posted 19 Aug 2019, 01:07 by info admin
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updated 19 Aug 2019, 01:07
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1 Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain 2 Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
On-surface LEGO-like synthesis of graphene-based nanostructures unites the sturdiness of covalently bonded networks with the easy tunability of molecular materials. In particular, the use of chemically modified molecular precursors (“multi-color LEGO bricks”) may serve as a valuable tool to enhance the optical, electronic and magnetic functionalities of graphene nanoribbons. However, before chemically doped GNRs can be used in practical applications, an atomic level understanding and control of their properties depending on the nature, location and periodicity of the dopants is required. Using density functional theory (DFT), and in close collaboration with our experimental colleagues, a detailed electronic characterization of chemically doped GNRs has been carried out. We have studied the properties of GNRs of various shapes (armchair, chiral, chevron) doped with different chemical groups, e.g. boron dimers, fluorine atoms or amino groups, located either in the central backbone or at the edge of the ribbon. Our findings are compared with scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission (ARPES) data. Depending on the doping mechanism, various effects are observed and interpreted, such as electron confinement, enhanced substrate-GNR interaction, energy gap modification, or semiconductor-to-metal transition. [1-3]
[1] E. Carbonell-Sanromà et al., Nano Letters 17, 50 (2017) [2] E. Carbonell-Sanromà et al., J. Phys. Chem. C 122, 16092 (2018) [3] M. Panighel et al., (in preparation); Li et al. (in preparation)
Aran Garcia-Lekue is an Ikerbasque Researcher at Donostia International Physics Center (DIPC) in San Sebastian (Spain). She received his PhD degree in Physics from the University of the Basque Country UPV/EHU (Spain). After finishing her Ph.D, she held postdoctoral positions at the Surface Science Research Center (SSRC) of the University of Liverpool (UK) and at the Berkeley National Laboratory (US). She joined DIPC as a Gipuzkoa Research Fellow Gipuzkoa in 2007, and became an Ikerbasque Researcher in 2012. Her research line is focused on the simulation of electron transport at the nanoscale, and on the theoretical investigation of electron processes at nanostructured surfaces. In the last years, she has been very active in the study of electronic and transport properties of graphene-based materials.
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posted 19 Aug 2019, 00:58 by info admin
N. Mishra1,2, S. Forti1, F. Fabbri1,2, L. Martini1, C. McAleese3, B. Conran3, P.R. Whelan4,5, A. Shivayogimath4,5, Lars Buß6, Jens Falta6, I. Aliaj7, S. Roddaro7,8, J. I. Flege6,9, P. Bøggild4,5, K.B.K. Teo3 and C. Coletti1,2,*
1Center for Nanotechnology Innovation @ NEST, Istituto Italiano di Tecnologia , Piazza San Silvestro 12,56127 Pisa, Italy
2Graphene Labs, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy 3AIXTRON Ltd., Buckingway Business Park, Anderson Rd, Swavesey, Cambridge CB24 4FQ, UK
4DTU Physics, Ørsteds Plads 345C, 2800 Kgs. Lyngby, Denmark
5Center for Nanostructured Graphene (CNG), Ørsteds Plads 345C, 2800 Kgs. Lyngby, Denmark
6Institute of Solid State Physics, University of Bremen, Bremen-28334, Germany
7NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy
8Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, 56127, Pisa (PI), Italy
9Brandenburg University of Technology Cottbus-Senftenberg, Chair of Applied Physics and Semiconductor Spectroscopy, Konrad-Zuse-Str. 1, 03046 Cottbus, Germany
[1] N. Mishra, et al., arxiv (2019).
[2] M. A. Fanton, et al., ACS Nano (2011).
[3] J. Hwang, et al., ACS Nano (2013).
Neeraj Mishra is a post-doctoral fellow in 2D material engineering group of Dr. Camilla Coletti in Istituto Italiano di Tecnologia (IIT), Pisa, Italy. His main work is the growth of Graphene on insulators like h-BN and sapphire by the CVD method and its characterization. Besides, he has experience of the growth of Graphene on copper and different technique of Gr transfer. He has highly experienced with chemical vapor deposition using different growth systems including Aixtron-BM and characterization techniques such as scanning tunneling microscopy, Atomic force microscopy, and Raman spectroscopy.
