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Contributed and young researchers 2019

Itai Epstein: Extremely Efficient Light-Exciton Interaction in a Monolayer Semiconductor Van der Waals Heterostructure Cavity

posted 27 Jun 2019, 09:20 by info admin   [ updated 28 Jun 2019, 02:48 ]

ICFO - The Institute of Photonic Sciences, Castelldefels 08860, Barcelona, Spain

Semiconductor two-dimensional materials, such as transition-metal-dichalcogenides (TMDs), exhibit extraordinary optoelectronic properties due to their unique band structure and the ability to tune their charge carrier density. The combination of a direct bandgap together with large exciton binding energies leads to an optical response that is completely dominated by the supported excitons in these materials.  
In this work, we demonstrate a TMD-based high quality Van der Waals heterostructure cavity, in which the achieved interaction strength is unitary and can be controlled both electrically and optically. Figure 1 shows the non-trivial behavior of the temperature dependent interaction strength via the FWHM of the exciton supported by the monolayer TMD.  We develop a theoretical framework to describe the light-exciton-cavity interaction, which fully supports the experimental results.  
The heterostructure cavity also enables the excitation of a large photo-excited excitonic population while still maintaining low optical power. This high density of excitons allows the observation of high order excitonic complexes, with ultra-low continuous-wave (CW) laser power excitation down to few nW.  Finally, we show that by modifying the structure of the cavity we are able to fully tailor the interaction strength from 0-100%.  
This enhanced light-exciton interaction paves the way to possible polaritonic condensation in monolayer semiconductors and excitonic optoelectronic devices based on 2D semiconductors. 

Itai Epstein is a postdoctoral researcher at the group of Prof. Frank Koppens at ICFO - The institute of photonic sciences, Barcelona, Spain. His postdoctoral research is devoted to studying light-matter interaction in 2D materials, such as graphene, transition-metal-dichalcogenides (TMDs), and hexagonal-boron-nitride (hBN). In particular, he is working on experimentally exposing new polaritonic phenomena in these materials, and new methods to achieve strong light-2D matter interaction. His PhD research, under the supervision of Prof. Ady Arie, at Tel Aviv University, focused on the investigation of surface plasmon waves in the scope of fundamental wave phenomena, and his M.Sc research, under the supervision of Prof. Yossi Rosenwaks, at Tel Aviv University, focused on the investigation of electronic transport properties of thin film transistors. 

Jan-Philip Joost: Correlation Effects and the Topological Band Structure of Graphene Nanoribbon Heterojunctions

posted 28 May 2019, 10:23 by info admin

Jan-Philip Joost1, Antti-Pekka Jauho2, Michael Bonitz1
1Institute of Theoretical Physics and Astrophysics, University of Kiel, Germany
2CNG, DTU Physics, Technical University of Denmark

Topological insulators are a central theme in modern solid state physics as they combine an insulating bulk with robust in-gap boundary states. Recent progress in the atomically precise bottom-up synthesis of finite length graphene nanoribbons (GNRs) has opened up a new way to realize such topological materials.
The existence of localized in-gap states in heterojunctions of topologically distinct GNRs was predicted theoretically [1] and later confirmed experimentally [2]. It was found that GNRs composed of alternating segments of 7- and 9-armchair GNRs (AGNRs) exhibit new bulk bands and end states that differ qualitatively from the band structures of pristine 7- and 9-AGNRs. Most theoretical modelling of 7/9-ANGRs has been restricted to DFT/LDA or tight binding (TB) methods which are known to underestimate or completely ignore correlation effects, respectively. Since electronic correlations are known to be very important in GNRs [3], a quantitative analysis must go beyond these simple theories. Here, we present simulations of 7/9-AGNRs based on a Green functions method with second Born and GW self-energies [3] applied to an effective Hubbard model. We find remarkable quantitative agreement with the experimental local dI/dV measurements of Ref.[2]. Especially the description of the topological end states is greatly improved by the inclusion of correlations compared to the TB and LDA results.
Moreover, our approach can be easily extended to extract the transport properties of these systems.

