Origin of exotic ferromagnetic behavior in exfoliated layered transition metal dichalcogenides MoS2 and WS2.

Jan Luxa,Martin Pumera,Ondřej Jankovský,Zdeněk Sofer,Miroslav Maryško,David Sedmidubský,Rostislav Medlín

doi:10.1039/c5nr05757d

Abstract

Bulk layered transition metal dichalcogenides (TMDs) show diamagnetic properties. When exfoliated, the materials' band gap increases and changes from an indirect band gap to a direct one. During the exfoliation, the TMDs may undergo a phase transition from 2H to 1T polymorph, which is likely electronically driven and accompanied by a metal-insulator transition. A significantly higher efficiency of the exfoliation was observed using sodium naphthalenide compared to butyllithium. Moreover we demonstrate that the exfoliation has a dramatic influence on the magnetic properties of two TMDs, MoS2 and WS2. These materials become partly ferromagnetic upon exfoliation, which is a highly unexpected behavior. Exotic ferromagnetism is generally observed on samples with a high degree of exfoliation, which indicates the association of this effect with defects formed on the edges of dichalcogenide sheets. Such an exotic ferromagnetic behavior, if properly understood and brought under material engineering control, shall open the door to new applications of these materials.

Highlights

Layered transition metal dichalcogenides (TMDs) have received revived interest in the past few years being dubbed as the “ graphene” due to their possible applications in energy generation and their highly anisotropic material properties.[1,2,3,4] In contrast to graphene, where the layer is single atom thick and truly 2-dimensional, MoS2 sheets consist of transition metals sandwiched between two layers of chalcogen atoms
While butyllithium is commonly used for the chemical exfoliation of transition metal dichalcogenides (TMDs),[20,21] only little has been reported on TMD exfoliation using sodium naphthalenide.[22]
Before the discussion of exotic magnetic properties of exfoliated TMDs we thoroughly examined the exfoliated TMDs by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), selective area electron diffraction (SAED) and surface area measurement by methylene blue adsorption

Summary

Introduction

Layered transition metal dichalcogenides (TMDs) have received revived interest in the past few years being dubbed as the “ graphene” due to their possible applications in energy generation and their highly anisotropic material properties.[1,2,3,4] In contrast to graphene, where the layer is single atom thick and truly 2-dimensional, MoS2 (or WS2) sheets consist of transition metals sandwiched between two layers of chalcogen atoms.

Methods

Results

Conclusion