Exfoliation and characterization of MoS2 single layer microcrystals
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In this thesis, we have mechanically exfoliated single layer and few-layer Molybdenum disulfide (MoS2) flakes on Si/SiO2 substrates using scotch tape. The flakes were characterized using optical microscopy, Raman spectroscopy, photoluminescence spectroscopy and reflectivity spectroscopy at room temperature and 77 K. Raman spectroscopy was used to determine the number of layers of MoS2 flakes using the frequency difference between the two Raman modes E2g1 and A1g as an accurate indicator of layer number. We have studied the layer sensitivity of the two Raman modes and the photoluminescence intensity of one, two and three layer MoS2 flakes at room temperature. The enhancement of the Photoluminescence intensity for single layer MoS2 flake is observed and can be attributed to the transition from indirect to direct band gap for single layers. Photoluminescence and reflectivity spectra of single layer MoS2 flake were then studied. The photoluminescence and reflectivity work at 77 K were carried out in a micro-PL set up. For photoluminescence 488 nm laser excitation was used. Two direct excitonic peaks (A and B excitions) were observed in the photoluminescence and reflectivity spectra; these have been attributed to the splitting of the valence band at K-point in Brillouin zone due to spin-orbit coupling.