Graphene has been attracting growing attentions in physics, chemistry, and device applications after the discovery of micromechanically cleaved graphene sheet by A. Geim and K. Novoselov, who were awarded the 2010 Nobel Prize in Physics. The electronic structure of graphene, which is described in terms of massless Dirac fermions, brings about unconventional electronic properties, which are not only an important basic issue in condensed matter physics but also a promising target of cutting-...