High frequency transmission properties of printed graphene

The microwave properties of graphene are of interest because of possible graphene applications in microwave components and devices. High frequency transmission properties of monolayer graphene have been studied recently. Printing techniques are competitive alternatives to conventional photolithography for the production of electronic devices with advantages of low cost, ease of mass production, and flexibility. It is important to know the microwave properties of printed graphene in term of its usage in printed radio components. This paper presents experimental work on study of high frequency transmission properties of printed graphene from DC to 5 GHz. Test structures were copper coplanar waveguides (CPW) fabricated on FR4 substrates. Three CPWs were placed on each substrate. One CPW was used as a reference. Central parts of two others CPWs (30 mm and 10 mm long correspondently) were replaced by electrodes with the same geometries but made of printed graphene. Graphene electrodes were printed by screen printing method. Commercial Vorbeck inks S301 and S701 were used in the printing process [4]. Graphene structures were printed on PET ST 506 flexible substrates. Silver paste was used to make a contact between copper and graphene parts of the CPW-line. The S-parameters of CPWs were measured over the frequency range DC-5 GHz using a network analyser. CPW lumped element models were created which includes lumped element models of copper and graphene parts of CPW and contact resistance between them. Contact resistance of the interface between copper and graphene were calculated from the resistance difference of the short and long graphene CPW-lines. Transmission properties of printed graphene were extracted by fitting experimental results to the model. Result shows that high frequency sheet resistance (defined as the real part of sheet impedance) of printed graphene depends on number of printed layers. The lowest RF sheet resistivity 9.1 Ohm/square was obtained for S301 inks printed in three layers. This value is in good agreement with the DC sheet resistance extraction. It was observed that in some CPWs insertion loss was smaller at higher frequencies than it could be expected for materials with such resistance. Possible explanation of this effect is that resistance of printed graphene could decrease at higher frequencies due to capacitive coupling between graphene flakes. Achievable parameters of RF components such antennas and CPWs made of printed graphene are estimated.