News & Events

Speaker: Stephen John Pennycook
Affiliation: U. Tennessee, USA
Abstract Details: The aberration-corrected scanning transmission electron microscope (STEM) now allows direct, real space imaging at atomic resolution with low accelerating voltages to minimize damage. In two-dimensional materials such as graphene and transition metal dichalcogenides, atom-by-atom characterization of atomic position, atomic species, chemical bonding and optical and electronic properties has become feasible. Furthermore, through direct momentum transfer, the STEM probe can also reveal the dynamics of small clusters. Movies of a Si6 cluster in a graphene nanopore show conformational changes, which in combination with calculations based on density functional theory, reveals the energy landscape of the cluster. Metallic transition metal chalcogenide nanowires can be sculpted directly from their respective dichalcogenide monolayer sheets. The wires are metallic, with Ohmic contacts to the surrounding two-dimensional layers, and are self-healing against beam damage. In solar cells, the same combination of atomic level microscopy and theory reveals new directions to improve cell effciency.
About the Speaker: Stephen J. Pennycook is a Professor in the Dept. of Materials Science and Engineering, University of Tennessee and the Dept. of Materials Science and Engineering, North Carolina State University. Before his retirement from Oak Ridge National Laboratory in December 2013 he was Corporate Fellow in the Materials Science and Technology Division and leader of the Scanning Transmission Electron Microscopy Group. He received his PhD in physics from the Cavendish Laboratory, University of Cambridge in 1978, joining Oak Ridge National Laboratory in 1982. Pennycook is a Fellow of the American Physical Society, the American Association for the Advancement of Science, the Microscopy Society of America, the Institute of Physics and the Materials Research Society, and is recipient of the Microbeam Analysis Society Heinrich Award, the Materials Research Society Medal, the Institute of Physics Thomas J. Young Medal and Award and the Materials Research Society Innovation in Characterization Award. He has 38 books and book chapters, over 400 publications in refereed journals and has given over 200 invited presentations on the development and application of atomic resolution Z-contrast microscopy and electron energy loss spectroscopy.
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Speaker: G. Vignale
Affiliation: U. Missouri-Columbia, USA
Abstract Details: Semiconductor quantum wells, inter-metallic interfaces, layered oxides, and monolayer materials are all promising platforms for the observation of spin-charge conversion due to strong spin-orbit interaction in the quasi two dimensional electron liquid they host. In this talk I focus on two closely related effects that can occur in these materials, namely the conversion of charge current to spin current (spin Hall effect) and the generation of spin polarization from an electric current (Edelstein effect). Together with their inverses (in the sense of Onsager reciprocity relations), these effects constitute a useful set of tools for spintronic applications. The theoretical challenge is to provide a unified treatment of the different mechanisms at work, including spin precession, spin relaxation, electron-impurity scattering and electron-electron scattering. In this talk I show how the SU(2) drift-diffusion theory allows such a unified treatment, and I describe its application to the interpretation of recent experiments on spin-charge conversion at the interface between two non-magnetic metals (Ag/Bi)†. I conclude with a brief review of our recent results on the generation of ordered spin structures from density-modulated structures and on the impact of spatially-dependent spin-orbit couplings on the process of spin-charge conversion.
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Speaker: Yang Bo
Affiliation: Yang Bo
Abstract Details: We show that the holomorphic wavefunction descriptions of the fractional quantum hall states, first pioneered by Laughlin, Moore-Read and others, can be more naturally described by the Jack polynomial formalism. Within this formalism not only the ground states of the various fractional quantum hall fluids can be constructed, the techniques can also be applied to zero-mode edge states and both charged and neutral bulk excitations. As an explicit example, we construct model wavefunctions for a family of single-quasielectron states supported by the nu = 1/3 fractional quantum Hall (FQH) fluid. (see attached PDF for detailed abstract).
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Abstract Details: Do join us for the GRC year end party 2013!
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Speaker: Aleksandr Rodin
Abstract Details: Graphene plasmonics is a rapidly developing field of study. In this talk, I will discuss what makes graphene such an attractive material for plasmons. Following this, I will show and describe experimental results of real-space plasmon imaging using near- field microscopy. I will focus on two types of geometries: half-planes and triangular flakes. Theoretical basis to describe the results will be provided. Finally, I will talk about the effect that grain boudaries and line defects have on plasmons. A. S. Rodin, Z. Fei, A. S. McLeod, M. Wagner, A. H. Castro Neto, M. M. Fogler, D. N. Basov, Plasmonic hot spots in triangular tapered graphene microcrystals', accepted to Phys. Rev. B (2013) Z. Fei, A. S. Rodin, W. Gannett, S. Dai, W. Regan, M. Wagner, M. K. Kiu, A. S. McLeod, G. Dominguez, M.Thiemens, M. M. Fogler, A. H. Castro-Neto, F. Keilmann, A. Zettl, R. Hillenbrand, M. M. Fogler, D. N. Basov, Electronic and plasmonic phenomena at grain boundaries in chemical vapor deposited graphene', Nature Nanotech. 8, 821-825 (2013) Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, 'Gate-tuning of graphene plasmons revealed by infrared nano-imaging', Nature 487, 82-85 (2012).
