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3 results on '"Mohn, Michael Johannes"'

1. Inert Liquid Exfoliation and Langmuir-Type Thin Film Deposition of Semimetallic Metal Diborides

Author

Synnatschke, Kevin, Müller, Alina, Gabbett, Cian, Mohn, Michael Johannes, Kelly, Adam G., Mosina, Kseniia, Wu, Bing, Caffrey, Eoin, Cassidy, Oran, Backes, Claudia, Sofer, Zdenek, Kaiser, Ute, and Coleman, Jonathan N.

Abstract

Graphite is one of only a few layered materials that can be exfoliated into nanosheets with semimetallic properties, which limits the applications of nanosheet-based electrodes to material combinations compatible with the work function of graphene. It is therefore important to identify additional metallic or semimetallic two-dimensional (2D) nanomaterials that can be processed in solution for scalable fabrication of printed electronic devices. Metal diborides represent a family of layered non-van der Waals crystals with semimetallic properties for all nanosheet thicknesses. While previous reports show that the exfoliated nanomaterial is prone to oxidation, we demonstrate a readily accessible inert exfoliation process to produce quasi-2D nanoplatelets with intrinsic material properties. For this purpose, we demonstrate the exfoliation of three representative metal diborides (MgB2, CrB2, and ZrB2) under inert conditions. Nanomaterial is characterized using a combination of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, IR, and UV–vis measurements, with only minimal oxidation indicated postprocessing. By depositing the pristine metal diboride nanoplatelets as thin films using a Langmuir-type deposition technique, the ohmic behavior of the networks is validated. Furthermore, the material decomposition is studied by using a combination of electrical and optical measurements after controlled exposure to ambient conditions. Finally, we report an efficient, low-cost approach for sample encapsulation to protect the nanomaterials from oxidation. This is used to demonstrate low-gauge factor strain sensors, confirming metal diboride nanosheets as a suitable alternative to graphene for electrode materials in printed electronics.

Published
2024
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2. Energy-filtered TEM and low-loss EELS of 2D materials

Author

Mohn, Michael Johannes, Kaiser, Ute, Koch, Christoph T., European Union (EU), and Horizon 2020

Subjects
Low-dimensional semiconductors, DDC 530 / Physics, Plasmon, Graphen, Niederdimensionales System, Condensed Matter::Materials Science, Durchstrahlungselektronenmikroskopie, Heterostructures, ddc:530, Dielectrics, Dichtefunktionalformalismus, Graphene, Electron energy loss spectroscopy, Elektronen-Energieverlustspektroskopie, Heterostruktur, Dielektrische Eigenschaft, Transmission electron microscopy, Density functionals
Abstract

In this work, we perform aberration-corrected energy-filtered TEM (EFTEM) and low-loss electron energy-loss spectroscopy (EELS) with two-dimensional (2D) materials. In particular, the carbon K-edge EFTEM signal is analyzed for a monolayer of graphene, and the low-loss EELS spectra of graphene and molybdenum disulfide (MoS���) heterostructures are investigated by the technique of momentum-resolved EELS. With EFTEM in the Cc/Cs-corrected ���SALVE III��� low-voltage transmission electron microscope, we demonstrate lattice contrast in zero-loss, plasmon-loss, and C-K-edge images. Both bright-atom contrast and dark-atom contrast are observed in all three cases, and focus series show that even for the C-K edge, contrast inversions can be caused by a change in the defocus. For an analysis of the image contrast in direct space, all raw EFTEM data are averaged over a large number of unit cells to improve the signal-to-noise-ratio. In the low-loss regime of EELS (0���50 eV), our momentum-resolved experimental data for 2D heterostructures of graphene and MoS��� can be understood by a combination of ab initio simulations and dielectric model calculations. Therefore, the low-loss EELS signals of 2D multilayers and heterostructures are described on the basis of time-dependent density functional theory calculations for the constituent monolayers. Expanding on a layered-electron gas model for graphene, multilayers and heterostructures with MoS��� are eventually modeled by microscopic dielectric calculations that are in good agreement with our experimental EELS data. The necessity of models beyond the layered electron-gas model is demonstrated by calculations for multilayer and bulk MoS���.

Published
2020

3. Dielectric properties of graphene/MoS2 heterostructures from ab initio calculations and electron energy-loss experiments

Author

Mohn, Michael Johannes, Hambach, Ralf, Wachsmuth, Philipp, Giorgetti, Christine, Kaiser, Ute, European Union (EU), and Horizon 2020

Subjects
DDC 530 / Physics, Physics::Optics, 2-dimensional systems, Time-dependent DFT, Plasmons (Physics), Niederdimensionales System, Condensed Matter::Materials Science, Two-dimensional systems, Dielectric properties, Dichtefunktionaltheorie, Density functional theory, Heterostructures, ddc:530, Dichtefunktionalformalismus, Electron energy loss spectroscopy, Heterostruktur, Dielektrische Eigenschaft, Density functionals
Abstract

High-energy electronic excitations of graphene and MoS2 heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/MoS2/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack., acceptedVersion

Published
2018

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