Transport through graphene nanoconstrictions.

摘要

Graphene is a recently-discovered material with remarkable physical properties, including high mechanical strength and thermal conductivity and the highest known room-temperature mobilities of any material. These properties make graphene an attractive candidate for next-generation high-frequency transistors, but extended sheets of graphene also display a non-zero minimum conductivity, making it impossible to fabricate transistors with high on-off ratios out of extended graphene sheets. However, the long narrow nanostructures known as graphene nanoribbons offer the possibility of high on-off ratios when used as transistors. Their properties were initially explained in terms of quantum confinement. We have fabricated and measured the transport properties of graphene nanoribbons of a variety of lengths, ranging from micron scales to very short constrictions tens of nanometers in length. We find that nanoribbons and constrictions of all lengths display transport properties consistent with the formation of quantum dots throughout the length of the ribbon. In very short constrictions, we measure transport consistent with the presence of only one or two quantum dots, with strong coupling to the bulk graphene leads. We present a model for formation of quantum dots in our constrictions due to background potential fluctuations in the presence of a confinement gap. Annealing experiments allow us to vary the density and arrangement of impurities in the vicinity of our ribbons; we find that annealing changes certain transport properties systematically in a manner consistent with our disorder-nucleated quantum dot model of transport.