凯发

One missing element in the required developments towards large scale mass-production of high quality graphene is a quick and accurate method of quality control of the electrical properties of graphene which may be applied in, or close to, the graphene growth process. Further, well-defined standards and protocols for accurate measurement are also needed, for reproducible characterisation of different types of graphene materials, grown or exfoliated using various production routes. Necessary electrical measurement techniques for graphene and other 2-D materials must be identified, developed and standardized at the national and International level to overcome the gap in metrology. 

In this talk I describe a non-contact method using microwave resonance which addresses this standards issue. I describe the technique, estimate its accuracy and future developments. In essence it relies on three distinct factors.  First, although graphene has a high 3D conductivity the 2D sheet resistance of graphene is comparable with the impedance of free space (m0/e0)1/2.  Second, a monolayer or even a few-layer thick sample of graphene does not have a significant attenuation effect on electric fields which are parallel to its surface.  Third, as a diamagnet, its relative permeability can be assumed to be close to unity. These factors, when taken together, suggest a novel non-contacting method of measuring the conductivity. We have developed a microwave resonance technique, based on the perturbation of a high Q factor dielectric resonator by the presence of a nearby sample of graphene.  We have shown that it is possible to convert this technique by a method of substitution, into an accurate and fast method for deriving sheet resistance (or equivalently, 2D conductivity) without the need for patterning or making contacts. We measure first the centre frequency and Q factor of the dielectric resonator on its own, then with a sample of graphene, on a non-conducting substrate, placed nearby and finally with an identical bare substrate in the same position.  The presence of the graphene produces a change in Q value. 

These measurements are sufficient to provide accurate determination of the graphene sheet resistance [1].  The reproducibility of our measurements is at the level of a few percent.  From comparison of the same measurements made with our microwave technique and conventional van der Pauw measurements on the same samples it is clear that the absolute accuracy is better than 10%. We have compared measurement on a range of graphene samples grown by chemical vapour deposition (CVD), high temperature decomposition of SiC and reduction of graphene oxide, having a sheet resistance range of some four orders of magnitude. 

In a recent development we have also demonstrated how we may use the same technique to measure the graphene mobility and carrier density without the need of electrical contacts.  This method has been patented.