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New technique controls the charge carrier density in graphene

Scientists at Chalmers University of Technology, Sweden, have made a breakthrough in controlling the charge carrier density in graphene.

Scientists are able to dope graphene over large areas that enables the study of the physics at the Dirac point, where graphene is charge neutral and electron transport studies are not obscured by mesoscopic effects and charge disorder. The method involves the use of polymers to assist in the assembly of acceptor molecules on graphene.

At present the creating high quality graphene involves heating up silicon carbide (SiC) in the presence of inert Argon gas. This technique results in so-called epitaxial single layer graphene on the surface of silicon carbide (epigraphene). Compared to graphene grown by other methods, epigraphene grows as a single crystal over the entire silicon carbide substrate, anticipating higher electronic quality with respect to polycrystalline graphene grown by other methods.

However, on SiC, the graphene layer interacts very strongly with the substrate resulting in very large transfer of electrons from SiC into the graphene layer.

To fully exploit the properties of graphene, it's necessary to have the capability to tune the amount of electrons transferred from the substrate. For epigraphene, the very strong interaction between the graphene layer and SiC substrate makes it difficult to control at will the amount of electron transfer (i.e. doping), and this has been a major challenge in studying and exploiting the properties of what is a very promising material.

Scientists have now discovered a method to carefully assemble molecules on the surface of epigraphene to dope it with unprecedentedly high control, while keeping intact its electronic quality. The method employs polymers to assist in the assembly of electron-accepting molecules (i.e. dopants) on the surface of graphene; once in contact with graphene, dopant molecules withdraw electrons from graphene and this interaction keeps the molecules in place. To achieve this, polymers are mixed with dopant molecules, and this blend is then deposited on the epigraphene substrate by simple spin-coating, which is a standard and scalable microfabrication step performed in ambient conditions.

“By carefully choosing the polymer, the molecular dopant and the substrate, we have obtained rather exciting results. With a simple spin-coating step, which is routine work in cleanroom environment, we can heal epitaxial graphene and produce a centimeter-scale Dirac material with very high electronic quality. The electronic disorder that we measure in the doped graphene is lower than that of microscopic graphene flakes encapsulated by hexagonal Boron Nitride (hBN), which is the method that thus far results in the highest quality graphene”, explained Samuel Lara-Avila at the Quantum Device Physics Laboratory at the Department of Microtechnology and Nanoscienc at Chalmers.

The researchers used a combination of the common polymer poly methyl methacrylate (PMMA) and the well-known F4TCNQ acceptor molecule to form the dopant blend. They discovered that when the dopant blend is deposited on epigraphene, molecular dopants (i.e. F4TCNQ) diffuse through the polymer and spontaneously assemble at the epigraphene-polymer interface, making a densely packed layer of 3-4 molecules per square nanometer.

Surprisingly, this dense assembly of molecules results in epigraphene doped close to the Dirac point with an exceptionally small amount of charge inhomogeneity, i.e. disorder.

Using epigraphene, these results simultaneously show the potency of this method, because epitaxial graphene grown on SiC is particularly difficult to dope while at the same time maintaining its electronic quality.

The research was a collaborative effort between groups at the Department of Microtechnology and Nanoscience and the Department of Chemistry and Chemistry Engineering at Chalmers. It was jointly supported by the Swedish Foundation for Strategic Research (SSF), Knut and Alice Wallenberg Foundation, Chalmers Excellence Initiative Nano, the Swedish Research Council (VR), the Swedish-Korean Basic Research Cooperative Program of the NRF, and European Union’s Horizon 2020 research and innovation program.

Author
Neil Tyler

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