Synthesis of programmable graphene nanoribbons from perfectly sequenced polymers of molecular precursors

Graphene nanoribbons (GNRs), nanometer-wide strips of graphene, are interesting because of their versatile electronic, optical, and magnetic properties. Progress in bottom-up synthesis over the last decade has enabled different types of GNRs to be realized by chemically designing molecular precursors that can be linked into molecular chains (i.e., polymers) and then subjected to cyclodehydrogenation to produce GNRs with atomically-precise widths, dopants, and edge configurations. Despite this progress, there currently exists no technique for “rationally” synthesizing GNRs that exhibit engineered precursor sequences that go beyond the simplest A-B-A-B-A-B pattern and that have well-defined length. More complex sequencing is necessary to create the types of reproducible heterojunctions required for useful nanodevice functionality (e.g., designed sequences of GNR p-n junctions, metallic segments, and quantum dots). Here we describe a new method for preparing diverse GNR structures from different molecular building blocks that can be attached in any order, thus yielding the first truly “sequenced” GNRs. This method enables precise control over GNR length, shape, and sequence of internal structural elements. The technique used to accomplish this is called protecting-group-aided iterative synthesis (PAIS), and involves monomer-by-monomer construction of polymers to yield well-defined molecular sequences. GNR polymers created using PAIS were deposited onto Au(111) for subsequent cyclodehydrogenation using matrix-assisted direct (MAD) transfer, a technique that allows clean surface deposition of large molecules from solution. The resulting complete, fully sequenced GNRs were imaged using low-temperature bond-resolved scanning tunneling microscopy (BRSTM). I will show images of atomically-precise GNR heterojunctions produced using PAIS that would not be possible using other more conventional GNR synthesis techniques. PAIS opens the door to future GNRs having complex multi-heterojunction layouts designed for quantum device functionality.