The discovery of archetypal two-dimensional (2D) materials provides enormous opportunities in both fundamental breakthroughs and device applications, as evident by the research booming in graphene, atomically thin transition-metal chalcogenides, and few-layer black phosphorous in the past decade. Here, we report a new, large family of semiconducting dialkali-metal monochalcogenides (DMMCs) with an inherent A$_{2}$X monolayer structure, in which two alkali sub-monolayers form hexagonal close packing and sandwich the triangular chalcogen atomic plane. Such unique lattice structure leads to extraordinary physical properties, such as good dynamical and thermal stability, visible to near-infrared light energy gap, high electron mobility (e.g. $1.87times10^{4}$ cm$^{2}$V$^{-1}$S$^{-1}$ in K$_{2}$O). Most strikingly, DMMC monolayers (MLs) host extended van Hove singularities near the valence band (VB) edge, which can be readily accessed by moderate hole doping of $sim1.0times10^{13}$ cm$^{-2}$. Once the critical points are reached, DMMC MLs undergo spontaneous ferromagnetic transition when the top VBs become fully spin-polarized by strong exchange interactions. Such gate tunable magnetism in DMMC MLs are promising for exploring novel device concepts in spintronics, electronics and optoelectronics.
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