume fraction decreases as pyrolysis temperature increases.

The phenomenon of creep in concrete, even after 40 years of experimental and theoretical research, is not fully understood. While it is commonly agreed that the viscoelastic nature of the primary hydration product, calcium-silicate-hydrate (C-S-H) gel, is largely responsible for concrete creep, the physical, nano-scale origin behind this viscoelastic nature is not clear. In order to determine if atomic-scale structural reorganization of C-S-H occurs during creep, in situ neutron scattering was used to continuously examine cementitious samples subject to uniaxial compression over a period of 9 months. For this in situ study, a custom load-frame was designed and implemented at the Nanoscale Ordered Materials Diffractometer (NOMAD) beamline at the Spallation Neutron Source (SNS), Oak Ridge National Lab. Pair distribution function (PDF) analysis of total neutron scattering was performed to follow the evolution of changes in the local structure of C-S-H, resulting in novel insights for the mechanism of concrete creep.

The quantum dimer magnet (QDM) is the canonical example of "quantum magnetism". This state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced "triplon" Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb₂Si₂O₇ [1]. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet zero field ground state with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of H_c1 ~0.4 T and H_c2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode (gapless to within = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field (H > 1.2T) part of the phase diagram in Yb₂Si₂O₇ is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.
[1] Gavin Hester, H. S. Nair, T. Reeder, D. R. Yahne, T. N. DeLazzer, L. Berges, D. Ziat, J. A. Quilliam, J. R. Neilson, A. A. Aczel, G. Sala, and K. A. Ross. A Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb₂Si₂O₇. Phys. Rev. Lett., (in press) (2019)

The European Spallation Source is currently under construction in Lund, Sweden. It is designed to provide world-leading performance, with instruments optimized for the long-pulse time structure of the facility, making full use of what will be the world’s brightest neutron beams for the study of materials ranging from biological systems and soft matter to engineering materials, structural chemistry and magnetism.
An overview will be given of the state of the construction project, as well as key design features of the facility, allowing an unprecedented degree of flexibility and performance for the instruments. 15 instruments are under construction, covering SANS, reflectometry, imaging, engineering diffraction, powder and single-crystal diffraction, chopper spectrometers, and crystal-analyser spectrometers. Highlights of instrument design aspects will be raised, which further enhance the scientific impact of the instruments.

Neutron scattering has become a vital tool for studying materials across many scientific fields and applications, including automotive engines, batteries, data storage, geology, polymers, and biomedicine. In the United States, the Office of Science of the US Department of Energy (DOE) supports two major neutron scattering user facilities at Oak Ridge National Laboratory (ORNL): the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS). SNS is currently undergoing a proton power upgrade (PPU) to double its power capability to 2.8 MW. This upgrade, which will be completed in 2024, will deliver 2 MW of proton beam to the existing First Target Station (FTS) at the SNS, resulting in a significant increase in thermal neutron brightness to enable new capabilities for materials research in the thermal energy (shorter wavelength) range. The PPU project will also provide the addition power required to operate a planned Second Target Station (STS) for SNS. STS will provide pulsed beams of cold (longer wavelength) neutrons with unprecedented peak brightness and containing broad ranges of neutron energies. These beams will provide wholly new capabilities for the study of a broad range of materials using neutron scattering and support users in many fields of research—materials science, physics, chemistry, geology, biology, and engineering, among others—and in industry. When it is fully built out, the capabilities offered by the twenty-two instruments planned for the STS will complement those of HFIR and the FTS. The STS has a pivotal role to play in extending the reach of neutron scattering to new transformative opportunities for discovery science and applications that require time-resolved examination of nonequilibrium processes in dynamic hierarchical systems over greatly increased length, energy, and time scales. To illustrate the extraordinary potential of the STS to impact a broad spectrum of scientific fields, examples of first experiments to be conducted at the STS will be described.

The first Neutron Spin Echo (NSE) instrument has been in user operation for 40 years now and the newest one: WASP at ILL has just started commissioning. I will use this occassion to review how the basic design of the NSE spectrometers have developed during these years. The technical and scientific capabilities of the new instrument will be presented as well.
All functioning Neutron Spin Echo spectrometers use the basic IN11A design where the precession field is generated by long solenoids along the neutron beam. This construction limits the angular coverage and count rate of the instruments. Last century there have been two tries to make a wide angle coverage neutron spin echo instrument. IN11C at ILL is equipped with a flattened solenoid and has been in use since its creation. It has a 30 degree-wide angular coverage but a very limited resolution. This instrument was practically trading intensity for resolution. The SPAN instrument at HZB used a pair of coils in the anti Helmholtz configuration creating an azimuthally symmetric magnetic field; which, in theory, could allow a nearly 360 degree detector coverage. WASP uses an improved SPAN construction, and it aims to have a 500 times higher detected intensity than IN11A while the resolution remains the same.
The long construction has finished in 2018, and the three weeks of commissioning were promising. We have echo in all detectors up to one third nominal field integral, and the detected intensity is 500x of IN11A. By the time of the conference we hope to present detailed characterisation of the instrument.

