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The organic–inorganic hybrid perovskites have been widely explored as key functional components for energy harvesting/conversion applications due to their superior photovoltaic properties. However, material stability issues, such as temperature induced instability of hybrid perovskite crystals during normal device operating conditions, limit their practical application. Here we conduct molecular dynamics simulations to investigate the thermal instability in pristine as well as defective crystals of the prototypical organic–inorganic hybrid perovskite, methylammonium lead iodide (MAPbI3). We show that the accumulation of tilting and splitting of PbI6 octahedra with increasing temperatures initiates the instability in pristine MAPbI3 crystals. Point defects can accelerate the inception of local lattice instability, and the crystals with such defects in the concentration range typically observed in perovskite-based devices undergo an irreversible instability at much lower temperatures. Two-dimensional defects such as grain boundaries in polycrystalline MAPbI3 crystals further decrease their crystal instability temperature to about 550 K, in good agreement with experimental measurements. Finally, we demonstrate that thermal instability in MAPbI3 thin films originates from their free surfaces at much lower temperatures due their increased free energies. We also investigate the structural evolution of the local lattice and show that Born and Lindemann crystal instability criteria coincide in initiating the instability. The key insights obtained from this work can usher a rational design of highly stable hybrid perovskites for their robust and reliable photovoltaic applications. [ABSTRACT FROM AUTHOR]

Lithium metal has been intensively investigated as a promising anode for next generation rechargeable Li metal batteries (LMBs). However, the safety concern on Li anodes caused by uncontrolled Li dendrite growth in liquid electrolytes hinders their application. Herein, a novel poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) based gel polymer electrolyte (GPE) with an asymmetric structure has been designed and developed to effectively retard the growth of lithium dendrites. Atomistic simulations confirm the strong interactions between PF6− and dipoles in the polymer matrix, which can anchor PF6− in the GPE and slow down its mobility to prevent space charge formation. In addition, this unique asymmetric membrane with a channel upper layer greatly enhances the mobility of Li+ in the GPE due to its low tortuosity and high porosity. The synergistic effect of the ion-dipole interaction and asymmetric structure increases the Li+ transference number to 0.66 and ionic conductivity to 3.36 mS cm−1 (20 °C). Using this superior asymmetric GPE, Li‖Li symmetric cells show more stable cycle performance than those using a liquid electrolyte. Li‖LiFePO4 batteries with the asymmetric GPE also deliver an impressive electrochemical performance, i.e., coulombic efficiency of 99.5% at 2C after 600 cycles. In consequence, this novel asymmetric GPE possesses potential application in high energy LMBs. [ABSTRACT FROM AUTHOR]

Caesium silver bismuth halide double perovskites, in the first instance Cs2AgBiBr6, were recently introduced to the field of emerging photovoltaics as environmentally friendly, non-toxic and thermodynamically stable photoabsorber materials. However, the wide indirect bandgaps of these materials indicate the need for bandgap engineering and enhancing the light absorption to improve the photovoltaic performance. The present work demonstrates that this can be achieved via modification of the double perovskite with sulphide to obtain caesium silver bismuth sulphobromide materials, Cs2AgBiBr6−2xSx, which have been synthesised as pin-hole-free uniform films and systematically investigated herein. Notable enhancements in the intrinsic light absorption for 0 ≤ x ≤ 0.2 are demonstrated, as are the improvements by up to 50% in the photocurrent density of the corresponding thin-film solar cells. The devices based on the films with the nominal composition Cs2AgBiBr5.8S0.1 delivered a short-circuit current density of 3.0 ± 0.3 mA cm−2 and a power conversion efficiency of 1.9 ± 0.1% (cf. 2.1 ± 0.2 mA cm−2 and 1.3 ± 0.1%, respectively, for the control cells based on the sulphide-free Cs2AgBiBr6). Equally important, caesium silver bismuth sulphobromides demonstrate excellent stability against all common environmental stimuli, viz. heat, light, and humidity. [ABSTRACT FROM AUTHOR]

Non-covalent functionalization of graphene materials with responsive polymers is a promising approach for synthesizing new, hybrid composites with improved dispersibility and functional properties. However, the interplay between various components of the hybrid systems, their structural configurations, and stimuli-responsive behavior are not yet well understood at the atomic level. Here, we investigate the temperature-responsive behavior of physisorbed poly(N-isopropylacrylamide) (PNIPAM) on to graphene (G) and graphene oxide (GO) sheets in aqueous solution using large scale molecular dynamics simulations. It was observed that PNIPAM can be spontaneously anchored to the surfaces of both G and GO at 290 K with a macromolecular coil shape. However, the configuration of PNIPAM on G is markedly different in comparison with that on GO, leading to its distinct thermoresponsive behavior. Specifically, the adsorption on G gives rise to an increase in the temperature of the coil-to-globule transition when compared to the native polymer, the origin of which can be interpreted in terms of the interactions and the solvation behavior. The results obtained here are of significance to the design and manipulation of graphene-based stimuli-responsive hybrid systems with optimal functional properties. [ABSTRACT FROM AUTHOR]

The energetic stability of layered covalent organic frameworks (COFs) in slipped structures and the experimental control of interlayer slipping (Q. Fang, Z. Zhuang, S. Gu, R. B. Kaspar, J. Zheng, J. Wang, S. Qiu and Y. Yan, Nat. Commun., 2014, 5, 4503) suggest that the interlayer slipping could be used as a design parameter to enhance the gas adsorption and separation properties of COFs. In this work, we have systematically studied the effect of interlayer slipping on CO2 adsorption and CO2/N2 separation in microporous TpPa1 and mesoporous TpBD and PI-COFs using quantum mechanical and grand canonical Monte Carlo simulations. We found that the slipping affects the number of preferred CO2 adsorption regions and corresponding adsorption energies, resulting in a drastic variation in the adsorption uptake. Our detailed analysis of the heat of adsorption, density distribution and energy landscape reveals that the effect of slipping on CO2 uptake is non-monotonous. We explain this behavior using a simplified model that also provides an optimal range of slipping distances to increase the gas storage performance of COFs. Our results show that the optimized slipped COF structures have approximately three times higher CO2 working capacity and CO2/N2 selectivity as compared to eclipsed structures. The highest CO2 working capacity of 5.8 mol kg−1 and CO2 : N2 separation selectivity of 197 (at 1 bar and 298 K) were observed for slipped PI-COF-2 and TpBD COFs, respectively, which are higher than those for any other COFs reported to date. The molecular insight presented here is qualitatively applicable to other similar slipped COFs and is useful for the development of COFs for enhanced gas storage and separation applications. [ABSTRACT FROM AUTHOR]

Herein we describe a large-scale, non-injection “one-pot” batch method for producing large quantities of novel colloidal Ag–Ag8GeS6 heteronanostructures. Using a suite of analytical techniques, including high resolution TEM, HAADF-STEM, XEDS mapping, and XRD, the formation mechanism of the nanostructures is elucidated. The formation is discovered to occur in three stages comprising nucleation, phase separation of the metal and semiconductor components, and the final segregation of the metal and semiconductor components to form the Janus nanostructure. The high ionic mobility and chemical reactivity of Ag enables the self-regulated formation of Janus nanostructures in optimized one-pot reactions – a phenomenon that is almost unique to silver-based systems. As such, silver-based systems are ideal candidates to study the formation of Janus nanostructures. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) magnetic materials offer a promising platform for nanoscale spintronics and for exploration of novel physical phenomena. Here, we predict a diverse range of magnetic orders in cobalt-based 2D single septuple layers CoX 2 Y 4 , namely, CoBi 2 Te 4 , CoBi 2 Se 2 Te 2 , CoBi 2 Se 4 , and CoSb 2 Te 4 . Notably, CoBi 2 Te 4 presents intrinsic non-collinear antiferromagnetism (AFM), while the others display collinear AFM. The emergence of AFM in all CoX 2 Y 4 materials is attributed to the antiferromagnetic 90° Co-Te(Se)-Co superexchange coupling. The origin of non-collinear/collinear orders lies in competing Heisenberg exchange interactions within the Co triangular lattice. A pivotal factor governing the non-collinear order of CoBi 2 Te 4 is the vanishingly small ratio of exchange coupling between next-nearest neighbour Co and the nearest neighbour Co ( J 2 / J 1 ∼ 0.01). Furthermore, our investigation into strain effects on CoX 2 Y 4 lattices demonstrates the tunability of the magnetic state of CoSb 2 Te 4 from collinear to non-collinear. Our prediction of the unique non-collinear AFM in 2D suggests the potential for observing extraordinary magnetic phenomena in 2D, including non-collinear scattering and magnetic domain walls.