Some organic molecules form crystals that have semiconducting behaviour. These crystals could become the foundations of the next generation of electronics, replacing the current silicon-based versions. But organic electronics do not yet perform at the level of the silicon-based counterparts. One route to improving their performance is through control of their crystal structure. To do this, we need accurate and fast methods to measure the structure of organic molecular crystals.
Direct measurement of the crytsal structure can be obtained by microscopical techniques. Transmission electron microscopy (TEM) allows the direct visualisation of individual atoms in a crystal structure. This has been used to improve our understanding of silicon-based materials and has benefitted traditional electronics enormously. Now TEM is being used to understand the structure of organic molecular crystals.
However, there are challenges to overcome. Principle amongst these is the damage caused by the electron beam. This beam is made from electrons that are accelerated to almost the speed of light, and millions of these electrons pass through the sample every second. The electrons interact with the molecules and disrupt its structure, rendering any TEM images useless.
There are ways to get around this problem. One way is to reduce the number of electrons passing through the sample, called the dose. Traditional dose-minimisation involves searching for the sample with a weak beam, where only 100 electrons pass through a square nanometre per second (100 e–/nm2/s). Once an interesting region has been found, the microscope can be focussed on a nearby region with a greater dose of around 10 000 e–/nm2/s. Finally, the beam is moved onto the region of interest, and the images captured.
This has produced many successful images of beam-sensitive materials in the TEM. However, for some of the thin molecular crystals, even the searching dose of 10 e–/nm2/s is enough to destroy the crystals.
Therefore, another approach is needed. This method uses TEM supports consisting of regularly spaced holes and a molecular film that is present uniformly across all the holes. First the microscope is focussed using an already destroyed part of the film. Then the beam is pointed away from the sample and the microscope programmed to move to another hole. After that, once the sample has stopped moving, the beam can be brought back and images instantly captured using the very first electrons that have passed through this part of the sample.
Using this new low-dose TEM imaging we were able to study how the organic molecule vanadyl phthalocyanine (VOPc) grew on graphene.