Annual report FOM programme nr. 157 'Two-dimensional semiconductor crystals' Foundation for Fundamental Research on Matter

Transcription

1 FOM Annual report 2015 FOM programme nr. 157 'Two-dimensional semiconductor crystals' Foundation for Fundamental Research on Matter Scanning tunneling microcopy image (4 nm x 4 nm) of germanene (the germanium analogue of graphene). The honeycomb lattice, which is composed of two interpenetrating triangular sub-lattices, is buckled. May 2016

2 Content 1. Scientific results Added value of the programme Personnel Publications TWOD01/14TWOD03/14TWOD TWOD Valorisation and outreach Vacancies... 4 Fact sheet as of 1 January Historical overview of input and output... 7 PhD defences... 7 Patents (new/changes)... 7 Overview of projects and personnel... 8 Workgroup FOM-D Workgroup FOM-N

3 1. Scientific results 2015 The FOM programme 'Two-dimensional semiconductor crystals' has been granted in December The programme encompasses four PhD students and four PD positions and as of 1 January 2016 two PhD positions (Nikos Papadopoulos/TUD and Rik van Bremen/UT) and one PD position (Jaap Kroes/RU) are filled, whereas a third PhD student (Michal Ochapski/UT) has agreed to start in March The first PD (RU, Jaap Kroes) started with first-principles calculations on the energetics, barriers and vibrational spectra of partially and fully hydrogenated hexagonal boron-nitride. During his PhD project at the EPFL in Lausanne he has obtained ample experience with density functional theory calculations. His first paper within the framework of the 2D FOM programme on partially and fully hydrogenated hexagonal boron-nitride has recently been submitted to Phys. Chem. Chem. Phys. The TUD PhD student (Nikos Papadopoulos) started his project on the fabrication and characterization of molybdenum disulfide layers in June Meanwhile he has become familiar with the chemical vapor deposition growth of molybdenum disulfide layers as well as various techniques to study the physical properties of this material. In addition, using chemical doping molybdenum disulfide was converted to its metallic poly-type (1T) phase and by exposure to laser radiation molybdenum disulfide could locally be transformed back to its semiconducting phase. This method allows to define a semiconducting molybdenum disulfide channel within a metallic polytype 1T host phase. Molybdenum disulfide devices and flakes have been characterized by several techniques, including Raman, photoluminescence and atomic force microscopy. The semiconducting to metallic phase transition of molybdenum disulfide has been studied with transport measurements and Raman spectroscopy. The next step will be to fabricate encapsulated singlelayer molybdenum disulfide (MoS 2) devices with 1T-sidecontacts as well as to try to observe superconductivity in 1T-LiMoS 2. In Twente four students (Lijie Zhang, Pantelis Bampoulis, Adil Acun and Caspar Walhout) have synthesized germanene (the germanium analogue of graphene) on Ge 2Pt crystals as well as molybdenum disulfide substrates. Scanning tunneling microscopy and spectroscopy experiments have been performed in order to determine the structural and electronic of germanene. Germanene layers grown on Ge 2Pt crystals exhibit a buckled honeycomb lattice, which is composed of two hexagonal sub-lattices that are displaced vertically by 0.2 Å. The nearest-neighbor distance of the atoms in the honeycomb lattice is 2.5±0.1 Å, i.e. very close to the predicted nearest-neighbor distance for germanene. The density of states of the germanene layers exhibits a well-defined V- shape, which is the hallmark of a two-dimensional Dirac system. Similar results are found for germanene layers synthesized on a molybdenum disulfide substrate. The official kick-off meeting of the programme will be held on 4 March 2016 with two renowned invited speakers from abroad: Dr. Patrick Vogt (Berlin) and Dr. Raphael Roldan (Madrid).The scientific programme is completed with several contributions from participants of 2D FOM programme. In 2015 two papers have appeared: a topical review on germanene and a letter on the Dirac nature of germanene. The topical review is a joint effort of the principal investigators of Radboud University (Prof. Katsnelson & Dr. Rudenko) and the University of Twente (Dr. Brocks and Prof. Zandvliet). Several papers are currently under review: a paper from RU (on hexagonal boron nitride, a paper on the temperature dependence of the density of states of germanene (UT) and a joint manuscript (RU/UT) on the synthesis and characterization of germanene on molybdenum disulfide. Finally, the groups of Van der Zant and Steele (TUD) are currently preparing a manuscript for submission

