Since its development over fifty years ago, transmission electron microscopy was generally used for thin, solid samples: liquid samples were incompatible with the vacuum inside the microscope. But now we can use microfabrication techniques to enable electron microscopy to provide high spatial and temporal resolution in liquids. The messy device shown here provided an early electron microscopic view of a materials growth process in water.
Image 1: Early liquid cell experimental set-up
More recent liquid cells consist of two ultrathin SiNx membranes that sandwich the liquid when clamped together in a specially designed sample holder. In situ liquid cell experiments can be applied to study a wide range of phenomena. Seeing every step of a reaction gives us a much better chance of understanding what is going on. Our ultimate goal is to understand materials growth and structure in liquids well enough to control the properties of the materials.
Image 2: Schematic of a typical liquid cell enclosure formed from two window chips
Early liquid cell experiments involved measurements of the electrochemical deposition of copper, the process used to fabricate conductive lines in microelectronic circuits. In the image sequence below, copper (dark areas) grows when a voltage is applied and the electrochemical characteristics (here, voltage and current vs. time) are measured simultaneously.
Image 3: Image series recorded during electrochemical deposition of copper at constant potential. The current during deposition is measured simultaneously.
Liquid cell TEM provides a unique window into many other materials reactions. We find electrochemical reactions particularly interesting: batteries as they charge and discharge, corrosion, and growth of compact layers and dendrites. We are also interested in nanobubble dynamics in water and nanoparticle formation from metal ions in solution. To help quantify observations made in the microscope, we also model the effects on the water itself due to irradiation by the high energy electrons.
Image 4: Liquid cell images of copper electrodeposition showing the onset of instability in the growth front, with a “heat map” showing the growth rate at each time and position
Image 5: Nucleation and growth of bubbles over a 2 second time interval. Bubble dynamics are relevant to catalysis, cavitation and corrosion
Project highlight: Metal nanoparticle dissolution
Inside a liquid cell, we can observe both growth and etching of nanoscale materials. We are particularly interested in the dissolution of alloy metal nanoparticles inside the liquid cell. The figure below shows TEM images of several bimetallic nanoparticles that we prepare using a wet chemical synthesis method then transfer into an electrochemical liquid cell. We can selectively etch away one component by tuning the electrochemical potential of the cell. We are also developing new liquid cell chips with additional capabilities which will open up more opportunities for future in situ studies.
Image 6: TEM images of several bimetallic nanoparticles illustrating selective etching