Electron Yield Detection Research Articles

Nanoscale Surface and Bulk Electronic Properties of Ti3C2Tx MXene Unraveled by Multimodal X-Ray Spectromicroscopy.

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few- or multilayered flakes remains unclear. In this work, scanning X-ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few-layered hydrofluoric acid-etched Ti3C2Tx MXenes and multilayered molten salt (MS)-etched Ti3C2Tx MXenes. Postmortem analysis of MS-etched Ti3C2Tx electrodes cycled in a Li-ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode-electrolyte interactions at the nanoscale.

Inducing molecular orientation in solution-processed thin films of fluorene-bithiophene-based copolymer: thermal annealing vs. solvent additive.

A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s → π* and S 1s → σ* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.

XAS studies of vanadium pentoxide thin films

Thin films of 200 nm of VOx RF magnetron sputtered on the SiO2 glass and Si substrates were studied with X-ray Absorption Spectroscopy in the XANES range of the oxygen K and vanadium L2,3 edges. The total electron yield (TEY) detection mode was used. The spectra were collected at several temperatures between room temperature and 300 °C for the unpolarized and linearly polarized beam to study polarization effects. The experimental data were compared with the theoretical model based on DFT-ROCIS (Density Functional Theory, Restricted Open shell Configuration Interaction with Singles) from the literature. Changes in the spectra with temperature, particularly visible at the oxygen K edge are attributed to the loss of surface oxygen and the corresponding changes of population of the single- double- and triple vanadium coordinated oxygen. The corresponding decrease of the surface stoichiometry leads to formation lower than V5+ valency of vanadium ions such as V4+ or V3+ i.e., containing d-electrons. It gives a tentative explanation for the occurrence of metallicity attributed in the literature to a metal–insulator transition (MIT).

(Invited) Core-Shell Fe3O4@CoFe2O4 Spinel Nanoparticles for the Oxygen Evolution Reaction in Alkaline Media

Transition metal oxides have attracted much attention as promising anode materials for anion exchange membrane water electrolysis (AEMWE). Herewith we introduce core-shell nanoparticles comprised of a Fe3O4 core and a CoFe2O4 shell as prospective catalysts for the oxygen evolution reaction (OER) at the cathode of an AEMWE. We show that confining the active component in a thin (~1-2 nm) shell and taking advantage from the core-shell synergistic interaction allows one to reach an exceptional OER activity of 2800 A at 1.65 V vs. RHE per gram of cobalt [1]. We further show that to correctly determine the OER activity, it is necessary to study the OER in a wide interval of the catalyst loadings [2].To shed light of the origin of the aforementioned synergistic effect between the core and the shell, the Fe3O4@CoFe2O4 nanoparticles were investigated using operando near edge X-ray absorption fine structure (NEXAFS) spectroscopy in a total electron yield (TYE) detection mode, under potential and during the OER conditions [3] . We documented a strong influence of the Fe3O4 core on the redox behavior of the CoFe2O4 shell under the OER conditions. Contrary to the previously studied Co3O4 anode, whose surface at high potentials has been shown to transform into CoOOH [4], the shell of the Fe3O4@CoFe2O4 nanoparticles maintains its spinel structure with Co(II) stabilized by the Fe(III) of the core. The results suggest the occurrence of a cation-redox OER mechanism involving cooperative redox behavior between Co and Fe atoms. It is remarkable that the core-shell structure is essentially preserved under the OER conditions.Figure 1. Left-hand side: cartoon showing the composition of the core-shell nanoparticle and charge transfer from Co to Fe occurring under positive polarization. Right-hand side: Fe L-edge difference spectra acquired at 1.2, 1.4 and 1.8 V vs RHE (the reference itself taken at 1.0 V vs. RHE). Reproduced from Ref. [3].Finally, we study the influence of the shell composition by replacing Co by Ni on the OER activity and stability of core-shell nanoparticles. Acknowledgements The authors are indebted to S. Hettler and R. Arenal of INMA (Zaragoza, Spain), S. Holdcroft of SFU (Vancouver, Canada) and J. Velasco-Vélez of Alba Synchrotron facility (Spain) for their cooperation. We thank the HZB für Materialien und Energie for the allocation of synchrotron radiation beamtime. Financial support from Jean-Marie-Lehn foundation is gratefully acknowledged.REFERENCES[1] L. Royer, S. Hettler, R. Arenal, T. Asset, B. Rotonnelli, A. Bonnefont, E. Savinova, B. Pichon, submitted.[2] L. Royer, J. Guehl, M. Zilbermann, T. Dintzer, C. Leuvrey, B. Pichon, E. Savinova, A. Bonnefont, submitted.[3] L. Royer, A. Bonnefont, T. Asset, B. Rotonnelli, J. Velasco-Vélez, S. Holdcroft, S. Hettler, R. Arenal, B. Pichon, E. Savinova, ACS Catalysis, https://doi.org/10.1021/acscatal.2c04512.[4] F. Reikowski, F. Maroun, I. Pacheco, T. Wiegmann, P. Allongue, J. Stettner, O. Magnussen, ACS Catal. 2019, 9 (5), 3811–3821; https://doi.org/10.1021/acscatal.8b04823. Figure 1

