Abstracts (contributed)

Ville Vierimaa: Spin Relaxation in Graphene: Long Ranged Scatterers

posted 21 Jun 2016, 05:15 by info admin

Ville Vierimaa, Zheyong Fan, Ari Harju
Department of Applied Physics, Aalto University, Helsinki, Finland

Graphene has spanned a lot of interest in the field of spintronics because of its small intrinsic spin-orbit coupling (SOC). In theory, the small SOC should lead to long spin relaxation times, allowing spin to carry information efficiently. However, there is a large discrepancy between the experiments and the theory as the measured relaxation times are a few orders of magnitude shorter than predicted.

In this work we have studied the effect of magnetic charged impurities on the spin relaxation in graphene. We model the impurities with Gaussian-shaped potential similar to [1], with an additional spin-dependent term in the potential. The focus has mostly been on the range of the potential, but the magnitude of the spin-dependent part and concentration of the impurities are also considered.

We found out that while both spin and charge relaxation times decrease with increasing defect size, spin does so faster. Therefore, a small concentration of large puddles is expected to relax spin fast but have minor effect on the charge transport properties. This result is similar to what is observed in the typical experiments and could be used to explain the short observed relaxation times.

[1] A. Rycerz, J. Tworzydlo, and C. W. J. Beenakker, EPL, 79, 57003 (2007)

Ville Vierimaa is a PhD student at the Department of Applied physics at Aalto University. He is part of the Quantum Many-Body Physics group, led by Ari Harju. He received his MSc from Aalto in early 2016 and is currently working on quantum transport calculations on graphene. The work is focused towards the spin and spin-dependent transport properties.

Naveen Kaushik: Impact of charge transfer at the TiO2-MoS2 interface on electron transport in MoS2

posted 3 Jun 2016, 01:34 by Peter Boggild

Centre of Excellence in Nanoelectronics, Department of Electrical Engineering, Indian Institute of Technology Bombay

Molybdenum disulfide (MoS2) has attracted significant research interest due to its sizable bandgap and ultrathin nature. However, high contact resistance [1] and lack of methods for chemical doping, especially p-type [2], have emerged as fundamental bottlenecks for devices based on MoS2. 
In this work we demonstrate a novel technique to lower the contact resistance on MoS2 by introducing an ultrathin TiO2 layer between metal and MoS2 [3]. The reduction in contact resistance and constant Schottky barrier height was attributed to an increased n-type doping at metal-MoS2 contact. Interfacial doping has been validated by first-principles calculations, showing the metallic behavior of the TiO2-MoS2 interface due to inter-layer charge transfer and associated interfacial strain. Electrical and materials studies also show that TiO2 increases the charge density in the MoS2 layer underneath which results in improved transistor on-current besides nearly contact resistance-free field effect mobility. In summary, this work presents a significant advance in contact engineering of transition metal dichalcogenides by enabling contact resistance reduction through interfacial n-type doping of MoS2. Unlike other doping techniques reported previously, this is an air stable and area selective technique that allows doping of different transistor regions such as contact and channel separately.

[1] N. Kaushik, A. Nipane, F. Basheer, S. Dubey, S. Grover, M. Deshmukh and S. Lodha Schottky barrier heights for Au and Pd contacts to MoS2, "Applied Physics Letters, 105, 113505 (2014). 
[2] A. Nipane, D. Karmakar, N. Kaushik, S. Karande and S. Lodha Few Layer MoS2 p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation, "accepted for publication in ACS Nano, (2016).
[3] N. Kaushik, D. Karmakar, A. Nipane, S. Karande and S. Lodha Interfacial n-Doping Using an Ultrathin TiO2 Layer for Contact Resistance Reduction in MoS2, "ACS Applied Materials and Interfaces, ACS-AMI, 8 (1), pp 256263, (2016). 

Naveen Kaushik is currently a PhD student in the Department of Electrical Engineering at the Indian Institute of Technology Bombay. His research focuses on contact and doping strategies for transition metal dichalcogenides (TMDs), especially MoS2. He holds an M.Tech in solid state electronic materials from Indian Institute of Technology Roorkee, where he worked on TCAD simulations of FinFET-based SRAM circuits. He was awarded a Prime Minister Fellowship during his PhD given to the top 100 doctoral students by the government of India. He also got the Malhotra Weikfield national award for nanoscience in 2016 for his work done on TMDs. 

