The function of many semiconductor devices (e.g. transistors, solar cells, diodes) is based on interfacing two dissimilar semiconductor materials with different properties (for the sake of simplicity, let’s say they have different colors: purple and orange). This interface is called a heterojunction. Currently, such devices are made from bulky semiconductors but research aims at replacing these materials with smaller, more energy-efficient alternatives, one of which are graphene nanoribbons.
Previously, researchers successfully fabricated heterojunctions consisting of two different types of graphene nanoribbons. This was achieved by combining two types of molecular building blocks which can link up and form a linear structure, the nanoribbon. However, since both types of building blocks are compatible with each other, the nanoribbons consisted of a random sequence of building blocks, which renders them useless for applications in functional devices. Instead, a nanoribbon containing only a single heterojunction is desirable.
We were able to improve on the other researchers’ strategy and produce nanoribbons with a controlled sequence of building blocks, featuring only a single heterojunction, which is an important step on the path to integrating graphene nanoribbons in functional electronic devices.
The idea behind our approach is to modify the building blocks (molecules) so that the two different species are no longer compatible with members of the opposite species. This results in two homogeneous nanoribbons (one purple, one orange) which are linked by a special, third building block: an adapter piece.
The above description is certainly oversimplified in several aspects. For example, instead of changing “compatibility” by changing the shape of puzzle-like connectors, we changed the atoms which lend the molecular building blocks their ability to bond. Specifically, bromine atoms were replaced by iodine atoms. The following graphic shows the three molecular precursors we used as well as the structure of the resulting graphene nanoribbon. The difference between the two sides of the heterojunction lies in the width of the nanoribbon.
Before the reaction of the three molecules, we deposited them onto a very clean surface of an inert gold sample and then induced the coupling reaction by heating the entire system. The fact that the molecular building blocks and the resulting nanoribbon are immobilized on a surface allowed us to study individual nanoribbon structures with a scanning tunneling microscope (STM).
Below is an image of one such heterojunction. The colors are artificially added and simply highlight the fact that the two halves of the nanoribbon have different properties such as different widths.