Total electron yield (TEY) detection mode Cr K-edge XANES spectroscopy as a direct method to probe the composition of the surface of darkened chrome yellow (PbCr1-xSxO4) and potassium chromate paints

The darkening of chromate-pigments, including chrome yellows (PbCr1-xSxO4), is a surface phenomenon affectinglate 19th-early 20thc. paintings, such as those by Van Gogh. Exploring analytical strategies that contribute to a deep understanding of darkening is therefore significant for the long-term conservation of unique masterpieces.Here, we examined the capabilities of Cr K-edge XANES spectroscopy collected at the same time in X-ray fluorescence yield (XFY) and total electron yield (TEY) detection modes to selectively study the surface composition of darkened oil paint mock-ups composed of chrome yellow (PbCr0.2S0.8O4) or potassium chromate.By discussing advantages and drawbacks in using XFY/TEY modes in relation to XFY µ-XANES analysis from sectioned samples, we aim at assessing if TEY-XANES spectroscopy: (i) is a selective surface method to determine the abundance of different Cr-species from paint fragments; (ii) can contribute to optimize the analytical strategy by limiting time consuming sample preparation procedures; (iii) can decrease the probability of radiation damage.

Digging deeper: Buried layers and interfaces studied by modified total electron yield and soft x-ray absorption spectroscopy

We report on the soft x-ray absorption spectroscopy investigation of thin film capacitors using a modified total electron yield detection mode. This mode utilizes two ammeters instead of one as commonly employed in the classical total electron yield scheme to measure photocurrents of devices under soft x-ray irradiation. The advantage of this configuration over the surface sensitive classical total electron yield mode is that it can provide information from buried layers and interfaces up to a thickness equal to the penetration depth of soft x-rays. The method can be easily adapted to existing synchrotron end stations. We investigate dielectric capacitors with dissimilar electrodes to assess the feasibility of the modified total electron yield method. Furthermore, in operando soft x-ray absorption spectroscopy measurements are performed on ferroelectric capacitors under bias and using two ammeters. The experimental results are discussed in terms of the external and internal photoemission processes and their distribution in thin film capacitors under an external bias condition. The proposed detection method opens the way to perform electronic and chemical state analyses of the buried interfaces and layers in various devices like multiferroic tunnel junctions, memristive devices, etc., during operation under an applied bias.

Magnetic moment orientation and in-depth distribution of dysprosium near the surface of DyCo4.6 thin films from x-ray circularly polarized absorption

We have investigated the dysprosium distribution and its magnetic moment orientation at the region near the surface of ${\mathrm{DyCo}}_{4.4}$ and ${\mathrm{DyCo}}_{4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy. X-ray magnetic circular dichroism spectroscopy of the films at the Dy ${M}_{4,5}$ and Co ${L}_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientations ranged from ${0}^{\ensuremath{\circ}}$ to ${70}^{\ensuremath{\circ}}$ with respect to the normal to the sample. The measurements showed an apparent partial decoupling between the cobalt and dysprosium magnetic sublattices. At RT, the magnetic moment per atom of dysprosium was below the minimum value expected if all dysprosium moments were Antiferromagnetic (AF) coupled to cobalt. At 2 K, the cobalt sublattice presented a surprisingly stronger magnetic anisotropy than the dysprosium sublattice. A detailed analysis of the circularly polarized spectra of the Dy ${M}_{5}$ edge, based on the deconvolution of the spectra in their related parallel, antiparallel, and transverse to ${J}_{z}$ spectral components, demonstrates that the spectra are composed by dysprosium with different magnetic moment distributions. The fit of the Dy ${M}_{5}$ spectra using the ${J}_{z}$ spectral components evidenced a gradation of dysprosium concentration due to segregation at the region probed by TEY. The topmost layer was magnetically uncoupled from cobalt. At RT, $25%$ of the dysprosium magnetic moments in the underlayer were found averaged oriented in the same direction of cobalt. The expected weak magnetic coupling of these dysprosium atoms to cobalt should explain the surprisingly lower magnetic anisotropy of the dysprosium sublattice compared to that of cobalt probed by TEY at 2 K.

