KMC-1: a high resolution and high flux soft x-ray beamline at BESSY

Unraveling the Role of Perovskite in Buried Interface Passivation

Interfaces in perovskite solar cells play a crucial role in their overall performance, and therefore, detailed fundamental studies are needed for a better understanding. In the case of the classical n-i-p architecture, TiO2 is one of the most used electron-selective layers and can induce chemical reactions that influence the performance of the overall device stack. The interfacial properties at the TiO2/perovskite interface are often neglected, owing to the difficulty in accessing this interface. Here, we use X-rays of variable energies to study the interface of (compact and mesoporous) TiO2/perovskite in such a n-i-p architecture. The X-ray photoelectron spectroscopy and X-ray absorption spectroscopy methods show that the defect states present in the TiO2 layer are passivated by a chemical interaction of the perovskite precursor solution during the formation of the perovskite layer and form an organic layer at the interface. Such passivation of intrinsic defects in TiO2 removes charge recombination centers and shifts the bands upward. Therefore, interface defect passivation by oxidation of Ti3+ states, the organic cation layer, and an upward band bending at the TiO2/perovskite interface explain the origin of an improved electron extraction and hole-blocking nature of TiO2 in the n-i-p perovskite solar cells.

Oxidation of Aqueous Phosphorous Acid Electrolyte in Contact with Pt Studied by X-ray Photoemission Spectroscopy

The oxidation of the aqueous H3PO3 in contact with Pt was investigated for a fundamental understanding of the Pt/aqueous H3PO3 interaction with the goal of providing a comprehensive basis for the further optimization of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Ion-exchange chromatography (IEC) experiments suggested that in ambient conditions, Pt catalyzes H3PO3 oxidation to H3PO4 with H2O. X-ray photoelectron spectroscopy (XPS) on different substrates, including Au and Pt, previously treated in H3PO3 solutions was conducted to determine the catalytic abilities of selected metals toward H3PO3 oxidation. In situ ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) combined with the "dip-and-pull" method was performed to investigate the state of H3PO3 at the Pt|H3PO3 interface and in the bulk solution. It was shown that whereas H3PO3 remains stable in the bulk solution, the catalyzed oxidation of H3PO3 by H2O to H3PO4 accompanied by H2 generation occurs in contact with the Pt surface. This catalytic process likely involves H3PO3 adsorption at the Pt surface in a highly reactive pyramidal tautomeric configuration.

A Versatile Approach to Electrochemical In Situ Ambient-Pressure X-ray Photoelectron Spectroscopy: Application to a Complex Model Catalyst

We present a new technique for investigating complex model electrocatalysts by means of electrochemical in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Using a specially designed miniature capillary device, we prepared a three-electrode electrochemical cell in a thin-layer configuration and analyzed the active electrode/electrolyte interface by using "tender" X-ray synchrotron radiation. We demonstrate the potential of this versatile method by investigating a complex model electrocatalyst. Specifically, we monitored the oxidation state of Pd nanoparticles supported on an ordered Co₃O₄(111) film on Ir(100) in an alkaline electrolyte under potential control. We found that the Pd oxide formed in the in situ experiment differs drastically from the one observed in an ex situ emersion experiment at similar potential. We attribute these differences to the decomposition of a labile palladium oxide/hydroxide species after emersion. Our experiment demonstrates the potential of our approach and the importance of electrochemical in situ AP-XPS for studying complex electrocatalytic interfaces.

Quantification of the Activator and Sensitizer Ion Distributions in NaYF4 :Yb3+ , Er3+ Upconverting Nanoparticles Via Depth-Profiling with Tender X-Ray Photoemission

The spatial distribution and concentration of lanthanide activator and sensitizer dopant ions are of key importance for the luminescence color and efficiency of upconverting nanoparticles (UCNPs). Quantifying dopant ion distributions and intermixing, and correlating them with synthesis methods require suitable analytical techniques. Here, X-ray photoelectron spectroscopy depth-profiling with tender X-rays (2000-6000 eV), providing probe depths ideally matched to UCNP sizes, is used to measure the depth-dependent concentration ratios of Er³⁺ to Yb³⁺ , [Er³⁺ ]/[Yb³⁺ ], in three types of UCNPs prepared using different reagents and synthesis methods. This is combined with data simulations and inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurements of the lanthanide ion concentrations to construct models of the UCNPs' dopant ion distributions. The UCNP sizes and architectures are chosen to demonstrate the potential of this approach. Core-only UCNPs synthesized with XCl₃ ·6H₂ O precursors (β-phase) exhibit a homogeneous distribution of lanthanide ions, but a slightly surface-enhanced [Er³⁺ ]/[Yb³⁺ ] is observed for UCNPs prepared with trifluroacetate precursors (α-phase). Examination of Yb-core@Er-shell UCNPs reveals a co-doped, intermixed region between the single-doped core and shell. The impact of these different dopant ion distributions on the UCNP's optical properties is discussed to highlight their importance for UCNP functionality and the design of efficient UCNPs.

