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2021 Fall Meeting

Functional materials


Interface phenomena in emerging electronic and energy technologies

The functionality of electronic devices is often governed by their interface properties. Consequently, the design and optimization of functional interfaces is one of the key challenges in the development of novel energy and electronic technologies. This symposium covers recent advances in interface engineering and analysis in emerging technologies.


The functionality of electronic devices, including thin film transistors, LEDs as well as a wide variety of solar cells and batteries, is to a large part governed by their interface properties. Consequently, the development of next-generation technologies requires novel device structures with tailored interfaces. For many applications tunable, multi-property-functionality is required of the respective contact materials. In modern device structures the number of functional layers is often reduced, so that contact materials need to fulfill multiple purposes, ranging from surface passivation over charge collection to the formation of charge-selective electrical contacts. Constraints in processing compatibility as well as the use of abundant and non-toxic materials pose additional challenges to the development of novel devices. The synthesis of hetero-structures containing multiple metastable or volatile materials, such as flexible polymer substrates (e.g. in flexible solar cells or wearables), is particularly challenging and requires novel synthesis routes (e.g. HIPIMS sputtering), to avoid degradation of the underlying materials.

As operating conditions are becoming more demanding, research in interface degradation and related defect physics has become more relevant than ever. The increased availability of HAXPES and environmental ESCA techniques has enabled the study of buried interfaces as well as semiconductor-electrolyte interfaces in near operating conditions providing valuable insights in critical processes, such as changes in the electronic band alignment or chemical reactions resulting in the formation of interface defects.

A detailed understanding of interfacial phenomena and processes is crucial to improve performance and durability of novel device structures. Combined with contact material innovation and novel synthesis techniques for the preparation of thin-film heterostructures these insights will help drive the development of next-generation technologies.

The goal of this symposium is to provide a dedicated platform for a multidisciplinary community of materials and device scientists, to discuss recent advances and future needs in interface engineering, manufacturing and advanced analytical techniques.

Hot topics to be covered by the symposium:

  • Electrode-Electrolyte Interfaces: Energy storage
  • Semiconductor-Electrolyte Interfaces: Catalysis
  • New Device Structures for Emerging Photovoltaic Materials
  • Interface Phenomena in Emerging Transistor Technologies
  • Multifunctional Nanolayers and 2D Materials for Interface Engineering
  • Defect Science and Stability at Interfaces
  • Near Ambient Characterization of Surfaces and Interfaces
  • Hard X-ray Photoemission Studies on Buried Interfaces and Depth Profiling
  • Advanced Analytical Techniques for Surface and Interface Analysis
  • Theoretical Studies and Computational Approaches for Interface Design and Analysis

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10:50 Welcome message and introduction to the Symposium    
Interfaces and Materials for Electronics - I : Tim Veal
Authors : Elvira Fortunato
Affiliations : Universidade NOVA de Lisboa, Portugal

Resume : TBA

Authors : Regina Dittmann
Affiliations : Peter Grünberg Insitut (PGI-7), Forschungszentrum Jülich GmbH & JARA-FIT

Resume : Memristive devices are promising candidates for future data storage and neuromorphic computing to overcome the scaling and power dissipation limits of classical CMOS technology. In particular, memristive devices can act as hardware representative of synapses in neuromorphic circuits. Therefore, the microscopic origin of the switching and failure mechanisms of memristive devices and to find methods to tune their properties is of key relevance. By performing X-ray photoelectron emission spectroscopy (X-PEEM) we could directly identify the modulation of the oxygen vacancies within the Schottky-barrier to the top electrode interface as the origin of the switching process (Bäumer et. al. Nature Commun. (2016)). Moreover we demonstrate that the introduction of an interface layer with a large activation energy for oxygen diffusion at the interface to the top electrode strongly enhances the retention of memristive SrTiO3 devices (Bäumer et. al. Nature Commun. (2015), F.V.E. Hensling, et al., Solid State Ionics (2018)). Engineering the interface to the top electrode offers a promising pathway for the rational design of memristive synapses with tailored plasticity, ranging from short-term to long-term plasticity. On the other hand, retention enhancement interface layers can result in a reduction of the switching speed not only by changing the voltage and temperature distribution in the cell, but also by influencing the rate-limiting-step of the switching kinetics. In particular, we demonstrate that by introducing a retention enhancement interface layer, the kinetics are no longer determined by the interface exchange reaction between switching oxide and active electrode, but depend on the oxygen ion migration in the additional interface layer (Siegel et al., Adv. Funct. Mater. (2020)). Thus, the oxygen migration barrier in the interface layer determines the switching speed. This trade-off between retention and switching speed is of general importance for rational engineering of memristive devices.

Authors : H.R.J. Cox1*, M. Buckwell2, W.H. Ng1, D.J. Mannion1, A. Mehonic1, P. R. Shearing2, S. Fearn3 & A.J. Kenyon1
Affiliations : 1: Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK; 2: Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK; 3: Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

Resume : We report the measurement of oxygen movement in silicon oxide-based ReRAM devices using a novel Secondary Ion Mass Spectrometry (SIMS) normalisation technique that allows the measurement of ionic movement with unparalleled sensitivity. Resistance switching in oxide-based intrinsic ReRAM devices is driven by oxygen movement in metal-oxide-metal (MIM) stacks under electric fields, hence understanding the nature of that movement and the factors driving it is crucial to the development of next generation ReRAM devices. In particular, the reversible movement of oxygen across interfaces with electrodes is a major factor determining device cycling endurance and the stability of conductive filaments formed during resistance switching. However, the sensitivity of existing analysis techniques at the nanometre scale is too limited to systematically examine complex interactions, oxygen exchange, formation of novel layers, and the role of ambient conditions. Here we present a new nanoscale analysis method that enables us to monitor oxygen movement in ReRAM devices with a sensitivity beyond what has previously been possible. SIMS, while being the most sensitive surface analysis technique, suffers from difficulties in comparing quantitatively differences in composition between different samples – particularly when those differences are of the order of only a few atomic percent or less. Our technique allows us for the first time to observe the movement of 16O across electrode-oxide interfaces in electrically biased silicon oxide (SiOx) ReRAM stacks, measuring bulk concentration changes in a continuous profile with unprecedented sensitivity. We present modelling of the electric fields in ReRAM devices which, for the first time, uses real measurements of both interface roughness and electrode porosity. This supports our findings and helps to explain how and where oxygen from ambient moisture enters devices during operation.

Authors : M. Censabella, V. Iacono, A. Scandurra, K. Moulaee, G. Malandrino, G. Neri, F. Ruffino, S. Mirabella
Affiliations : M. Censabella Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, via S. Sofia 64, 95123 Catania, Italy CNR-IMM via S. Sofia 64, 95123 Catania, Italy; V. Iacono Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, via S. Sofia 64, 95123 Catania, Italy CSFNSM - Centro Siciliano di Fisica Nucleare e Struttura della Materia, Via S. Sofia 64 95123 Catania; A. Scandurra Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, via S. Sofia 64, 95123 Catania, Italy; K. Moulaee Department of Engineering, University of Messina and INSTM Research Unity, C.da Di Dio, I-98166, Messina, Italy; G. Malandrino Dipartimento di Scienze Chimiche, Università di Catania, and INSTM UdR Catania, Viale A. Doria 6, I-95125 Catania, Italy; G. Neri Department of Engineering, University of Messina and INSTM Research Unity, C.da Di Dio, I-98166, Messina, Italy; F. Ruffino Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, via S. Sofia 64, 95123 Catania, Italy CNR-IMM via S. Sofia 64, 95123 Catania, Italy CSFNSM - Centro Siciliano di Fisica Nucleare e Struttura della Materia, Via S. Sofia 64 95123 Catania; S. Mirabella Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, via S. Sofia 64, 95123 Catania, Italy CNR-IMM via S. Sofia 64, 95123 Catania, Italy CSFNSM - Centro Siciliano di Fisica Nucleare e Struttura della Materia, Via S. Sofia 64 95123 Catania;

Resume : In the last years the demand of gas sensors has increased due to the air pollutants caused by industrial processes, motor vehicles and waste incinerators. In particular, in these high temperature combustion processes Nitric Oxide (NO) is produced, a colorless and odorless gas, harmful for human health. Among the various type of sensors, a key role is played by Metal Oxide Semiconductors (MOx), which change their electrical characteristics with the change in surrounding gas atmosphere. In addition, nanostructured MOxs, with high surface-to-volume ratio and high exposed surface, improve the sensor’s properties in terms of response speed, sensitivity and selectivity. Among the most studied semiconductors, there is copper oxide (CuO), in particular cupric oxide, a low cost p-type semiconductor with excellent catalytic properties and high stability. It finds important devices applications in catalysis, in energy storage and in gas sensing. Anyway, the success of such technologies implies knowledge and control over the nanostructures’ properties (shape, sizes, structure and crystallinity) and, consequently, the knowledge of interface interactions between adsorbate and adsorbent. To meet these requirements, we have produced ligand-free CuO nanostructures (Ns) with desired size, composition and shape, by laser-based synthesis method. After drop-casting of solutions on interdigitated electrode and annealing process, we performed the gas sensing measurements, exposing the CuO Ns-based sensor to NO in a temperature range from 50°C to 400 °C. In particular, the sensor responds to the gas in different ways as a function of temperature: showing an oxidant or reducing behaviour depending on whether it is at low or high temperature. We explained these interface phenomena, attributing a key role to oxygen and to high catalytic activity of CuO Ns as regards the reactions involving nitrogen monoxide. We found these redox surface reactions and, by employing the Langmuir adsorption model, we extrapolated the respective activation energies. The deep knowledge of the physical processes temperature-based, such as adsorption and desorption behaviour, and the structural effects can revolutionize the next-generation sensor’s field.

