Structural Transformations in Few-Layer MnPSe₃ Stimulated by Thermal Annealing and Electron Irradiation

Transition-metal phosphorus trichalcogenides (TMPTs) are inorganic materials with exciting properties, such as inherent magnetism combined with the electronic band gap. Due to their layered structure, these materials can be exfoliated into ultrathin sheets, which show properties different from their bulk counterparts. In this work, we present an experimental study supported by first-principles calculations focused on tuning the properties of freestanding few-layer MnPSe₃ by local structural transformations stimulated by electron beam irradiation and thermal annealing under high vacuum conditions in a transmission electron microscope (TEM). In both cases, we observe the emergence of α- or γ-MnSe crystal structures. Using different TEM methods, we systematically investigate the irradiation-induced structural modifications. The results are rationalized with the help of ab initio calculations, which predict that the elastic knock-on threshold for removing selenium is significantly higher than that for phosphorus. Nevertheless, an increased sputtering rate of Se compared to P was detected by complementary spectroscopic experiments in this ternary compound, indicating that inelastic damage mechanisms play the dominant role within the low-voltage region. Moreover, the locally formed MnSe phases arise after the complete degradation of the host MnPSe₃ structure. First-principles calculations predict that the electronic and magnetic properties of 2D MnSe structures depend on the crystal types, facets, that is, orientation of the crystallographic planes of the parent bulk material, and crystallite thickness.