Chirality Effects and Semiconductor versus Metallic Nature in Halide Nanotubes.

A density functional theory study of the electronic structure of nanostructures based on the hexagonal layers of LuI3 is reported. Both bulk and slabs with one to three layers exhibit large and indirect bandgaps. Different families of nanotubes can be generated from these layers. Semiconducting nanotubes of two different chiralities have been studied. The direct or indirect nature of the optical gaps depends on the chirality, and a simple rationalization of this observation based on band folding arguments is provided. Remarkably, a metastable form of the armchair LuI3 nanotubes can be obtained under a structural rearrangement such that some iodine atoms are segregated toward the center of the nanotube forming chains of dimerized iodines. These nanotubes having an Lu2N I5N backbone are predicted to be metallic and should be immune toward a Peierls distortion. The iodine chains in the inner part of the nanotubes are weakly bound to the backbone so that it should be possible to remove these chains to generate a new series of neutral Lu2N I5N nanotubes which could exhibit interesting magnetic behavior. Because the LuI3 structure occurs for a large number of lanthanide and actinide trihalides, a tuning of the optical, transport, and probably magnetic properties of these new families of nanotubes can be a challenging prospect for future experimental studies.

Plurality of excitons in Ruddlesden-Popper metal halides and the role of the B-site metal cation.

We investigate the effect of metal cation substition on the excitonic structure and dynamics in a prototypical Ruddlesden-Popper metal halide. Through an in-depth spectroscopic and theoretical analysis, we identify the presence of multiple resonances in the optical spectra of a phenethyl ammonium tin iodide, a tin-based RPMH. Based on ab initio calculations, we assign these resonances to distinct exciton series that originate from the splitting of the conduction band due to spin-orbit coupling. While the splitting energy in the tin based system is low enough to enable the observation of the higher lying exciton in the visible-range spectrum of the material, the higher splitting energy in the lead counterpart prevents the emergence of such a feature. We elucidate the critical role played by the higher lying excitonic state in the ultrafast carrier thermalization dynamics.

Study of Optoelectronic Features in Polar and Nonpolar Polymorphs of the Oxynitride Tin-Based Semiconductor InSnO2N.

In view of its potential applicability in photoconversion processes, we here discuss the optoelectronic features of the recently proposed tin-based oxynitride material for (photo)catalysis, InSnO2N. In detail, by combining Density Functional and Many-Body Perturbation Theory, we compute the electronic and optical properties discussing how they vary from the nonpolar phase to the more stable polar one. After providing a detailed, unbiased, description of the optoelectronic features of the two phases, we have finally calculated the Spectroscopic Limited Maximum Efficiency and obtained data that further witness the relevance of InSnO2N for solar energy conversion processes.

Tailoring High-Entropy Oxides as Emerging Radiative Materials for Daytime Passive Cooling.

In the framework of intense research about high-entropy materials and their applications in energy-oriented technologies, in the present work, we discuss the potential applicability of selected oxides and of the alloys they form at different concentrations for daytime radiative cooling implementation. In particular, by combining density functional theory and the finite difference method, we provide an unbiased, scattering-free description of structural, electronic, and dynamic features of the best candidates, showing the required strong radiative properties for passive cooling while offering the benefits of affordability and compatibility with commercial coating fabrication processes.

The impact of face-mask mandates on all-cause mortality in Switzerland: a quasi-experimental study.

