Transporte elétrico em nanoestruturas de grafeno: influência da funcionalização, da geometria e da dopagem do substrato

Abstract

In this work we fabricated and investigated electrical transport properties in nanoribbons, quantum dots and Hall bars fabricated from graphene. We studied the effect of doping the edges of graphene nanostructures, such as nanoribbons and quantum dots, by functionalizing these regions using specific molecules. The effect of doping is measured by the variation of the gate voltage associated with the charge neutrality point of the graphene nanostructure before and after doping. To this end we fabricated dozens of graphene nanoribbons with different widths and lengths and measured their electrical conductivity as a function of applied gate voltage. We also studied the transport mechanisms in graphene nanoribbons fabricated by electron beam lithography. These nanoribbons have disordered edges and, at low temperature, they present a region of strongly suppressed conductance in their G(Vg) curves that may be associated with a transport gap. In the gap region the transport in the nanoribbons is due to hopping trough localized states. This type of transport is thermally activated and is characterized by variable range hopping at low temperatures and by simply activated hopping trough first neighbors at higher temperature. We fabricated and analyzed the electrical transport in graphene devices made on top of lightly doped silicon substrates. The electrical resistance as a function of gate voltage, measured at low temperature (77K), presents a huge variation from what one expects for a device fabricated on a highly doped silicon substrate. We show that this variation may be explained by the charge distribution in the substrate. Depending on the applied gate voltage and on the illumination of the device, the charges in the substrate go through accumulation, depletion and inversion regimes, and this strongly affects the variation of the graphene resistance with gate voltage. We observe that avalanche breakdown may also occur in the silicon substrate in these devices and may be identified by changes in the resistance of graphene. We demonstrate that an optical switch may be fabricated with graphene, which works by controlling the resistance variations caused by illumination and by applied gate voltage.