Authors : Eoghan Vaughan, Daniela Iacopino, Pei Shee Tan, Joanna Tierny, Niall Burke.
Affiliations : Tyndall National Institute; IT Tralee

Resume : Porous graphitic carbon electrodes were fabricated by laser scribing of commercial polyimide sheets. The process was performed by a simple one-step procedure using visible wavelength laser irradiation from a low-cost hobbyist laser engraver. The obtained electrodes displayed a highly porous morphology, rich in three-dimensional interconnected networks and edge planes, suitable for electrochemical sensing applications. Spectral characterization by Raman and XPS spectroscopies revealed a crystalline graphitic carbon structure with high percentage of sp2 carbon bonds. Extensive electrochemical characterization performed with outer-sphere [Ru(NH3)6]3+ and inner-sphere [Fe(CN)6]4-, Fe2+/3+ and dopamine (DA) redox mediators showed quasi-reversible electron transfer at the graphitic carbon surface dominated by mass diffusion process. Fast heterogeneous electron-transfer rates, higher than similar carbon-based materials and comparable to other graphitic carbon electrodes obtained by infrared laser irradiation, were obtained for these electrodes. Biosensing capabilities of the bare electrodes were first investigated by their ability to simultaneously detect Dopamine, Ascorbic Acid, and Uric Acid in solution. The electrodes were functionalised with 1-pyrenebutyric acid (PBA) and EDC/NHS to create a linker that enables the immobilisation of antigen (anti IL-6) onto the surface for the detection of IL-6. The linear range of the resulting biosensor was 10 pg/ml to 500 pg/ml in the buffer. The gas sensing capabilities of such electrodes have been investigated by examining its response to various concentrations of Phenol gas. Initial results indicate that these electrodes have exciting potential as electrochemical gas sensors for VOCs. Moreover, the compatibility with lightweight, portable and handheld instrumentation makes such electrodes highly promising for the realization of low-cost disposable sensing platforms for point-of-care applications.

12:45 Q&A session / Break    
Interfaces and Materials for Electronics - II : Claudia Cancellieri
Authors : Payal Wadhwa (1), Alessio Filippetti (1,2)
Affiliations : (1) Department of Physics, University of Cagliari, Italy (2) Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali, Cagliari, Italy

Resume : The field-effect control of spin-orbit coupling and/or ferromagnetic order in 2D ultrathin electron gas trapped in oxide heterostructures is at the core of fundamental mechanisms potentially enabling the implementation of visionary quantum nanotechnologies, from topological states to spin-orbit qubits, from spin-charge conversion to spin transistors. The accurate theoretical description of these phenomena is quintessential complement of the experimental characterization and device implementation. In this study, our prototype host material is the STO/LAO interface, in which the spontaneous 2DEG formation and a robust Rashba effect are largely attested in literature. We explored the Sr-Eu substitution as key ingredient to ignite long-range magnetism; in fact, bulk EuTiO3 is a well known antiferromagnet which becomes ferromagnetic upon electron doping. Our description based on advanced ab-initio calculations and a chemical rendering based on Wannier functions, describes the key factors driving the ferromagnetic order at the interface, including Ti 3d - O 2p and Eu 4d - O 2p orbital coupling. As the magnetic electron gas is tightly localized at the interface layer, it can be easily manipulated by field effect, enabling the control of magnetic coupling strength and critical temperature. Together with electronic and magnetic properties, our study analyzes in detail transport and topological properties of the interface in both (001) and (111) orientations, specifically focusing on the description of anomalous Hall effect and edge states. A careful comparison with the experimental evidence will be provided as well.

Authors : C. Kalha (1), S. Bichelmaier (2), T. Lee (3), P. Kumar-Thakur (3), N. K. Fernando (1), J. Gutierrez (4), S. Mohr (4), L. E. Ratcliff (5) and Anna Regoutz (1)
Affiliations : (1) Department of Chemistry, University College London, 20 Gower Street, WC1H 0AJ, UK. (2) Technische Universität Wien, Department of Computational Chemistry, Getreidemarkt 9/165, 1060 Vienna, Austria. (3) Diamond Light Source Ltd., Harwell Science & Innovation Campus, Oxfordshire, OX1 3QR, UK. (4) Barcelona Supercomputing Center (BSC), C/ Jordi Girona 29, Barcelona 08034, Spain. (5) Department of Materials, Imperial College London, London SW7 2AZ, UK.

Resume : The progressive miniaturisation of feature sizes in microelectronic devices has increased the risk of exposing the system to severe service conditions, such as higher power densities and consequently higher local temperatures. These devices consist of a complex multi-metallic-layered architecture and when subjected to such conditions, detrimental inter-diffusion phenomena can occur between adjacent layers, compromising the integrity, functionality and reliability of these components. The problem of interdiffusion is particularly persistent in power semiconductor devices that employ a copper (Cu) metallisation scheme and a titanium-tungsten (TiW) diffusion barrier. The TiW barrier is required to isolate the copper from the silicon substructure but high thermal events can induce a segregation of titanium out of the barrier layer and diffusion into the overlaid copper layer, leading to the degradation and failure of the barrier. Additionally, the TiW/Cu interface can act as a sink for the accumulation of oxygen due to the strong gettering ability of titanium, which further adds an additional risk to the system and often promotes delamination of the layers. Here, un-patterned, Si/SiO2/TiW and Si/SiO2/TiW/Cu thin film stacks annealed for varying durations at 400°C under an inert atmosphere were characterized using a combination of soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES). Combining the two techniques provided the opportunity to non-destructively study both the titanium diffusion mechanism, and the oxidation behaviour of TiW at multiple depths. The findings were able to systematically showcase the dependence of the titanium surface enrichment and oxidation behaviour on the annealing duration, and provided a detailed account of the degradation and failure mechanisms associated with these TiW/Cu heterostructures. Overall the combinatorial XPS characterisation approach worked well as the respective techniques complemented each other’s shortcoming and therefore the approach holds promise for the characterisation of other multi-metallic systems where similar problems exist.

Authors : Leanne A. H. Jones (1), Warda Rahim (2), Jack E. N. Swallow (3), Nicole Fleck (1), Anna Regoutz (2), Pardeep K. Thakur (4), Tien-Lin Lee (4), David O. Scanlon (2,4,5), Tim D. Veal (1), Vin R. Dhanak (1)
Affiliations : (1) Stephenson Institute for Renewable Energy and Department of Physics,University of Liverpool, Liverpool L69 7ZF, United Kingdom (2) Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK (3) Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH (4) Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus,Didcot, Oxfordshire OX11 0DE, United Kingdom (5) Thomas Young Centre, University College London, Gower Street, London, WC1E 6BT, UK

Resume : Transition metal dichalcogenides (TMDs) are of great interest due to their two-dimensional structure analogous of that of graphene. However, as many of the TMDs possess a band gap, they are suitable for many applications that graphene could not be utilised for, including optoelectronics, transistors, biosensors, gas sensors, electrocatalysis and in energy storage. The rhenium dichalcogenides are unique members of the TMD family as their properties do not change drastically with the number of layers whereas other TMDs have layer dependent properties. This is due to weak interlayer coupling which arises due to the in-layer anisotropy. Here, the core levels, valence band (VB), work function and ionisation potentials (IP) of ReS2 and ReSe2 were investigated using x-ray photoemission spectroscopy (XPS). Furthermore, soft XPS (SXPS) and hard XPS (HAXPES) were utilised in order to take advantage of the probing depth and the change in photoionisation cross-sections. This allowed the investigation into the contributions of different orbitals to the valence band as well as investigating overlapping core level features. From the core level analysis, three peaks are used to fit the S 2p/ Se 3d core levels and are, for the first time, thought to be associated with three chalcogen environments within the structure. This is despite many papers indicating multiple chalcogen peaks, but not identifying or acknowledging them. The lineshape in the vicinity of the Re 4f7/2 core level is seen to become increasingly more Lorentzian with increasing photon energy, implying that a different core level is present rather than an additional chemically-shifted component of the Re 4f core level. This extra intensity at the same binding energy (42.35 and 41.90 eV for the sulphide and selenide, respectively) as the Re 4f7/2 is identified as being from the Re 5p3/2 core level which is not usually reported in the literature due to its low relative photoionisation cross-section at lower photon energies. The Re 5p3/2 binding energy is found to fit the trend of the 5p states in the period 6 transition elements. The valence band spectra agree well with the cross-section-corrected density-functional-theory-calculated VB density of states. Multiple photon energy measurements of the valence band allowed for the different contributions of the VB to be probed due to the photonionisation cross-sections. Furthermore, coupling the position of the valence band maximum with the work function gives the value for the ionisation potential which allows for the natural band alignment of these materials to be plotted which gives useful information for the device application of these materials. The ionisation potential values calculated for ReS2 and ReSe2 were 5.98 eV and 5.53 eV, respectively. The decrease in IP value from the sulphide to the selenide can be explained by the contributions from the valence band and the orbital configuration energies of the chalcogen p orbital.

