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Paulina
Majchrzak
,
Klara
Volckaert
,
Antonija Grubišić
Čabo
,
Deepnarayan
Biswas
,
Marco
Bianchi
,
Sanjoy K.
Mahatha
,
Maciej
Dendzik
,
Federico
Andreatta
,
Signe S.
Grønborg
,
Igor
Markovic
,
Jonathon M.
Riley
,
Jens C.
Johannsen
,
Daniel
Lizzit
,
Luca
Bignardi
,
Silvano
Lizzit
,
Cephise
Cacho
,
Oliver
Alexander
,
Dan
Matselyukh
,
Adam S.
Wyatt
,
Richard T.
Chapman
,
Emma
Springate
,
Jeppe V.
Lauritsen
,
Phil D. C.
King
,
Charlotte
Sanders
,
Jill A.
Miwa
,
Philip
Hofmann
,
Soeren
Ulstrup
Open Access
Abstract: The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS
. These timescales are explained by Auger recombination involving MoS
and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.
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Jul 2021
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Theoretical Physics
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Frank M. F.
De Groot
,
Hebatalla
Elnaggar
,
Federica
Frati
,
Ru-Pan
Wang
,
Mario U.
Delgado-Jaime
,
Michel
Van Veenendaal
,
Javier
Fernandez-Rodriguez
,
Maurits W.
Haverkort
,
Robert J.
Green
,
Gerrit
Van Der Laan
,
Yaroslav
Kvashnin
,
Atsushi
Hariki
,
Hidekazu
Ikeno
,
Harry
Ramanantoanina
,
Claude
Daul
,
Bernard
Delley
,
Michael
Odelius
,
Marcus
Lundberg
,
Oliver
Kuhn
,
Sergey I.
Bokarev
,
Eric
Shirley
,
John
Vinson
,
Keith
Gilmore
,
Mauro
Stener
,
Giovanna
Fronzoni
,
Piero
Decleva
,
Peter
Kruger
,
Marius
Retegan
,
Yves
Joly
,
Christian
Vorwerk
,
Claudia
Draxl
,
John
Rehr
,
Arata
Tanaka
Open Access
Abstract: This review provides an overview of the different methods and computer codes that are used to interpret 2p x-ray absorption spectra of 3d transition metal ions. We first introduce the basic parameters and give an overview of the methods used. We start with the semi-empirical multiplet codes and compare the different codes that are available. A special chapter is devoted to the user friendly interfaces that have been written on the basis of these codes. Next we discuss the first principle codes based on band structure, including a chapter on Density Functional theory based approaches. We also give an overview of the first-principle multiplet codes that start from a cluster calculation and we discuss the wavefunction based methods, including multi-reference methods. We end the review with a discussion of the link between theory and experiment and discuss the open issues in the spectral analysis.
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Apr 2021
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[22876]
Abstract: Fly ash represents a promising alternative source of rare earth elements (REE). However, information on REE containing mineral phases and their association with other fly ash components, vital for REE recovery from fly ash, is currently lacking. Herein, the mass fraction, distribution, crystallography and solid-state chemistry of REE, U and Th in Nigerian simulated fly ash samples were characterised using a range of laboratory and synchrotron x-ray based analytical techniques to underpin future extraction methodologies. Inductively coupled plasma mass spectrometry following full-acid digest of forty-five samples revealed recoverable average total REE content which ranged between 442 mgkg−1and 625 mgkg−1, comprising over 30 wt% of the critical REE Nd, Eu, Tb, Dy, Y and Er. These REE within the fly ash samples were found to be most frequently associated with discrete monazite, xenotime and Y-bearing zircon mineral particles, with the former the most detected, which could be beneficiated through gravity separation. Analysis of monazite particles isolated from the composite samples through a complimentary suite of analytical synchrotron radiation techniques revealed a core-shell pattern, with the shell rich in colocalised Ce, Nd and La, and the core enrich in both U and Th. Ce in monazite was found to exist in a mixed trivalent and tetravalent oxidation state, with the monazite structure amorphized due to the high temperature combustion process. Such results demonstrate the strong co-association and physical distribution of REE, U and Th within monazite in fly ash; knowledge of which can subsequently be used to optimise or develop a more selective, cost-effective and environmentally friendly solvent extraction methodology, by targeting the strongly colocalised and surface bound REE in fly ash monazite particles.
