X-ray based techniques for sustainable energy related materials

Resume : The sudden relevance of metal-halide perovskite (HaP) materials in photovoltaics (PV) and the strong influence of composition, process parameters, and environmental effects on the resulting device behavior make it both a highly promising technology and one that is difficult to optimize in a systematic, empirical fashion. To investigate the relationships between structure ? chemical and electronic ? and function of HaPs in thin-film PV, our research group employs a large toolchest of x-ray spectroscopic techniques. Hard x-ray photoemission spectroscopy (HAXPES) at synchrotron sources can provide a depth-resolved picture or electronic structure, and in the case of materials with organic components the rate of photon-induced decomposition can be reduced. In situ treatment of HaP-related films during HAXPES measurements reveals changes in electronic structure occurring during formation ? and thermal decomposition ? of a HaP. Near-ambient pressure HAXPES was employed as part of an overall drive to understand, and ultimately to mitigate, the environmental degradation of HaP materials. Resonant Inelastic soft X-ray Scattering of the organic component of a metal-organic HaP can be used to examine projected partial density of states and molecular dynamic effects on the electron structure. By employing these advanced characterization techniques to specimens of relevant, real-world photovoltaic, we can understand performance limiting factors at a fundamental level.

Resume : In the polymer electrolyte membrane (PEM) fuel cells, the family of Pt-rare earth (RE) and lanthanide alloys, such as Pt3Y, Pt5Gd, Pt5Tb, etc. have shown great potential to catalyze the oxygen reduction reaction (ORR) with an activity up to an order of magnitude higher than Pt. Bulk samples of these alloys have shown to have up one order of magnitude higher ORR activity compared to pure Pt. In our recent work on these alloys we have successfully fabricated nanometer thin films of Pt-RE alloys and evaluated then in a real fuel cell setup. The fuel cell measurements confirm the increased ORR activity seen in previously in half-cells and also highlights the importance of catalyst structure to optimize performance. Detailed materials characterization using synchrotron X-ray photoelectron spectroscopy (XPS) of the catalyst surface show the formation of a Pt overlayer, a few monolayers thick. For a sputtered Pt3Y thin film, the majority of Y at the surface is oxidized and is removed by acid pretreatment. X-ray diffraction (XRD) measurements indicate very small grain sizes and large microstrain as well as a change in the effective lattice parameter of Pt, reflecting the loss of Y from the alloy. Understanding more about the de-alloying process and the structure of the formed overlayer will aid in the development of these catalysts for long term application in fuel cells.

Resume : The importance of sodium in Cu(In,Ga)Se2 absorbers used for thin film solar cells is well known, however, its role is not fully understood. We examined Na from different sources using x-ray photoelectron spectroscopy. The origin of the chemical shifts in the various experimental XPS peak components observed in the case of Na from SLG substrate (Na-SLG) and Na as post-deposition (Na-PDT) on CIGSe surfaces has been interpreted in terms of initial and final state contributions to the binding energy shifts w.r.t. the metallic states. It has been shown how Auger parameter analysis of the XPS data can be utilized to determine chemical states of elements, in this case, for Na from the two sources. Na as post-deposition brought about a Se-enrichment and InyOz formation at the CIGSe surface. Charge sharing between Se and Ga indicated towards formation of a Na-containing complex of the form (Na,Ga)(Se,OH)(InyOz). Another component of Na may have formed a secondary phase NaxCu(1-x)(In,Ga)Se2 due to reduced [Cu]/[In] ratio at the Na-PDT CIGSe surface and charge sharing with Cu, In, Ga and Se. Elemental Na from Na-PDT also reacted with O to form NaOH. On the other hand, no interaction of Na in Na-SLG with any of the CIGSe-related components has been found. In this case, oxides of Na like NaxO and Na2CO3 and uncompensated Na+ ions have been identified at the CIGSe surface. Band-alignment studies of the Na-SLG CIGSe and Na-PDT CIGSe surfaces with a Zn(O,S) buffer layer showed a higher VBM and interface-induced band-bending leading to a type-inversion (to n-type) at the Na-SLG CIGSe/Zn(O,S) interface causing a reduced Voc and an increased FF. On the other hand, a reduced VBM at the Na-PDT CIGSe surface resulted in an increased effective p-type doping at the absorber surface and a corresponding enhanced Voc. In this case, a decreased FF was obtained due to a reduced work function at the CIGSe surface as a result of Na deposition.