Authors : Yunxia Zhou, Jun Zhu, Shengqiang Zhou
Affiliations : Yunxia Zhou, Helmholtz Zentrum Dresden Rossendorf, University of Electronic Science and Technology of China; Jun Zhu, University of Electronic and Technology of China; Shengqiang Zhou, Helmholtz Zentrum Dresden Rossendorf

Resume : Feasible interface lattice design is a really key issue for high-quality hybrid of functional oxide thin films on GaAs semiconductor substrates. But interfacial defects induced by lattice mismatch cause the problem to become challenging. Here, we reported a novel sub-titanium oxide (Ti2.5O3) thin film epitaxially grown on GaAs substrate using pulsed laser deposition, the high-quality Ti2.5O3/GaAs heterostructure significantly reduced the lattice mismatch between titanium sub-oxides and the GaAs substrate. Besides, our work theoretically and experimentally demonstrated that high crystalline Ti2.5O3 (010) film can be grown layer-by-layer on GaAs (001) substrate with highly compatible interfaces. Extremely low lattice mismatch values of 0.3% and 0.6% along different orientations can be achieved in combination with the notably suppressed formation of arsenic oxides (AsOx) and gallium oxides (GaOx) between Ti2.5O3/GaAs interfaces. Owing to the favorable interface and high crystalline, integrated BaTiO3(250 nm)/STO/Ti2.5O3/GaAs heterostructure demonstrates hysteresis loops with a remnant polarization of 9.85 µC/cm2 at 600 kV/cm and a small leakage current density of 1×10−5 A/cm2 at -500 kV/cm. Not only the excellent performances pave the path for the further application of Ti2.5O3/GaAs heterostructure in electronics, but also the unique strategy gives a good inspiration for coupling other functional oxides on GaAs with expected excellent performances.

Authors : Cara-Lena Nies, Suresh Kondati Natarajan, Michael Nolan
Affiliations : Tyndall National Institute; Synopsys Denmark; Tyndall National Institute

Resume : Copper is used as interconnects in semiconductor devices but cannot keep up with the ever-decreasing size of transistors. This has created an “interconnect bottleneck”. While many alternatives to Cu interconnects are being considered, it is also important to extend the lifetime of copper for as long as possible. One way to achieve this is with single ultra-thin materials that combine the properties the diffusion barrier and seed liners to facilitate deposition of Cu in high-aspect ratio vias. The interactions at the interface between Cu and the candidate barrier/liner material are essential to determining if this material will promote Cu deposition. We explored TaN that is surface doped with Ru, Co, and W as a potential barrier/liner material. Through density functional theory we study the interactions at the Cu/modified-TaN interface, for Cu structures of varying sizes and different contents of Ru, Co, and W. This allows us to gain insight into the nucleation and growth mechanism of Cu thin films on modified TaN. Initial results show that the difference in ionic radius between Ta, Ru, Co and W leads to different surface morphologies when each metal is doped into the surface. Ab initio molecular dynamics and activation energies for atom migration show that Ru content in the substrate can be tailored to minimise upward migration of copper atoms compared to pure TaN. Lateral migration of atoms is promoted, which encourages the growth of a 2D film, required for interconnect structures. Finally, we will explore how this differs for Cu/Co-TaN and Cu/W-TaN.

Authors : F. La Mattina(1), C. Cancellieri(1), T. Gagnidze(1), A. Chickina(2), M. Caputo(2), V. Strocov(2), M. Bon(1), C. Pignedoli(1), D. Passerone(1), M. Rossel(1), G.-L. Bona(1), and A. Shengelaya(3).
Affiliations : (1) EMPA, Swiss Federal Laboratories for Materials Testing and Research, Uberlandstrasse 129, 8600 Dubendorf, Switzerland; (2)Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland; (3) Department of Physics , Tbilisi State University, Tbilisi, Georgia

Resume : The dielectric response of an insulator at interface with High Temperature Superconductors (HTSs) may result in a special charge state that affects charge-pairing mechanism for the superconducting state. The case of FeSe monolayer on top of SrTiO3 (STO) where an optical phonon of the insulators give rise to polarons with the electrons of the monolayer, is a remarkable example of this interaction. In a more general approach, Muller and Shengelaya (2013) have been suggested that ultra-thin layers of copper-oxide HTSs sandwiched between high-dielectric-constant insulator layers could provide a potential pathway to high-Tc superconductivity. In such structures polarization of the dielectric material could reduce the Coulomb repulsion between the charged clusters or stripes formed in the pseudogap phase resulting in an increase of the superconducting critical temperature. Here we present a study of SrTiO3/YBa2Cu3O7-  (STO/YBCO ) interface. We use soft-X-ray angle-resolved photoelectron spectroscopy (SX-ARPES) to access electronic states at the buried interface. Measurements were carried out at ADRESS beam line of Swiss Light Source (SLS) in an energy range 400 < h < 1000 eV and about 40meV of energy resolution. We measured the electronic band structure of YBCO states present at the interface of ultra-thin coatings (STO) through their spectral function A(omega,k). The Fermi surface projection obtained in YBCO shows that the presence of STO cap layers modify the symmetry of the FS projections from 2D toward 1D states. Since a similar phenomenon has been observed in STO grown on polar LaAlO3, we suggest that this is a general property of the high-  STO grown on charged surfaces, in which a distortion of its oxygen octahedron (ferroelectric distortion) may compensate the polar mismatch.

Authors : Serrano, Z.*(1), Cortinhal M.(1), Carioca, R.(1), Martins R.(1), Barquinha, P.(1), Bundaleski, N.(2), Fortunato, E.(1) & Deuermeier, J(1)
Affiliations : (1) CENIMAT/i3N, Department of Materials Science, NOVA School of Science and Technology (FCT-NOVA) and CEMOP/UNINOVA, NOVA University Lisbon, Campus de Caparica, 2829-516 Caparica, Portugal; (2) CEFITEC, Department of Physics, NOVA School of Science and Technology (FCT-NOVA), NOVA University Lisbon, Campus de Caparica, 2829-516 Caparica, Portugal * lead presenter

Resume : Indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) are the first choice for pixel driver circuits in commercial large-area active matrix displays. Controlling the electrical properties of IGZO is generally achieved by adjusting the oxygen content and the cation mix, also affecting the device stability. Most importantly, the chemical and electronic characteristics of the interface between the semiconductor and the gate dielectric play the key role in determining the device performance. X-ray photoelectron spectroscopy (XPS) is typically used to measure the energy band alignment between materials quantitatively and/or to trace compositional changes at interfaces qualitatively. For this work a quantitative analysis of IGZO composition in top-gated TFTs was conducted, beyond the standard approach based on sensitivity factors. The IGZO channel material was deposited by confocal radio-frequency magnetron sputter deposition. The gate dielectrics Al2O3 (at 200°C and 150°C) and Ta2O5 were deposited by atomic layer deposition. In this sequence, an increase in channel conductance (i.e. a more negative turn-on voltage) was observed. These variations are correlated with changes in the IGZO cation mix and oxygen deficiency at the interface.

