Substrate Effects on Chemical Properties of Graphene: An Interview with Prof. Michael Strano and Dr. Qing Hua Wang

In this Thought Leader interview with Will Soutter, Prof. Michael Strano and Dr. Qing Hua Wang talk about their work on graphene, and explain how the properties of the atomically thin layers of carbon depend on what type of material they are placed on.

What made you decide to study graphene?

Graphene has so many interesting and unusual physical properties in terms of its electronic transport properties, mechanical strength, thermal conductivity, etc. It's also a purely two-dimensional substance that makes it unique and different from conventional materials that have an exterior and a bulk. Because it's only one atomic layer thick, but can be made into rather large areas, it provides really interesting opportunities from a materials perspective and from a chemistry perspective.

Can you outline the findings of your current research for us?

In our current work, we have found that the chemical properties of graphene are strongly affected by the substrate on which it rests. That is, depending on what material the underlying substrate is, the chemical reactivity on the top side of graphene can change quite drastically. The reason is that graphene is so thin it is readily affected by its surroundings. In particular, if there are charged impurities in the underlying substrate – which we can control by changing the surface chemistry of the substrate – they can cause electrons and holes in the graphene to cluster together in the graphene, forming what are known as electron–hole puddles. Within these puddles, which can be as small as a few nanometers across or as large as several hundred nanometers, graphene's local chemical reactivity can be very high if there a lot of electrons, or very low if there is a lack of electrons. Once we know this, we can actually make patterns on the substrate over many square centimeters before graphene is placed onto it, in order to control the level of electron–hole puddles, and thus achieve precise spatial control of chemical reactions in graphene.

Figure 1. (A) Large-area single-layer graphene is transferred onto a substrate that is patterned in alternating regions of bare SiO₂ (pale blue) and octadecyltrichlorosilane (OTS) (orange), which shields the graphene from charged impurities in the SiO₂ that induced electron–hole puddles in graphene. (B) After reaction with a diazonium salt, which forms covalently attached groups on the graphene, the reaction is strongest in regions where graphene is resting on SiO₂. The reactivity pattern reflects the initial substrate pattern. (C) Raman spectroscopy is used to map the degree of reaction, which varies spatially, by tracking the intensity ratio of the D peak and G peak. The stripes in blue are the low reactivity regions where graphene was resting on OTS, and the stripes in red are the high reactivity regions on SiO₂.

What implications will this have for future graphene research?

Our work and the work of other groups has shown that we must pay close attention to graphene's surroundings, because materials on top of and underneath graphene can significantly affect its electronic and chemical properties. Graphene is not just a single uniform layer; it can interact in complex ways with its environment. We think this opens up new areas of research to really take advantage of graphene's sensitivity towards its surrounding environment.

Are there any applications for graphene in commercial devices which are made possible by these new findings?

This research is currently aimed at getting a better fundamental understanding of the chemical properties of graphene, but there are likely implications for graphene-based biosensing because we can spatially pattern biomolecules without resorting to harsh methods involving etching or photolithography. There are also implications for adding graphene-based coatings to a variety of materials that can be then chemically functionalized, for instance to make materials biocompatible or to protect metals from corrosion.

What areas will your research be moving into next?

We have several members of the our group studying graphene, and we're looking at how different chemical treatments affect the electronic transport in graphene devices, how the behaviour of bilayer graphene differs from monolayer graphene under different chemical treatments, and how graphene and carbon nanotubes interact when brought into contact with each other. A lot of what we are doing is exploring the fundamental properties of graphene and other nanomaterials because there are so many fascinating things to study, but at the same time we are still engineers and want to find interesting new applications.

Where can people find out more about your work?

Our current results are published in Nature Chemistry, volume 4, pp. 724-732. Our other  publications can be found listed on the Strano group website.