Iteratively Synthesized, Atomically Precise Graphene Nanoribbons with Heteroatoms

The semi-metallic electronic structure of graphene has been the major problem in the fabrication and integration of graphene electronic devices. While the thermal, optical, mechanical, and electronic transport properties of graphene vastly outperform silicon, the very low on-off current ratio prevents 2D planar graphene from being used as a controllable channel in field effect transistors (FETs). Graphene nanoribbons (GNRs), laterally confined, 1D structures of graphene exhibit a wide variety of bandgaps while maintaining similar physical properties. For example, GNRs are predicted to have electron mobilities and thermal conductivity over twenty-five times higher than that of Silicon. However, limitations in current GNR synthesis techniques prevent the use of heteroatoms from being incorporated into a length-controlled ribbon. Here, I discuss a solution-based iterative coupling technique capable of synthesizing GNR copolymers and GNRs with heteroatoms. This increase control over GNR synthesis arises from the use of tetramethyl <i>N</i>-methyliminodiacetic acid (TIDA) protected haloboronic acid difunctionalized GNR building blocks. By utilizing optimized catch-and-release purification methods and automated machinery, we’ve demonstrated the ability to create user-defined, atomically precise, length controlled GNR copolymers with no human intervention in as little as eight hours. Because TIDA-protected boronic acids can be deprotected under mild, basic conditions, these GNR building blocks can be synthesized with the inclusion of heteroatoms without degradation. GNR building blocks containing Oxygen, Sulfur, and Nitrogen have already been synthesized. The inclusion of these electron-rich species into the graphene lattice alters the electronic band structure and ribbon conductivity, effectively n-type doping the local environment. These synthesized GNRs and GNR copolymers are then exfoliated onto metallic and semi-conducting surfaces in situ in ultra-high vacuum (UHV) using dry-contact transfer. Finally, these on-surface GNRs probed using scanning tunneling microscopy (STM) to better understand their electronic structure, on-surface behavior, and stability. This procedure has the potential to open the floodgates to hundreds of novel GNRs and GNR heterostructures that can be used in post-silicon devices.