  The asymmetric properties of Janus two-dimensional materials commonly depend
on chemical effects, such as different atoms, elements, material types, etc.
Herein, based on carbon gene recombination strategy, we identify an intrinsic
non-chemical Janus configuration in a novel purely sp$^2$ hybridized carbon
monolayer, named as Janus-graphene. With the carbon gene of tetragonal,
hexagonal, and octagonal rings, the spontaneous unilateral growth of carbon
atoms drives the non-chemical 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 $p_z$ orbital. The unprecedented
half-auxeticity in Janus-graphene extends intrinsic auxeticity into pure sp$^2$
hybrid carbon configurations. With the unique half-auxeticity emerged in the
non-chemical Janus configuration, Janus-graphene enriches the functional carbon
family as a promising candidate for micro/nanoelectronic device applications.
