Self-assembly on graphene

Imagine a sculptor, stood inside his studio, a large block of marble in the centre of the floor. They want to create a statue. They approach the block and start removing pieces, discarding material until, after many hours, they have the finished article.

Now imagine an alternative reality. In this one, the studio floor starts empty. The sculptor is throwing tiny blocks of marble into the middle of the floor. The blocks start to assemble, and again, over many hours, a statue is constructed. In this reality, something has caused the blocks to assemble themselves into a grander structure.

The first of these realities is a an example of what we would call in nanotechnology a top-down approach. We start with a larger object and gradually remove material until we have the finished article. The second reality demonstrates the bottom-up approach, and, while it is unlikely to ever happen in the sculptor’s studio, it is very common when making nanomaterials.

In bottom-up synthesis the interactions between the individual blocks arrange each other into a grander structure. These structures can be used for many applications and their properties are determined by the arrangement of the blocks. To understand the final structure formed in bottom-up synthesis, it is important to understand the processes that make it.

There is a crucial moment in bottom-up synthesis. Imagine the blocks are forming on the floor, one next to the other. At some point, they will start to stack on top of each other. We would call this the 2D to 3D transition. It is flat when it is 2D, and at this transition point it becomes 3D.

This is interesting because lots of information is available about 2D structures formed through bottom-up synthesis. This is driven by the tools we have available for examining them. The best of these is scanning tunnelling microscopy (STM). In STM a sharp tip—so sharp the tip’s end is often a single atom—approaches the surface. We apply a voltage between the tip and the surface and then measure the electrons tunnelling between the two. This tunnelling current is very sensitive to the distance the electrons have to travel, and so STM gives us that important surface structure information. The difficulty arises when that film becomes thicker: that 2D to 3D transition. After a few layers, the information is all mixed and the information is muddled.

This is where other techniques would be used. In transmission electron microscopy (TEM), electrons pass through a material and are collected on the other side. These electrons are affected by their interactions with the material, and so can provide that structural information. TEM samples still need to be thin, but they can be up to 200 nm thick and still give useful information.

There is, however, a challenge to this approach, and its the reason TEM is not that popular for studying bottom-up synthesis. The electrons we use damage the material when they pass through because they have been accelerated to high speeds. This is particularly true for sensitive organic materials, the most commonly used materials for bottom-up synthesis. The damage can be so severe that all the information about the sample is lost almost immediately after being exposed to the energetic electrons.

We have been developing solutions to this problem. We use automation in the TEM to expose the sample to damaging electrons for the shortest possible time. More details of this are available in another post. Here’s how we used it to study bottom-up synthesis.

We were looking at two molecules TMA and TPA, and how they self-assemble on graphene. To start with we looked at how they self-assembled on graphene grown on copper. The graphene-copper surface is ideal for STM because the copper is flat and conducting. The STM showed how TMA and TPA arranged in their 2D structures. These patterns were expected and have been seen by other researchers before.

Scanning tunnelling microscopy of TMA (a) and TPA (c) on graphene. The chemical structures are shown in (b) and (d).

Now we have a look at them in the TEM. We deposited the TMA and TPA onto graphene that is now freely supported on a TEM grid. When we do this for a thin layer we see that the molecules have the same structure as those seen in the STM.

The surprising thing happens when we start to add more molecules onto the graphene. For TMA the diffraction patterns we take look no different: the molecules are arranging into layers just like the first one, and simply stacking on top of each other. The only change is that the patterns become easier to see as there is a greater signal from more molecules.

This is different for TPA: the patterns we measure start to change. Looking further we find that it is forming its bulk structure—that is the structure that large numbers of molecules form when they are allowed to arrange freely. Instead of adding more and more layers, a transition has occurred. The key point here is that TMA does not go through the same transition. It just keeps stacking molecules as before in their layers. It does not do the same transition to its bulk structure as TPA does.

Why does this happen for two molecules that are chemically very similar? We think that the difference lies within the different bulk structures of the two molecules. For TPA, the bulk structure is similar to the one seen for the single layers: the molecules are just tilted and packed closer together. However, for TMA, the bulk structure is distinctly different: it consists of molecules in planes that are interwoven. There is no easy way for the 2D layers to transform into this structure, and so it simply keeps packing layers onto each other.

This result implies that different 3D structures can be made by self-assembly by choosing the molecules in the beginning. The molecules are chemically similar but have a different transition.