Surface Characterization of the Structural Defects in MoS2 Atomic Layers Formed by Lithium Intercalation

Two-dimensional molybdenum disulfide (2D MoS₂) has become a promising material towards the next-generation photovoltaic solar cells, optoelectronic circuits, and sensors due to its excitonic properties. Top-down synthesis methods -such as employed in the present study- are ideal for scaling up 2D material fabrication; however, these methods can introduce defects, phase changes, and, concomitantly, properties that are fundamentally different from their pristine analogs. In the case of the chemical exfoliation method, bulk MoS₂ undergoes a semiconducting to metallic phase change and loses sulfur to form vacancies. Characterization of these changes is necessary for the appropriate application of MoS₂ in electronic and optical devices. We exfoliate MoS₂ monolayers by intercalating bulk MoS₂ powder with lithium ions. As previously reported, the lithium ions: 1) increase interlayer spacing; 2) react with the MoS₂ to form LiS₂; and 3) donate charge to the MoS₂. These actions respectively result in: 1) weakened Van der Waals interactions such that the MoS₂ monolayers can be exfoliated via sonication; 2) sulfur vacancies, and 3) a semiconducting to metallic phase transformation. In this study, we map the electronic properties of 2D MoS₂ with Raman spectroscopy and conductive atomic force microscopy (C-AFM) to analyze local phase changes resulting from the chemical exfoliation described above. This mapping will provide a detailed understanding of transport phenomena and electronic properties to be applied in the controlled design of next-generation devices.