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials

There is a high fundamental interest in the surface and bulk chemistry of quantum dot (QD) solids, as they have proven to be very promising materials in optoelectronic devices. The choice of surface ligands for quantum dots in solid devices determines many of the film properties, as the ligands influence for example the doping density, chemical stability and charge transport. Lead halide ligands have developed as the main ligand of choice for lead sulfide quantum dots, as they have been shown to passivate quantum dot surfaces and enhance the chemical stability. In this study, we successfully varied the ligand composition on the surface of PbS quantum dot films from pure lead iodide to pure lead bromide and investigated its influence on the chemical and electronic structure of the QD solids using hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we developed a surface treatment to prevent the surface oxidation of a bulk PbS reference sample. Through measurements of this sample and of lead halide reference samples, we were able to assign the contributions of different chemical bonding to the Pb 4f core level and of different atomic orbitals to the valence band spectral shape of the QD materials. Overall, we found that the valence band edge position was very similar for all different iodide:bromide ratios and that all investigated compositions were able to protect the quantum dot surfaces within solid films from oxidation. However, the ligand composition significantly influences the sample stability under X-rays. The iodide rich QD solids showed the highest stability with very little to no chemical changes over several hours of X-ray exposure, while the bromide rich QD solids changed already within the first hour of exposure.

Chemical and electronic structure of the heavily intermixed (Cd,Zn)S:Ga/CuSbS2 interface

The interface formation and chemical and electronic structure of the (Cd,Zn)S:Ga/CuSbS2 thin-film solar cell heterojunction was studied using hard X-ray photoelectron spectroscopy (HAXPES) of the bare absorber and a buffer/absorber...

Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge

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.

By measuring the resonant S-KLL Auger decay we investigate the attosecond charge transfer in PbS quantum dots of different sizes. The results show both intra- and interparticle charge transfer, where the former shows a strong QD size dependence.

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.

The lower energy diffraction and scattering side-bounce beamline for materials science at the Canadian Light Source

A new diffraction beamline for materials science has been built at the Canadian Light Source synchrotron. The X-ray source is an in-vacuum wiggler with a 2.5 T peak magnetic field at 5.2 mm gap. The optical configuration includes a toroidal mirror, a single side-bounce Bragg monochromator, and a cylindrical mirror, producing a sub-150 µm vertical × 500 µm horizontal focused beam with a photon energy range of 7-22 keV and a flux of 10¹² photons per second at the sample position. Three endstations are currently open to general users, and the techniques available include high-resolution powder diffraction, small molecule crystallography, X-ray reflectivity, in situ rapid thermal annealing, and SAXS/WAXS. The beamline design parameters, calculated and measured performance, and initial experimental results are presented to demonstrate the capabilities for materials science.

Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO₃. However, the use of MoO₃ as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO₃/CH₃NH₃PbI_3-xCl_x (MAPbI_3-xCl_x) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO₃ deposition, significant chemical interaction is induced at the MoO₃/MAPbI_3-xCl_x interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI₂ on the MoO₃ cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO₄, PbN₂O₂, and PbO as a result of the MAPbI_3-xCl_x decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO₃/MAPbI_3-xCl_x interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO₃ as a hole-transport material and in which there is a direct contact between MoO₃ and perovskite.

Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation

In recent years Microwave Kinetic Inductance Detectors (MKIDs) have emerged as one of the most promising novel low temperature detector technologies. Their unrivaled scalability makes them very attractive for many modern applications and scientific instruments. In this paper we intend to give an overview of how and where MKIDs are currently being used or are suggested to be used in the future. MKID based projects are ongoing or proposed for observational astronomy, particle physics, material science and THz imaging, and the goal of this review is to provide an easily usable and thorough list of possible starting points for more in-depth literature research on the many areas profiting from kinetic inductance detectors.

NaF/RbF-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers: Distinct Surface Modifications Caused by Two Different Types of Rubidium Chemistry

The underlying beneficial mechanism of heavy alkali postdeposition treatment (PDT) of Cu(In,Ga)Se₂ thin-film solar cell absorbers that led to new record efficiencies in recent years is studied using photoelectron spectroscopy. Excitation energies between 40.8 eV and 6 keV were used to examine the near-surface region of Cu(In,Ga)Se₂ thin-film solar cell absorbers that underwent NaF and combined NaF/RbF PDT. The already Cu-deficient surface region after NaF PDT, which is modeled as a Cu:(In + Ga):Se = 1:5:8 phase, shows further depletion after NaF/RbF PDT and seems to incorporate some Rb. Additionally, we have found strong indications for the NaF/RbF PDT-induced formation of a Rb-In-Se-type compound with a 1:1:2 stoichiometry partially covering the absorber surface. The electronic Cu(In,Ga)Se₂ structure is modified due to the RbF treatment, with a pronounced shift in the valence band maximum away from the Fermi level in the immediate vicinity of the surface.

