“We have studied how molecules assemble on graphene when the carbon sheet itself is placed on an insulator,” explains team member Hsin-Zon Tsai at the University of California at Berkeley. “The electronic properties of graphene are very different to those of most other materials, which make the molecules assemble on the carbon sheet in unexpected ways.”

The researchers, led by Michael Crommie, Marvin Cohen and Steven Louie of UC Berkeley and the Lawrence Berkeley National Laboratory, obtained their results using scanning tunnelling and atomic force microscopy to study close-packed 2D islands of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) molecules, which are negatively charged, at the surface of a graphene layer supported by BN.

Molecules assemble to form islands

“When the tip of the STM (which is just a sharp metal needle) is brought sufficiently close to a sample surface, the electrons in the tip jump to the sample surface thanks to quantum mechanical tunnelling,” says Tsai. “This allows us to image the surface of graphene at the atomic scale, and we used this technique to help us identify how molecular islands self assemble over distances of nanometres. The AFM method is different in that it detects atomic-level force interactions between the tip and sample. By attaching a single CO molecule to the AFM tip, it becomes highly sensitive and can even resolve single chemical bonds within a molecule.”

Together these techniques are very useful for showing how molecules assemble to form islands and how they align relative to each other and the graphene surface, he adds.

F4TCNQ could be used to engineer graphene’s electronic properties through surface doping, that is, the transfer of electrons from graphene to the F4TCNQ molecules. “F4TCNQ has been employed before now as an electron acceptor in organic and inorganic semiconductors, so we knew that it would very likely charge up on graphene, which is what we wanted,” says Tsai.

Creating functional charge patterns in devices on the nanoscale

“You might think that the negatively charged F4TCNQ molecules would repel each other on the graphene surface, but this is not what happens,” he tells nanotechweb.org. “Instead, electrons from the graphene get ‘sucked’ into the F4TCNQ molecules, causing the molecules to stick more strongly to the graphene sheet below them. This additional force is strong enough to overcome the repulsive interactions between the negatively-charged F4TCNQ molecules.”

According to the team, which includes researchers from the Korea Institute for Advanced Study, the National Institute for Materials Science in Tsukuba, Japan, and the National University of Singapore, this island formation mechanism might apply to other molecule adsorbate systems exhibiting charge transfer in poorly screened substrates, like the insulator studied in this work. “These charged molecular islands might be used to engineer charge carrier density and create functional charge patterns in new devices at the nanometre scale,” says Tsai.

The research is detailed in ACS Nano DOI: 10.1021/acsnano.5b05322.