Antonija Grubisic Cabo: Electron dynamics in epitaxial single layer MoS2/Au(111) and a MoS2/graphene heterostructure

posted 3 Jun 2016, 00:39 by info admin

Antonija Grubisic Cabo (1), Soren Ulstrup (2), Jill A. Miwa (1), Jonathon M. Riley (3), Signe S. Gronborg (1), Jens C. Johannsen (4), Cephise Cacho (5), Oliver Alexander (5), Richard T. Chapman (5), Emma Springate (5), Marco Bianchi (1), Maciej Dendzik (1), Jeppe V. Lauritsen(1), Phil D. C. King (3) and Philip Hofmann (1)

1. Aarhus University, Denmark, 2. Berkeley National Laboratory, USA, 3. St. Andrews University, UK,4. EPFL, Switzerland, 5. Rutherford Appleton Laboratory, UK

Time- and angle-resolved- photoemission spectroscopy has been used to directly measure the excited carriers dynamics in epitaxial single layer MoS2 grown on either Au(111) or on graphene. For MoS2/Au(111) we determine an ultrafast (50 fs) extraction of excited free carriers via the metal and ascertain a direct quasiparticle band gap of 1.95 eV, which is significantly smaller than the theoretically estimated value for free-standing MoS2. This can be explained by a strong renormalisation of the band gap [1]. For the model 2D heterostructure of MoS2 on graphene, we are able to determine the layer-resolved band structure and separate the excited carrier dynamics
of MoS2 and graphene. On top of a static band gap reduction in MoS2 due to the screening of the nearby graphene, our results reveal a pronounced dynamic renormalisation of the quasiparticle band gap, reducing it by up to approximately 400 meV on femtosecond timescales following optical excitation. This results from a persistence of strong electronic interactions despite the environmental screening by the doped graphene [2]. These results show a large degree of tuneability of the electronic structure and electron dynamics within the van der Waals heterostructure.

[1] A. Grubisic Cabo, J. A. Miwa et al., Nano Letters 15, 5883-5887 (2015)
[2] S. Ulstrup, A. Grubisic Cabo et al., submitted to ACS Nano (2016)

Antonija Grubisic Cabo is a PhD student at the Interdisciplinary Nanoscience Center (iNANO) at Aarhus University. Her main research focus is on the possibility of using graphene in industrial applications and on the electronic structure and excited electron dynamics of novel 2D materials such as transition metal dichalcogenides. Antonija has experience in various UHV techniques, with special dedication to synchrotron- and laser-based techniques such as Angle Resolved Photoemission Spectroscopy (ARPES) and Time-Resolved ARPES (TR-ARPES). Prior to her PhD, she has received her M.S. in Physics in 2014 from the University of Zagreb, Croatia, where she worked mainly with complex magnetic systems.

Jan Los: Scaling behavior of elastic properties of defected graphene

posted 2 Jun 2016, 07:18 by Peter Boggild

J.H. Los, R.C. Ouwersloot, A. Fasolino, M.I. Katsnelson, Theory of Condensed Matter, Institute of Molecules and Materials, Radboud University, Nijmegen, The Netherlands

As recently explicitly demonstrated by simulation [1], the elastic properties of pristine graphene depend on the system size as a power law. For example, for a system of 1 cm^2, the in-plane elastic constants are about 100 times (!) smaller while the out-of-plane elastic constant, i.e. the bending rigidity, is about 10000 times (!!) larger
than for a system of nanometer size. This anomalous behavior, rooted in the theory of membranes, originates from the strong anharmonic coupling between large out-of-plane modes and in-plane modes. This coupling is in fact what stabilizes graphene as a relatively flat phase.

Considering graphene with defects, such as single vacancies,or polycrystalline graphene, other length scales come into play. This not only affects the bare elastic properties but also their size dependence. I will give an overview of the situation regarding the scaling behavior of the elastic properties of defected graphene systems, providing amongst others an explanation for the recently, experimentally observed strong increase of the Young modulus of graphene with a low density of single vacancies [2].

[1] J.H. Los, A. Fasolino, and M. I. Katsnelson, Phys. Rev. Lett. 116, 015901 (2016).
[2] G. Lopez-Polin, C. Gomez-Navarro, V. Parente, F. Guinea, M. I. Katsnelson, F. Perez-Murano, and Julio Gomez-Herrero, Nature Physics, 26-31 (2015).