[1] T. Cao et al., Phys. Rev. Lett. 119, 076401 (2017)
[2] D. J. Rizzo et al., Nature 560, 204-208 (2018)
[3] J.-P. Joost et al., Phys. Stat. Sol. (b) (2019), doi: 10.1002/pssb.201800498

Jan-Philip Joost is currently a Ph.D. student in the group of Michael Bonitz at the University of Kiel. He received his B.Sc. and M.Sc. in Physics at the University of Kiel. His research interest is the study of equilibrium properties as well as non-equilibrium electron dynamics in graphene nanostructures by means of nonequilibrium Green functions.

Martha Scheffler: Graphene-coating of Platinum nanoparticles

posted 28 May 2019, 05:03 by info admin   [ updated 29 May 2019, 02:57 ]

Martha Scheffler, Andrew Cassidy, and Liv Hornekaer
Department of Physics and Astronomy, Aarhus University

Despite the great potential that has been reported for core-shell metal-metal nanoparticles in catalytic processes and biomedical applications, similar reports on graphene-coated nanoparticles are lacking. Graphene coatings prevent nanoparticles from sintering, increase the temperature stability, and can prevent corrosive species from blocking the catalytically active sites. For some single crystal systems, graphene coatings have been shown to have beneficial effects, improving reaction efficiency. 
I will present measurements on graphene-coated Platinum nanoparticles. Pt nanoparticles of tens of nm in diameter were grown on a HOPG support by MBE. Subsequent annealing in ethylene yields a graphene coating layer on the nanoparticles that is tested on its catalytic and reactive properties. I will show scanning tunneling microscopy data, describing the shape and size of the nanoparticles and their graphene coating layer, XPS data to compare the chemical properties before and after coating, and TPD data proving the protective properties of the coating against exposure to CO.

Martha Scheffler is a postdoctoral member of Liv Hornekaer’s group at Aarhus University. Her current project is the coating of metal nanoparticles (used e.g. for photonics or catalysis) with graphene. By studying the effects of graphene functionalization on the coated nanoparticles with spectroscopic and microscopic methods, she hopes to enhance the plasmonic properties. She graduated with a diploma in physics (MSc) from Dresden Technical University in 2011. Subsequently, she started her PhD studies in the group of Christian Hess/Bernd Buechner at the Institute for Solid State and Materials Research Dresden (IFW Dresden). After obtaining a PhD in physics (Dr.rer.nat), she shortly joined the group of Martin Knupfer as a postdoctoral researcher (IFW Dresden).

Alireza Taghizadeh: Anomalous excitonic signature in nonlinear spin Hall current of monolayer TMDs

posted 27 May 2019, 13:17 by info admin   [ updated 28 May 2019, 04:47 ]

Ålborg University and Technical University of Denmark

The spin Hall effect (SHE) is a physical phenomenon, in which moving spin-up/-down charges are separated and accumulate at opposite boundaries of a surface [1]. The SHE is an extraordinary tool for studying fundamental physics as well as for spintronics, where the spin degree of freedom is manipulated. Monolayer TMDs are intriguing candidates for investigating the SHE, due to their large spin-orbit coupling and long spin relaxation time [2].  
Recently, we theoretically predict the existence of a dc SHE in ordinary monolayer TMDs (1H phase) that emerges from a nonlinear optical process, in which a strong time-dependent field induces a spin current at zero frequency via optical rectification. Two distinct mechanisms contribute to this photo-induced spin current, a purely interband part, and a mixed inter-/intraband contribution. Analogous to the linear optical absorption, excitons modify the SHE spectrum significantly by introducing strong discrete resonances. Remarkably, the direction of the excitonic spin current is inverted by varying the temperature, as the relative strengths of inter- and intraband contributions change.  We provide numerical data for MoS2 and WSe2, but the main findings hold true for other TMDs. Our results pave the way for generating spin currents in ordinary TMDs without the need for doping or external fields. 