Resume : Hybrid organic-inorganic lead halide perovskite (HaP) materials have attracted substantial research interest over the last decade, primarily due to their outstanding opto-electronic properties which are beneficial for solar cells and LED’s. HaP-based solar cells and LED’s are currently undergoing technology commercialization world-wide. In spite of the substantial research efforts invested by the global community, we currently have an incomplete understanding of the intrinsic carrier scattering mechanisms which dictate carrier mobility. One type of scattering mechanism could be due to electronic hybridization between the organic and inorganic sub-lattices, and one technique of choice for investigating this open question is X-ray absorption spectroscopy (XAS). Utilizing synchrotron-based X-ray absorption near edge structure (XANES), performed on in-vacuum cleaved single crystals at a beam-line which is designed to provide low photon fluxes for measuring radiation-sensitive materials, we find evidence that the nitrogen-derived unoccupied electronic levels, originating from the methylammonium organic cation, hybridize differently to the inorganic lead-iodide/lead-bromide sub-lattices. These findings may yield insights on the differences in electron scattering mechanisms, and possibly ambient stability, between methylammonium lead tri-iodide (MAPbI3) and methylammonium lead tri-bromide (MAPbBr3), two prototypical members of this class of materials.

Resume : Synchrotron radiation techniques, in particular x-ray diffraction and absorption, are nowadays well-established tools to study the structure of both crystalline and disordered semiconductor materials, including low dimensional systems such as nanostructures as well as the local environment of dopants and dilute isoelectronic impurities in a semiconductor matrix. More recently, the exploitation of these techniques in situ, made possible by the high brilliance of the modern synchrotrons and the availability of dedicated reactors, has opened the way to the investigation of the growth process itself. Here, we review some recent results of the synchrotron radiation characterization of III-Vs semiconductors with potential for solar energy harvesting. This includes nitride semiconductors (InxGa1-xN) with direct bang gap spanning the entire solar spectrum from near-infrared to near-ultraviolet, dilute nitrides alloys (GaAs1-yNy, InxGa1-xAs1-yNy) which constitute a model study case for the analysis of the detailed structure of defects, and other systems in which heavier anions were incorporated to tailor the optical band gap (GaAs1-ySby, GaAs1-yBiy, etc.). The phenomena of atom clustering, hydrogen incorporation, and their effects on the optical properties of these materials for post-growth band-gap engineering have been addressed by several synchrotron radiation techniques coupled with advanced theoretical calculations. Finally, we show the advantages of the use of the same techniques in situ in the analysis of the atomic layer deposition growth and the structure-property relationship of an archetype oxide (ZnO) employed as transparent conductive electrode in both inorganic and dye-sensitized hybrid solar cells.

Resume : In this study we would like to present an application of laboratory-based near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) for in-situ and operando characterizations of heterogeneous metal-oxides based catalysts and gas sensors. We show that this tool can be successfully used for study of both the model thin films in-situ prepared in high vacuum and the real nanostructured layers. First, the lab-based NAP-XPS was used for narrowing the gap between the basic knowledge of structural and physico-chemical properties of Au/CeO2 catalysts in the vacuum and in the realistic gaseous environments (under operating conditions). A basic understanding of the mechanisms of simple catalytic C1 reactions, such as water gas shift reaction and CO oxidation, at the molecular level was achieved. Together with NAP-XPS technique other powerful characterization techniques such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and synchrotron radiation photoelectron spectroscopy (SRPES) were used in this work. Another example of application of the lab-based NAP-XPS is the study of sensing mechanisms of nanostructured tin oxide and copper oxide chemiresistors. The nanostructured layers exhibit large surface-to-volume ratio with high concentration of active surface sites for chemisorption. The SnO2 nanostructured layers were prepared by pulsed laser deposition while CuOx nanowires were obtain by the thermal oxidation method. Scanning and transmission electron microscopies (SEM, TEM) and atomic force microscope (AFM) were used to characterize the morphology of the as-prepared oxides. A simple gas sensors were realized by depositing of SnO2 and CuOx on a special fused silica substrates and were investigated in-operando under exposure to the reactive atmosphere at elevated temperatures by means of NAP-XPS. Small modification of the equipment allowed us to follow the resistivity of the nanostructured films simultaneously with NAP-XPS measurements during the gases exposure. The unique combination of thin film resistivity measurement and photoelectron study under higher pressure of investigated gases brings a new insight into sensoric properties and catalytic processes on the surface of studied sensors.

