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Nearly free electron(NFE)states on smooth atomic surfaces in 2D materials are an important kind of electronic states.As states mainly distributed in free space,they can provide ideal transport channels without nuclear scattering.To use such ideal channels,NFE states should be partially occupied to become two-dimensional electron gas(2DES)states.Unfortunately,NFE states are typically unoccupied and electron doping is required to downshift them towards the Fermi level.It is thus very desirable to obtain nucleus-free(NF)2DES without relying on doping.Inspired by a recently identified electride,we suggest that Ca2N monolayer comes with intrinsic NF-2DES state,as confirmed by our first-principles calculations.NF-2DES state makes the material surface very active.We predict dissociative adsorption of ambient molecules,such as O2 and H2O,on Ca2N monolayer,accompanying a significant charge transfer from NF-2DES to adsorbates.Effective encapsulation is thus a requirement for any practical application of this material.Boron nitride monolayer is found to be too thin to protect NF-2DES state from oxygen adsorption.An effective encapsulation can be provided by graphane,where NF-2DES state remains intact between Ca2N and graphane layers regardless of molecule adsorption on graphane surface.Well encapsulated intrinsic NF-2DES state designed in this study is expected to open a new avenue in two-dimensional electronics.