During his Ph.D. has experience of growth SWCNTs and MWCNTs from waste plastics. He has completed his Ph.D. under the guidance of Prof. Maheshwar Sharon and worked on the project of University Grant Commission (UGC) title entitled “Scaling up of conversion technology of waste plastics into CNM and burnable Wax”. During the Ph.D. program, he was selected for MIUR scholarship in Soft Material Design Laboratory, Istituto Italiano di Tecnologia (IIT), Genova, Italy on a project entitled “Transparent carbon nanotubes conductive networks for flexible electronics and polymer solar cell technology” in the year 2010.
He has done his Bachelor (B.Sc. in Chemistry), M.Sc. (Physical chemistry) and Ph.D. in chemistry from the University of Mumbai. He has several papers in good journals. |
posted 14 Aug 2019, 05:44 by info admin
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updated 14 Aug 2019, 05:45
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P. Schmidt1, E. T. Icking1,2, L. Banszerus1,2,
C. Steiner1, C. Rogge1, K. Watanabe3, T. Taniguchi3,
B. Beschoten1 and C. Stampfer1,2 1JARA-FIT and 2nd Institute of Physics, RWTH
Aachen University, Germany, EU
1,2Peter Grünberg Institute (PGI-9),
Forschungszentrum Jülich, Germany, EU
3National Institute for Materials Science, 1-1
Namiki, Tsukuba, 305-0044, Japan  One of the most
unique characteristics of bilayer graphene (BLG) is the possibility to tune the
low-energy electron dispersion relation by applying an external electric field.
This allows for opening up a band gap, modifying the band curvatures and even
changing the topology of the Fermi surface. It has been shown that local
graphite gates improve the device quality significantly compared to devices
equipped with a global silicon back gate [1]. This can be explained by the
formation of conductive edge channels due to fringe fields of the far separated
global back gate and a lower disorder potential for local graphite gates
efficiently screening charge disorder in the SiO2. While this current
technology already allows for the confinement of charge carriers in zero or one
dimensions [2], novel, more complex device geometries require structured top
and back-gates [3]. This requires structuring of the graphite gate, which
results in contaminated interfaces and therefore could lead to a reduced device
quality. On the other hand, using a lithographically defined local gold gate
results in a high disorder potential due to its rough surface. To avoid both
problems, we present a technique to flip the encapsulated graphene with a
pre-defined gate structure so that the rough surface of the gold is not at the
interface to the graphene stack. Bias spectroscopy and temperature dependent
transport measurements of a flipped gold gate device are also presented and
compared to the graphite counterpart.
[1] H. Overweg, et al.: Nano Lett. 2018, 18, 1, 553-559
[2] L. Banszerus et al.: Nano Lett. 2018, 18, 8, 4785-4790
[3] J. Li, et al.: Science 2018, 362, 6419, 1149-1152 Philipp
Schmidt is currently a Master student in the group of Christoph Stampfer at
RWTH Aachen University. He is interested in mesoscopic electron transport in
bilayer graphene nanostructures and the nanofabrication of 2D heterostructures.
During his
bachelor studies he investigated the dephasing properties of a graphene-based
nanomechanical resonator coupled to a microwave cavity.
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posted 13 Aug 2019, 02:01 by info admin
Tero S. Kulmala1, Nils Goedecke1, Xiaorui Zheng2, Edoardo Albisetti2, Elisa Riedo2
1 SwissLitho AG Technoparkstrasse 1, Zurich, Switzerland
2Tandon School of Engineering, New York University, New York, NY, USA.
Forming high-quality electrical contacts is a key issue in fabricating high-performance 2D material electronic devices. However, predominant fabrication processes (i.e. electron beam lithography followed by metal evaporation and lift-off) typically yield poor quality non-ohmic metal contacts with high Schottky barriers and large contact resistances [1]. Here, we show that NanoFrazor (thermal scanning probe) lithography that relies on thermal decomposition of polymer resists [2] can be used to pattern high-quality metal contact electrodes to monolayer MoS2 with high reproducibility, sub-10-nm resolution, and a throughput comparable to high-resolution electron beam lithography [3]. The approach offers simultaneous in situ imaging and patterning as well as superior markerless alignment accuracy [4] and does not utilize high-energy charged particle beams. We developed a variety of lift-off metallization processes with different resists and solvents achieving gaps between metal electrodes below 10nm. Using this technique, we have patterned both top-gated and back-gated field-effect transistors with metal electrodes in direct contact with monolayer MoS2. These devices exhibit vanishing Schottky barrier heights (around 0 meV, Figure 1), record-high on/off ratios of 1010, no hysteresis, and subthreshold swings as low as 64 mV per decade without the use of negative capacitors or hetero-stacks.
