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In heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we study a series consisting of multilayer, bilayer and monolayer N2O on Ni(111) and employ Auger-photoelectron coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface (not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogen atoms of N2O are chemically inequivalent we can assign the intensity present in the bilayer and monolayer cases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio, molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of the surface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donation from the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail.

Implementation of 2D materials is one of the promising routes for improving the efficiency and stability of organic solar cells (OSCs). Due to their tunable optical and electronic properties, MXenes, a family of 2D transition metal carbides and nitrides, have attracted considerable attention and demonstrated their potential for next-generation solar cells. In this work, Ti3C2Tx MXene was added into ZnO precursors and applied as a modified composite electron transport layer (ETL) in PM6:N3-based inverted OSCs. The nonhalogenated solvent o- xylene was employed as the active layer solvent for the development of stable, efficient, and eco-friendly OSCs. By optimizing the concentration of Ti3C2Tx in the ZnO ETL, the solar cells exhibited power conversion efficiencies (PCEs) of 14.1 and 13.7% for 0.5 and 2 wt % MXene, respectively, as compared to neat ZnO layer devices with a PCE of 14.9%. Interestingly, the MXene-based PM6:N3 OSC devices showed superior device stability compared to the reference cells. It is demonstrated that the MXene introduced in the composite ZnO-based ETL mitigates the photocatalytic decomposition of the organic active layer on the ZnO surface, as analyzed via optical spectroscopy and hard X-ray photoelectron spectroscopy, which appears as a main reason for improved device stability. We thus report on the usage of MXene in green solvent-processed OSCs to enhance the lifetime of solar cells and thus address an important bottleneck in high-performance nonfullerene acceptor solar cells.

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., QC 20240201

The internal magnetic field in a ferromagnetic compound, Y2Co12P7 with TC = 150 K, was studied with µ+SR using a powder sample down to 2 K. The wTF-µ+SR measurements revealed the presence of a sharp magnetic transition at TC = 151 K, and the ZF-µ+SR measurements clarified the formation of static magnetic order below TC. The presence of two muon spin precession signals in the ZF-µ+SR spectrum below TC indicates the existence of the two different muon sites in the lattice. By considering the muon sites and local spin densities at the muon sites predicted with DFT calculations, the ordered magnetic moments of Co were successfully determined.

Oxygen is a commonly overlooked element influencing the properties of many metal oxides. By combining several analytical in situ techniques and theoretical calculations, we demonstrate that oxygen plays a vital part in the phase formation and crystallization of Al2TiO5-based chemical vapor-deposited coatings. Rutherford backscattering spectrometry (RBS) corroborates a polymorphic transformation during crystallization. Subsequent hard X-ray photoelectron spectroscopy (HAXPES) shows that crystallization occurs through a displacive (diffusionless) mechanism. Coupled with theoretical calculations, the crystallization and co-formation of Al2TiO5, Al6Ti2O13, and Al16Ti5O34 are suggested to be driven by the migration of oxygen ions and their corresponding vacancies.

A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination., QC 20230412

The conductivity of organic polymer heterojunction devices relies on the electron dynamics occurring along interfaces between the acceptor and donor moieties. To investigate these dynamics with chemical specificity, spectroscopic techniques are employed to obtain localized snapshots of the electron behavior at selected interfaces. In this study, charge transfer in blends (by weight 10, 50, 90, and 100%) of p-type polymer P(g(4)2T-T) (bithiophene-thiophene) and n-type polymer BBL (poly(benzimidazo-benzo-phenanthroline)) was measured by resonant Auger spectroscopy. Electron spectra emanating from the decay of core-excited states created upon X-ray absorption in the donor polymer P(g(4)2T-T) were measured in the sulfur KL2,3L2,3 Auger kinetic energy region as a function of the excitation energy. By tuning the photon energy across the sulfur K-absorption edge, it is possible to differentiate between decay paths in which the core-excited electron remained on the atom with the core-hole and those where it tunneled away. Analyzing the competing decay modes of these localized and delocalized (charge-transfer) processes facilitated the computation of charge-transfer times as a function of excitation energy using the core-hole clock method. The electron delocalization times derived from the measurements were found to be in the as/fs regime for all polymer blends, with the fastest charge transfer occurring in the sample with an equal amount of donor and acceptor polymer. These findings highlight the significance of core-hole clock spectroscopy as a chemically specific tool for examining the local charge tunneling propensity, which is fundamental to understanding macroscopic conductivity. Additionally, the X-ray absorption spectra near the sulfur K-edge in the P(g(4)2T-T) polymer for different polymer blends were analyzed to compare molecular structure, orientation, and ordering in the polymer heterojunctions. The 50% donor sample exhibited the most pronounced angular dependenc, Funding Agencies|Vetenskapsr?det [2020-06409]; Swedish Research Council

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.

