Since　the　discovery　of　graphene,　two-dimensional　(2D)　layered　nanomaterials　have　been　receiving　continuous　attention　owing　to　their　extraordinary　properties　and　promising　applications　in　nano　electronics.　However,　many　2D　nanomaterials　are　gapless　or　possess　a　small　band-gap　(＜=　2　eV),　which　greatly　restricts　their　applications.　Here,　by　means　of　ab　initio　calculations　and　molecular　dynamics　simulcations,　we　report　a　class　of　emerging　2D　semiconductors,　mono-　and　few-layer　arsenic　trichalcogenides　(As2S3　and　As2Se3),　with　a　broad　band-gap　range　from　2.06　eV　to　3.18　eV,　which　can　be　manipulated　by　the　number　of　layers　or　external　strains.　Interestingly,　under　moderate　tensile　strain,　the　nanolayers　undergo　a　transition　from　indirect　to　direct　band-gap　semiconductors.　More　importantly,　these　2D　semiconductors　exhibit　suitable　band-edge　alignment　and　desirable　optical　absorption,　suggesting　their　potential　applications　for　photocatalysis　and　optoelectronics.　Thanks　to　the　small　exfoliation　energies,　these　2D　layered　materials　could　be　fabricated　from　experiments　feasibly　and　serve　as　promising　candidates　in　constructing　van　der　Waals　heterostructures　for　future　nanoelectronics.　(C)　2017　Elsevier　B.V.　All　rights　reserved.