Surface interactions in MoS2: a study on the effects of contact on a 2D semiconductor

Abstract

This work investigates the influence of surface interactions in the properties of the 2D semiconductor MoS2 when integrated in heterostructures. The work is done experimentally, it qualitatively and quantitatively describes the interaction of thin exfoliated MoS2 under different contact conditions. In the first experiment, the MoS2 was placed in a gold substrate and contacted from the top by a metallic Atomic Force Microscope (AFM) tip. The current-voltage curve of the tip-MoS2-gold heterostructure was measured for different contact forces, an improvement on previously proposed circuit model is done by means of the new "3-diode" model. Through the proposed model we are able to measure the variation of the valence band energy in the MoS2 with respect to the tip's contact force - the rates of change are 0.21, 0.23, and 0.78 meV/nN for the few-layers, three-layers, and two-layers MoS2, respectively. In the second experiment, we placed the MoS2 on top of a gold nanoparticle (NP) supercrystal - a plasmonic structure with high light-matter interaction. We measure and analyse the optical behaviour of the heterostructure with a micro-absorbance setup, noting the excitons in the MoS2 are drastically affected by the evanescent dielectric screening of the supercrystal, with a blue-shift of a few hundred meV in the excitons' resonance energy. The plasmonic modes seem not to vary with the presence of the monolayer semiconductor, but by growing the number of MoS2 layers, the influence of the dipole moment from the excitons leads to a red shift in plasmonic resonance. This thesis also includes a step-by-step description of the supercrystal fabrication procedure developed by collaborators, we show how to grow gold nanoparticles, functionalize their surface and use them to grow supercrystals. This is done in an attempt to spread the knowledge and usage of this new material with record high light-matter interaction.