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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Controlling the structure and ferroic properties of strained epitaxial NiTiO3 thin films on sapphire by post-deposition annealing

Ferroelectrically induced weak ferromagnetism had been predicted earlier in compounds MTiO3 (M = Fe, Mn, Ni) with the LiNbO3-type structure. In order to stabilize this metastable structure by oxide heteroepitaxy, we attempted to grow epitaxial NiTiO3 films on the nonpolar, non-magnetic substrate Al2O3 by pulsed laser deposition. Given the structural imperfections of the as-deposited films arising from the large lattice mismatch, which resulted in weak ferroic ordering, we investigated the effect of post-synthesis annealing on the films’ properties. Our structural data from x-ray diffraction and electron microscopy suggest that the crystalline quality of the NiTiO3 films was greatly improved by annealing the films at 600-1000 oC for 8 hours, with new phase formation along the interface at 1000 oC. Our physical property characterization indicates increased ferromagnetism and polarization after 600 oC, but a change in the magnetic state to likely ferrimagnetism, and a loss of polarization after the 1000 oC treatment. Our results suggest that the ferroic domain properties of the NiTiO3 films can be favorably altered by post-synthesis heat treatment at the proper temperature to manipulate interfacial strain and enhance the multiferroic properties. Improvement in the ferroic properties can be explained by a combination of crystallite growth, interfacial strain relief, and thermodynamic phase stability arguments. At the same time, exceeding temperatures may drive the system to a thermodynamically more favored crystalline order, and eventually to phase segregation and decomposition.

Varga, Tamas↗
DOE OSTI2018

Hydrogen Bonding Controls the Structural Evolution in Perovskite-Related Hybrid Platinum(IV) Iodides

In this paper, we describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A 2 PtI 6 , where A is one of the following amines: ammonium, NH 4 + ; methylammonium, CH 3 NH 3 + ; formamidinium, CH(NH 2 ) 2 + ; guanidinium, C(NH 2 ) 3 + . These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A 2 Pt$\square$I 6 ($\square$ indicates a vacant site) structure in the K 2 PtCl 6 archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA) 2 PtI 6 , crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl 3 structure: the K 2 MnF 6 structure. A combination of solid-state 195 Pt and 1 H NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. Finally, the calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I–Pt–I connectivity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Transient Symmetry Controls Photo Dynamics near Conical Intersections

Excited-state chemistry lacks generalised symmetry rules. With many femtochemistry studies focused on individual cases, it is hard to build up the same level of chemical intuition for excited-states as exists for ground states. Here, we unravel the degrees of freedom involved in ultrafast internal conversion (IC) by mapping the vibrational coherence of the initial wavepacket, and the dependence on molecular symmetry in various cyclic tertiary amines. Molecular symmetry plays an important role in the preservation of vibrational coherence in the transit from one electronic state to another. We show here that it is sufficient for the molecule to simply have the possibility of a more symmetric structure to achieve the preservation of vibrational coherence. It can be transient and still lead to preservation. Furthermore, this finding provides an additional angle on how symmetry influences electronic transitions and an additional piece to the puzzle of establishing symmetry-based selection rules for excited-state processes.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Manipulating Triplet Yield through Control of Symmetry-Breaking Charge Transfer

The efficiency of an organic solar cell depends on the efficacy of exciton diffusion and dissociation processes, and this can be enhanced by reducing the exciton binding energy and increasing the exciton lifetime. Zinc chlorodipyrrin (ZCl) complexes exhibit reduced exciton binding energy due to ultrafast generation of intramolecular charge transfer (ICT) states via symmetry breaking charge transfer in polar media. This paper explores the fate of the ICT states using nanosecond transient absorption. In cyclohexane, ZCl undergoes intersystem-crossing to produce triplets with ~8 ns time constant (~30% yield) and no ICT states are generated. However, in more polar solvents, triplets are generated within 1 ns via ICT state recombination with ~3 times higher yield than produced via ISC. Furthermore, this high triplet yield in toluene (89%) and acetonitrile (76%) via ICT state recombination is a beneficial pathway to spin-protect the excited state decay for additional charge generation from triplet excited states.

14 SOLAR ENERGY↗
DOE OSTI2018

Tunable Polaron Distortions Control the Extent of Halide Demixing in Lead Halide Perovskites

Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I$^-$ rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br$^-$/I$^-$ methylammonium (MA + ) lead halide perovskite (MAPb(Br x I 1- x ) 3 ) nanoplates, eliminating effects from extended structural defects. Even in these nanoplates, we find that phase separation occurs, resulting in I$^-$rich clusters that are nucleated stochastically and stabilized by polarons. Upon lowering the electron-phonon coupling strength by partially exchanging MA + for Cs + , a phase-separated steady state is not reached, nevertheless transient I$^-$ clustering still occurs. Our results, supported by multiscale modeling, demonstrate that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics of which depends on the electron-phonon coupling strength.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