Molecular Doping of Graphene

Abstract

Graphene is considered as one of the most promising materials for post silicon electronics, as it combines high electron mobility with atomic thickness [Novoselov et al. Science2004, 306, 666−669. Novoselov et al. Proc. Natl. Acad. Sci. U.S.A.2005, 102, 10451−10453]. The possibility of chemical doping and related excellent chemical sensor properties of graphene have been demonstrated experimentally [Schedin et al. Nat. Mater.2007, 6, 652−655], but a microscopic understanding of these effects has been lacking, so far. In this letter, we present the first joint experimental and theoretical investigation of adsorbate-induced doping of graphene. A general relation between the doping strength and whether adsorbates are open- or closed-shell systems is demonstrated with the NO₂ system: The single, open shell NO₂ molecule is found to be a strong acceptor, whereas its closed shell dimer N₂O₄ causes only weak doping. This effect is pronounced by graphene's peculiar density of states (DOS), which provides an ideal situation for model studies of doping effects in semiconductors. We show that this DOS is ideal for “chemical sensor” applications and explain the recently observed [Schedin et al. Nat. Mater.2007, 6, 652−655] NO₂ single molecule detection.