The European Spallation Source ESS in Sweden is currently in construction and will receive first neutrons in 2021. The start of the user program will be in 2023. Even now, during construction of the facility, we are encountering interesting material problems, some of which can be solved best using neutron scattering. This presentation will show examples for ESS material research that supports construction and leads towards operation. The data shown throughout the contribution has been recorded at currently operating spallation sources such as SNS, ISIS and JPARC.
Material degradation due to radiation is a limiting factor for the lifetime of high power spallation sources. This well-recognized fact leads to increased research efforts in finding the best materials for construction of ESS and understanding what happens with the materials on the atomic scale when exposed to a radiation field. These material investigations are greatly aided by the use of neutrons. Examples given are connected to understanding the radiation hardness of greases and oils, investigating the effect of purity and grain size of beryllium on its effectiveness as neutron reflector as well as solving issues related to the ortho- to parahydrogen conversion in the cold moderators.
With ESS moving towards the start of the user program in 2023, we are beginning to collaborate with local scientists and small local industry on scientific projects using neutron scattering as main analysis tool. This helps to educate the local user community in the usefulness of neutrons and us in preparing the user laboratory infrastructure for the future. We will show an example using neutron vibrational spectroscopy to observe bisphenol A adsorbed on sand and clay. This is currently an area of great interest to the water industry as these materials are part of water filters potentially suitable to remove bisphenol A from sewage water. This project benefitted from the use of neutrons as the bisphenol A adsorbed on clay and sand cannot easily be detected using optical spectroscopy or X-ray methods.

In situ small angle neutron scattering under flow (flow-SANS) has become a critical tool for developing processing-structure-property relationships of complex fluids and soft matter. However, sample environments and associated measurement methods for flow-SANS have largely limited these measurements to steady state flows and simple rheometric deformations (pure shearing or elongation) that fail to capture the complex nonlinear and time-varying deformations encountered during processing. To bridge this gap, we have developed a novel fluidic four-roll mill (FFoRM) that, in combination with recent capabilities for spatiotemporal measurements, enables in situ SANS measurements under programmable quasi-two dimensional flows within a single device.[1] Here, we highlight the unique capabilities of “FFoRM-SANS” by its application to understand flow-induced ordering during the processing of rod-like polymeric and colloidal fluids. Interpretation of the rich data sets obtained are interpreted using a new, general framework for model-free estimation of orientation probability distribution functions (OPDFs) from scattering of orientable particles. Doing so enables parameter-free quantification of the effective structure factor that captures the dominant contributions of flow-interaction coupling on orientational order. We find that the effective structure factor depends significantly on the flow history of the fluid, and thus provides rational criteria for designing processing flows for improved or controlled ordering.
[1] P.T. Corona, N. Ruocco, K. Weigandt, L.G. Leal and M.E. Helgeson, “A fluidic four-roll mill for in situ neutron scattering under arbitrary two-dimensional deformations”, Scientific Reports, 2018, 8(1): 15559.

Wormlike micelles (WLMs) are self-assembly systems formed by amphiphilic surfactants immersed in water. Understanding the structural origin of their rheological properties is important for scientific as well as technological reasons. Small-angle neutron scattering (SANS) has been used to investigate the conformational characteristics of WLMs under macroscopic deformations. Upon increasing the applied stress, the angular dependence of SANS intensities develops progressively due to the flow-induced alignment at a molecular level. In existing analysis schemes, an assumption of the angular distribution function is often required to extract the orientation of aligned WLMs from their anisotropic SANS spectra. The validity of the assumption is, however, not known a priori.

To bypass this intrinsic constraint, we have developed a rigorous approach based on the spherical harmonics expansion (SHE); this approach allows us to extract the conformational information of the aligned WLMs from their scattering intensity profiles without any model fitting. The feasibility of this approach is verified theoretically. Based on a case study of the hexadecyltrimethylammonium bromide (CTAB)/sodium salicylate WLM system, we demonstrate that our method not only facilitates the quantitative scattering studies of deforming materials but also provides insightful information regarding their deformation behavior at the molecular level based on the symmetric properties of real spherical harmonics.

*YS and TE acknowledge support from the US Department of Energy, Office of Science, Basic Energy Science, Materials Science and Engineering Division.

There has been much interest in understanding the structural characteristics of deforming materials that are manifest on different length scales. In the context of scattering investigation, the domain of interest can be decomposed into a microscopic region within which the local configurational translation and rotation can be treated in detail, and a larger regime where the description of global shape envelope is appropriate. From a perspective of geometric interpretation at either the microscopic or the macroscopic level, a central issue is how to quantitatively determine the structural characteristics from the anisotropic scattering intensity in a model-free manner.
We present a general approach to deal with this problem, which is valid to different deformation conditions and mathematically rigorous. In the mean-field limit, we first demonstrate that the radius of gyration is indeed the source term of intra-particle structure factor. By expanding the spectral anisotropy using the real spherical harmonic expansion (RSHE), we derive the exact mathematical expression of 2^nd order gyration tensor in terms of the anisotropic spatial correlation functions. Based on the same RSHE scheme, we derive the orientational distribution functions to describe the local configurational alignment. Theoretical benchmark studies demonstrate that our developed approach not only facilitates quantitative scattering studies of flowing materials based on the symmetric properties of real spherical harmonics, but also provides insightful information regarding the deformation behavior of materials at the molecular level.

We look into methodology development based on neutron scattering techniques to “visualize” complex phenomena. This visualization must integrate scattering experimental analysis, simulation, and theory to provide real space images of such phenomena. And their potential use in exploring energy and engineering processes is highly promising. Exploratory neutron scattering methodology development for the fields of Nuclear Data and Concentrating Solar Power will be discussed in the presentation.

