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"2D materials advances: from large scale synthesis and controlled heterostructures to improved characterization techniques, defects and applications" — Zhong Lin: Amber McCreary, Natalie Briggs, Shruti Subramanian, Kehao Zhang, Yifan Sun, Xufan Li, Nicholas J Borys, Hongtao Yuan, Susan K Fullerton-Shirey, Alexey Chernikov, Hui Zhao, Stephen McDonnell, Aaron M Lindenberg, Kai Xiao, Brian J LeRoy, Marija Drndić, James C M Hwang, Jiwoong Park, Manish Chhowalla, Raymond E Schaak, Ali Javey, Mark C Hersam, Joshua Robinson and Mauricio Terrones; IOP Science, 12/08/16.
The rise of two-dimensional(2D) materials research took place following the isolation of graphene in 2004. These new 2D materials include transition metal dichalcogenides, mono-elemental 2D sheets, and several carbide- and nitride-based materials. The number of publications related to these emerging materials has been drastically increasing over the last five years. Thus, through this comprehensive review, we aim to discuss the most recent groundbreaking discoveries as well as emerging opportunities and remaining challenges. This review starts out by delving into the improved methods of producing these new 2D materials via controlled exfoliation, metal organic chemical vapor deposition, and wet chemical means.We look into recent studies of doping as well as the optical properties of 2D materials and their heterostructures. Recent advances towards applications of these materials in 2D electronics are also reviewed, and include the tunnel MOSFET and ways to reduce the contact resistance for fabricating highquality devices. Finally, several unique and innovative applications recently explored are discussed as well as perspectives of this exciting and fast moving field.
"Direct and continuous strain control of catalysts with tunable battery electrode materials" — Haotian Wang: Shicheng Xu, Charlie Tsai, Yuzhang Li, Chong Liu, Jie Zhao, Yayuan Liu, Hongyuan Yuan, Frank Abild-Pedersen, Fritz B. Prinz, Jens K. Nørskov, Yi Cui; Science , 11/25/16.
We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.
"Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3" — F. Chen: Y. Zhu, S. Liu,Y. Qi, H. Y. Hwang, N. C. Brandt, J. Lu, F. Quirin, H. Enquist, P. Zalden, T. Hu, J. Goodfellow, M.-J. Sher, M. C. Hoffmann, D. Zhu, H. Lemke, J. Glownia, M. Chollet, A. R. Damodaran, J. Park, Z. Cai, I. W. Jung, M. J. Highland, D. A. Walko, J. W. Freeland, P. G. Evans, A. Vailionis, J. Larsson, K. A. Nelson, A. M. Rappe, K. Sokolowski-Tinten, L. W. Martin, H. Wen, and A. M. Lindenberg; Physical Review B, 11/22/16.
The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.
"All-optical materials design of chiral edge modes in transition-metal dichalcogenides" — Martin Claassen: Chunjing Jia, Brian Moritz & Thomas P. Devereaux; Nature Communications, 10/10/16.
Monolayer transition-metal dichalcogenides are novel materials which at low energies constitute a condensed-matter realization of massive relativistic fermions in two dimensions. Here, we show that this picture breaks for optical pumping—instead, the added complexity of a realistic materials description leads to a new mechanism to optically induce topologically protected chiral edge modes, facilitating optically switchable conduction channels that are insensitive to disorder. In contrast to graphene and previously discussed toy models, the underlying mechanism relies on the intrinsic three-band nature of transition-metal dichalcogenide monolayers near the band edges. Photo-induced band inversions scale linearly in applied pump field and exhibit transitions from one to two chiral edge modes on sweeping from red to blue detuning. We develop an ab initio strategy to understand non-equilibrium Floquet–Bloch bands and topological transitions, and illustrate for WS2 that control of chiral edge modes can be dictated solely from symmetry principles and is not qualitatively sensitive to microscopic materials details.
"Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields" — Barry D. Bruner: Zdeněk Mašín, Matteo Negro, Felipe Morales, Danilo Brambila, Michele Devetta, Davide Faccialà, Alex G. Harvey, Misha Ivanov, Yann Mairesse, Serguei Patchkovskii, Valeria Serbinenko, Hadas Soifer, Salvatore Stagira, Caterina Vozzi, Nirit Dudovich and Olga Smirnova; Farady Discussions, 09/26/16.
High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.
"Direct Intertube Cross-Linking of Carbon Nanotubes at Room Temperature" — Yunxiang Gao: Hongwei Chen, Jun Ge, Jingna Zhao, Qingwen Li, Jianxin Tang, Yi Cui, and Liwei Chen; Nano Letters, 09/22/16.