Resume : A detailed mechanism of oxidation processes on the InP surface, which can reveal a clear correlation between electronic properties, oxide morphology and local chemical oxygen environment is necessary for tuning optoelectronic properties of InP-based photoelectrodes via surface oxidation. Our approach is to take advantage of recent advances in surface-sensitive techniques, i.e., ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to track the dynamic electronic, morphological and chemical structures of the O2/InP interface that allows one to access surface reaction processes under ambient conditions. We also integrate our XPS study with DFT calculations and direct spectroscopic simulations [1] which allow to obtain explicit mechanistic understandings of the O2/InP(001) interface and the evolution of InP(001) oxidation under different O2 pressures and temperatures. Our very preliminary results showed that introduction of O2 onto InP(001) surface at a low temperature regime leads to the exclusive formation of a mixture of In and crosslinked PO4 via bridging oxygen that aggregates on the surface of InP(001) with monolayer thickness. At higher temperatures, these InPOx sub-units begin to be linked amorphously across the surface forming 2-dimentional structure via bridging oxygen atoms. We also observed that different oxidation processes are activated at different O2 pressures and temperatures, thus they give rise to different types of oxides and we are continuing to investigate them. Therefore, the surface oxide composition may effectively be controlled by leveraging thermodynamics against kinetic considerations. [1] Pham, T. A.; Zhang, X.; Wood, B. C.; Prendergast, D.; Ptasinska, S.; Ogitsu, T. Integrating Ab Initio Simulations and X-ray Photoelectron Spectroscopy: Toward a Realistic Description of Oxidized Solid/Liquid Interfaces. J. Phys. Chem. Lett. 2018, 9, 194-203

Resume : Different combinations of cathode and anode materials have been investigated for improving the energy content of Li-ion battery materials. One of the dominant factors of performance is related to the achieved battery voltage depending on the charging state of the battery. The voltage difference of the electrochemical cell is given by the change in the Gibbs energy for the exchange of Li which will again will depend on the exchange of electrons and ions between the two phases. A systematic experimental study on the relative influence of both contributions on the measurable battery voltage have not been performed yet. Therefore we will present in this contributions surface science studies of well-defined thin film electrodes which allows to compare changes in electronic structure and related electronic surface potentials with voltage measurements. In addition, we will present first experiments on the direct measurements of the ionic work function and of complete battery set-ups. A number of different thin film cathode materials with different composition of the layered oxides and olivine structural family have been prepared in recent years mostly by sputter deposition inside an integrated UHV system. Subsequently, the electronic structure and surface potentials as e. g. the electronic work function and the changes induced with charging have been determined by applying photoelectron spectroscopy. The experiments allow to relate energetic changes in the Fermi level positions to the measured battery voltage and thus deduce the relative contribution of the different electronic chemical potential to the battery voltage. Based on our data we conclude that the major contribution to the battery voltage is related to electronic work function variations of the different electrode materials. In addition, we have performed first experiments by experimentally determining the electronic and ionic work function of electrode materials. Such experiments allow to directly determine the chemical potential of the Li atom as given by the electronic and ionic contribution in relation to the vacuum level. With a Born-Haber cycle we can show that the experimentally determined values are in reasonable correspondence to the battery voltage. Finally we will present in situ quasi operando electron spectra (SXPS and XAS) on complete solid thin film battery structures using NaCoO2 as cathode, which allow to investigate the changes in surface composition and surface potentials directly after performing the polarization of the cathode. These experiments indicate that the higher stability regime of NaCoO2 with Na loss in contrast to LiCoO2 is related to a favourable involvement of O2p states to the valence band maximum involved in hole transfer. Summarizing our results we conclude that the performance of Li-ion batteries depend on the relative changes of bonding contributions for electrons and ions within the used electrodes with electrons providing the larger part to the battery voltage. Measured variations in performance must be related to deviations of the rigid band behavior of the electrode materials and to surface effects occurring in the given battery set-up.

