Coulomb-blockade transport in single-crystal organic thin-film transistors

Coulomb-blockade transport-whereby the Coulomb interaction between electrons can prohibit their transport around a circuit-occurs in systems in which both the tunnel resistance, R-T, between neighbouring sites is large (much greater than h/e(2)) and the charging energy, E-C (E-C = e(2)/2C, where C is the capacitance of the site), of an excess electron on a site is large compared to kT. (Here e is the charge of an electron, k is Boltzmann's constant, and h is Planck's constant.) The nature of the individual sites-metallic, superconducting, semiconducting or quantum dot-is to first order irrelevant for this phenomenon to be observed(1). Coulomb blockade has also been observed in two-dimensional arrays of normal-metal tunnel junctions(2), but the relatively large capacitances of these micrometre-sized metal islands results in a small charging energy, and so the effect can be seen only at extremely low temperatures. Here we demonstrate that organic thin-film transistors based on highly ordered molecular materials can, to first order, also be considered as an array of sites separated by tunnel resistances. And as a result of the subnanometre sizes of the sites (the individual molecules), and hence their small capacitances, the charging energy dominates at room temperature. Conductivity measurements as a function of both gate bias and temperature reveal the presence of thermally activated transport, consistent with the conventional model of Coulomb blockade.