In Nuclear Data, improvements in determination of the thermal scattering law of moderator materials (measuring, calculating, and validating) are important for accurate prediction of neutron thermalization in nuclear systems. In this work a new methodology for producing thermal scattering libraries from the experimental data for polyethylene (C2H4)n is discussed and expanded to other systems such as Lucite and water. Double differential scattering cross section (DDSCS) experiments were performed at the Spallation Neutron Source of Oak Ridge National Laboratory (SNS ORNL). New scattering kernel evaluations, based on phonon spectrum for (C2H4)n, are created using the NJOY2016 code. Two different methods were used: direct and indirect geometry neutron scattering at ARCS and SEQUOIA, and VISION instruments, respectively, where the phonon spectrum was derived from the dynamical structure factor S(Q,omega) obtained from the measured DDSCS. In order to compare and validate the newly created library, the experimental setup was simulated using MCNP6.1. Compared with the current ENDF/B-VII.1, the resulting Rensselaer Polytechnic Institute (RPI) (C2H4)n libraries improved both double differential scattering and total scattering cross sections. A set of criticality benchmarks containing (C2H4)n from HEU-MET-THERM resulted in an overall improved calculation of Keff , although the libraries should be tested against benchmarks more sensitive to (C2H4)n. The Density Functional Theory (DFT)+oClimax (a package provided by ORNL) method is used and is shown to be most comprehensive method for analysis of moderator materials. The importance of DFT+oClimax method lies in the fact that it can be validated against all data measured at VISION, ARCS and SEQUOIA, and experimental total scattering cross section measurements.

In Concentrating Solar Power, we are developing neutron measurements for in-situ interface corrosion kinetics and molten salt properties. This research will provide fundamental data for material selection including the molten salt systems for both nuclear and solar applications. The presentation will focus on the development of in-situ neutron techniques for fundamental understanding of the mechanisms of molten salt corrosion, and the micro-structural response of containment alloys thereto, to measure the surface corrosion kinetics. We are working on realization and initial application of in-situ techniques for measuring molten salt fundamental properties including molten salt structure, dynamics, and salt density, etc. and the micro-structural and -chemical response of containment alloys to corrosive molten salt environments. The two neutron techniques involved are Vibrational Spectroscopy and Neutron Reflectometry. They both have unique characteristics for complex liquids at high temperatures. Moreover, while we are designing and manufacturing sample environments for harsh environments (high temperature and corrosive), they will provide great first-of-the-kind experimental data for molten salt systems.

We are developing neutron optical devices whose task is to increase the throughput of neutron instruments. For neutron imaging, we demonstrate designs of two types of neutron microscopes. Like an optical microscope, the neutron microscope consists of a condenser and an image forming optics. Neutrons are focused on the sample by the condenser mirrors while the image forming mirrors focus transmitted neutrons on the detector [1]. The condenser is designed to maximize the neutron flux and to obtain desired beam divergence at the sample. The condenser consists of axisymmetric confocal paraboloid and a hyperboloid mirrors, which are concentrically nested. The image-forming optics are designed using different combinations of the confocal nested ellipsoid and hyperboloid mirrors called Wolter mirrors. Both optics are being manufactured for installation at NIST. The design of magnification-10 mirrors should achieve the spatial resolution of about 10 μm. Importantly, the resolution of the microscope is determined by the mirrors rather than by the beam collimation as in conventional pinhole imaging, leading to possible dramatic improvements in the signal rate and resolution. Also, in contrast with pinhole imaging, in the microscope, the samples are placed far from the detector to allow for bulky sample environment. For diffraction, we are developing analyzers for polychromatic cold and thermal diffractometers. Polychromatic incident beam allows for multiplexing analyzer crystals in order to cover relatively large solid angles. Specifically, we optimized axisymmetric PG analyzers inspired by the very first neutron-diffraction experiment by Mitchell and Powers [2]. In addition, we are adopting bent single-crystal wafers following the developments of the so-called “thickness focusing” [3,4]. The combination of the polychromatic beam [5] and focusing geometry allows increasing both the signal rate and resolution. The proposed instruments will have much higher throughput than existing instruments and thus enable diffractometers at compact neutron sources.

[1] P. Jorba, M. Schulz, D.S. Hussey, M. Abir, M. Seifert, V. Tsurkan, A. Loidl, C. Pfleiderer, B. Khaykovich, High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni3Al and HgCr2Se4 under pressure, Journal of Magnetism and Magnetic Materials. 475 (2019) 176–183. doi:10.1016/j.jmmm.2018.11.086.
[2] D.P. Mitchell, P.N. Powers, Bragg Reflection of Slow Neutrons, Physical Review. 50 (1936) 486–487. doi:10.1103/PhysRev.50.486.2.
[3] A.D. Stoica, M. Popovici, C.R. Hubbard, Neutron imaging with bent perfect crystals. I. Imaging conditions, Journal of Applied Crystallography. 34 (2001) 343–357. doi:10.1107/S0021889801005106.
[4] A.D. Stoica, M. Popovici, W.B. Yelon, R. Berliner, Position-sensitive analysis in curved-crystal three-axis neutron spectrometry (quasielastic scattering case), Journal of Applied Crystallography. 33 (2000) 147–155. doi:10.1107/S0021889899012947.
[5] A. Percival, The white beam steady-state diffractometer: A next-generation neutron diffraction strain scanner, MSc Thesis, Queen’s University, Ontario, Canada, 2009. http://hdl.handle.net/1974/1783.

Complete structural characterization of colloidal nanocrystals is challenging due to rapid variation in the electronic, vibrational, and elemental properties across the nanocrystal surface. While traditional characterization techniques such as electron microscopy and X-ray scattering can provide detailed information about the inorganic nanocrystal core, these techniques provide little information about the molecular ligands coating the nanocrystal surface. Moreover, because most models for scattering data are parametrically nonlinear, uncertainty estimates for parameters are challenging to formulate robustly. Using both PbO- and PbCl₂-derived, oleate-capped PbS quantum dots as a model system, we demonstrate the capability of small angle neutron scattering (SANS) in resolving core, surface layer, ligand-shell, and solvent structure for well-dispersed nanocrystals using a single technique. We quantify a ∼0.3 nm thick surface PbCl_x layer on the PbCl₂-derived quantum dots, previously only hypothesized. Global fitting of the SANS data across a solvent deuteration series enables unique determination of the spatial distribution of each material. Molecular dynamics simulations were used to develop a coarse-grained form factor describing the scattering length density profile of ligand-stabilized nanocrystals in solution. We introduce an affine invariant Markov chain Monte Carlo method to efficiently perform nonlinear parameter estimation for the form factor describing such dilute solutions. This technique yields robust uncertainty estimates. This experimental design is broadly applicable across colloidal nanocrystal material systems including emergent perovskite nanocrystals and the parameter estimation protocol significantly accelerates characterization and provides new insights into the atomic and molecular structure of colloidal nanocrystals.