16:00 Q&A session    
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Interfaces and Materials for Energy-Storage - I : Gustav Graeber
Authors : Julia Maibach (1), Ida Källquist (2), Lydia Gehrlein (1), Maria Hahlin (2)
Affiliations : 1 Institute for Applied Materials - Energy Storage Systems, Karlsruhe Institute of Technology; 2 Department of Physics and Astronomy, Uppsala University

Resume : Post mortem photoelectron spectroscopy (PES) analyses have contributed significantly to our understanding of electrode/electrolyte interfaces in Li-ion batteries. For these types of measurements, cycled battery electrodes are removed from the battery, the excess liquid electrolyte is rinsed off and the sample then transferred to the PES instrument to be characterized in UHV conditions. Using ambient pressure photoelectron spectroscopy (APPES) especially at synchrotron sources, we now have the possibility to acquire more realistic information about the reactions of the electrode with the electrolyte since the experiments can be conducted at elevated pressures and with liquid electrolyte present. This liquid phase introduces new challenges to the PES experiments and we addressed these in firstly characterizing Li-ion battery electrodes [1] and electrolytes [2, 3] using APPES as a precondition to achieve reliable studies on working batteries. In this presentation, we will focus on our recent findings on electrochemical potential differences over the solid/liquid interface using operando APPES [4] at the HIPPIE beamline at MAX IV (Lund, Sweden). We will show our approach using operando ambient pressure photoelectron spectroscopy to follow changes in electrochemical potential difference over the solid/liquid interface by measuring the kinetic energy shifts of the electrolyte core levels as a function of applied external voltage to the working electrode. Even without direct access to the electrode/electrolyte interface, we can clearly distinguish between electrode polarization and charge transfer as we will show for to Li-ion battery model systems. [1] J. Maibach, C. Xu, S. K. Eriksson, J. Åhlund, T. Gustafsson, H. Siegbahn. H. Rensmo, K. Edström, M. Hahlin, Rev. Sci. Inst. 86 (2015), 044101. [2] J. Maibach, I. Källquist, M. Andersson, S. Urpelainen, K. Edström, H. Rensmo, H. Siegbahn, M. Hahlin, Nature Comm. 10 (2019), 3080. [3] P.m. Dietrichs, L. Gehrlein, J. Maibach, A. Thissen, Crystals (2020), 10, 1056. [4] I. Källquist, F. Lindgren, M.-T. Lee, A. Shavorskiy, K. Edström, H. Rensmo, L. Nyholm, J. Maibach, M. Hahlin, under review at ACS Applied Materials & Interfaces.

Authors : Robert Weatherup
Affiliations : University of Oxford

Resume : Lithium-ion batteries (LIBs) are key to the transition from fossil fuels towards increased use of renewable energy sources. Although LIBs are already widely used in portable electronics and the rapidly expanding electric vehicle market, more widespread deployment requires improved cycle-lifetimes. These are currently limited by side-reactions that occur primarily at the electrode-electrolyte interfaces, and understanding the nature of these reaction is critical to the design of materials solutions to mitigate LIB degradation. However, obtaining chemical information with nm-scale interface sensitivity is a significant challenge given these interfaces are typically buried between a bulk electrode and dense electrolyte environment. 1,2 We report here detailed ex-situ studies of Ni-rich LiNixMnyCo1-x-yO2 (NMC) cathode materials cycled vs. Graphite in full cells. These materials offer improved energy densities, but for commercial applications their rapid capacity fade must be addressed. To understand this, we connect electrochemical signatures of cell degradation with surface chemistry changes taking place on the electrodes as revealed with Hard X-ray Photoelectron Spectroscopy (HaXPES). We find that the main cause of the capacity fading during the first few hundred cycles is electrolyte reduction, followed by an increasing contribution from loss of active NMC material. It is well established that transition metals in the solid electrolyte interphase (SEI) can cause ongoing electrolyte reduction. Our studies reveal that the relative rates of plating of different transition metals change as cycling proceeds. Mn is found to be plated more rapidly in the beginning, whereas Ni becomes a more significant contribution at higher cycle numbers. This change is found to correlate with a lowering of the oxidation state of transition metal species at the surface of the NMC and the formation of a high-impedance rocksalt layer. Despite the insights, ex-situ measurements suffer from potential ambiguity due to changes to the electrode surfaces occurring during glovebox disassembly, and cannot capture intermediate species involved in interface degradation. We therefore introduce several complementary interface-sensitive approaches for performing operando x-ray photoelectron and absorption spectroscopy (XPS/XAS).2-5 These rely on reaction cells sealed with X-ray/electron-transparent membranes such as thin (<100 nm) silicon nitride or graphene membranes that remain impermeable to liquids. We demonstrate how these approaches can monitor the evolution of solid-liquid interfaces under electrochemical control, 5 including solid-electrolyte interphase (SEI) formation on Li-ion battery anodes. We further discuss our recent progress in applying HaXPES to study electrode-electrolyte interfaces in all-solid-state batteries.6 References 1. Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229. 2. Weatherup et al. Top. Catal. 2018, 61, 2085. 3. Velasco-Velez et al. Angew. Chemie 2015, 54, 14554. 4. Weatherup et al. J. Phys. Chem. Lett. 2016, 7, 1622. 5. Weatherup et al. J. Phys. Chem. B 2018, 122, 737. 6. Brugge et al. J. Mater. Chem. A 2020, 8, 14265

Authors : T. Amelal, M. Futscher, J. Patidar, Y. Romanyuk, and S. Siol
Affiliations : Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland

Resume : All-solid-state-batteries (ASSBs) with Li anodes are promising candidates for the next generation of batteries due to their superior energy and power density. Their functionality strongly depends on the stability of the different components. This includes the interface between the electrolyte and the anode. At this interface, the formation of dendrites has a detrimental effect on efficiency and lifespan. This instable behavior still poses a challenge for the widespread use of Li-based ASSBs. Recently, researchers from Samsung demonstrated that Ag-C composite interlayers could effectively suppress dendrite growth in Li-based ASSBs [1]. However, the exact mechanism governing this behavior remains unclear. We study the plating behavior of Li at the interface between a solid electrolyte and a Cu electrode. This simplified model system allows us to gain deeper insights into the underlying effects that cause inhomogeneous Li plating. We demonstrate the ability of amorphous carbon (a-C) thin films, deposited by direct current magnetron sputtering (DCMS), to stabilize the interface of the solid electrolyte, resulting in homogeneous Li plating on the Cu electrode. We show that the temperature during a-C deposition strongly effects the most influential properties of the carbon films, i.e. density, conductivity and microstructure. The consequential correlation between these three parameters during DCMS makes it difficult to determine which of them is most crucial. The use of high power impulse magnetron sputtering (HiPIMS) allows us to vary these thin film properties over a wider range and to investigate the influence of density, conductivity and microstructure separately. References: [1] Y.-G. Lee et al., High-energy long-cycling all-solid-state lithium metal batteries enabled by silver–carbon composite anodes, Nat. Energy 5, 299–308, 2020

Authors : Shira Haber, Rosy, Arka Saha, Malachi Noked, Michal Leskes
Affiliations : Department of Molecular Chemistry and Materials Science, Weizmann Institute of Science, Israel

Resume : The electrode – electrolyte interface in rechargeable batteries has a central role in the battery’s performance. A plethora of degradation processes, including electrolyte decomposition, structural and mechanical transformations lead to loss of energy and power densities. These processes can be controlled through deposition of thin protective layers at the electrode interface which prevent degradation, while enabling efficient ion transport across the interface. However, rational design of such interphases is limited by the scarcity of analytical tools that can probe few nanometers thick, heterogenous and disordered layers. Here I will present a new approach to examine thin interphases and gain atomic level insight into their composition, 3D structure and lithium ion transport properties by using solid state nuclear magnetic resonance (ssNMR) spectroscopy. The approach is based on (i) 10-104 fold increase in ssNMR sensitivity provided by Dynamic nuclear polarization (DNP), a process in which the high electron spin polarization is transferred to surface nuclei in the sample, enabling the detection of otherwise invisible nanometer-thick layers, and (ii) tracking 6Li-7Li isotope exchange processes across the electrode-electrolyte interface. I will describe the application of this approach to a novel surface treatment for high energy cathodes, here lithium rich LiNixMnyCozO2 (NMC), which leads to substantial improvements in rate performance and capacity retention[1]. Specifically, I will show how the combination of DNP and ssNMR provides a detailed chemical map of the surface composition and structure of this lithium-silicate protection layer. The permeability of the coating and the role of lithiated interphases was assessed by 6,7Li exchange experiments on coated and uncoated NMC and further compared to Electrochemical Impedance Spectroscopy (EIS) results[2]. The combination of structural insight from high sensitivity ssNMR and lithium exchange provide unique insight at the atomic-molecular level which is crucial for designing new materials for high energy battery cells. [1] Alkylated LixSiyOz Coating for Stabilization of Li-rich Layered Oxide Cathodes. Rosy*, S. Haber, E. Evenstein, A. Saha, O. Brontvein, Y. Kratish, D.B.Zhivotovskii, Y. Apeloig, M. Leskes*, M. Noked*, Energy Storage Materials (2020) 33, 268-275 [2] Structure and Functionality of an Alkylated LixSiyOz Interphase for High-Energy Cathodes from DNP-ssNMR Spectroscopy. S. Haber, Rosy, A. Saha, O. Brontvein, R. Carmieli, A. Zohar, M. Noked, M. Leskes*, J. Am. Chem. Soc. (2021) 143, 12, 4694–4704

Authors : Swallow, J. E. N.*(1), Head, A. R.(2), Jones, E.(1), Gibson, J. S.(1), van Spronsen, M. A.(3), Held, G.(3), Eren B.(4) & Weatherup, R. S.(1)
Affiliations : (1) Department of Materials, University of Oxford, Parks Road, Oxford, Oxfordshire, OX1 3PH, United Kingdom (2) Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton 11973, New York, United States (3) Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom (4) Department of Chemical and Biological Physics, Weizmann Institute of Science, 234 Herzl Street, 76100 Rehovot, Israel

