Tailoring Magnetically Active Defect Sites in MoS2 Nanosheets for Spintronics Applications

We show that it is possible to tune the magnetic properties in MoS2 without any intentional magnetic dopants but by simply controlling the magnetically active defect sites. The layer 0.0 thickness and defect-density-controlled MoS2 (2H-phase) nano-sheets are obtained by annealing a wet-chemical precursor from 500 to 900 degrees C. High-resolution transmission electron microscopy images show the presence of planar and edge defects arising from abruptly terminating edges, bends, tears, and folding of nanosheets. In addition, Mo5+ and sulfur vacancy point defects are predominant, as evidenced from the electron paramagnetic resonance and X-ray photoelectron spectra. Nanosheets gradually become more crystalline and devoid of defects upon annealing, with MoS2 (900 degrees C) becoming >70% defect-free. The interlayer spacing decreases monotonically from similar to 6.351 to 6.231 angstrom with annealing; however, the lattice parameter a increases from 3.107 to 3.172 angstrom. These structural changes induce a large local strain (similar to 9%) in MoS2 (500 degrees C) nanosheets compared to the small strain (similar to 3%) present in MoS2 (900 degrees C). Raman spectroscopy further manifests control on the defect density, with the intensity ratio of the defect-induced LA(M) mode (similar to 227 cm(-1)) relative to the E-2g(1g) and A(1g) modes decreasing monotonically with the annealing temperature. We demonstrate that magnetically active defect sites can be tuned to enhance the ferromagnetism. Magnetization (at 5 K) systematically varies with 1 order higher M-s (=0.10 emu/g) found in MoS2 (500 degrees C) nanosheets compared to MoS2 (900 degrees C). Interestingly, temperature-dependent magnetization shows a ferromagnetic-like transition around 120 K, which becomes more pronounced in defect-rich MoS2. The ferromagnetic interaction is more likely due to a bound magnetic polaron made up of a spin 1/2 Mo5+ ion with trapped carriers present at the sulfur vacancies. These findings open a new way of exploring spintronic devices such as spin-field-effect switches, spin valves, magnetic sensors, and ultrathin high-density data storage devices using MoS2 2D nanosheets by controlling magnetically active defect sites.