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Aug 2020
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B22-Multimode InfraRed imaging And Microspectroscopy
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Diamond Proposal Number(s):
[17501]
Abstract: Fourier transform infrared (FT-IR) spectroscopic imaging and microscopy of single living cells are established label-free technique for the study of cell biology. The constant driver to improve the spatial resolution of the technique is due to the diffraction limit given by IR wavelength making subcellular study challenging. Recently we have reported, with the use of a prototype ZnS transmission cell made of two hemispheres, that the spatial resolution is improved by the factor of the refractive index of ZnS, achieving a λ/2.7 spatial resolution using the synchrotron– IR microscopy with a 36x objective with numerical aperture (NA) of 0.5. To refine and to demonstrate that the ZnS hemisphere transmission device can be translated to standard bench-top FT-IR imaging systems, we have, in this work, modified the device to achieve a more precise path length, which has improved the spectral quality of the living cells, and showed for the first time that the device can be applied to study live cells with three different bench-top FT-IR imaging systems. We applied focal plane array (FPA) imaging, linear array, and a synchrotron single point scanning method and demonstrated that in all cases, subcellular details of individual living cells can be obtained. Results have shown that imaging with the FPA detector can measure the largest area in a given time whilst measurements from the scanning methods produced a smoother image. Synchrotron single point mapping produced the best quality image and has the flexibility to introduce over sampling to produce images of cells with great details, but it is time consuming in scanning mode. In summary, this work has demonstrated that the ZnS hemispheres can be applied in all three spectroscopic approaches to improve the spatial resolution without any modification to the existing microscopes.
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Feb 2020
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15716]
Open Access
Abstract: We use 9 nm and 15 nm thin membranes for determining the effective attenuation length of photoelectrons in silicon. One side of silicon membranes was covered with a thin film of aluminium and exposed to X-rays with energies from 3 to 8 keV. We recorded Al 1s and 2s photoelectrons that were (a) emitted from the Al film directly and (b) transmitted through the membranes. With the help of the ratio of both yields, we obtained values for the effective attenuation length (EAL) of electrons with kinetic energies up to 7.9 keV in silicon. The experimentally determined EAL values are smaller than obtained from different predictive equations. Using a power law fit View the MathML sourceEAL(k,p)=kEkinp to the experimental and predicted EAL values we find that mainly different is the pre-factor of the power law, k, while the exponent, i.e. the dependence on kinetic energy Ekin is represented well. Our study underlines the feasibility of using membranes for investigating surfaces under (near) ambient pressure conditions by photoelectron spectroscopy and points out the advantages of employing hard X-rays.
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Mar 2018
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I06-Nanoscience
I10-Beamline for Advanced Dichroism
Theoretical Physics
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Diamond Proposal Number(s):
[10191, 11500, 11501, 13047]
Abstract: X-ray detected ferromagnetic resonance (XFMR) has recently emerged as a powerful synchrotron-radiation-based tool able to study the element-selective magnetization dynamics. Magnetic and chemical contrast in XFMR is obtained by X-ray magnetic circular dichroism (XMCD), while the phase difference between the magnetization precessions is monitored using stroboscopic probing. A unique property of time-resolved XFMR is the visualization of the magnetization precession for each individual layer in a magnetic device. Measurement of the amplitude and phase response of the magnetic layers gives a clear signature of spin-transfer torque (STT) coupling between ferromagnetic layers due to spin pumping.
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Oct 2017
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[8449, 15122]
Abstract: The normal incidence x-ray standing waves technique is one of the most well-established methods for investigating the geometric structure at interfaces and surfaces. It is able to measure vertical positions and distances of individual atomic species with highest precision (typically <0.02 Å). These data not only yield valuable structural information, but also represent an excellent benchmark for density functional theory and ab initio calculations. Non-dipolar effects are well known to strongly affect the result, in particular when light elements are involved. A correction mechanism for these effects is established, but in its commonly-used form it is based on one essential restriction, namely the assumption of perfect normal incidence of the x-rays with respect to the relevant lattice planes of the crystal. Here, we show that small deviations from normal incidence, as they are unavoidable in typical experimental setups, lead to significant systematic errors in the NIXSW results. The magnitude of this effect depends on the specific conditions in a non-linear way and may reach up to 5%, corresponding to several tenths of an Ångström in the adsorption height. We present a straightforward way of accounting for this effect, and demonstrate that recording the photoelectron yield in an angular-resolved mode is indispensable, since the correction parameters strongly depend on the electron take-off angle.
