I15-1-X-ray Pair Distribution Function (XPDF)
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Xiao
Hua
,
Alexander S.
Eggeman
,
Elizabeth
Castillo-Martinez
,
Rosa
Robert
,
Harry S.
Geddes
,
Ziheng
Lu
,
Chris J.
Pickard
,
Wei
Meng
,
Kamila M.
Wiaderek
,
Nathalie
Pereira
,
Glenn G.
Amatucci
,
Paul A.
Midgley
,
Karena W.
Chapman
,
Ullrich
Steiner
,
Andrew L.
Goodwin
,
Clare
Grey
Diamond Proposal Number(s):
[17315]
Abstract: Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF2 and CuF2. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F− sublattices and that of LiF is established. Initial lithiation of FeF3 forms FeF2 on the particle’s surface, along with a cation-ordered and stacking-disordered phase, A-LixFeyF3, which is structurally related to α-/β-LiMn2+Fe3+F6 and which topotactically transforms to B- and then C-LixFeyF3, before forming LiF and Fe. Lithiation of FeF2 and CuF2 results in a buffer phase between FeF2/CuF2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.
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Jan 2021
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[17315]
Abstract: The recent introduction of organometal halide perovskites to solar cells has significantly enhanced the power conversion efficiency of alternative photovoltaic devices, revolutionizing the development of photovoltaic technologies. To produce perovskite thin films with high device performances, various fabrication methodologies have been developed leading to thin films with different surface structures and crystal morphologies. Tremendous efforts have been devoted to characterizing macro- and microscopic structures within these films to better understand the processing-property-performance relationship. However, their atomic structure and its influence on device performance remains poorly understood. To this end, we employed pair distribution function analysis of X-ray total scattering data to obtain crystallographic and compositional information of methylammonium-lead-iodide (MAPbI3) thin films. This analysis revealed the ubiquitous presence of two near-amorphous intermediate phases with local structures that share subtle but significant correlations with the PbI2 precursor and the desired perovskite phase. The structure transformation from these intermediates to the perovskite deviates from the intuitive belief where the molecular cations get inserted between the sheets of layered PbI2 upon the crystallization of perovskite. This knowledge offers critical insight into the perovskite formation thermodynamics and reveals an important link between the short-range structure of the thin films and their corresponding device performance.
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Apr 2019
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I22-Small angle scattering & Diffraction
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James A.
Dolan
,
Karolina
Korzeb
,
Raphael
Dehmel
,
Karl C.
Gödel
,
Morgan
Stefik
,
Ulrich
Wiesner
,
Timothy D.
Wilkinson
,
Jeremy J.
Baumberg
,
Bodo D.
Wilts
,
Ullrich
Steiner
,
Ilja
Gunkel
Diamond Proposal Number(s):
[13448]
Abstract: The efficacy with which solvent vapor annealing (SVA) can control block copolymer self‐assembly has so far been demonstrated primarily for the simplest class of copolymer, the linear diblock copolymer. Adding a third distinct block—thereby creating a triblock terpolymer—not only provides convenient access to complex continuous network morphologies, particularly the gyroid phases, but also opens up a route toward the fabrication of novel nanoscale devices such as optical metamaterials. Such applications, however, require the generation of well‐ordered 3D continuous networks, which in turn requires a detailed understanding of the SVA process in terpolymer network morphologies. Here, in situ grazing‐incidence small‐angle X‐ray scattering (GISAXS) is employed to study the self‐assembly of a gyroid‐forming triblock terpolymer during SVA, revealing the effects of several key SVA parameters on the morphology, lateral order, and, in particular, its preservation in the dried film. The robustness of the terpolymer gyroid morphology is a key requirement for successful SVA, allowing the exploration of annealing parameters which may enable the generation of films with long‐range order, e.g., for optical metamaterial applications.
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Sep 2018
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[13448]
Open Access
Abstract: Block copolymer (BCP) self-assembly is a promising route to manufacture functional nanomaterials for applications from nanolithography to optical metamaterials. Self-assembled cubic morphologies cannot, however, be conveniently optically characterized in the lab due to their structural isotropy. Here, the aligned crystallization behavior of a semicrystalline-amorphous polyisoprene-b-polystyrene-b-poly(ethylene oxide) (ISO) triblock terpolymer was utilized to visualize the grain structure of the cubic microphase-separated morphology. Upon quenching from a solvent swollen state, ISO first self-assembles into an alternating gyroid morphology, in the confinement of which the PEO crystallizes preferentially along the least tortuous pathways of the single gyroid morphology with grain sizes of hundreds of micrometers. Strikingly, the resulting anisotropic alignment of PEO crystallites gives rise to a unique optical birefringence of the alternating gyroid domains, which allows imaging of the self-assembled grain structure by optical microscopy alone. This study provides insight into polymer crystallization within a tortuous three-dimensional network and establishes a useful method for the optical visualization of cubic BCP morphologies that serve as functional nanomaterial templates.
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Aug 2017
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I22-Small angle scattering & Diffraction
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Sandeep
Pathak
,
Alessandro
Sepe
,
Aditya
Sadhanala
,
Felix
Deschler
,
Amir
Haghighirad
,
Nobuya
Sakai
,
Karl C.
Goedel
,
Samuel D.
