Heterostructures of transition metal oxides perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial two-dimensional electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm)/STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the electronic structure trend under an increase of the confining potential with using soft X-ray angle-resolved photoemission spectroscopy combined with ab initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3dt2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands.

Hysteresis and memory-effects are an issue for the development of a reliable solar technology based on hybrid perovksites. In this context, we study the effect of surfaces on the electric mobility of charged point-defects at room conditions by molecular dynamics simulations. At variance with the bulk, for which the ionic current is linear and reversible upon inversion of the electric field, we find that the presence of surfaces gives rise to accumulation of charged point-defects and memory effects. Surfaces act as trapping regions for iodine defects with fast accumulation and much slower release kinetics (by at least two orders of magnitude). The calculated release times are of hundreds of millisecond for both vacancies and interstitials at MAI- and PbI2-terminated surfaces, respectively and are consistent with the decays of ionic currents observed in time-resolved Kelvin Probe Force Microscopy experiments. As we do not find ferroelectric polarization and memory effects on ionic currents in perfect crystalline bulks at room conditions, we conclude that surfaces (and likely interfaces or boundaries) are key factors giving rise to hysteresis. Present results are complemented by an analytic model of the effect of surfaces on point defects and of the kinetics of ionic polarization for the design of functional films with better control of hysteretic ionic currents. © 2019 Elsevier Ltd

We study the electronic transport coefficients and the thermoelectric figure of merit ZT in n-doped Mg3Sb2 based on density-functional electronic structure and Bloch-Boltzmann transport theory with an energy- and temperature-dependent relaxation time. Both the lattice and electronic thermal conductivities affect the final ZT significantly, hence we include the lattice thermal conductivity calculated ab initio. Where applicable, our results are in good agreement with existing experiments, thanks to the treatment of lattice thermal conductivity and the improved description of electronic scattering. ZT increases monotonically in our T range (300-700 K), reaching a value of 1.6 at 700 K; it peaks as a function of doping at about 3 x 10(19) cm(-3). At this doping, ZT > 1 for T > 500 K.

We discuss the effects of different approximations to thermal conductivity and relaxation time on the thermoelectric figure of merit ZT in n-doped Mg3Sb2. We use density-functional electronic structure and Bloch-Boltzmann transport theory with an energy-dependent relaxation time. ZT is monotonically increasing with T in our range (300-700 K) with a maximum value of similar to 1.5.

One of the key properties of lead-halide perovskites employed in solar cells is the defect tolerance of the materials, in particular regarding intrinsic point defects, which mainly form shallow traps. Considering that high luminescence yields and photovoltaic performance are obtained by simple solution processing from commercial chemicals, it is commonly anticipated that the defect tolerance - at least to a considerable degree - extends to grain boundaries and extrinsic defects, i.e. impurities, as well. However, the effect of impurities has hardly been investigated. Here, we intentionally introduce small quantities of bismuth (10 ppm to 2%) in solution to be incorporated in the perovskite films based on mixed cation mixed anion compositions. We observe that Bi impurities in the %-regime reduce charge carrier collection efficiency and, more importantly, that the open-circuit voltage decreases systematically with impurity concentration even in the ppm regime. This strong defect intolerance against Bi impurities comes along with reduced electroluminescence yields and charge carrier lifetimes obtained from transient photoluminescence experiments. Calculations based on molecular dynamics and density functional theory predict delocalized (approximate to 0.16 eV) and localized deep (approximate to 0.51 eV) trap states dependent on the structural arrangement of the surrounding atoms. Structural characterization supports the idea of Bi being present as a homogeneously spread point defect, which substitutes the Pb2+ by Bi3+ as seen from XPS and a reduction of the lattice parameter in XRD. Sensitive measurements of the photocurrent (by FTPS) and surface photovoltage (SPV) confirm the presence of tail states. Photoelectron spectroscopy measurements show evidence of a deep state. These results are consistent with the common idea of shallow traps being responsible for the reduced charge collection efficiency and the decreased fill factor, and deeper traps causing a substantial reduction of the open-circuit voltage. As Bi is only one potential impurity in the precursor salts used in perovskite solar cell fabrication, our findings open-up a research direction focusing on identifying and eliminating impurities that act as recombination centers - a topic that has so far not been fully considered in device optimization studies.

Metal halide perovskites are maturing as materials for efficient, yet low cost solar cells and light-emitting diodes, with improving operational stability and reliability. To date however, most perovskite-based devices contain Pb, which poses environmental concerns due to its toxicity; lead-free alternatives are of importance to facilitate the development of perovskite-based devices. Here, the germanium-based Ruddledsen-Popper series (CH3(CH2)3NH3)2(CH3NH3)n-1GenBr3n+1 is investigated, derived from the parent 3D (n = ?) CH3NH3GeBr3 perovskite. Divalent germanium is a promising, nontoxic alternative to Pb2+ and the layered, 2D structure appears promising to bolster light emission, long-term durability, and moisture tolerance. The work, which combines experiments and first principle calculations, highlights that in germanium bromide perovskites the optical bandgap is weakly affected by 2D confinement and the highly stereochemically active 4s2 lone pair preludes to possible ferroelectricity, a topic still debated in Pb-containing compounds.

