A group of researchers from Denmark, UK and Spain within the Graphene Flagship project, explains in a recent review paper why the graphene industry needs better and faster electrical characterisation methods. The Graphene Flagship is a large European project, with more than hundred research groups collaborating on development of novel graphene technologies and applications.
Just 5 years after the first announcement that graphene could be isolated at all, Rod Ruoff (2009) and Samsung (2010) showed that graphene can be synthesized in a deceptively simple way; by decomposing hydrocarbons at high temperature, leaving single layer graphene sheets to crystallise on a copper surface.
Today, just 7 years later, graphene sheets are produced and used in large quantities – or areas – for instance for cell phone touch screens, according to Chinese researchers.
While large-area fabrication is taking off fast, the methods for quality control are lagging behind – and this is particularly true with respect to the electronic properties that are central to many applications.
Electrical measurements are most often done by turning the graphene film into a number of electrical devices, where field effect measurements give the “key performance indicators” of conductivity, carrier density and mobility. Depending on the number of devices, and the time spent on measuring, such tests can also give an idea about the variability. The two main drawbacks are ; (1) the process is fundamentally destructive – the graphene is irreversible damaged in the process, and (2) the throughput is many orders of magnitude smaller than the CVD-based fabrication of the graphene in the first place creating a bottleneck.
Researchers at the Technical University of Denmark and at the National Physics Laboratory in UK have over the past several years developed a number of fast, large-scale, non-destructive characterisation techniques of electronic properties that they believe have the potential to become game changing technologies.
A recent review focuses on one of these: terahertz time-domain spectroscopy (THz-TDS).
THz-TDS shoots terahertz pulses through the graphene and measures how much the film absorbs. The absorption spectrum up to 2 THz depends distinctly on the conductivity as well as on the scattering time – a measure of the average time the carrier spend between collision with obstacles.
Knowing these two, the carrier density and mobility can be computed. The technique has been meticulously verified against electrical measurements and is now being proposed as a metrology standard, in collaboration with the Spanish company DasNano, who are the first to manufacture terahertz-based conductivity mapping equipment for graphene.
Peter Bøggild, professor at DTU puts it like this: “Trivially, there can be no industry without quality, and there can be no quality without quality control. Non-contact mapping is fast and non-destructive, so anyone interested in consistency, reproducibility and reliability of graphene films, should pay attention.”
The review paper is available as open access at IOP 2D Materials.