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Invited abstracts

Eric Pop: Electronic, Thermal, and Unconventional Applications of 2D Materials

posted 7 Apr 2017, 08:52 by Peter Boggild

Stanford University, USA, Electrical Engineering, Materials Science & Engineering, Precourt Institute for Energy

Two-dimensional (2D) materials have applications in low-power electronics and energy-conversion systems. These are also rich domains for both fundamental discoveries as well as technological advances. This talk will present recent highlights from our research on graphene, BN, and transition metal dichalcogenides (TMDs). We have studied graphene from basic transport measurements and simulations, to the recent wafer-scale demonstration of analog dot product nanofunctions for neural networks [1]. We are also growing [2] and evaluating the electrical, thermal, and thermoelectric properties of TMDs including MoS2, MoSe2, HfSe2, and WTe2. Recent results include low-resistance contacts, 10-nm scale transistors [3], and high-field transport studies including velocity saturation. We have also examined the anisotropic thermal conductivity of these materials, for unconventional applications to thermal switches and thermal routing. If time permits, I will also discuss nanoscale thermoelectric effects in transistors and phase-change memory (PCM), which could enable energy-efficient operation. Our studies reveal fundamental limits and new applications that could be achieved through the co-design and heterogeneous integration of 2D nanomaterials.

1. N.C. Wang, S.K. Gonugondla, I. Nahlus, N.R. Shanbhag, E. Pop, "GDOT: A Graphene-Based Nanofunction for Dot-Product Computation," IEEE VLSI Tech. Symp., Jun 2016, Honolulu HI

2. K.K.H. Smithe, C.D. English, S.V. Suryavanshi, E. Pop, "Intrinsic Electrical Transport and Performance Projections of Synthetic Monolayer MoS2 Devices," 2D Materials 4, 011009 (2017)

3. C.D. English, K.K.H. Smithe, R.L. Xu, E. Pop, "Approaching Ballistic Transport in Monolayer MoS2 Transistors with Self-Aligned 10 nm Top Gates," IEEE Intl. Electron Devices Meeting (IEDM), Dec 2016, San Francisco CA

Eric Pop is an Associate Professor of Electrical Engineering (EE) and Materials Science & Engineering (by courtesy) at Stanford University. He was previously on the faculty of the University of Illinois Urbana-Champaign (2007-13) and also worked at Intel (2005-07). His research interests are at the intersection of nanoelectronics, nanomaterials, and energy. He received his PhD in EE from Stanford (2005) and three degrees from MIT (MEng and BS in EE, BS in Physics). His awards include the 2010 PECASE from the White House, the highest honor given by the US government to early-career scientists and engineers. He is also a recipient of Young Investigator Awards from the ONR, NSF CAREER, AFOSR, DARPA, and of several best paper/poster and teaching/advising awards. He is an IEEE Senior member, he served as the General Chair of the Device Research Conference (DRC), and on program committees of the VLSI, IRPS, MRS, IEDM, and APS conferences. In a past life, he was a DJ at KZSU 90.1 FM from 2001-04. Additional information about the Pop Lab is available online at http://poplab.stanford.edu.

Sarah Haigh: Understanding 2D Material Heterostructures at the Atomic Scale using Transmission Electron Microscopy

posted 7 Apr 2017, 08:50 by Peter Boggild

S.J. Haigh, A. Rooney, E. Prestat, E. Khestanova, R. Dryfe, M Velický, R.V. Gorbachev, R. Boya, Y. Cao, I. Grigorieva, R. Nair, K. Novoselov, F. Withers, A.K. Geim, National Graphene Institute, University of Manchester, Manchester, M13 9PL, UK

2D crystals can be layered together to create new van der Waals crystals with bespoke properties. However, the performance of such materials is strongly dependent on the quality of the crystals and their interfaces at the atomic scale. Transmission electron microscopy (TEM) is the only technique able to characterize the nature of buried interfaces in these engineered van der Waals crystals and hence to provide insights into the optical, electronic and mechanical properties. I will report the use of TEM imaging technique to aid the development of 2D heterostructures. I will review our work on traditional heterostructures as well as those where individual planes of atoms have been effectively removed to produce nanochannels. The latter provide unique opportunities to help understand water flow at the nanoscale.[1] I will show the use of TEM to reveal how high pressure confinement in such channels can drive chemical transformations in aqueous salts [2]. I will further demonstrate the use of STEM characterization to understand the unusual structure of Franckeite, a natural mineral phase composed of incommensurate 2D layers, which has promising electrochemical properties when exfoliated to few-layer thickness [3].