Resume : There is an urgent and intense drive to develop alternative energy vectors as part of the transition towards a low-carbon economy in order to limit the extent of man-made climate change. Methanol is already widely used as a feedstock for the chemical industry, and is also a promising liquid fuel, either used directly (or as a mixture with gasoline) for combustion engines and fuel cells, or as a means of safely and efficiently storing and transporting hydrogen for on-board generation when needed[1]. If the methanol can be produced using captured CO2, and hydrogen supplied by electrochemical water-splitting from renewable sources, it has the added benefit of having a net-zero impact on carbon emissions. However, at present the majority of methanol production is based on natural gas as a carbon containing feedstock. This consists of three principle steps: (1) the production of synthesis gas (syngas), i.e., a H2, CO, and CO2 mixture, (2) the conversion of syngas into crude methanol, and (3) distillation of the product to the desired purity level. The conversion of syngas into methanol can involve several reactions; carbon monoxide hydrogenation, carbon dioxide hydrogenation, and the forward/reverse water gas shift reaction. At pressures and temperatures used in industrial production of methanol (around 50-100 bar and 200-300 degrees Celsius) hydrogenation of carbon dioxide is the energetically favourable reaction pathway for methanol generation. Cu-based heterogeneous catalysts are commonly employed, allowing for the optimised selectivity of the process in moderate reaction conditions to be achieved. In these reactions carbon monoxide is thought to be a source material for carbon dioxide formation, as well as acting as a scavenger for the oxygen atoms in water, which are an inhibitor for the active metal sites[2]. Here, we investigate whether there is any secondary function of carbon monoxide in methanol generation, other than converting water into carbon dioxide via the forward water gas shift reaction. To do this we employ ambient pressure x-ray photoelectron spectroscopy in the mbar pressure range[3], in conjunction with total electron yield x-ray absorption spectroscopy in a custom-built high pressure flow cell allowing us to reach ~1 bar. We observe the changes in chemical state of the Cu-catalyst surface and the absorbed species when carbon monoxide is included in or excluded from the gas mixture during the reaction, allowing us to determine the secondary role (if any) of carbon monoxide in the generation of methanol. [1] B. Eren, C. G. Sole, J. S. Lacasa, D. Grinter, F. Venturini, G. Held, C. S. Esconjauregui, R. S. Weatherup, Phys. Chem. Chem. Phys., 2020, 22, 18806 [2] N. D. Nielsen, A. D. Jensen, J. M. Christensen, J. Catal. 2021, 393, 324-334 [3] B. Eren, R. S. Weatherup, N. Liakakos, G. A. Somorjai, M. B. Salmeron, J. Am. Chem. Soc. 2016, 138, 8207–8211.

16:00 Q&A session    
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Interfaces and Materials for Energy-Conversion - I : Ute Cappel, Selina Olthof
Authors : Monica Morales-Masis
Affiliations : MESA+ Institute for Nanotechnology, University of Twente

Resume : Sputtered transparent conducting oxides (TCOs) are widely accepted transparent electrodes for several types of high-efficiency solar cells, from research to industrial applications. However, the high kinetic energies of the arriving species during sputtering may damage sensitive functional layers beneath, affecting interface formation and final device performance. This has motivated the search of alternative deposition techniques, allowing the ‘soft’ arrival of particles during thin film formation. In this presentation we discuss the potential of pulsed laser deposition (PLD) as an alternative damage-free physical vapor deposition technique. PLD is operated at high deposition pressures promoting thermalization of particles, and therefore reducing the kinetic energy of ablated species. We developed broadband transparent and high mobility TCOs using wafer-scale (4-inch) PLD with deposition rates on par (>4 nm/min) with lab-scale RF sputtering. The optoelectronic properties and microstructure of these Indium-based TCOs were studied and compared to sputtered counterparts. These high mobility TCOs were furthermore applied as rear electrode in buffer-free semi-transparent halide perovskite solar cells and on top of Silicon Heterojunction Solar Cells. In the case of semitransparent perovskite solar cells, the TCO is directly deposited on top of sensitive C60/BCP transport layers leading to improved stabilized maximum power point efficiency (15.1%) as compared to the cells with sputtered TCO electrodes (11.9%)(Y. Smirnov et al. Adv. Mater. Technol. 2021, 6, 2000856.). Enabled high short circuit currents leads to the possibility to reduce the amount of indium by reducing the thickness of the TCO films compared to standard ITO. On the other hand, higher deposition pressures during PLD fabrication offer a promising way to mitigate the sputter-induced damage for the deposition of rear transparent electrodes relevant for the development of high-efficiency tandem solar cells.

Authors : Xian’e Li,* Qilun Zhang, Jianwei Yu, Ye Xu, Rui Zhang, Chuanfei Wang, Huotian Zhang, Simone Fabiano, Xianjie Liu, Jianhui Hou, Feng Gao, Mats Fahlman*
Affiliations : Xian’e Li,* Qilun Zhang, Chuanfei Wang, Simone Fabiano, Xianjie Liu, Mats Fahlman* Laboratory of Organic Electronics, Department of Science and Technology (ITN), Linköping University, Norrköping SE-60174, Sweden; Jianwei Yu, Rui Zhang, Huotian Zhang, Feng Gao Biomolecular and organic electronics, Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping SE-58183, Sweden; Ye Xu, Jianhui Hou Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

Resume : Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is an essential property that has a strong influence on the charge generation and recombination process in organic photovoltaic devices. However, such ELA is largely undetermined for the recently developed high-efficiency non-fullerene-based organic solar cells, and hence the debate regarding efficient zero driving force D-A systems remain inconclusive. Here, ELA and its depth-dependent variation at high performing donor/non-fullerene-acceptor interfaces are studied by fabricating and characterizing D-A quasi-bilayers from sequential spin-coating and D-A planar bilayers from Langmuir-Schäfer layer-by-layer deposition. Significant vacuum level (VL) shifts are found at all of the D-A interfaces, which reduces the interfacial energetic offsets and increases the energies of the charge transfer (CT) states. The VL shifts are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL-shift-enhanced energy gap at the heterojunctions correlate well with the measured charge transfer energy and open circuit voltage of corresponding organic solar cell devices, reconciling the conflicting observations of large energy level offsets inferred from neat films and large charge transfer energies of donor - non-fullerene acceptor systems.

Authors : Huw Shiel, Theodore D. C. Hobson, Oliver S. Hutter, Laurie J. Phillips, Matthew J. Smiles, Leanne A. H. Jones, Thomas J. Featherstone, Jack E. N. Swallow, Pardeep K. Thakur, Tien-Lin Lee, Vin R. Dhanak, Jonathan D. Major, Ken Durose, Timothy D. Veal
Affiliations : Stephenson Institute for Renewable Energy, Department of Physics, University of Liverpool, Liverpool, L69 7ZF, UK Diamond Light Source, Harwell Science & Innovation Campus, Didcot, OX11 0DE, UK

Resume : Band alignments are a critical aspect of PV device performance, with conduction band offsets playing a crucial role in determining quantities such as the open-circuit voltage and short-circuit current of the cell. Photoemission techniques such as XPS and UPS are a popular choice used for measuring band alignments, but a reliable measurement can be difficult when measuring films that are directly comparable to high efficiency devices. Factors such as uneven interfaces, interdiffusion, and surface contamination are often present in PV devices and these are difficult to account for using conventional photoemission methods. Antimony selenide (Sb2Se3) is a promising emerging material for use in photovoltaics (PV). It has excellent optical properties, cheap and earth abundant materials and has improved rapidly in the last 7 years, from 2% to over 9% efficient. Sb2Se3 has an unusual nanoribbon structure which is key to its excellent carrier transport properties, however these nanoribbons must be oriented correctly to achieve good efficiencies. This orientation is extremely sensitive to deposition conditions and the choice of substrate. For this reason, many popular deposition techniques that would allow deposition of ultra-thin films or in-situ deposition during photoemission measurements are not likely to accurately represent a device relevant interface. Furthermore, due to the widespread use of fluorine doped tin oxide (which is extremely rough) as a transparent conducting layer in Sb2Se3 solar cells, depositing layers thin enough to carry out a Kraut method measurement is not possible while achieving complete coverage. Here we present a number of studies carried out on different interfaces within antimony selenide solar cells using hard x-ray photoemission spectroscopy (HAXPES). Using the superior probing depth of HAXPES, samples identical to those used in high-efficiency devices were used for valence band offset measurements employing the Kraut method. The alignment between Sb2Se3 and two different window layers is investigated and compared to the results of band alignment measurements obtained through natural alignment measurements and Anderson’s rule. The results for the CdS were similar for the two methods, however the offset between Sb2Se3 and TiO2 was significantly different for the Kraut method (-0.82 eV) and Anderson’s rule (0.11 eV). Additionally the valence band offset between Sb2Se3 and the back contact contaminant layer of Sb2O3 is measured with the Kraut method (-1.9 eV) and compared to the band alignment measured using the difference spectrum for an oxidised and an in-situ cleaved bulk crystal (-1.72 eV). This study provides an in-depth look at a variety of interfaces within the device structure of one of the most exciting emerging solar cell technologies. It presents measurements of quantities that are vital to the optimisation of Sb2Se3 devices as well as discussion of the advantages and compromises of a number of different approaches.

