New publication: Quantum transport through MoS2 constrictions defined by photodoping
Journal of Physics: Condensed Matter 30, 205001(2018) We present a device scheme to explore mesoscopic transport through molybdenum disulfide (MoS2) constrictions using photodoping. The devices are based on van-der-Waals heterostructures where few-layer MoS2 flakes are partially encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer graphene flake to fabricate electrical contacts. Since the as-fabricated devices are insulating at low temperatures, we use photo-induced remote doping in the hBN substrate to create free charge carriers in the MoS2 layer. On top of the device, we place additional metal structures, which define the shape of the constriction and act as shadow masks during photodoping of the underlying MoS2/hBN heterostructure. Low temperature two- and four-terminal transport measurements show evidence of quantum confinement effects.
Race report Bonn Marathon 2018
Our institute participated successfully in the "Deutsche Post Marathon 2018" in Bonn. Together with 13,000 other participants, over 200,000 spectators and best weather, Markus (1:35:29), Alex (1:52:06) and Luca (1:54:21) completed the Half Marathon course while Michael ran the full Marathon in 3:23:16. Congratulations!
The Söllerhaus-Workshop 2018 from the 28th to the 31st of March of the 2nd Institute of Physics A and B focusing on "2D materials" was great fun with many interesting talks and discussions and lots of good snow. For more information on the program see here.
New publication: Tailoring Mechanically Tunable Strain Fields in Graphene
Nano Letters 18, 1707 (2018) There are a number of theoretical proposals based on strain engineering of graphene and other two-dimensional materials, however purely mechanical control of strain fields in these systems has remained a major challenge. The two approaches mostly used so far either couple the electrical and mechanical properties of the system simultaneously or introduce some unwanted disturbances due to the substrate. Here, we report on silicon micromachined comb-drive actuators to controllably and reproducibly induce strain in a suspended graphene sheet in an entirely mechanical way. We use spatially resolved confocal Raman spectroscopy to quantify the induced strain, and we show that different strain fields can be obtained by engineering the clamping geometry, including tunable strain gradients of up to 1.4%/μm. Our approach also allows for multiple axis straining and is equally applicable to other two-dimensional materials, opening the door to investigating their mechanical and electromechanical properties. Our measurements also clearly identify defects at the edges of a graphene sheet as being weak spots responsible for its mechanical failure.
New publication: Mesoporous manganese phthalocyanine-based materials for electrochemical water oxidation via tailored templating
Catal. Sci. Technol. 8, 1517 (2018) Electrochemical water splitting using non-noble metals as catalysts is of increasing importance for the future energy sector. In particular, efficient catalysts for the demanding oxygen evolution reaction present a major challenge. As a contribution to this field, tailored mesoporous hard template materials based on manganese phthalocyanine were prepared. The preparation method proved to be crucial to achieve suitable physicochemical properties as a basis for high catalytic activity. The materials show overpotentials between 490 and 590 mV at 10 mA cm−2 This can be mainly attributed to efficient graphitization, high Mn dispersion and tailored oxidation states.
New book chapter publication: Characterization of Graphene by Confocal Raman Spectroscopy
Confocal Raman Microscopy, pp 177-184 (2018), Part of the Springer Series in Surface Sciences Confocal Raman spectroscopy has emerged as a key characterization technique in graphene research, as with this technique important material characteristics can be obtained locally and noninvasively. In this chapter, the fundamentals of the Raman spectrum of graphene are reviewed and the utilization of Raman spectroscopy for graphene characterization is demonstrated. In this regard, we show how crucial properties of graphene samples, i.e. doping , strain , defect-density and layer-number can be extracted from the Raman spectrum. Accessing these quantities is highly relevant for monitoring, understanding and improving graphene synthesis processes and device fabrication techniques for research and emerging industrial applications.
Sebastian Staacks, Heidrun Heinke & Christoph Stampfer have been nominated for the "Lehrpreis Physik 2018"
Sebastian Staacks, Heidrun Heinke and Christoph Stampfer have been nominated by the Fachschaft Physik for the "Lehrpreis Physik 2018" in the category of "Beste unterstützende Lehre" for their work on the physics app "phyphox".
New publication: Out-of-plane heat transfer in van der Waals stacks through electron–hyperbolic phonon coupling
Nature Nanotechnology 13, 41 (2018) Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities and superior electrical and optoelectronic properties. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements, we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism using distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.
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