[1] M. Dyakonov and V. Perel, Phys. Lett. A 35, 459 (1971).
[2] X. Xu, W. Yao, D. Xiao, and T. F. Heinz, Nat. Phys. 10, 343 (2014).

Alireza Taghizadeh is a postdoc researcher at Center for Nanostructures Graphene (CNG) in Aalborg University (AAU) and Technical University of Denmark (DTU). His main research interests lie in the fields of nanophotonics and physics of low-dimensional systems such as 2D materials and carbon nanotubes. He has a multidisciplinary background in electronics, optics, and physics. He graduated with honor (ranked 1st) in two B.Sc. programs of physics and electrical engineering in 2009. Having accomplished his master thesis in microelectronics, he graduated excellently (ranked 1st) in 2011. He received his Ph.D. degree in 2016 from DTU, where he exclusively studied various nonphotonic devices such nanolasers for optical interconnect applications. In 2015, he had a research stay in U.C. Berkeley, USA, working on a project about optical antenna-enhanced nanoLEDs. After Ph.D. graduation, he continues his researches at DTU as a postdoc for one year, working on novel electromagnetic phenomena in nanoscale systems. Since the beginning of 2017, he is actively doing research in the field of 2D materials, particularly their excitonic effects and nonlinearities. This includes a post-doctoral fellowship at the QUSCOPE center of excellence in AAU, which is followed by another postdoc at the CNG center in DTU and AAU. He is the author of 16 journal articles and 2 international patents. 

Avik Ghosh: Electronic switching using Tunable Dirac fermion optics

posted 27 May 2019, 12:14 by info admin   [ updated 12 Aug 2019, 02:41 ]

Charles Brown Dept of Electrical and Computing Engineering and the Dept of Physics at the University of Virginia

The unconventional flow of electrons in 2D Dirac cone systems provides unique opportunities to realize nontrivial optical analogues of their electronic counterparts. Using quantum simulations of electron flow as well as careful junction fabrication [1] and transport experiments from our collaborators - we can now demonstrate the ability to steer electrons using the negative index Veselago effect [2], collimate them at junctions using Klein tunneling [3] and rotate their transmission lobes to demonstrate the analogue of Malus' law for polarizer-analyzers [4]. Other examples such as antiKlein tunneling and electronic Brewster angles still remain to be seen experimentally. In all these examples, as well as analogous transport studies of Neel skyrmions along racetracks, the key underlying physics is driven by the conservation of topological charge carried by the spins and pseudospins. These symmetry effects give us additional degrees of tunability that are quite unconventional for electronic switching. For instance, the ability to collimate electron flow can be used to engineer a gate-tunable transport gap in bulk graphene [6], which is necessary to beat the fundamental Boltzmann limit on electronic switching. Such a gap also helps us tune the junction resistance over 3 orders of magnitude, pushing it well beyond typical contact resistances, making the output current saturate and giving us a high RF f_max power gain [6]. A PN junction on a 3D topological insulator can help polarize the transmitted spins and control the intrinsic charge-to-spin current ratio through spin-momentum locking [7].

[1] ACS Nano article ASAP 2019.
[2] Science, vol. 353 :6307 , pp. 1522-1525, 2016
[3] PNAS 2019, in press
[4] Physical Review B, vol. 86 , pp. 155412, 2012
[5] ACS Nano, vol. 7 :11 , pp. 9808-9813, 2013
[6] Nature Scientific Reports 7, 9714, 2017.
[7] Physical Review Letters, vol. 114 , pp. 176801, 2015

Avik Ghosh is Professor at the Charles Brown Dept of Electrical and Computing Engineering and the Dept of Physics at the University of Virginia. He did his PhD in condensed matter theory at the Ohio State University, and a postdoctoral fellowship in Electrical Engineering at Purdue University. He is the UVA site-director of the NSF-Industry University Cooperative Center on Multifunctional Integrated Systems Technology (MIST). Ghosh has authored 125+ refereed papers and a book (“Nanoelectronics – a Molecular View”, World Scientific 2016) in the area of computational nano-materials and devices. He has given over 125 invited lectures worldwide. He is Fellow of the Institute of Physics (IOP), senior member of the IEEE, and has received the IBM Faculty Award, the NSF CAREER Award, a 2006 best paper award from the Army Research Office, and UVA’s All University Teaching Award. His group’s work with Columbia University on negative refractive index behavior in graphene was voted by the editors of Physics World as one of the top 10 research breakthroughs of 2016.