BACKGROUND: Whereas there is strong evidence that wearing a face mask is effective in reducing the spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), evidence on the impact of mandating the wearing of face masks on deaths from coronavirus disease 2019 (COVID-19) and all-cause mortality is more sparse and likely to vary by context. Focusing on a quasi-experimental setting in Switzerland, we aimed to determine (i) the effect of face-mask mandates for indoor public spaces on all-cause mortality; and (ii) how the effect has varied over time, and by age and sex. METHODS: Our analysis exploited the fact that between July and October 2020, nine cantons in Switzerland extended a face-mask mandate at different time points from being restricted to public transportation only to applying to all public indoor places. We used both a Difference-in-Differences approach with fixed-effects for canton and week and an event-study approach. RESULTS: In our main Difference-in-Differences model, the face-mask mandate was associated with a 0.3% reduction in all-cause mortality [95% confidence interval (CI): -3.4% to 2.7%; P = 0.818]. This null effect was confirmed in the event-study approach and a variety of robustness checks. Combining the face-mask mandate with social distancing rules led to an estimated 5.1% (95% CI: -7.9% to -2.4%; P = 0.001) reduction in all-cause mortality. CONCLUSIONS: Mandating face-mask use in public indoor spaces in Switzerland in mid-to-late 2020 does not appear to have resulted in large reductions in all-cause mortality in the short term. There is some suggestion that combining face-mask mandates with social distancing rules reduced all-cause mortality.

Ab Initio Study of Graphene/hBN Van der Waals Heterostructures: Effect of Electric Field, Twist Angles and p-n Doping on the Electronic Properties.

In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.

Bidimensional Engineered Amorphous a-SnO2 Interfaces: Synthesis and Gas Sensing Response to H2S and Humidity.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal chalcogenides (MCs), despite their excellent gas sensing properties, are subjected to spontaneous oxidation in ambient air, negatively affecting the sensor's signal reproducibility in the long run. Taking advantage of spontaneous oxidation, we synthesized fully amorphous a-SnO2 2D flakes (≈30 nm thick) by annealing in air 2D SnSe2 for two weeks at temperatures below the crystallization temperature of SnO2 (T < 280 °C). These engineered a-SnO2 interfaces, preserving all the precursor's 2D surface-to-volume features, are stable in dry/wet air up to 250 °C, with excellent baseline and sensor's signal reproducibility to H2S (400 ppb to 1.5 ppm) and humidity (10-80% relative humidity (RH)) at 100 °C for one year. Specifically, by combined density functional theory and ab initio molecular dynamics, we demonstrated that H2S and H2O compete by dissociative chemisorption over the same a-SnO2 adsorption sites, disclosing the humidity cross-response to H2S sensing. Tests confirmed that humidity decreases the baseline resistance, hampers the H2S sensor's signal (i.e., relative response (RR) = Ra/Rg), and increases the limit of detection (LOD). At 1 ppm, the H2S sensor's signal decreases from an RR of 2.4 ± 0.1 at 0% RH to 1.9 ± 0.1 at 80% RH, while the LOD increases from 210 to 380 ppb. Utilizing a suitable thermal treatment, here, we report an amorphization procedure that can be easily extended to a large variety of TMDs and MCs, opening extraordinary applications for 2D layered amorphous metal oxide gas sensors.

Materials Design and Optimization for Next-Generation Solar Cell and Light-Emitting Technologies.

We review some of the most potent directions in the design of materials for next-generation solar cell and light-emitting technologies that go beyond traditional solid-state inorganic semiconductor-based devices, from both the experimental and computational standpoints. We focus on selected recent conceptual advances in tackling issues which are expected to significantly impact applied literature in the coming years. Specifically, we consider solution processability, design of dopant-free charge transport materials, two-dimensional conjugated polymeric semiconductors, and colloidal quantum dot assemblies in the fields of experimental synthesis, characterization, and device fabrication. Key modeling issues that we consider are calculations of optical properties and of effects of aggregation, including recent advances in methods beyond linear-response time-dependent density functional theory and recent insights into the effects of correlation when going beyond the single-particle ansatz as well as in the context of modeling of thermally activated fluorescence.

Carbon Nanotubes/Regenerated Silk Composite as a Three-Dimensional Printable Bio-Adhesive Ink with Self-Powering Properties.

