Berries, kale, and green tea – these foods have been touted for their health benefits because of their high amounts of antioxidants, compounds that prevent oxidation and in turn, prevent disease. When free radicals – unstable molecular species with an unpaired electron – accumulate in the body, they can cause a state of oxidative stress, which is linked to chronic diseases such as heart disease and cancer. Consuming foods that are high in antioxidants is thought to help neutralize free radicals to reduce the risk of these diseases.
It turns out that the same concept can be applied to polymer electron microscopy. When imaging soft materials such as polymers, electron beam damage limits the resolution. One theory for the damage mechanism is that when the electron beam interacts with the polymer, it creates free radicals (white circles below), which then diffuse around and cause further damage to the material. In our work, we show that the addition of antioxidants (green circles) into a polymer specimen can neutralize these free radicals, thus minimizing beam damage and improving resolution.
In particular, the polymers investigated in this work are solution-processable conjugated polymers, which have applications in many optoelectronics such as organic light emitting diodes, organic photovoltaics, organic field-effect transistors, and bioelectronics. To date, imaging of this class of polymers has been limited to the 16-22 angstrom lamellar stacking, or the distance between polymer chains lying flat next to each other. By adding antioxidants to reduce damage, we were able to push the resolution limit to 3.6 angstroms, enabling imaging of π-π stacking, the distance between chains stacked on top of each other. By opening the doors to imaging π-π stacking, an important pathway for charge transport, we hope that this work will lead to improved investigations into the relationship between electron transport and morphology in organic electronics.
Until now, the most common strategy for minimizing beam damage in soft materials has been imaging at cryogenic conditions. Our work demonstrates that the addition of antioxidants can reduce damage at room temperature, which could lead to opportunities in high resolution in situ experiments of soft matter.
It is our hope that this work can improve resolution in different soft materials systems while also inspiring further work on the potential of additives as a solution to electron beam damage.