  Designing an efficient membrane for He purification is quite crucial in
scientific and industrial applications. Ultrathin membranes with intrinsic
pores are highly desirable for gas purification because of their controllable
aperture and homogeneous hole distribution. Based on the first-principles
density function theory and molecular dynamics simulations, we demonstrate that
the compressively strained graphitic carbon nitride (CN) can effectively purify
He from Ne and Ar. Under a -6% strain, the CN monolayer with a suitable pore
size presents an easily surmountable barrier for He (0.11 eV) but formidable
for Ne (0.51 eV) and Ar (2.45 eV) passing through the membrane, and it exhibits
exceptionally high selectivity of 5.17*10^6 for He/Ne and 1.89*10^39 for He/Ar,
as well as excellent He permeance of 1.94*10^7 GPU at room temperature,
superior to those of porous graphene and C2N membrane. Our results confirm that
strain-tuned CN membrane could be potentially utilized for He separating from
other noble gases.