Biopolymers are very important class of materials as they are environmentally friendly. Chitosan is a very popular biopolymer and it has several advantages such as excellent film forming capacity, biodegradability and nanoparticle forming capability as it has reducing and stabilizing effect on metal ions. Understanding the interactions between ions and chitosan can help us make better membranes that can be potentially used in battery technology. Here, the use of Small Angle Neutron Scattering (SANS) in chitosan gels is explored. SANS is a useful technique for the characterization of biological materials. SANS experiments involve scattering of a monochromatic neutron beam from the sample and measures the scattered neutron intensity as a function of scattering vector. The wavelength of the neutron beam used was 5.2 Å with a resolution (Δλ/λ) of about 15%. All of the data were collected in the accessible Q range of 0.017−0.35 Å⁻¹. All of the SANS data were corrected for the background, the empty cell contribution, and solvent contribution, and were normalized using standard procedure. Chitosan solution was prepared by adding 1% (w/v) of chitosan powder, 1.5% (w/v) of acetic acid to D₂O. Previous study by us has shown that we can use neutron scattering to determine the radius of gyration (Rg) of chitosan and the size of the gold nanoparticles. Using SANS and SEM images we could show that the size of the gold nanoparticles templated on chitosan is comparable to that of the Rg of chitosan when the chitosan solution contained 1Mm HAuCl4 and was heated to form red colored solution. Also, we have shown that increasing the concentration of gold in chitosan solution to 3mM and more resulted in the formation of gels. When the gels where heated it resulted in the formation of rose red colored liquid. When chitosan-HAuCl₄ gels where incubated at 25° C the gels collapsed in time. After ten days when the samples where imaged it was observed that micron sized gold particles had formed. The particles where mostly hexagonal in shape. The size of these particles are much larger than the Rg of the chitosan and therefore this result implies that chitosan acts as a template for the gold ions to nucleate and form micron sized particles. The formation and the collapse of the gels was attributed to the electrostatic interactions between the gold ions and the chitosan unit with the gold ions acting as bridges. This in turn made the liquid viscous thus forming gels. SANS was used to study the formation of the gels. SANS results showed that the Rg for chitosan film is lesser than that for chitosan in solution. Also, the Rg for chitosan-gold gels was the same as that of chitosan in solution. There was no correlation length observed in chitosan solution, however it was present in chitosan-gold gels. This result indicates that the chitosan units are localized in the gel phase. These gels have potential applications in detectors.

Within the polymer semiconductor family, poly(9,9-dioctylfluorene) (PFO) has attracted interest for applications such as light emitting diodes and lasers due to its efficient pure-blue electroluminescence. It has been shown that PFO can adopt a unique and interesting conformer termed as ‘β phase’.1,2 This conformer exhibits efficient light emission and spontaneous emission.3–5 However it also emits light at different wavelengths to the other blue emitting material conformers, and therefore can pose challenges in the creation of polymer LED’s. It has also been shown to act as a charge transport trap,6 therefore electron-phonon coupling and slower dynamics are likely to impact photoemission. We look to provide insight into the β phase dynamics to help with characterisation. We use temperature-resolved neutron diffraction and spectroscopy techniques to study β phase and to propose a crystal structure for it. We explore the dynamics using quasi-elastic and inelastic neutron spectroscopy in combination with molecular dynamics simulations and quantum chemical calculations. Combining the neutron data with computational methods allows us to test various polymer-solvent compound/co-crystal structures as candidates for β phase and explore its dynamical features.

Block copolymer micelles enable the formation of widely tunable self-assembled structures in liquid phases, with applications ranging from drug delivery to personal care products to nanoreactors. In order to understand fundamental aspects of micelle assembly and dynamics, the structural properties and solvent uptake of biocompatible poly(ethylene oxide-b-ε-caprolactone) (PEO-PCL) diblock copolymers in deuterated water (D₂O) / tetrahydrofuran (THF_d8) mixtures were investigated with a combination of small-angle neutron scattering (SANS), nuclear magnetic resonance (NMR), and transmission electron microscopy. PEO-PCL block copolymers, of varying molecular weight yet constant block ratio, formed spherical micelles through a wide range of solvent compositions. We investigated the effect of solvent composition on the unimer content, aggregation number, degree of solvent swelling of the micelle core, and micelle size parameters. Furthermore, we explored the impact of guest molecules, such as model drug compounds, on the micelle self-assembly. Finally, we are developing a time-resolved NMR technique, complemented by existing time-resolved SANS methods, to quantify chain exchange dynamics in these systems.

The nanoparticle superlattices (NPSLs) can provide new emerging properties through collective coupling between nanoparticles, which are strongly dependent on the lattice symmetry of NPSLs as well as nanoparticle composition. Various methods have been developed for the synthesis of NPSLs, which include slow solvent evaporation, DNA-mediation, electrostatic interaction and others. In our group, we developed for the first time a micelle-assisted method for forming exceptionally ordered NPSLs which are inherently sensitive to environmental conditions, and used it to demonstrate the thermally reversible structural symmetry transitions of NPSLs. We applied this method for one-dimensional [1] and spherical [2] nanoparticles, respectively, which show a spectrum of lattice symmetries depending on particle size ratio, composition, and temperature. The structural details and phase transitions are investigated by neutron and x-ray small scattering techniques. The maximization of free volume entropy is considered as the main driving force for the formation of superlattices, which is well supported by theoretical free energy calculations.

