NPG Asia Materials featured highlight | doi:

Doping graphene: Molecular tuning

Graphene — an atomic layer of carbon in which the atoms are arranged in a honeycomb structure — has excellent electronic properties for many device applications. However, controlling the concentration and type of charge carriers in graphene has proved to be challenging. Now, Yuan Ping Feng and colleagues¹ in Singapore and China report that the adsorption of organic molecules onto graphene could allow the electronic properties of graphene to be controlled and tuned. Their first-principles calculations show that a charge-transfer complex is created with the electron–acceptor molecule tetracyanoethylene (TCNE), allowing the electronic properties of graphene to be tuned by adjusting the coverage of adsorbed molecules.

TCNE, consisting of cyano groups in an ethene structure, is a strong electron acceptor and readily forms charge-transfer complexes by ‘pulling’ electrons from neighboring molecules. Feng and colleagues found that TCNE molecules adsorbed to graphene float about 0.3 nm above the carbon layer and do not form chemical bonds with the substrate. 

Instead, the TCNE molecules draw electrons from the graphene layer, resulting in the formation of a charge-transfer complex between the two layers. The increase in electron density on the TCNE is concentrated on the nitrogen atoms of the cyano groups and the central carbon atoms, whereas the charge-depleted area on the graphene is delocalized across the carbon atoms in the vicinity of the organic molecules. 

The researchers found that the TCNE molecules do not interact with one another at 1.04% coverage, whereas interaction is appreciable at 4.20% coverage. Although the interaction among TCNE molecules reduces the charge transfer from graphene, the positive charge is still enhanced in the graphene even at higher rates of coverage, effectively ‘p-doping’ areas of the carbon layer. In this way, it could be possible to tune the electronic properties of the graphene by altering the surface coverage of TCNE molecules.

“Our work demonstrates that the electronic structure of graphene can be controlled over a wide range by covering with organic molecules,” says Feng. “This creates the possibility of building all-graphene electronic devices by creating local p- and n-type regions with patterned organic molecules," (Fig. 1). "All-graphene electronic devices are attractive as the material doesn’t need to be cut or interfaced with leads, which is difficult.”

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

Top Home