The article presents significant advances in the synthesis of atomically precise graphene nanostructures. By combining solution synthesis of molecular precursors, on-surface synthesis of graphene nanostructures, and first-principles calculations, this study presents a new chemical route for the realization of nanometer scale porous graphene nanoribbons by means of controlled, sequential reactions.

What did the researchers discover?

In this study, we demonstrated that the migration of phenyl groups can be an efficient strategy for the on-surface synthesis of novel graphene-derived nanomaterials. This approach allows for very precise control of the internal transformations occurring in these materials.

In summary, our research succeeded in selectively inducing the formation of [18]-annulene pores at the edges of graphene nanoribbons (GNRs). GNRs consist of atomically thin, planar carbon nanostructures, which could be obtained if one had a “nanoscissor” that could cut graphene strips into any shape with atomic precision. However, this nanofabrication tool does not yet exist, so one has to rely on LEGO-like synthetic chemistry, in which molecular building blocks designed by solution chemistry are self-assembled and transformed using catalytic surfaces and special thermal processes. Typically, the edges of GNRs are straight, following a particular direction of the graphene atomic structure, although more complex open-edged structures have recently been achieved.

In our study, periodic arrays of pores were achieved by migration of phenyl groups located at the edges of the nanoribbons. The resulting pores are in the form of [18]-annulene rings, a molecular structure consisting of 18 carbon atoms arranged in a ring, as shown in the picture below.

 Figure 1. Structure of the obtained [18]-annulene GNRs.

What is the main novelty of this study in its field?

Our paper describes a breakthrough in the on-surface synthesis of graphene nanostructures, a field that has made significant progress in recent years. This research focuses on the fabrication of atomically precise structures from diverse molecular precursors, which act as building blocks that are self-assembled on metallic surfaces. Until recently, the reactions used for this kind of synthesis were limited to some strategies like cross-coupling or cyclisation reactions, which involve the cleavage of some specific atoms or groups.

In this study, we move beyond by adding a different type of reaction to the synthetic toolbox that involves not only the activation of sites by cleavage, but also the internal migration of specific groups. In particular, we used the migration of phenyl groups attached to the nanoribbons to generate these [18]-annulene-type pores at the edges of the structure. Altogether, the introduction of this new type of reaction in a sequential manner opens the way for the realization of graphene nanostructures with a higher degree of structural complexity.

How was the porous structure achieved?

The manufacturing pathway of this porous nanomaterial was conducted sequentially, starting from specific DBP-DBBA molecular precursors (shown below). These precursors were our initial building blocks. The process starts by depositing them on a gold surface at room temperature. This surface is then gradually heated to different temperatures, resulting in a series of chemical transformations:

-First, when heated to 200°C, the precursors undergo cleavage of their C-Br bonds, generating free radicals that assemble to form linear polymers. This reaction is known as Ullmann coupling.

-Then, when heated to a higher temperature (around 400°C), the polymers are converted into graphene nanoribbons, resulting in a flat, aromatic structure. In this case, the involved reaction is an internal cyclodehydrogenation.

-At the end, the main novelty of our study occurs when the temperature is increased to 450°C. A migration of phenyl groups to adjacent positions in the nanoribbon structure takes place. This migration triggers an additional dehydrogenation coupling reaction that finally generates the desired pores.

Figure 2. Simplified scheme of sequential chemical transformations, starting from the precursors DBP-DBBA.

Are there any factors that could improve the reaction’s performance?

One of the key findings from our study is that anchoring GNRs to a stepped surface significantly increases the yield of pore formation by impeding the lateral interribbon coupling that blocks the phenyl migration. By structuring the gold surface into terraces rather than keeping it flat, this approach prevents the formation of undesired bonds between vicinal nanoribbons, optimizing the process. Using a stepped gold surface to anchor the nanoribbons resulted in a pore formation yield of more than 60%.

Figure 3. Representative scanning tunnelling microscopy (STM) image of the [18]-annulene GNRs obtained in a stepped gold surface.

 How was the atomic structure of the new nanomaterial tested?

The arrangement of the new material was verified using bond-resolved scanning tunnelling microscopy (BR-STM), an advanced technique that enables the detailed observation of atomic structure with extremely high resolution. Using this technique, it was possible to observe and confirm that the transformations occurring in the nanoribbon were indeed due to the migration of phenyl groups. In most cases, the migration only occurs for one phenyl, resulting in the formation of the pore. We could also find that, eventually, the migrated phenyl group does not find a counterpart to close the pore (shown below).

Computational calculations were also conducted to predict the behaviour of atoms during the chemical reactions. Density functional theory (DFT) showed that phenyl migration is thermodynamically feasible within the temperature range used, aligning with the experimental observations.

Figure 4. STM image of a GNR section exhibiting the formation of two pores (1) and an unpaired migrated phenyl (1´). The corresponding structure is shown at the right.

Conclusions and next steps

In summary, this work represents a significant advancement in the on-surface synthesis of graphene-derived nanomaterials. We have successfully developed a new chemical reaction sequence that allows the creation of porous GNRs with atomic precision.

Additionally, the strategy of stepped surfaces could also be applied to other nanomaterial synthesis processes in order to improve reaction yield and assembly precision. This approach could help avoid unwanted interactions between nanoribbons, facilitating the creation of more complex structures. It is a strategy with significant potential.

We firmly believe that our findings could be useful or have a big impact on a wide range of technologies. These include the realization of advanced electronic devices and the development of new filtration and chemical sensing systems.