Helium tunneling through nitrogen-functionalized graphene pores: Pressure- and temperature-driven approaches to isotope separation

Recently, we showed that nitrogen-functionalized nanopores obtained by removing two rings from a perfect graphene sheet provide suitable barriers for a separation of fermionic helium-3 from its bosonic counterpart helium-4 [J. Phys. Chem. Lett.2012, 3, 209–213]. In this follow-up Article, we provide potential curves for helium passing through several different types of pores, discuss the relation of the barrier height to the effective pore size, give estimations for bound states of helium attached to the pores, and analyze the effects of isomeric and stoichiometric variations of the pore-rim nitrogen-passivation on the gas separation performance. Slight deviations in the tunneling probability for the two helium isotopes can lead to a high selectivity at an industrially acceptable gas flux if the gas temperature is kept sufficiently low. We also recently showed that the mass-dependence of quantum tunneling and zero-point energy differences at the top of the potential energy barrier allow for a classically prohibited steady-state thermally driven isotope separation [Chem. Phys. Lett.2012, 521, 118–124]. The nitrogen-passivated nanopores studied here give rise to larger steady-state isotopic enrichment than that in previous work and are dominated by zero-point energy differences at both high and low temperatures.