4 2. Added value of the programme As described in section 1 the programme has already resulted in two joint manuscript (one publicshed and a second one in the pipeline). We envisage that the projected collaborations will be further strengthen in the coming years. 3. Personnel The programme originally encompasses five PhD students and three PD positions, but one of the PhD positions (RU Katsnelson) has been converted to a PD position. As of 1 January 2016 two PhD positions and one PD position are filled, whereas a third PhD student has agreed to start in March We hope to fill the fourth (and last) PhD position soon. We anticipate that one of the three PD positions (RU) will be filled in 2016, whereas the other two PD positions (RUG and UT) are scheduled for Publications 14TWOD01/14TWOD03/14TWOD07 - Topical review (open access) A. Acun, L. Zhang, P. Bampoulis, M. Farmanbar, M. Lingenfelder, A. van Houselt, A.N. Rudenko, G. Brocks, B. Poelsema, M.I. Katsnelson and H.J.W. Zandvliet, Germanene: the germanium analogue of graphene, Journal Physics Condensed Matter 27, (2015). 14TWOD07 - L. Zhang, P. Bampoulis, A. van Houselt and H.J.W. Zandvliet, Two-dimensional Dirac signature of germanene, Applied Physics Letters 107, (2015). 5. Valorisation and outreach Not applicable. 6. Vacancies One PhD position (Brocks/Kelly, UT). We are also seeking a suitable candidate for the second RU PD position (Katsnelson/Fasolino). The remaining two PD positions (RUG and UT) are scheduled for

5 Fact sheet as of 1 January 2016 FOM /1 datum: APPROVED FOM PROGRAMME Number 157. Title (code) Executive organisational unit Programme management Two-dimensional semiconductor crystals (TWOD) BUW Prof.dr.ir. H.J.W. Zandvliet Duration Cost estimate M 1.6 Concise programme description a. Objectives In this programme we aim to synthesize and characterize two-dimensional materials that have a sizeable band gap and appreciable charge carrier mobilities. We will propose, design, characterize and implement elementary field-effect based electronic devices that rely on two-dimensional semiconductors. Particular attention will be paid to (1) the opening/tuning of a band gap, (2) encapsulation, (3) field-effect characteristics, (4) artificial multilayers and (5) magnetic doping. We will focus on silicene, phosphorene and transition-metal (di)chalcogenides (e.g., molybdenum disulfide), and explore in a concerted and coherent way, new fundamental science and applications of these novel two-dimensional semiconductors. b. Background, relevance and implementation In the past decade a new exciting class of materials has been developed, which is not threedimensional, but two-dimensional in nature. Graphene is the most famous example of this new class of materials. It exhibits a wealth of exotic and intriguing properties, which has resulted in a myriad of scientific breakthroughs. However, graphene also suffers from a severe drawback: it is gapless, implying that it cannot take over the leading role that silicon plays in the current microelectronic industry. There are several two-dimensional materials such as silicene, phosphorene and transition-metal (di)chalcogenides that have a band gap (or band gap can be opened up in these materials). Twodimensional semiconductors are very appealing for modern electronics, which is basically twodimensional, as the functionality of devices is dominated by what occurs at the interfaces of semiconductors. Manipulating charge carrier densities and transport is often hindered rather than assisted by having to use bulk semiconductors. Employing two-dimensional semiconductors would enable to enter a new regime and open doors to exciting new physics and applications. We envisage these two-dimensional semiconductor crystals as the gateway to a wealth of novel and exciting phenomena with a strong potential for technological applications. A balanced and well-chosen arsenal of experimental and theoretical techniques that includes transport measurements, scanning tunneling microscopy & spectroscopy, photoemission electron - 5 -

6 spectroscopy, low energy electron microscopy and diffraction, near edge X-ray absorption fine structure, photoluminescence and Raman spectroscopy, density functional theory and quantum many body theory calculations will used to address these tantalizing challenges. In the starting phase of the programme, PhD students will pay visits of about 1-2 weeks to the other research groups. In addition, several experimental PhD and postdoc projects are executed in two research groups, the majority in the host group, but another part at the second location. Network meetings are held every 9-12 months at one of the research groups. The programme leader will organize an international workshop at the start and at the end of the programme in order to give an additional boost to the knowledge level and ambition of the junior scientists (PhD and postdocs) and to cement future networks and collaborations conducive to their further careers. Funding salarispeil cao tot bedragen in k > 2021 Totaal FOM-basisexploitatie FOM-basisinvesteringen Doelsubsidies NWO Doelsubsidies derden Totaal Source documents and progress control a) Original programme proposal: FOM b) Ex ante evaluation: FOM c) Decision Executive Board: FOM Remarks The final evaluation will be based on the self-evaluation report initiated by the programme leader and is foreseen for JM par. HOZB Subgebieden: 50% COMOP, 50% NANO - 6 -

7 Historical overview of input and output personnel (in fte) finances* (in k ) Input WP/V WP/T PhD NWP PhD theses refereed publications other publications & patents Output presentations * After closing the financial year. PhD defences 2015 None. Patents (new/changes) 2015 None

8 Overview of projects and personnel Workgroup FOM-D-44 Leader Organisation Project leader(s) Project (title + number) Prof.dr.ir. H.S.J. van der Zant Delft University of Technology Field-effect devices (14TWOD04) FOM employees on this project Name Position Start date End date N. Papadopoulos PhD 1 June May 2019 Workgroup FOM-N-24 Leader Organisation Project leader(s) Project (title + number) Prof.dr. A. Fasolino Radboud University Nijmegen Structural and thermal properties (14TWOD02) FOM employees on this project Name Position Start date End date J. Kroes WP/T 1 April March