I15-Extreme Conditions
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Diamond Proposal Number(s):
[16390]
Abstract: Electromechanical actuation in piezoceramics is usually enhanced by creating chemically homogeneous materials with structurally heterogeneous morphotropic phase boundaries, leading to abrupt changes in ion displacement directions within the perovskite unit cell. In the present study, an alternative mechanism to enhance electromechanical coupling is found in both chemically and structurally heterogeneous BiFeO3−BaTiO3 lead-free piezoceramics. Such a mechanism is observed in a composition exhibiting core-shell type microstructure, associated with donor-type substitution of Ti4+ for Fe3+, and is primarily activated by thermal quenching treatment. Here, we describe the use of in-situ high-energy synchrotron X-ray powder diffraction upon the application of a high electric field to directly monitor the ferroelectric and elastic interactions between these composite-like components, formed as core and shell regions within grains. Translational short or long-range ordering is observed in the BiFeO3‒depleted shell regions which undergo significant structural alterations from pseudocubic Pm-3m relaxor-ferroelectric in slow-cooled ceramics to rhombohedral R3c or R3m with long-range ferroelectric order in the quenched state. The strain contributions from each component are calculated, leading to the conclusion that the total macroscopic strain arises predominantly from the transformed shell after quenching. Such observations are also complemented by investigations of microstructure and electrical properties, including ferroelectric behaviour and temperature-dependent dielectric properties.
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Jul 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[13569]
Open Access
Abstract: An understanding of the thin film growth modes of substrate-induced polymorphs allows a deeper insight into the origin of this class of materials. Their onset of crystallisation, the subsequent crystal growth, the evolution of the thin film morphology and the transfer to the equilibrium bulk structure are still not fully understood. This work investigates the thin film formation of a conjugated molecule with terminal alkyl chains. Thin films of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene were grown by physical vapor deposition on silicon oxide surfaces with varying the film thicknesses from the sub-monolayer regime up to 33 layer thick films. Additionally, the substrate temperature and deposition rate were varied. The films were investigated by atomic force microscopy, X-ray reflectivity and grazing incidence X-ray diffraction. The first growth stage is a closed monolayer with a thickness of 3 nm formed by upright-standing molecules. It is found that the substrate-induced crystal structure is already formed within the first monolayer and continues its growth up to the largest investigated film thickness. The characteristic morphology is terraced islands over the whole thickness range. On top of the first monolayer a morphology with several terrace levels appears, which is associated with a rapid increase of the surface roughness. At larger film thicknesses (≥13 nm) the number of terrace steps does not increase significantly, so that the surface roughness only increases slowly. This work shows that molecules with terminal alkyl chains can form a substrate-induced phase up to large film thicknesses without the appearance of the equilibrium bulk phase.
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Jun 2019
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[11145]
Open Access
Abstract: In this work we successfully developed a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron donor (D) unit, and dibenzothiophene-S,S-dioxide as the acceptor (A), linked in D–A and D–A–D structures. In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted molecules 6 and 7 in particular show extremely high TADF efficiency in solution and solid film, even with a singlet–triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.
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May 2019
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I11-High Resolution Powder Diffraction
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Abstract: Two series of new apatite-type silicate materials were synthesised and characterised with the aim of achieving white light emission from single-phase phosphors. The Gd9.33(SiO4)6O2 host was doped systematically with Dy3+, Tb3+ and Eu3+ to tune the emission to the white light region. Eight new phosphors with emission very near the ideal white light point are reported. The best properties are found for the 0.5%Tb, 0.03%Eu co-doped Gd9.33(SiO4)6O2, with colour coordinates (0.340, 0.341) and a correlated colour temperature of 5190 K, corresponding to the white light region. The temporal decay of emission from the doped ions was measured following pulsed excitation of the Gd3+ host. An evaluation of energy transfer and quenching effects in the phosphors has been made based on the lifetimes so obtained. In particular, a comparison of the Eu3+ lifetimes in the Dy,Eu co-doped phosphors with those in the Tb,Eu-containing materials suggests quenching of the 5D0 excited state of Eu3+ by the Dy3+ ions. These observations imply that the combination of Eu3+ and Dy3+ ions may not provide optimal efficiencies in phosphors, and that the combination of Eu3+ and Tb3+ is likely to be superior for optimising the emission properties.
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May 2019
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Abstract: As photovoltaics have grown to become one of the dominant renewable energy generating technologies, attention has fallen upon thin-film materials as a route to lightweight, flexible and portable solar cells. NaSbS2 has recently been proposed as a non-toxic, earth abundant solar absorber for thin-film cells. In this study, we use a combined theoretical and experimental approach to characterize and assess the electronic and optical properties of NaSbS2 as a promising solar absorber. Our results, utilising two theoretical efficiency metrics, demonstrate that NaSbS2 may be limited for use in single-junction cells by a forbidden band gap and slow absorption onset. Other features of its electronic structure, however, indicate that the material may still be promising in thermoelectric applications.
