Daniel Mierwaldt

Electronic structure of Pr1−xCaxMnO3

The electronic structure of

An endstation for resonant inelastic X-ray scattering studies of solid and liquid samples.

{\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}

ETEM Studies of Electrodes and Electro-catalysts

has been investigated using a combination of first-principles calculations, x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions,

Role of oxygen vacancies for resistive switching in noble metal sandwiched Pr0.67Ca0.33MnO3-δ

x=0,1/2,1

Developing an in situ environmental TEM set up for investigations of resistive switching mechanisms in Pt-Pr1-xCaxMnO3-δ-Pt sandwich structures

, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study have been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the co-existence of magnetic orders (respectively, to the occupation of local orbitals). A consistent treatment of the Coulomb interaction indicates a partial cancellation of Coulomb parameters and supports the dominance of the electron-phonon coupling.

Ex-Situ Analysis and In-Situ Environmental TEM Studies of Manganite Perovskites for Catalytic Water Splitting

A novel experimental setup is presented for resonant inelastic X-ray scattering investigations of solid and liquid samples in the soft X-ray region for studying the complex electronic configuration of (bio)chemical systems. The uniqueness of the apparatus is its high flexibility combined with optimal energy resolution and energy range ratio. The apparatus enables investigation of chemical analyses, which reflects the chemical imprints. The endstation is composed of a main sample chamber, a sample holder for either solid or liquid jet delivery system, and a soft X-ray grating spectrometer for 210-1250 eV with a resolving power of ∼1000. It combines for the first time liquid jet technology with a soft X-ray spectrometer based on the variable line spacing principle. This setup was commissioned at the soft X-ray beamline P04 at PETRA III of the Deutsches Elektronen-Synchrotron in Hamburg which is currently the most brilliant storage-ring-based X-ray radiation source in the world. The first results of liquid and solid samples show that this setup allows the detection of photons across an energy range of ∼300 eV. This covers simultaneously the emission lines of life-important elements like carbon, nitrogen and oxygen in a shot-based procedure.

In Situ Electrochemical Electron Microscopy Study of Oxygen Evolution Activity of Doped Manganite Perovskites

Environmental TEM is an excellent tool for gaining insight into the atomic and electronic structure of electro-catalysts under operating conditions. Several electrochemical reactions such as oxidation/reduction processes of electrodes, heterogeneous gas phase catalysis of water splitting/oxygen evolution and electrochemical corrosion processes of materials have been studied in some pioneering experiments which will be summarized in this chapter. These experiments often reveal a strong change of the electrode due to the adsorption of gas species from the environment as well as due to the impact of the electron beam. We show that inelastic scattering of the high-energy electrons can induce electric potentials in the studied samples influencing the observed state of the catalyst. After an introduction to electrochemistry and ETEM investigations, we address, experimentally and theoretically, beam-induced potentials, their dependence on several parameters such as electron flux, electric conductivity, and geometry of samples, aiming at learning how to disentangle them from radiation damage. Our second focus is to control the electric potential distribution within and around samples by dedicated electrical TEM sample holders. To illustrate how this can be achieved, we present the results of a bias-controlled electro-corrosion experiment. We will discuss some of the main experimental and theoretical challenges for the development of controlled electrochemistry studies in transmission electron microscopes.

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

Non-volatile resistance change under electric stimulation in oxides is a promising path to next generation memory devices. However, the underlying mechanisms are still not fully understood. We report here on the study of switching in Pr0.67Ca0.33MnO3-δ (PCMO) films sandwiched by noble metal Pt electrodes, where electrode oxidation can be excluded. In order to develop an understanding of the switching induced oxygen migration, its initial concentration is modified by post-annealing of the deposited PCMO films. The oxygen distribution is obtained by manganese valence determination using spatially resolved electron energy loss spectroscopy in scanning transmission electron microscopy mode. We observe correlations between virgin state resistance, resistive switching properties, oxygen vacancy distribution, and stress/strain state of the PCMO films and propose a simplified interface resistance model based on the measured valence distribution. It assumes a linear correlation of oxygen vacancy concentration with...

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Non-volatile resistance change under electric stimulation in many metal-oxides is a promising path to next generation memory devices. However, the underlying mechanisms are still not fully understood. In situ transmission electron microscopy experiments provide a powerful tool to elucidate these mechanisms. In this contribution, we demonstrate a TEM lamella geometry for in situ biasing with two fixed electrode contacts ensuring low and stable contact resistances. We use Pr1-xCaxMnO3-δ sandwiched by Pt electrodes as model system. The evolution of manganese valence state during electric stimulation in different environments is mapped by means of electron energy loss spectroscopy with high spatial resolution in STEM. Correlation of Mn valence with local oxygen content is found. In addition to electrically driven switching, beam-induced redox reactions in oxygen environment are observed. This effect might be restricted to thin lamellae. In general, our results support that bulk oxygen electromigration is the relevant mechanism for non-volatile resistive switching in PCMO.

Environmental TEM Investigation of Electrochemical Stability of Perovskite and Ruddlesden-Popper Type Manganite Oxygen Evolution Catalysts

Catalytic water splitting, i.e. the electrochemical decomposition of water into molecular hydrogen and oxygen allows for conversion and storage of solar or electrical energy in chemical energy and, thus, represents an important contribution to the development of renewable energy sources. The oxygen evolution half reaction 2H2O+4h4H + +O2 is the bottleneck of water-splitting since it requires the coupled transfer of 4 electron holes from the catalyst surface to the water molecule. This half reaction was comprehensively studied on perovskite catalysts [1], because they provide high chemical flexibility and the opportunity to selectively tune and adjust the electronic structure to the water oxidation potential by Aand B-site doping. Furthermore, electronic and electron-lattice correlations influence the electronic and crystal structure and may result in the formation of polaronic charge carriers, well known in particular for manganites. However, the impact of such correlation effects on catalytic activity is not yet understood.