Spotlights Cite This: J. Phys. Chem. Lett. 2018, 9, 2105−2105
Spotlights: Volume 9, Issue 8
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FIRST-PRINCIPLES COMPUTED RATE CONSTANT FOR THE O + O2 ISOTOPIC EXCHANGE REACTION NOW MATCHES EXPERIMENT
IN SITU MONITORING THE UPTAKE OF MOISTURE INTO HYBRID PEROVSKITE THIN FILMS As any gardener knows, water is crucial for success, but there can be too much of a good thing. This is true in many other areas, including photovoltaics. The presence of water at low moisture levels influences perovskite film formation and can even be beneficial for solar cell performance, but it can be detrimental as well, as was found for a mixed halide perovskite solar cell stored in 55% relative humidity for 1 day without encapsulation. Encapsulation is one of several means of preventing ingression of moisture into perovskite thin films that have been studied, and some material compositions have been found to be less prone to hydration. Research is needed to understand fully the kinetics of water uptake in perovskite films, e.g., to enable cost-effective industrial device production in ambient atmospheres. To that end, Schlipf et al. (10.1021/ acs.jpclett.8b00687) studied the ingression of moisture into MAPI films under different humidity levels in the surrounding atmosphere. They used in situ grazing incidence small-angle neutron scattering (in situ GISANS) and heavy water (D2O) to follow the kinetics of changes in film morphology. Their MAPI films show the capability of incorporating up to ∼50 vol % water at the highest humidity levels, suggesting intermediate amorphous phases. The authors were surprised to find that water entered the perovskite films even at low humidity. Hydration and dehydration of the films were accompanied by various morphological changes, so protection of hybrid perovskite thin films against ambient humidity during device fabrication and operation is necessary.
Ozone plays a key role in the Earth’s atmosphere, so it is no surprise that it is among the most extensively studied small polyatomic molecules. Nearly 40 years ago the stratospheric ozone O3 molecule was shown to present a strong enrichment in heavy isotope 18O, referred to as mass-independent fractionation (MIF), but a full explanation of the anomalous isotope effect for ozone formation is still needed to further our understanding of ozone chemistry, production, lifetime, and loss in the atmosphere. Guillon et al. (10.1021/acs.jpclett.8b00661) used a full quantum mechanical treatment and found quantitative agreement of theoretical rates of the 18O + 16 16 O O reaction with the latest measurements. Their results demonstrate the high quality of the interaction potential between oxygen atoms at the “spectroscopic accuracy” level and the importance of correctly describing the numerous metastable states of O3*. Because the O3* ro-vibrational population is one of the major characteristics for the thermodynamic models used to interpret ozone measurements in the atmosphere by groundbased and satellite instruments, the findings may help improve the accuracy and reliability of ozone monitoring.
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SURPRISING STABILITY OF CUBANE UNDER EXTREME PRESSURE
The behavior of molecular solids subjected to pressure is of particular interest in physical chemistry. Under extreme conditions, the free-energy change associated with compression can alter the bonding patterns of molecular systems dramatically, leading to unexplored chemical reaction pathways and the formation of new materials, most famously the diamond. Recent interest has turned to the critical roles of volume of activation and intermolecular distance in the organic reactions of small molecules under pressure. For example, cubane, the smallest platonic hydrocarbon, is considered an energetic molecule, and its spontaneous explosion at a pressure of 3 GPa and room temperature has been reported, but more information is needed about its vibrational and structural behavior under pressure. As reported in their Letter, Huang et al. (10.1021/acs.jpclett.8b00395) found that cubane shows no unimolecular decomposition or intermolecular reaction up to a pressure of 60 GPa. These results demonstrate that reaction pathways with zero or positive volume of activation will be forbidden under pressure even with an immensely strained chemical structure. The findings also suggest that the lack of a translatory lattice phonon can hamper the triggering dynamics for the pressure-induced polymerization of molecular cubane. Using powder X-ray diffraction analysis, the authors found that no phase transition has occurred up to 60 GPa, showing that the bulk modulus of solid cubane is determined from the fitted equation of state. © 2018 American Chemical Society
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Published: April 19, 2018 2105
DOI: 10.1021/acs.jpclett.8b01113 J. Phys. Chem. Lett. 2018, 9, 2105−2105