In-situ analysis of ion-induced physicochemical change of Si surface by secondary electron yield detection

We report the variation of secondary electron yield with ion fluence during 10 keV N+1 and Ar+1 ions bombardment on a Si (1 0 0) surface at normal and oblique incidence. The initial high secondary electron yield from Si surface is observed due to the native oxide layer on Si for both nitrogen and argon ion bombardment. The secondary electron yield is expected to decrease and stabilize for continue ion bombardment, which is observed for the argon ion irradiation. However, a significant variation of the electron yield with continuing nitrogen ion bombardment is observed. Ex situ measurement of ion bombarded surfaces by Atomic Force Microscope shows that the variation of secondary electron yield is correlated with the surface topography development due to ion bombardment. Also, the reaction of the projectile with target atom alters the secondary electron emission in case of N+ ion bombardment. The mean free path length of the secondary electron is estimated using the Parilis-Kishinevsky theory and the variation is presented. Real-time physicochemical changes of Si surface due to low energy ion bombardment have been probed in-situ by monitoring the secondary electron emission yield measurements.

Realization of a scanning soft X-ray microscope for magnetic imaging under high magnetic fields.

For the purpose of imaging element- and shell-specific magnetic distributions under high magnetic fields, a scanning soft X-ray microscope has been developed at beamline BL25SU, SPring-8, Japan. The scanning X-ray microscope utilizes total electron yield detection of absorbed circularly polarized soft X-rays in order to observe magnetic domains through the X-ray magnetic circular dichroism effect. Crucially, this system is equipped with an 8 T superconducting magnet. The performance and features of the present system are demonstrated by magnetic domain observations of the fractured surface of a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet.

Sum rule distortions in fluorescence-yield x-ray magnetic circular dichroism

The quantitative analysis of $3d$ transition metal ${\mathrm{L}}_{2,3}$ edge x-ray magnetic circular dichroism (XMCD) spectra and the related sum rules are compared for measurements with electron yield and fluorescence yield detection. Multiplet calculations on divalent ions show that fluorescence yield detected sum rule derived expectation values of ${\mathrm{L}}_{z}$ and ${\mathrm{S}}_{z}$ show noticeable deviations and detection angle dependence. We show that small deviations of the polarization dependent fluorescent decay values lead to significant deviations in the ${\mathrm{L}}_{z}$ and ${\mathrm{S}}_{z}$ sum rule values. Fe and Co experimental XMCD spectra of a supported 10 nm ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ thin film are measured simultaneously by both electron and fluorescence yield. The deviations shown in the experimental data are well explained by the calculations and are shown to mainly depend on the polarization dependent total decay. We conclude that fluorescence yield detected x-ray magnetic circular dichroism is unsuitable for quantitative analysis of the ${\mathrm{L}}_{z}$ and ${\mathrm{S}}_{z}$ sum rule values.

Erbium(III) Coordination at the Surface of an Aqueous Electrolyte

Grazing-incidence (GI) X-ray absorption spectroscopy (XAS) under conditions of total external reflection is used to explore the coordination environment of the trivalent erbium ion, Er(3+), at an electrolyte-vapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that the Er(3+) ions have a simple hydration shell with an average Er-OH2 bond distance of 2.33(1) Å, consistent with previous descriptions of the aquated cation, [Er(OH2)8](3+). No other correlations are observed in the electrolyte EXAFS (extended X-ray absorption fine structure) data acquired at room temperature. In contrast, the coordination of the Er(3+) ions at the electrolyte-helium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the Er-OH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6-7 O atoms at 2.36(1) Å and a second one of 3 Cl atoms at 2.89(2) Å, suggesting the formation of a neutral [(H2O)6-7ErCl3] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to Er-Er correlations (in possible combination with contributions from distant Er-O and Er-Cl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) Å, confirming the near-surface-enrichment of Er(3+) as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082). Here, the strong associations between the Er-aqua-chloro entities at the electrolyte-vapor interface are shown to be consistent with the formation of domains of polynuclear cluster motifs, such as would arise through hydrolysis reactions of the aquated Er(3+) cations. The local structural results and the calculated surface coverage are of relevance to understand the myriad reactions involved in the hydrometallurgical process of solvent extraction (SX) for metal purification, which involves the transfer of a selected metal ion, like Er, across an interface from an aqueous electrolyte to an organic phase.