1. Boya et al, Nature, 538, (2016) p. 222.

2. Vasu et al, Nature Communications, 7, (2016), 12168.

3. Velický et al, Nature Communications, 8, (2017), 14410.

Sarah Haigh is a Reader in Material Science and Director of the Electron Microscopy Centre at the University of Manchester, UK. Her research interests focus on improving our understanding of nanomaterials properties using transmission electron microscope (TEM) imaging and analysis techniques. She worked as consultant application specialist to JEOL UK before moving to Manchester in 2010. She completed her PhD in 2008 at the University of Oxford working on developing exit wave reconstruction in the TEM. She now leads a research group (5 PhD student, 6 MSc Students and 3 postdocs) centred on TEM imaging of nanomaterials. She has a particular interest in imaging of materials under more realistic environments and was recently awarded an ERC starter grant to develop this area of microscopy. She has more than 100 peer reviewed papers of which 78 are from the last 3 years. She is Chair of the Institute of Physics EMAG group, a member of council for the Royal Microscopy Society (UK) and a freeman of the Worshipful Company of Armourers and Brasiers. Awards include the IOM3 Silver Award (2014), RMS Medal for Innovation in Applied Microscopy (2016) and Rosenhain Medal (2017),

Alexey Chernikov: Coulomb engineering in 2D materials

posted 7 Apr 2017, 08:46 by Peter Boggild

Department of Physics, University of Regensburg, Germany

Since the discovery of graphene, a single sheet of carbon atoms, research focused on two-dimensional (2D) van der Waals materials evolved rapidly due the availability of atomically thin, thermally stable crystals with intriguing physical properties. The 2D materials naturally inherit major traits associated with systems of reduced dimensionality: strongly enhanced interactions, efficient light-matter coupling, and sensitivity to the environment. In particular, the considerable strength of the Coulomb forces introduces a rich variety of many-body phenomena including significant renormalization of the bandgap and the emergence of tightly bound exciton quasi-particles.

In this talk, I will show how atomically-thin crystals offer an alternative approach to nanoscale bandgap engineering, based on the local tuning of the Coulomb interaction and the environmental sensitivity of 2D materials. I will demonstrate how careful tailoring of the surrounding dielectric environment allows us to tune the electronic bandgap of single layers of semiconducting transition-metal dichalcogenides by many 100’s of meV and present an in-plane dielectric heterostructure as an illustration. The unique advantages of the Coulomb engineering in 2D, including nanometer sensitivity and a high flexibility of resulting dielectric heterostructures, will be further discussed. Finally, I will give a brief outlook towards new pathways for manipulating and designing electronic bandgaps in the 2D plane.

Alexey Chernikov received his Ph.D. from the University of Marburg (Germany) for the work on the optical properties of semiconducting materials and external cavity semiconducting lasers. With a Feodor-Lynen Fellowship from the Alexander von Humboldt Foundation, he joined the group of Tony F. Heinz at the Columbia University (New York, USA) in 2013 to study Coulomb phenomena in atomically-thin 2D systems. Currently, he leads a research group at the University of Regensburg (Germany) funded by the Emmy-Noether Initiative of the German Research Foundation. His research is focused on fundamental interactions of electronic and excitonic many-body states in nanostructured matter.  

Alexander Balatsky: Driven Dirac Materials

posted 7 Apr 2017, 04:51 by Peter Bøggild

Nordita and Los Alamos

Recent rapid developments in time resolved probes and in manipulation of quantum matter in time domain open opportunities to control correlations and instabilities of electronic states in time domain. I will discuss applications of these ideas to Dirac Materials[1]. Dirac Materials exhibit nodes in the spectra that result in the strong energy dependence of the Density of States (DOS). Hence the driven and nonequilibrium Dirac Materials offer a platform for investigation of collective instabilities of Dirac nodes via controlled tuning of the coupling constants with drive. I will present the results of  investigation of the many body instabilities, like excitonic instabilities, in driven Dirac Materials[2]. Recent optical pump experiments are consistent with the creation of long lived states away from equilibrium in Dirac Materials [2] and hence pave the way to tunable interactions in Dirac Materials.

1. T. O. Wehling, et.al. Dirac materials. Advances in Physics, 63(1):1–76, (2014).

2. Christopher Triola, et.al,  Excitonic Gap Formation in Pumped Dirac Materials, arXiv:1701.04206, (2017).

A.Balatsky is a Professor of Theoretical Physics at Nordita and a Director of the Institute for Materials Science at Los Alamos. He got his PhD at the Landau Institute for Theoretical Physics, 1987. After a PD at Urbana Champaign with D. Pines he moved to Los Alamos as an Oppenheimer Fellow. Awards include  fellow of  the American Physical Society (2003), Los Alamos Fellow (2005) American  Association for Advanced of Science (2013) and ITS Senior Fellow at ETHZ (2016). Main research interests include superconductivity, superfluidity and Dirac Materials. Recent focus of research at our group has been on Dirac materials and dynamic orders including odd frequency superconductivity and driven collective states in Dirac Materials.  