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Jan 2019
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[15968]
Abstract: A novel High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) study at the Tb L3-edge for the rhabdophane-type LaPO4·nH2O:Tb3+ and the monazite-type LaTbPO4:Tb3+ nanorods is reported. We have observed that the changes in the white-line intensity follow a similar trend to that previously observed in the photoluminescence behaviour of these materials. Those changes have been assigned to the localization of the terbium 5d states and the modification induced by the hybridization with the next neighbours as the terbium content is varied. The interplay between electronic (d-states localization) and structural (local disorder) effects is used to explain the different optical behaviour between the two series of nanorods. XAS results indicate that the local ordering is a key factor influencing the different emission efficiency, quenching effect and decay lifetimes observed experimentally for both series of nanorods. The present results indicate that the long lifetime decay process is accounted within a standard single Tb3+ ion scenario, where the structural disorder, favoured by the presence of water molecules in the structure, aids the non-radiative recombination processes. Implications for future use of the white-line intensity in the HERFD-XAS spectra in characterizing other luminescent materials are also discussed.
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Oct 2018
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B18-Core EXAFS
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Delphine
Malarde
,
Ian D.
Johnson
,
Ian J.
Godfrey
,
Michael J.
Powell
,
Giannantonio
Cibin
,
Raul
Quesada-cabrera
,
Jawwad A.
Darr
,
Claire J.
Carmalt
,
Gopinathan
Sankar
,
Ivan P.
Parkin
,
Robert G.
Palgrave
Diamond Proposal Number(s):
[17147]
Open Access
Abstract: Monoclinic vanadium(IV) oxide [VO2(M)] is a widely studied material due to its thermochromic properties and its potential use for energy-efficient glazing applications. VO2(M) nanoparticles can be a great advantage for energy-efficient glazing as below 50 nm the nanoparticles poorly interact with visible wavelengths – resulting in an increase in visible light transmittance whilst maintaining the thermochromic response of the material. The direct synthesis of VO2(M) nanoparticles with effective thermochromic properties will be a step forward towards industrial applications of this material. Unfortunately, many of the synthesis processes reported so far involve multiple steps, including post-treatment, and the synthesis is not always reproducible. In this study, we present the first direct synthesis of pure monoclinic VO2 nanoparticles by continuous hydrothermal flow synthesis (CHFS). TEM images showed that nanoparticles in the size range of 30-40 nm were produced. The VO2(M) nanoparticles also showed good thermochromic properties with a solar modulation (∆Tsol) of 3.8%, as established by UV/Vis spectroscopy. A range of analytical methods was used to characterise the materials, including X-ray absorption spectroscopy (XANES and EXAFS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The influence of niobium (Nb) doping on the physical and thermochromic properties of the VO2 nanoparticles was also explored. Previous work has shown a sharp metal-to-semiconductor transition of VO2 upon incorporation of Nb dopant. The results of the current work suggested these changes are likely due to changes on the local structure of the oxide.
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Oct 2018
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[12408]
Abstract: Spin crossover complexes are one of the most studied classes of molecular switches and have attracted considerable attention for their potential technological use as active units in new multifunctional devices. A fundamental step towards their practical implementation is their effective processability into thin films. Crucially, the physical property of technological interest shown by these materials in the bulk phase has to be retained once they are deposited on a solid surface. These conditions are not easily satisfied by most of the intrinsically fragile coordination compounds, either because the material processing methods can compromise their molecular structure, or the interaction between the molecule and the surface can induce drastic changes into the resulting properties. Herein, we report the identification of a novel high-vacuum processable spin-crossover complex, [Fe(qnal)2] (qnal = quinoline-naphthaldehyde), and the preparation of a 50 nm sublimated film of this molecular switch on gold. X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) were used to investigate the composition and the temperature- and light-induced spin-crossover of the deposited material, providing full evidence on the capability of this molecular system to be efficiently processed into nanometric films with retention of its switchable magnetic properties.
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Jul 2018
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Open Access
Abstract: Transparent conducting oxides have widespread application in modern society but there is a need to move away from the current ‘industry champion’ tin doped indium oxide (In2O3:Sn) due to high costs. Antimony doped tin(IV) oxide (ATO) is an excellent candidate but is limited by its opto-electrical properties. Here, we present a novel and scalable synthetic route to ATO thin films that shows excellent electrical properties. Resistivity measurements showed that at 4 at% doping the lowest value of 4.7 × 10−4 Ω cm was achieved primarily due to a high charge carrier density of 1.2 × 1021 cm−3. Further doping induced an increase in resistivity due to a decrease in both the carrier density and mobility. Ab initio hybrid density functional theory (DFT) calculations show the thermodynamic basis for the tail off of performance beyond a certain doping level, and the appearance of Sb(III) within the doped thin films.
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Jun 2018
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I09-Surface and Interface Structural Analysis
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Jia-ye
Zhang
,
Weiwei
Li
,
Robert L. Z.
Hoye
,
Judith
Macmanus-driscoll
,
Melanie
Budde
,
Oliver
Bierwagen
,
Le
Wang
,
Yingge
Du
,
Matthew
Wahila
,
Louis F. J.
Piper
,
Tien-lin
Lee
,
Holly
Edwards
,
Vinod R.
Dhanak
,
Hongliang
Zhang
Diamond Proposal Number(s):
[16005]
Abstract: NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (< 0.05 cm2 V−1s−1). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T <330 K, and variable range hopping at T <200 K. A combination of x-ray photoemission and O K-edge x-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization.
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Jan 2018
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