Strong Anharmonicity at the Origin of Anomalous Thermal Conductivity in Double Perovskite Cs2 NaYbCl6.

Anomalous thermal transport of Cs2 NaYbCl6 double-halide perovskite above room temperature is reported and rationalized. Calculations of phonon dispersion relations and scattering rates up to the fourth order in lattice anharmonicity have been conducted to determine their effective dependence on temperature. These findings show that specific phonon group velocities and lifetimes increase if the temperature is raised above 500 K. This, in combination with anharmonicity, provides the microscopic mechanism responsible for the increase in lattice thermal conductivity at high temperatures, contrary to the predictions of phonon transport theories based on solely cubic anharmonicity. The model accurately and quantitatively reproduces the experimental thermal conductivity data as a function of temperature.

Terbium and Europium Chlorocyananilate-Based 2D Coordination Polymers.

Two-dimensional layered coordination polymers based on the hetero-substituted 3-chloro-6-cyano-2,5-dihydroxybenzoquinone ligands, hereafter ClCNAn2- anilate, and LnIII ions (Tb and Eu) are reported. Compounds 1 and 2, formulated as Ln2(ClCNAn)3(DMSO)6 (LnIII = Tb, 1; Eu, 2), and their related intermediates 1' and 2', formulated as Ln2(ClCNAn)3(H2O)x·yH2O (x + y likely = 12, Ln = Tb, 1'; and Eu, 2'), were prepared by a conventional one-pot reaction (the latter) and recrystallized from DMSO solvent (the former). Polyhydrated intermediates 1' and 2' show very similar XRPD patterns, while, despite their common stoichiometry, 1 and 2 are not isostructural. Compound 1 consists of a 2D coordination framework of 3,6 topology, where [Tb(DMSO)3]III moieties are bridged by three bis-chelating ClCNAn2- ligands, forming distorted hexagons. Ultrathin nanosheets of 1 were obtained by exfoliation via the liquid-assisted sonication method and characterized by atomic force microscopy, confirming the 2D nature of 1. The crystal structure of 2, still showing the presence of 2D sheets with a "hexagonal" mesh and a common (3,6) connectivity, is based onto flat, non-corrugated slabs. Indeed, at a larger scale, the different "rectangular tiles" show clear roofing in 1, which is totally absent in 2. The magnetic behavior of 1 very likely indicates depopulation of the highest crystal-field levels, as expected for TbIII compounds.

Exciton dissociation in 2D layered metal-halide perovskites.

Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA2PbI4 (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

Stabilization of Inorganic Perovskite Solar Cells with a 2D Dion-Jacobson Passivating Layer.

Inorganic metal halide perovskites such as CsPbI3 are promising for high-performance, reproducible, and robust solar cells. However, inorganic perovskites are sensitive to humidity, which causes the transformation from the black phase to the yellow δ, non-perovskite phase. Such phase instability has been a significant challenge to long-term operational stability. Here, a surface dimensionality reduction strategy is reported, using 2-(4-aminophenyl)ethylamine cation to construct a Dion-Jacobson 2D phase that covers the surface of the 3D inorganic perovskite structure. The Dion-Jacobson layer mainly grows at the grain boundaries of the perovskite, effectively passivating surface defects and providing favourable interfacial charge transfer. The resulting inorganic perovskite films exhibit excellent humidity resistance when submerged in an aqueous solution (isopropanol:water = 4:1 v/v) and exposed to a 50% humidity air atmosphere. The Dion-Jacobson 2D/3D inorganic perovskite solar cell (PSC) achieves a power conversion efficiency (PCE) of 19.5% with a Voc of 1.197 eV. It retains 83% of its initial PCE after 1260 h of maximum power point tracking under 1.2 sun illumination. The work demonstrates an effective way for stabilizing efficient inorganic perovskite solar cells.

Solid-State Nuclear Magnetic Resonance of Triple-Cation Mixed-Halide Perovskites.

Mixed-cation lead mixed-halide perovskites are the best candidates for perovskite-based photovoltaics, thanks to their higher efficiency and stability compared to the single-cation single-halide parent compounds. TripleMix (Cs0.05MA0.14FA0.81PbI2.55Br0.45 with FA = formamidinium and MA = methylammonium) is one of the most efficient and stable mixed perovskites for single-junction solar cells. The microscopic reasons why triple-cation perovskites perform so well are still under debate. In this work, we investigated the structure and dynamics of TripleMix by exploiting multinuclear solid-state nuclear magnetic resonance (SSNMR), which can provide this information at a level of detail not accessible by other techniques. 133Cs, 13C, 1H, and 207Pb SSNMR spectra confirmed the inclusion of all ions in the perovskite, without phase segregation. Complementary measurements showed a peculiar longitudinal relaxation behavior for the 1H and 207Pb nuclei in TripleMix with respect to single-cation single-halide perovskites, suggesting slower dynamics of both organic cations and halide anions, possibly related to the high photovoltaic performances.

Combined Experimental/Theoretical Study on the Luminescent Properties of Homoleptic/Heteroleptic Erbium(III) Anilate-Based 2D Coordination Polymers.

The synthesis, structural and photophysical characterization, and theoretical studies on homo/heteroleptic neutral 2D-layered coordination polymers (CPs), obtained by combining the ErIII ion with chlorocyananilate (ClCNAn) and/or tetrafluoroterephthalate (F4BDC) linkers, are herein reported. The structure of the heteroleptic ErIII-based CP, formulated as [Er2(ClCNAn)2(F4BDC)(DMSO)6]n (1) is also reported. 1 crystallizes in the triclinic P1̅ space group, and the structure consists of neutral 2D layers formed by ErIII ions linked through the two linkers oriented in such a way that the neighboring 2D layers are eclipsed along the a axis, leading to parallelogram-like cavities. Photophysical measurements highlight the prominent role of chlorocyananilate linkers as optical antennas toward lanthanide ions, while wave-function-theory analysis supports the experimental findings, providing evidence for the effect of ligand substitution on the luminescence properties of homo/heteroleptic 2D CPs.

Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH3NH3PbBr3 ferroelastic single crystals.

Hybrid lead halides CH3NH3PbX3 (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled. Here, we investigate, using a combined experimental and theoretical approach, the possible presence of electric polarization in tetragonal and orthorhombic CH3NH3PbBr3 (T-MAPB and O-MAPB). We found that T-MAPB does not sustain spontaneous polarization but, under an external electric field, it is projected into a metastable, ionic space-charge electret state. The electret can be frozen on cooling, producing a large and long-lasting polarization in O-MAPB. Molecular dynamics simulations show that the ferroelastic domain boundaries are able to trap charges and segregate ionic point defects, thus playing a favorable role in the stabilization of the electret. At lower temperatures, the lack of ferroelectric behavior is explained using first principles calculations as the result of the tight competition among many metastable states with randomly oriented polarization; this large configurational entropy does not allow a single polar state to dominate at any significant temperature range.

Silicon-based fluorescent platforms for copper(ii) detection in water.

The potential of silicon-based fluorescent platforms for the detection of trace toxic metal ions was investigated in an aqueous environment. To this aim, silicon chips were first functionalized with amino groups, and fluorescein organic dyes, used as sensing molecules, were then covalently linked to the surface via formation of thiourea groups. The obtained hybrid heterostructures exhibited high sensitivity and selectivity towards copper(ii), a limit of detection compatible with the recommended upper limits for copper in drinking water, and good reversibility using a standard metal-chelating agent. The fluorophore-analyte interaction mechanism at the basis of the reported fluorescence quenching, as well as the potential of performance improvement, were also studied. The herein presented sensing architecture allows, in principle, tailoring of the selectivity towards other metal ions by proper fluorophore selection, and provides a favorable outlook for integration of fluorescent chemosensors with silicon photonics technology.

Cystic echinococcosis in the endemic island of Sardinia (Italy): has something changed?

Cystic echinococcosis (CE) is a disease of livestock and humans caused by the larval stage of the Echinococcus granulosus sensu lato (s.l.) species complex and occurs especially in areas where sheep breeding is widespread. This survey evaluates, after more than a decade since the last epidemiological survey, the current situation and trend of CE in sheep of the highly endemic island of Sardinia (Italy). From 2012 to 2018, out of a total of 1416 sheep, 65.3% were found positive for CE. Cyst fertility has increased slightly from 10.3% in 2006 (Scala et al. 2006) to 11.7% in the present survey (χ2 = 1.07; P = 0.30018) with a peak of 14.0% in the province of Sassari. Overall, 6815 hydatids were found and classified as sterile (26.2%), fertile (14.4%), caseous (5.3%), or calcified (54.1%) based on their morphology. All isolates were identified as E. granulosus sensu stricto (former G1/G3, strains). Results shown as the implementation of new European Council Regulations and in particular of 21/2004 that established a system for the individual identification and tracking of sheep could be a step forward towards a better control of animal movement, preventing illegal slaughtering on the island as well as in other countries experiencing similar problems. Even if at first sight the general situation of Sardinia regarding CE seems a little better compared with the past, our findings indicate that in some Provinces the epidemiological situation is not improved since 2006.

Surveillance and control of African Swine Fever in free-ranging pigs in Sardinia.

African swine fever (ASF) is a notifiable infectious disease, caused by the ASF virus (ASFV), which is a DNA virus belonging to the family Asfarviridae, genus Asfivirus. This disease has gained importance in the last decade after its spread in several countries in Eastern and Central Europe, and more recently, in China. Despite the efforts made to eradicate it, ASF is still present on the Mediterranean island of Sardinia (Italy) and has been since 1978. ASF risk factors on the island have been analysed in previous studies; the role of free-ranging pigs in virus persistence has been suggested, but has not been fully elucidated. The most recent eradication plan provides more stringent measures to combat free-ranging pigs and any kind of illegality in the pig sector. From December 2017 to June 2018, a total of 29 depopulation actions were performed in 13 municipalities in central Sardinia, during which 2,281 free-ranging pigs were culled and more than 50% of them were tested for ASFV and antibody presence (1,218 and 1,416, respectively). A total of 651 pigs were seropositive, with a mean seroprevalence of 53.4% (CI 95% = 50.6-56.3), and 38 were ASFV positive (virus prevalence = 2.6%; CI 95% = 2.1-3.0). To the best of our knowledge, the present study is the first to provide a complete evaluation of this millennial system of pig farming and ASFV prevalence in free-ranging pigs. Furthermore, it has emphasised the necessity of combining the maintenance of an epidemiological surveillance program with continuous education of farmers and other people involved in pig husbandry, based on cultural and economic aspects.

Bifacial Diffuse Absorptance of Semitransparent Microstructured Perovskite Solar Cells.

An optical radiometry technique enabling simultaneous transmittance and reflectance measurements from both sides of a device was used to investigate bifacial diffuse absorptance of neutral-colored semitransparent perovskite solar cells based on a thin film of microsized perovskite islands. In such microstructured solar cells, diffuse irradiance was more effectively absorbed than direct irradiance at near-normal incidence, in contrast to reference solar cells comprising a continuous perovskite thin film. Experimental findings were discussed in ray-optic approximation in relation to the surface texture of the active layer, highlighting the role of light trapping. This absorptance spectroscopy technique is envisaged to find wide applicability to bifacial solar cells for building-integrated photovoltaics and other bifacial light-harvesting systems.

Investigation of Dimethylammonium Solubility in MAPbBr3 Hybrid Perovskite: Synthesis, Crystal Structure, and Optical Properties.

The possible existence of mixed methylammonium (MA)/dimethylammonium (DMA) lead bromide hybrid perovskites of general formula MA1- xDMA xPbBr3 (0 ≤ x ≤ 1) was investigated. A combined X-ray diffraction and solid-state nuclear magnetic resonance approach indicates that DMA can be incorporated up to about x = 0.30 while retaining the cubic lattice of MAPbBr3. By increasing the DMA content ( x), the absorption shows a progressive blue shift and the band gap moves from about 2.17 eV ( x = 0) to about 2.23 ( x = 0.30) with a concomitant slightly faster recombination in the mixed cation powders.

Novel Physical Vapor Deposition Approach to Hybrid Perovskites: Growth of MAPbI3 Thin Films by RF-Magnetron Sputtering.

Solution-based methods represent the most widespread approach used to deposit hybrid organic-inorganic perovskite films for low-cost but efficient solar cells. However, solution-process techniques offer limited control over film morphology and crystallinity, and most importantly do not allow sequential film deposition to produce perovskite-perovskite heterostructures. Here the successful deposition of CH3NH3PbI3 (MAPI) thin films by RF-magnetron sputtering is reported, an industry-tested method to grow large area devices with precisely controlled stoichiometry. MAPI films are grown starting from a single-target made of CH3NH3I (MAI) and PbI2. Films are single-phase, with a barely detectable content of unreacted PbI2, full surface coverage and thickness ranging from less than 200 nm to more than 3 µm. Light absorption and emission properties of the deposited films are comparable to as-grown solution-processed MAPI films. The development of vapor-phase deposition methods is of interest to advance perovskite photovoltaic devices with the possibility of fabricating perovskite multijunction solar cells or multicolor bright light-emitting devices in the whole visible spectrum.

Optical determination of Shockley-Read-Hall and interface recombination currents in hybrid perovskites.

Metal-halide perovskite solar cells rival the best inorganic solar cells in power conversion efficiency, providing the outlook for efficient, cheap devices. In order for the technology to mature and approach the ideal Shockley-Queissier efficiency, experimental tools are needed to diagnose what processes limit performances, beyond simply measuring electrical characteristics often affected by parasitic effects and difficult to interpret. Here we study the microscopic origin of recombination currents causing photoconversion losses with an all-optical technique, measuring the electron-hole free energy as a function of the exciting light intensity. Our method allows assessing the ideality factor and breaks down the electron-hole recombination current into bulk defect and interface contributions, providing an estimate of the limit photoconversion efficiency, without any real charge current flowing through the device. We identify Shockley-Read-Hall recombination as the main decay process in insulated perovskite layers and quantify the additional performance degradation due to interface recombination in heterojunctions.

Absorption F-sum rule for the exciton binding energy in methylammonium lead halide perovskites.

Advances of optoelectronic devices based on methylammonium lead halide perovskites depend on understanding the role of excitons, whether it is marginal as in inorganic semiconductors, or crucial, like in organics. However, a consensus on the exciton binding energy and its temperature dependence is still lacking, even for widely studied methylammonium lead iodide and bromide materials (MAPbI3, MAPbBr3). Here we determine the exciton binding energy based on an f-sum rule for integrated UV-vis absorption spectra, circumventing the pitfalls of least-squares fitting procedures. In the temperature range 80-300 K, we find that the exciton binding energy in MAPbBr3 is EB = (60 ± 3) meV, independent of temperature; for MAPbI3, in the orthorhombic phase (below 140 K) EB = (34 ± 3) meV, while in the tetragonal phase the binding energy softens to 29 meV at 170 K and stays constant up to 300 K. Implications of binding energy values on solar cell and LED workings are discussed.

Synergic combination of the sol-gel method with dip coating for plasmonic devices.

Biosensing technologies based on plasmonic nanostructures have recently attracted significant attention due to their small dimensions, low-cost and high sensitivity but are often limited in terms of affinity, selectivity and stability. Consequently, several methods have been employed to functionalize plasmonic surfaces used for detection in order to increase their stability. Herein, a plasmonic surface was modified through a controlled, silica platform, which enables the improvement of the plasmonic-based sensor functionality. The key processing parameters that allow for the fine-tuning of the silica layer thickness on the plasmonic structure were studied. Control of the silica coating thickness was achieved through a combined approach involving sol-gel and dip-coating techniques. The silica films were characterized using spectroscopic ellipsometry, contact angle measurements, atomic force microscopy and dispersive spectroscopy. The effect of the use of silica layers on the optical properties of the plasmonic structures was evaluated. The obtained results show that the silica coating enables surface protection of the plasmonic structures, preserving their stability for an extended time and inducing a suitable reduction of the regeneration time of the chip.

Correlated electron-hole plasma in organometal perovskites.

Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.

Optical Sensitivity Gain in Silica-Coated Plasmonic Nanostructures.

Ultrathin films of silica realized by sol-gel synthesis and dip-coating techniques were successfully applied to predefined metal/polymer plasmonic nanostructures to spectrally tune their resonance modes and to increase their sensitivity to local refractive index changes. Plasmon resonance spectral shifts up to 100 nm with slope efficiencies of ∼8 nm/nm for increasing layer thickness were attained. In the ultrathin layer regime (<10 nm), which could be reached by suitable dilution of the silica precursors and optimization of the deposition speed, the sensitivity of the main plasmonic resonance to refractive index changes in aqueous solution could be increased by over 50% with respect to the bare plasmonic chip. Numerical simulations supported experimental data and unveiled the mechanism responsible for the optical sensitivity gain, proving an effective tool in the design of high-performance plasmonic sensors.

Innovative composite films of chitosan, methylcellulose, and nanoparticles.

Plastic is readily available and inexpensive, so it is becoming the main material for packaging. Unfortunately plastics do not biodegrade and, if reduced in small pieces, contaminate soil and waterways. In the present work, natural films composed of chitosan, methylcellulose, and silica (SiO(2)) nanoparticles (NPs) were developed as new packaging materials. The effect of the incorporation of NPs into the polymeric film matrix was evaluated. An excellent improvement of the mechanical properties was obtained for nanostructured films with a composition of CH:MC 50:50 and NPs 1% w/v that make these materials able to replace plastics and derivatives, reducing environmental pollution.

Shaping mesoporous films using dewetting on X-ray pre-patterned hydrophilic/hydrophobic layers and pinning effects at the pattern edge.

Ordered mesoporous silica micrometer-sized structures have been fabricated via selective dewetting of the coating sol on a hydrophilic/hydrophobic fluorinated silica substrate, which had been pre-patterned using deep X-ray lithography with a synchrotron radiation source. We have observed that deposition of mesoporous films on the pre-patterned areas can be used as a design tool for obtaining regions of specific geometry and dimensions. The evaporation of the solution in constrained conditions because of pinning at the pattern edges gives layers with thicker edges. This edge effect appears dependent upon the dimension of the pre-patterned hydrophilic/hydrophobic layer; in smaller patterns, the evaporation is too fast and thickening of the edges is not observed. We have used infrared imaging, optical profilometry, and atomic force microscopy to characterize the patterned layers and the edge effect, produced by pinning at the border of the microstructures.