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Jul 2017
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B23-Circular Dichroism
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Diamond Proposal Number(s):
[1681, 2074]
Open Access
Abstract: Previous clinical research has suggested high-affinity binding of flavopiridol (FLAP) to human blood serum proteins,
specifically either human serum albumin (HSA) or human alpha-1-acid glycoprotein (hAGP), when compared to fetal bovine
serum albumin (BSA) or bovine alpha-1-acid glycoprotein (bAGP) used in pre-clinical assays. This high-affinity binding was
suggested as the reason for its poor human clinical trial performance as a treatment for chronic lymphocytic leukaemia (CLL).
Using three biophysical techniques, specifically circular dichroism (CD), isothermal calorimetry (ITC) and fluorescence spectroscopy,
I show that FLAP does not have an overly high-affinity for either fetal BSA, HSA, bAGP or hAGP. I therefore suggest
an alternate hypothesis that models the albumin and alpha-1-acid glycoprotein (AGP) binding sites at the different protein concentrations
used in the fetal bovine pre-clinical assay and human physiological conditions. I use analytical ultracentrifugation
(AUC) experiments to determine the validity of the theoretical models. The models can also be altered to account for the elevated
AGP levels and reduced albumin levels seen in human cancer patients. Major differences in the concentrations of free
available FLAP are observed between the fetal bovine pre-clinical model and human physiological conditions. A number of
recommendations can therefore be made on how future pre-clinical assay studies should be conducted.
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Jun 2017
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Abstract: In this account the application of synchrotron radiation based X-ray photoelectron spectroscopy (XPS) for the investigation of electrochemically active gas-solid and liquid-solid interfaces will be discussed. The potential of Near Ambient Pressure XPS (NAP-XPS) for the estimation of the electronic surface structure of electrochemically active interfaces will be described by two examples. Thereto the oxygen evolution reaction (OER) over Pt and IrOx anodes will be introduced. In particular the analysis of XP core level spectra of IrOx requires the development of an appropriate fit model. Furthermore the design of reaction cells based on proton exchange membranes (PEM) and on electron transparent graphene membranes, which enables the investigation of liquid-gas and liquid-solid interfaces under electrochemical relevant conditions will be discussed. In the last part of this article a perspective to the EMIL project at the synchrotron radiation facility BESSY will be given. The purpose of this project is the implementation of two new beamlines enabling X-ray photoelectron spectroscopy in the X-ray regime from 80 eV − 8 keV under reaction conditions. The extension to the so called tender X-ray regime will allow the release of higher kinetic energy photoelectrons which have a higher inelastic mean free path compared to photoelectrons excited by soft X-ray radiation and therefore will enable the investigation of solid-liquid interfaces under electrochemical reaction conditions.
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Mar 2017
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B22-Multimode InfraRed imaging And Microspectroscopy
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Diamond Proposal Number(s):
[9755]
Abstract: By 2050, it is estimated that the global population will have surpassed 9 billion people, presenting a significant challenge with regards to food security. In order to provide sufficient quantities of nutritious food in the future, it is necessary to improve agricultural productivity by up to 70%. Nutrient deficiencies are one particular threat to food security that can have a negative impact on crop yield and quality. Currently the standard agricultural approach to prevention is to supply an excess macronutrient fertiliser, such as nitrate or phosphate, during crop production. However, the efficiency of this approach is poor as deficiencies of specific nutrients, such as Ca, are not prevented in this circumstance, and fertiliser use is associated with a host of adverse environmental impacts. Herein, we describe a novel method to detect Ca deficiency using synchrotron radiation-based Fourier-transform infrared (FTIR) microspectroscopy in live and fixed tissue of the model plant Commelina communis, as a precursor to targeted nutrient remediation in the field.
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Mar 2017
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