Resume : An important element in the reduction of CO2 is the change of vehicles with internal combustion engines to electric battery powered vehicles. The as such produced renewable energy can be used for individual mobility as well as for a temporary intermediate storage of excess energy. A viable electric mobility concept requires however stable cycle batteries with high specific energy (minimising weight, maximising driving range). Li ion batteries are widely used in applications such as mobile phones and laptops, and will likely be key to future electromobility. An alternative promising battery is the lithium sulfur battery with a potential twofold energy density increase. The requirements for such batteries present major challenges; e.g. energy capacity, deactivation/stability and safety. A detailed understanding of the charge, discharge and deactivation mechanisms are thus required, preferably quantitative and spatially resolved. X-ray absorption spectroscopy (XAS) is a characterisation technique which provides detailed electronic and structural information on the material under investigation, in a time- and spatially resolved manner. Here, I will explain the strengths and limitations of XAS for battery research. A novel operando XAS cell design will be described [1], including the challenges to perform reliable experiments (electrochemically and spectroscopically). The cell allows time and spatial resolved XAS, providing insights in the type, location and reversibility of the intermediates formed in electrodes and electrolyte separately. Obtained insights in cycling and deactiviation mechanisms for the different battery types will be discussed [1-6] and future research directions described. [1] Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H.A. Gasteiger, M. Tromp, J. Electrochem. Soc. 162(7): A1146-A1155, 2015. [2] Y. Gorlin, M. U. M. Patel, A. Freiberg, Q. He, M. Piana, M. Tromp, H. A. Gasteiger, J. Electrochem. Soc. 2016, 163(6), A930-A939. [3] J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger, M. Tromp, J. Mater. Chem. A 2016, 4, 18300-18305. [4] A. T. S. Freiberg, A. Siebel, A. Berger, S. M. Webb, Y. Gorlin, H. A. Gasteiger, M. Tromp, J. Phys. Chem. C 2018, 122, 10, 5303-5316. [5] A. Berger, A. T. S. Freiberg, R. J. Thomas, M. U. M. Patel, M. Tromp, H. Gasteiger, Y. Gorlin, J. Electrochem. Soc. 2018, 165(7), A1288-A1296. [6] R. Jung, F. Linsenmann, R. J. Thomas, J. Wandt, S. Solchenbach, F. Maglia, C. Stinner, H. A. Gasteiger, M. Tromp, J. Electrochem. Soc. 2019, 166(2): A378-A389.

Resume : The use of built-in electric fields present in p-n junctions is inherently limited by the Shockley-Queisser limit. An alternative method to efficiently separate charge carriers are in ferroelectric materials due to their switchable photoelectric response and a bulk-photovoltaic effect that can generate photovoltages larger than its bandgap. To improve their performance and potential for application in solar cells, we focus on narrowing the bandgap of the model BaTiO3 by doping with transition metals to better match the solar spectrum. Using a combination of soft and hard x-ray spectroscopies and density functional theory (DFT) calculations, we reveal this compound’s electronic structure and the modifications induced by doping. Additionally, we fabricate solar cell devices and reverse the polarization in-situ to monitor the changes to the electronic band alignment at the metal/ferroelectric interface and derive the conduction and valence band offsets. Lastly, ferroelectric photovoltaics are also studied under monochromatic illumination to simultaneously understand the mechanism for photovoltaic voltage generation and interface profile during in-operando measurements. The x-ray methodologies used for ferroelectric photovoltaics can reveal mechanisms to their operation in devices and guide in design of epitaxial ferroelectrics.

Resume : MXenes are a new class of 2D materials consisting of transition metals carbides and nitrides that demonstrate extraordinary properties for electrochemical energy storage applications. In particular, Ti3C2Tx MXenes have shown very large capacitance in sulfuric acid due to the surface redox (pseudocapacitive) charging mechanism1. Intercalation of Ti3C2Tx by cations induces a larger spacing between the material sheets2 and might alter the oxidation state which would eventually result in a different surface redox mechanism. Tracking the changes associated with the average oxidation of Ti atoms of Ti3C2Tx in situ/operando in various environments could provide more insights into their energy storage mechanism. X-ray Absorption Spectroscopy (XAS) using soft X-rays is an element-sensitive technique that is very sensitive to surface chemistry and transition metal oxidation states. Herein, we present an XAS study of pristine, Li-, Na-, K-, and Mg-intercalated Ti3C2Tx in different environments. Electrochemical reaction coupled with in operando XAS at the Ti L-edge was conducted to probe the changes in the oxidation state of Ti in Na-, K-, and Mg-Ti3C2Tx MXenes in sulfuric acid. This study provides a basis for electrochemical performance of MXenes by applying the Ti L-edge sensitivity to probe the differences in the oxidation state of Ti atoms. References: 1. M. R. Lukatskaya et al., Adv. Energy Mater 2015, 5, 1500589 2. Muckley, E. S. et al., ACS Nano 2017, 11, 11118–11126