[1] Sung-Hwan Lim, Taehoon Lee, Younghoon Oh, Theyencheri Narayanan, Bong June Sung and Sung-Min Choi, “Hierarchically Self-Assembled Hexagonal Honeycomb and Kagome Superlattices of Binary 1D Colloids”, Nature Communications 8, 360 (2017)
[2] Jae-Min Ha, Sung-Hwan Lim, Jahar Dey, Sang-Jo Lee, Min-Jae Lee, Shin-Hyun Kang, Kyeong Sik Jin, and Sung-Min Choi, “Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions”, Nano Letters 19, 2313, (2019)

Soft interface characteristic of a strong response to external stimuli represents a grand challenge central to many technical applications, ranging from electrochemistry to colloidal science, heterogeneous catalysis, nanoscience, corrosion, tribology, surface science, biology, and energy production. However, there is a dearth of techniques capable of probing both spatial arrangement and chemical property at interfaces, especially if the solution contains a mixture of ions and organic and biomolecules. As complementary to contrast variation neutron scattering, resonant x-ray scattering (RXS) by merging x-ray scattering and x-ray absorption spectroscopy in a single experiment has developed into an important technique, which yields orders of magnitude more contrast between materials than non-resonant x-ray scattering and provides the unique sensitivity to chemical configurations and spatial relationships of organic functional groups (resonant scattering centers) at important 1–1000 nm length scales. This added resonant sensitivity represents a qualitatively different scattering mechanism than those active in non-resonant x-ray, neutron, and electron scattering.
Here we applied RXS to investigate a breadth of interfaces from Hard-Soft to Soft-Soft: solid-solid interfaces of polystyrene (PS) films on the silicon substrate, solid-liquid interfaces of silica nanoparticles surface grafted poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) zwitterionic polymer brushes (SiO₂-PMPC) in solutions, and polymer interfaces of symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. It is found that the chemical interfacial width of PS thin film at the substrate interface arising from the preferential orientation is about one order of magnitude of the polymer radius of gyration (R_g). Moreover, although the ion-induced changes of PMPC thickness are not apparent in aqueous solutions, their chain conformations like polyzwitterion distribution and correlation varied, dependent on salt types, ionic strengths and ion valences. As each monomer has no net charge, the chains are not in extended configurations at low salt concentrations. In contrast to simple polyelectrolytes, they expand when salt is added since the local attractions between positive and negative ions are screened, thereby producing a rich science base in understanding the configurations, ionic distribution, and in interfacial interactions of zwitterionic polymer brushes in a variety of relevant and important ionic environments. Regarding the PS-b-PMMA diblock polymers with lamellar microdomains normal to the substrate, their even order scatterings should be significantly reduced or extinct as a result of symmetric extinction. By adjusting beam energies close to the absorption edge of the constituent atoms, the intensity of odd order scatterings remains intact, while that of even order scatterings changes as a function of beam energies. Quantitative analyses on these RXS data around the individual absorption edge of constituent components allow us to determine electron density profiles at interfaces, independent of the models. This, in turn, establishes the relationships of interfacial structures and physical properties of polymers and soft condensed matter, which can be strongly influenced by inherent structural disorder ranging over length scales from molecular functional groups to characteristic conformation and crystallization scales up to sample dimensions.

Proteins form the basis for important materials in today’s society, including a wide variety of biocatalysts and biosensors that are critical to the production of chemicals, medicine, and national defense. In many of these materials as well as in other applications such as antifouling coatings, self-assembled biomaterials, and protein crystallization, the interactions between proteins and polymers are critical to the functioning of the device. However, measurement of these interactions has traditionally been quite difficult, requiring arduous thermodynamic measurements to estimate interaction parameters in a variety of different competing theoretical frameworks.

Here, we explore several approaches to better understand protein-polymer interactions through the use of neutron scattering methods. First, we explore measurements of the form factor of protein-polymer bioconjugates, where the shape of the flexible polymer coil attached to the protein can provide information on the protein-polymer interaction. Measurements on a series of different polymers are performed, and data fitting allows the general behavior of these molecules in solution to be understood. Measurements are interpreted in the light of coarse-grained molecular simulations of the bioconjugate materials.

Second, cross structure factors are measured for protein-polymer blends at relatively high concentration in order to provide insight into the physics under more strongly interacting conditions. Measurements were performed for three different model systems using contrast variation small-angle neutron scattering (CV-SANS), and cross structure factors were extracted from the data. These were analyzed through systematic comparison to the dilute solutions of protein and polymer as well as through inverse Fourier transforms to provide real space information.

Finally, several experiments suggest that hydration plays a key role in governing interactions between proteins and polymers, especially at high concentration where there is a competition for water. SANS measurements of high concentration solution, combined with a new analysis method, enable estimation of hydration numbers from the traditional dilute regime through the semidilute and into a relatively concentrated regime, allowing an understanding of how hydration number changes with concentration to be gained. Comparison of this neutron data with other spectroscopic data and self-assembly measurements provides insight into how hydration governs the molecular interactions that drive these processes.

