Computational Study of Halide Perovskite-Derived A2_2BX6_6 Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

The electronic structure and energetic stability of A$_2$BX$_6$ halide compounds with the cubic and tetragonal variants of the perovskite-derived K$_2$PtCl$_6$ prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A$_2$BX$_6$ compounds with A = K, Rb and Cs, and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se and Te and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A$_2$BX$_6$ compounds for potential photovoltaic applications

The Bright Side and the Dark Side of Hybrid Organic Inorganic Perovskites

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher energy bright and lower energy dark electrons (holes). The spatial separation of the dark electrons and the dark holes and the k-space separation of the bright and the dark charge carriers reduce electron hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets

Vacancy-Ordered Double Perovskite Cs2TeI6 Thin Films for Optoelectronics

Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an anti-solvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250°C, had an optical band gap of ~1.5 eV, absorption coefficients of ~6 x 104 cm-1, carrier lifetimes of ~2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV and had p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material

Materials Deforming Near their Ideal Strength

In recent years it has been shown that it is possible to design materials with strengths approaching their theoretical ideal limit. This is an intriguing development; materials typically fail at stresses that are several orders of magnitude below their theoretical limits of strength. The development of engineering alloys with usable strengths near the ideal limit would have profound technological implications. The most common approach used to increase a material's strength, is grain refinement. This method has been used to produce nanograined hollow nanospheres of CdS. Under nanoindentation these spheres show remarkable strength and deformation properties. The stresses and strains in the shells are studied with linear elastic finite element analyses and from this a failure criterion is developed. The stresses predicted by the failure criteria are 2.2 GPa, which is very large for an inherently brittle material. We compare the failure stress to the calculated ideal strength for CdS, calculated using density functional theory. Comparing the stress predicted by the failure criteria to the ideal strength shows that the hallow nanospheres approach 70% of their ideal strength. In 2003 a new Ti, Nb based alloy "Gum Metal" was introduced by Toyota Research Corp. This alloy has strength approaching the ideal limit even in bulk form. Moreover, the material deforms in a novel fashion without the obvious participation of dislocations. A Ti-V alloy has been chosen to study the properties of this type of alloy. The BCC ideal yield surface is examined as a function of composition. Dislocation core structures are also examined as a function of composition. The results explain some experimental observations in this novel system

Electronic Warfare in the Suwalki Gap: Facing the Russian “Accompli Attack”

The Joint Operating Environment 2035 predicts that for the foreseeable future, U.S. national interests will face challenges from both persistent disorders and states contesting international norms.1 One of these outfalls could be “accompli” attacks from near-peer and peer states to exploit disorder, challenge international norms, and enjoy a quick advance with a limited resistance that cannot be realistically reversed. The rapid attack could establish territorial gains requiring a large-scale land war to liberate—with the imminent threat of an escalation to nuclear war—and the potentially massive cost in life, pain, and devastation to reverse the attacker’s gains could be used to get negotiation leverage for the attacker in a final peace settlement. The attacker could also escalate the conflict once its territorial objectives are reached by declaring that a counteroffensive by the North Atlantic Treaty Organization (NATO) could face a tactical nuclear response, practically denying the Alliance the option to free the occupied territory with conventional military means

Rational design : a high-throughput computational screening and experimental validation methodology for lead-free and emergent hybrid perovskites

Perovskite solar cells, with efficiencies of 22.1%, are the only solution-processable technology to outperform multicrystalline silicon and thin-film solar cells. Whereas substantial progress has been made in scalability and stability, toxicity concerns drive the need for lead replacement, intensifying research into the broad palette of elemental substitutions, solid solutions, and multidimensional structures. Perovskites have gone from comprising three to more than eight (CH3NH3, HC(NH2)2, Cs, Rb, Pb, Sn, I, Br) organic and inorganic constituents, and a variety of new embodiments including layered, double perovskites, and metal-deficient perovskites are being explored. Although most experimentation is guided by intuition and trial-and-error-based Edisonian approaches, rational strategies underpinned by computational screening and targeted experimental validation are emerging. In addressing emergent perovskites, this perspective discusses the rational design methodology leveraging density functional theory-based high-throughput computational screening coupled to downselection strategies to accelerate the discovery of materials and industrialization of perovskite solar cells.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio

Association of missing paternal demographics on infant birth certificates with perinatal risk factors for childhood obesity

Background: The role of fathers in the development of obesity in their offspring remains poorly understood. We evaluated associations of missing paternal demographic information on birth certificates with perinatal risk factors for childhood obesity. Methods: Data were from the Linked CENTURY Study, a database linking birth certificate and well-child visit data for 200,258 Massachusetts children from 1980–2008. We categorized participants based on the availability of paternal age, education, or race/ethnicity and maternal marital status on the birth certificate: (1) pregnancies missing paternal data; (2) pregnancies involving unmarried women with paternal data; and (3) pregnancies involving married women with paternal data. Using linear and logistic regression, we compared differences in smoking during pregnancy, gestational diabetes, birthweight, breastfeeding initiation, and ever recording a weight for length (WFL) ≥ the 95th percentile or crossing upwards ≥2 WFL percentiles between 0–24 months among the study groups. Results: 11,989 (6.0 %) birth certificates were missing paternal data; 31,323 (15.6 %) mothers were unmarried. In adjusted analyses, missing paternal data was associated with lower birthweight (β -0.07 kg; 95 % CI: −0.08, −0.05), smoking during pregnancy (AOR 4.40; 95 % CI: 3.97, 4.87), non-initiation of breastfeeding (AOR 0.39; 95 % CI: 0.36, 0.42), and with ever having a WFL ≥ 95th percentile (AOR 1.10; 95 % CI: 1.01, 1.20). Similar associations were noted for pregnancies involving unmarried women with paternal data, but differences were less pronounced. Conclusions: Missing paternal data on the birth certificate is associated with perinatal risk factors for childhood obesity. Efforts to understand and reduce obesity risk factors in early life may need to consider paternal factors