Second harmonic study of thermally oxidized mono- and few-layer 2H-MoS2

Abstract

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

Figures (8)

• Figure 1: Crystal structure of 2H-MoS2 (top) top view with indications of the armchair and zigzag directions and corresponding orientation of the nonlinear tensor elements $d_{yyy}$ and $d_{yxx}$, (bottom) side view 2H stacking.
• Figure 2: Optical images of MoS2 flakes on SiO2/Si-substrate before (left) and after (right) 6h-oxidation at 300℃. Here, the regions for the 1L and 3L system are labeled. Structural modifications due to oxidation become visible in a slight change of contrast. Especially at the edges of the regions of different thicknesses a significant modification is observed.
• Figure 3: Absolute Raman shift difference progression with layer number for MoS2 on SiO2/Si-substrate. The shift difference is taken between the phonon modes E$^{1}_{2g}(\Gamma)$ and A$_{1g}(\Gamma)$, which undergo a red- (blue-) shift with increasing layer number, respectively.
• Figure 4: Absolute SH-signal for untreated and oxidized 2H-MoS2 samples with respect to the layer number. Changes due to oxidation are indicates via arrows. Measurements are performed @ 2mW laser power and 1s integration time.
• Figure 5: DFT band structure of 1L -3L for pristine MoS2 and the influence of full exchange of the top S layer with O atoms (oxidized). With a coupling constant of $\lambda = 75$ meV zhu2011nguyen2018, the effect of SOC is small in comparison with the changes introduced by oxidization and thus can be safely neglected here.