Anregungen in graphenbasierten Materialien: Magnetooptische Untersuchungen im THz- und subTHz-Bereich
Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Zusammenfassung der Projektergebnisse
We perform extensive comparative studies of natural and manmade graphene based samples. In case of natural graphite, we performed detailed studies on natural graphite samples from different origins revealing quantum oscillations above 8 T as intrinsic origin of natural graphite. It turned out to be a great material for studying the (ωτ = 1)-transition in carbon based materials, which is not fully understood yet. These investigations are still on going and will be published as well. A deeper theoretical understanding of the quantum oscillations beyond the DC-limit will be developed (maybe it is already existing and we would just apply it). In case of manmade graphene based materials we perform very detailed studies on large area, especially on single layer hydrogen intercalated graphene. These measurements were investigated simultaneously by two methods: Contacted using a large area Hall bar and contactless microwave absorption from the bar. Shubnikov-de Haas oscillations are exhibited by both methods up to 53 K. The sheet density determined by both methods were within experimental error, showed minimal temperature dependence, were consistent with room temperature Hall measurements of the sample before bar fabrication and are consistent with electronic transport measurements by others using the Hall effect. Further analysis of the SdH oscillations produced by both methods yielded a Berrys phase of 0.45 (modulo 2π), consistent with monolayer graphene. The mobility found by microwave absorption measurements was about 2-fold less than the mobility found in Hall measurements. The difference is most likely due to sample inhomogeneity, where the Hall measurements tend to overestimate the conductance along favorable conduction channels in contrast to the microwave absorption measurements which obtain contributions from all regions of the sample. This work demonstrates microwave absorption as a valuable characterization technique with the potential for use in the production environment for graphene, or similar materials. This is due to the contactless nature of the method, which greatly simplifies the measurement process and the critical characterization data produced. We further used the technique in studies of different samples like hydrogen intercalated bilayer graphene, buffer layer graphene, chemically modified graphene samples etc. These investigations are still on going and will be published as well. Many of the observed spectral features are still not fully understand and additional measurements and calculations are planned to prove or disprove our hypotheses. Furthermore, significant steps toward real application of graphene based bolometers was made. We tested graphene bolometers in spectroscopy of molecular magnets (Figure 8). The performance is very promising and opens new ways how to perform very sensitive measurements in strong magnetic at tiny amount of samples. In overall conclusion, the beauty of THz spectroscopy is in very high sensitivity to even small changes during preparation of graphene based materials, which is on one hand very great feature but on the other hand put lot of demand on investigation of the origin. For these reasons we worked in very close collaboration whit people producing graphene in order to optimize the fabrication processes and understand the origin. The results shown here are only highlights of the project, the work is still ongoing and other papers are foreseen.
Projektbezogene Publikationen (Auswahl)
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Magnetic and HFEPR Studies of Exchange Coupling in a Series of it mu-Cl Dicobalt Complexes, Inorg. Chem., 56, 2417−2425 (2017)
J.-J. Liu, S.-D. Jiang, P. Neugebauer, J. van Slageren, Y. Lan, W. Wernsdorfer, B.-W. Wang, S. Gao
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Multi-frequency rapid-scan HFEPR, J. Magn. Reson., 296, 138–142 (2018)
Laguta, O.; Tucek, M.; van Slageren, J., P. Neugebauer
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Ultra-broadband EPR spectroscopy in field and frequency domains, Phys. Chem. Chem. Phys., 20, 15528 – 15534 (2018)
P. Neugebauer, D. Bloos, R. Marx, P. Lutz, M. Kern, D. Aguilà, J. Vaverka, O. Laguta, C. Dietrich, R. Clérac, J. van Slageren
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PhD thesis, IPC Univeristy of Stuttgart, 2019. Magneto-optische Untersuchungen der elektrischen Transporteigenschaften von Graphen und Graphit durch sub-Terahertz-Spektroskopie
Dominik Bloos
