Berkeley Lab director Paul Alivisatos (left) and physicist Alex Zettl at TEAM I, the world’s most powerful electron microscope, where a they led a collaboration that produced the first atomic-scale, real-time movies of nanocrystal growth in liquids. Photo by Roy Kaltschmidt, Berkeley Lab |
They
won’t be coming soon to a multiplex near you, but movies showing the
growth of platinum nanocrystals at the atomic-scale in real-time have
blockbuster potential. A team of scientists with the Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University of California (UC)
Berkeley has developed a technique for encapsulating liquids of
nanocrystals between layers of graphene so that chemical reactions in
the liquids can be imaged with an electron microscope. With this
technique, movies can be made that provide unprecedented direct
observations of physical, chemical and biological phenomena that take
place in liquids on the nanometer scale.
“Watching
real-time chemical reactions in liquids at the atomic-scale is a dream
for chemists and physicists,” says Jungwon Park, a member of the team
who holds joint appointments with Berkeley Lab’s Materials Sciences
Division and UC Berkeley’s chemistry department. “Using our new graphene
liquid cell, we’re able to capture a small amount of liquid sample
under a high vacuum condition for taking real-time movies of
nanoparticle growth reactions. Since graphene is chemically inert and
extremely thin, our liquid cell provides realistic sample conditions for
achieving high resolution and contrast.”
Park was the lead author, along with Jong Min Yuk, of a paper in the journal Science
that describes this research titled “High-Resolution EM of Colloidal
Nanocrystal Growth Using Graphene Liquid Cells.” The research was done
as a collaboration between the research groups of Paul Alivisatos,
director of Berkeley Lab and UC Berkeley’s Larry and Diane Bock
Professor of Nanotechnology, and Alex Zettl, who holds joint
appointments with Berkeley Lab’s Materials Sciences Division and UC
Berkeley’s Physics Department where he directs the Center of Integrated
Nanomechanical Systems. Both are corresponding authors of the Science paper
along with Jeong Yong Lee of Korea’s Advanced Institute of Science and
Technology (KAIST). Other authors were Peter Ercius, Kwanpyo Kim, Daniel
Hellebusch and Michael Crommie.
In
using a beam of electrons rather than a beam of light for illumination
and magnification, electron microscopes can “see” objects hundreds and
even thousands of times smaller than what can be resolved with an
optical microscope. However, electron microscopes can only operate in a
high vacuum as molecules in the air disrupt the electron beam. Since
liquids evaporate in high vacuum, liquid samples must be hermetically
sealed in special solid containers—called cells—with a viewing window
before they can be imaged in an electron microscope. Until now, such
liquid cells have featured viewing windows made from silicon nitride or
silicon oxide. While this has permitted studies of some nanoscale
phenomena in liquids, the silicon-based cell windows are too thick to
allow strong penetration by the electron beam and this has limited
resolution to only a few nanometers. In addition to not allowing true
atomic-resolution, the thick silicon-based cell windows also appear to
perturb the natural state of the liquid or sample suspended in the
liquid.
“Graphene
is single carbon atom in thickness, making it one of the thinnest known
membranes,” says Park, a member of the Alivisatos’ research group. “It
does not scatter the electron beam but lets it pass through.
Furthermore, graphene is also very strong and impermeable, as well as
being chemically non-reactive, and this helps protects the sample in the
liquid cell from the high-energy beam of an electron microscope.”
To
make their graphene liquid cell, the Alivisatos-Zettl collaboration
encapsulated a platinum growth solution between two laminated graphene
layers that were suspended over holes in a conventional transmission
electron microscope (TEM) grid. The graphene was grown on a copper foil
substrate via chemical vapor deposition and then directly transferred
onto a gold TEM mesh with a perforated amorphous carbon support. The
platinum growth solution was pipetted directly atop two graphene-coated
TEM grids facing in opposite directions.