Maria C. Asensio: Electronic and Chemical nano-imaging of wonder materials beyond graphene

posted 7 Apr 2017, 04:44 by Peter Bøggild   [ updated 7 Apr 2017, 09:27 by Peter Boggild ]

Synchrotron SOLEIL & University Paris-Saclay, FRANCE

Recently, remarkable progress has been achieved in modern microscopies. However, even if they have attained exceptional results, the problem of providing powerful high-resolution spectroscopic tools for probing at nano- and  mesoscopic-scale still remains. This is particularly the case for an innovative and powerful technique named k-nanoscope or NanoARPES (Nano Angle Resolved Photoelectron Spectroscopy). This cutting-edge nanoscope is able to determine the momentum and spatial resolved electronic structure, disclosing the implications of heterogeneities and confinement on the valence band electronic states typically present close to the Fermi level, with not more than 15-20 eV of binding energy. 

In this presentation, the more relevant results of the recently built ANTARES nanoscope beamline at the synchrotron SOLEIL will be disclosed [1]. In particular, nanoARPES findings describing the electronic band structure of mono-atomic exfoliated graphene on SiO2 substrates, epitaxial and polycrystalline monolayer graphene films grown on copper and SiC [2] will be presented and Graphene/MoS2 heterostrustures. Electronic and chemical mapping with high energy, momentum and lateral resolution have provided relevant features like gap-size, doping, effective mass, Fermi velocity and electron-phonon coupling.among other properties.  Finally, special mentions will be dedicated to the recently reported results on the spin-charge separation in metallic MoSe2 grain boundary [3].

[1] C. Chen et al., Nature Communications, 6 (2015) 8585

[2] I, Razado-Colambo et al., Nature Scientific Reports 6 (2016) 27261

[3] Y. Ma, et al., Nature Communications 8 (2017) 14231.

Currently, Permanent Research Staff of the SOLEIL French synchrotron source and the Université Paris-Saclay, Professor Maria Asensio is also Permanent staff of the Institute of Material Science of Madrid, in Spain working in the area of electronic structure determination by using low energy Synchrotron radiation. Maria Asensio commenced her academic career in Argentina, where she finished her PhD degree in Surface Science. Then she held a Senior Lecturer in Physics at the Autonomous University of Madrid, followed by positions at the University of Warwick in England and at the Fritz Haber Institute of the Max-Planck in Berlin, Germany, collaborating in several large European Research Projects. Asensio’s research comprises studying the application of a wide-ranged conventional and Synchrotron Radiation Based techniques devoted to the characterization of advanced materials, in the area of Solid State Physics. Lately, she has conceived an innovative chemical and electronic imaging technique combining angle resolved photoemission and microscopy, named “k-nanoscope or NanoARPES”. She is author of more than 278 publications and had more than 100 invited talks.

Saroj Prasad Dash: Creation and Control of Spin Current in 2D Materials Heterostructures

posted 7 Apr 2017, 04:41 by Peter Bøggild   [ updated 7 Apr 2017, 09:26 by Peter Boggild ]

Chalmers University of Technology, Gothenburg, Sweden

Exploiting the spin degrees of freedom of electrons in solid state devices is considered as one of the alternative state variables for information storage and processing beyond the charge based technology. However, one of the primary challenges in this field is the efficient creation, transport and control of spin polarization at room temperature. In this regard, two-dimensional (2D) atomic crystals and their heterostructures provide an ideal platform for spintronics. Recently, we demonstrated a long distance spin transport over 16 µm and spin lifetimes up to 1.2 ns in large area CVD graphene at room temperature [1]. In order to achieve an efficient spin injection into graphene, we used further used h-BN tunnel barriers with large tunnel spin polarization up to 65 % at room temperature [2]. More recently, we demonstrated gate control of spin polarization by employing graphene/MoS2 heterostructures at room temperature [3]. Our findings demonstrate  all-electrical spintronic device at room temperature with the creation, transport and control of the spins in 2D materials heterostructures, which can be key building blocks in future device architectures.

[1] MV Kamalakar, G. Chris, A Dankert, SP Dash; Nature Communication, 6, 6766 (2015).

[2] MV Kamalakar, A Dankert, P. Kelly, SP Dash; Scientific Reports, 6, 21168 (2016).

[3] A Dankert, SP Dash; arXiv:1610.06326 (2016). Under review in Nature Nano.

Saroj Prasad Dash is an Associate Professor and group leader at Dept. Microtechnology and Nanoscience, Chalmers University of Technology, Sweden. His research focuses on spin and quantum transport in graphene, semiconductors, topological insulators, and van der Waals heterostructures of 2D materials. He received a PhD in Physics from Max Planck Institute (Stuttgart, Germany) in 2007. His previous positions include postdocs at Uni. of Twente and Uni. of Groningen in Netherlands for three years. He has published >33 high impact articles (citations:1637, h-index:18, avg. citation per article: 50) and given >35 invited talks in international conferences and seminars. He is presently principal investigator of several Swedish and European Union (EU) projects including the Graphene Flagship. He is presently also an editorial board member of Nature publishing Scientific Reports.

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