Heterojunctions – the interface formed between two solid-state materials with differing electronic properties – are the basic building block of microelectronic devices. Now researchers have devised a means of creating heterojunctions that are precise on the atomic scale in tiny strips of graphene known as nanoribbons, which could enable the design of a new generation of more efficient and powerful nanoelectronics [Nguyen et al., Nature Nanotechnology (2017), doi: 10.1038/nnano.2017.155].
“We want to find materials that will ultimately outperform current silicon-based technology, which is reaching its fundamental physical limits,” explains Felix R. Fischer, who led the research with Steven G. Louie, Michael F. Crommie and colleagues from the University of California at Berkeley, Lawrence Berkeley National Laboratory, The University of Texas at Austin, and the Kavli Energy NanoSciences Institute. “The key is the ability to build up heterostructures with atomic control.”
Bottom-up synthesis of graphene nanoribbons provides a means of fabricating very precise structures with complete control over dopants. But, until now, the formation of heterojunctions relied on co-polymerization of two different molecular precursors, which produced two corresponding nanoribbons linked together at random points on the substrate surface.
“We need to get away from this ‘Hail Mary’ approach, where we cross our fingers and hope for the best,” says Fischer. “Instead, we have devised a way of making heterojunctions at precise positions on graphene nanoribbons.”
The researchers’ new approach relies on a single precursor molecule, which contains a carbonyl group. Under the right conditions, the precursor forms fluorenone graphene nanoribbons decorated along the edges with the carbonyl groups. However, these chemical groups can be removed from specific regions of the nanoribbons by heating or applying an electric field between the sample and the tip of a scanning tunneling microscope. Since the bandgaps of graphene nanoribbons with and without the carbonyl groups are different, very precise heterojunctions spanning less than a nanometer can be fabricated.
“We remove the functional groups selectively to create heterostructures in a homogeneous material – rather like writing the band structure onto a strip of paper,” explains Fischer.
The researchers confirmed that the heterojunctions coincide with the change from regions of unfunctionalized (carbonyl-free) graphene to sections with carbonyl groups using a technique they developed called bond-resolved scanning tunneling microscopy (BRSTM). The imaging probe determines local chemical structure at the same time as spectroscopy measurements provide an indication of the local band gap.
To be sure, the researchers also compared their experimental results with theoretical simulations based on ab initio calculations. Happily, the simulated properties of functionalized and unfunctionalized graphene nanoribbons using density functional theory (DFT) show the same trends in bandgap structure as the team’s experimental observations.
“This work offers a simple way to fabricate a wide range of molecular heterojunctions based on graphene nanoribbons,” comments Cinzia Casiraghi of the University of Manchester. “Scanning probe microscopy and simulations indicate the formation of type II heterojunctions, which is the same formed in the InAs–AlSb system,” she points out.
She cautions, however, that we are still far away from being able to use this method in real applications because the synthesis requires ultra-high vacuum and high temperature conditions and a gold substrate.
This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.10.007.