Colloidal nanocrystals make their film debut
by Kaitlin Duffey
Most scientists will never be film stars, and neither will their research subjects. But two graduate students at UC Berkeley have come close. By combining two kinds of films—video microscopy, and atomically-thin carbon sheets—Jungwon Park of the chemistry department and Jong Min Yuk of the physics department have opened a new window onto nature, allowing them to record the first-ever movies of atoms forming crystals inside a liquid.
Their camera was a transmission electron microscope, or TEM, which works by focusing a beam of electrons onto a very thin sample. Some electrons are deflected by the atoms of the sample while others pass through to a detector to create an image. TEMs have allowed scientists to see fine details of crystal lattices and biological structures, but its scope has been limited to solid specimens. A liquid sample will evaporate under the vacuum inside a TEM unless it is confined within a tiny compartment called a cell, but the atoms of the cell block the electron beam and blur the image. Biomolecules floating in their natural aqueous environments, for example, have been impossible to view—until now. To create a TEM cell that blocks the minimum number of electrons, Park and Yuk used the slimmest material possible: single-atom-thick sheets of carbon, or graphene.
Graphene liquid cells are an inspiring example of what can be achieved when scientists from different fields come together. Park, a member of Professor Paul Alivisatos’s group, was studying the mechanism of colloidal (liquid-suspended) nanocrystal growth using a TEM, but the 25-nanometer-thick silicon nitride cells he was using did not allow him to see individual atoms. Yuk, a visiting scholar in Professor Alex Zettl’s group, was experimenting with sandwiching solid materials between layers of graphene. The two met fortuitously on the soccer field.
“After playing soccer, I told Jungwon about my experiments and he suddenly became very excited,” Yuk recalled. Through subsequent conversations, they developed the idea of covering liquid droplets with graphene.
“We started dreaming about putting a liquid sample in between two layers” of graphene, said Park. “We thought it would be really difficult, but it was worth trying.” With Park’s intuition for colloidal liquids and Yuk’s expertise in making graphene, implementing their idea was easier than they expected.
The resulting experiment was strikingly elegant. Drops of platinum solution—the nanocrystal precursor—were placed onto two sheets of graphene. When one sheet was overlaid upon the other the drops were trapped between them, forming tiny liquid-filled blisters. When the TEM was focused onto one of these blisters, the electron beam turned positively-charged platinum ions into neutral atoms, initiating crystal growth. The transparent graphene skin allowed this growth to be imaged with unprecedented resolution.
Park and Yuk used the world’s highest-resolution microscope, the Transmission Electron Aberration-Corrected Microscope I at Lawrence Berkeley National Laboratory, to film their graphene-encased crystals. Many hours were spent depositing graphene layers onto the microscope stage, scanning the stage to find a cell, focusing the beam, and hoping to see crystal formation. Initially, Yuk had doubts about whether they could zero in on a single crystal inside the relatively large bubble of liquid. “I was afraid,” he said, “that since we focused in the center of the bubble, we wouldn’t be able to see a nanoparticle that was slightly above or below.”
When they finally captured images of a crystal forming, they were exhilarated. “We kept repeating the process for 16 hours, and after all that effort we ended up with a perfect growth movie. . . . It was a really exciting moment,” said Park.
Yuk shook his head and smiled. “I was so surprised. I thought, ‘I’m the first man to watch the actual growth of particles inside a liquid!’ They looked like bacteria, moving around and then coming together. It was amazing!”
The movies provided direct observational insight into how quickly colloidal nanocrystals grow, how their shapes evolve, and how small crystals came together to form larger ones. “Before, chemists just made particles inside a flask, pointed to it, and said ‘there’s a nanocrystal in there,’ but no one really knew what was going on inside the flask. Graphene is a great viewing window, just like a fishbowl. Now we can watch the fish inside the bowl,” explained Yuk.
Nanotechnology is not the only field that will benefit from the invention of graphene blisters. Any liquid can be encased in graphene and observed at the atomic scale using a TEM. Park and Yuk’s work was published in Science on April 6, 2012, and since then many other research groups have begun using graphene liquid cells. Members of the Alivisatos and Zettl groups have continued to collaborate in exploring new applications of the cells. They have succeeded in imaging aqueous solutions, and have begun working with biological samples. The next graphene-clad movie stars may be proteins folding or DNA strands replicating . . . stay tuned!