Electron yield soft X-ray photoabsorption spectroscopy under normal ambient-pressure conditions.

Ambient-pressure soft X-ray photoabsorption spectroscopy (XAS) was demonstrated to be applicable to the chemical analysis of hydrated transition-metal compounds. For this purpose, even under ambient-pressure conditions, electron yield detection XAS (EY-XAS), based on a simple drain-current set-up, was used to overcome a weakness in fluorescence yield detection XAS (FY-XAS), which does not give a pure soft XAS. The feasibility of EY-XAS was investigated and it was clarified that the EY-XAS under ambient-pressure conditions corresponds to the mixed data of the total EY and conversion EY spectra. Normal ambient-pressure EY-XAS analysis was applied to anhydrous (CoCl2) and to hydrated (CoCl2·6H2O) cobalt chloride at the Co L23-edge. The present measurements demonstrated the ability to unambiguously distinguish the different chemical states of cobalt ions, relying upon spectral differences that indicate octahedral/quasi-octahedral structural changes as a result of hydration/dehydration reactions.

Quantification of strain through linear dichroism in the Si 1s edge X-ray absorption spectra of strained Si1−xGex thin films

We have quantitatively measured the angle dependence in the Silicon 1s X-ray absorption spectra of strained Si1−xGex thin films prepared by epitaxial growth on Si(100) substrates, through surface sensitive total electron yield detection. The linear dichroism difference extracted from these angle dependent X-ray absorption spectra is proportional to the degree of strain, as measured separately by Raman spectroscopy. This quantitative relationship provides a means to measure the compressive strain in Si1−xGex thin films. This strain-dependent X-ray absorption spectroscopy has the potential to realize a semiconductor strain metrology through high spatial resolution X-ray spectromicroscopy.

A versatile partial electron yield detector with large acceptance angle and well-defined threshold energy and gain

We describe a partial electron yield detector for X-ray absorption studies with a large angle of acceptance of π sr, a well-defined electron cut-off energy due to a spherical retardation optics, and a linear, well reproducible gain. The detector operates without electron multipliers and is assembled from standard replacement parts of conventional LEED optics.

Interfacial effects in manganite thin films with different capping layers of interest for spintronic applications

We report on the use of surface-sensitive techniques (x-ray absorption spectroscopy and x-ray magnetic circular dichroism) to investigate interfacial effects in sputtered manganite thin films, La${}_{2/3}$Ca${}_{1/3}$MnO${}_{3}$ (LCMO) and La${}_{2/3}$Sr${}_{1/3}$MnO${}_{3}$ (LSMO), with different capping layers [MgO, LaAlO${}_{3}$ (LAO), SrTiO${}_{3}$ (STO), NdGaO${}_{3}$ (NGO), and Au]. To ensure surface sensitivity, data were acquired by using the total electron yield detection mode. It is found that LSMO and LCMO films exhibit similar behavior when capped with oxide layers but dissimilar effects when a metallic capping is used. Almost bulklike Mn valence at the interface was observed in the case of LAO capping. However, a notorious increase of the Mn oxidation state was detected for both MgO and NGO capping layers. In contrast, metallic Au and STO capping promotes a reduction of the Mn oxidation state. These results are correlated with the x-ray magnetic circular dichroism data, showing a concomitant decrease of the saturation magnetization at the interface in those cases where a modification of the Mn oxidation state is observed.

Interfacial effects in La2/3Sr1/3MnO3 thin films with different complex oxide capping layers

Interfacial effects in sputtered La2/3Sr1/3MnO3 thin films with different capping layers (MgO, LaAlO3, SrTiO3, NdGaO3, and Au) have been locally investigated by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn L3,2-edge. Data were acquired by using the total electron yield detection mode thus guaranteeing maximum sensitivity to the interface. The data show that LaAlO3 capping almost does not modify the bulklike Mn valence at the interface. In case of SrTiO3 and Au, the presence of divalent Mn is detected, whereas MgO and NdGaO3 capping lead to an increase of the Mn valence oxidation state. The modification of the nominal Mn valence state leads to depressed surface magnetization.

