Microwave characterization of large area graphene using a TE<inf>01δ</inf> dielectric resonator

Abstract

Graphene is a thinnest conducting material in the world. Its ability to conduct electricity is higher than of other materials at room temperature.In addition with its high transparency graphene may find a variety of industrial applications if high quality large area monolayers can be produced. The chemical vapour deposition (CVD) method is considered to be the most promising technique to synthesise graphene on a large scale.The requirements for graphene conductivity and sheet resistance vary from application to application, starting from touch screens with requirements to sheet resistance to be 200 - 500 Ωsq-1 and finishing by solar cells where those values have to be less than50 Ωsq-1. The determination of sheet resistance by conventional probe or van der Pauw methods requires deposition of contacts. It reveals a number of problems when applied to such ultrathin material like graphene. There is a demand of fast and contact free methods of conductivity determination of large graphene samples. Microwave methods can be non-invasive and represent an accurate alternative to the DC methods. In this work we propose a microwave resonator method for the assessment of graphene sheet resistance. A microwave dielectric resonator operating with TE01δat 10 GHz was designed and two different configurations were tested. In both the shielded and open case(Fig. 1) a dielectric BZT (barium-zirconium-tantalate)-resonator of cylindrical shape was arranged inside a metal cavity. The top wall of the cavity contains a circular metal aperture of diameter 7.5 mm, which is slightly larger than the diameter of the dielectric puck. The graphene sample is placed on top of the aperture. The aperture helps to focus the evanescent electric field leaking from the top of the dielectric cylindrical puck onto the graphene sample. In the shielded configuration (Fig. 1a) the metal lid confines the cavity to avoid microwave losses due to radiation. The open configuration (Fig. 1b) without the lid enables convenient and fast assessment of the conductivity of graphene samples of random size with cross section above 8 mm x 8 mm.