Conjugated polymers have attracted a keen interest over the past decade for their potential applications as semiconductors in various types of devices: organic light emitting diodes, organic solar-cells, organic field-effect transistors, etc. One of the potential advantage of polymeric semiconductors over their inorganic counterpart is the possibility of processing them from solution. To increase solubility, alkyl side chains are added to the conjugated backbones of the polymers. The side chains are therefore not involved in the frontier molecular orbitals. Thus, they do not impact directly optoelectronic properties. However, they do impact the polymer conformation and the polymer packing, which in turn impact the optoelectronic processes and the subsequent emergent macroscopic properties of the material. Recently, the side chains have also been engineered to transport ions in the solid-state, opening a new range of applications such as batteries or bio-sensors. Because polymers are soft materials, a range of dynamics occurs over an extended time scale, from femtosecond to millisecond, and are likely to modulate the impact of microstructure on the optoelectronic properties of the material.
Vibrational dynamics have been evidenced to impact absorption, inner reorganization energy, charge transfer between similar molecules and between different molecules at an heterojunction, delocalization, and so more generally charge transport and charge separation processes. Slower dynamics on the picosecond to nanosecond time-scale includes side-chain reorientation and backbone torsion. These dynamics are impacted the conformation of the chains and thus, modulates the efficiency of the opto-electronic processes. It has been shown for instance that the global charge transport network changes over timescale competing with charge carrier lifetime. Furthermore, these slow dynamics are temperature-dependent and therefore, can be activated during device operation. For instance, molecular diffusion becomes predominant above the glass transition of the material and is a known degradation mechanism.
We present a detailed microscopic study of the structure-dynamics relationship of both regio-regular (RR) and regio-random (RRa) poly(3-hexylthiophene) (P3HT) using elastic, quasielastic, spin-echo and inelastic neutron scattering techniques. We use deuteration to modulate the coherent and incoherent cross-sections of the materials, beyond a contrast variation purpose, allowing particularly to access both self-motions and collective dynamics of the materials. Measurements are underpinned by quantitative numerical simulations using classical molecular dynamics (MD) simulations, as well as molecular and periodic density functional theory based quantum chemical (QC) calculations. MD simulations reproduced well the large structural features and slow motions, but provided a limited description of molecular vibrations. MD shed light on differences in collective dynamics between Q-values linked with the π-π stacking and the lamellar stacking of the polymer, with the crystalline phase being the most impacted. Molecular QC described well the high-energy vibrational features, while periodic QC allowed to describe the mid-energy vibrational range. We show that this extensive combined approach of neutron-based measurements and multi-computational calculations allows to fully map the structural dynamics of conjugated polymers such as P3HT.

Polymeric aerogels, more specifically polyimide aerogels (PIA), provide excellent mechanical properties relative to traditional silica aerogels while maintaining thermal stability. NASA has investigated PIA paired with ionic liquids as novel mechanically robust electrolyte systems for next generation batteries. Common laboratory techniques provide insight into the relationship between polyimide structure and surface area, porosity, and pore volume of the aerogel; however, these measurements are traditionally conducted sans solvent. Because of this, the impact of the ionic liquid on the nanoscale structure of PIA is unclear. To determine the impact of solvent presence, we use small angle neutron scattering to determine the skeletal size and composition of solvated PIA.Careful analysis of SANS data from PIA/ionic liquid constructs surprisingly shows that the ionic liquid penetrates the polyimide skeleton. This unexpected structure clearly impacts charge transport and therefore performance of the aerogel as a battery component. This mixing behavior must be more fully understood to rationally utilize these promising materials in devices and further experiments are planned to elucidate the impact of polyimide and ionic liquid structure, as well as fabrication procedure on the pore size, structure, and mixing behavior in PIA/ionic liquid aerogels. These results therefore broaden the characterization tools of polymeric aerogels and provide pathways to correlate a broad range of their structural characteristics to their performance.

Experimentally exploring the molecular exchange in polyelectrolyte complexes is crucial to understanding the kinetic behaviors of many ionic systems. Polyelectrolyte complex (PEC) micelles are a type of nanoscale assemblies whose core is usually composed of ionic complexes and corona is constituted by neutral and hydrophilic polymers. Here, we present the employment of time-resolved small-angle neutron scattering (TR-SANS) technique to assess the chain exchange rate in polyelectrolyte complex micelles in aqueous solutions. The aim is to explore the dependency of the molecular exchange rate on polymer block length, system temperature, and solution salinity. We investigate two micelle systems that are prepared through the ionic complexation between poly (styrene sulfonate) (PSS) and a polypeptide, poly (ethylene oxide)-block-poly (L-lysine) (PEO-b-PLL), or a synthetic diblock polyelectrolyte, poly (ethylene oxide)-block-poly (vinyl benzyl trimethyl ammonium chloride) (PEO-b-PVBTMA). Neutron contrast is obtained by deuterium labeling the PVBTMA block and PSS block. Measurements performed up to 34 hours after the mixing of deuterium-rich micelles and hydrogen-rich micelles imply that the PEC micelles may not exchange molecules with their surroundings due to the strong ionic interaction between oppositely charged groups. Neither the elevation of system temperature nor increase of solution salinity, two common ways to weakening the strength of electrostatic interactions, seem to unlock the molecular exchange at the experimented time scale. We postulate that it is the large energy barrier stemming from the breakup of ionic pairs in polyelectrolyte complexes that prevents the chain expulsion or micelle fusion from happening. We anticipate these findings would provide fundamental insights on the origin of morphological complexities and nonequilibrium phenomena in polyelectrolyte-based self-assemblies.