Resume : Complete physico-chemical operando characterization of electrochemical devices in whole, or it?s constituent materials separately, is necessary to guide the development and to improve the performance. High brilliance synchrotron X-ray sources play a crucial role in this respect as they act as a probe with relatively high penetration power and low damage potential. In this contribution the new possibilities of using using high energy, high intensity X-rays to probe model fuel cell catalysts and energy conversion devices will be presented. HESXRD (High Energy Surface X-ray Diffraction) [1] and TDS (Transmission Surface Diffraction) [2] provide ideal tools to study structural changes during reaction conditions on single crystal model electrodes. The main advantage of both techniques is the possibility to follow the structural changes precisely with atomic resolution. While HESXRD is ideally used to determine exact atomic position, the TSD is easier to use and allows studies with high spatial resolution. For example, HESXRD can be used to follow the atomic movement of Pt atoms during electrochemical oxidation and dissolution with very high precision, explaining the different catalyst degradation behaviors and suggesting possible routes to improve its durability [3-5]. The TSD is an excellent tool to study advanced 2D catalysts. To study fuel cells or batteries as a whole, elastic scattering techniques, such as WAXS and SAXS, can be employed as they can provide important complementary information to more standard X-ray imaging and tomography. The advantage is that the chemical contrast and sensitivity at atomic and nm scales is superior. Coupling these technique with the tomographic reconstruction (XRD-CT and SAXS-CT) is much less common as it requires bright synchrotron sources, fast 2D detectors and advanced instrumentation, but allows 3D imaging of operational devices with unprecedented chemical sensitivity [6]. This can be demonstrated, as an example, on imaging of standard 5cm2 fuel cells during operation. The change in morphology and atomic arrangement of the catalysts, PEM hydration and water distribution can be followed in one experiment as a function of operating conditions. Furthermore, the fundamental processes leading to the catalyst aging can be assessed with high temporal and spatial resolution. These advanced scattering techniques open a door to holistic investigations of operational devices, which are needed to successfully incorporate new materials at the device level. [1] J. Gustafson et al., Science 343, 758 (2014) [2] F. Reikowski et al., J. Phys. Chem. Lett., 5, 1067-1071 (2017) [3] J. Drnec et al, Electrochim. Acta, 224 (2017), [4] M. Ruge et al, J. Am. Chem. Soc., 139 (2017) [5] Chattot et al., Nature Materials, 17(2018) [6] A. Vamvakeros et al., Nat. Commun., 9, 4751 (2018)

Resume : Since its initial conception as a possible photovoltaic (PV) candidate around the turn of the 1930s1, 2 cuprous oxide (Cu2O) has been known to the photovoltaic community. However, the material has an intrinsic p-type conductivity due to the presence of Cu vacancies acting as acceptors,3 rendering the creation of p-n homojunctions as challenging to produce. The search for inexpensive and non toxic materials for greener energy production has now renewed interest in Cu2O for PV applications. Here we have prepared Fe-doped ZnS films with various Fe concentration by room temperature molecular beam epitaxy (MBE). The optical bandgaps of these films were determined from optical transmission spectroscopy data. A Cu2O overlayer was then produced in-situ within a custom photoemission system. Using photoelectron spectroscopy techniques (XPS/UPS) the positions of the core levels and valence band maxima (VBM) in the bulk materials were obtained. By observing the core level binding energies at the ZnS:Fe/Cu2O heterojunction interface, relative to the bulk positions of the constituent materials, the offsets between the conduction bands and valance bands (EC and EV respectively) can be calculated. For a at6% Fe dopedZnS/Cu2O the offsets were calculated to be EC = 0.22eV andEV = 0.93eV, showing an improvement over undoped ZnS/Cu2O heterojunctions, whilst bringing the conduction band offset within the desired range of 0.4 > EC > 0 eV for high efficiency photovoltaic heterojuctions4 1. Grondahl, L. O. Geiger, P. H. Journal of the A.I.E.E. 1927, 46, 215-222. 2. Grondahl, L. O. Reviews of Modern Physics 1933, 5, 141-168. 3. Raebiger, H. Lany, S. Zunger, A. Physical Review B 2007, 76, 045209 4. T. Minemoto, T. Matsui, H. Takakura, Y. Hamakawa, T. Negami, Y. Hashimoto, T. Uenoyama, and M. Kitagawa, Energy Materials and Solar Cells 67, 83 (2001)