Viktoria Ritter: Silicene passivation by few-layer graphene

posted 27 May 2019, 12:03 by info admin   [ updated 27 May 2019, 13:13 ]

Viktoria Ritter1, Jakob Genser1, Daniele Nazzari1, Ole Bethge2, Emmerich Bertagnolli1, and Alois Lugstein1
1 Institute of Solid State Electronics, Technische Universität Wien, Gußhausstraße 25-25a, 1040 Vienna, Austria
2 Infineon Technologies Austria AG, Siemensstraße 2, 9500 Villach, Austria
Contact: viktoria.ritter@tuwien.ac.at

Silicene is of foremost interest as it combines an ultra-high carrier mobility with the unique opportunity to tune the bandgap. Due to its intrinsic instability, the synthesis of silicene requires ultra-high vacuum (UHV) conditions and it oxidizes within minutes when exposed to ambient conditions.
Here, we demonstrate the encapsulation of 4x4 silicene grown on Ag(111) by few-layers graphene (FLG) flakes, allowing its stabilization for up to 48h. Natural graphite is exfoliated mechanically on top of a vacuum-compatible polyimide adhesive tape and inserted into a specially designed UHV chamber. After Si evaporation, the formation of silicene is verified via LEED analysis. Consecutively, FLG flakes are mechanically transferred atop of the silicene layer. A Raman analysis of the encapsulated silicene is performed under ambient conditions. The acquired data are reported in Figure 1. It shows the well-known peaks for exfoliated FLG, labelled, respectively, as D, G, G* and 2D, alongside two additional peaks located at 216 cm-1 and 515 cm-1, labelled as A and E. The intensity of the A mode, which is related to the out-of-plane vibration of optical phonons (ZO), was interpolated to obtain a heatmap showing the presence of silicene, as shown in Figure 2. Notably, silicene can be detected under the whole capping layer, with the exclusion of those areas that are close to the FLG edges. This is probably caused by a non-ideal adhesion of the FLG to the silver substrate. Polarization-dependent measurements, shown in Figure 3, demonstrate that the symmetry properties of silicene are unaltered by the capping process, as a result of a weak interaction with the encapsulation layer.

[1] A. Molle, C. Grazianetti, L. Tao, D. Taneja, M. H. Alam, and D. Akinwande, Chem. Soc. Rev. 47, 6370 (2018).
[2] D. Solonenko, O. D. Gordan, G. Le Lay, H. Şahin, S. Cahangirov, D. R. T. Zahn, and P. Vogt, 2D Mater. 4, 015008, (2016).

Viktoria Ritter, M.Sc. B.Sc. received her Master's degree in Geomaterials and Geochemistry from the Technical University Munich as well as from the Ludwigs-Maximilians University. After completing her thesis about Covalent Organic Frameworks analysis via Scanning Tunelling Microscopy she joined the silicene research group at the Vienna University of Technology. Currently she is pursuing her PhD in electrical engineering at the Institute for Solid State Electronics focusing on the passivation of silicene by further 2D-materials.

