Tl 2 S: a metal-shrouded two-dimensional semiconductor.

Recently, the family of metal-shrouded two-dimensional (2D) crystals has expanded rapidly, although most of these crystals are metallic. Using first-principles, we identify a semiconducting 2D metal-shrouded crystal, namely the Tl2S monolayer, which is found to be thermally and dynamically stable, and should be readily exfoliated experimentally. Most importantly, Tl2S monolayers exhibit compelling electronic and photovoltaic properties, i.e., modulable bandgaps of 1.89-2.31 eV, strong light absorption and ideal photo-electricity transduction properties. Additionally, the unique metal-shrouded structure, that has rarely been found in other 2D semiconductors, endows monolayer Tl2S with the potential to form excellent contacts with electrode materials in device applications. An intriguing large conduction band spin-valley coupling is also obtained in 2H-Tl2S due to the strong spin-orbit coupling and breaking of inversion symmetry. These exotic properties render Tl2S an excellent candidate for a wide range of applications in electronic and photovoltaic devices.