Influence of Lithium Intercalation on Magnetic Properties of MoS2 Nanosheets

Abstract

MoS2 is a member of a large class of 2-dimensional transition metal dichalcogenides (TMDs) with general formula MX2 (M-transition metal, X-chalcogen atom). In these 2D materials X-M-X monolayer comprises of M-layer sandwiched between two X-layers. Monolayers are held by weak van der Waals interaction in bulk compounds. Among various TMDs reported MoS2 has received great attention due to its exclusive properties like large spin-orbit coupling, layer dependent bandgap (1.2 eV to 1.8 eV) and high on/off current ratio making it a promising candidate for spintronics devices.1 Bulk MoS2 is diamagnetic in nature, hence it is necessary to tune its ferromagnetic properties to make it a potential candidate for spintronics applications. Several experiments like transition metal doping, inducing defects in MoS2 and structural phase transition have been shown to influence the ferromagnetic properties.MoS2 exists in different polymorphs, namely 1T, 2H and 3R. Mo in 2H and 3R phase resides in trigonal prismatic geometry. In this coordination electrons in d-orbital pair up oppositely resulting in a net zero magnetic moment. Whereas, in 1T phase Mo residing in octahedral geometry stabilizes in spin-polarized state giving net magnetic moment of 2μB per MoS2. This provides a pathway for enabling MoS2 ferromagnetic through structural phase transition (i.e. 3R/2H to 1T). Intercalating ions within the interlayer sites between adjacent MoS2 layers which takes place with an associated charge transfer to molybdenum enable such structural phase transition. This process leads to the trigonal prismatic (2H/3R) coordination of molybdenum unstable. Hence the system stabilizes in octahedral geometry (1T) via intralayer atomic plane gliding through transverse S-layer displacement. In this work, we utilize MoS2 nanosheets synthesized from wet chemical method to investigate the magnetic properties of Li+ intercalated MoS2 nanosheets. MoS2 nanosheets with different degrees of disorder can be obtained by controlling the annealing temperature.2 Planar and edge defect-rich MoS2 (MS-DR) are obtained upon annealing at 500 °C and defect-suppressed ordered layers of MoS2 nanosheets (MS-DS) are obtained upon annealing at 900 °C for 1 h. The former shows larger magnetization (0.10 emu/g) compared to the latter (0.03 emu/g). After lithium intercalation these MoS2 nanosheets show enhanced magnetization, 0.37 emu/g (for MS-DR) and 0.22 emu/g (MS-DS). Since the deintercalation process retains Li within the nanosheets based on the extent of defects present in MoS2, these nanosheets retain ferromagnetism even after deintercalation process. This presentation will discuss in detail the ferromagnetic properties of MoS2 nanosheets upon Li intercalation/deintercalation process.