Tl2S: a metal-shrouded two-dimensional semiconductor

Abstract

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.