Janus graphene: A two-dimensional half-auxetic carbon allotrope with a nonchemical Janus configuration

We propose a nonchemical intrinsic Janus configuration and identify it in a pure $s{p}^2$ hybridized carbon monolayer, termed Janus graphene. The spontaneous unilateral growth of carbon atoms drives the nonchemical Janus configuration in Janus graphene, which is totally different from the chemical effect in common Janus materials such as MoSSe. A structure-independent half-auxetic behavior is mapped in Janus graphene, that the structure maintains expansion whether stretched or compressed, which lies in the key role of the $p_z$ orbital. With the unique half-auxeticity that emerged in the nonchemical Janus configuration, Janus graphene enriches the functional carbon family as a promising candidate for micro/nanoelectronic device applications.