Polymer nanocomposites (PNCs) with attractive polymer–nanoparticle (NP) interactions, are often reported to disperse individually, enabling the investigation of interfacial effects on the rheological properties without contribution from particle percolation. By taking advantage of selective isotope labeling of the chains, neutron scattering techniques uniquely allow the possibility of investigating the structural and dynamical properties of the polymers in the matrix. In particular, high-resolution QuasiElastic Neutron Scattering (QENS) techniques, such as backscattering and Neutron Spin Echo, allow directly observing the chain motions at the nanoscale by simultaneously accessing a broad range of time-scale (from sub-nanoseconds to hundred nanoseconds), and length-scales (from monomer size to entanglement mesh sizes); specifically important for polymers are the localized fast dynamics on sub-monomer level, the segmental dynamics at the monomer level and the entangled/collective dynamics at larger scales. These information, as well as those obtained using X-ray Photon Correlation Spectroscopy (X-PCS), provide microscopic insight relevant for the understanding of the rheological properties of the PNC.

Here, I will present results on the nanoscale dynamics of polymer chains in composites with dynamically asymmetric interphases, [1] with NPs of sizes comparable to that of the polymer coils in the matrix, [2] and which have been subject to large deformation. [3]

[1] E. Senses, et al., Sci. Rep., 6, 29326 (2016).
[2] E. Senses, et al., Phys. Rev. Lett, 118, 147801 (2017).
[3] E. Senses, et al., Soft Matter 13, 7922 (2017)

Recent advances in synthetic chemistry made it possible to precisely control the topology of polymers that determines many of their physical properties. Ring, star, comb, bottlebrush and hyperbranched polymers exhibit structural relaxation that is not observed in their linear chain counterparts. In particular, star shaped polymers in which many linear chains share a common center are particularly interesting as the monomer density towards the center increases dramatically at high functionalities, making a portion of a star macromolecule dense and impenetrable, akin to hard-spheres. Thus, depending on the number and the length of arms, these polymers can display both soft and hard nanoparticle character, making them ideal to study the effect of interphases in polymer nanocomposites. In our work [1], we used high-glass transition temperature (T_g) deuterated star shaped polymers as nanofillers and dispersed them in miscible low-T_g hydrogenated linear matrices to create a new kind of polymer nanocomposites in which the interpenetration between the fillers and the matrix, and therefore, the bulk rheological properties, are systematically varied via filler architecture at the same particle concentration. The resulting ‘architecturally engineered’ nanocomposites transition from the well-known simple linear blends to conventional hard sphere-polymer nanocomposites. We investigated these nanocomposites using static (SANS) and quasielastic neutron scattering (backscattering and spin-echo) measurements over a wide range of time and length scales in the glassy and melt state of the composites. The localized and segmental dynamics as well as chain-chain entanglements are all modified by compactness of star-shaped fillers. The observed microscopic changes manifest at the macroscopic level as softening and stiffening depending on the functionality and sizes of the macromolecules, thus offers a novel approach for tuning the physical properties of polymer based nanocomposites for advanced materials applications.

[1] Senses, Erkan, et al. ACS nano 12.11 (2018): 10807-10816.

In this work, nanoparticles with preferential wetting to one of the liquid phases are demonstrated to arrest the spinodal decomposition of binary solvent mixture. A physical gel is formed and thus termed as solvent segregation driven gel (SeedGel). Small angle neutron scattering (SANS) experiments are conducted to identify the partition of particles by contrast matching the scattering length density of one liquid component to that of the nanoparticles. During the solvent phase separation, nanoparticles are found to be jammed within the phase that is rich in one of the liquid components. Compared to bicontinuous interfacially jammed emulsion gel (Bijel), SeedGel greatly relaxes the rigid restriction on the neutral wettability of the nanoparticles to the two solvents. Additionally, SeedGel shows reversible thermal response to quasi-static temperature ramping. The multi-length scale characterization by SANS and ultra-small angle neutron scattering (USANS) reveal that both the particle-particle distance and the domain size can be directly manipulated by temperature change and are strongly correlated with the gel transition. The SeedGel formation mechanism is also applicable to sub-10 nm particles, which possesses a great potential to link the self-assembled structure with the unique properties of small particles, such as quantum dots. Moreover, SeedGel can be molded to various shapes due to the enhanced mechanical strength. Subsequent drying of the solvents results in nano-porous medium up to centimeter size with neither nanometer nor micrometer cracking. In conclusion, a new mechanism of arresting liquid-liquid spinodal decomposition with colloidal particles is proposed. The strategy for preparing SeedGel can serve as a versatile, yet simple method to assemble particles to porous materials that can be implemented to various applications that favor high surface area.

Pursuing hardware upgrades to provide brighter beams for material studies has been the paramount goal of every neutron scattering facility. Here, we present an alternative route to circumvent the limitation of neutron flux using recent advancements in artificial intelligence (AI), namely image super-resolution deep convolutional neural network (CNN). The feasibility of accelerating data collection has been demonstrated by using small angle neutron scattering data collected from the Extended Q-range Small Angle Neutron Scattering (EQ-SANS) instrument at Spallation Neutron Source (SNS). Data collection time can be reduced by increasing the size of binning of the detector pixels at the sacrifice of resolution. High-resolution scattering data is then reconstructed by using AI deep super-resolution learning method. This technique can not only improve the productivity of neutron scattering instruments by speeding up the experimental workflow but also enable capturing kinetic changes and transient phenomenon of materials that are currently inaccessible by existing neutron scattering techniques. Then, we will show how Machine Learning can be used to assist users in selecting appropriate scattering models.

A 5 nm thick membrane separates the cell from its surrounding environment. Life places extreme demands on the material properties of these thin membranes by requiring that they are both rigid enough to define the cell structure yet flexible enough to undergo dramatic changes in cell shape during processes like endocytosis and cell division. In turn, the properties of the biomembranes are determined by the unique characteristics of the thousands of chemically distinct lipid molecules that make up the membrane. A long-standing challenge in biophysics is to link the complex and highly regulated lipid diversity to the membrane properties and ultimately cell function. This talk will highlight new insights from neutron scattering towards understanding the role of lipid diversity in tuning the properties of model biomembranes. We will demonstrate how subtle changes in lipid composition, such as mixing hydrophobic tail lengths or adding charged headgroups, can have significant and unexpected effects on the membrane elastic properties. The results reveal the complex and interwoven relationship between lipid membrane composition, structure and dynamics.

