I15-1-X-ray Pair Distribution Function (XPDF)
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Sahil
Tippireddy
,
Feridoon
Azough
,
Vikram
Vikram
,
Animesh
Bhui
,
Philip
Chater
,
Demie
Kepaptsoglou
,
Quentin
Ramasse
,
Robert
Freer
,
Ricardo
Grau-Crespo
,
Kanishka
Biswas
,
Paz
Vaqueiro
,
Anthony V.
Powell
Diamond Proposal Number(s):
[30162]
Open Access
Abstract: Chalcopyrite, CuFeS2 is considered one of the promising n-type thermoelectric materials with high natural abundance as a mineral. In this work, partial substitution of germanium in materials CuFe1−xGexS2, (0.0 ≤ x ≤ 0.10), leads to an almost six-fold enhancement of thermoelectric properties. X-Ray photoelectron spectroscopy (XPS) reveals that germanium is present in two oxidation states: Ge2+ and Ge4+. The stereochemically-active 4s2 lone-pair of electrons associated with Ge2+ induces a local structural distortion. Pair-distribution function (PDF) analysis reveal that Ge2+ ions are displaced from the centre of the GeS4 tetrahedron towards a triangular face, leading to pseudo-trigonal pyramidal coordination. This distortion is accompanied by lattice softening and an increase of the strain-fluctuation scattering parameter (ΓS), leading to a decrease in thermal conductivity. Phonon calculations demonstrate that germanium substitution leads to the appearance of resonant phonon modes. These modes lie close in energy to the acoustic and low-energy optical modes of the host matrix, with which they can interact, providing an additional mechanism for reducing the thermal conductivity. The weak chemical bonding of germanium with sulphur also leads to localized electronic states near the Fermi level which results in a high density-of-states effective mass, enabling a relatively high Seebeck coefficient to be maintained, despite the reduced electrical resistivity. This combination produces an almost three-fold improvement in the power factor, which when coupled with the substantial reduction in thermal conductivity, leads to a maximum figure-of-merit, zT ∼ 0.4 at 723 K for CuFe0.94Ge0.06S2.
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Oct 2022
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I09-Surface and Interface Structural Analysis
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Hongxia
Wang
,
Meiyan
Cui
,
Gaoliang
Fu
,
Jiaye
Zhang
,
Xingyu
Ding
,
Irene
Azaceta
,
Matthew
Bugnet
,
Demie M.
Kepaptsoglou
,
Vlado K.
Lazarov
,
Víctor A.
De La Pena O'Shea
,
Freddy E.
Oropeza
,
Kelvin H. L.
Zhang
Abstract: The design of heterostructured transition metal-based electrocatalysts with controlled composition and interfaces is key to increasing the efficiency of the water electrolysis and the elucidation of reaction mechanisms. In this work, we report the synthesis of well-controlled vertically aligned Ni/NiO nanocomposites consisting of Ni nanoclusters embedded in NiO, which result in highly efficient electrocatalysts for overall water splitting. We show that such a high catalytic efficiency toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) originates from a synergetic effect at Ni/NiO interfaces that significantly reduces the energy barrier for water dissociation, and favours the formation of reactive H* intermediates on the Ni side of the interface, and OHads on the NiO side of the interface. A study of water chemisorption based on near-ambient pressure photoelectron spectroscopy indicates that the abundant hetero-interfaces in Ni/NiO nanocomposite promote the dissociation of water with a three-fold increase in the surface concentration of OHads compared with pure NiO. Density functional theory calculations indicate that Ni/NiO interface leads to the reduction of the water dissociation energy barrier due to a high concentration of oxygen vacancies at NiO side of the interface, whereas the formation of highly active metallic Ni sites with an optimal value of Gibbs free energy of H* (ΔGH* = −0.16 eV) owes to a favourable adjustment of the electron energetics at the interface, thus accelerating the overall electrochemical water splitting.
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Sep 2022
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E02-JEM ARM 300CF
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Diamond Proposal Number(s):
[17997]
Abstract: Single-atom catalytic sites may have existed in all supported transition metal catalysts since their first application. Yet, interest in the design of single-atom heterogeneous catalysts (SACs) only really grew when advances in transmission electron microscopy (TEM) permitted direct confirmation of metal site isolation. While atomic-resolution imaging remains a central characterization tool, poor statistical significance, reproducibility, and interoperability limit its scope for deriving robust characteristics about these frontier catalytic materials. Here, we introduce a customized deep-learning method for automated atom detection in image analysis, a rate-limiting step toward high-throughput TEM. Platinum atoms stabilized on a functionalized carbon support with a challenging irregular three-dimensional morphology serve as a practically relevant test system with promising scope in thermo- and electrochemical applications. The model detects over 20,000 atomic positions for the statistical analysis of important properties for establishing structure–performance relations over nanostructured catalysts, like the surface density, proximity, clustering extent, and dispersion uniformity of supported metal species. Good performance obtained on direct application of the model to an iron SAC based on carbon nitride demonstrates its generalizability for single-atom detection on carbon-related materials. The approach establishes a route to integrate artificial intelligence into routine TEM workflows. It accelerates image processing times by orders of magnitude and reduces human bias by providing an uncertainty analysis that is not readily quantifiable in manual atom identification, improving standardization and scalability.
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Mar 2022
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I09-Surface and Interface Structural Analysis
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M.
Chrysler
,
J.
Gabel
,
T.-L.
Lee
,
A. N.
Penn
,
B. E.
Matthews
,
D. M.
Kepaptsoglou
,
Q. M.
Ramasse
,
J. R.
Paudel
,
R. K.
Sah
,
J. D.
Grassi
,
Z.