Nils is a trained biophysicist with a Master’s Degree from Humboldt University Berlin (1999) and a Ph.D. in Analytical Chemistry from Imperial College London (2005). Throughout his scientific career he applied microsystem and surface engineering to a broad range of technical development projects with biological context. In that capacity he developed lab-on-a-chip devices for DNA forensics at MIT (2003-2005) as well as microfluidics for single-cell analysis at ETH Zurich (2005-2011). His research at University Hospital Balgrist (2012-2017) focused on sensor arrays for measuring cell mechanics. Nils joined SwissLitho in summer 2018, where he is part of the sales team trying to identify new customers and applications for the NanoFrazor technology. |
posted 12 Aug 2019, 07:40 by info admin
Johannes H. J. Martiny, Kristen Kaasbjerg, and Antti-Pekka Jauho Center for Nanostructured Graphene (CNG), DTU Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark johmar@nanotech.dtu.dk
Valleytronics – the proposed field of information processing based on the electron valley index in e.g. graphene – relies on the control of protected valley currents [1]. In this work we consider the model of graphene gated via a periodic array of holes in a dielectric resting on a bottom electrode. We are inspired by the recent transport experiments in such structures [2]. Using tight‐binding supercell calculations and an unfolding procedure [3], we demonstrate how the electronic structure of this system corresponds to a gapped graphene structure with an associated valley Hall effect. We characterize the valley polarized currents by extracting the valley Hall conductivity from the unfolded Berry curvature of occupied states, and find that these currents become tunable by the gate‐potential. We furthermore perform Boltzmann conductivity calculations in order to characterize the valley Hall angle and make predictions for the indirect detection of such currents in nonlocal transport experiments when the Fermi level is tuned close to the band edge. Finally, we demonstrate the stability of the valley Hall effect in these systems when realistic potentials are considered which include the effect of disorder in the dielectric.
[1] J. R. Schaibley et al., Nature Reviews Materials, 1 (2016) 16055 [2] C. Forsythe et al., Nature Nanotechnology, 13 (2018) 566‐571 [3] T. Olsen and I. Souza, Physical Review B, 92 (2015) 125146
Johannes H. J. Martiny is a PhD student at DTU Physics in the group of Prof. Antti‐Pekka Jauho. He received his master degree in condensed matter theory from the Niels Bohr Institute at Copenhagen University in 2016. His current research interests include valley Hall effect in graphene superlattices, nonlocal signatures of topological currents in 2D materials, impurity‐induced magnetization in iron based superconductors, and Anderson localization of inter‐edge coupled edge states in 2D topological insulators. |
posted 12 Aug 2019, 07:35 by info admin
DTU Physics, Technical University of Denmark
Graphene based 2D catalysts hold great promise for CO2 reduction to CO and CH4. Recent experimental investigations1 show metal doped Iron-Nitrogen-Carbon (Fe-N-C) catalysts are able to reduce CO2 to CO at low overpotentials and with high selectivity. However, modelling these materials in an electrochemical environment poses several open challenges. In this work, we present a theoretical investigation on Fe-N-C catalysts which includes the effect of potential, interfacial pH, change in local spin states and quantum capacitance to properly elucidate the mechanism for CO2 reduction. Energies are benchmarked to higher levels of theory and experiment to test the applicability of commonly used methods in computational catalysis to these class of materials. We find that the electronic structure of Fe-N-C resembles graphene more than it does a metal, with significantly fewer states at the fermi level. Charge dependence[2] of binding energies of key intermediates depend on the position of the highest energy d-orbital with respect to the fermi level. Using computed reaction energetics coupled with mean-field kinetic models, we are able to ascertain the mechanism for CO2 reduction and compare our results with experimental findings. We extend this analysis to other Metal-Nitrogen-Carbon systems and rational design principles are proposed.
