The Nanoaquarium: A New Paradigm in Electron Microscopy

| Since its invention, the electron microscope has facilitated numerous advances in a plethora of disciplines ranging from materials science, physics, and chemistry, to biology. Traditional electron microscopy must be carried out, however, in a high vacuum environment that does not allow for real time imaging of processes in liquid media. Consequently, traditional electron microscopy has been restricted to painstaking “postmortem” investigations on dry or frozen samples without any guarantee that an image is captured at the “right” moment. Static images also do not provide information on process dynamics, and the sample preparation may adversely impact the structure of the object to be imaged. The ability to image dynamic processes in liquid media is certain to be transformative, lead to new discoveries, and provide a better understanding of many important processes at the nanoscale. To overcome the limitations of traditional electron microscopy, there has been a growing interest in recent years in developing means for wet electron microscopy that will allow one to image samples in real time in their native environment and observe processes in situ as they take place. We briefly survey recent efforts pertaining to wet electron microscopy and then describe in greater detail the work of our group with a custom-made, micro-fabricated liquid cell dubbed the “nanoaquarium”. The nanoaquarium sandwiches a thin liquid layer, ranging in thickness from tens of nanometers to a few microns, between two thin, electron-transparent, silicon nitride membranes. The liquid cell is hermetically sealed from the vacuum environment of the electron microscope. The thin liquid layer scatters only a small fraction of the electrons and allows one to image objects suspended in the liquid with high resolution. We describe briefly the imaging of oriented assembly of colloidal crystals, diffusion limited aggregation of nanoparticles, and electrochemical processes. 1Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104– 6315, USA. 2IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA.