Composition dependence of X-ray stability and degradation mechanisms at lead halide perovskite single crystal surfaces.

The multiple applications of lead halide perovskite materials and the extensive use of X-ray based techniques to characterize them highlight a need to understand their stability under X-ray irradiation. Here, we present a study where the X-ray stability of five different lead halide perovskite compositions (MAPbI3, MAPbCl3, MAPbBr3, FAPbBr3, CsPbBr3) was investigated using photoelectron spectroscopy. To exclude effects of thin film formation on the observed degradation behaviors, we studied clean surfaces of single crystals. Different X-ray resistance and degradation mechanisms were observed depending on the crystal composition. Overall, perovskites based on the MA+ cation were found to be less stable than those based on FA+ or Cs+. Metallic lead formed most easily in the chloride perovskite, followed by bromide, and only very little metallic lead formation was observed for MAPbI3. MAPbI3 showed one main degradation process, which was the radiolysis of MAI. Multiple simultaneous degradation processes were identified for the bromide compositions. These processes include ion migration towards the perovskite surface and the formation of volatile and solid products in addition to metallic lead. Lastly, CsBr formed as a solid degradation product on the surface of CsPbBr3.

Interface Energy Alignment between Lead Halide Perovskite Single Crystals and TIPS-Pentacene.

At present, there is a huge development in optoelectronic applications using lead halide perovskites. Considering that device performance is largely governed by the transport of charges across interfaces and, therefore, the interfacial electronic structure, fundamental investigations of perovskite interfaces are highly necessary. In this study, we use high-resolution soft X-ray photoelectron spectroscopy based on synchrotron radiation to explore the interfacial energetics for the molecular layer of TIPS-pentacene and lead halide perovskite single crystals. We perform ultrahigh vacuum studies on multiple thicknesses of an in situ formed interface of TIPS-pentacene with four different in situ cleaved perovskite single crystals (MAPbI3, MAPbBr3, FAPbBr3, and CsxFA1-xPbBryI3-y). Our findings reveal a substantial shift of the TIPS-pentacene energy levels toward higher binding energies with increasing thickness, while the perovskite energy levels remain largely unaffected regardless of their composition. These shifts can be interpreted as band bending in the TIPS-pentacene, and such effects should be considered when assessing the energy alignment at perovskite/organic transport material interfaces. Furthermore, we were able to follow a reorganization on the MAPbI3 surface with the transformation of the surface C 1s into bulk C 1s.

Direct Measurements of Interfacial Photovoltage and Band Alignment in Perovskite Solar Cells Using Hard X-ray Photoelectron Spectroscopy.

A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination.

Order-disorder phase transition and molecular dynamics in the hybrid perovskite [(CH3)3NH][Mn(N3)3].

We present a temperature-dependent Raman scattering study of a [(CH3)3NH][Mn(N3)3] hybrid organic-inorganic azide-perovskite, in which we have analysed in detail the wavenumber and full width at half-maximum (FWHM) of lattice modes and internal modes of the NC3 skeleton, N3- and CH3 molecular groups. In general, the modes exhibited unusual behaviour during the phase transitions, including discontinuity in the phonon wavenumber, bandwidth, and unconventional shifts upon temperature variation. Spectral features on heating reveal the absence of significant distortions in the NC3 skeleton and a relatively restricted order-disorder process of the TrMA+ cations. On the other hand, linewidth anomalies of the δNC3 and νasNC3 modes have been attributed to the molecular dynamics of encapsulated cations. The unconventional blue shift of the symmetric stretching modes of azide ligands indicates the weakening of the intermolecular interactions between the TrMA+ cations and azido-bridges, and the strengthening of the intramolecular bonds. Additionally, we have used differential scanning calorimetry to confirm the subtle monoclinic to monoclinic (P21/c â†’ C2/c) phase transition at around 330 K; and the phase transition to trigonal structure (R3¯m) above 359 K, whose associated entropy variation turns to be |ΔS| ∼ 22.3 J·kg-1 K-1 and displays a barocaloric (BC) tunability |δTt/δP| ∼ 3.17 K kbar-1, according to our estimations using the Clausius-Clapeyron method. Although the obtained values of entropy change and BC tunability are very close to those reported on formate-perovskites and other important caloric materials, those parameters are much lower than the giant entropy change of ∼80 Jkg-1 K-1 and large BC tunability ∼12 K kbar-1 observed for the analogue azide-perovskite [(CH3)4N][Mn(N3)]3 (TMAMnN3). Very interestingly, our combined study shed light to understand such different behaviour, as they reveal that the hydrogen bonds created between the TrMA+ cations and the framework prevent an extensive order-disorder process that is needed to obtain large entropy changes and large BC coefficients as it occurs in the case of related azide-perovskites with no H-bonds between the A cations (for example TMA) and the framework.

Electronic Structure and Chemical Bonding in Methylammonium Lead Triiodide and Its Precursor Methylammonium Iodide.

A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with ab initio molecular dynamics (AIMD) simulations based on density functional theory, and the theoretical results are compared to experimental probes. The occupied valence bands of a MAPI single crystal and MAI powder are probed with X-ray photoelectron spectroscopy, and the conduction bands are probed from the perspective of nitrogen K-edge X-ray absorption spectroscopy. Combined, the theoretical simulations and the two experimental techniques allow for a dissection of the electronic structure unveiling the nature of chemical bonding in MAPI and MAI. Here, we show that the difference in band gap between MAPI and MAI is caused chiefly by interactions between iodine and lead but also weaker interactions with the MA+ counterions. Spatial decomposition of the iodine p levels allows for analysis of Pb-I σ bonds and π interactions, which contribute to this effect with the involvement of the Pb 6p levels. Differences in hydrogen bonding between the two materials, seen in the AIMD simulations, are reflected in nitrogen valence orbital composition and in nitrogen K-edge X-ray absorption spectra.

Anomalous and colossal thermal expansion, photoluminescence, and dielectric properties in lead halide-layered perovskites with cyclohexylammonium and cyclopentylammonium cations.

A detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl- or Br- anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK-1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials.

There is a high fundamental interest in the surface and bulk chemistry of quantum dot (QD) solids, as they have proven to be very promising materials in optoelectronic devices. The choice of surface ligands for quantum dots in solid devices determines many of the film properties, as the ligands influence for example the doping density, chemical stability and charge transport. Lead halide ligands have developed as the main ligand of choice for lead sulfide quantum dots, as they have been shown to passivate quantum dot surfaces and enhance the chemical stability. In this study, we successfully varied the ligand composition on the surface of PbS quantum dot films from pure lead iodide to pure lead bromide and investigated its influence on the chemical and electronic structure of the QD solids using hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we developed a surface treatment to prevent the surface oxidation of a bulk PbS reference sample. Through measurements of this sample and of lead halide reference samples, we were able to assign the contributions of different chemical bonding to the Pb 4f core level and of different atomic orbitals to the valence band spectral shape of the QD materials. Overall, we found that the valence band edge position was very similar for all different iodide:bromide ratios and that all investigated compositions were able to protect the quantum dot surfaces within solid films from oxidation. However, the ligand composition significantly influences the sample stability under X-rays. The iodide rich QD solids showed the highest stability with very little to no chemical changes over several hours of X-ray exposure, while the bromide rich QD solids changed already within the first hour of exposure.

The Complex Degradation Mechanism of Copper Electrodes on Lead Halide Perovskites.

Lead halide perovskite solar cells have reached power conversion efficiencies during the past few years that rival those of crystalline silicon solar cells, and there is a concentrated effort to commercialize them. The use of gold electrodes, the current standard, is prohibitively costly for commercial application. Copper is a promising low-cost electrode material that has shown good stability in perovskite solar cells with selective contacts. Furthermore, it has the potential to be self-passivating through the formation of CuI, a copper salt which is also used as a hole selective material. Based on these opportunities, we investigated the interface reactions between lead halide perovskites and copper in this work. Specifically, copper was deposited on the perovskite surface, and the reactions were followed in detail using synchrotron-based and in-house photoelectron spectroscopy. The results show a rich interfacial chemistry with reactions starting upon deposition and, with the exposure to oxygen and moisture, progress over many weeks, resulting in significant degradation of both the copper and the perovskite. The degradation results not only in the formation of CuI, as expected, but also in the formation of two previously unreported degradation products. The hope is that a deeper understanding of these processes will aid in the design of corrosion-resistant copper-based electrodes.

Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces.

A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI3 and Csx FA1- x PbI3 ) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI3 and assign these to MA+ ions in an MAI-terminated surface and to MA+ ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI2 -terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.

Sensitivity of Nitrogen K-Edge X-ray Absorption to Halide Substitution and Thermal Fluctuations in Methylammonium Lead-Halide Perovskites.