Tailoring ultra-fast charge transfer in MoS2

Charge transfer dynamics are of importance in functional materials used in devices ranging from transistors to photovoltaics. The understanding of charge transfer in particular of how fast electrons tunnel away from an excited state and where they end up, is necessary to tailor materials used in devices. We have investigated charge transfer dynamics in different forms of the layered two-dimensional material molybdenum disulphide (MoS2, in single crystal, nanocrystalline particles and crystallites in a reduced graphene oxide network) using core-hole clock spectroscopy. By recording the electrons in the sulphur KLL Auger electron kinetic energy range we have measured the prevalence of localised and delocalised decays from a state created by core excitation using X-rays. We show that breaking the crystal symmetry of the single crystal into either particles or sheets causes the charge transfer from the excited state to occur faster, even more so when incorporating it in a graphene oxide network. The interface between the MoS2 and the reduced graphene oxide forms a Schottky barrier which changes the ratio between local and delocalised decays creating two distinct regions in the charge transfer dependent on the energy of the excited electron. Thereby we show that ultra-fast charge transfer in MoS2 can be tailored, a result which can be used in the design of emergent devices.

Impact of SnF2 Addition on the Chemical and Electronic Surface Structure of CsSnBr3

We report on the chemical and electronic structure of cesium tin bromide (CsSnBr₃) and how it is impacted by the addition of 20 mol % tin fluoride (SnF₂) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF₂ improves coverage of the titanium dioxide substrate by CsSnBr₃ and decreases the oxidation of Sn^II to Sn^IV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF₂-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn^II/Sn^IV ratio. Notwithstanding some variability in the data, we conclude that SnF₂ addition decreases the energy difference between the VB maximum of CsSnBr₃ and the Fermi level, which we explain by defect chemistry considerations.

The Doping Mechanism of Halide Perovskite Unveiled by Alkaline Earth Metals

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

Tunability of MoO3 Thin-Film Properties Due to Annealing in Situ Monitored by Hard X-ray Photoemission

The chemical and electronic structure of MoO3 thin films is monitored by synchrotron-based hard X-ray photoelectron spectroscopy while annealing from room temperature to 310 °C. Color-coded 2D intensity maps of the Mo 3d and O 1s and valence band maximum (VBM) spectra show the evolution of the annealing-induced changes. Broadening of the Mo 3d and O 1s spectra indicate the reduction of MoO3. At moderate temperatures (120-200 °C), we find spectral evidence for the formation of Mo5+ and at higher temperatures (>165 °C) also of Mo4+ states. These states can be related to the spectral intensity above the VBM attributed to O vacancy induced gap states caused by partial filling of initially unoccupied Mo 4d-derived states. A clear relation between annealing temperature and the induced changes in the chemical and electronic structure suggests this approach as a route for deliberate tuning of MoO3 thin-film properties.

Optimized highly efficient multilayer-coated blazed gratings for the tender X-ray region

The optimized design of multilayer-coated blazed gratings (MLBG) for high-flux tender X-ray monochromators was systematically studied by numerical simulations. The resulting correlation between the multilayer d-spacing and grating blaze angle significantly deviated from the one predicted by conventional equations. Three high line density gratings with different blaze angles were fabricated and coated by the same Cr/C multilayer. The MLBG with an optimal blaze angle of 1.0° showed a record efficiency reaching 60% at 3.1 keV and 4.1 keV. The measured efficiencies of all three gratings were consistent with calculated results proving the validity of the numerical simulation and indicating a more rigorous way to design the optimal MLBG structure.

Combining diffusive gradients in thin films (DGT) and spectroscopic techniques for the determination of phosphorus species in soils

A wide range of methods are used to estimate the plant-availability of soil phosphorus (P). Published research has shown that the diffusive gradients in thin films (DGT) technique has a superior correlation to plant-available P in soils compared to standard chemical extraction tests. In order to identify the plant-available soil P species, we combined DGT with infrared and P K- and L_2,3-edge X-ray adsorption near-edge structure (XANES) spectroscopy. This was achieved by spectroscopically investigating the dried binding layer of DGT devices after soil deployment. All three spectroscopic methods were able to distinguish between different kinds of phosphates (poly-, trimeta-, pyro- and orthophosphate) on the DGT binding layer. However, infrared spectroscopy was most sensitive to distinguish between different types of adsorbed inorganic and organic phosphates. Furthermore, intermediates of the time-resolved hydrolysis of trimetaphosphate in soil could be analyzed.

