Fractional and Integer Quantum Conductance at 300K and Zero Magnetic Field in Polymer-Suspended Graphene Nanoplatelets

M. Orlowski
Virginia Tech,
United States

Keywords: fractional quantum conductance, graphene nanoribbons

Summary:

Quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. In devices with medium concentration of graphene platelets, integer multiples of conductance Go=2e2/h (=12.9 kΩ)-1 are observed. In some devices partial quantized conductance steps are observed, including a series with (n/7)×Go steps. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10. We attribute the quantized conduction to the existence of a 1D electron waveguide along the conductive path. Such a memory device can be viewed as a realization of a quantum memristor. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10. The graphene-based devices have been fabricated on a thermally oxidized silicon wafer using standard semiconductor processes. The thermally oxidized silicon wafer was first cleaned using acetone and subsequently rinsed in isopropanol and deionized water. The wafer was then patterned photolithographically and deposited with a bottom gold (Au) electrode using electron-beam physical vapor deposition (EBPVD). The graphene nanoplatelet (GNP) powder provided by PPG Industries was first dispersed into toluene and further exfoliated for one hour by ultrasonication. P3HT was subsequently dissolved into the GNP solution and ultrasonication for another hour followed. According to the manufacturer of GNP, GNPs comprise one or more layers of one-atom-thick planar sheets of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice. The number of stacked layers is typically between 5 and 30. The lateral dimension of the flakes ranges from 100 nm to a few μm. The graphene platelet ribbons are substantially flat, but when made thin by exfoliation they can be curved, curled or buckled indicating a single flake or a stack of a few carbon sheets. Ultrasonication has been found to be a very effective method in overcoming the van der Waals force between the individual carbon sheets leading to uniform dispersions of single flakes or very thin stacks of flakes. The resulting P3HT(GNP) dispersion was then drop-deposited onto the bottom metal Au electrode and covered by islands of Cu electrodes, also using EBPVD, to form Au/P3HT(GNP)/Cu devices. The thickness uniformity of the P3HT(GNP) film is +/- 50 nm. Spin coating of the dispersion has been tried but resulted in poor adhesion to the subjacent Au electrode. The graphene concentrations in the P3HT(GNP) solutions vary from 0.05mg/ml to 0.2mg/ml. The volume of the P3HT solution is 10 ml. The devices have an area of (2/3) mm × (2/3) mm. The thicknesses of the layers are 60 nm, 700 nm, 150 nm, for Au, P3HT(GNP), and Cu, respectively. The P3HT(GNP) layer is sufficiently thick to accommodate non-horizontally oriented GNP ribbons. For each concentration, ten devices with the same structure and the same active area were fabricated. For comparison and assessment of the role of carbon nano-platelets, six Au/P3HT/Cu structures have been fabricated with no graphene nano-platelets.