After my PhD in the theory of condensed matter group at the Radboud University in Nijmegen (Netherlands), and having worked as a researcher in different locations in Europe on various topics in the field of theory of condensed matter, modelling and simulation, I am now back in the group where I did my PhD. My current research activities concentrate on graphene/2D systems, their (statistical-)mechanical properties, development of effective interatomic interaction models enabling large scale simulation.

Jose M. Caridad: Electron transport in periodically modulated graphene

posted 2 Jun 2016, 06:23 by Peter Boggild

Department of Micro- and Nanotechnology, Technical University of Denmark

Being a high mobility and ambipolar conductor, graphene has been proposed to replace and complement a broad range of electronic devices including high-frequency communication devices or photodetectors [1].
However, graphene unique properties involving the relativistic character of its charge carriers are rarely utilized, specifically at application-relevant conditions. In particular, the presence of negative refraction of Dirac fermions in graphene [2] is of particular relevance to develop novel quantum devices based on electron optics.
The present work addresses the guidance of relativistic carriers by employing periodic potential modulation (np junctions); demonstrated by the anisotropic propagation of charge carriers in these systems. Guiding effects survive even at ambient conditions, crucial for the technological development of novel electronic systems based on Dirac-fermions.
[1] Palacios, T. Graphene electronics: Thinking outside the silicon box. Nature Nanotech. 6, 464 (2011).
[2] Lee, G-H.; Park, G-H.& Lee. H-J. Observation of negative refraction of Dirac fermions in graphene. Nature Physics, 11, 925 

José Caridad is currently a postdoctoral researcher at the Technical University of Denmark. He is primarily interested in the optical and electronic properties of low dimensional systems such as graphene. His research focuses not only in understanding the fundamental properties of these devices, but also their industrial scalability and performance at application-relevant conditions. He received his BSc and MSc degrees in Physics from the University of Salamanca (2008) and a PhD in Physics (2014) from Trinity College Dublin.

Chengyi Hou: Graphene-Paper-Origami as a Multi-Responsive Water-Driven Actuator

posted 1 Jun 2016, 07:15 by info admin

1 Department of Chemistry, Technical University of Denmark, Kongens Lyngby, Denmark. 
2 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China.

Mechanical actuators driven by water, which can timely respond to multiple stimuli with  a large deformation and can generate high stress, have promising applications in artificial muscles, motors, and electromagnetic generators. However, matching all these requirements in a single device remains a challenge. In this talk, I present an attempt of fabricating low-cost graphene nanomaterial based construct that enables to undergo reversible deformation with high performance [1, 2]. The device is based on monolayered graphene papers consisting of gradient hybrid reduced graphene oxide (rGO) and graphene oxide (GO) components. The overall fabrication procedure can be carried out at room temperature with economical, scalable, and environmentally friendly advantages. A functional device ("Graphene Origami") composed of this graphene paper can (i) adopt predesigned shapes, (ii) walk, and (iii) turn a corner. These processes can be remote-controlled by gentle light or heating. It is believed that this self-folding material holds potential for a wide range of applications such as sensing, artificial muscles, and robotics.

[1] Sci. Adv. 2015;1:e1500533 (Highlighted by Nature, Science, Nano Today, etc.)
[2] Sci. Rep. 2015, 5, 9503 | DOI: 10.1038/srep09503

I received my Ph.D from Donghua University, China in 2014. I am currently a H.C. Ørsted-Marie Curie research fellow at DTU, Denmark. My research focuses on design, three-dimensional assembly and property of stimuli-responsive graphene-based hybrids and nanomaterials. The potential applications of these new materials as electronic skin, micro-reactors, artificial muscle and three-dimensional biological scaffolds have been systematically investigated. Over 20 of my research articles on this topic have been published on Science Advances, Advanced Materials, Nano Energy, Scientific Reports, Nanoscale, Journal of Materials Chemistry, Carbon etc. Relative publications have been highlighted by Nature, Science, C&EN, MaterialsViewsChina and etc.

