Monolayer ThSi2N4: An indirect-gap semiconductor with ultra-high carrier mobility

The recently synthesized ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ monolayer [Science 369, 670 (2020)] exhibit outstanding environmental stability, moderate band gap, and excellent mechanical properties, which opens up a new avenue for the explorations of two-dimensional (2D) ${\mathrm{MA}}_{2}{\mathrm{Z}}_{4}$ materials. Inspired by this finding, we perform comprehensive structural predictions of ${\mathrm{MSi}}_{2}{\mathrm{N}}_{4}$ monolayers with lanthanide and actinide metals at the M site. Using the CALYPSO structural search method and first-principles calculations, we identify seven ${\mathrm{MSi}}_{2}{\mathrm{N}}_{4}$ monolayers with robust ambient stabilities, four of which are metals (M = Tm, Lu, Pa, Np) and three are semiconductors (M = Ce, Th, U). Of particular interest is the ${\mathrm{ThSi}}_{2}{\mathrm{N}}_{4}$ monolayer, which is an indirect-gap semiconductor with ultra-high electron mobility of 14384 ${\mathrm{cm}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Our results enrich the 2D ${\mathrm{MA}}_{2}{\mathrm{Z}}_{4}$ family and offer insights into the design and synthesis of novel multifunctional materials.