Reactive metal-organic interfaces studied with hard x-ray photoelectron spectroscopy: controlled formation of metalloporphyrin interphase layers during metal vapor deposition onto porphyrin films

Interfaces between organic semiconductors and metallic layers are ubiquitous in organic (opto-) electronic devices and can significantly influence their functionality. Here, we studied in situ prepared metal-organic interfaces, which were obtained by vapor deposition of metals (Co, Fe) onto organic semiconductor films (2H-tetraphenylporphyrin), with hard x-ray photoelectron spectroscopy. In these systems, the interphase zones, which are formed by diffusion and reaction of the metal in the organic material, can be clearly distinguished spectroscopically from the unreacted organic bulk, since they comprise the corresponding metalloporphyrins, CoTPP and FeTPP. In order to gain control over the thickness of the interphase layers, we varied process parameters such as sample temperature and metal-atom flux during interface preparation. We found that the temperature of the organic film during metal deposition was the only parameter that significantly influenced the formation of the interphase layers: their thicknesses were typically ~0.5 nm for deposition at 90 K, compared to ~1 nm at 300 K, irrespective of metal atom flux and chemical nature of the metal atom (Fe versus Co). Notably, these values are significantly smaller than the thicknesses of other metal/organics interphase regions reported in the literature.

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Oxidation-Induced Changes in the ALD-Al2O3/InAs(100) Interface and Control of the Changes for Device Processing

InAs crystals are emerging materials for various devices like radio frequency transistors and infrared sensors. Control of oxidation-induced changes is essential for decreasing amounts of the harmful InAs surface (or interface) defects because it is hard to avoid the energetically favored oxidation of InAs surface parts in device processing. We have characterized atomic-layer-deposition (ALD) grown Al₂O₃/InAs interfaces, preoxidized differently, with synchrotron hard X-ray photoelectron spectroscopy (HAXPES), low-energy electron diffraction, scanning tunneling microscopy, and time-of-flight elastic recoil detection analysis. The chemical environment and core-level shifts are clarified for well-embedded InAs interfaces (12 nm Al₂O₃) to avoid, in particular, effects of a significant potential change at the vacuum-solid interface. High-resolution As 3d spectra reveal that the Al₂O₃/InAs interface, which was sputter-cleaned before ALD, includes +1.0 eV shift, whereas As 3d of the preoxidized (3 × 1)-O interface exhibits a shift of -0.51 eV. The measurements also indicate that an As₂O₃ type structure is not crucial in controlling defect densities. Regarding In 4d measurements, the sputtered InAs interface includes only a +0.29 eV shift, while the In 4d shift around -0.3 eV is found to be inherent for the crystalline oxidized interfaces. Thus, the negative shifts, which have been usually associated with dangling bonds, are not necessarily an indication of such point defects as previously expected. In contrast, the negative shifts can arise from bonding with O atoms. Therefore, specific care should be directed in determining the bulk-component positions in photoelectron studies. Finally, we present an approach to transfer the InAs oxidation results to a device process of high electron mobility transistors (HEMT) using an As-rich III-V surface and In deposition. The approach is found to decrease a gate leakage current of HEMT without losing the gate controllability.

Control of TiN oxidation upon atomic layer deposition of oxides

The interfaces of a physical-vapor deposited (PVD)-TiN electrode with atomic-layer deposited (ALD) HfO2 layers were studied using photoelectron spectroscopy with high kinetic energies of photoelectrons enabling nondestructive in-depth chemical profiling and phase analysis. Our results reveal the presence of only TiNxOy at the TiN/ALD-HfO2 interface with no measurable traces of the TiO2 phase. By contrast, the interface formed by ALD of HfO2 on top of PVD TiN contains both TiO2 and TiNxOy compounds and may be compared to an HfO2/TiN interface with intentional ALD TiO2 interlayer (IL) formation prior to HfO2 growth. Pre-growth of ALD Al2O3 IL drastically reduces the TiO2 and TiNxOy amounts present at the HfO2/TiN interface, which can be ascribed to oxygen scavenging from the initially oxidized TiN surface by energetically more favorable Al-O bonds. The present study demonstrates that the amount of TiO2 phase can be effectively controlled, i.e., increased or decreased, during the ALD process enabling engineering of vacancy-mediated processes.

Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations

We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb²⁺. We investigate the chemical distribution and electronic structure of solution processed CH₃NH₃PbI₃ perovskite structures containing Na⁺, Cu⁺, and Ag⁺, which are lower valence metal ions than Pb²⁺ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag⁺ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag⁺ and Cu⁺, Na⁺ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.

Electronic Structure Characterization of Cross-Linked Sulfur Polymers

Cross-linked polymers of elemental sulfur are of potential interest for electronic applications as they enable facile thin-film processing of an abundant and inexpensive starting material. Here, we characterize the electronic structure of a cross-linked sulfur/diisopropenyl benzene (DIB) polymer by a combination of soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES). Two different approaches for enhancing the conductivity of the polymer are compared: the addition of selenium in the polymer synthesis and the addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) during film preparation. For the former, we observe the incorporation of Se into the polymer structure resulting in a changed valence-band structure. For the latter, a Fermi level shift in agreement with p-type doping of the polymer is observed and also the formation of a surface layer consisting mostly of TFSI anions.