Investigation of the structural changes in Li1−xFePO4 upon charging by synchrotron radiation techniques

A systematic study on a LiFePO4 sample with 600 nm particle size was carried out to investigate the electronic and crystal structure changes during charge and their relationships with the charging curve. In situX-ray diffraction data collected at 0.1C rate during first charge shows that the crystal FePO4 is observed at x = 0.3 in Li1−xFePO4, with significant delay than the beginning of the charge plateau. On the other hand, in situ X-ray absorption spectra for the Fe K-edge collected at 0.1C during first charge show that the average oxidation of Fe2+ to Fe3+ almost synchronized with the charging curve. In addition, the ex-situ soft X-ray absorption data for the Fe LII–III edges using surface sensitive partial electron yield detection mode show that the lithium content changes at the surface of the sample particles synchronized or ahead to the charge curve, while the data collected at the same time using bulk sensitive fluorescence yield detection mode show that the lithium content changes in the bulk of the particles lag behind the charging curve. The contents of FePO4 and LiFePO4 calculated from in situXRD in relating to the charge curve are quite different than those calculated from in situXAS and ex-situ soft X-ray absorption data. This tells us that cautious need to be taken when using XRD data alone to track the state of charge of LiFePO4 samples, significant delay in observing the new crystal phase formation could occur, especially at relatively high charging rate.

Structural Evolution of Nuclear Glasses under Forcing Conditions (Irradiation, Alteration)

AbstractAssessing the long-term behavior of nuclear glass implies the prediction of their long-term performance, and more precisely of their evolution under irradiation and during interaction with water. After briefly recalling the major characteristics of the local and medium-range structure of borosilicate glasses of nuclear interest, we will present some structural features observed under forcing conditions. Specific structural tools (EXAFS/XANES, Neutron/x-ray diffraction, solid state spectroscopic methods…) are correlated with numerical simulations to determine the local structure of glass and provide selective information on glass surface using total electron yield detection. During alteration in near- or under-saturated conditions, some elements such as Fe change coordination, as other elements such as Zr only suffer structural modifications in under-saturated conditions. These structural modifications may explain the chemical dependence of the initial alteration rate and the transition to the residual regime. They also illustrate the molecular-scale origin of the processes at the origin of the glass-to-gel transformation. Molecular scale processes help in predicting the properties of new generations of nuclear glasses required by future production of nuclear energy. Under irradiation, various structural effects are observed, including coordination change, ion migration or disorder effects. These studies show that glasses with a simplified composition do not show the same behavior as more realistic glasses. Molecular dynamics (MD) simulations provide complementary information on elastic effects. Recent direct evidence for B-coordination change under external irradiation together with structural models derived from MD sheds light on the structural mechanisms at the origin of radiation-induced modifications of glass properties, emphasizing the importance of the thermal regime in the cascade core.

Improvement in XAFS beamline BL01B1 at SPring-8

Beamline BL01B1 was constructed for conventional XAFS measurements using a bending magnet light source at SPring-8 in 1997 [1]. BL01B1 covers a wide energy range from 3.8 to 113 keV using an adjustable inclined double-crystal monochromator and a double mirror system. Here, we describe a newly developed apparatus and the current status of the BL01B1. A time-efficient QEXAFS method was developed to measure high quality spectra up to high-k regions in the shortest possible time. The method involves sweeping the monochromator continuously at a variable angular speed depending on the k-region using a VME stepping motor controller. During scanning, a pair of crystals of the monochromator rotates without translation motion like a channel-cut crystal. This QEXAFS method was applied to the fluorescence mode using a 19-element Ge detector combined with a digital X-ray processor system. A new conversion electron yield detector having a rotational sample stage was developed for single crystalline samples. Angular oscillations of 4 degrees in the sample rotation stage around the axis normal to the sample surface successfully remove diffraction noise in the XAFS spectra.

Size-dependent magnetic spin and orbital moments of Fe nanoparticles deposited onto Co/W(110)

In situ x-ray magnetic circular dichroism (XMCD) measurements by means of total electron-yield detection have been carried out on mass-filtered Fe nanoparticles with diameters ranging from 6 to 10 nm upon deposition onto Co/W(110). The shape as well as the crystallographic structure has independently been investigated by means of high-resolution transmission electron microscopy. An evaluation of the XMCD spectra revealed bulklike magnetic spin moments but strongly enhanced magnetic orbital moments after correcting for electron-yield saturation effects. The observed size dependence of the magnetic moments cannot be understood in simple core shell models where the core is assumed to be bulklike and only the outer shell possesses enhanced values according to their reduced coordination at the surface. Instead, size-dependent distortions of the crystal lattice due to surface and interfacial tension as well as mesoscopic relaxations at the surface facets of the nanoparticles are considered as the origin of the observed large orbital moments.