Julio Gomez: Preparation of ultralow percolation threshold graphene composites

posted 1 Jun 2016, 07:10 by info admin

Julio Gomez (a); Javier Perez (a); Elvira Villaro (b,c)
a) Avanzare Innovacion Tecnologica S.L., Avda Lentiscares 4-6. 26370 Navarrete, Spain 
b) Instituto de Tecnologías Químicas Emergentes de La Rioja; San Francisco, 11 Navarrete, Spain. c) Departamento de Química Inorgánica y Técnica, UNED. Senda del Rey, 9, Madrid, Spain

The graphene material market, bulk graphene, graphene nanoplatelets and graphene films, will grow to 350 € million in 2025. Their application in composites is the largest segment, followed by energy storage [1]. Several reviews analysed the applications of the different graphene and related products in in composites.[2, 1b]. Currently, a variety of techniques have been developed to prepare graphene materials. In this presentation, 3 different methods for the production of bulk graphene or reduce graphene oxide: liquid exfoliation, reduced graphene oxides and high expansion were compared with other carbon materials. The complete characterization of pristine graphene and highly reduce graphene oxide, will be presented. 
Different types of graphene materials with variation in lateral size, defects and defects concentration, thickness, etc, have been used to obtain graphene-thermoplastic and thermoset composites studying the electrical, thermal conductivity and fire retardant properties of the composites. Related to electrical properties, some of these composites show ultralow percolation threshold limits, lower than the previously reported values, also obtaining very high electrical conductivity, opening a new range of applications and markets. We have also obtained high thermal conductive composites align with the best published result for graphene composites. Other factors as processing technique have been analysed due to their extremely high importance in the final results. 

[1] a) Zh Ma, R. Kozarsky, M. Holman., GRAPHENE MARKET UPDATE. LUX RESEARCH (2014). b) Ferrari A Cet al  Nanoscale 7 (2015) 4598–810, c) M. Peplow, Nature 522, (2015), 268 
[2] a) P Samorì, I A Kinloch, X Feng and V Palermo, 2D Mater. 2 (2015) 030205 b) R. J. Young, I. A. Kinloch, L. G., Kosty. S. Novoselov, Composites Science and Technology, 72 (2012) 1459–1476

Julio Gomez is the Founder of AVANZARE, company producer of graphene and other 2D materials and its composites . He is primarily interested in the mechanical properties of atomically thin materials such as graphene. He received his B.S. degree in Chemistry from Universidad Complutense de Madrid (1995), receiving the  and a Ph.D. in Chemistry (2000) from University of La Rioja where he studied the preparation and electrical and optical properties of nanosize metallic clusters,  and a postdoctoral researcher position in the Laboratoire de Synthèse Organique, University of Nantes-CNRS  After finishing his Ph.D, he spent 3 years as assistant Professor in Universidad de La Rioja and 2 years as an Area Manager in the research centre CIDETEC studying electrochemical systems before joining AVANZARE at the end of 2004. His awards include among others the best B.S. degree in Chemistry in 1995 award in the University Complutense de Madrid, the best PhD degree in Science and Technology award in the University of La Rioja from the years 1999-2000, National award Entrepreneur of the year 2008 in Spain, best product NANOAWARDS 2008 (USA), F&S best practices in innovation 2013 (UK). Author of 43 papers H-index 22.

Lapo Bogani: Graphene-molecular magnet hybrids for molecular spintronics: from single-molecule effects to control of coherent spin currents

posted 1 Jun 2016, 06:54 by info admin

Lapo Bogani, Department of Materials, University of Oxford, 16 Parks Road, OX1 6RP, Oxford, UK

The problem of how flowing electrons interact with single spins is a fundamental one, which conceptually determines the working principles and performances of spintronic devices. The influence of the graphene environment on the spin systems has yet to be unraveled, as well as how molecular systems can be used to control spin currents or single magnetic molecules. 
Here we concentrate on our approach of using graphene as a conductor that can interact with molecular magnets. We first explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets on graphene. While the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated.[1] We quantify the effect of the perturbed phonon environment on the classical dynamics of molecular spins, and we then show that the presence of the conduction channel can strongly alter the quantum properties. We then show how to inject coherent spin currents in graphene planes and how molecular spins can be used to control them.[2] We show that the Hanle precession can be strongly influenced by the interaction with molecular spins, which can introduce an additional level of control for the spintronic response.[2] Eventually, we show how nanoscale gaps can be created in graphene to accommodate single molecule magnets, and what parameters dominate the anisotropy of such single-molecule spintronic devices.

[1] C.Cervetti et al., Nature Mat., 2016, 15, 164-169. 
[2] C. Cervetti et al., submitted. 

