Graphene liquid cell transmission electron microscopy (TEM) is a technique that can visualize nanoscale dynamics in liquid in real time. The liquid cell TEM technique has bypassed the high vacuum limitations for TEM operation by using electron transparent membranes to encapsulate small volumes of solution. These days, graphene is commonly used as the membrane, which has made atomic resolution imaging of materials in liquids commonplace. This emerging technique has already shown great promise in becoming a powerful characterization tool for the nanomaterials community and beyond, but as with any new technique, careful analysis and understanding of the system are critical for further development.
A major hurdle in the liquid cell TEM technique is understanding and controlling the effects of water radiolysis in the solution from the ionizing electron beam. A variety of reactive species are formed once imaging begins, and understanding how these species interact with the solution and sample is critical to developing this technique. This is a daunting task, as the solution is hermetically sealed within the graphene sheets, the volume within the liquid cell is typically less than an attoliter, and the radical chemistry only occurs when the electron beam is turned on, which greatly limits the methods that can be used to study the solution. Together, these factors make it an extremely challenging task to study the solutionchemistry of graphene liquid cells.
This thesis will give an overview of the solution chemistry thought to occur in graphene liquid cells, describe some experimental techniques that can be used to probe this chemistry, and provide experimental results expanding our understanding of the chemistry occurring in these sealed pockets. In Chapter 1, the development of the liquid cell TEM technique, as well as the fundamentals of radiation chemistry, how it is thought to occur within the context of liquid cell TEM, and the current methods used to understand the radiation chemistry will be provided. In Chapter 2, we studied how halides, common additives in nanocrystal synthesis and well known radical scavengers, altered the dynamics of a model system, gold nanocrystal etching in graphene liquid cells. Correlative pulse radiolysis measurements determined the active oxidant in the gold nanocrystal etching mechanism. These studies revealed the power of using experimental measurements to describe the system. In Chapter 3, electron energy loss spectroscopy was used to directly measure the solvated species inside a graphene liquid cell. These studies revealed significant concentration of the encapsulated solution, which was also shown to affect the kinetics of oxidation of the active oxidant. Overall, using these experimental methods to measure the solution species in graphene liquid cells have greatly enhanced our knowledge of the system, and how to better design the liquid cell around the desired experiments. In Chapter 4, an outlook of the state of graphene liquid cells and suggestions to adapt the design of the system and modeling of the solution to the experimental results presented herein are given.
