Photolumineszenz- und Raman-Spektroskopie von zweidimensionalem MoS2

In this work the electronic and vibrational properties of two-dimensional molybdenum disulfide (MoS2) are examined using Ramanand photoluminescencespectroscopy (PL). Spatially resolved measurements of suspended MoS2 and Si/SiO2 supported MoS2 show an increase of the charge carrier concentration of the MoS2 in the supported areas. Furthermore the indirect band gap energy of bilayer MoS2 is decreased by 80meV by the substrate. Besides the spatially resolved PL measurements we performed temperature depended PL measurements of singleand bilayer MoS2. These measurements indicate that the additional PL peak in the spectra of single-layer MoS2 which occurs at low temperatures relates to the indirect band gap instead of bound excitons. Furthermore the Raman resonance profiles (RRP) of singleand fewlayer MoS2 were measured. These RRPs show that fewlayer MoS2 behaves similar to multiple separate single layers in terms of the Raman scattering selection rules for excitation energies below 2.4 eV. We explain this effect by considering the effect of the spatial extent of the exciton wave functions onto the Raman scattering selection rules. Besides the experimental section we provide a method based on group theory which allows the systematic classification of all phonon modes in few-layer crystals with inversion symmetry and/or horizontal reflection symmetry outgoing from the phonon modes of the corresponding single-layer crystal. Each phonon mode of the single-layer leads to multiple, in terms of energy nearly degenerated phonons of the few-layer systems whose total number equals the number of layers of the few-layer. We show that these phonons can always be divided into two groups, showing opposite transformation properties of the displacement pattern under inversion and/or horizontal mirroring. These results allow to understand the observation of additional Raman modes in the spectra of few-layer MoS2 and other two-dimensional materials, which are not present for the bulk or single-layer. Besides the phonons of few-layer MoS2, the phonons of few-layer graphene and black phosphorus are systematically classified to demonstrate the method. Liste der Veröffentlichungen 1. Interlayer resonant Raman modes in few-layer MoS2. N. Scheuschner, R. Gillen, M. Staiger und J. Maultzsch Physical Review B 91 235409 (2015) 2. Photoluminescence of freestanding singleand few-layer MoS2. N. Scheuschner, O. Ochedowski, A. -M. Kaulitz, R. Gillen, M. Schleberger und J. Maultzsch Physical Review B 89 125406 (2014) 3. Resonant Raman profiles and μ-photoluminescence of atomically thin layers of molybdenum disulfide. N. Scheuschner, O. Ochedowski, M. Schleberger und J. Maultzsch Physica Status Solidi (B) 249 2644-2647 (2012) 4. Splitting of monolayer out-of-plane A1 Raman mode in few-layer WS2. M. Staiger, R. Gillen, N. Scheuschner, O. Ochedowsk, F. Kampmann, M. Schleberger, C. Thomsen und J. Maultzsch Physical Review B 91 195419 (2015) 5. Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets. C. Backes, R. J. Smith, N. McEvoy, N. C. Berner, D. McCloskey, H. C. Nerl, A. O’Neill, P. J. King, T. Higgins, D. Hanlon, N. Scheuschner, J. Maultzsch, L. Houben, G. S. Duesberg, J. F. Donegan, V. Nicolosi und J. N. Coleman Nature Communications 5 4576 (2014) 6. Effect of contaminations and surface preparation on the work function of single layer MoS2. O. Ochedowski, K. Marinov, N. Scheuschner, A. Poloczek, B. K. Bussmann, J. Maultzsch und M. Schleberger Beilstein Journal of Nanotechnology 5 291-297 (2014) 7. Electronic characterization of single-layer MoS2 sheets exfoliated on SrTiO3. B. K. Bussmann, K. Marinov, O. Ochedowski, N. Scheuschner, J. Maultzsch und M. Schleberger MRS Proceedings 1474 (2012) doi:10.1557/opl.2012. 1463 8. Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation. O. Ochedowski, K. Marinov, G. Wilbs, G. Keller, N. Scheuschner, D. Severin, M. Bender, J. Maultzsch, F. J. Tegude und M. Schleberger Journal of Applied Physics 113 214306 (2013) 9. Graphene on Si(111)7x7. O. Ochedowski, G. Begall, N. Scheuschner, M. El Kharrazi, J. Maultzsch und M. Schleberger Nanotechnology 23 405708 (2012) 10. Fundamental Insights into the Degradation and Stabilization of Thin Layer Black Phosphorus. G. Abellán, S. Wild, V. Lloret, N. Scheuschner, R. Gillen, U. Mundloch, J. Maultzsch, M. Varela, F. Hauke und A. Hirsch Journal of the American Chemical Society Just Accepted (2017) doi:10.1021/jacs.7b04971