Carbon nanotubes (CNTs) have long been regarded as an efficient free radical scavenger because of the large-conjugation system in their electronic structures. Hence, despite abundant reports on CNT reacting with incoming free radical species, current research has not seen CNT itself displaying the chemical reactivity of free radicals. Here we show that reactive free radicals can in fact be generated on carbon nanotubes via reductive defluorination of highly fluorinated single-walled carbon nanotubes (FSWNTs). This finding not only enriches the current understanding of carbon nanotube chemical reactivity but also opens up new opportunities in CNT-based material design. For example, spacer-free direct intertube cross-linking of carbon nanotubes was previously achieved only under extremely high temperature and pressure or electron/ion beam irradiation. With the free radicals on defluorinated FSWNTs, the nanotubes containing multiple radicals on the sidewall can directly cross-link with each other under ambient temperature through intertube radical recombination. It is demonstrated that carbon nanotube fibers reinforced via direct cross-linking displays much improved mechanical properties.
"Scaling of the Stress and Temperature Dependence of the Optical Anisotropy in Ba(Fe1−xCox)2As2" — C. Mirri: A. Dusza, S. Bastelberger, J.-H. Chu, H.-H. Kuo, I. R. Fisher, L. Degiorgi; Journal of Superconductivity and Novel Magnetism, 09/15/16.
We revisit our recent investigations of the optical properties in the underdoped regime of the title compounds with respect to their anisotropic behavior as a function of both temperature and uniaxial stress across the ferro-elastic tetragonal-to-orthorhombic transition. By exploiting a dedicated pressure device, we can tune and control uniaxial stress in situ thus changing the degree of detwinning of the samples in the orthorhombic SDW state as well as pressure-inducing an orthorhombicity in the paramagnetic tetragonal phase. We discover a hysteretic behavior of the optical anisotropy; its stress versus temperature dependence across the structural transition bears testimony to the analogy with the magnetic-field versus temperature dependence of the magnetization in a ferromagnet when crossing the Curie temperature. In this context, we find furthermore an intriguing scaling of the stress and temperature dependence of the optical anisotropy in Ba(Fe1−xCox)2As2.
"Dual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System" — Zhuoyu Chen: Hongtao Yuan, Yanwu Xie, Di Lu, Hisashi Inoue, Yasuyuki Hikita, Christopher Bell, and Harold Y. Hwang; Nano Letters, 09/08/16.
Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson–Schrödinger sub-band model. In particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. Our study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.
"Superconducting Gap Anisotropy in Monolayer FeSe Thin Film" — Y. Zhang: J. J. Lee, R. G. Moore, W. Li, M. Yi, M. Hashimoto, D. H. Lu, T. P. Devereaux, D.-H. Lee, and Z.-X. Shen; Physical Review Letters, 09/08/16.
Superconductivity originates from pairing of electrons near the Fermi energy. The Fermi surface topology and pairing symmetry are thus two pivotal characteristics of a superconductor. Superconductivity in one monolayer (1 ML) FeSe thin film has attracted great interest recently due to its intriguing interfacial properties and possibly high superconducting transition temperature over 65 K. Here, we report high-resolution measurements of the Fermi surface and superconducting gaps in 1 ML FeSe using angle-resolved photoemission spectroscopy. Two ellipselike electron pockets are clearly resolved overlapping with each other at the Brillouin zone corner. The superconducting gap is nodeless but moderately anisotropic, which puts strong constraint on determining the pairing symmetry. The gap maxima locate on the dxy bands along the major axis of the ellipse and four gap minima are observed at the intersections of electron pockets. The gap maximum location combined with the Fermi surface geometry deviate from a single d-wave, extended s-wave or s± gap function, suggesting an important role of the multiorbital nature of Fermi surface and orbital-dependent pairing in 1 ML FeSe. The gap minima location may be explained by a sign change on the electron pockets, or a competition between intra- and interorbital pairing.
"Defying Stereotypes with Nanodiamonds: Stable Primary Diamondoid Phosphines" — Oana Moncea: Oana Moncea, Maria A. Gunawan, Didier Poinsot, Hélène Cattey, Jonathan Becker, Raisa I. Yurchenko, Ekaterina D. Butova, Heike Hausmann, Marina Šekutor, Andrey A. Fokin, Jean-Cyrille Hierso, and Peter R. Schreiner; Journal of Organic Chemistry, 08/25/16.
Direct unequal C–H bond difunctionalization of phosphorylated diamantane was achieved in high yield from the corresponding phosphonates. Reduction of the functionalized phosphonates provides access to novel primary and secondary alkyl/aryl diamantane phosphines. The prepared primary diamantyl phosphines are quite air stable compared to their adamantyl and especially alkyl or aryl analogues. This finding is corroborated by comparing the singly occupied molecular orbital energy levels of the corresponding phosphine radical cations obtained by density functional theory computations.