Two-dimensional materials(including graphene,MoS2,etc.)represent a promising direction for electronic and photonic devices,benefting from their unique properties such as extremely high mobility and ultrathin body.In this talk I will present our works on electronic devices based on 2D atomic and molecular semiconducting crystals.In the frst part,I will talk about our recent efforts to understand the charge transport in MoS2,and to improve the performance of MoS2 transistors towards its intrinsic limit.We provide direct evidence that sulfur vacancies exist in the material,introducing localized midgap donor states.At low carrier density,the charge transport in MoS2 is by electron hopping through these localized states.Our results suggest that the current MoS2 device performance is still limited by intrinsic defects and charge impurities.Using a thiol chemistry,we are able to repair the sulfur vacancies and signifcantly reduce the charged impurities and traps.As a result,high mobility greater than 80cm2 V-1 s-1 is achieved in backgated monolayer MoS2 feld-effect transistors at room temperature.In the second part,I will present our recent work on 2D molecular crystals.We demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy,with precisely controlled thickness down to monolayer,large-area single crystal,low process temperature and patterning capability.Monolayer dioctylbenzothienoben-zothiophene molecular crystal feld-effect transistors on boron nitride show record-high carrier mobility up to 10cm2V-1s-1 and aggressively scaled saturation voltage around 1V,comparable to that of metal dichalcogenides.Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.