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Speaker: Hyun-Jong Chung
Affiliation: Dept. Physics, Konkuk University, Korea
Abstract Details: Since its gapless band structure keeps from turning-off the devices in traditional way, until recently, applications for the graphene transistors are limited to analog amplifiers which does not have to be turned off during the operation. Last year new device structures have been proposed to solve the issue: graphene barristor [1] and graphene tunneling transistor [2]. Both structures have hetero junction between graphene and semiconductor or insulator. In this talk, the research on graphene transistors will be reviewed towards RF (Radio Frequency) applications and also the research on graphene barristor or tunneling transistor will be reviewed towards logic applications. Finally, their potentials and limits as the next generation devices will be discussed. [1] H. Yang, J. Heo, S. Science, 336, 1140-1143 (2012) [2] L. Britnell, R. V. Gorbachev, R. Jalil, et al., Science, 335, 947-950 (2012)
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Speaker: Klaus Ensslin
Affiliation: ETH Zurich
Abstract Details: Transport experiments through graphene nanoribbons prepared both on SiO substrates as well as on BN substrates show that edge disorder is most likely the dominant disorder mechanism giving rise to the observed transport gap around the Dirac point. Graphene nanostructures deposited on AlGaAs heterostructures allow the tuning of both electronic systems with respect to each other. Such systems may be useful for sensitive charge detection and for better characterization of disorder in graphene nanostructures. Bilayer graphene has been prepared between two layers of BN and high mobilities have been achieved as documented by the observation of broken symmetry states in the quantum Hall effect. We discuss the prospects of local split gates on bilayer graphene giving rise to locally gapped regions for the realization of tunable electronic nanostructures. This work was done in collaboration with D.Bischoff, T. Ihn, C. Rössler, P. Simonet, and A. Varlet.
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Speaker: Christopher T. Nelson
Affiliation: University of California, Berkeley
Abstract Details: Ferroelectric oxides possess a spontaneous polarization which can be oriented with an applied electric field. This can be harnessed directly as a next-generation non-volatile memory or indirectly by coupling across heterojunctions to produce extrinsic electric- field controlled materials properties. The polarization switching process underpinning these applications is poorly understood at device-relevant sub-micrometer length scales. This is primarily due to a high sensitivity to defects, many of which are atomic-scale and inhomogenous. In this talk I will discuss our use of transmission electron microscopy (TEM) to study nanoscale ferroelectric switching of two prominent exemplar materials: PbZr0.2Ti0.8O3 (PZT) and BiFeO3 (BFO). I will provide a brief introduction to the operating principle and capabilities of TEM for in situ and ex situ characterization of transition metal oxides. This includes atomic- scale resolution of chemical and structural information which we use to calculate unit- cell maps of polarization distribution. I will then discuss the ferroelectric switching we observe in-situ, especially as it relates to macroscale switching properties. Of particular interest are preferential sites for domain nucleation and pinning, the presence of inactive regions, and the stability of switched domains.
About the Speaker: Dr. Christopher T. Nelson is affiliated with the University of California at Berkeley and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE).
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Speaker: Narasimha Boddeti
Affiliation: SUTD
Abstract Details: Graphene, despite being highly stiff, adheres strongly to substrates and surrounding structures with high conformity. The extent to which graphene conforms to its surrounding structures effects its thermal and electrical properties. Hence, understanding the role of adhesion is necessary for reliable and efficient design of not only graphene based nano-mechanical devices but other applications as well. We analyzed series of experiments involving micro-cavities covered by graphene membranes adhered to the substrate along the micro-cavity edge/s. In each of these experiments, the suspended region of the graphene membrane is subjected to a pressure load through trapped pressurized gas inside the micro-cavity. Through our analysis, we describe the overall mechanics and thermodynamics of the graphene/substrate/gas system in each experiment highlighting the role played by adhesion while accounting for the non-linear mechanics of the graphene membranes. In the first set of experiments, we determined the adhesion energy and the Young’s moduli of graphene membranes with different thicknesses with our analysis. The second set of experiments is an improvisation of the first experiment which helped us realize graphene nano-mechanical structures that can switch shape/size via pressure load and adhesion. An extended analysis is developed to completely describe the mechanics. The third set using the same structures as in the second experiment lead us to pull-in of graphene membranes under the influence of surface interactions between graphene and the substrate. This pull-in behavior is captured by our analytical model and made use of to determine the magnitude of the interactions.
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