Three-dimensional periodic graphene nanostructures
Graphene is a basic building block for many known and emerging carbon-based materials whose properties largely depend on the way how graphene is structured at the nanoscale. Synthesis of novel three-dimensional (3D) nanostructures of graphene would be an important advance for fundamental research and various applications. In this paper, we discuss the possibility of fabricating complex 3D graphene nanostructures by growing graphene on pre-synthesized nanostructured metal templates by chemical vapor deposition (CVD) and then etching away the metal. In the proof-of-concept experiments, we study this idea using two types of metal nanostructures, inverse opals and slanted nanopillar arrays grown by a glancing angle deposition (GLAD) technique. Due to the elevated temperatures used in typical CVD growth, nanostructures can be easily damaged during the growth process. Therefore, we systematically study the roles of different parameters, such as the composition, morphology and crystallinity of a nanostructured metal, as well as the CVD growth temperature and different carbon sources to grow graphene on metal nanostructures and at the same time preserve their integrity. In particular, we show that nanostructures with large crystalline domains can withstand high temperature CVD, whereas polycrystalline nanostructures, such as nanopillars grown by GLAD, suffer damage even at the low growth temperature of 500 °C. In the case of such thermally sensitive nanostructures, a careful selection of a highly reactive carbon source that could form graphene at lower temperatures becomes crucial. Furthermore, the selection of a metal is also important, as cobalt nanostructures are shown to be more resistant to thermal damage than their nickel counterparts. Finally, we could successfully remove the nanostructured metal templates to form free-standing graphene-based inverse opals and hollow graphene nanopillars. The findings presented in this paper could facilitate synthesis of other 3D graphene nanostructures.