We show that spin-orbit coupling in methylammonium lead-iodide perovskite generates Fermi surfaces of peculiar topology: for the low charge-injection regime of interest for photovoltaic applications, the Fermi surfaces are donuts (ring tori), which evolve into apples (spindle tori) as the band population is raised, with a vortex spin texture that indicates the dominance of the Rashba effect. This material is a significant example of a bulk system, where despite the lack of any macroscopic field, a strong Rashba effect shows up, originated by a local dipole field on Pb and I orbitals, with the field direction locked to the vortex point loci in k-space. Remarkably, the same Fermi surface topology and spin helicity characterize both electrons and holes: this makes the presence of Rashba compatible with the direct band gap behavior described by photoluminescence experiments.

Surface properties are often assessed with measurements of the contact angle of a water drop. The process is however flawed for the very important class of hybrid perovskite materials, extensively employed in solar cells and optoelectronics research, because they are water soluble and their surface degrades during contact angle measurements. While hybrid perovskites are considered to be highly hydrophilic, a contact angle with water of 83° can be measured, as if they were almost hydrophobic. By combining experiments and simulations, the actual value is explained as the result of the interaction of water with degraded superficial layers that form over sub-millisecond time scale at the water/perovskite interface. The models are validated against contact angle measurements for water on a variety of substrates, and are referenced to with the Young-Dupré relation between liquid-solid adhesion and contact angle. Present work reconciles the hydrophilic nature of methylammonium lead iodide with the apparent hydrophobic behavior in contact angle measurements, proposing a methodology for the study of contact angle on evolving substrates.

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications", which took place within the 2018 European Materials Research Society fall meeting.

Ferroelectric materials are characterized by a spontaneous polar distortion. The behavior of such distortion in the presence of free charge is the key to the physics of metallized ferroelectrics in particular, and of structurally polar metals more generally. Using first-principles simulations, here we show that a polar distortion resists metallization and the attendant suppression of long-range dipolar interactions in the vast majority of a sample of 11 representative ferroelectrics. We identify a meta-screening effect, occurring in the doped compounds as a consequence of the charge rearrangements associated to electrostatic screening, as the main factor determining the survival of a noncentrosymmetric phase. Our findings advance greatly our understanding of the essentials of structurally polar metals, and offer guidelines on the behavior of ferroelectrics upon field-effect charge injection or proximity to conductive device elements. © 2018 American Physical Society.

Dialkali halides are materials of great interest from both fundamental and technological viewpoints, due to their wide transparency range. The accurate determination of their electronic, excitation and optical properties in bulk and low dimensional systems is therefore of crucial importance. Moreover, it is a challenge from the theoretical point of view to deal with quasiparticle band structure calculations for such large energy gap materials, requiring very expensive methods for achieving a desirable accuracy. Here we report electronic quasiparticle band structures for three representative bulk fluorides, BaF2, CaF2 and CdF2, calculated using two low computational cost methods, the DFT-1/2 and the PSIC schemes, which have been relatively little explored by the theoretical community so far. Our results, compared with both available experimental data and previous heavyweight DFT-GW self-energy calculations, demonstrate a satisfactory accuracy for the examined compounds, at a level comparable with the perturbative G0W0 approach. Remarkably, both our proposed methods scale quite similarly to standard local density functional approaches, thus resulting in a large saving of computational effort with respect to the computationally heavyweight GW. Our results open up the perspective of the computational exploration of much bigger fluoride systems. As a significant proof of concept of this capability, we also calculated the quasiparticle properties of the (1 1 1) surfaces of all the three systems under study. Very good agreement with experiment was found. © 2018 IOP Publishing Ltd.

We review the fundamental aspects related to ab-initio band structure calculations for the SrTiO 3/LaAlO 3 interface, analyzing capabilities and limits of the most advanced approaches, using available experiments as a reference. In particular, we discuss accuracy and failures for what concern the description of electronic, transport, and thermoelectric properties of oxide heterostructures. Despite evident shortcomings, our overview assesses the usefulness and the satisfying quality of ab-initio methods as an efficient approach for oxide heterostructure design and analysis. © Springer International Publishing AG, part of Springer Nature 2018.