In this study, regenerated silk (RS) obtained from Bombyx Mori cocoons is compounded with carboxyl-functionalized carbon nanotubes (f-CNTs) in an aqueous environment for the fabrication of functional bio-adhesives. Molecular interactions between RS and carboxyl groups of CNTs result in structural increase of the ß-sheet formation, obtaining a resistant adhesive suitable for a wet biological substrate. Moreover, the functionalization of CNTs promotes their dispersion in RS, thus enabling the production of films with controlled electrical conductivity. The practical utility of such a property is demonstrated through the fabrication of a piezoelectric device implanted in a rat to monitor the breathing in vivo and to be used as a self-powered system. Finally, RS/f-CNTs were used as a printable biomaterial ink to three dimensionally print bilayer hollow tubular structures composed of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and RS. Initial tests carried out by seeding and growing human skin fibroblasts demonstrated that the 3D printed bilayer hollow cylindrical structures offer a suitable surface for the seeded cells to attach and proliferate. In general, the herein proposed RS/f-CNT composite serves as a versatile material for solvent-free dispersion processing and 3D printing, thus paving a new approach to prepare multifunctional materials with potential applications of great interest in sealing biological substrates and implantable devices for regenerative medicine.

Engineering Graphene Oxide/Water Interface from First Principles to Experiments for Electrostatic Protective Composites.

From the global spread of COVID-19 we learned that SARS-CoV-2 virus can be transmitted via respiratory liquid droplets. In this study, we performed first-principles calculations suggesting that water molecules once in contact with the graphene oxide (GO) layer interact with its functional groups, therefore, developing an electric field induced by the heterostructure formation. Experiments on GO polymer composite film supports the theoretical findings, showing that the interaction with water aerosol generates a voltage output signal of up to -2 V. We then developed an electrostatic composite fiber by the coagulation method mixing GO with poly(methyl methacrylate) (PMMA). These findings could be used to design protective fabrics with antiviral activity against negatively charged spike proteins of airborne viruses.

A Scalable Method for Thickness and Lateral Engineering of 2D Materials.

The physical properties of two-dimensional (2D) materials depend strongly on the number of layers. Hence, methods for controlling their thickness with atomic layer precision are highly desirable, yet still too rare, and demonstrated for only a limited number of 2D materials. Here, we present a simple and scalable method for the continuous layer-by-layer thinning that works for a large class of 2D materials, notably layered germanium pnictides and chalcogenides. It is based on a simple oxidation/etching process, which selectively occurs on the topmost layers. Through a combination of atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction experiments we demonstrate the thinning method on germanium arsenide (GeAs), germanium sulfide (GeS), and germanium disulfide (GeS2). We use first-principles simulation to provide insights into the oxidation mechanism. Our strategy, which could be applied to other classes of 2D materials upon proper choice of the oxidation/etching reagent, supports 2D material-based device applications, e.g., in electronics or optoelectronics, where a precise control over the number of layers (hence over the material's physical properties) is needed. Finally, we also show that when used in combination with lithography, our method can be used to make precise patterns in the 2D materials.

Effect of organic cation states on electronic properties of mixed organic-inorganic halide perovskite clusters.

We study the effect of organic cation-centered states in mixed organic-inorganic halide perovskites on the bandstructure and optical properties. Clusters of methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3) and of MAPbI3 (MAPbBr3) in which an organic cation was substituted with formamidinium (FA) and guanidinium (GA) are studied with density functional theory and time-dependent density functional theory. This model permitted comparing bandstructure and optical properties with different organic cations computed with GGA and hybrid functionals. We find that while with MA and GA, cation-centered states are deep in the conduction band, with FA, organic cation-centered states are introduced within as little as 0.5 eV of the conduction band maximum, which are expected to influence electronic and optical properties of perovskites in solar cells and other optoelectronic devices. There is qualitative agreement between a GGA and a hybrid functional; however, the use of a hybrid functional leads to a slightly higher offset of the cation-centered states from the conduction band edge, a different order of electronic states, and much better localization of cation-centered states. Analysis of optical absorption spectra suggests that occupation by photoexcitation of FA-centered states and formation of transient formamidinium species is possible in both I and Br-based perovskites.

Nature of the Electronic and Optical Excitations of Ruddlesden-Popper Hybrid Organic-Inorganic Perovskites: The Role of the Many-Body Interactions.

The knowledge of the exact nature of the electronic and optical excitations of Ruddlesden-Popper organic-inorganic halide perovskites (RPPs) is particularly relevant in view of their usage in optoelectronic devices. By means of parameter-free quantum-mechanical simulations, we unambiguously demonstrate the dominant role of many-body Coulomb interaction, as recently proposed by Blancon et al. Indeed, focusing on the first two terms ( n = 1,2) of the Pb-based buthylammonium series, in the form of both isolated nanosheet and repeated bulk-like quantum well, we observe large band gap renormalization and strongly bound excitons with binding energies up to ∼1 eV in the thinnest isolated nanosheet. Notably, taking into account electronic correlation beyond density functional theory, we obtain exciton reduced masses similar to the corresponding 3D bulk counterpart and large Rashba splitting of the same order of the value reported by Zhai et al. in a recent experimental work.

First-principles investigation of the Lewis acid-base adduct formation at the methylammonium lead iodide surface.

We have here performed a campaign of ab initio calculations focusing on the anchoring mechanism and adduct formation of some Lewis bases, both aliphatic and aromatic, on a PbI2-rich flat (001) methylammonium lead iodide (MAPI) surface. Our goal is to provide theoretical support to the recently reported experimental techniques of MAPI surface passivation via Lewis acid-base neutralization and similarly of MAI·PbI2·(Lewis base) adduct formation. We tested several X-donor bases (X = :N, :O, :S), paying attention to the thermodynamic stability of the final MAPI·base adducts and to their electronic properties. Factors that impact on the passivation mechanism are the directionality of the Lewis base lone pair and its enhanced/reduced overlap with MAPI Pb p orbitals, the dipole moment of the base and, similarly, the electronegativity of the X donor atom. Also non-covalent interactions, both at the surface side (intra, MAPI) and at the very interface (inter, MAPI·Lewis base), seem to contribute to the stability of the final adducts. Here we show that the thermodynamic stability does not necessarily correspond to the most effective base → acid dative bond formation. Starting from a low coverage (12.5% of the undercoordinated Pb atoms available at the surface are passivated) this paper paves the way towards the study of cooperative and steric effects among Lewis bases at higher coverages representing, to the best of our knowledge, one of the very first studies focusing on the molecular anchoring on the surfaces of this very important class of materials.

Role of Quantum-Confinement in Anatase Nanosheets.

Despite most of the applications of anatase nanostructures rely on photoexcited charge processes, yet profound theoretical understanding of fundamental related properties is lacking. Here, by means of ab initio ground and excited-state calculations, we reveal, in an unambiguous way, the role of quantum confinement effect and of the surface orientation, on the electronic and optical properties of anatase nanosheets (NSs). The presence of bound excitons extremely localized along the (001) direction, whose existence has been recently proven also in anatase bulk, explains the different optical behavior found for the two orientations - (001) and (101) - when the NS thickness increases. We suggest also that the almost two-dimensional nature of these excitons can be related to the improved photoconversion efficiency observed when a high percentage of (001) facet is present in anatase nanocrystals.

A Novel Nanoporous Graphite Based on Graphynes: First-Principles Structure and Carbon Dioxide Preferential Physisorption.