Lene Gammelgaard: Lithographic band structure engineering of graphene

posted 27 May 2019, 11:56 by info admin   [ updated 12 Aug 2019, 02:26 ]

Lene Gammelgaard, Bjarke Sørensen Jessen, Peter Bøggild,  Department of Physics, Technical University of Denmark, Kongens Lyngby, Denmark

Two-dimensional materials such as graphene allow direct access to the entirety of atoms constituting the crystal. While this makes lithographic shaping particularly attractive as a tool for band structure engineering through quantum confinement effects, edge disorder and contamination have so far limited progress towards experimental realization. We demonstrate here band structure engineering by direct, ultra-dense lithographic patterning of graphene [1]. Specifically, we have fabricated a triangular superlattice of etched holes with a period of 35 nm and a separation of 12-15 nm in a graphene sheet encapsulated in hexagonal boron nitride. We observe a distinct magnetotransport regime in the nanostructured graphene, with a nonlinear Landau level fan; in contrast to pristine graphene, and a band gap of 156 meV, which can be tuned with an external magnetic field. The rich magnetotransport measurements are accurately described by both tight-binding simulations and an analytical model based on Dirac fermions in ring geometries [2]. Furthermore, we observe strong indications that the lithographically engineered band structure at the main Dirac point is cloned to a satellite peak that appears due to moiré interactions between the graphene and the encapsulating hexagonal boron nitride. Band structure design in two-dimensional materials by top-down patterning enables the exploration of many exciting predictions and opportunities such as valleytronics [2] and spin qubits [3], as well as potential platform for “twistronic” circuits.

[1] B. S. Jessen, L. Gammelgaard, et al., Nat. Nano. 14, 340-346, (2019)
[2] M. R. Thomsen, et al., Phys. Rev. B, 95, 235427, (2017)
[3] T. G. Pedersen et al. Phys. Rev. Lett, 473,136804, (2008)

Lene Gammelgaard obtained her PhD in experimental physics at the Technical University of Denmark (DTU) in 2016 with the title “2D Material Device Architectures: Process Optimization and Characterization”. She is currently a postdoctoral researcher in the group of Prof. Peter Bøggild at DTU Physics. Her interests are physics and fabrication of nanodevices, including stacking of van der Waals heterostructures, cleanroom fabrication of devices by shaping and electrical contacting, and fabrication of dense nano-structures by electron beam lithography.

Maria Iliut: Graphene-enhanced elastomers and their applications

posted 27 May 2019, 08:22 by info admin

Grafine Ltd, University of Manchester, UK

Elastomers, particularly natural rubber, are widely used in applications such as tires, gloves, clothing, medical devices, etc. We demonstrate that the mechanical properties of elastomers can be improved by the incorporation of graphene fillers. Elastomers are compounded either an aqueous formulation called a latex, or as a solid. In order to maximise the impact on mechanical properties by the addition of a small volume fraction of graphene, it is essential to ensure uniform dispersion of the graphene flakes in the elastomer matrix. Upon optimisation, it is possible to improve an elastomer by making it simultaneously 50% stronger, 50% more elastic and 50% more hard wearing. 
A graphene-modified elastomer can then be formed into a number of shapes using, most popularly, dip-moulding of latex or compression moulding of solid rubber. I will discuss the fabrication and testing of dip-moulded elastomer-graphene composite products such as condoms and gloves, as well as compression moulded products such as shoe soles.  
I will present the specific case of athletic footwear, where the rubber-graphene composite gives markedly improved grip on tough terrains while simultaneously improving durability. The graphene-enhanced athletic shoes from Inov-8 Ltd. have been available for purchase, worldwide, since July 2018. I will also talk about the future potential of graphene-enhanced elastomers, and a new spin-out company Grafine Ltd. for this purpose. 

Dr. Maria Iliut is the founder and CTO of Grafine Ltd. and a post-doctoral research associate at the University of Manchester, in the Nano-functional Materials Group. Maria was awarded a PhD (2013), MSc (2010) and BSc (2008) from Babeș-Bolyai University, Romania. Her research involves the development of graphene-enhanced polymer composites and their applications. She has published over 20 papers in international peer-reviewed journals.  

