Modifications of Transition Metal Dichalcogenides by Defect and Interface Engineering
Who: Prof Matthias Batzill, Department of Physics, University of South Florida, USA
When: Wed, 20-Nov-2019, 11am
Where: Physics Conference Room (S11-02-07)
Host: Prof Andrew Wee
Modifying and manipulating properties of 2D materials is central for achieving the potential of these materials. In our group we aim at synthesizing 2D materials by molecular beam epitaxy and study approaches for controlled modifications by alloying, doping, one-dimensional modifications (edges or grain boundaries) or interfacing with dissimilar materials. In the first part of the talk we present our studies on the formation of metallic mirror twin grain boundaries in MoSe21 or MoTe22 by incorporation of excess Mo into the lattice (see Figure). Very high density of MTB networks can be obtained in MoTe2 that effectively metallizes the material and thus may act as a metallic contact patch.3 Such line defects may also increase electrocatalytic properties for hydrogen evolution reactions.4 On a more fundamental level, we show that these 1D metallic grain boundaries host a one-dimensional electron gas and we present the first angle resolved photoemission (ARPES) studies of the electronic properties of such line defects. These studies show evidence for the presence of Tomonaga-Luttinger Liquid behavior.5 The second part of the talk focuses on the differences in charge density wave properties of metallic TMDCs in the monolayer compared to bulk materials. Here we focus on the layer dependent variations of the properties of VSe26 and VTe2.7 For both materials we observe a charge density wave (CDW) behavior in the monolayer limit compared to their bulk properties. From ARPES and DFT we can also show that CDW competes with ferromagnetic ordering in monolayer VSe2.6 In VTe2 we observe a structural transition from a distorted-1T’’ structure in the bulk to a simple hexagonal 1T structure in the monolayer. This structural transition is accompanied by a CDW instability for the monolayer that gives rise to a 4×4 CDW at low temperatures, in contrast to bulk VTe2 that does not exhibit a CDW instability.7
Figure: Line defect formation in 2H-MoTe2 with increasing Mo-deposition and the defect-induced band gap states measured by ARPES, showing linear (1D) Fermi-surface and dispersing bands.
About the speaker:
Dr. Batzill graduated from the University of Goettingen, Germany and obtained a PhD in Physics from the University of Newcastle upon Tyne, UK in 1999. He went on to do post-doc research at the Universities of Southern California in Los Angeles and Tulane University in New Orleans, before starting his independent research group at the University of South Florida (USF) in Tampa, USA in 2006, where he currently is a full Professor of Physics. He received a NSF-CAREER award and NSF creative extension award. He also has been a Hans-Fischer Fellow of the Technical University of Munich, Germany and won several outstanding research achievement and faculty awards at USF. He gave over 80 invited talks and published more than 120 publications with an H-index of 42 (google scholar). His research expertise is in the physics and chemistry of surfaces and interfaces with current focus on 2D materials.
Exotic Hadrons: A New Particle Spectroscopy?
Who: Prof Luciano Maiani and Dr Veronica Riquer, Shanghai Jiao Tong University and Universita’ di Roma La Sapienza
When: Fri, 6-Dec-2019, 10.30am
Where: Physics Conference Room (S11-02-07)
Host: Assoc Prof Phil Chan and Dr Cindy Ng
Over the last decade, new particles have been discovered that do not fit the established rules that mesons are quark-antiquark and baryons three quark states. We introduce the main alternatives thus far proposed, hadron molecules bound by the nuclear forces and multiquark compact states bound by the primary QCD forces. There are interesting, evolving theoretical methods (quark model, lattice, Born-Oppenheimer) and experiments planned at future hadron and lepton colliders, to explore this very lively field of research. Particular attention will be devoted to the emerging field of hadrons with two heavy quarks (non-exotic and exotic).
About the speakers:
Dr. Veronica Riquer was born in Mexico. She earned her PhD in theoretical physics at UNAM in 2002 and after she obtained a Postdoc Conacyt fellowship at CERN (2003-2005) and a research contract INFN in Rome (2003-2005). From 2009 is consultant of the UNESCO ICTP in Trieste (Italy). She is now managing for the establishment of a UNESCO ICTP associated institute in Mexico.
In the period 2005-2015 she was co-coordinator of the HELEN and EPLANET projects which provided more than 3000 Months*Person fellowships for exchanges between 22 Latin American Institutions (Argentina, Brazil, Chile, Colombia, Peru, Venezuela, Cuba, Mexico) and EU Institutions (Italy, France, Germany, Spain, Portugal, UK), in particular concerning projects at CERN (Switzerland) and at the Cosmic Ray Observatory Pierre Auger (Argentina).
