凯发

Little work has been reported on the properties and applications of graphene at microwave frequencies. We discuss a novel microwave characterization method for graphene films and also near-field microwave excitation and readout as an important technique for graphene nanomechanical systems (NEMS) resonators which promise a range of sensing applications. Many different methods for preparing graphene thin films have appeared. There is a great variability in the quality of films prepared, and even when identical methods are used the film properties between successive batches may be quite different. Our non-contact method for conductivity and sheet resistance measurements of graphene uses a high Q microwave dielectric resonator perturbation technique to provide fast and accurate measurement non-invasively. The dynamic range of the microwave conductivity measurements makes this technique sensitive to a wide variety of imperfections and impurities, without the need for film patterning or contact fabrication. The graphene samples are supported on a low-loss dielectric substrate, suspended in the near-field region of a small high Q sapphire puck microwave resonator. The presence of the graphene perturbs both centre frequency and Q value of the microwave resonator.  The measured data is interpreted in terms of real and imaginary components of the permittivity, and by calculation, the conductivity and sheet resistance of the graphene.  Results are presented for graphene samples grown by three different methods:  reduced graphene oxide (GO), chemical vapour deposition (CVD) and epitaxial graphene on SiC. We also report fabrication and measurement of a number of different graphene mechanical resonators based on transferred material onto different lithographically patterned substrates. CVD grown graphene films are of increasingly good quality and, following growth on a metal thin film catalyst, can then be transferred to any arbitrary supporting substrate. The transfer process is critical. These resonators are typically circular drum membranes with diameters ranging from 0.5 to 40 micrometres. We report experimental data on these drums using a variety of microwave excitation and readout methods. An important issue is that there is strong coupling between graphene and microwave fields. This relates to the relatively close matching of the impedance of free space, or a confined geometry like a microwave resonator, and the sheet resistance of high quality graphene,[1] making the microwave method particularly suitable for application to graphene NEMS resonator based sensors.[2]

References:

[1] L. Hao, J. Gallop, S. Goniszewski, O. Shaforost, N. Klein and R. Yakimova, ‘Non-contact method for measurement of the microwave conductivity of graphene’, Appl. Phys. Lett. Vol.103, 123103 (2013) 

[2] Ling Hao, Stefan Goniszewski,  Jie Chen and John Gallop, ‘Microwave excitation and readout of nano- and micron scale cantilevers’, Applied Surface Science vol. 258  pp. 2192-5 (2012).