Resume : We present a detailed investigation of the phase formation dynamics of the chlorine derived one-step synthesis of MAPbI3 perovskite. In a specially designed atmosphere controlled in-situ reactor, synchrotron based in-situ quick scanning X-ray absorption spectroscopy (QEXAFS), X-ray diffraction (XRD) as well as X-ray fluorescence (XRF) are applied in combination with in-situ optical reflection and photoluminescence spectroscopy. This enables us to correlate the evolution of the chemical reaction from XAS, the formation of crystalline phases from XRD and the chemical composition from XRF with the evolution of the optoelectronic properties of the film on one single timeline. We observe a delayed formation of crystalline MAPbI3 and attribute it to a concentration threshold in the decreasing chlorine content of the sample. Our results give detailed insight into the formation process and can help to provide a mechanistic understanding of the reactions and intermediates involved. This knowledge can be utilized to optimize synthesis strategies and to tune the optoelectronic properties of organic-metal-halide perovskite semiconductors. As an outlook we will discuss new developments for the upcomeing in-situ multi-parameter experimental platform at the Balder beamline at MAX IV.

Resume : Oxynitrides are among some of the most promising materials for hydrogen generation by visible light driven water splitting. The effect of nitrogen substitution into the oxygen sites of the pristine oxide leads to the reduction of the band gap, shifting the absorption edge towards the visible light energy range, enabling a more efficient utilisation of the solar spectrum. The oxynitrides are well known photoelectrodes and have been studied extensively in the form of powders. However, specific material properties cannot be probed with powder samples. The polycrystalline oxynitride powders do not provide any well-defined surface to allow the detailed studies of the solid-liquid interface where photoelectrochemical reactions take place. For that reason we investigate oxynitride thin films as ideal model systems to allow the investigation of the surface and interface. Using grazing incidence X-ray absorption spectroscopy (GIXAS) we are able to distinguish the surface from the bulk and, track the changes in the electronic and local structure of the cations situated at the surface of the oxynitride film as a consequence of the water splitting process. These changes are limited to the first 3nm of a 100nm film and can help to achieve a fundamental understanding of the physiochemical processes occurring at the solid-liquid interface, where the electrochemical reaction takes place. Leading to the development of new materials with better performance. In the future, operando measurements will be performed using a commissioned cell designed specifically for GIXAS at solid-liquid interfaces.

Resume : X-ray photoelectron spectroscopy has played an enormous role in developing our understanding of a range of interesting materials, including energy materials, over the last five decades, primarily arising from its ability to energy-resolve electrons ejected from any sample due to its interaction with sufficiently high energy photons, thereby defining one of the most powerful spectroscopic techniques. However, when combined with space-resolution to achieve a microscopy based on this spectroscopy, it can provide us with unique information, not available otherwise. In my presentation, I shall describe two qualitatively different ways1,2 to achieve such a space-resolution to provide complementary insights into diverse systems. In the first part of the presentation, I shall focus on an elusive metastable phase, existing only as small patches in chemically exfoliated few-layer, thermodynamically stable 2H phase of MoS2 that is believed to influence critically properties of MoS2 based devices. Its electronic structure is little understood in absence of any direct experimental data and conflicting claims from theoretical investigations. I shall present data3,4 to conclusively resolve this issue based on probing the electronic structure of chemically exfoliated few layer systems using spatially resolved photoemission spectroscopy and show that the dominant belief in the community is qualitatively incorrect, requiring reinterpretations of almost all existing literature claims. In the second approach, I shall explore the rapidly expanding field that deal with emergent properties at a variety of interfaces formed in heterostructured materials, including optically active nanomaterials. It is in general difficult to probe directly the nature of such interface states since these are typically buried under a depth and represent a very small volume fraction of the entire sample. We have shown2,5-10 in recent times that x-ray photoelectron spectroscopy with a widely tunable photon energy can be used very effectively to obtain spatially resolved electronic structure information, thereby making it the ideal probe to investigate interface properties in diverse heterostructured systems. I illustrate this by discussing electronic properties of a few interesting systems, covering, based on the availability of time, LaAlO3-SrTiO3, magnetic properties at tunnel junctions with CoFeB and MgO, and optical properties of highly luminescent semiconductor nanoparticles with integrated internal interfaces. References: 1. D. D. Sarma et al., Phys. Rev. Lett. 93, 097202 (2004). 2. D. D. Sarma et al., Chem. Mater. 25, 1222 (2013). 3. Banabir Pal et al., Phys. Rev. B 96, 195426 (2017). 4. Debasmita Pariari et al., Unpublished results. 5. S. Sapra et al., J. Phys. Chem. B 110, 15244 (2006). 6. Pralay K. Santra et al., J. Am. Chem. Soc. 131, 470 (2009). 7. D. D. Sarma et al., J. Phys. Chem. Lett. 1, 2149 (2010). 8. Banabir Pal et al., J. Electron Spectrosc. Relat. Phenom. 200, 332 (2015). 9. Sumanta Mukherjee et al., Phys. Rev. B 91, 085311 (2015). 10. Sumanta Mukherjee, et al., EPL (Europhys. Lett.) 123, 47003 (2018).

Resume : Silicon (Si) is ubiquitous in photovoltaic (PV) solar devices due to its abundance, low cost, and non-toxicity. Despite these advantages, the optical absorption of Si leaves much to be desired. This has generated increased interest in exotic phases of Si that retain the advantages of Si while having increased optical properties. One such phase is the rhombohedral (r8-Si) phase. Modelling studies suggest r8-Si has a significantly increased optical absorption, particularly across the solar spectrum, when compared to the types of Si currently used for PV applications. This phase is formed via pressure application and is known to be stable between 2 – 9 GPa under conventional diamond anvil cell (DAC) compression. Unfortunately, r8-Si is not stable at ambient conditions and no significant amounts of r8-Si have been observed after DAC decompression. However, under point-loading a mixed structure comprised of r8-Si alongside a secondary phase (bc8-Si) can be recovered. In our work, we use a novel sample preparation method to investigate this bc8/r8 mixed structure using x-ray diffraction (XRD). The relative volume of these phases was determined, with r8-Si being the dominant phase comprising around 70% of the mixed structure. Further, a distortion within the unit cell was observed along the direction of point-loading. Theoretical calculations together with these observations suggest the recovered material contains an intrinsic compression of around 4 GPa that stabilizes the r8-Si.

Resume : Grazing incidence X-ray diffraction (GIXD) is a powerful method frequently applied to characterize thin films and surfaces. Depending on the sample's crystallite alignment, special care is required during the measurement. While a static measurement at one azimuth is sufficient for 2D powders, epitaxially grown samples with defined in-plane alignment of the crystallites require data collection at all sample azimuths, i.e. 0°–360°. The rotation is necessary to obtain all information for polymorphic phase, texture and epitaxy analysis. Our state of the art measurements collect 180 or 360 images which then require a thorough processing. GIDVis [1], a software package developed in our group, allows analysis of GIXD data in general, but more importantly, calculation of pole figures, i.e. spatial distributions of all net planes accessible in the reciprocal space volume of an experiment. The power of our approach is demonstrated by two examples: In the first case, pentacenequinone (P2O) on Ag(111) is studied and its crystal structure is determined. GIXD pole figures are used to evaluate the epitaxial relationship of the two materials. In the second example, we apply our technique to a metal organic framework (MOF) deposited on a Cu(OH)2 nanobelt surface, which provides information on the alignment of the nanobelts, and to what extent the alignment of the surface is transferred to the MOF. [1] B. Schrode et al., Journal of Applied Crystallography 2019, 52, DOI 10.1107/S1600576719004485

Resume : In the era of searching for efficient renewable energy sources, toxic and greenhouse gases monitoring or building specialized low-power consumption (opto-) electronics, the discovery of new and evolution of known materials becomes crucial. ZnO as a wide band gap semiconductor with high optical transparency, sensing properties and electrical conductivity, could be the answer for nowadays needs. In this work, we present fundamental physical and chemical properties of ZnO thin layers prepared by sol-gel synthesis followed by spin-coat deposition and subsequently by thermal processing. Various chemical composition of sols and post-deposition drying or annealing processes impacted the layers’ surface topography and bulk crystalline structure as monitored by atomic force microscopy and X-ray diffraction. The effect of morphological structure reorganisation induced by layer processing were also revealed by vibrational investigations using Raman spectroscopy. Detailed chemical composition obtained by X-ray photoemission spectroscopy (XPS) analysis manifested modification of O 1s, Zn 2p and adventitious C 1s species due to presence of defects depending on sol type and thermal processing. Simultaneous XPS and ultraviolet photoemission spectroscopy (UPS) studies of the ZnO valence band region revealed tendency for reaching stable intrinsic character of oxide electronic structure. The XPS and UPS results corroborated with observed oxide’s propensity for structural neutralization of defects and building of ordered, crystalline structure after thermal post-processing.