1. Varela, A. S. et al. Electrochemical reduction of CO2 (CO2RR) on Metal-Nitrogen-doped carbon (MNC) catalysts. ACS Catal. (2019). 2. Gauthier, J. et al. Unified Approach to Implicit and Explicit Solvent Simulations of Electrochemical Reaction Energetics doi:10.26434/chemrxiv.8396954.v1
Sudarshan Vijay is a PhD student at the Catalysis Theory group at the Physics department of the Technical University of Denmark. He works on mechanistic understanding and computational catalyst discovery of CO2 reduction (CO2R) to CO and further reduced products on single atom catalysts on graphene supports. Previously, he obtained his MS from Carnegie Mellon University, Pittsburgh where he worked on using computational methods to elucidate mechanisms at play for transport of hydroxide ions through an anion exchange membrane. |
posted 12 Aug 2019, 07:12 by info admin
C. Struzzi, N.A. Vinogradov, Y. Niu, A.B. Preobrajenski, A.A. ZakharovMAX IV Laboratory - Lund University, Sweden
The Spectroscopic PhotoElectron and Low Energy Electron Microscope (SPELEEM) is installed at the dedicated beamline MaxPEEM at MAX IV synchrotron, Lund (Sweden). The key feature of the SPELEEM relies in the availability of different contrast mechanisms allowing imaging structural, chemical, electronic, and magnetic properties with spatial resolutions in the nanometer range. The recent upgrade of the microscope with an aberration corrector improves the spatial resolution of the microscope by an order of magnitude (down to 2nm resolution in some modes of operation) and increases the transmission by a similar factor. The samples can be investigated in operando condition by exploiting the detection at video-rates and monitoring real-time dynamical processes at the surface at elevated temperatures. Sampling structures from the micrometer down to the nanometer scale is feasible thanks to the large range of available field of view (from 0.75 μm up to 100 μm). The capabilities of this powerful instrument allow surface studies in a wide range of disciplines, e.g. materials science, nano-science, heterogeneous catalysis, corrosion science, polymer science. To give a taste of the main characteristics of the different SPELEEM imaging modes and to emphasize the capabilities of the instrument, few research examples are presented, e.g. intercalation of germanium at the SiC-graphene interface and graphene protection to oxidation of copper foil.
Dr. Claudia Struzzi is an experimental physicist with experience in surface science. After the Master’s Degree in Physics at the Sapienza University of Rome (Italy) in 2012, she worked 18 months at the ARPES beamline BaDElPh, at ELETTRA synchrotron in Trieste (Italy), where she contributed to the operation, optimization, maintenance and upgrade of the beamline, providing support to the external users and performing research activities. At the end of 2013, she was awarded by the Belgian Funds for Scientific Research (FNRS) with a fellowship to start the PhD thesis (defended in 2017). Her work focused on fluorination of carbon nanostructures (graphene and carbon nanotubes) and their implementation in gas sensing devices to test their sensing properties to ppm concentration of NO2 and NH3 gases. During the PhD, she gained experience with various spectroscopic and microscopic characterization techniques. Immediately after the PhD, she joined MAX IV synchrotron in Lund (Sweden) as PostDoc Researcher working at MaxPEEM beamline where the AC-SPELEEM microscope is installed. In 2018, she won the “Prix de Chimie appliquée 2018” that is a biennial price issued by FNRS for an original PhD thesis concerning new concepts and applications in the field of industrial chemistry. |
posted 12 Aug 2019, 07:07 by info admin
Mathias Geisler1,2, Martijn Wubs1,2, N. Asger Mortensen2,3,4, Sanshui Xiao1,2, Nicolas Stenger1,2 1Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 2Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 3Center for Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark 4Danish Institute for Advanced Study, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
Tailoring the interaction between excitons in semiconducting materials and electromagnetic fields focused inside optical cavities is the cornerstone of many applied and fundamental researches in nanophotonics. The coupling strength between matter and light can range from the weak to the strong regime, where light and matter can no longer be treated independently but instead as hybridized states sharing both properties of light and matter. In order to reach that regime, the coherent energy exchange between the excitons and the optical field must happen faster than any dissipation processes. With the recent discovery of new low-dimensional materials such as transition metal dichalcogenides (TMDC), hosting excitons with binding-energies above 0.3 eV and exhibiting large transition dipole moments, it has been shown experimentally, in combination with plasmonic nanostructures, that it is possible to reach the strong-coupling regime at room temperature [1, 2]. Here, we report the observation of strong coupling at room temperature between plasmons in monocrystalline gold nanodisks deposited on top of tungsten disulfide (WS2). With dark- and bright-field spectroscopy we show for monolayer WS2 a clear Rabi splitting of 108 meV which is at the onset of the strong coupling regime. We use multilayer WS2 to push this coupling deeper into the strong-coupling regime and reach a splitting as high as 175 meV. To our knowledge, this is the highest Rabi splitting reported for TMDC materials coupled to plasmonic cavities [3]. This investigation could lead to the design of novel ultra-compact and efficient light sources. [1] J. Wen, et al., “Room-Temperature Strong Light-Matter Interaction with Active Control in Single Plasmonic Nanorod Coupled with Two-Dimensional Atomic Crystals,” Nano Lett. 17, 4689-4697 (2017). [2] M. Stührenberg, et al., “Strong Light-Matter Coupling between Plasmons in Individual Gold Bi-pyramids and Excitons in Mono- and Multilayer WSe2,” Nano Lett. 18, 5938-5945 (2018). [3] M. Geisler, et al., “Single-Crystalline Gold Nanodisks on WS2 Mono- and Multilayers for Strong Coupling at Room Temperature,” ACS Nanophotonics 6, 994-1001 (2019).