Lapo Bogani is currently ERC and Royal Society group leader at the University of Oxford. He is primarily interested in the magnetic and electronic properties of nanoscale devices, with particular attention to the integration of molecular magnetic systems in nanoelectronic devices. He received his M.S. and Ph.D. degrees from The University of Florence, Italy (2006) working on the magnetic properties of one-dimensional materials. The then Moved to CNRS Grenoble with an individually-driven Marie Curie fellowship, where he was one of the main proponents of molecular spintronic systems (2009) using carbon nanomaterials. He then received the prestigious Sofja Kovalevskaja research prize of the Alexander von Humboldt Stiftung, which he developed at the University of Stuttgart, Germany. He joined the faculty at the Department of Materials, University of Oxford, in 2015. His awards and distinctions include, among the others, the Burghen award of the European Academy of Sciences, the Olivier Kahn prize for molecular magnetism, the Nasini and Semerano prizes of the Italian Chemical Society and the Nicholas Kurti European Science Prize. His personal grants include, among others, the Marie Curie and Royal Society research fellowships and the Sofja Kovalevskaja and ERC Starting grants.

Oren Regev: Thermally conductive graphene-based composites

posted 1 Jun 2016, 06:48 by info admin

Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

The rapidly increasing device densities in electronics calls for efficient thermal management. If successfully exploited, graphene, which possesses extraordinary thermal properties, can be commercially utilized to produce polymer composites with ultrahigh thermal conductivity (TC). The total potential of graphene to enhance TC, however, is restricted by the large interfacial thermal resistance between the polymer-mediated graphene boundaries [1] We report a facile and scalable dispersion of commercially available graphene nano platelets (GnPs) in a polymer matrix, which forms composite with ultra-high TC of 12.4 W/mK (vs. 0.2 W/mK for the neat polymer) [2] This ultra-high TC is achieved by applying high compression forces during the dispersion that results in the closure of gaps between adjacent GnPs with large lateral dimensions and low defect densities. We also found strong evidence for a thermal percolation threshold. Finally, the addition of electrically insulating nano-boron nitride to the thermally conductive GnP-polymer composite significantly reduces its electrical conductivity (to avoid short circuit) and synergistically increases the TC [3] The efficient dispersion of commercially available GnPs in polymer matrix provides the ideal framework for substantial progress toward the large-scale production and commercialization of GnP-based thermally conductive composites.

1. Shtein, M., Pri Bar, I., Varenik, M., Regev, O., Anal. Chem. 2015, 87, 4076-4080.
2. Shtein, M., Nadiv, R., Buzaglo, M., Kahil, K., Regev, O., Chem. Mater. 2015, 27 2100.
3. Shtein, M., Nadiv, R., Buzaglo, M., Regev, O., ACS Appl. Mater. Interfaces 2015, 7, 23725.

Oren Regev is currently a full Professor of Chemical Engineering at the Ben-Gurion University of the Negev. He is primarily interested in the dispersion of nanocarbons and their application. He received his PhD in 1992 from the Technion, IIT and was a visiting scientist in Lund, Eindhoven and Texas A&M universities. He has published over 100 papers (>3000 citations) and two patents.

Ari Harju: Dominant source of disorder in graphene: Charged impurities or ripples?

posted 1 Jun 2016, 06:40 by info admin

Ari Harju, Andreas Uppstu, and Zheyong Fan, Department of Applied Physics, Aalto University, Finland

Experimentally produced graphene sheets exhibit a wide range of mobility values. Both extrinsic charged impurities and intrinsic ripples (corrugations) have been suggested to induce long-range disorder in graphene and could be a candidate for the dominant source of disorder. Here, using large-scale molecular dynamics and quantum transport simulations, we find that the hopping disorder and the gauge and scalar potentials induced by the ripples are short-ranged, in strong contrast with predictions by continuous models, and the transport fingerprints of the ripple disorder are very different from those of charged impurities. We conclude that charged impurities are the dominant source of disorder in most graphene samples, whereas scattering by ripples is mainly relevant in the high carrier density limit of ultraclean graphene samples (with a charged impurity concentration < 10 ppm) at room and higher temperatures.

http://arxiv.org/abs/1605.03715, submitted to Physical Review Letters

Ari Harju leads a Quantum many-body physics group at Department of Applied Physics, Aalto University, Helsinki, Finland. The group is part of an Academy of Finland Center of Excellence. He has authored 100+ international peer-reviewed journal articles, including high-rank journals like Nature Physics.

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