  The understanding of many nanoscale processes occurring in liquids such as
colloidal crystal formation, aggregation, nanowire growth, electrochemical
deposition, and biological interactions would benefit greatly from real-time,
in situ imaging with the nanoscale resolution of transmission electron
microscopes (TEMs) and scanning transmission electron microscopes (STEMs).
However, these imaging tools cannot readily be used to observe processes
occurring in liquid media without addressing two experimental hurdles: sample
thickness and sample evaporation in the high vacuum microscope chamber. To
address these challenges, we have developed a nano-Hele-Shaw cell, dubbed the
nanoaquarium. The device consists of a hermetically-sealed, 100 nm tall,
liquid-filled chamber sandwiched between two freestanding, 50 nm thick, silicon
nitride membranes. Embedded electrodes are integrated into the device. This
fluid dynamics video features particle motion and aggregation during in situ
STEM of nanoparticles suspended in liquids. The first solution contains 5 nm
gold particles, 50 nm gold particles and 50 nm polystyrene particles in water.
The second solution contains 5 nm gold particles in water. The imaging was
carried out with a FEI Quanta 600 FEG Mark II with a STEM detector. In the
footage of the multi-particle solution, note that the 50 nm gold particles
prominently decorate the clusters and are clearly distinguished. In the footage
of the 5 nm gold particles, diffusion-limited aggregation is observed.
Individual particles and small clusters are seen diffusing throughout the field
of view, bumping into each other and bonding irreversibly to form a fractal
structure. The rate of aggregation and the fractal dimension of the aggregates
are consistent with light scattering measurements, indicating that the electron
beam does not greatly alter the observed phenomenon.
