Strain induced structural changes and magnetic ordering in thin MoS2 flakes as a consequence of 1.5 MeV proton ion irradiation

Abstract

We report the effects of 1.5 MeV proton ion irradiation on thin MoS2 (Molybdenum disulfide) flakes. The XRD (X-ray diffraction) analysis of both the unirradiated and irradiated samples revealed a known hexagonal crystalline phase, with the peaks of the irradiated samples slightly shifting towards lower diffraction angle. The change in the peak position can be attributed to the defects created by the proton ion beam indicating strain in the relevant phases. All the irradiated MoS2 samples showed distinct and increasing room temperature ferromagnetism, as compared to the unirradiated MoS2 sample. The presence of any kind of impurities and transition metal element in the irradiated and unirradiated samples, has been ruled out by X-ray photoelectron spectroscopy analysis and it further confirms sulfur vacancies in the sample post irradiation. Raman spectroscopy analysis of the samples revealed a red shift in the fundamental vibrational modes of MoS2 indicating presence of tensile strain in the samples after irradiation. The observed ferromagnetic ordering has been attributed to locally aligned magnetic dipole moments induced by sulfur vacancies and defects produced by tensile strain as a consequence of proton beam irradiation.