Nicolas Stenger is currently an Associate Professor with the Department of Photonics Engineering at the Technical University of Denmark. Previously, he received his MSc in 2004 and PhD in Condensed Matter Physics in 2008 from the University of Strasbourg, France, before embarking on invisibility cloaking research as a postdoctoral fellow at the Karlsruhe Institute of Technology, Germany. In 2012 he moved to Denmark and was awarded a fellowship from the Lundbeck Foundation to explore quantum effects in plasmonic nanostructures. Nicolas is a member of the Center for Nanostructured Graphene (DTU CNG) and his research activities are mainly focused on strong light-matter interactions between plasmonic cavities and semiconducting two-dimensional materials. |
posted 12 Aug 2019, 07:03 by info admin
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updated 12 Aug 2019, 07:08
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Raluca-Maria Stan, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
Theoretical studies on single-layer (SL) NbS2 have shown that the inclusion of different many-body effects has a significant impact on the electronic structure of this system [1]. This might be a unique case for experimentally tuning the different many-body interactions--by the choice of substrate or doping--in order to reach a desired ground state. We present a SL NbS2 that has been epitaxially synthesized on Au (111) substrate [2]. Complementary techniques like scanning tunneling microscopy, low-energy electron diffraction indicate excellent crystalline quality and the NbS2 lattice is aligned with respect to the Au (111) substrate. Furthermore, the SL NbS2 has a hexagonal structure with a measured lattice constant of (3.29±0.03) Å which is in agreement with the lattice constant of the bulk parent [3]. The electronic structure of SL NbS2 investigated by means of angle-resolved photoemission spectroscopy reveals two electron pockets crossing the Fermi level. The metallic character has been shown and the general shape of the band structure is consistent with the 1H configuration. The observed band broadening can be due to hybridization with the substrate or strong many-body effects in the system.
[1] E. G. C. P. van Loon, M. Rösner, et al., npj Quantum Materials 3, 32 (2018) [2] R. M. Stan, S. K. Mahatha et al., Phys. Rev. Mat. 3, 044003 (2019) [3] K. F. Mak, C. Lee, et al., Phys. Rev. Lett. 105, 136805 (2010)
Raluca-Maria Stan received her B.Sc. and M.Sc. in Physics at Alexandru Ioan Cuza University of Iasi, in Romania and moved to Denmark in 2016 where she continued with her career in physics. She is currently in her last year of her PhD program at Aarhus University. She is supervised by Assoc. Prof. Jill Miwa and Prof. Philip Hofmann. She is interested in 2D materials, particularly single-layer (SL) transition metal dichalcogenides (TMDCs) because of their exotic physics and correlated effects such as charge and spin ordering, Mott insulating states and superconductivity which may behave differently compared to their bulk counterparts. Her PhD project involves the growth of SL TMDCs on Au (111) in order to investigate the structural configuration and the electronic properties of these systems. |
posted 12 Aug 2019, 06:58 by info admin
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updated 12 Aug 2019, 07:08
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DTU Physics, Technical University of Denmark
At the atomic scale, correlation
between form and function is key. Electron transport is no exception to the
rule. Any subtle change in lattice structure, defect or external contamination
can change radically the transmission of a given low-dimensional system. However,
making accurate theoretical predictions of the transport properties of a single
configuration can be very computation heavy, which prohibits rapid testing.
This ultimately makes the development of an intuition for the phenomena
involved at the atomic-scale an extremely slow process.
By leveraging the possibilities
offered by Virtual Reality, we provide a novel approach which aims at making
the teaching and research in 2D materials more approachable, by allowing for
fast experimentation and prototyping. As a demonstrator, we have, using various
tricks of the trade, designed a tool that outputs real-time electron transport
calculations, based on the user defined structures. Niels
Pichon is currently a Master student at DTU Physics, studying fluorographene in
Peter Bøggild’s group. Being a VR and AI enthusiast, he has been working as
student assistant on developing the DevinaVR software environment. Joachim
Sødequist is also a Master Student at DTU Physics, focusing mostly on
theoretical condensed matter physics. He has been working on the real time
transport calculation for the last few months as a research assistant. |
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