P. Patoka, G. Ulrich, A. E. Nguyen, L. Bartels, P. A. Dowben, V. Turkowski, T. S. Rahman, P. Hermann, B. Kästner, A. Hoehl, G. Ulm, and E. Rühl
Optics Express, Vol. 24, Issue 2, pp. 1154-1164 (2016)
Nanoscale plasmonic phenomena observed in single and bi-layers of molybdenum disulfide (MoS2) on silicon dioxide (SiO2) are reported. A scattering type scanning near-field optical microscope (s-SNOM) with a broadband synchrotron radiation (SR) infrared source was used. We also present complementary optical mapping using tunable CO2-laser radiation. Specifically, there is a correlation of the topography of well-defined MoS2 islands grown by chemical vapor deposition, as determined by atomic force microscopy, with the infrared (IR) signature of MoS2. The influence of MoS2 islands on the SiO2 phonon resonance is discussed. The results reveal the plasmonic character of the MoS2 structures and their interaction with the SiO2 phonons leading to an enhancement of the hybridized surface plasmon-phonon mode. A theoretical analysis shows that, in the case of monolayer islands, the coupling of the MoS2 optical plasmon mode to the SiO2 surface phonons does not affect the infrared spectrum significantly. For two-layer MoS2, the coupling of the extra inter-plane acoustic plasmon mode with the SiO2 surface transverse phonon leads to a remarkable increase of the surface phonon peak at 794 cm−1. This is in agreement with the experimental data. These results show the capability of the s-SNOM technique to study local multiple excitations in complex non-homogeneous structures.