@article{MartenssonFoehlischSvensson2022, author = {Martensson, Nils and F{\"o}hlisch, Alexander and Svensson, Svante}, title = {Uppsala and Berkeley}, series = {Journal of vacuum science \& technology : JVST ; an AVS journal / A}, volume = {40}, journal = {Journal of vacuum science \& technology : JVST ; an AVS journal / A}, number = {4}, publisher = {American Institute of Physics}, address = {New York}, issn = {0734-2101}, doi = {10.1116/6.0001879}, pages = {11}, year = {2022}, abstract = {The development of modern photoelectron spectroscopy is reviewed with a special focus on the importance of research at Uppsala University and at Berkeley. The influence of two pioneers, Kai Siegbahn and Dave Shirley, is underlined. Early interaction between the two centers helped to kick-start the field. Both laboratories have continued to play an important role in the field, both in terms of creating new experimental capabilities and developing the theoretical understanding of the spectroscopic processes.}, language = {en} } @article{BornJohanssonLeitneretal.2022, author = {Born, Artur and Johansson, Fredrik O. L. and Leitner, Torsten and Bidermane, Ieva and Kuehn, Danilo and Martensson, Nils and F{\"o}hlisch, Alexander}, title = {The degree of electron itinerancy and shell closing in the core-ionized state of transition metals probed by Auger-photoelectron coincidence spectroscopy}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {24}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {32}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/d2cp02477b}, pages = {19218 -- 19222}, year = {2022}, abstract = {Auger-photoelectron coincidence spectroscopy (APECS) has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d-shell is localized (atomic-like) and with a self-convoluted valence band for band-like (itinerant) materials as explained using the Cini-Sawatzky model. For transition metals, the 3d band changes from band-like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self-convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini-Sawatzky model.}, language = {en} } @article{CappelSvanstromLanzilottoetal.2017, author = {Cappel, Ute B. and Svanstrom, Sebastian and Lanzilotto, Valeria and Johansson, Fredrik O. L. and Aitola, Kerttu and Philippe, Bertrand and Giangrisostomi, Erika and Ovsyannikov, Ruslan and Leitner, Torsten and F{\"o}hlisch, Alexander and Svensson, Svante and Martensson, Nils and Boschloo, Gerrit and Lindblad, Andreas and Rensmo, Hakan}, title = {Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells}, series = {ACS applied materials \& interfaces}, volume = {9}, journal = {ACS applied materials \& interfaces}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.7b10643}, pages = {34970 -- 34978}, year = {2017}, abstract = {Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAP-bI(3))(0.85)(MAPbBr(3))(0.15)) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.}, language = {en} } @article{GiangrisostomiOvsyannikovSorgenfreietal.2018, author = {Giangrisostomi, Erika and Ovsyannikov, Ruslan and Sorgenfrei, Nomi and Zhang, Teng and Lindblad, Andreas and Sassa, Yasmine and Cappel, Ute B. and Leitner, Torsten and Mitzner, Rolf and Svensson, Svante and Martensson, Nils and F{\"o}hlisch, Alexander}, title = {Low Dose Photoelectron Spectroscopy at BESSY II}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {224}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2017.05.011}, pages = {68 -- 78}, year = {2018}, abstract = {The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances.}, language = {en} } @article{KuehnMuellerSorgenfreietal.2019, author = {K{\"u}hn, Danilo and M{\"u}ller, Moritz and Sorgenfrei, Nomi and Giangrisostomi, Erika and Jay, Raphael Martin and Ovsyannikov, Ruslan and Martensson, Nils and Sanchez-Portal, Daniel and F{\"o}hlisch, Alexander}, title = {Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2}, series = {Scientific reports}, volume = {9}, journal = {Scientific reports}, number = {488}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-018-36637-0}, pages = {9}, year = {2019}, abstract = {For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.}, language = {en} }