Exciton-Dominated Core-Level Absorption Spectra of Hybrid Organic-Inorganic Lead Halide Perovskites

In a combined theoretical and experimental work, we investigate X-ray absorption near-edge structure spectroscopy of the I L₃ and the Pb M₅ edges of the methylammonium lead iodide (MAPbI₃) hybrid inorganic-organic perovskite and its binary phase PbI₂. The absorption onsets are dominated by bound excitons with sizable binding energies of a few hundred millielectronvolts and pronounced anisotropy. The spectra of both materials exhibit remarkable similarities, suggesting that the fingerprints of core excitations in MAPbI₃ are essentially given by its inorganic component, with negligible influence from the organic groups. The theoretical analysis complementing experimental observations provides the conceptual insights required for a full characterization of this complex material.

A new approach towards the study of thermal decomposition and formation processes of nanoalloys: the double complex salt [Pd(NH3)4][PtCl6]

The thermal decomposition process of the [Pd(NH₃)₄][PtCl₆] double complex salt was investigated using in situ XAFS, XPS, HAXPES, and XRD.

Evidence for Chemical and Electronic Nonuniformities in the Formation of the Interface of RbF-Treated Cu(In,Ga)Se2 with CdS

We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se₂ (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface's structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.

Mechanisms of photoinduced magnetization in Pr0.6Ca0.4MnO3 studied above and below charge-ordering transition temperature

We report the effect of photonic field on the electronic and magnetic structure of a low bandwidth manganite [Formula: see text] [Formula: see text]MnO₃ (PCMO) thin film. In particular, the present study confirmed a mechanism that was recently proposed to explain how optical excitation can bias or directly activate the metamagnetic transition associated with the colossal magnetoresistance (CMR) effect of PCMO. The transition is characterized by a shift in the dynamic equilibrium between ferromagnetic (FM) and antiferromagnetic clusters, explaining how it can be suddenly triggered by a sufficient external magnetic field. The film was always found to support some population of FM-clusters, the proportional size of which could be adjusted by the magnetic field and, especially in the vicinity of a thermomagnetic irreversibility, by optical excitation. The double exchange mechanism couples the magnetic degrees of freedom of manganites to their electronic structure, which is further coupled to the ion lattice via the Jahn-Teller mechanism. In accordance, it was found that producing optical phonons into the lattice could lower the free energy of the FM phase enough to significantly bias the CMR effect.

Identifying the nature of surface chemical modification for directed self-assembly of block copolymers

In recent years, block copolymer lithography has emerged as a viable alternative technology for advanced lithography. In chemical-epitaxy-directed self-assembly, the interfacial energy between the substrate and each block copolymer domain plays a key role on the final ordering. Here, we focus on the experimental characterization of the chemical interactions that occur at the interface built between different chemical guiding patterns and the domains of the block copolymers. We have chosen hard X-ray high kinetic energy photoelectron spectroscopy as an exploration technique because it provides information on the electronic structure of buried interfaces. The outcome of the characterization sheds light onto key aspects of directed self-assembly: grafted brush layer, chemical pattern creation and brush/block co-polymer interface.

Re-distribution of oxygen at the interface between γ-Al2O3 and TiN

Interface of TiN electrode with γ-Al2O3 layers was studied using near edge X-ray absorption fine structure, conventional X-ray photoelectron spectroscopy and photoelectron spectroscopy with high energies. Despite the atomic-layer deposited Al2O3 being converted into thermodynamically-stable polycrystalline cubic γ-phase by high-temperature (1000 or 1100 °C) anneal, our results reveal formation of a thin TiNxOy (≈1-nm thick) interlayer at the interface between γ-Al2O3 film and TiN electrode due to oxygen scavenging from γ-Al2O3 film. Formation of the TiO2 was not observed at this interface. As environmental effect, a strong oxidation resulting in formation of a TiO2(1.4 nm)/TiNxOy(0.9 nm) overlayers on the top of the TiN electrode is traced. Development of O-deficiency of γ-Al2O3 is observed and related to the polarization anisotropy due to the preferential orientation of spin states involved in the X-ray absorption in the plane parallel to the surface. Investigation of the TiN electrode reveals the predominantly "stretched" octahedra in its structure with the preferential orientation relative the interface with γ-Al2O3. This anisotropy can be correlated with ≈200 meV electron barrier height increase at the O-deficient TiN/γ-Al2O3 interface as compared to the TiN/γ-Al2O3 barrier formed under abundant oxidant supply condition as revealed by internal photoemission of electrons from TiN into the oxide.

Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices

A covalently linked organic dye-cobaloxime catalyst system based on mesoporous NiO is synthesized by a facile click reaction for mechanistic studies and application in a dye-sensitized solar fuel device. The system is systematically investigated by photoelectrochemical measurements, density functional theory, time-resolved fluorescence, transient absorption spectroscopy, and photoelectron spectroscopy. The results show that irradiation of the dye-catalyst on NiO leads to ultrafast hole injection into NiO from the excited dye, followed by a fast electron transfer process to reduce the catalyst. Moreover, the dye adopts different structures with different excited state energies, and excitation energy transfer occurs between neighboring molecules on the semiconductor surface. The photoelectrochemical experiments also show hydrogen production by this system. The axial chloride ligands of the catalyst are released during photocatalysis to create the active sites for proton reduction. A working mechanism of the dye-catalyst system on the photocathode is proposed on the basis of this study.

Formation of a K-In-Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se₂ (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a K-In-Se species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber.

Synthesis of Dispersible Mesoporous Nitrogen-Doped Hollow Carbon Nanoplates with Uniform Hexagonal Morphologies for Supercapacitors

In this study, dispersible mesoporous nitrogen-doped hollow carbon nanoplates have been synthesized as a new anisotropic carbon nanostructure using gibbsite nanoplates as templates. The gibbsite-silica core-shell nanoplates were first prepared before the gibbsite core was etched away. Dopamine as carbon precursor was self-polymerized on the hollow silica nanoplates surface assisted by sonification, which not only favors a homogeneous polymer coating on the nanoplates but also prevents their aggregation during the polymerization. Individual silica-polydopamine core-shell nanoplates were immobilized in a silica gel in an insulated state via a silica nanocasting technique. After pyrolysis in a nanoconfine environment and elimination of silica, discrete and dispersible hollow carbon nanoplates are obtained. The resulted hollow carbon nanoplates bear uniform hexagonal morphology with specific surface area of 460 m²·g^-1 and fairly accessible small mesopores (∼3.8 nm). They show excellent colloidal stability in aqueous media and are applied as electrode materials for symmetric supercapacitors. When using polyvinylimidazolium-based nanoparticles as a binder in electrodes, the hollow carbon nanoplates present superior performance in parallel to polyvinylidene fluoride (PVDF) binder.

Pronounced Surface Band Bending of Thin-Film Silicon Revealed by Modeling Core Levels Probed with Hard X-rays

Enhancing the probing depth of photoemission studies by using hard X-rays allows the investigation of buried interfaces of real-world device structures. However, it also requires the consideration of photoelectron-signal attenuation when evaluating surface effects. Here, we employ a computational model incorporating surface band bending and exponential photoelectron-signal attenuation to model depth-dependent spectral changes of Si 1s and Si 2s core level lines. The data were acquired from hydrogenated boron-doped microcrystalline thin-film silicon, which is applied in silicon-based solar cells. The core level spectra, measured by hard X-ray photoelectron spectroscopy using different excitation energies, reveal the presence of a 0.29 nm thick surface oxide layer. In the silicon film a downward surface band bending of eVbb = -0.65 eV over ∼6 nm obtained via inverse modeling explains the observed core level shifts and line broadening. Moreover, the computational model allows the extraction of the "real" Si 1s and Si 2s bulk core level binding energies as 1839.13 and 150.39 eV, and their natural Lorentzian line widths as 496 and 859 meV, respectively. These values significantly differ from those directly extracted from the measured spectra. Because band bending usually occurs at material surfaces we highly recommend the detailed consideration of signal integration over depth for quantitative statements from depth-dependent measurements.

Core electron excitations in U(4+): modelling of the nd(10)5f(2)→nd(9)5f(3) transitions with n = 3, 4 and 5 by ligand field tools and density functional theory

Ligand field density functional theory (LFDFT) calculations have been used to model the uranium M4,5, N4,5 and O4,5-edge X-ray absorption near edge structure (XANES) in UO2, characterized by the promotion of one electron from the core and the semi-core 3d, 4d and 5d orbitals of U(4+) to the valence 5f. The model describes the procedure to resolve non-empirically the multiplet energy levels originating from the two-open-shell system with d and f electrons and to calculate the oscillator strengths corresponding to the dipole allowed d(10)f(2)→ d(9)f(3) transitions appropriate to represent the d electron excitation process. In the first step, the energy and UO2 unit-cell volume corresponding to the minimum structures are determined using the Hubbard model (DFT+U) approach. The model of the optical properties due to the uranium nd(10)5f(2)→nd(9)5f(3) transitions, with n = 3, 4 and 5, has been tackled by means of electronic structure calculations based on the ligand field concept emulating the Slater-Condon integrals, the spin-orbit coupling constants and the parameters of the ligand field potential needed by the ligand field Hamiltonian from Density Functional Theory. A deep-rooted theoretical procedure using the LFDFT approach has been established for actinide-bearing systems that can be valuable to compute targeted results, such as spectroscopic details at the electronic scale. As a case study, uranium dioxide has been considered because it is a nuclear fuel material, and both atomic and electronic structure calculations are indispensable for a deeper understanding of irradiation driven microstructural changes occurring in this material.