The performance of hybrid perovskite materials in solar cells crucially depends on their electronic properties, and it is important to investigate contributions to the total electronic structure from specific components in the material. In a combined theoretical and experimental study of CH3NH3PbI3-methylammonium lead triiodide (MAPI)-and its bromide cousin CH3NH3PbBr3 (MAPB), we analyze nitrogen K-edge (N 1s-to-2p*) X-ray absorption (XA) spectra measured in MAPI and MAPB single crystals. This permits comparison of spectral features to the local character of unoccupied molecular orbitals on the CH3NH3 + (MA+) counterions and allows us to investigate how thermal fluctuations, hydrogen bonding, and halide-ion substitution influence the XA spectra as a measure of the local electronic structure. In agreement with the experiment, the simulated spectra for MAPI and MAPB show close similarity, except that the MAPB spectral features are blue-shifted by +0.31 eV. The shift is shown to arise from the intrinsic difference in the electronic structure of the two halide atoms rather than from structural differences between the materials. In addition, from the spectral sampling analysis of molecular dynamics simulations, clear correlations between geometric descriptors (N-C, N-H, and H···I/Br distances) and spectral features are identified and used to explain the spectral shapes.

X-ray stability and degradation mechanism of lead halide perovskites and lead halides.

Lead halide perovskites have become a leading material in the field of emerging photovoltaics and optoelectronics. Significant progress has been achieved in improving the intrinsic properties and environmental stability of these materials. However, the stability of lead halide perovskites to ionising radiation has not been widely investigated. In this study, we investigated the radiolysis of lead halide perovskites with organic and inorganic cations under X-ray irradiation using synchrotron based hard X-ray photoelectron spectroscopy. We found that fully inorganic perovskites are significantly more stable than those containing organic cations. In general, the degradation occurs through two different, but not mutually exclusive, pathways/mechanisms. One pathway is induced by radiolysis of the lead halide cage into halide salts, halogen gas and metallic lead and appears to be catalysed by defects in the perovskite. The other pathway is induced by the radiolysis of the organic cation which leads to formation of organic degradation products and the collapse of the perovskite structure. In the case of Cs0.17FA0.83PbI3, these reactions result in products with a lead to halide ratio of 1 : 2 and no formation of metallic lead. The radiolysis of the organic cation was shown to be a first order reaction with regards to the FA+ concentration and proportional to the X-ray flux density with a radiolysis rate constant of 1.6 × 10-18 cm2 per photon at 3 keV or 3.3 cm2 mJ-1. These results provide valuable insight for the use of lead halide perovskite based devices in high radiation environments, such as in space environments and X-ray detectors, as well as for investigations of lead halide perovskites using X-ray based techniques.

Raman Spectroscopy Studies on the Barocaloric Hybrid Perovskite [(CH3)4N][Cd(N3)3].

Temperature-dependent Raman scattering and differential scanning calorimetry were applied to the study of the hybrid organic-inorganic azide-perovskite [(CH3)4N][Cd(N3)3], a compound with multiple structural phase transitions as a function of temperature. A significant entropy variation was observed associated to such phase transitions, |∆S| ~ 62.09 J·kg-1 K-1, together with both a positive high barocaloric (BC) coefficient |δTt/δP| ~ 12.39 K kbar-1 and an inverse barocaloric (BC) coefficient |δTt/δP| ~ -6.52 kbar-1, features that render this compound interesting for barocaloric applications. As for the obtained Raman spectra, they revealed that molecular vibrations associated to the NC4, N3- and CH3 molecular groups exhibit clear anomalies during the phase transitions, which include splits and discontinuity in the phonon wavenumber and lifetime. Furthermore, variation of the TMA+ and N3- modes with temperature revealed that while some modes follow the conventional red shift upon heating, others exhibit an unconventional blue shift, a result which was related to the weakening of the intermolecular interactions between the TMA (tetramethylammonium) cations and the azide ligands and the concomitant strengthening of the intramolecular bondings. Therefore, these studies show that Raman spectroscopy is a powerful tool to gain information about phase transitions, structures and intermolecular interactions between the A-cation and the framework, even in complex hybrid organic-inorganic perovskites with highly disordered phases.

Intubación orotraqueal en paciente con divertículo traqueal conocido / Orotracheal Intubation in a Patient with Known Tracheal Diverticulum

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Diimidazolium Halobismuthates [Dim]2[Bi2X10] (X = Cl-, Br-, or I-): A New Class of Thermochromic and Photoluminescent Materials.

