Abstract
Nanoscale plasmonic phenomena observed in single and bi-layers of molybdenum disulfide (MoS(2)) on silicon dioxide (SiO(2)) 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 CO(2)-laser radiation. Specifically, there is a correlation of the topography of well-defined MoS(2) islands grown by chemical vapor deposition, as determined by atomic force microscopy, with the infrared (IR) signature of MoS(2). The influence of MoS(2) islands on the SiO(2) phonon resonance is discussed. The results reveal the plasmonic character of the MoS(2) structures and their interaction with the SiO(2) 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 MoS(2) optical plasmon mode to the SiO(2) surface phonons does not affect the infrared spectrum significantly. For two-layer MoS(2), the coupling of the extra inter-plane acoustic plasmon mode with the SiO(2) 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.
Highlights
Among the 2D electronic materials that have received increased attention recently are the transition metal dichalcogenides
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
This result slightly differs from the expected theoretical thickness of 0.615 nm [53], which is commonly ascribed to a combination of different hygroscopicity/water accumulation on the SiO2 substrate and the MoS2 film as well as the atomic force microscope (AFM) tapping mode, which has been reported to produce lower accuracy than in contact mode measurements, which has been derived from the characterization of graphene samples [54]
Summary
Among the 2D electronic materials that have received increased attention recently are the transition metal dichalcogenides. Recent experiments by Fourier-transform infrared (FTIR) nanoscopy and nanoimaging on exfoliated graphene demonstrated that confining IR radiation to a nanosized scattering object results in an increased coupling to the in-plane momentum component and enables the optical excitation of plasmons [36,37,38,39]. These experiments used the near-field based technique namely scattering-type scanning near-field optical microscopy (s-SNOM) [40,41,42,43]. This leads to a significant increase in intensity in the s-SNOM spectrum, as shown here
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