Highly efficient blazed grating with multilayer coating for tender X-ray energies

For photon energies of 1 - 5 keV, blazed gratings with multilayer coating are ideally suited for the suppression of stray and higher orders light in grating monochromators. We developed and characterized a blazed 2000 lines/mm grating coated with a 20 period Cr/C- multilayer. The multilayer d-spacing of 7.3 nm has been adapted to the line distance of 500 nm and the blaze angle of 0.84° in order to provide highest efficiency in the photon energy range between 1.5 keV and 3 keV. Efficiency of the multilayer grating as well as the reflectance of a witness multilayer which were coated simultaneously have been measured. An efficiency of 35% was measured at 2 keV while a maximum efficiency of 55% was achieved at 4 keV. In addition, a strong suppression of higher orders was observed which makes blazed multilayer gratings a favorable dispersing element also for the low X-ray energy range.

LiNi(0.5)Mn(1.5)O4 high-voltage cathode coated with Li4Ti5O12: a hard X-ray photoelectron spectroscopy (HAXPES) study

A Li4Ti5O12 (LTO) film was coated as buffer layer onto a LiNi0.5Mn1.5O4 (LNMO) high-voltage cathode, and after cycling of the cathode in a battery electrolyte, the LTO film was investigated by means of synchrotron radiation based hard X-ray photoelectron spectroscopy (HAXPES). By tuning the photon energy between 2 keV and 6 keV, we obtained non-destructive depth profiles of the coating material with probing depths ranging from 6 nm to 20 nm. The coating was found to be covered by a few nanometers thin surface layer resulting from electrolyte decomposition. This layer consisted predominantly of organic polymers as well as metal fluorides and fluorophosphates. A positive influence of the Li4Ti5O12 coating with regard to the size and stability of the surface layer was found. The coating itself consisted of a uniform mixture of Li(I), Ti(IV), Ni(II) and Mn(IV) oxides that most likely adopted a spinel structure by forming a solid solution of the two spinels LiNi0.5Mn1.5O4 and Li4Ti5O12 with Li, Mn, Ni and Ti cations mixing on the spinel octahedral sites. The diffusion of Ni and Mn ions into the Li4Ti5O12 lattice occurred during the heat treatment when preparing the cathode. The doping of Li4Ti5O12 with the open d-shell ions Ni(2+) (d(8)) and Mn(4+) (d(3)) should increase the electronic conductivity of the coating significantly, as was found in previous studies. The complex signal structure of the Ti 2p, Ni 2p and Mn 2p core levels provides insight into the chemical nature of the transition metal ions.

Unveiling the Hybrid n-Si/PEDOT:PSS Interface

We investigated the buried interface between monocrystalline n-type silicon (n-Si) and the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (

PEDOT

PSS), which is successfully applied as a hole selective contact in hybrid solar cells. We show that a post-treatment of the polymer films by immersion in a suitable solvent reduces the layer thickness by removal of excess material. We prove that this post-treatment does not affect the functionality of the hybrid solar cells. Through the thin layer we are probing the chemical structure at the n-Si/

PEDOT

PSS interface with synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). From the HAXPES data we conclude that the Si substrate of a freshly prepared hybrid solar cell is already oxidized immediately after preparation. Moreover, we show that even when storing the sample in inert gas such as, e.g., nitrogen the n-Si/SiOx/

PEDOT

PSS interface continues to further oxidize. Thus, without further surface treatment, an unstable Si suboxide will always be present at the hybrid interface.

Interface Engineering to Create a Strong Spin Filter Contact to Silicon

Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on silicon is a perfect route to enrich silicon nanotechnology with spin filter functionality. To date, the inherent chemical reactivity between EuO and Si has prevented a heteroepitaxial integration without significant contaminations of the interface with Eu silicides and Si oxides. We present a solution to this long-standing problem by applying two complementary passivation techniques for the reactive EuO/Si interface: (i) an in situ hydrogen-Si (001) passivation and (ii) the application of oxygen-protective Eu monolayers-without using any additional buffer layers. By careful chemical depth profiling of the oxide-semiconductor interface via hard x-ray photoemission spectroscopy, we show how to systematically minimize both Eu silicide and Si oxide formation to the sub-monolayer regime-and how to ultimately interface-engineer chemically clean, heteroepitaxial and ferromagnetic EuO/Si (001) in order to create a strong spin filter contact to silicon.

Passivation Layer and Cathodic Redox Reactions in Sodium-Ion Batteries Probed by HAXPES

The cathode material P2-Nax Co2/3 Mn2/9 Ni1/9 O2, which could be used in Na-ion batteries, was investigated through synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Nondestructive analysis was made through the electrode/electrolyte interface of the first electrochemical cycle to ensure access to information not only on the active material, but also on the passivation layer formed at the electrode surface and referred to as the solid permeable interface (SPI). This investigation clearly shows the role of the SPI and the complexity of the redox reactions. Cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; all are in the 4+ state at the end of charging. Reduction to Co(3+), Ni(3+), and Mn(3+) occurs upon discharging and, at low potential, there is partial reversible reduction to Co(2+) and Ni(2+). A thin layer of Na2 CO3 and NaF covers the pristine electrode and reversible dissolution/reformation of these compounds is observed during the first cycle. The salt degradation products in the SPI show a dependence on potential. Phosphates mainly form at the end of the charging cycle (4.5 V), whereas fluorophosphates are produced at the end of discharging (2.0 V).