We present a novel family of polyhalide salts of Bi(III) with the general formula [Dim]2[Bi2X10], where Dim2+ is the diimidazolium cation (C9H14N4)2+ and X is Cl-, Br-, or I-. Single-phase materials are easily obtained by means of a mild solution chemistry method performed at room temperature. This [Dim]2[Bi2X10] family exhibits a crystal structure based on halobismuthate [Bi2X10]4- dimers, built by distorted {BiX6} octahedra interconnected by edge sharing, and sandwiched between two diimidazolium cations. The optical band gaps displayed by these materials (1.9-3.2 eV) allow their classification as semiconductors. Additionally, the three halides display photoluminescence with emission in the visible range. The behavior of [Dim]2[Bi2I10] is particularly interesting, as it shows an optical band gap of 1.9 eV, a broad band photoluminescence emission, and a relatively long emission lifetime of 190 ns. Moreover, the iodide and bromide compounds also exhibit a reversible solid state thermochromism, being the first example of a bromobismuthate with this property. The diimidazolium cations play an important structural role by stabilizing the crystal structure and balancing the charges of the [Bi2X10]4- dimers. Furthermore, density functional theory calculations suggest that they play a key role in the thermochromic behavior. Therefore, compounds [Dim]2[Bi2X10] (X = Cl-, Br-, or I-) represent a very versatile family in which the optical band gap can be tuned by changing the halide or temperature. This makes them promising new materials for different optoelectronic applications, in particular for obtaining new solar absorbers.

[(CH3)2NH2]7Pb4X15 (X = Cl- and Br-), 2D-Perovskite Related Hybrids with Dielectric Transitions and Broadband Photoluminiscent Emission.

We have prepared two new lead halides with the novel general formula of DMA7Pb4X15 (DMA = [(CH3)2NH2]+ and X = Cl- or Br-) by using an easy route under mild conditions at room temperature. These compounds exhibit an unprecedented crystal structure, are formed by layers of distorted [PbX6] octahedra, which share corners and faces, and contain intercalated DMA cations. Very interestingly, they display dielectric transitions, which are related to a partial order-disorder process of the DMA cations between 160 and 260 K. Additionally, these new layered hybrids exhibit a broadband photoluminiscent emission, which is related to the structural distortions of the [PbX6] octahedra. These findings not only open up large possibilities for future optoelectronic applications of these materials, but they also offer a novel playground for an easy modulation of electrical and optical properties of hybrid organic-inorganic materials. We anticipate that this novel A7Pb4X15 formula can be adequate to tune the family of the hybrid lead halides using other alkylammonium cations, such as methylammonium, formamidinium, or ethylammonium, to improve their photoelectronic properties.

Effect of snoring and obstructive respiratory events on sleep architecture in adolescents.

OBJECTIVE: To evaluate the effect of snoring and obstructive respiratory events on the distribution of sleep stages and arousals in a nonselected group of adolescents from the general population. DESIGN: Cross-sectional study. SETTING: Randomly selected secondary schools in Seville, Spain. Patients A general population sample of 43 adolescents (mean [SD] age, 13.6 [1.77] years). INTERVENTIONS: A questionnaire for the investigation of sleep-related breathing disorders was administered. Symptoms were evaluated according to a 4-point frequency scale. Snorers answered "sometimes" or "often" to the question about snoring, and nonsnorers answered "never" or "rarely." All subjects underwent standard polysomnography at the sleep laboratory. RESULTS: Twenty-eight subjects were snorers; 15 were nonsnorers. No statistically significant differences were noted between both groups in the percentages of sleep stages, arousal index, awakenings, or wakefulness during sleep. Snorers showed a significantly higher number of respiratory arousals than nonsnorers (mean [SD], 1.14 [1.5] vs 0.33 [0.6], P<.05). However, neither the apnea-hypopnea index (AHI) nor the oxygen desaturation index correlated with the arousal index. Twelve snorers (27.5%) had an AHI of 2 or more; 13 nonsnorers (30.2%) had an AHI of less than 2. Snorers with some obstructive respiratory events had a significantly higher number of awakenings, a lower percentage of stage 4 sleep, and a higher number of respiratory events compared with nonsnorers. However, the total number of arousals and the arousal index were similar for both groups. Wakefulness during sleep tended to be longer in snorers than in nonsnorers although differences were not significant. The percentage of respiratory events that terminated with an arousal was greater in snorers who had an AHI of 2 or more than in nonsnorers who had an AHI of less than 2 (mean [SD], 8.4% [9.5%] vs 4.9% [11.53%], P<.05). CONCLUSIONS: These data indicate normal sleep architecture in the adolescents. Although snorers as well as adolescents with some polysomnographic abnormality showed a higher number of respiratory arousals than control subjects, most obstructive events did not terminate with a cortical arousal, which may suggest that adolescents share with younger children this mechanism for preserving sleep architecture.