Authors : Siarhei Zhuk, Sebastian Siol
Affiliations : Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland

Resume : Novel functional materials are required to for the development of next-generation electronic devices. Ternary metal nitrides are considered a promising, but still underexplored, class of materials with many predicted compounds yet to be experimentally synthesized. [1] Combinatorial physical vapor deposition screening an extremely effective way to discover new phases in such complex phase spaces. This is because application of composition and temperature gradients during the deposition process enables to cover wide range of synthesis phase diagram in a single deposition step. [2] Typically, X-ray diffraction (XRD) and X-ray fluorescence mapping techniques are employed in combination to charac-terize the phase formation and composition of combinatorial thin film samples. [3] In this study, combination of X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAXPES) were used to complement accelerated phase discov-ery in Zn-V-N thin films deposited by combinatorial reactive co-sputtering. Using a HAX-PES system equipped with monochromatic Cr-Kα X-ray source (5.41 KeV) allows us to characterize electronic structure and composition up to 20 nm below the surface. [4] In a contrast, an XPS measurement setup using a monochromatic Al-Kα excitation source (1.48 KeV) can probe only up to 10 nm. Thus, results of HAXPES analysis are less affected by the presence of surface oxides or adsorbates, which typically complicate surface analysis of nitrides. For the Zn-V-N material system a new potentially stable phase was discovered by XPS mapping characterization by analysis of the modified Zn Auger parameter (AP). The latter was calculated using the kinetic energy of the Zn LMM Auger line as well as the bind-ing energy of the Zn 2p3/2 core level emission. The Zn AP showed a distinct shift for certain samples, which correlated directly with the occurance of a new phase. This was confirmed using XRD/XRF analysis. In addition, XPS and HAXPES were used to invstigate the chemical state and Fermi level position in the new compound. These results highlight how surface analysis can produce important insights for the discovery of new nitride materials, even when the samples exhibit surface oxides or adsorbates. Specifically, measurements of the Auger parameter can be used to identify new phases and changes in the microstructure in previsouly underexplored material systems. References [1] W. Sun, C.J. Bartel, E. Arca, S.R. Bauers, B. Matthews, B. Orvañanos, B.R. Chen, M.F. Toney, L.T. Schelhas, W. Tumas, J. Tate, A. Zakutayev, S. Lany, A.M. Holder, G. Ceder, Nat. Mater. 18 (2019) 732–739. [2] Y. Han, S. Siol, Q. Zhang, A. Zakutayev, Chem. Mater. 29 (2017) 8239–8248. [3] E. Arca, J.D. Perkins, S. Lany, A. Mis, B.R. Chen, P. Dippo, J.L. Partridge, W. Sun, A. Holder, A.C. Tamboli, M.F. Toney, L.T. Schelhas, G. Ceder, W. Tumas, G. Teeter, A. Zakutayev, Mater. Horizons 6 (2019) 1669–1674. [4] S. Siol, J. Mann, J. Newman, T. Miyayama, K. Watanabe, P. Schmutz, C. Cancellieri, L.P.H. Jeurgens, Surf. Interface Anal. 52 (2020) 802–810.

Authors : Markus Frericks (a;b), Christof Pflumm (c), Eric Mankel (a;b), Thomas Mayer(a;b), Wolfram Jaegermann(a;b)
Affiliations : a) Technical University of Darmstadt, Materials and Earth Sciences, Surface Science Laboratory, Otto-Berndt-Str. 3, 64287 Darmstadt; b) InnovationLab GmbH, Speyerer Str. 4, 69115 Heidelberg; c) Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt;

Resume : For measuring the electronic surface and interface properties, photoelecton spectroscopy is a powerful tool due to its high surface sensitvty and the direct access to the electronic structure. However, at homointerfaces the spectral emission results from the same material in sublayer and adsorption layer. Thus, it is nearly impossible to separate their signal from each other. In the presented work, we approached this challenge at the homointerfaces of organic hole transport materials. The studied hole transport materials are 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA), a common material from literature, and a state-of-the-art triarylamine-based molecule synthesized at Merck KGaA. The inverstigated interfaces combine p-doped and undoped layers of the same hole transport material. We performed in-situ experiments of stepwise layer-by-layer deposition and used X ray as well as ultraviolet photoelectron spectroscopy for characterization. To analyze the obtained spectra, we applied an advanced modelling technique. The model is based on the approximative desciption of the density of states. It allows for a detailed analysis and discussion of interfacial effects beyond the usual practice. As a result of our experiments, we find an unexpected space charge region in the p doped layer. Using our model, we show that interdiffusion of the dopant molecule can be ruled out as explenation for this space charge region. Instead, we present a strongly increased number of states in the undoped layer at the very interface as possible origin for the observed space charge region, which evidently is a result from the deposition process.

Authors : Gaëlle A. L. Andreatta, Agata Lachowicz, Julien Gay, Nicolas Blondiaux, Antonin Faes, Xavier Lefèvre, Leonardo Pires da Veiga, Brett Kamino, Adriana Paracchino
Affiliations : Centre Suisse d'Electronique et de Microtechnique CSEM SA

Resume : Interfacial engineering is recognized as critical in the improvement of processing, performance, stability and even safety of multi-layered systems such as solar cells or lithium-ion batteries. But it is arguably one of the least understood aspect of such systems. Our goal is to investigate how the design of material interfaces in multilayered systems can be used for processing, or performance enhancement, while ensuring that the functionalization methodology is fully characterized and can be easily upscaled. Functional self-assembled monolayers (SAMs) based on phosphonic acids have proved a versatile platform for metals and metal-oxides modifications, with applications ranging from corrosion inhibition [1,2], low-cost metallization of solar cells [3], lithium-ion battery development [4] and high efficiency perovskite solar cells [5,6]. We will show through several applications the importance of the SAMs deposition parameters and chemistries on their functional properties. For photovoltaics (PV) applications, one such example is a novel, simple and cost-effective method for patterning transparent conductive oxides (TCO) in order to form solar cell?s metal grid by copper plating. The investigated chemistry and explored deposition parameters on indium tin oxide have resulted in anti-corrosion surfaces with exceptional coverage of the TCO by the SAM. Deposited layers at optimized processing conditions have demonstrated excellent masking properties for electroplating and methods have been adapted for up-scaled tests done on 6? solar cells. In perovskite-based PV, recent research has shown the potential of SAMs as hole-transporting layers [5,6]. We will show how the deposition parameters affect the bonding of SAMs at the interface and novel characterization results using quartz crystal microbalance, attenuated total reflectance Fourier-transform infrared spectroscopy and wettability measurements. Finally, interfacial layers on key battery interfaces such as cathode/electrolyte is under investigation to improve performance, safety, and sustainability of these complex systems. We will show methods and characterization of the SAMs functionalization of the cathodes materials and how they affect performance and electrochemical properties. [1] R. Quiñones et al. Thin Solid Films, 516 (2008) 8774 [2] C. R. Perkins, J. Phys. Chem. C 113 (2009) 18276 [3] G. A. L. Andreatta et al. Thin Solid Films 691 (2019) 137624 [4] B. G. Nicolau et al. ACS Applied Materials and Interfaces, 5 (2018), 1701292 [5] A. Al-Ashouri et al. Energy Environ. Sci., 2019, 12, 3356--3369 [6] A. Al-Ashouri et al. Science (2020) 370, 1300?1309

Authors : Rhys M. Kennard,# Clayton J. Dahlman,# Ryan A. DeCrescent,^ Jon A. Schuller,~ Kunal Mukherjee,# Ram Seshadri,#* and Michael L. Chabinyc#
Affiliations : # Materials Department, University of California, Santa Barbara ^ Department of Physics, University of California, Santa Barbara ~ Department of Electrical and Computer Engineering, University of California, Santa Barbara * Department of Chemistry and Biochemistry, University of California, Santa Barbara

Resume : Hybrid perovskites are being commercialized for solar cells using roll-to-roll processing, and are attractive for flexible optoelectronics. This raises questions about how bending impacts the structure and stability of thin films. Here, we examine how sub-grain interfaces respond to bending in MAPbI3 (methylammonium lead iodide), the prototypical halide perovskite. MAPbI3 is a ferroelastic, which means that it forms sub-grain domains with identical crystal structure and different crystallographic orientations. These sub-grain domains are separated by interfaces called twin walls, which are known in other materials to contain point defects in higher concentrations than the surrounding domains do. Bending MAPbI3 moves the twin walls, which changes the proportions of the sub-grain domains. Bending MAPbI3 films outwards causes faster degradation to the undesired PbI2 phase. This degradation is correlated with nucleation of new sub-grain domains that form to accommodate the applied strain: domain nucleation also increases the number of twin walls and consequently, the number of point defects. The roles played by twin walls in influencing ion migration, carrier trapping, and degradation are discussed. [1] [1] Kennard, R. M., Dahlman, C. J., DeCrescent, R. A., Schuller, J. A., Mukherjee, K., Seshadri, R., & Chabinyc, M. L. Ferroelastic Hysteresis in Thin Films of Methylammonium Lead Iodide. Chem. Mater., 2020, 33, 298-309.