In the graphene liquid cell, opposing graphene sheets form a sealed liquid nanoscale reaction chamber that is transparent to an electron microscope beam. The cell allows nanocrystal growth, dynamics and coalescence to be captured in real time at atomic resolution via a transmission electron microscope. |
“Upon
wetting the system, the solution wicks between the graphene and
amorphous carbon layers, allowing one of the graphene sheets to detach
from its associated TEM grid,” says co-author Kim, a member of the Zettl
research group. “Because the van derWaals interaction between graphene
sheets is relatively strong, liquid droplets ranging in thickness from
six to 200 nanometers can be securely trapped in a pocket or blister
between the graphene sheets.”
To
test their graphene liquid cells, the collaborators used the world’s
most powerful electron microscope, the TEAM I at the National Center for
Electron Microscopy (NCEM), which is housed at Berkeley Lab. TEAM
stands for Transmission Electron Aberration-corrected Microscope and the
TEAM I instrument is capable of producing images with a half-angstrom
resolution, which is less than the diameter of a single hydrogen atom.
With TEAM I and their new graphene liquid cells, the Alivisatos-Zettl
collaboration was able to directly observe at the highest resolution
possible to date and with minimal sample perturbation, the growth of
nanocrystals of platinum, one of the best metal catalysts in use today.
“Direct
atomic-resolution imaging allowed us to visualize critical steps in the
platinum nanocrystal growth process, including a host of previously
unexpected phenomena, such as site-selective coalescence, structural
reshaping after coalescence, and surface faceting,” says Park.
Three
years ago, Park and Alivisatos were part of a team that used another
TEM at NCEM and liquid cells featuring silicon nitride windows to record
the first ever images of colloidal platinum nanocrystals growing in
solution at subnanometer resolution. Their results showed that while
some crystals in solution grew steadily in size via classical nucleation
and aggregation—meaning molecules collide and join together—others grew
in fits and spurts, driven by “coalescence events,” in which small
crystals randomly collide and fuse together into larger crystals.
Despite their distinctly different growth trajectories, these two
processes ultimately yielded nanocrystals of approximately the same size
and shape.
“In
that earlier study, however, we lacked the resolution to fully
understand how these nanoparticles merge and reorganize their shape in
the coalescence growth trajectory,” Park says. “With the graphene liquid
cells we used in this study, we were able to resolve the oriented
coalescence along a specific crystal direction and see how they
reorganized their overall structure into a final shape.”
With
the graphene liquid cells and the greater resolution of TEAM I, the
Alivisatos-Zettl collaboration was able to observe that most coalescence
events proceed along the same crystallographic direction—the {111}
plane of the crystal. This points to a specific nanocrystal orientation
for coalescence not seen before in metal nanoparticles.
“We
were able to resolve atomistic arrangement at the moment two of the
platinum nanoparticles merged and visualize oriented attachment, a
phenomenon known to be one the of major growth mechanisms of anisotropic
particles,” Park says. “This oriented coalescence could be one of the
formation mechanisms behind another phenomenon we observed, twin
boundaries, which occurs when nanoparticles merge together along the
same {111} direction but on a mirror plane in the crystal.”
In
the future, the collaborators plan to use their graphene liquid cells
to study the growth of many different types of nanoparticles, including
metals, semiconductors and other useful materials. The graphene cells
could also be applied to biomaterials, such as DNA and proteins, which
exist naturally in solution.
“The
one atom thick graphene membranes are ideal for liquid encapsulation,”
says co-author Ercius, the NCEM staff member who ran the TEAM I
microscope for this study. “When combined with the aberration corrected
imaging of TEAM I, we can reach the ultimate in image contrast and
resolution for in-situ liquid experiments. The graphene liquid cell
technique could be easily applied to other electron microscopes and I
think it will become instrumental in answering questions regarding the
synthesis of materials in liquids at the atomic scale.”
This
work was supported by the U.S. Department of Energy’s Office of
Science, and in part through the National Research Foundation of Korea.
High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells
