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"Vertical-Substrate MPCVD Epitaxial Nanodiamond Growth" — Yan-Kai Tzeng: Jingyuan Linda Zhang, Haiyu Lu, Hitoshi Ishiwata, Jeremy Dahl, Robert M. K. Carlson, Hao Yan, Peter R. Schreiner, Jelena Vuckovic, Zhi-Xun Shen, Nicholas Melosh, and Steven Chu; American Chemical Society, 02/09/17.
Color center-containing nanodiamonds have many applications in quantum technologies and biology. Diamondoids, molecular-sized diamonds have been used as seeds in chemical vapor deposition (CVD) growth. However, optimizing growth conditions to produce high crystal quality nanodiamonds with color centers requires varying growth conditions that often leads to ad-hoc and time-consuming, one-at-a-time testing of reaction conditions. In order to rapidly explore parameter space, we developed a microwave plasma CVD technique using a vertical, rather than horizontally oriented stage-substrate geometry. With this configuration, temperature, plasma density, and atomic hydrogen density vary continuously along the vertical axis of the substrate. This variation allowed rapid identification of growth parameters that yield single crystal diamonds down to 10 nm in size and 75 nm diameter optically active center silicon-vacancy (Si-V) nanoparticles. Furthermore, this method may provide a means of incorporating a wide variety of dopants in nanodiamonds without ion irradiation damage.
"Core–Shell Nanoparticle Coating as an Interfacial Layer for Dendrite-Free Lithium Metal Anodes" — Wei Liu: Weiyang Li, Denys Zhuo, Guangyuan Zheng, Zhenda Lu, Kai Liu, and Yi Cui; ACS Central Science, 02/08/17.
Lithium metal based batteries represent a major challenge and opportunity in enabling a variety of devices requiring high-energy-density storage. However, dendritic lithium growth has limited the practical application of lithium metal anodes. Here we report a nanoporous, flexible and electrochemically stable coating of silica@poly(methyl methacrylate) (SiO2@ PMMA) core−shell nanospheres as an interfacial layer on lithium metal anode. This interfacial layer is capable of inhibiting Li dendrite growth while sustaining ionic flux through it, which is attributed to the nanoscaled pores formed among the nanospheres. Enhanced Coulombic efficiencies during lithium charge/ discharge cycles have been achieved at various current densities and areal capacities.
"Strong ferromagnetic exchange interaction under ambient pressure in BaFe2S3" — Meng Wang: S. J. Jin, Ming Yi, Yu Song, H. C. Jiang, W. L. Zhang, H. L. Sun, H. Q. Luo, A. D. Christianson, E. Bourret-Courchesne, D. H. Lee, Dao-Xin Yao, and R. J. Birgeneau; Phys. Rev. B, 02/03/17.
Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasione-dimensional antiferromagnetic ladder compound BaFe2S3, where a superconducting transition was observed under pressure [H. Takahashi et al., Nat. Mater. 14, 1008 (2015); T. Yamauchi et al., Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest-neighbor ferromagnetic exchange interaction (SJR = −71 ± 4 meV) along the rung direction, an antiferromagnetic SJL = 49 ± 3 meV along the leg direction, and a ferromagnetic SJ2 = −15 ± 2 meV along the diagonal direction. Our data demonstrate that the antiferromagnetic spin excitations are a common characteristic for the iron-based superconductors, while specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials.
"Catalytic oxidation of Li2S on the surface of metal sulfides for Li−S batteries" — Guangmin Zhou: Hongzhen Tian, Yang Jin, Xinyong Tao, Bofei Liu , Rufan Zhang, Zhi Wei Seh, Denys Zhuo, Yayuan Liu, Jie Sun, Jie Zhao, Chenxi Zu, David Sichen Wu, Qianfan Zhang, and Yi Cui; Proceedings of the National Academy of Sciences, 01/31/17.
Polysulfide binding and trapping to prevent dissolution into the electrolyte by a variety of materials has been well studied in Li−S batteries. Here we discover that some of those materials can play an important role as an activation catalyst to facilitate oxidation of the discharge product, Li2S, back to the charge product, sulfur. Combining theoretical calculations and experimental design, we select a series of metal sulfides as a model system to identify the key parameters in determining the energy barrier for Li2S oxidation and polysulfide adsorption. We demonstrate that the Li2S decomposition energy barrier is associated with the binding between isolated Li ions and the sulfur in sulfides; this is the main reason that sulfide materials can induce lower overpotential compared with commonly used carbon materials. Fundamental understanding of this reaction process is a crucial step toward rational design and screening of materials to achieve high reversible capacity and long cycle life in Li−S batteries.
"Sulﬁphilic Nickel Phosphosulﬁde Enabled Li2S Impregnation in 3D Graphene Cages for Li–S Batteries" — Guangmin Zhou: Jie Sun, Yang Jin, Wei Chen, Chenxi Zu, Rufan Zhang, Yongcai Qiu, Jie Zhao, Denys Zhuo, Yayuan Liu, Xinyong Tao, Wei Liu, Kai Yan, Hye Ryoung Lee, and Yi Cui; Advanced Materials, 01/30/17.
A 3D graphene cage with a thin layer of electrodeposited nickel phosphosulfide for Li2S impregnation, using ternary nickel phosphosulphide as a highly conductive coating layer for stabilized polysulfide chemistry, is accomplished by the combination of theoretical and experimental studies. The 3D interconnected graphene cage structure leads to high capacity, good rate capability and excellent cycling stability in a Li2S cathode.
"Modular soft x-ray spectrometer for applications in energy sciences and quantum materials" — Yi-De Chuang: Yu-Cheng Shao, Alejandro Cruz, Kelly Hanzel, Adam Brown, Alex Frano, Ruimin Qiao, Brian Smith, Edward Domning, Shih-Wen Huang, L. Andrew Wray, Wei-Sheng Lee, Zhi-Xun Shen, Thomas P. Devereaux, Jaw-Wern Chiou, Way-Faung Pong, Valeriy V. Yashchuk, Eric Gullikson, Ruben Reininger, Wanli Yang, Jinghua Guo, Robert Duarte, and Zahid Hussain; Review of Scientific Instruments, 01/27/17.
Over the past decade, the advances in grating-based soft X-ray spectrometers have revolutionized the soft X-ray spectroscopies in materials research. However, these novel spectrometers are mostly dedicated designs, which cannot be easily adopted for applications with diverging demands. Here we present a versatile spectrometer design concept based on the Hettrick-Underwood optical scheme that uses modular mechanical components. The spectrometer’s optics chamber can be used with gratings operated in either inside or outside orders, and the detector assembly can be reconfigured accordingly. The spectrometer can be designed to have high spectral resolution, exceeding 10 000 resolving power when using small source (∼1 µm) and detector pixels (∼5 µm) with high line density gratings (∼3000 lines/mm), or high throughput at moderate resolution. We report two such spectrometers with slightly different design goals and optical parameters in this paper. We show that the spectrometer with high throughput and large energy window is particularly useful for studying the sustainable energy materials. We demonstrate that the extensive resonant inelastic X-ray scattering (RIXS) map of battery cathode material LiNi1/3Co1/3Mn1/3O2 can be produced in few hours using such a spectrometer. Unlike analyzing only a handful of RIXS spectra taken at selected excitation photon energies across the elemental absorption edges to determine various spectral features like the localized dd excitations and non-resonant fluorescence emissions, these features can be easily identified in the RIXS maps. Studying such RIXS maps could reveal novel transition metal redox in battery compounds that are sometimes hard to be unambiguously identified in X-ray absorption and emission spectra. We propose that this modular spectrometer design can serve as the platform for further customization to meet specific scientific demands.
"High-pressure behavior of A2B2O7 pyrochlore (A=Eu, Dy; B=Ti, Zr)" — Dylan R. Rittman: Katlyn M. Turner, Sulgiye Park, Antonio F. Fuentes, Jinyuan Yan, Rodney C.Ewing, and Wendy L. Mao; Journal of Applied Physics , 01/24/17.
In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼ 50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼ 41 GPa for B = Ti and ∼ 16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A =Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu2Zr2O7 as compared with the initially defect-fluorite structured Dy2Zr2O7.
"Ultrafast light-induced symmetry changes in single BaTiO3 nanowires" — Yi-Hong Kuo: Sanghee Nah, Kai He, Te Hu, Aaron M. Lindenberg; Journal of Materials Chemistry C, 01/23/17.
The coupling of light to nanoscale ferroelectric materials enables novel means of controlling their coupled degrees of freedom and engineering new functionality. Here we present femtosecond time-resolution nonlinear-optical measurements of light-induced dynamics within single ferroelectric barium titanate nanowires. By analyzing the time-dependent and polarization-dependent second harmonic intensity generated by the nanowire, we identify its crystallographic orientation and then make use of this information in order to probe its dynamic structural response and change in symmetry. We show that photo-excitation leads to ultrafast, non-uniform modulations in the second order nonlinear susceptibility tensor, indicative of changes in the local symmetry of the nanostructure occurring on sub-picosecond time-scales.
"Cu2ZnSnSe4 Photovoltaic Absorber Layers Evaluated by Transmission X-Ray Microscopy Tomography: Composition Fluctuations on the Length Scale of Grains" — Dennis S. Pruzan: Anna E. Caruso, Yijin Liu, Yu Lin, Carolyn Beall, Ingrid Repins, Michael F. Toney, and Michael A. Scarpulla; RRL Solar, 01/31/17.
The origins of open-circuit voltage deficits in Cu2ZnSnSe4-based solar cells have been an intense topic of research over the past few years as device efficiencies have never approached those of CuInGaSe2based cells despite the materials sharing similar crystal and electronic structures. In this work, we use transmission X-ray microscopy tomography to investigate the length scales over which elemental fluctuations occur. We find and show evidence of micron-scale Cu to Zn anti-correlations over a previously inaccessible combination of resolution and sample size that is consistent with the length scale of grains in this material. This result yields further insight into the causes of the large open-circuit voltage deficits regularly seen in these devices as well as the challenges of achieving compositional homogeneity in this material.
"Buckled two-dimensional Xene sheets" — Alessandro Molle: Joshua Goldberger , Michel Houssa, Yong Xu, Shou-Cheng Zhang and Deji Akinwande; Nat Mater, 01/16/17.
Silicene, germanene and stanene are part of a monoelemental class of two-dimensional (2D) crystals termed 2D-Xenes (X = Si, Ge, Sn and so on) which, together with their ligand-functionalized derivatives referred to as Xanes, are comprised of group IVA atoms arranged in a honeycomb lattice — similar to graphene but with varying degrees of buckling. Their electronic structure ranges from trivial insulators, to semiconductors with tunable gaps, to semi-metallic, depending on the substrate, chemical functionalization and strain. More than a dozen different topological insulator states are predicted to emerge, including the quantum spin Hall state at room temperature, which, if realized, would enable new classes of nanoelectronic and spintronic devices, such as the topological field-effect transistor. The electronic structure can be tuned, for example, by changing the group IVA element, the degree of spin–orbit coupling, the functionalization chemistry or the substrate, making the 2D-Xene systems promising multifunctional 2D materials for nanotechnology. This Perspective highlights the current state of the art and future opportunities in the manipulation and stability of these materials, their functions and applications, and novel device concepts.