13:00 Q&A session / Break    
Interfaces and Materials for Energy-Conversion - II : Monica Morales-Masis
Authors : Ute Cappel
Affiliations : KTH - Royal Institute of Technology

Resume : Lead halide perovskites have drastically changed the solar cell research field due to their ease of synthesis and high power conversion efficiencies, which now reach over 25%. Improving stability and understanding degradation pathways in these devices is of high importance for their further development and potential commercialisation. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful tools for obtaining chemical and electronic structure information of material surfaces as well as interfaces. By combining measurements with visible illumination and/or dosing of atmospheric gasses, photo-induced reactions and therefore the stability of materials can be studied in-situ. However, the X-rays themselves used for measurement can also cause changes in the perovskite materials. In this presentation, I will show how we were able to establish the mechanism and kinetics of X-ray induced changes in perovskite materials [1]. When taking these effects into account, it is possible to investigate the electronic and chemical structure of perovskite surfaces and interfaces by photoelectron spectroscopy. By studying clean surfaces of perovskite single crystals, it was possible to establish the electronic structure of pure perovskites. Furthermore, I will show results of studies of the interface formation and interface degradation of a perovskite active layer with metals such as silver [2] and copper. Reactions with these metals can lead to a degradation of the perovskite materials. References: 1 Svanström et al. Phys. Chem. Chem. Phys. 23, 12479–12489 (2021). 2 Svanström et al., ACS Appl. Mater. Interfaces 12, 7212-7221 (2020).

Authors : Regan G. Wilks
Affiliations : Department Interface Design, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany

Resume : Routes to insight-driven device performance optimization can be revealed through characterization and fundamental understanding of the chemical and electronic structures of each component in a multilayer structure as well as their interaction at interfaces. The tailored combination of complementary electron and x-ray spectroscopic studies as well as advanced modelling techniques can be applied to analysis of an endless variety of energy conversion and storage devices. It is particularly useful in the study of complex materials in which composition, process parameters, and environmental effects on the device behavior make them difficult to optimize in a systematic, empirical fashion. This type of advanced characterization can clarify the relationships between macroscale properties by providing a fundamental framework of the processes through which they are determined. To investigate the surface, bulk, and interface properties of energy materials, our research group employs a large toolchest of x-ray spectroscopic techniques. Synchrotron-based techniques such as resonant inelastic x-ray scattering (RIXS) provide a powerful probe of electronic structure and dynamics in materials but must be used with caution and evaluated carefully to separate the desired information from artefacts of the measurement and preparation. In this contribution, a RIXS study of organic structures in different metal halide perovskite thin films will be presented and compared, and the effects of ultrafast dynamics as well as damage by the soft x-rays will be presented. The obtained information will be discussed in the context of the device behavior as well as the results of other characterization techniques.

Authors : Selina Olthof
Affiliations : Institute of Physical Chemistry, University of Cologne, 50939 Germany

Resume : In optoelectronic devices, the function and performance depends crucially on the proper alignment of the energy level landscape throughout the device, allowing for efficient charge transport across the various interfaces. For applications containing halide perovskites as active layer it turned out that such interfaces can show rather complex behavior. On the one hand, interface dipoles and band bending occur. But more importantly, the perovskite composition and formation can be significantly influenced by chemical reactions taking place at these interfaces. In this talk I will summarize our work on a variety of metal-oxides. We use photoelectron spectroscopy to analyze which components are responsible for the strong interface chemistry. For this, we looked at a variety of different perovskites (i.e. organic vs. inorganic ones, I vs. Br, etc.) as well as the individual perovskite precursors. Overall, I will show how photoelectron spectroscopy measurements can help to probe and understand the processes going on at these various bottom contact materials which should ultimately help to improve the stability of perovskite related devices.

Authors : Bojar, A.*(1,3), Marchat, C. (1,3), Alvarez, J.(1,2,3), Alamarguy, D.(1,2), Jaffre, A.(1,2), Guillemoles, J.-F.(3,4), Kleider, J.-P.(1,2,3), Schulz, P.(3,4)
Affiliations : (1) Université Paris-Saclay, CentraleSupélec, CNRS, Laboratoire de Génie Electrique et Electronique de Paris, 91192, Gif-sur-Yvette, France; (2) Sorbonne Université, CNRS, Laboratoire de Génie Electrique et Electronique de Paris, 75252, Paris, France; (3) IPVF, Institut Photovoltaïque d’Ile-de-France, 18, Boulevard Thomas Gobert, 91120 Palaiseau France; (4) CNRS, École Polytechnique, IPVF, UMR 9006, 18, Boulevard Thomas Gobert, 91120 Palaiseau, France; * lead presenter

Resume : Perovskite-silicon tandem solar cells are a promising way for overcoming the single-cell efficiency limit. To date, only a few studies were dedicated to a direct investigation of immediate perovskite-silicon interfaces. While in most tandem solar cell designs, these two materials are not in direct contact, knowledge of the carrier transport and band alignment at their interface would allow for a better understanding of their compatibility and attainable performance levels, guiding the development of perovskite-silicon tandem solar cells in monolithic device architectures with adapted tunnel-recombination junctions between the two sub-cells. In this study, we used Kelvin force probe microscopy and photoemission spectroscopy to study the energetics of the perovskite layer deposited directly on the c-Si substrate of different doping type. Our results reveal a relative shift of the perovskite’s Fermi level solely depending on the doping type of the silicon substrates. We also studied the wavelength-dependent surface photovoltage of these samples, which allowed us to effectively vary the probe depth in the sample and discern the contribution from each interface to the overall effect measured under white light illumination. Depending on where the photocarriers are generated (at the perovskite surface, at the perovskite/Si interface or just in Si), different SPV signals are observed: at the perovskite/Si interface, the signal depends on Si doping type, while at the surface the SPV is always negative indicating downward surface band bending. These results can give insights on the band alignment at the interface, whether it has carrier extracting or blocking properties, and hence, give the guidelines for perovskite/Si monolithic tandem solar cell design.

Authors : Krishanu Dey, Satyaprasad P. Senanayak, Youcheng Zhang, Bart Roose, Ravichandran Shivanna, Weiwei Li, Dibyajyoti Ghosh, Zahra Andaji-Garmaroudi, Judith L. MacManus-Driscoll, Henning Sirringhaus, Sam Stranks
Affiliations : Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK; Los Alamos National Laboratory, New Mexico 87545, United States; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK; Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK / Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB3 0AS, United Kingdom

Resume : Riding on the coat tails of rapid developments in single junction and tandem solar cells, newer applications have also been investigated with metal halide perovskite materials in recent years, including light emitting diodes (LEDs), lasers, photodetectors and hard radiation detectors. This has also inspired the community to explore the field-driven lateral charge transport properties of perovskites for possible implementation in field effect transistors (FETs), which form the bedrock of modern electronics. FETs are three-terminal devices that probe the movement of charges at the interface of semiconducting channel with a dielectric, which can be modulated with voltages applied at the gate and the drain terminals (both w.r.t. the source terminal). Although the literature on perovskite FETs is still limited when compared to other applications, it still throws up interesting insights on the current challenges faced in the field. Firstly, most of the high mobility FETs with 3D perovskite channels contain methylammonium (MA) in its composition, which is known to be thermally unstable and also introduces the additional problem of dipolar disorder which reduces the carrier mobility. Secondly, ionic movement on the application of gate voltage screens the electric field at the interface and thereby result in reduced gate-modulation of carriers, leading to lower mobility at room temperature. The combined effect of these two mechanisms of dipolar disorder and ionic screening is the well-known negative temperature coefficient of mobility observed in perovskite FETs, where highest mobilities are obtained only at low temperatures. Finally, despite the extensive use of 2D tin based perovskites as semiconducting channels in FETs, similar approaches have not yielded results for 3D compositions owing to their semi-metallic nature. In this talk, I will present our work which addresses all these challenges using a simple strategy of alloying tin (Sn) with lead (Pb) in FA-Cs perovskite compositions. We show that incorporation of even a small amount of Sn in FACsPbI3 transforms the channel from n-type to p-type. This is supported by the fact that Sn addition contributes to the generation of additional density of states predominantly at the valence band maximum, thereby p-doping the perovskite. Moreover, there is a monotonic rise in the field-effect mobility as the Sn-content is increased from 0% to 50%, with the champion mobility reaching 5.4 cm2/Vs at room temperature. To the best of our knowledge, this is the highest reported mobility so far for MA-free 3D perovskite thin film FETs. It is important to note that we could not observe any gate modulation for 75% and 100% Sn contents. Since the facile oxidation of Sn2 to Sn4 is a major issue in the field of Sn-based perovskites, I will also show the characterization of Sn4 defects in our films and their effect on the device performance. Most importantly, we see a reversal from negative temperature-coefficient of mobility (for Pb perovskites) to a positive temperature-coefficient of mobility (for mixed Pb-Sn perovskites), which indicate a suppression of ionic screening effect and dipolar disorder. I will discuss our proposed mechanism for this observation, which can possibly be exploited for further developments of perovskite FETs in future. Finally, I will present impressive operational and environmental stability results for mixed Pb-Sn FETs with different A-site cations, which further reinforces the need to go MA-free in future.

