Robust ferromagnetism in Mn and Co doped 2D-MoS2 nanosheets: Dopant and phase segregation effects

Abstract

We investigate the structural and ferromagnetic properties of Mo1-xMnxS2 and Mo1-xCoxS2 (x = 0, 0.025, 0.05, 0.075, 0.1 and 0.25) nanosheets synthesized by a wet chemical process. X-ray diffraction (XRD) patterns confirm the 2H polytype of MoS2 for both pristine and doped nanosheets. From X-ray absorption near-edge spectral (XANES) studies, Mn doping in MoS2 is found to be very ineffective even for x up to 0.25 and is found to be in Mn4+ oxidation state. Cobalt doping in MoS2 is very effective and found in a mixed oxidation state of 2(+)/3(+). For x > 0.05, Co9S8 secondary phase is discerned in XRD. XANES and extended X-ray absorption fine structure study further confirm the absence of Co, Mn clusters. All the MoS2 nanosheets are multi-layered (similar to 10 to 20 layers) as discerned from Raman and high-resolution transmission electron microscopy (HRTEM) studies. HRTEM images also show the presence of planar and edge defects in MoS2 nanosheets. These defects arise from the bends, tears, folds and edges of the nanosheets. In addition to these defects, sulphur vacancy point defects are predominant as evidenced from electron paramagnetic resonance signal at g = 2.03. Pristine MoS2 nanosheets exhibit weak ferromagnetism [magnetization (MS) = 19 x 10(-3) emu/g and 1.3 x 10(-3) emu/g at T = 5 K and 300 K] mostly arising from the planar defects. While the magnetization increases roughly 6 (3) times higher for Mn-doped MoS2 compared to the pristine MoS2 nanosheets at 300 K (5 K). For Co-doped MoS2 nanosheets 7 times increase in MS is found at 300 K, however, the changes are minimal at 5 K. A small change in MS for Mo1-xMnxS2 nanosheets is due to ineffective Mn substitution at Mo sites. On the contrary, Co doped samples exhibit strong paramagnetic contribution at 5 K. Despite effective substitution of Co in MoS2, the saturation magnetization values obtained after subtracting the paramagnetic component are slightly less compared to Mn doped samples. This is most probably due to the dominant antiferromagnetic interaction and the Co9S8 phase segregation. Thus, our experiments show that controlled Mn and Co doping in the low concentration (x < 0.05) is a promising candidate for enabling ferromagnetism in MoS2 nanosheets for spintronics and sensor applications.