Formation of an interphase layer during deposition of cobalt onto tetraphenylporphyrin: a hard X-ray photoelectron spectroscopy (HAXPES) study

Chemical depth profiling of a metal/porphyrin interface with Hard X-ray Photoelectron Spectroscopy (HAXPES) reveals the formation of a 1.6 nm thick interphase layer.

Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy – a comparative study

Modifications at the CIGSe/CdS interface due to KF-PDT are studied using synchrotron radiation.

Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se₂ Surfaces--Reason for Performance Leap?

Direct and inverse photoemission were used to study the impact of alkali fluoride postdeposition treatments on the chemical and electronic surface structure of Cu(In,Ga)Se2 (CIGSe) thin films used for high-efficiency flexible solar cells. We find a large surface band gap (E(g)(Surf), up to 2.52 eV) for a NaF/KF-postdeposition treated (PDT) absorber significantly increases compared to the CIGSe bulk band gap and to the Eg(Surf) of 1.61 eV found for an absorber treated with NaF only. Both the valence band maximum (VBM) and the conduction band minimum shift away from the Fermi level. Depth-dependent photoemission measurements reveal that the VBM decreases with increasing surface sensitivity for both samples; this effect is more pronounced for the NaF/KF-PDT CIGSe sample. The observed electronic structure changes can be linked to the recent breakthroughs in CIGSe device efficiencies.

Observation and Mediation of the Presence of Metallic Lead in Organic-Inorganic Perovskite Films

We have employed soft and hard X-ray photoelectron spectroscopies to study the depth-dependent chemical composition of mixed-halide perovskite thin films used in high-performance solar cells. We detect substantial amounts of metallic lead in the perovskite films, which correlate with significant density of states above the valence band maximum. The metallic lead content is higher in the bulk of the perovskite films than at the surface. Using an optimized postanneal process in air, we can reduce the metallic lead content in the perovskite film. This process reduces the amount of metallic lead and a corresponding increase in the photoluminescence quantum efficiency of the perovskite films can be observed. This correlation indicates that metallic lead impurities are likely a key defect whose concentration can be controlled by simple annealing procedures in order to increase the performance for perovskite solar cells.

Analysis of electronic structure and its effect on magnetic properties in (001) and (110) oriented La0.7Sr0.3MnO3 thin films

Epitaxial thin films of half-metallic oxide La0.7Sr0.3MnO3 (LSMO) have been grown in two crystalline orientations, one with the c-axis out-of-plane, the (001) orientation, and one with the c-axis in-plane, the (110) orientation. For the (110) oriented growth, there is no polar discontinuity at the substrate-film interface and hence no dead layer formation, which improves ferromagnetic interaction in the LSMO, making it highly attractive for spintronic applications. In our experiments, with x-ray diffraction, x-ray photoelectron spectroscopy and magnetic measurements, we have demonstrated that in the (110) oriented LSMO the lattice is more relaxed, leading to less deformation of electronic density around the La atom or in the MnO6 octahedra. This improved crystal and electronic structure improves the ferromagnetic properties of the films, making the Curie temperature higher by almost 15 K, which is of potential interest for spintronics. However, substrate strain induced magnetic anisotropy causes domain formation with out-of-plane components in these films, which poses some concern for practical spintronic devices.

Improved performances of nanosilicon electrodes using the salt LiFSI: a photoelectron spectroscopy study

Silicon is a very good candidate for the next generation of negative electrodes for Li-ion batteries, due to its high rechargeable capacity. An important issue for the implementation of silicon is the control of the chemical reactivity at the electrode/electrolyte interface upon cycling, especially when using nanometric silicon particles. In this work we observed improved performances of Li//Si cells by using the new salt lithium bis(fluorosulfonyl)imide (LiFSI) with respect to LiPF6. The interfacial chemistry upon long-term cycling was investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100-800 eV) and hard X-rays (2000-7000 eV) from two different synchrotron facilities and in-house XPS (1486.6 eV). We show that LiFSI allows avoiding the fluorination process of the silicon particles surface upon long-term cycling, which is observed with the common salt LiPF6. As a result the composition in surface silicon phases is modified, and the favorable interactions between the binder and the active material surface are preserved. Moreover a reduction mechanism of the salt LiFSI at the surface of the electrode could be evidenced, and the reactivity of the salt toward reduction was investigated using ab initio calculations. The reduction products deposited at the surface of the electrode act as a passivation layer which prevents further reduction of the salt and preserves the electrochemical performances of the battery.