| 1. |
- Gao, Jiajia, et al.
(författare)
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Electrochemical impedance and X-ray absorption spectroscopy analyses of degradation in dye-sensitized solar cells containing cobalt tris(bipyridine) redox shuttles
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 24:31, s. 18888-18895
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Tidskriftsartikel (refereegranskat)abstract
- Electrochemical impedance spectroscopy (EIS) is a commonly used steady-state technique to examine the internal resistance of electron-transfer processes in solar cell devices, and the results are directly related to the photovoltaic performance. In this study, EIS was performed to study the effects of accelerated ageing, aiming for insights into the degradation mechanisms of dye-sensitized solar cells (DSSCs) containing cobalt tris(bipyridine) complexes as redox mediators. Control experiments based on aged electrolytes differing in concentrations of the redox couple components and cation co-additives were conducted to reveal the correlation of the cell degradation with external and internal properties. The failure modes of the cells emerged as changes in the kinetics of charge- and ion-transfer processes. An insufficient concentration of the redox complexes, in particular Co(iii), was found to be the main reason for the inferior performance after ageing. The related characterization of electrolytes aged outside the solar cell devices confirms the loss of active Co(iii) complexes in the device electrolytes. A new EIS feature at low frequencies emerged during ageing and was analysed. The new EIS feature demonstrates the presence of an unexpected rate-limiting, charge-transfer process in aged devices, which can be attributed to the TiO2/electrolyte interface. High-resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) was performed to identify the reduction of a part of Co(iii) to Co(II) after ageing, by investigating the Co K absorption edge. The HERFD-XAS data suggested a partial reduction of Co(iii) to Co(ii), accompanied by a difference in symmetry of the reduced species.
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| 2. |
- Ghosh, Anirudha, et al.
(författare)
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Magnetic circular dichroism in the dd excitation in the van der Waals magnet CrI3 probed by resonant inelastic x-ray scattering
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:11
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Tidskriftsartikel (refereegranskat)abstract
- We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The dd excitation energies are similar on the two sides of the ferromagnetic transition temperature, T-C similar to 61 K, although MD in RIXS is predominant at 0.4 T magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intraorbital dd excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.
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| 3. |
- Grubisic-Cabo, Antonija, et al.
(författare)
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In Situ Exfoliation Method of Large-Area 2D Materials
- 2023
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Ingår i: Advanced Science. - : Wiley. - 2198-3844. ; 10:22
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Tidskriftsartikel (refereegranskat)abstract
- 2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.
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| 4. |
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| 5. |
- Li, Cong, et al.
(författare)
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Emergence of Weyl fermions by ferrimagnetism in a noncentrosymmetric magnetic Weyl semimetal
- 2023
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.
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| 6. |
- Malik, Rameez Saeed, 1987-, et al.
(författare)
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Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:10
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Tidskriftsartikel (refereegranskat)abstract
- We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy Co2FeAl, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (tau(M)) in films of Co2FeAl is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by tau(R), is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter (alpha). Based on these results we argue that for Co2FeAl the remagnetization process is dominated by magnon dynamics, something which might have general applicability.
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| 7. |
- Man, Gabriel J., et al.
(författare)
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A-site cation influence on the conduction band of lead bromide perovskites
- 2022
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Ingår i: Nature Communications. - : Nature Research. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the sigma-pi energy splitting, and find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure. The A-cation influence on the mechanism of slow hot carrier cooling in perovskites is controversial. Here, Man et al. resolve a debated issue regarding A-cation influence on the electronic structure of lead halide perovskites.
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| 8. |
- Mukherjee, Soham, et al.
(författare)
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Interplay between Growth Mechanism, Materials Chemistry, and Band Gap Characteristics in Sputtered Thin Films of Chalcogenide Perovskite BaZrS3
- 2023
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 6:22, s. 11642-11653
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Tidskriftsartikel (refereegranskat)abstract
- The prototypical chalcogenide perovskite BaZrS3, characterized by its direct band gap, exceptionally strong light-harvesting ability, and good carrier transport properties, provides fundamental prerequisites for a promising photovoltaic material. This inspired the synthesis of BaZrS3 in the form of thin films, using sputtering and rapid thermal processing, aimed at device fabrication for future optoelectronic applications. Using a combination of short- and long-range structural information from X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we have elucidated how, starting from a random network of Ba, Zr, and S atoms, thermal treatment induces crystallization and growth of BaZrS3 and explained its impact on the observed photoluminescence (PL) properties. We also provide a description of the electronic structure and substantiate the surface material chemistry using a combination of depth-dependent photoelectron spectroscopy (PES) using hard X-ray (HAXPES) and traditional Al K alpha radiation. From the knowledge of the optical band gap of BaZrS3 thin films, synthesized at an optimal temperature of 900 C-degrees, and our estimation of the valence band edge position with respect to the Fermi level, one may conclude that these semiconductor films are intrinsic in nature with a slight n-type character. A detailed understanding of the growth mechanism and electronic structure of BaZrS3 thin films helps pave the way toward their utilization in photovoltaic applications.
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| 9. |
- Mukherjee, Soham, et al.
(författare)
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Structure and Electronic Effects from Mn and Nb Co-doping for Low Band Gap BaTiO3 Ferroelectrics
- 2021
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:27, s. 14910-14923
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the doping-induced local structural and electronic effects in the recently developed low band gap room temperature ferroelectric Mn-Nb co-doped BaTiO3. Experimental and theoretical Raman spectroscopies are utilized to quantify the Ti off-centering, identified to be the intrinsic origin of ferroelectricity in these systems. Mn and Nb exhibit contrasting doping behaviors that have remarkable effects on BaTiO3 functionality. Jahn-Teller distorted Mn3+ is primarily associated with lowering of the bulk band gap, while charge-compensating Nb5+ off-centers within the O-6 octahedra, creating a polar mode that stabilizes the ferroelectric ground state. The charge neutral aliovalent Mn3+-Nb5+ pair effectively couples to the inherent ferroelectric instability of the BaTiO3 lattice, restoring some spontaneous polarization lost by doping Mn3+ (d(4)) ions at Ti4+ (d(0)) sites.
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| 10. |
- Phuyal, Dibya, 1985-, et al.
(författare)
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Electronic structure of the charge disproportionate and metallicity in LaFe1-xMoxO3 studied by resonant x-ray spectroscopies
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A metal-insulator transition and its intriguing properties were recently reported based on doping of the antiferromagnetic insulator LaFeO3. Through doping at the B site by Mo to the insulating perovskite LaFeO3 exhibits a coexistence of an antiferromagnetic ordering with room-temperature metallicity at higher doping levels, an unusual ground state for an oxide perovskite. Replacing Fe by Mo in the Fe-O6 octahedra enhances the separation of the two 2 Fe-O bonds relative to the orthorhombic LaFeO3, largely retaining the centrosymmetric character of the Fe sites, as confirmed through extended x-ray absorption fine structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. Resonant photoemission was recorded for both Fe and Mo to obtain element specific spectral information on the projected 3d and 4d partial density of states. The data shows gradual increase of spectral weight at the Fermi level that is attributed to Mo d states, while the Fe d states primarily reside at higher binding energy in the valence band, with significant hybridization with O 2p states. The RIXS spectra show strong electron correlation effects from d-d inelastic features and broad charge-transfer excitations for x = 0.2 where the compound becomes metallic. This is in contrast to the insulating parent LaFeO3 compound and insulating samples with lower Mo content. The coexistence of both bound and continuum excitations observed in the RIXS spectra provides strong evidence for charge disproportionation with features that are linked to the disproportionate Fe sites.
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