The relation among structure, electric transport, and photovoltaic effect is investigated for a $\mathrm{pn}$ heterojunction with strong correlation interactions. A perovskite interface is chosen as a model system consisting of the $p$-doped strongly correlated manganite ${\mathrm{Pr}}_{0.64}{\mathrm{Ca}}_{0.36}{\mathrm{MnO}}_{3}$ (PCMO) and the $n$-doped titanate SrTi${}_{1\ensuremath{-}y}$Nb${}_{y}$O${}_{3}$ ($y=0.002$ and 0.01). High-resolution electron microscopy and spectroscopy reveal a nearly dislocation-free, epitaxial interface and give insight into the local atomic and electronic structure. The presence of a photovoltaic effect under visible light at room temperature suggests the existence of mobile excited polarons within the band-gap-free PCMO absorber. The temperature-dependent rectifying current-voltage characteristics prove to be mainly determined by the presence of an interfacial energy spike in the conduction band and are affected by the colossal electroresistance effect. From the comparison of photocurrents and spatiotemporal distributions of photogenerated carriers (deduced from optical absorption spectroscopy), we discuss the range of the excited polaron diffusion length.
Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization will thus be of great significance for efficient light energy conversion, e.g., in photovoltaic devices. Ultrafast optical pump–probe experiments in the manganite Pr 0.65 Ca 0.35 MnO 3, a photovoltaic, thermoelectric, and electrocatalytic material with strong polaronic correlations, reveal an ultraslow recombination dynamics on a nanosecond‐time scale. The nature of long living excitations is further elucidated by photovoltaic measurements, showing the presence of photodiffusion of excited electron–hole polaron pairs. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. Escape from this state is possible via a slow dipole‐forbidden recombination process or via rare thermal fluctuations toward a conical intersection followed by a radiation‐less decay. The strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron.
Abstract Despite significant advancements in materials design for renewable energy devices, the fundamental understanding of the underlying processes in many materials remains limited, particularly in complex, inhomogeneous systems and interfaces. In such cases, in situ studies with high spatial and energy resolution are essential for uncovering new insights into excitation, dissipation, and conversion processes. Recent progress in in situ atomic scale methods has greatly enhanced the understanding of energy materials. Here, key advances are reviewed, including in situ, environmental and ultra‐fast transmission electron microscopy, scanning probe techniques, single‐photon‐resolved infrared spectroscopy, velocity‐resolved molecular kinetics, and in situ grazing‐incidence X‐ray spectroscopy. These techniques enable the study of energy conversion with spatial resolution from nanometers down to individual atoms, energy resolution down to meV, and single‐quantum detection. Especially they enable access to processes that involve multiple degrees of freedom, strong coupling, or spatial inhomogeneities. They have driven a qualitative leap in the fundamental understanding of energy conversion processes, opening new avenues for improving existing materials and designing novel clean and efficient energy materials in photovoltaics, friction, and surface chemistry and (photo‐)electrochemistry.
Studying the oxygen evolution reaction (OER) on anisotropic perovskite oxides with different surface orientations improves our understanding of how surface structure, chemistry, catalytic stability, and activity are related. Here, we present a comparative electrochemical study of (001)- and (010)-oriented epitaxial thin films of Ruddlesden–Popper (n = 1) Pr0.5Ca1.5MnO4 (RP-PCMO) grown on Nb:SrTiO3 (STNO) substrates of different orientations with using buffer layers. The results on epitaxial films are compared with those of the RP-PCMO powder. The OER activity and stability are studied using cyclic voltammetry and rotating ring disk electrodes under alkaline conditions. In addition, an analysis of their surface structure and chemistry by AFM and XPS before and after electrochemical measurements was carried out. The RP-PCMO powder shows high stability during electrochemical cycling. In contrast, the two differently oriented thin films are both corroding, while the (010)-oriented films reveal a higher activity and stability than the (001)-oriented ones. The variation in electrochemical activity and stability of the polycrystalline and single-crystalline RP-PCMO electrodes of different orientations is related to their different surface chemistry. In particular, it depends on the different Mn/Pr/Ca ratios and their different rates of Ca leaching, which is governed by the orientation-dependent Ca hydroxide surface concentration.
