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"Structure and chemistry of epitaxial ceria thin films on yttria-stabilized zirconia substrates, studied by high resolution electron microscopy" — Robert Sinclair: Sang Chul Lee, Yezhou Shi, William C. Chueh; Ultramicroscopy, 03/18/17.
We have applied aberration-corrected transmission electron microscopy (TEM) imaging and electron energy loss spectroscopy (EELS) to study the structure and chemistry of epitaxial ceria thin films, grown by pulsed laser deposition onto (001) yttria-stabilized zirconia (YSZ) substrates. There are few observable defects apart from the expected mismatch interfacial dislocations and so the films would be expected to have good potential for applications. Under high electron beam dose rate (above about 6000 e-/Å2s) domains of an ordered structure appear and these are interpreted as being created by oxygen vacancy ordering. The ordered structure does not appear at lower lose rates (ca. 2600 e-/Å2s) and can be removed by imaging under 1 mbar oxygen gas in an environmental TEM. EELS confirms that there is both oxygen deficiency and the associated increase in Ce3+ versus Ce4+ cations in the ordered domains. In situ high resolution TEM recordings show the formation of the ordered domains as well as atomic migration along the ceria thin film (001) surface.
"Pressure-Induced Metallization of the Halide Perovskite (CH3NH3)PbI3" — Adam Jaffe: Yu Lin, Wendy L. Mao, Hemamala I. Karunadasa; Journal of the American Chemical Society, 03/14/17.
We report the metallization of the hybrid perovskite semiconductor (MA)PbI3 (MA = CH3NH3 + ) with no apparent structural transition. We tracked its bandgap evolution during compression in diamond-anvil cells using absorption spectroscopy and observed strong absorption over both visible and IR wavelengths at pressures above ca. 56 GPa, suggesting the imminent closure of its optical bandgap. The metallic character of (MA)PbI3 above 60 GPa was confirmed using both IR reflectivity and variable-temperature dc conductivity measurements. The impressive semiconductor properties of halide perovskites have recently been exploited in a multitude of optoelectronic applications. Meanwhile, the study of metallic properties in oxide perovskites has revealed diverse electronic phenomena. Importantly, the mild synthetic routes to halide perovskites and the templating effects of the organic cations allow for fine structural control of the inorganic lattice. Pressure-induced closure of the 1.6 eV bandgap in (MA)PbI3 demonstrates the promise of the continued study of halide perovskites under a range of thermodynamic conditions, toward realizing wholly new electronic properties.
"Field-theoretic simulations of random copolymers with structural rigidity" — Shifan Mao: Quinn MacPherson, Jian Qin, Andrew J. Spakowitz; The Royal Society of Chemistry, 03/13/17.
Copolymers play an important role in a range of soft-materials applications and biological phenomena. Prevalent works on block copolymer phase behavior use flexible chain models and incorporate interactions using a mean-field approximation. However, when phase separation takes place on length scales comparable to a few monomers, the structural rigidity of the monomers becomes important. In addition, concentration fluctuations become significant at short length scales, rendering the mean-field approximation invalid. In this work, we use simulation to address the role of finite monomer rigidity and concentration fluctuations in microphase segregation of random copolymers. Using a field-theoretic Monte-Carlo simulation of semiflexible polymers with random chemical sequences, we generate phase diagrams for random copolymers. We find that the melt morphology of random copolymers strongly depends on chain flexibility and chemical sequence correlation. Chemically anti-correlated copolymers undergo first-order phase transitions to local lamellar structures. With increasing degree of chemical correlation, this first-order phase transition is softened, and melts form microphases with irregular shaped domains. Our simulations in the homogeneous phase exhibit agreement with the density–density correlation from mean-field theory. However, conditions near a phase transition result in deviations between simulation and mean-field theory for the density–density correlation and the critical wavemode. Chain rigidity and sequence randomness lead to frustration in the segregated phase, introducing heterogeneity in the resulting morphologies.
"Reviving the lithium metal anode for high-energy batteries" — Dingchang Lin: Yayuan Liu & Yi Cui; Nat Nano, 03/07/17.
Lithium-ion batteries have had a profound impact on our daily life, but inherent limitations make it difficult for Li-ion chemistries to meet the growing demands for portable electronics, electric vehicles and grid-scale energy storage. Therefore, chemistries beyond Li-ion are currently being investigated and need to be made viable for commercial applications. The use of metallic Li is one of the most favoured choices for next-generation Li batteries, especially Li–S and Li–air systems. After falling into oblivion for several decades because of safety concerns, metallic Li is now ready for a revival, thanks to the development of investigative tools and nanotechnology-based solutions. In this Review, we first summarize the current understanding on Li anodes, then highlight the recent key progress in materials design and advanced characterization techniques, and finally discuss the opportunities and possible directions for future development of Li anodes in applications.
"Temperature-dependent optical properties of titanium nitride" — Justin A. Briggs: Gururaj V. Naik, Yang Zhao, Trevor A. Petach, Kunal Sahasrabuddhe, David Goldhaber-Gordon, Nicholas A. Melosh, and Jennifer A. Dionne; Applied Physics Letters, 03/06/17.
The refractory metal titanium nitride is promising for high-temperature nanophotonic and plasmonic applications, but its optical properties have not been studied at temperatures exceeding 400 ͦ C. Here, we perform in-situ high-temperature ellipsometry to quantify the permittivity of TiN films from room temperature to 1258 ͦ C. We find that the material becomes more absorptive at higher temperatures but maintains its metallic character throughout visible and near infrared frequencies. X-ray diffraction, atomic force microscopy, and mass spectrometry confirm that TiN retains its bulk crystal quality and that thermal cycling increases the surface roughness, reduces the lattice constant, and reduces the carbon and oxygen contaminant concentrations. The changes in the optical properties of the material are highly reproducible upon repeated heating and cooling, and the room-temperature properties are fully recoverable after cooling. Using the measured high-temperature permittivity, we compute the emissivity, surface plasmon polariton propagation length, and two localized surface plasmon resonance figures of merit as functions of temperature. Our results indicate that titanium nitride is a viable plasmonic material throughout the full temperature range explored.
"Theoretical Investigation of 2D Layered Materials as Protective Films for Lithium and Sodium Metal Anodes" — Hongzhen Tian: Zhi Wei Seh, Kai Yan, Zhongheng Fu, Peng Tang, Yingying Lu, Ruifeng Zhang, Dominik Legut, Yi Cui, and Qianfan Zhang; Advanced Energy Materials, 02/17/17.
Rechargeable batteries based on lithium (sodium) metal anodes have been attracting increasing attention due to their high capacity and energy density, but the implementation of lithium (sodium) metal anode still faces many challenges, such as low Coulombic efficiency and dendrites growth. Layered materials have been used experimentally as protective films (PFs) to address these issues. In this work, the authors explore using first-principles computations the key factors that determine the properties and feasibility of various 2D layered PFs, including the defect pattern, crystalline structure, bond length, and metal proximity effect, and perform the simulations on both aspects of Li+ (Na+) ion diffusion property and mechanical stability. It is found that the introduction of defect, the increase in bond length, and the proximity effect by metal can accelerate the transfer of Li+ (Na+) ion and improve the ionic conductivity, but all of them make negative influences on the stiffness of materials against the suppression of dendrite growth and weaken both critical strains and critical stress. The results provide new insight into the interaction mechanism between Li+ (Na+) ions and PF materials at the atomic level and shed light onto exploring a variety of layered PF materials in metal anode battery systems.
"A half-wave rectified alternating current electrochemical method for uranium extraction from seawater" — Chong Liu: Po-Chun Hsu, Jin Xie, Jie Zhao, Tong Wu, Haotian Wang, Wei Liu, Jinsong Zhang, Steven Chu and Yi Cui; Nature Energy, 02/17/17.
In total there is hundreds of times more uranium in sea water than on land, but extracting it for use in nuclear power generation is challenging due to its low concentration (∼3 ppb) and the high salinity background. Current approaches based on sorbent materials are limited due to their surface-based physicochemical adsorption nature. Here we use a half-wave rectified alternating current electrochemical (HW-ACE) method for uranium extraction from sea water based on an amidoximefunctionalized carbon electrode. The amidoxime functionalization enables surface specific binding to uranyl ions, while the electric field can migrate the ions to the electrode and induce electrodeposition of uranium compounds, forming charge-neutral species. Extraction is not limited by the electrode surface area, and the alternating manner of the applied voltage prevents unwanted cations from blocking the active sites and avoids water splitting. The HW-ACE method achieved a ninefold higher uranium extraction capacity (1,932 mg g−1 ) without saturation and fourfold faster kinetics than conventional physicochemical methods using uranium-spiked sea water
"Ubiquitous strong electron–phonon coupling at the interface of FeSe/SrTiO3" — Chaofan Zhang: Zhongkai Liu, Zhuoyu Chen, Yanwu Xie, Ruihua He, Shujie Tang, Junfeng He, Wei Li, Tao Jia, Slavko N. Rebec, Eric Yue Ma, Hao Yan, Makoto Hashimoto, Donghui Lu, Sung-Kwan Mo, Yasuyuki Hikita, Robert G. Moore, Harold Y. Hwang, Dunghai Lee & Zhixun Shen; Nature Communications, 02/10/17.
The observation of replica bands in single-unit-cell FeSe on SrTiO3(STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.
"Coexistence of Replica Bands and Superconductivity in FeSe Monolayer Films" — S. N. Rebec: T. Jia, C. Zhang, M. Hashimoto, D.-H. Lu, R. G. Moore, and Z.-X. Shen; Physical Review Letters, 02/10/17.
To elucidate the mechanisms behind the enhanced Tc in monolayer (1 ML) FeSe on SrTiO3 (STO), we grew highly strained 1 ML FeSe on the rectangular (100) face of rutile TiO2, and observed the coexistence of replica bands and superconductivity with a Tc of 63 K. From the similar Tc between this system and 1ML FeSe on STO (001), we conclude that strain and dielectric constant are likely unimportant to the enhanced Tc in these systems. A systematic comparison of 1 ML FeSe on TiO2 with other systems in the FeSe family shows that while charge transfer alone can enhance Tc, it is only with the addition of interfacial electron-phonon coupling that Tc can be increased to the level seen in 1 ML FeSe on STO.
"Magnetism and local symmetry breaking in a Mott insulator with strong spin orbit interactions" — L. Lu: M. Song, W. Liu, A. P. Reyes, P. Kuhns, H. O. Lee, I. R. Fisher & V. F. Mitrović; Nature Communications, 02/09/17.
Study of the combined effects of strong electronic correlations with spin-orbit coupling (SOC) represents a central issue in quantum materials research. Predicting emergent properties represents a huge theoretical problem since the presence of SOC implies that the spin is not a good quantum number. Existing theories propose the emergence of a multitude of exotic quantum phases, distinguishable by either local point symmetry breaking or local spin expectation values, even in materials with simple cubic crystal structure such as Ba2NaOsO6. Experimental tests of these theories by local probes are highly sought for. Our local measurements designed to concurrently probe spin and orbital/lattice degrees of freedom of Ba2NaOsO6 provide such tests. Here we show that a canted ferromagnetic phase which is preceded by local point symmetry breaking is stabilized at low temperatures, as predicted by quantum theories involving multipolar spin interactions.