Ubiquitous graphene is a stricly 2D material representing an ideal adsorbing platform due to its large specific surface area as well as its mechanical strength and resistance to both thermal and chemical stresses. However, graphene as a bulk material has the tendency to form irreversible agglomerates leading to 3D graphitic structures with a significant decrease of the area available for adsorption and no room for gas intercalation. In this paper, a novel nanoporous graphite formed by graphtriyne sheets is introduced; its 3D structure is theoretically assessed by means of electronic structure and molecular dynamics computations within the DFT level of theory. It is found that the novel layered carbon allotrope is almost as compact as pristine graphite but the inherent porosity of the 2D graphyne sheets and its relative stacking leads to nanochannels that cross the material and whose subnanometer size could allow the diffusion and storage of gas species. A molecular prototype of the nanochannel is used to accurately determine first-principles adsorption energies and enthalpies for CO2, N2, H2O, and H2 within the pores. The proposed porous graphite presents no significant barrier for gas diffusion and shows a high propensity for CO2 physisorption with respect to the other relevant components in both pre- and postcombustion gas streams.

Structural and electronic features of small hybrid organic-inorganic halide perovskite clusters: a theoretical analysis.

We herein present the results of a series of calculations performed on some representative cluster models of hybrid organic-inorganic halide perovskites, (MA)jPbkXl (l = 2j + k; MA = methylammonium, +CH3NH3; X = halide). In particular, aimed at finding possible analogies with the bulk, we focused our initial attention on neutral clusters of iodides (X = I) constituted by an increasing number of Pb atoms (k = 1, 2, 8, 12). For the single octahedron (k = 1), we similarly extended our calculations to mixed Br-/I-terminated and fully Br-terminated octahedra, finding similar miscibilities for the two dimensionally different systems (i.e., the cluster and bulk). When increasing the size of the models, we found an unequivocally evident relationship between the total dielectric dipole moment of the investigated cluster and the wavefunction spatial distribution of the frontier molecular orbitals. This result rationalizes the structural and electronic properties of such zero-dimensional systems and supports the results previously obtained via linear scaling ab initio methods for very large supercells, i.e., the localization at the nanoscale of the wavefunction of the frontier orbitals as a function of the local fluctuations of the potential, which are mainly associated with the organic cation orientation.

The Effects of the Organic-Inorganic Interactions on the Thermal Transport Properties of CH3NH3PbI3.

Methylammonium lead iodide perovskite (CH3NH3PbI3), the most investigated hybrid organic-inorganic halide perovskite, is characterized by a quite low thermal conductivity. The rotational motion of methylammonium cations is considered responsible for phonon transport suppression; however, to date, the specific mechanism of the process has not been clarified. In this study, we elucidate the role of rotations in thermal properties based on molecular dynamics simulations. To do it, we developed an empirical potential for CH3NH3PbI3 by fitting to ab initio calculations and evaluated its thermal conductivity by means of nonequilibrium molecular dynamics. Results are compared with model systems that include different embedded cations, and this comparison shows a dominant suppression effect provided by rotational motions. We also checked the temperature dependence of the vibrational density of states and specified the energy range in which anharmonic couplings occur. By means of phonon dispersion analysis, we were able to fully elucidate the suppression mechanism: the rotations are coupled with translational motions of cations, via which inorganic lattice vibrations are coupled and scatter each other.

Zero-Dimensional Hybrid Organic-Inorganic Halide Perovskite Modeling: Insights from First Principles.

We discuss the properties of zero dimensional (cluster) hybrid organic-inorganic halide perovskite in view of their possible applicability in photovoltaics, light-emitting, and lasing devices. To support the need of theoretical investigations of such systems and pave the way for future investigations of clusters with different orientations, terminations, and compositions, we have assembled and characterized some zero dimensional models of methylammonium lead iodide, MAPbI3, by "cutting" its bulk. Interesting properties of such clusters that have been here theoretically investigated include their charge distribution, bandgap, wave function localization, and reduced effective mass. The surface orientation/termination and the organic/inorganic cation ratios have been discussed together with the roles they play in determining the electronic properties of such clusters. Also in agreement with experiments, it emerges that surface termination is crucial in determining the structural and optoelectronic properties of this largely overlooked, dimensionally reduced class of materials. Analogies and differences between clusters and bulk are discussed.