Graphene Electronic Circuits with Atomic Precision

Rapid progress has been made in high-performance graphene devices for the study of new physical phenomena^1,2,3,4,5. Mainstream applications, however, require a nonzero band gap^{6, 7}. This effect can be introduced in a variety of ways, one of the most prominent being the preparation of narrow strips of graphene known as graphene nanoribbons (GNRs)^8,9,10. The properties of these GNRs sensitively depends on width, orientation, and edge geometry^{5, 9, 11,12,13}. Armchair-terminated GNRs are either metallic or semiconducting, depending on their exact atomic width: 3 N or 3 N + 1 (where N is an integer) atom wide armchair GNRs are semiconductors, and those in the 3 N + 2 family are metallic<sup>5, 9,

14,15,16,17,18</sup>. This extreme sensitivity to the detailed atomic structure also implies that traditional fabrication methods, such as e-beam lithography, are not precise enough to fabricate structures of sufficient quality. This limitation has been overcome with the on-surface, bottom-up synthesis of atomically precise GNRs. In this route, precursor molecules containing halogen atoms are evaporated onto a metal substrate, typically Au(111), and heated to form polymeric chains via Ullman coupling. These chains are converted into fully aromatic GNRs through a cyclo dehydrogenation step occurring at a higher temperature than the initial polymerization step⁹.
The wide variety of atomically precise GNRs that can be prepared through on-surface synthesis¹⁰ has brought the concept of GNR electronics closer to reality with the first prototype transistors having been demonstrated¹⁹. For future applications, direct integration of electrical contacts and functional electronic components such as diodes and tunnel barriers into a single ribbon would be highly advantageous. This can be realized by synthesizing GNR hetero structures using a combination of different precursor molecules. For example, semiconductor-semiconductor hetero structures in single GNRs have been demonstrated through synthesis from precursors that give rise to segments of different widths²⁰ or segments with sub situational nitrogen doping²¹.
Here, we use the bottom-up approach to fabricate a metal-semiconductor junction and a tunnel barrier in a single GNR by utilizing atomically perfect connections between 5- and 7-atom wide segments (denoted as 5-GNR and 7-GNR, respectively). Not only are such junctions relevant for developing contacts in all-GNR devices, they also provide an additional route to create diodes and transistors^{10, 19,20,21}. We characterize the atomic scale geometry and electronic structure by combined atomic force microscopy (AFM), scanning tunneling microscopy (STM), and conductance measurements. The GNR equivalent of a tunnel barrier constitutes a first step toward complete electronic devices built into a single GNR.

Results