As a theoretical physicist, she worked in QCD (light scalar mesons, hidden charm mesons and exotic hadrons) and in Higgs physics in the minimal SUSY extension of the Standard Theory.
Prof. Luciano Maiani was born in Rome (Italy) where he obtained his university Degree in physics. He is also Doctor Honoris Causa c/o many Universities in Aix-Marseille (France), Saint Petersburg (Russia), Bratislava (Slovakia), Warsaw (Poland) and Puebla (Mexico). Luciano Maiani is now Emeritus Professor of Rome University “La Sapienza” and fellow of the Italian Accademia Nazionale dei Lincei and of the American Physical Society.
He was President of many Italian research and government Institutions like the National Institute for Nuclear Physics (INFN), the National Research Council (CNR), the Technical Scientific Committee, Applied Research Fund, the National Committee for Large Risks. At International level he was first President of the Council of the European Organization for Nuclear Research (CERN) and after Director General of CERN from 1999 to 2003. He chaired many International Scientific Committees like for the UNESCO International Center for Theoretical Physics (ICTP). He was very active in promoting international cooperation having been President of the High Energy Physics Latinoamerican Network (HELEN 2005-2009 ) and of the European Particle physics Latin American Network (EPLANET 2011-2016). He is now also President of the International General Balzan Prize for achievement in the field of humanties, natural science, culture and peace.
As a theoretical physicist, he is worldwide known for the prediction of charmed subatomic particles (the so-called GIM effect with the Nobel Laureate S. Glashow and with J. Iliopoulos) which were later discovered and awarded with an other Nobel Prize. He also developed fundamental theories for the Higgs boson, for the Supersymmetry, for the Quantum Chromodynamics and for exotic hadrons.
First-Principles Investigation Of Charged Dopants, Defects, And Defect Complexes In 2D Transition Metal Dichalcogenides
Who: Dr Anne Marie Tan, University of Florida, USA
When: Wed, 18-Dec-2019, 11am
Where: Physics Conference Room (S11-02-07)
Host: Prof Feng Yuan Ping
Two-dimensional (2D) semiconductor materials, such as transition metal dichalcogenides (TMDCs), monochalcogenides, and phosphorene have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. Understanding the effect of impurities, defects, and dopants on their electronic properties is crucial for the selection of materials and choice of suitable synthesis and processing conditions to tailor the carrier concentration, character, and mobility.
Density functional theory (DFT) calculations of point defects in solids is a mature field with a proven record of experimentally validated predictions. However, modeling charged defects in single-layer materials with common plane-wave DFT approaches poses additional challenges which lead to the divergence of the energy with vacuum spacing. Recently, Freysoldt and Neugebauer developed a correction scheme which employs a surrogate model to restore the appropriate electrostatic boundary conditions for charged 2D materials. We perform DFT calculations and apply this correction scheme to accurately compute formation energies and charge transition levels associated with dopants, defects, and defect complexes in technologically relevant TMDCs such as MoS2 and WSe2. We identify dopants which can form defect complexes with intrinsic defects such as vacancies, acting as compensating defects and modifying the electronic properties of the individual defects. An example of this is the Re dopant – S vacancy complex in MoS2 which we predict to be thermodynamically favorable and which has been observed experimentally. By analyzing their electronic structures, we are able to gain insights into the effect of defects on bonding and magnetism. Finally, we are developing a Python-based workflow to facilitate efficient high-throughput calculations of charged point defects in 2D materials and interfacing it with existing materials databases to create a new database of defects in 2D and layered bulk materials.
About the speaker:
Dr. Anne Marie Tan is currently a Post-Doctoral Associate in the Hennig group at the University of Florida, where she develops and applies computational tools to model, predict, and analyze 2D materials. Her research focuses on using first-principles density functional theory (DFT) to study the energetics and electronic properties of charged point defects and dopants in 2D semiconductor materials such as transition metal dichalcogenides. Prior to this, Anne Marie received her Ph.D. in Materials Science and Engineering in 2018 from the University of Illinois at Urbana-Champaign. During her Ph.D., Anne Marie also studied defects, developing a method to evaluate the lattice Green function for dislocations in crystalline materials, and applying it within a flexible boundary condition approach coupled with DFT to compute dislocation core structures in metals, intermetallics, and semiconductors. Anne Marie is also an alumna of the National University of Singapore, where she earned her B.Eng. in Materials Science and Engineering in 2012, and was awarded the Lee Kuan Yew Gold Medal, Institution of Engineers Singapore (IES) Gold Medal, and Materials Research Society (Singapore) Medal for outstanding academic achievement.
Date / Location: Thursday 26 December 2019 / NUS football FieldPlease visit website: SEAAN Meeting 2019 for more information.