Neumonías adquiridas en la comunidad en pacientes mayores de 60 años. Incidencia de gérmenes atípicos y evolución clinicorradiológica / Community acquired pneumonia in patients older than 60 years. Incidence of atypical agents and clinical-radiological progression

FUNDAMENTO: Se estudiaron de forma prospectiva 75 pacientes mayores de 60 años con neumonías adquiridas en la comunidad (NAC) seguidos de forma ambulatoria, evaluando la incidencia de gérmenes atípicos, las características clinicorradiológicas, su evolución y los datos diferenciales con neumonías de pacientes más jóvenes. MÉTODO: Se realizaron protocolos de evaluación clinicorradiológica en la primera visita, y durante el seguimiento en dos revisiones más. El diagnóstico etiológico se efectuó por serología (en la primera visita y a las tres semanas). RESULTADOS: Se incluyó inicialmente a 85 pacientes mayores de 60 años, de los que fueron seguidos un total de 75 casos no hospitalizados. Además, se incluyó en el estudio comparativo a 216 pacientes con edad igual o menor de 60 años, controlados de forma ambulatoria en el mismo espacio temporal. En el primer grupo la incidencia de gérmenes atípicos fue del 33,3 por ciento, siendo el patógeno más frecuente Coxiella burnetii (13,3 por ciento), seguido de virus y Legionella pneumophila. No se diagnosticó ningún caso debido a Mycoplasma pneumoniae. La forma de presentación radiológica más frecuente fue la condensación alveolar (85 por ciento). En el análisis comparativo entre las dos poblaciones (mayores o menores de 60 años), encontramos escasas diferencias clínicas (disnea más frecuente en población mayor, escalofríos en población más joven) y en semiología respiratoria (crepitantes más frecuentes en población mayor), no hallando diferencias en el resto de los datos clinicorradiológicos o de laboratorio. La evolución clínica y radiológica f e satisfactoria, siendo necesario el ingreso hospitalario en sólo dos casos (2,7 por ciento). CONCLUSIONES: Encontramos un elevado número de casos debidos a gérmenes atípicos en las NAC de pacientes mayores de 60 años. Con seguimiento ambulatorio, la evolución clinicorradiológica ha sido satisfactoria en la mayoría de ellos, por lo que no consideramos la edad un factor fundamental para decidir la hospitalización (AU)

Estudio prospectivo de 221 neumonías adquiridas en la comunidad seguidas de forma ambulatoria. Etiología y evolución clinicorradiológica / Prospective study of 221 community acquired pneumonias followed up in an outpatient clinic. Etiology and clinical-radiological progression

FUNDAMENTO: Se estudiaron de forma prospectiva las neumonías adquiridas en la comunidad seguidas en una consulta neumológica extrahospitalaria, con objeto de determinar la etiología, las características clinicorradiológicas y su evolución con unas pautas de actuación y tratamiento protocolizadas. PACIENTES Y MÉTODO: Se realizaron protocolos de evaluación clínica, diagnóstico etiológico por serología (en la primera visita y a las tres semanas) y, en los casos necesarios, por fibrobroncoscopia (cepillo microbiológico protegido), así como protocolos de evolución clínica y radiológica (hasta tres revisiones) tras pautas de tratamiento empírico previamente establecidos. RESULTADOS: Se incluyó inicialmente a 240 pacientes, siendo seguidos un total de 221 casos. Obtuvimos el diagnóstico etiológico en 86 pacientes (39 por ciento), siendo el germen más frecuente Coxiella burnetii (12,2 por ciento), seguido de Mycoplasma pneumoniae y Legionella pneumophila. Se diagnosticaron dos casos de Sptreptococcus pneumoniae. La forma de presentación radiológica más frecuente fue la condensación alveolar (86 por ciento). Se emplearon como tratamiento empírico inicial macrólidos (71 por ciento) o cefalosporinas de segunda generación (22 por ciento). La evolución clínica y radiológica fue satisfactoria en la mayoría de los pacientes, siendo necesario el ingreso hospitalario en dos casos (< 1 por ciento del total). CONCLUSIONES: En las neumonías adquiridas en la comunidad en régimen ambulatorio encontramos un elevado número de gérmenes 'atípicos'. En pacientes ambulatorios creemos apropiado el tratamiento empírico con macrólidos o cefalosporinas de segunda generación (AU)