First-principles calculations of the geometry and electronic structure of electron- and hole-doped C60 in the field-effect transistor structure

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We investigate the geometry and electronic structure of field-effect-doped C60 by carrying out first-principles calculations based on the density-functional theory. To reproduce field-effect doping, we employ a structural model which consists of a single C60 (111) layer and an electrode layer. In this model, we assume that the carrier concentration is higher than one electron or hole per unit cell. We find that the charge distribution on the electrode side hemisphere of the C60 molecule, say the north hemisphere, is changed by field-effect doping while that on the south hemisphere is hardly changed. In particular, on the hexagonal face nearest to the electrode, the π-like electron density is considerably increased and decreased in electron- and hole-doped C60, respectively. Therefore, the C-C bonds on this face are elongated with increasing the carrier concentration both in electron- and hole-doped C60. Furthermore, it is found that the potential energy induced by field-effect doping is large enough to change the band dispersion and the density of states. In addition, we discuss the charge distribution at lower carrier concentrations with two models; one is the model consisting of three C60 layers and the other is the continuum model.
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