Stranks
,
Nakita
Noel
,
Michael
Price
,
Sven
Hüttner
,
Nicholas A.
Hawkins
,
Richard H.
Friend
,
Ullrich
Steiner
,
Henry J.
Snaith
Diamond Proposal Number(s):
[8459]
Abstract: Recently, solution-processable organic–inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (i.e., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance.
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Mar 2015
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[8657]
Open Access
Abstract: The electronic structure of low temperature, solution-processed indium–zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm2 V−1 s−1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.
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Mar 2015
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I07-Surface & interface diffraction
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Demet
Asil
,
Brian J.
Walker
,
Bruno
Ehrler
,
Yana
Vaynzof
,
Alessandro
Sepe
,
Sam
Bayliss
,
Aditya
Sadhanala
,
Philip C. Y.
Chow
,
Paul E.
Hopkinson
,
Ullrich
Steiner
,
Neil C.
Greenham
,
Richard H.
Friend
Diamond Proposal Number(s):
[8657]
Abstract: Semiconductor nanocrystals are promising materials for printed optoelectronic devices, but their high surface areas are susceptible to forming defects that hinder charge carrier transport. Furthermore, correlation of chalcogenide nanocrystal (NC) material properties with solar cell operation is not straightforward due to the disorder often induced into NC films during processing. Here, an improvement in long-range ordering of PbSe NCs symmetry that results from halide surface passivation is described, and the effects on chemical, optical, and photovoltaic device properties are investigated. Notably, this passivation method leads to a nanometer-scale rearrangement of PbSe NCs during ligand exchange, improving the long-range ordering of nanocrystal symmetry entirely with inorganic surface chemistry. Solar cells constructed with a variety of architectures show varying improvement and suggest that triplet formation and ionization, rather than carrier transport, is the limiting factor in singlet fission solar cells. Compared to existing protocols, our synthesis leads to PbSe nanocrystals with surface-bound chloride ions, reduced sub-bandgap absorption and robust materials and devices that retain performance characteristics many hours longer than their unpassivated counterparts.
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Dec 2014
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[8657]
Abstract: A cellulose acetate thin film composite (CA TFC) membrane was developed by direct casting of CA onto an ultrafiltration membrane support. The flux through the CA TFC membrane was, as expected, inversely proportional to the selective layer thickness. The membrane with the lowest thickness of the CA film (217 nm) initially had poor salt rejection (~55%) but relatively high permeability (~0.5 l m?2 h?1 bar?1). In contrast to asymmetric CA reverse osmosis membranes that are developed in 4 °C water baths, the CA TFC membranes were developed by subsequent swelling in room temperature water baths, thermal annealing at 81 °C, and with room-temperature incorporation of silver nanoparticles onto CA TFC surfaces by chemical reduction. The change in the physical properties as a result of the swelling process was examined with ellipsometry, grazing incidence X-ray scattering and contact angle measurements. A combined swelling and annealing treatment was found to improve salt rejection to an acceptable reverse osmosis salt rejection level (~94%) without significant deterioration of flux. The swelling time was found to generally improve the membrane performance in terms of flux and surface roughness whilst the silver nanoparticle treatment reduced bacterial surface coverage by four orders of magnitude.
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Nov 2013
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I22-Small angle scattering & Diffraction
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Abstract: The miscibility and aggregation of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) in a polymer matrix is of great importance for the development of fullerene-based organic photovoltaic cells (OPVs). In this study we have systematically investigated the loading of PCBM in regioregular and regiorandom P3HT (poly(3-hexylthiophene-2,5-diyl). Using optical microscopy, we demonstrate the partial miscibility of PCBM in thermally annealed P3HT films and relate it to the relative crystallinity of P3HT. The low polydispersity and the nearly 100% regioregularity of a self-synthesized P3HT allowed a detailed X-ray characterization as a function of PCBM content, revealing a superstructure of periodic amorphous and crystalline lamellar domains of fully chain extended polymer chains. PCBM dissolves in the amorphous interlamellar P3HT regions (partially index-matching the X-ray scattering contrast) up to a threshold, above which PCBM aggregates start to form. These results show that crystallization of P3HT into 10-nm-wide lamellar domains sets the main length scale in P3HT/PCBM structure formation. PCBM is displaced into the amorphous intralamellar regions, swelling the lamellar stack. This structure formation by crystallization, which is intrinsic to most semicrystalline polymers, followed by the enrichment, segregation, and crystallization of PCBM provides an interdigitated structure, which is conceptually ideal for excitonic solar cells.
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May 2013
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[7205]
Abstract: Fractionated crystallization (FC), a chain-sorting mechanism by length, is identified in well-defined, low molecular weight, and defect-free regioregular poly(3-hexylthiophene) by X-ray scattering and calorimetry of bulk samples. While wide-angle X-ray scattering (WAXS) qualitatively suggests that the degree of crystallinity is similar in all investigated samples, the melting enthalpies are largely different. We ascribe this to intricacies in the integration of the melting and crystallization peaks in calorimetric experiments, which is caused by FC occurring over a large temperature range. The extent of FC decreases with increasing molecular weight and increases with increasing polydispersity. The temperature-dependent investigation of the long period LP and the (100)-WAXS reflection of a sample in which FC is absent allows to disentangle effects from main-chain and side-chain crystallization.
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Oct 2012
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