Organolead halide perovskites currently are the new front‐runners as light absorbers in hybrid solar cells, as they combine efficiencies passing already 20% with deposition temperatures below 100 °C ...and cheap solution‐based fabrication routes. Long‐term stability remains a major obstacle for application on an industrial scale. Here, it is demonstrated that significant decomposition effects already occur during annealing of a methylammonium lead triiode perovskite at 85 °C even in inert atmosphere thus violating international standards. The observed behavior supports the view of currently used perovskite materials as soft matter systems with low formation energies, thus representing a major bottleneck for their application, especially in countries with high average temperatures. This result can trigger a broader search for new perovskite families with improved thermal stability.
It is shown that significant degradation effects of CH3NH3PbI3 already occur at 85 °C, independent of the atmosphere, where humidity is a catalyst but not the origin of the degradation. This represents an important message for the perovskite community and leads to recommendations for future perovskite developments.
Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize ...their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.
Abstract
In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon ...platform. Here, we present asymmetric LaMnO
3
/BaTiO
3
/SrTiO
3
superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize
c
-axis oriented BaTiO
3
layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO
3
single film and conventional BaTiO
3
/SrTiO
3
superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO
3
as the thickness of BaTiO
3
increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.
Tin‐based perovskites are promising alternative absorber materials for lead‐free perovskite solar cells but need strategies to avoid fast tin (Sn) oxidation. Generally, this reaction can be slowed ...down by the addition of tin fluoride (SnF2) to the perovskite precursor solution, which also improves the perovskite layer morphology. Here, this work analyzes the spatial distribution of the additive within formamidinium tin triiodide (FASnI3) films deposited on top of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layers. Employing time‐of‐flight secondary ion mass spectrometry and a combination of hard and soft X‐ray photoelectron spectroscopy, it is found that SnF2 preferably accumulates at the PEDOT:PSS/perovskite interface, accompanied by the formation of an ultrathin SnS interlayer with an effective thickness of ≈1.2 nm.
The impact of the commonly used tin fluoride (SnF2) additive in Sn‐based perovskite solar cells on the poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/perovskite interface are analyzed. SnF2 is found to preferably precipitate at this interface where it forms a SnS interlayer of approximately 1.2 nm thickness induced by a chemical reaction with sulfur‐containing groups at the PEDOT:PSS surface.
Van der Waals (vdW) heterostructures continue to attract intense interest as a route of designing materials with novel properties that cannot be found in nature. Unfortunately, this approach is ...currently limited to only a few layers that can be stacked on top of each other. Here, we report a bulk vdW material consisting of superconducting 1H TaS2 monolayers interlayered with 1T TaS2 monolayers displaying charge density waves (CDW). This bulk vdW heterostructure is created by phase transition of 1T-TaS2 to 6R at 800 °C in an inert atmosphere. Its superconducting transition (T c) is found at 2.6 K, exceeding the T c of the bulk 2H phase. Using first-principles calculations, we argue that the coexistence of superconductivity and CDW within 6R-TaS2 stems from amalgamation of the properties of adjacent 1H and 1T monolayers, where the former dominates the superconducting state and the latter the CDW behavior.
We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in ...semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of 5.3±1.5 MV/cm is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.
Ferroelectric domain formation is an essential feature in ferroelectric thin films. These domains and domain walls can be manipulated depending on the growth conditions. In rhombohedral BiFeO3 thin ...films, the ordering of the domains and the presence of specific types of domain walls play a crucial role in attaining unique ferroelectric and magnetic properties. In this study, controlled ordering of domains in BiFeO3 film is presented, as well as a controlled selectivity between two types of domain walls is presented, i.e., 71° and 109°, by modifying the substrate termination. The experiments on two different substrates, namely SrTiO3 and TbScO3, strongly indicate that the domain selectivity is determined by the growth kinetics of the initial BiFeO3 layers.
Growth of the BiFeO3 thin films is shown to depend on the atomic termination of the SrTiO3 and TbScO3 substrates. Interface engineering with a single SrO layer on substrate surfaces enables the control of domain ordering and selectivity between 71° and 109° domain walls. Underlying mechanism is claimed to be growth dynamic during the initial phase of growth.
Lead based bulk piezoelectric materials, e.g., PbZrxTi1‐xO3 (PZT), are widely used in electromechanical applications, sensors, and transducers, for which optimally performing thin films are needed. ...The results of a multi‐domain Landau–Ginzberg‐Devonshire model applicable to clamped ferroelectric thin films are used to predict the lattice symmetry and properties of clamped PZT thin films on different substrates. Guided by the thermal strain phase diagrams that are produced by this model, experimentally structural transitions are observed. These can be related to changes of the piezoelectric properties in PZT(x = 0.6) thin films that are grown on CaF2, SrTiO3 (STO) and 70% PbMg1/3Nb2/3O3‐30% PbTiO3 (PMN‐PT) substrates by pulsed laser deposition. Through temperature en field dependent in situ X‐ray reciprocal space mapping (RSMs) and piezoelectric force microscopy (PFM), the low symmetry monoclinic phase and polarization rotation are observed in the film on STO and can be linked to the measured enhanced properties. The study identifies a monoclinic ‐rhombohedral MC‐MA‐R crystal symmetry path as the polarization rotation mechanism. The films on CaF2 and PMN‐PT remain in the same symmetry phase up to the ferroelectric‐paraelectric phase transition, as predicted. These results support the validity of the multi‐domain model which provides the possibility to predict the behavior of clamped, piezoelectric PZT thin films, and design films with enhanced properties.
Different symmetry phases are identified in PbZr0.6Ti0.4O3 ferroelectric thin films as function of the strain introduced by CaF2, SrTiO3 and PMN‐PT substrates. Measured strain‐temperature paths are successfully described by a multi‐domain LGD‐model that predicts the symmetry phases and phase transitions, and the observed enhanced piezoelectricity in one of the phases.
In this paper, with the example of two different polymorphs of KEu(MoO4)2, the influence of the ordering of the A-cations on the luminescent properties in scheelite related compounds (A′,A″) n ...(B′,B″)O4 m is investigated. The polymorphs were synthesized using a solid state method. The study confirmed the existence of only two polymorphic forms at annealing temperature range 923–1203 K and ambient pressure: a low temperature anorthic α-phase and a monoclinic high temperature β-phase with an incommensurately modulated structure. The structures of both polymorphs were solved using transmission electron microscopy and refined from synchrotron powder X-ray diffraction data. The monoclinic β-KEu(MoO4)2 has a (3+1)-dimensional incommensurately modulated structure (superspace group I2/b(αβ0)00, a = 5.52645(4) Å, b = 5.28277(4) Å, c = 11.73797(8) Å, γ = 91.2189(4)o, q = 0.56821(2)a*–0.12388(3)b*), whereas the anorthic α-phase is (3+1)-dimensional commensurately modulated (superspace group I1̅(αβγ)0, a = 5.58727(22) Å, b = 5.29188(18)Å, c = 11.7120(4) Å, α = 90.485(3)o, β = 88.074(3)o, γ = 91.0270(23)o, q = 1/2a* + 1/2c*). In both cases the modulation arises due to Eu/K cation ordering at the A site: the formation of a 2-dimensional Eu3+ network is characteristic for the α-phase, while a 3-dimensional Eu3+-framework is observed for the β-phase structure. The luminescent properties of KEu(MoO4)2 samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed.
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