Stacking thin layers of graphene and related materials (GRMs) leads to heterostructure devices with a variety of different electronic and optical properties, which can be tailored by careful design of the stack. Now, researchers from the Graphene Flagship have added a new option for tailoring the electronic properties, using molecular monolayers to create controllable periodic potentials on the surface of graphene. The research, published in Nature Communications, paves the way for new materials with specially designed electrical, magnetic, piezoelectric and optical functionalities.

Working at Flagship partner institutes the University of Strasbourg (France), CNRS (France), the Max Planck Institute for Polymer Research (Germany) and Dresden Technical University (Germany), and in collaboration with the University of Mons (Belgium) who are not involved in the Graphene Flagship, the researchers used organic molecules that self-assembled into ordered structures on the surface of graphene. This supramolecular strategy could be applied to other materials to create a new class of hybrid organic-inorganic materials with fully controllable structural and electronic properties.

Programmable Potentials

The researchers used a bottom-up approach to create the hybrid materials, allowing the molecular building blocks to self-assemble into a layer of repeating units that varied in one direction. They found that the molecular layer could significantly affect the electrical properties of graphene, changing the behaviour of graphene field-effect transistor devices. This modulation could lead to new types of devices which allow current to flow in restricted channels. Paolo Samorì (University of Strasbourg) is the Deputy Leader for the Graphene Flagship's Functional Forms and Coatings Work Package, and was involved in the work. "The mechanical superposition of different layered crystals has been proven to be a route towards the fabrication of heterostructures featuring 2D periodic potentials. Here, we showed that using supramolecular lattices makes it possible to tailor 1D periodic potentials in the resulting organic-inorganic hybrid heterostructures, thereby endowing anisotropic properties to these otherwise isotropic materials," he said.

The molecules consist of a long-chain tail and a reactive head with a small electric field caused by unequal distribution of the electrons in the head. In the self-assembly of the supramolecular lattice, the molecules line up next to each other, with the heads and tails lying in ordered rows (see illustration). The electric field of the heads influences the underlying graphene, while the regions covered by tails remain unaffected. The presence of the molecules leads to a periodic variation in the electric field in 1D, which can alter the behaviour of electric current in the graphene.

The properties of the molecule determine the specifics of the periodic potential. Using three different types of molecules, the researchers showed that the potentials can be fully controlled, with the size and orientation of the electric field in the head of the molecule determining the strength and type of effect in the graphene. By carefully designing the molecular layer, the electronic properties of the resulting hybrid structure can be fully tailored. Xinliang Feng (TU Dresden), Leader of the Functional Forms and Coatings Work Package, and co-author of the work, added "One can surely foresee the fabrication of artificial hybrid heterostructures exhibiting novel electrical, magnetic, piezoelectric and optical functionalities by taking full advantage of the infinite degrees of freedom offered by the design of the molecular building blocks."

New Hybrid Materials

This new approach to device design could be extended to other GRMs, enabling more complex multilayer heterostructures with new properties. For example, in semiconductor transition metal dichalcogenides, the periodic potentials could lead to a series of nanoscale junctions with distinct optical properties. "Ultimately, this could pave the way towards systems with unconventional physical and chemical properties for opto-electronics," said Samorì.

Vincenzo Palermo (CNR, Italy), Leader of the Graphene Flagship's Composites Work Package, commented "Scientists have always tried hard to tune the electrical properties of graphene. The Graphene Flagship demonstrated, with this and other works, that chemistry can be a powerful tool to create beautiful, self-assembled structures on graphene, modifying its nanoscale properties using light or (electro)chemical stimuli."

This result demonstrates the wide potential of hybrid systems that are yet to be fully explored. Andrea Ferrari (University of Cambridge, UK), is the Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel. He added "The Graphene Flagship has always been, since its start, about the entire family of graphene, related layered materials and hybrid systems. The latter have not been yet fully investigated, and this work represents an interesting proof of principle, showing how supramolecular chemistry opens another dimension in the already wide space covered by the properties of GRMs."

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