16:00 Q&A session / Break    
Advanced Characterization Techniques : Sebastian Siol
Authors : S. David Tilley
Affiliations : University of Zurich, Switzerland

Resume : In this talk, I will discuss two operando techniques for characterizing interfaces in solar water splitting cells. Our photoelectrodes are based on light absorbing, earth abundant photovoltaic materials, and they are typically coated with additional layers for improved photovoltage and/or corrosion protection (so-called "buried junction" cells). I will first discuss the dual working electrode technique, which enables the extraction of a photovoltaic JV curve from the water splitting data by measuring the surface potential of the electrode under operation and thereby deconvolutes the PV and electrocatalytic performance of the photoelectrode. I will then discuss our recent work with impedance spectroscopy, where we are able to assign and investigate the various elements in multilayered water splitting photocathodes under operation. These techniques uncover the limitations and problematic interfaces in practical water splitting cells and can quickly identify targets for performance improvement.

Authors : Glenn Teeter
Affiliations : NREL

Resume : Laboratory-based x-ray and ultraviolet photoelectron spectroscopies (XPS/UPS) are ubiquitous, powerful techniques for analyzing surfaces and interfaces to reveal compositions, chemical states, and electronic properties including work function and interfacial band offsets. Beyond these typical applications, in recent years there has been growing interest around in situ and operando XPS/UPS approaches that can elucidate dynamic interfacial phenomena, including charge- and mass-transfer processes relevant to renewable technologies including photovoltaics (PV) and batteries. This presentation will discuss recent efforts by researchers at the National Renewable Energy Laboratory (NREL) to develop and apply in situ and operando approaches to probe interfacial charge transfer and phase transformations relevant to the formation and evolution of the solid-electrolyte interphase (SEI) in silicon-anode-based lithium-ion batteries (LIBs). These experiments are enabled by the so-called virtual electrode (VE) approach, in which electrochemical currents are driven during VE-XPS measurements on exposed interfaces via a combination of low-energy Li+ ion and electron guns and ultraviolet-based photoemission. In the area of thin-film PV, results will be presented that demonstrate the use of pulsed-light-bias operando XPS (op-XPS) to assess photovoltage decay transients on exposed interfaces in Cd(Se,Te)- and hybrid-perovskite-based solar cells, and correlate these observations with changes in the interfacial properties that accompany stress-induced degradation processes.

Authors : Michael Odelius [1], Chinnathambi Kamal [1,2,3], Cody Sterling [2] , Axel Erbing [2] , Abhijeet Gangan [4]
Affiliations : [1] Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden [2] Theory and Simulations Laboratory, HRDS, Raja Ramanna Centre for Advanced Technology, Indore - 452013, India [3] Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai - 400094, India [4]

Resume : In the presentation, I will discuss methods and results used for realistic modelling of material surfaces and simulations of X-ray spectra. Results include the mixed molecular and dissocative hydration of the TiO2(110) surface [1] and the influence of surfaces and interfaces on the structure and electonic structure in hybrid lead perovskites. Water adsorption on metal oxide surfaces often exhibits a strong coverage and temperature dependence related to different modes of adsorption under different conditions. In particular, dissocation induced by hydrogen bonding can create complex structures. In close collaboration with experiment [1], we have investigated mixed molecular and dissocative hydration of the defect-free TiO2(110) surface and examined the energetics and O1s core-level spectra of different hydrogen bonding motifs. The spectral variations in the experiments at varying coverage and temperature could be rationalized by a clear dependence of the chemical shift in O1s of molecular water to the hydrogen bond environment. Recent developments in the synthesis of hybrid lead perovskites (HLPs) open for studies of clean surfaces of single crystals, adsorption and regular interfaces. These model experiments creates a link between the material aspects of devices and what is possible to model theoretically. Hence we are developing models for open surfaces of HLPs and interfaces between HLPS and metal oxides relevant for processes in solar cell devices. Results on structure and electronic structure are interfaces to experiments through simulations of X-ray spectra. [1] C. Kamal, N. Stenberg, L. E. Walle, D. Ragazzon, A. Borg, P. Uvdal, N. V. Skorodumova, M. Odelius, A. Sandell, Core level binding energy reveals hydrogen bonding configurations of water adsorbed on TiO2(110) surface, Phys. Rev. Lett. 126 (2021) 016102. doi:10.1103/PhysRevLett.126.016102

Authors : Benjamin W. Schmidt, Kateryna Artyushkova, Jennifer E. Mann, John G. Newman, Risayo Inoue, Katsumi Watanabe, Hiromichi Yamazui, Anja Vanleenhove, Thierry Conard
Affiliations : Physical Electronics, 18725 Lake Dr E, Chanhassen, MN 55317, USA (BWS, KA, JEM, JGN);ULVAC-PHI, Inc, 2500 Hagisono, Chigasaki, Kanagawa, 253-8522, Japan (RI, KW, HY);IMEC , Kapeldreef 75, Leuven, 3001 Belgium (AV, TC)

Resume : X-ray Photoelectron Spectroscopy (XPS) is a widely used surface analysis technique with many well established industrial and research applications. The surface sensitivity (top 5-10 nm) of XPS and its ability to provide short-range chemical bonding information make the technique extremely popular in materials characterization and failure analysis laboratories. While its surface sensitivity is an important attribute, in some cases, the depth of analysis of XPS is not sufficient to analyze buried interfaces without first sputter etching the sample surface. However, sputter etching can often lead to alterations of the true surface chemistry. An alternative to sputter etching the sample is Hard X-ray Photoelectron Spectroscopy (HAXPES), available at some synchrotron facilities. By increasing the photon energy of the X-ray source, the mean free path of photoelectrons is increased, resulting in an increased information depth obtained from the sample. Depending on the energy used, these hard X-rays can provide depths of analysis three or more times than that of soft x-rays used on conventional XPS systems. In this presentation, we will present applications of a laboratory-based instrument, the PHI Quantes, for analysis of buried interfaces. The Quantes is equipped with two scanning microprobe monochromated X-ray sources, Al Kα (1486.6 eV) and Cr Kα (5414.8 eV), thus enabling both traditional XPS and HAXPES experiments in the same instrument. Combining both soft and hard X-ray analyses, we can gain an even better understanding of composition with depth and information at buried interfaces. We will demonstrate advances in quantification of HAXPES data and show thickness calculations for multilayered structures using StrataPHI software.

Authors : C. Cancellieri, R. Hauert, F. La Mattina, L. P.H. Jeurgens
Affiliations : Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland

Resume : Hard x-ray photoemission spectroscopy (HAXPES) has the big advantage of probing ma-terials deeper than standard XPS techniques. In this way, buried interfaces can be ac-cessed non-destructively and bulk chemical and electronic properties can be derived. The first commercial XPS/HAXPES systems, equipped with both soft and hard lab-based X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk-sensitive shallow core-levels can be ex-cited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. In this contribution, we will present a combined chemical state analysis of the cations and O anions in Ti and Al oxides thin films (~10-50 nm) produced by dif-ferent deposition methods (i.e. atomic layer deposition, anodization and thermal oxida-tion). By careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources, the Auger parameter (AP) for the cations and ani-ons were derived at various probing depths, i.e. at the outer surface or closer to the metal interface. The depth-resolved AP values for the thin oxide films were compared to their respective bulk crystalline reference phase(s). The resolved shifts in the cation and anion AP values as function of the deposition method /conditions could be linked to changes in the electronic polarizability between the different oxide polymorphs, which originate from tiny differences in the local electronic structures / coordination spheres around the core-ionized cations and anions.

18:30 Q&A session / Closing Remarks    

Symposium organizers
Anna REGOUTZUniversity College London

Department of Chemistry, 20 Gordon Street, London, WC1H 01J, UK
Maria HAHLINUppsala University

Department of Physics and Astronomy, Box 530, 752 20 Uppsala , Sweden

18 Boulevard Thomas Gobert, 91120 Palaiseau, France
Sebastian SIOLEmpa - Swiss Federal Laboratories for Materials Science and Technology

Überlandstrasse 129, 8600 Dübendorf, Switzerland