Introducing the 1H-Na 2 S monolayer as a new direct gap semiconductor with feature-rich electronic and magnetic properties.

In this work, a new direct gap semiconductor, the Na 2 S monolayer in the 1H-phase, with good stability and ionic character, has been explored using first-principles calculations. A Γ - Γ energy gap of 0.80 (1.48) eV is obtained using the standard PBE (hybrid HSE06) functional. The studied two-dimensional (2D) material possesses weak dynamical stability under compressive strain due to the sensitivity of the ZA mode. Meanwhile tensile strain has much more positive effects, where the stability is well retained up to a strain strength of 7%. Once external strain is applied, the band gap increases due to switching from lattice compression to lattice tension. Further exploration of defect engineering indicates that significant magnetism with magnetic moment of ±1 is induced by a single Na vacancy. The magnetic properties are mainly produced by S atoms around the defect site. In contrast, the paramagnetic nature is preserved with a single S vacancy. However, large energy gap reduction of up to 93.75% can be achieved with a defect concentration of 25%. This research introduces a new prospective 2D material similar to transition metal dichalcogenides for optoelectronic and spintronic applications, contributing to the continued efforts to develop novel multifunctional low-dimensional materials.