QC 20221011

Commercial use of lead halide perovskites requires improved thermal stability and therefore a better understanding of their degradation mechanisms. The thermal degradation of three clean perovskite single crystal surfaces (MAPbI(3), MAPbBr(3), FAPbBr(3)) was investigated using synchrotron-based photoelectron spectroscopy. Central findings are that the halide has a large impact on thermal stability and that the degradation of formamidnium results in the formation of a new organic species at the FAPbBr(3) crystal surface.

The magnetism of double perovskites is a complex phenomenon, determined from intra- or interatomic magnetic moment interactions, and strongly influenced by geometry. We take advantage of the complementary length and timescales of the muon spin rotation, relaxation, and resonance (mu+SR) microscopic technique and bulk ac/dc magnetic susceptibility measurements to study the magnetic phases of the LaCaNiReO6 double perovskite. As a result, we are able to discern and report ferrimagnetic ordering below TC = 102 K and the formation of different magnetic domains above TC. Between TC < T < 270 K, the following two magnetic environments appear, a dense spin region and a static-dilute spin region. The paramagnetic state is obtained only above T > 270 K. An evolution of the interaction between Ni and Re magnetic sublattices, in this geometrically frustrated fcc perovskite structure, is revealed as a function of temperature through the critical behavior and thermal evolution of microscopic and macroscopic physical quantities.

There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.

In this work, we present the CoESCA station for electron-electron coincidence spectroscopy from surfaces, built in a close collaboration between Uppsala University and Helmholtz-Zentrum Berlin at the BESSY II synchrotron facility in Berlin, Germany. We start with a detailed overview of previous work in the field of electron-electron coincidences, before we describe the CoESCA setup and its design parameters. The system is capable of recording shot-to-shot resolved 6D coincidence datasets, i.e. the kinetic energy and the two take off angles for both coincident electrons. The mathematics behind extracting and analysing these multi-dimensional coincidence datasets is introduced, with a focus on coincidence statistics, resulting in fundamental limits of the signal-to-noise ratio and its implications for acquisition times and the size of the raw data stream. The functionality of the CoESCA station is demonstrated for the example of Auger electron-photoelectron coincidences from silver surfaces for photoelectrons from the Ag 3d core levels and their corresponding MNN Auger electrons. The Auger spectra originating from the different core levels, 3d3/2 and 3d5/2 could be separated and further, the two-hole state energy distributions were determined for these Auger decay channels.

Organic-inorganic halide perovskites have been intensively reinvestigated due to their applications, yet the optoelectronic function of the organic cation remains unclear. Through organic-selective resonant Auger electron spectroscopy measurements on well-defined single-crystal surfaces, we find evidence for electronic coupling in the unoccupied states between the organic and inorganic sublattices of the prototypical hybrid perovskite, which is contrary to the notion based on previous studies that the organic cation is electronically inert. The coupling is relevant for electron dynamics in the material and for understanding optoelectronic functionality.

An investigation of the unoccupied electronic structure of the transition-metal dichalcogenide tin disulphide has been conducted using core-hole clock spectroscopy. Polarization-dependent x-ray absorption in the tender x-ray regime at the S K edge and maps of the resonant Auger spectra in the S KLL Auger kinetic energy range have been recorded. Supported with ab initio calculations of the unoccupied electronic structure, these allow us to relate resonances in the absorption cross section to excitations along various directions in the Brillouin zone. We observe anisotropy in the x-ray absorption cross section in polarization directions in plane and out of plane of the crystal. There is also anisotropy in the charge transfer dynamics as inferred from the coherent and noncoherent parts of the resonant Auger spectra. This approach can be generally used to interpret dynamics in unoccupied states, e.g., in layered structure or heterogenous interfaces.

Black phosphorus (BPh) is a layered material with strong in-plane anisotropy of its structural and electronic properties; in spite of the great potential of BPh for conceptually new devices in optoelectronics and plasmonics, its fundamental electronic excitations have not yet been fully elucidated. In order to discriminate collective (plasmons) and single-particle (inter band transitions) excitations, we investigate the energy-loss distribution of P 1s photoelectrons in hard X-ray photoelectron spectra of BPh over a wide energy range. The energy-loss function (ELF), averaged over the principal directions of the BPh crystal, has been retrieved by using a Fourier Transform analysis to eliminate multiple inelastic scattering events. At low loss energies (1-8 eV), weak unresolved energy loss peaks are well described by DFT calculated inter band transitions, showing some anisotropy in the dielectric function epsilon(omega, q) tensor of BPh. At high loss energies, the ELF is dominated by the collective excitation of valence electrons with a peak energy at 20.1 +/- 0.2 eV, and weak anisotropy is found in the DFT calculated Im(- 1/epsilon) tensor. The anomalously small peak energy (9.0 +/- 0.5 eV) of a weak surface plasmon resonance is attributed either to low surface electron density in the terminal phosphorene layer or to some anisotropic surface plasmon propagation.

Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome-like lattice of silicene on aluminum (1 1 1) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations., QC 20200916