Robust Ferromagnetism in Li-Intercalated and -Deintercalated MoS₂ Nanosheets: Implications for 2D Spintronics

Abstract

Li-intercalation and -deintercalation effects on the magnetic properties of defect-rich (DR) and defect-suppressed (DS) MoS2 nanosheets are investigated. Active defect sites take large Li (x ≈ 2 in LixMoS2) for DR-MoS2 compared to that for the DS-MoS2 (x ≈ 1.6). Li intercalation brings phase transition from 2H-MoS2 to 1T-LixMoS2 associated with a change in the spin state from spin (S) = 0 Mo4+ in trigonal prismatic geometry (TPG) to S = 3/2 Mo3+ in octahedral geometry (OG). Despite this phase transition, Mo5+ at the defect sites continues to stabilize in the TPG coordination with a change in the oxidation state to Mo4+, giving a net magnetic moment (m) of 0 μB. In contrast, Mo3+ in MoS2 yields m = 3 μB upon Li intercalation. These structural and chemical changes are consistent with the magnetic properties, with DR-MoS2-500 showing four times enhancement in saturation magnetization (MS) at 5 K (from 0.1 to 0.4 emu/g) upon Li intercalation. In contrast, DS-MoS2-900 exhibits low MS = 0.028 emu/g and shows one order increase in MS = 0.21 emu/g upon Li intercalation. Delithiation retains a significant fraction of Li at the defect sites. Distorted OG (DOG) with S = 1 Mo4+ still provides a ferromagnetic state for delithiated MoS2 nanosheets. This study reveals a favorable ferromagnetic ground state of the 1T phase, facilitating much larger magnetization in lithiated MoS2 nanosheets. Ferromagnetic contributions in pristine, lithiated, and delithiated MoS2 arise from S = 1/2 Mo5+ in TPG, S = 3/2 Mo3+ in OG, and S = 1 Mo4+ in DOG, respectively. In all of these cases, a bound magnetic polaron (BMP) is formed with spins from Mo-ions and trapped carriers present at the sulfur vacancies, and the extent of the BMP overlap seems to decide the magnetization values. The enhanced ferromagnetism demonstrated here paves the way for two-dimensional (2D) functional spintronics-based device applications.