Graphene and F4TCNQ

Sometimes referred to as a “wonder material,” graphene is a carbon allotrope that is only an atom thick, which is flat enough to be considered a 2D material, but it is extraordinarily strong. In fact, a single layer of graphene is about 100 times stronger than steel yet extremely flexible. Also, it is about 200x more electrically conductive than silicon and 10x more thermally conductive than copper.

Being able to efficiently mass produce graphene is a current industrial endeavor that we are working on, as it is rather expensive to produce.

Now, scientists from the Lawrence Berkeley National Laboratory have discovered a new mechanism for assembling two-dimensional molecular “islands” that have the potential to be used to modify graphene at the nanometer scale. These 2D islands are comprised of F4TCNQ molecules that trap electrical charge in ways that are potentially useful for graphene-based electronics.

Physicist Michael Crommie describes the study, wherein the F4TCNQ molecules coalesce into 2D close-packed islands. “The resulting islands could be used to control the charge-carrier density in graphene substrates, as well as to modify how electrons move through graphene-based devices. They might also be used to form precise nanoscale patterns that exhibit atomic-scale structural perfection unmatched by conventional fabrication techniques,” he asserts in the press release.

The Mechanism
Crommie, M. F., et al.

F4TCNQ is able to extract electrons from a substrate, thus changing the substrate charge-carrier density. Some studies investigated F4TCNQ adsorbed on graphene supported by a metal substrate. This creates a highly screened environment.

Crommie and his colleagues found that, unlike with metals, F4TCNQ molecules on graphene/boron nitride form 2D islands by a self-assembly mechanism caused by the long-range Coulomb interactions between the charged molecules.

“Negatively-charged molecules coalesce into an island, increasing the local work function above the island and causing additional electrons to flow into the island. These additional electrons cause the total energy of the graphene layer to decrease, resulting in island cohesion,” Crommie says.

Such a mechanism could allow fine-tuning graphene properties for device applications in the future.

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