Lapo Bogani: Topological effects in the quantum transport of molecular graphene nanoribbons

posted 27 May 2019, 07:59 by info admin

 Department of Materials, University of Oxford, Oxford, UK 

Fabricating devices that are nanometers long, yet defined down to the single-atom level, remains an enduring challenge in nanoscience. This degree of control is particularly appealing in graphene: graphene nanoribbons should produce field-effect-transistors with magnetic and topological effects solely when their edges are shaped with molecular precision.  Synthetic graphene nanoribbons now provide the necessary structural control, but their potential for quantum electronics remains unexplored. Here we report the observation of topological effects on the quantum transport of molecular graphene nanoribbons. The devices operate as ambi-polar transistors at room-temperature, and as single-electron transistors at low temperatures. Direct correspondence is established between the transport features and the morphology, with all features determined by the synthetic design. Molecularly-precise edges produce a magnetic field evolution that is completely opposite to non-molecular nanoribbons and the spin-orbit and coherent dephasing length are quantified, so that topology and time-reversal symmetry breaking become visible in a universality-class transition from symplectic to unitary. These results open the path to a new family of quantum electronic devices, where the graphene element can be atomically shaped, and synthetic chemistry allows the rational design of topological phenomena or applicative functionalities. 

Lapo Bogani obtained his PhD from the University of Florence, Italy. He moved to CNRS Grenoble with a personal Marie Curie fellowship, and then to Germany with a Sofja Kovalevskaja award of the Alexander von Humboldt Stiftung. In 2015 he moved to the University of Oxford with an ERC Starting Grant as Royal Society research fellow, and was appointed Professor in 2017. He is currently running an ERC Consolidator grant and he has been the recipient of a number of international awards, including the Kurti European research prize and the O. Kahn European award. His current research focuses on the transport and magnetic properties of molecular graphene nanostructures. 

Sergey Slizovskiy: Surface states of Bernal and rhombohedral graphite

posted 27 May 2019, 07:24 by info admin   [ updated 27 May 2019, 07:25 ]

National Graphene Institute, University of Manchester, M13 9PL, UK.

Graphite is semimetal allowing for unprecedented electrostatic control of its surface states via hBN encapsulation and gating. Graphite with Bernal (ABA) and rhombohedral (ABC) stacking have notably different electronic properties: Bernal graphite is a semimetal with small electron and hole Fermi surfaces and no surface states; Rhombohedral graphite is also a semimetal, but thin films thereof are insulating in the bulk and host 2D metallic surface states. We show that two types of surface states do appear at the surface of Bernal graphite upon electrostatic doping: (1) compressible surface states that form a 2D metal on the surface (red branch on the image) and (2) incompressible surface states that may appear above or below the Fermi level. The results are confirmed experimentally by quantum capacitance spectroscopy. For surface states in rhombohedral graphite films we calculate the phase diagram, predicting a pattern of gate-induced topological Lifshitz transitions between metallic surface states with different Fermi surface topology, we find a giant Berry curvature induced Hall coefficient and strong sensitivity of surface states to in-plane magnetic field. The theoretical results are confirmed by recent experiments carried in Manchester.

[1] J. Yin, S. Slizovskiy, Y. Cao, Sh. Hu, Y. Yang, I. Lobanova, B. Piot, S. Son,
S.Ozdemir, T. Taniguchi, K.Watanabe , K. Novoselov, F. Guinea, A. Geim , V.
Fal’ko , and A. Mishchenko, Nature Physics (2019), doi:10.1038/s41567-019-
0427-6 and a work in preparation

[2] S.Slizovskiy, E. McCann, M.Koshino, V.Fal’ko, in preparation

Sergey Slizovskiy is currently a postdoctoral member of theory group of Prof.
V.Fal'ko at National Graphene Institute, The University of Manchester, UK. He is interested in electronic properties of 2D materials, Quantum Hall Effect devices, topological insulators, edge states, Dirac and Weyl semimetals, topological Lifshitz transitions in interacting systems. He received his M.S. and PhD degrees in theoretical physics from Saint-Petersburg State University and a PhD in physics from Uppsala University, then spent 5 years at Loughborough University studying interacting electronic systems with strong spin fluctuations and graphene in magnetic field. His awards include EPSRC post-doctoral fellowship, PhD stipend of Russian noncommercial foundation Dynasty and RFBR (Russia) grants.

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