Tungsten trioxide (WO3) is a versatile material with widespread applications ranging from electrochromics and optoelectronics to water splitting and catalysis of chemical reactions. For technological applications, thin films of WO3 are particularly appealing, taking advantage from a high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this paper, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of WO3 thin films. By performing x-ray diffraction and low-temperature electrical transport measurements, we find changes in the WO3 lattice volume of up to 10% concomitantly with a resistivity drop of more than five orders of magnitude at room temperature as a function of increased oxygen deficiency. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of WO3 single-crystal thin films can all be purposely tuned by controlling the oxygen vacancy formation during growth.

Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La1-xSrxMnO3/LaAlO3/SrTiO3 (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La1-xSrxMnO3 layer into the 2DEG (two-dimensional electron gas) at the LaAlO3/SrTiO3 interface. By out-of-plane transport measurements, the tunneling across the LaAlO3 barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 x 10(13) cm(-2) corresponding to a magnetic moment of 0.038 mu(B) per Ti atom, tightly confined within the single SrTiO3 layer adjacent to LaAlO3. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La1-xSrxMnO3 dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La1-xSrxMnO3 epilayers grown on-top of LaAlO3/SrTiO3 is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells.

Controlling magnetism by using electric fields is a goal of research towards novel spintronic devices and future nanoelectronics. For this reason, multiferroic heterostructures attract much interest. Here we provide experimental evidence, and supporting density functional theory analysis, of a transition in La0.65Sr0.35MnO3 thin film to a stable ferromagnetic phase, that is induced by the structural and strain properties of the ferroelectric BaTiO3 (BTO) substrate, which can be modified by applying external electric fields. X-ray magnetic circular dichroism measurements on Mn L edges with a synchrotron radiation show, in fact, two magnetic transitions as a function of temperature that correspond to structural changes of the BTO substrate. We also show that ferromagnetism, absent in the pristine condition at room temperature, can be established by electrically switching the BTO ferroelectric domains in the out-of-plane direction. The present results confirm that electrically induced strain can be exploited to control magnetism in multiferroic oxide heterostructures.

Perovskites are a class of recently discovered crystals with a multitude of innovative applications. In particular, a lead role is played by organic-inorganic halide perovskites (OIHPs) in solar devices. In 2013 Science and Nature selected perovskite solar cells as one of the biggest scientific breakthroughs of that year. This book provides the first comprehensive account of theoretical aspects of perovskite solar cells, starting at an introductory level but covering the latest cutting-edge research. Theoretical Modeling of Organohalide Perovskites for Photovoltaic Applications aims to provide a theoretical standpoint on OIHPs and on their photovoltaic applications, with particular focus on the issues that are still limiting their usage in solar cells. This book explores the role that organic cations and defects play in the material properties of OIHPs and their effects on the final device, in addition to discussing the electric properties of OIHPs; the environmentally friendly alternatives to the use of lead in their structural and electronic properties; theoretical screening for OIHP-related material for solar-to-energy conversion; and the nature and the behavior of quasiparticles in OIHPs.

Perovskites are a class of recently discovered crystals with a multitude of innovative applications. In particular, a lead role is played by organic-inorganic halide perovskites (OIHPs) in solar devices. In 2013 Science and Nature selected perovskite solar cells as one of the biggest scientific breakthroughs of that year. This book provides the first comprehensive account of theoretical aspects of perovskite solar cells, starting at an introductory level but covering the latest cutting-edge research. Theoretical Modeling of Organohalide Perovskites for Photovoltaic Applications aims to provide a theoretical standpoint on OIHPs and on their photovoltaic applications, with particular focus on the issues that are still limiting their usage in solar cells. This book explores the role that organic cations and defects play in the material properties of OIHPs and their effects on the final device, in addition to discussing the electric properties of OIHPs; the environmentally friendly alternatives to the use of lead in their structural and electronic properties; theoretical screening for OIHP-related material for solar-to-energy conversion; and the nature and the behavior of quasiparticles in OIHPs.

Using ab initio band energy calculations and van Roosbroeck-Shockley recombination theory, we model transient photoluminescence for lead-iodide CH3NH3PbI3 perovskites. We provide clear evidence that the most important features of the photoluminescence process, i.e. strong absorption, low recombination rates, and long lifetimes, can be all coherently derived from the band-to-band recombination process, and the sometimes invoked contradiction between strong absorption and low recombination rate in a direct band-gap material has no reason to exist; optical gain and charge carrier lasing threshold are also reproduced in satisfactory agreement with the observed values, at least within a significant charge-injection range. Our description provides a solid theoretical assessment to the widespread but debated idea that photoluminescence in these materials is predominantly produced by unbound charge recombination.

The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics.

Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DESs), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO3/SrTiO3 interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO3 as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility. In this work, we use an amorphous WO3 overlayer to obtain a high-mobility 2DES in WO3/LaAlO3/SrTiO3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO3 and WO3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm(2) s(-1), and quantum oscillations of conductance.