Lipid bilayers, the main matrix of cell membranes, host a wide range of vital biological processes and are ubiquitous in a variety of research areas at the interface of biophysics, health care, and biotechnology. In order to understand the function of lipid membranes and fully utilize their potential in pharmaceutical and biotechnological applications, it is imperative to understand the phenomena that control key membrane functions, such as domain formation and protein recruitment. Among such phenomena, membrane dynamics, manifested in mechanical and diffusive membrane properties, remain poorly understood despite evidence of their active role in various cellular processes. This talk will focus on recent advances in neutron scattering characterization and MD simulations for interrogating molecular and collective dynamics in lipid membranes and their response to cholesterol content, lateral rearrangement, and protein binding events. The synergistic integration of neutron scattering and simulations, and the inherent capabilities of both techniques to probe selective dynamics, is offering significant insights into the delicate spatiotemporal functions of biomimetic lipid membranes.

Computational sequence prediction is a powerful design tool that has enabled discovery of non-natural peptide sequences that can assemble into coiled coil bundles that are antiparallel homotetramers containing strategically-placed solvent-exposed amino-acid side groups and chemically-reactive complementary functional groups. By taking advantage of the tunable interaction landscape coupled with orthogonal chemical-assembly pathways, the artificial coiled coils can be utilized as tunable anisotropic Molecular Legos or modular building-blocks to construct artificial biomaterials that display a rich hierarchical nanostructure. In this talk, we will discuss the design and construction of new sequences of robust 4 nm x 2 nm cylindrical coiled coil forming peptides that have been computationally designed to carry different net charge as a result of the dissociable amino-acid side-groups in the peptide sequence. Thus, an entire spectrum of charged bundles has been studied, consisting of bundles that carry a high net positive charge (+32) to those with a high net negative charge (-32) in order to systematically investigate the impact of the net charge and also local charge distribution on inter-bundle interactions as a function of solution conditions such as pH and ionic strength. We have employed Small Angle Neutron Scattering (SANS) extensively to characterize the computationally designed bundle dimensions and inter-bundle interactions since SANS is uniquely suited to studying soft-matter systems in their native solution state in a non-intrusive and non-destructive manner. SANS along with Small Angle X-ray scattering (SAXS) has yielded insight into the rich interaction landscape of charged bundles at the nanoscale. We show that all bundles at dilute concentrations yield a form-factor scattering profile that can be fit using a cylinder model to extract the bundle dimensions. The experimentally measured dimensions are directly compared to the expected dimensions calculated using the bundle PDB file. As the bundle concentration increases, a correlation peak or ‘ionomer peak’ is observed in the scattering profile, the strength of which increases with net charge and bundle concentration and is responsive to the presence of salt. The electrostatic repulsion interactions between bundles resulting in the correlation peak are modeled using both Effective Hard-Sphere (EHS) and the Hayter-Penfold (HP) structure factor, with the latter giving better fit results especially for highly charged bundles at higher concentrations. Lastly, we will show that the coiled coil bundle stability and solubility in water, i.e. the formation of stable coiled coil homotetramers in aqueous solution is dependent on a number of factors such as the net charge on the bundle, type of charge (negative versus positive), temperature and ionic strength, which is characterized for a specific peptide sequence via contrast-matching SANS measurements.

Internal dynamics such as the movements of hinge-bending domain motions of therapeutic monoclonal antibodies (mAb) are crucial for important bio-functionalities during targeted immune therapies. However, determining these functional dynamics via traditional spectroscopic techniques (e.g. NMR) at the spatial and temporal scales pertinent to a mAb is highly nontrivial primarily due to their anisotropic shape and large molecular weight. Currently, the commonly used theories have inaccurate assumptions and often do not apply to anisotropic proteins at high concentrations. Here, we have established a substantiated methodology using a unique combination of the neutron spin echo spectroscopy, an accurate dynamic decoupling theory, and the semi analytic colloidal theories for dilute suspensions to probe the internal dynamics of NISTmAb, RM 8670. Our findings show that at the short time limit when the contribution of the rotational motion is negligible, the effective diffusion coefficients obtained by NSE is intrinsically different from the long-time diffusion coefficient obtained by the dynamic light scattering, which is primarily due to the anisotropic shape of a mAb. A prominent difference between a rigid-body mAb diffusion and the overall effective diffusion of a NISTmAb at 3-5 nm length scales is observed. This concludes that the intra-particle domain motions contribute ~15% to the total effective diffusion of a NISTmAb within the NSE correlation time window. Our analysis also provides a theoretical description of the hydrodynamic parameter. The hydrodynamic effects of many different mAbs were evaluated and a universal method was established that describes the concentration dependence of the diffusion coefficients with different types of solution interactions. To our surprise, when the hydrodynamic volume fraction is considered, the hydrodynamic parameters collapses from values ranging from 7.6 to 11.2 to that of 2.5 to 3.3 approaching the hard sphere (HS) value, 1.83. This elucidates the importance of hydration and shapes in solution stability. Though a qualitative agreement is achieved between NSE experiments and atomistic molecular dynamic simulations, their intra-particle dynamics are overestimated. We attribute the difference to the force fields and water models used in the simulation. The findings of this work provide foundation for future theoretical and simulation work for predicting diffusion properties and their related macroscopic solution properties for proteins with various shapes, sizes, and interactions.