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

TL;DR

This study investigates how thermal oxidation affects the second harmonic generation in mono- and few-layer MoS2, revealing layer-dependent structural changes and demonstrating non-invasive monitoring of oxidation progress using SH microscopy.

Contribution

It provides a detailed analysis of oxidation-induced modifications in nonlinear optical response of MoS2 and introduces SH microscopy as a tool for monitoring oxidation in 2D materials.

Findings

Oxidation significantly alters the SH response of MoS2.

Oxidation depth is limited to the top layer and depends on the number of layers.

SH microscopy can non-invasively track oxidation progress in 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.