X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La 0.6 Sr 0.4 MnO 3 by analysis of three emission lines simultaneously at the detector, namely the O 2 p –1 s , Mn 3 s –2 p and Mn 3 d –2 p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3 s –2 p and Mn 3 d –2 p transitions when scanning the Mn L -edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3 s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.
Optically induced phase transitions of the manganite ${\mathrm{Pr}}_{1/3}{\mathrm{Ca}}_{2/3}\mathrm{Mn}{\mathrm{O}}_{3}$ have been simulated by using a model Hamiltonian that captures the dynamics of strongly correlated charge, orbital, lattice, and spin degrees of freedom. Its parameters have been extracted from first-principles calculations. Beyond a critical intensity of a femtosecond light pulse, the material undergoes an ultrafast and nonthermal magnetic phase transition from a noncollinear to collinear antiferromagnetic phase. The light-pulse excites selectively either a spin-nematic or a ferroelectric phase, depending on the light polarization. The behavior can be traced to an optically induced ferromagnetic coupling between Mn trimers, i.e., polarons which are delocalized over three Mn sites. The polarization guides the polymerization of the polaronic crystal into distinct patterns of ferromagnetic chains determining the target phase.
Understanding the influence of vibrational degrees of freedom on transport through a heterostructure poses considerable theoretical and numerical challenges. In this work, we use the density-matrix renormalization group (DMRG) method together with local basis optimization (LBO) to study the half-filled Holstein model in the presence of a linear potential, either isolated or coupled to tight-binding leads. In both cases, we observe a decay of charge-density-wave (CDW) states at a sufficiently strong potential strength. Local basis optimization selects the most important linear combinations of local oscillator states to span the local phonon space. These states are referred to as optimal modes. We show that many of these local optimal modes are needed to capture the dynamics of the decay, that the most significant optimal mode on the initially occupied sites remains well described by a coherent-state typical for small polarons, and that those on the initially empty sites deviate from the coherent-state form. Additionally, we compute the current through the structure in the metallic regime as a function of voltage. For small voltages, we reproduce results for the Luttinger parameters. As the voltage is increased, the effect of larger electron-phonon coupling strengths becomes prominent. Further, the most significant optimal mode remains almost unchanged when going from the ground state to the current-carrying state in the metallic regime.
The rich phase diagram of bulk Pr1-x Cax MnO3 resulting in a high tunability of physical properties gives rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo-cubic transition for x = 0.1 is studied in ion-beam sputtered thin films and differences to the respective bulk system are pointed out by employing in situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, that is, preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non-equilibrium conditions.
Investigating the interfaces between electrolytes and electrocatalysts during electrochemical water oxidation is of tremendous importance for an understanding of the factors influencing catalytic activity and stability. Here, the interaction of a wellestablished, nanocrystalline and mesoporous Ca-birnessite catalyst material (initial composition K0.2Ca0.21MnO2.21·1.4 H2O, initial Mn-Oxidation state ~+3.8) with an aqueous potassium phosphate buffer electrolyte at pH 7 was studied by using various electron microscopy and spectroscopy techniques. In comparison to electrolyte solutions not containing phosphate, Ca-birnessite electrodes show especially high and stable oxygen evolution activity in phosphate buffer. During electrolysis, partial ion substitutions of Ca2+ by K + and OH- / O 2- by HnPO4 (3-n)- were observed, leading to the formation of a stable, partially disordered Ca-K-Mn-HnPO4-H2O layer on the outer and the pore surfaces of the electrocatalyst. In this surface layer, Mn(III) ions are stabilized, which are often assumed to be of key importance for oxygen evolution catalysis. Furthermore, evidence for the formation of [Ca/PO4/H2O]- complexes located between the [MnO6] layers of the birnessite was found using Ca 2p and Ca L-edge the soft X-ray synchrotron-based spectroscopy. A possible way to interpret the obviously very favorable, “special relationship” between (hydrogen)phosphates and Ca-birnessites in electrocatalytic water oxidation would be that HnPO4 (3-n)- anions are incorporated into the catalyst material where they act as stabilizing units for Mn3+ centers and also as “internal bases” for the protons released during the reaction.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)