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Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported. Here we demonstrate that the selection of spacer cations (i.e., selective fluorination of phenethylammonium) affects the film properties of two-dimensional perovskites, leading to different device performance of two-dimensional perovskite solar cells (average n = 4). Structural analysis reveals that different packing arrangements and orientational disorder of the spacer cations result in orientational degeneracy and different formation energies, largely explaining the difference in film properties. This work provides key missing information on how spacer cations exert influence on desirable electronic properties and device performance of two-dimensional perovskites via the weak and cooperative interactions of these cations in the crystal lattice.

We report the crystal growth of well-faceted single crystals of methylammonium lead iodide, CH 3 NH 3 PbI 3 , and detailed single crystal neutron diffraction structural studies aimed at elucidating the orientation of the methylammonium (CH 3 NH 3 + ) cation in the tetragonal and cubic phases of the hybrid inorganic−organic perovskite. Room temperature experiments reveal a tetragonal structure where the protonated amine substituent (−NH 3 + ) of the cation is disordered in four positions, each preferentially located near the neighboring iodine of the [PbI 6 ] octahedra, while the methyl substituent (−CH 3 ) is disordered in eight positions located near the body position of the unit cell. High temperature experiments show a cubic structure where the cation aligns along the [011] (edge), the [111] (diagonal), and the [100] (face) directions of the unit cell. The resulting site occupancy ratio suggests the CH 3 NH 3 + cation resides primarily along the [011] direction, in agreement with reported DFT calculations. One important feature that was observed for both tetragonal and cubic structures measured at 295 and 350 K, respectively, is the middle point of the C−N bond being located off-center from the high symmetry sites in the crystal structure, induced by the formation of hydrogen bond-like interactions between the −NH 3 + substituent of the organic cation and the iodine atoms of [PbI 6 ] octahedra.

Magnetic honeycomb iridates are thought to show strongly spin-anisotropic exchange interactions which, when highly frustrated, lead to an exotic state of matter known as the Kitaev quantum spin liquid. However, in all known examples these materials magnetically order at finite temperatures, the scale of which may imply weak frustration. Here we show that the application of a relatively small magnetic field drives the three-dimensional magnet β-Li2IrO3 from its incommensurate ground state into a quantum correlated paramagnet. Interestingly, this paramagnetic state admixes a zig-zag spin mode analogous to the zig-zag order seen in other Mott-Kitaev compounds. The rapid onset of the field-induced correlated state implies the exchange interactions are delicately balanced, leading to strong frustration and a near degeneracy of different ground states.

Water cluster formation and methane adsorption within a hydrophobic porous metal organic framework is studied by in situ vibrational spectroscopy, adsorption isotherms, and first-principle DFT calculations (using vdW-DF). Specifically, the formation and stability of H2O clusters in the hydrophobic cavities of a fluorinated metal-organic framework (FMOF-1) is examined. Although the isotherms of water show no measurable uptake (see Yang et al. J. Am. Chem. Soc. 2011 , 133 , 18094 ), the large dipole of the water internal modes makes it possible to detect low water concentrations using IR spectroscopy in pores in the vicinity of the surface of the solid framework. The results indicate that, even in the low pressure regime (100 mTorr to 3 Torr), water molecules preferentially occupy the large cavities, in which hydrogen bonding and wall hydrophobicity foster water cluster formation. We identify the formation of pentameric water clusters at pressures lower than 3 Torr and larger clusters beyond that pressure. The binding energy of the water species to the walls is negligible, as suggested by DFT computational findings and corroborated by IR absorption data. Consequently, intermolecular hydrogen bonding dominates, enhancing water cluster stability as the size of the cluster increases. The formation of water clusters with negligible perturbation from the host may allow a quantitative comparison with experimental environmental studies on larger clusters that are in low concentrations in the atmosphere. The stability of the water clusters was studied as a function of pressure reduction and in the presence of methane gas. Methane adsorption isotherms for activated FMOF-1 attained volumetric adsorption capacities ranging from 67 V(STP)/V at 288 K and 31 bar to 133 V(STP)/V at 173 K and 5 bar, with an isosteric heat of adsorption of ca. 14 kJ/mol in the high temperature range (288-318 K). Overall, the experimental and computational data suggest high preferential uptake for methane gas relative to water vapor within FMOF-1 pores with ease of desorption and high framework stability under operative temperature and moisture conditions.

The demonstrated square-planar Pt(II)-complex has reduced triplet-triplet quenching and therefore a near unity quantum yield in the neat thin film. A non-doped phosphorescent organic light-emitting diode (PhOLED) based on this emitter achieves (31.1 ± 0.1)% external quantum efficiency without any out-coupling, which shows that a non-doped PhOLED can be comparable in efficiency to the best doped devices with very complicated device structures.

Two-dimensional (2D) organic−inorganic hybrid perovskites (OIHPs) have showed impressive stability, compared to their three-dimensional (3D) counterparts. However, tuning the chemical structure of the organic cations to simultaneously improve the device performance and stability of 2D OIHP solar cells is rarely reported. Here, we demonstrate that by introducing a classic noncovalent aryl-perfluoroaryl interaction, 2D OIHP solar cells with 1:1 mixed phenethylammonium (PEA) and perfluorophenethylammonium (F5-PEA) can achieve an efficiency of >10% with much enhanced stability using a simple deposition at low temperature without using any additives. The competing effects of surface morphology and crystal orientation with an increased amount of F5-PEA result in the highest efficiency at a 1:1 ratio, while singlecrystal studies reveal the expected aryl-perfluoroaryl interaction, accounting for the highest device stability of 2D OIHP solar cell at 1:1 ratio as well. This work provides an example where tuning the interactions of organic cations via molecular engineering can have a profound effect on device performance and stability of 2D OIHP solar cells.

Recent reports have linked severe lung injuries and deaths to the use of e-cigarettes and vaping products. Nevertheless, the causal relationship between exposure to vaping emissions and the observed health outcomes remains to be elucidated. Through chemical and toxicological characterization of vaping emission products, this study demonstrates that during vaping processes, changes in chemical composition of several commonly used vape juice diluents (also known as cutting agents) lead to the formation of toxic byproducts, including quinones, carbonyls, esters, and alkyl alcohols. The resulting vaping emission condensates cause inhibited cell proliferation and enhanced cytotoxicity in human airway epithelial cells. Notably, substantial formation of the duroquinone and durohydroquinone redox couple was observed in the vaping emissions from vitamin E acetate, which may be linked to acute oxidative stress and lung injuries reported by previous studies. These findings provide an improved molecular understanding and highlight the significant role of toxic byproducts in vapingassociated health effects.