Charge profile of surface doped C 60

We study the charge profile of a C60-FET (field effect transistor) as used in the experiments of Schön, Kloc and Batlogg. Using a tight-binding model, we calculate the charge profile treating the Coulomb interaction in a mean-field approximation. At low doping, the charge profile behaves similarly to the case of a continuous space-charge layer and becomes confined to a single interface layer for doping higher than ∼0.3 electron (or hole) per C60 molecule. The morahedral disorder of the C60 molecules smoothens the structure in the density of states. PACS. 73.25.+i Surface conductivity and carrier phenomena – 73.90.+f Other topics in electronic structure and electrical properties of surfaces, interfaces, thin films, and low-dimensional structures – 74.70.Wz Fullerenes and related materials Surface doping of high quality organic crystals has recently been achieved by Schön, Kloc and Batlogg [1–3]. This breakthrough has led to novel devices, new super-conductors and measurements of transport properties over unprecedented wide ranges of carrier concentration and temperature. A tunable field effect transistor (FET) is used to inject carriers near to the surface of the organic crystal. In this paper we report on calculations of the resulting charge profile. We chose to investigate the case of a C 60-crystal which has a special interest in view of the recent observations of superconductivity with a high T c (∼52 K) in hole doped samples. The FET used in the work of Schön et al. is illustrated in Figure 1a. In the present work, we consider a C 60 crystal (fcc lattice) with a [001] plane parallel to the gate. Undoped C 60 is a semiconductor with a 2 eV gap. However , when an electric potential is applied between the gate electrode and the source/drain electrodes, either electrons or holes accumulate on the interface between the C 60 crystal and the gate dielectric. This leads to a doping of the C 60 interface planes which allows current to flow parallel to the interface, between the source and the drain. The resistance of this channel can be measured as a function of temperature and doping. The FET can be represented as a planar capacitance with equal and opposite charges on the metal gate and the C 60 planes near to the interface as shown in Figure 1b. We will calculate the charge profile as a function of the total charge induced at the interface a (a) (b) Electron (hole) doped layers …