Zhu
,
A. X.
Gray
,
J. M.
Lebeau
,
S. R.
Spurgeon
,
S. A.
Chambers
,
P. V.
Sushko
,
J. H.
Ngai
Diamond Proposal Number(s):
[25582, 26487]
Abstract: We demonstrate that the interfacial dipole associated with bonding across the
SrTi
O
3
/
Si
heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into
SrTi
O
3
. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.
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Oct 2021
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NONE-No attached Diamond beamline
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Kenji
Nawa
,
Demie
Kepaptsoglou
,
Arsham
Ghasemi
,
Philip
Hasnip
,
Guillermo
Bárcena-González
,
Giuseppe
Nicotra
,
Pedro L.
Galindo
,
Quentin M.
Ramasse
,
Kohji
Nakamura
,
Susannah C.
Speller
,
Balati
Kuerbanjiang
,
Thorsten
Hesjedal
,
Vlado K.
Lazarov
Abstract: We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator
Bi
2
Te
3
on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the
Bi
2
Te
3
quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.
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Feb 2021
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I08-Scanning X-ray Microscopy beamline (SXM)
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Diamond Proposal Number(s):
[20900]
Open Access
Abstract: We report on the detection of primordial organic matter within the carbonaceous chondrite Maribo that is distinct from the majority of organics found in extraterrestrial samples. We have applied high-spatial resolution techniques to obtain C-N isotopic compositions, chemical, and structural information of this material. The organic matter is depleted in 15N relative to the terrestrial value at around δ15N ~ -200‰, close to compositions in the local interstellar medium. Morphological investigations by electron microscopy revealed that the material consists of µm- to sub-µm-sized diffuse particles dispersed within the meteorite matrix. Electron energy loss and synchrotron X-ray absorption near-edge structure spectroscopies show that the carbon functional chemistry is dominated by aromatic and C=O bonding environments similar to primordial organics from other carbonaceous chondrites. The nitrogen functional chemistry is characterized by C-N double and triple bonding environments distinct from what is usually found in 15N-enriched organics from aqueously altered carbonaceous chondrites. Our investigations demonstrate that Maribo represents one of the least altered CM chondrite breccias found to date and contains primordial organic matter, probably originating in the interstellar medium.
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Nov 2020
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: Tungsten bronze (TB) structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with a tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN), was prepared by the conventional mixed oxide route with some samples processed by Spark Plasma Sintering (SPS). The addition of MnO enabled the fabrication of high density BTN ceramics at a low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallizes with tetragonal symmetry (P4bm space group). High angle annular dark field-electron energy loss spectroscopy analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7 eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimization of the processing parameters, the ceramics achieved a high power factor of 280 μW/m K2 at 873 K. The BTN ceramics showed phonon-glass-type thermal conduction behavior with a low thermal conductivity of 1.7–1.65 W/m K at 300–873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing.
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Sep 2019
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I11-High Resolution Powder Diffraction
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Abstract: Donor-doped SrTiO3 ceramics are very promising n-type oxide thermoelectrics. We show that significant improvements in the thermoelectric power factor can be achieved by control of the nanostructure and microstructure. Using additions of B2O3 and ZrO2, high density, high quality Sr0.9Nd0.1TiO3 ceramics were synthesised by the mixed oxide route; samples were heat treated in a single step under reducing atmosphere at 1673 K. Synchrotron and electron diffraction studies revealed an I4/mcm tetragonal symmetry for all specimens. Microstructure development depended on the ZrO2 content; low level additions of ZrO2 (up to 0.3 wt%) led to a uniform grain size with transformation-induced sub-grain boundaries. HRTEM studies showed a high density of dislocations within the grains; the dislocations comprised (100) and (110) edge dislocations with Burger vectors of d(100) and d(110) respectively. Zr doping promoted atomic level homogenization and a uniform distribution of Nd and Sr in the lattice, inducing greatly enhanced carrier mobility. Transport property measurements showed a significant increase in the power factor, mainly resulting from the enhanced electrical conductivity while the Seebeck coefficients were unchanged. In optimised samples a power factor of 2.0 × 10−3 W m−1 K−2 was obtained at 500 K. This is an ∼30% improvement compared to the highest values reported for SrTiO3-based ceramics. The highest ZT value for Sr0.9Nd0.1TiO3 was 0.37 at 1015 K. This paper demonstrates the critical importance of controlling the structure at the atomic level and the effectiveness of minor dopants in enhancing the thermoelectric response.
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Nov 2018
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I09-Surface and Interface Structural Analysis
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S. A.
Chambers
,
Y.
Du
,
Z.
Zhu
,
J.
Wang
,
M. J.
Wahila
,
L. F. J.
Piper
,
A.
Prakash
,
J.
Yue
,
B.
Jalan
,
S. R.
Spurgeon
,
D. M.
Kepaptsoglou
,
Q. M.
Ramasse
,
P. V.
Sushko
Diamond Proposal Number(s):
[16630]
Abstract: Photoemission features associated with states deep in the band gap of n−SrTiO3(001) are found to be ubiquitous in bulk crystals and epitaxial films. These features are present even when there is little signal near the Fermi level. Analysis reveals that these states are deep-level traps associated with defects. The commonly investigated defects—O vacancies, Sr vacancies, and aliovalent impurity cations on the Ti sites—cannot account for these features. Rather, ab initio modeling points to these states resulting from interstitial oxygen and its interaction with donor electrons.
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Jun 2018
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I11-High Resolution Powder Diffraction
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Open Access
Abstract: The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesized. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.
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May 2018
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