Resume : An up-scalable chemical vapor deposition process (CVD, Figure 1) for methylammonium lead iodide (MAPI, CH3NH3PbI3) and formamidinium lead iodide (FAPI, CH5N2PbI3) used in thin film perovskite solar cells has been built-up and optimized. The procedure uses exclusively gaseous precursors to transform the inorganic salt, i.e. lead iodide (PbI2) to the respective perovskite films. The mechanism of perovskite formation has been confirmed using XPS and XRD. Further characterization techniques include SEM, UV-/Vis spectroscopy, photoluminescence and solar simulator measurements. For MAPI, efficiencies of up to 12.9% have been measured, with the process still being under optimization.

Resume : Many emerging technologies, including fuels cells, water splitting, and energy storage devices, are based on principles of electrocatalysis. Yet, our fundamental understanding of electrocatalysis lags behind classical heterogeneous catalysis which has been dominating chemical technology for a long time. In order to get insight into complex electrocatalytic processes at solid/liquid interfaces, we use well-defined model Pt/Co3O4(111) electrocatalysts prepared by surface science methods in ultrahigh vacuum and transfer these into an electrochemical environment without exposure to air. The corresponding approach allows investigating the Pt/Co3O4(111) electrocatalyst under electrochemical conditions by numerous techniques including synchrotron radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). In particular, we employ a dedicated experimental setup that combines an electrochemical cell and spectroscopic analysis to investigate the redox processes as a function of the electrode potential. Thus, the stability of Pt/Co3O4(111) electrocatalyst was characterized with respect to the oxidation state of the Pt particles and the degree of the reduction of the Co3O4(111) support. Various processes such as charge transfer, oxygen spillover, water dissociation, and reversible oxidation of Pt nanoparticles were identified under electrochemical conditions.

Resume : XANES is sensitive to the atomic structure of materials. Therefore, it is a good technique for a detection of defects and determination of crystal phases. The organic lead-halide perovskite and III-V semiconductor solar cells are in our focus. While nowadays a quality of the wurzite GaN wafers is very high, various defects in perovskites are unavoidable. The most unwanted are these defects which lead to the crystal degradation, and one such example is a deprotonation of the organic cation. Other most frequent defects in perovskites are vacancies of the organic cations and inorganic cations and anions. On the other hand, despite a wide application of the wurzite GaN crystals, the high pressure and high temperature growth leads to the mixed phases which are not yet well investigated. The most popular components, in addition to the wurzite phase, are B1 (6-fold coordinated rocksalt) and B2 (8-fold coordinated) cubic phases. However, a comparison of the calculated spectra with the recent synchrotron measurements suggests a more rich phase diagram.

Resume : Dynamic processes occurring in batteries are generally studied by ex situ modality. However, those processes that rule electrochemical energy storage in batteries should be studied under operating conditions. Operando experiments provide a realistic representation of the reaction behavior occurring at electrodes. The typical drawbacks of ex situ experiments due to sample transfer, such as the alteration of air or moisture-sensitive species, are avoided, and so are the relaxation reactions that may occur when the electric circuit is opened. Operando data collection allows to check the structural and electronic reversibility of a battery system, while at least one full cycle is performed. For all these reasons, ex situ studies of electrode materials are now complemented by operando measurements using complementary tools such as X-ray diffraction (XRD) and/or spectroscopic techniques such as X-ray absorption spectroscopy (XAS). X-ray absorption spectroscopy is a synchrotron radiation based technique that is able to provide information on local structure and electronic properties in a chemically selective manner. Operando synchrotron radiation powder diffraction experiments allow monitoring the extended structure of a material during the intercalation/release process. The potentiality of the joint XAS-PXRD approach in the newly proposed Prussian Blue-like cathodes materials [1] for Li and Na batteries is here underlined. This makes designing cathode materials with improved properties. [1] A. Mullaliu, M. Giorgetti et al. J. Phys. Chem. C 123(2019)8588; A. Mulllaiu, M. Giorgetti et al. Condensed Matter 3(2018)36; A. Mullaliu, M. Giorgetti at al. J. Phys. Chem. C 122(2018)15868.