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

Coulomb-blockade transport-whereby the Coulomb interaction between electron s 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 i s the capacitance of the site), of an excess electron on a site is large co mpared to kT. (Here e is the charge of an electron, k is Boltzmann's consta nt, and h is Planck's constant.) The nature of the individual sites-metalli c, superconducting, semiconducting or quantum dot-is to first order irrelev ant for this phenomenon to be observed(1). Coulomb blockade has also been o bserved in two-dimensional arrays of normal-metal tunnel junctions(2), but the relatively large capacitances of these micrometre-sized metal islands r esults in a small charging energy, and so the effect can be seen only at ex tremely low temperatures. Here we demonstrate that organic thin-film transi stors 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, consiste nt with the conventional model of Coulomb blockade.