Structure and magnetic properties of curved graphene networks and the effects of bromine and potassium adsorption

The structure and magnetic properties of nanoporous zeolite-templated carbon (ZTC) and its guest-adsorbed systems are investigated using potassium and bromine as guest species. In ZTC, nanopores formed by a three-dimensional curved-graphene network, whose structure is transcribed from the zeolite nanopore structure, are regularly arranged (periodicity: 1.4 nm). The regularity of the nanopore structure is preserved during vacuum heat treatment (<380°C) and potassium adsorption, though it is destroyed by bromine adsorption. The pristine ZTC has localized spins, whose origin is attributed to those created in the region having a negative curvature in the graphene network, as well as to the contribution of an edge state. The spin concentration is estimated as 1 spin/nanopore and is independent of the heat-treatment temperature up to 380°C. In potassium-adsorbed ZTC, potassium atoms forming networked clusters in the nanopores are magnetic and undergo a weak ferromagnetic transition at 24 K. The ZTC spins, which are not directly coupled with the potassium spins through exchange interaction, accurately mirror the magnetic behavior of the potassium spins.