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TAP TIMES -21- SEPTEMBER 2022
Nanoparticle growth determined
by defects, not difference in size
BERKELEY, Calif. — For decades, a
textbook process known as “Ostwald
ripening,” named for the Nobel Prize-
winning chemist Wilhelm Ostwald,
has guided the design of new materi-
als including nanoparticles – tiny ma-
terials so small they are invisible to
the naked eye.
According to this theory, small particles
dissolve and redeposit onto the surface of
large particles, and the large particles con-
tinue to grow until all of the small particles
have dissolved.
Vido demolishes old belief
But now, new video footage captured
by Berkeley Lab scientists reveals that
nanoparticle growth is directed not by dif-
ference in size, but by defects. The sci-
entists recently reported their findings in
Nature Communications.
“This is a huge milestone. We are rewriting
textbook chemistry, and it’s very exciting,”
said senior author Haimei Zheng, a senior
scientist in Berkeley Lab’s Materials Sci-
ences Division and an adjunct professor of
materials science and engineering at UC
Berkeley.
For the study, the researchers suspend-
ed a solution of cadmium sulfide (CdS)
nanoparticles with cadmium chloride
(CdCl2) and hydrogen chloride (HCl) in a
custom liquid sample holder.
The researchers exposed the solution with
an electron beam to produce Cd-CdCl2
core-shell nanoparticles (CSNPs) — which
look like flat, hexagonal discs – where cad-
mium atoms form the core, and cadmium
chloride forms the shell.
Using a technique called high-resolution
liquid cell transmission electron microsco-
py (LC-TEM) at the Molecular Foundry, the
researchers captured real-time, atomic-
scale LC-TEM videos of Cd-CdCl2 CSNPs
ripening in solution.
Key experiment
In one key experiment, an LC-TEM vid-
eo shows a small Cd-CdCl2 core-shell
nanoparticle merging with a large Cd-Cd-
Cl2 CSNP to form a larger Cd-CdCl2 CSNP.
However, the di-
rection of growth
was guided not
by a difference
in size but by a
crack defect in
the shell of the
Dr. Heimel Zheng, LBL initially larger
CSNP.
“The finding was very unexpected, but
we’re very happy with the results,” said
Qiubo Zhang, first author and postdoctoral
researcher in the Materials Sciences Divi-
sion.
Highest resolution recorded
The researchers say that their work is the
highest resolution LC-TEM video ever re-
corded.
The advance — monitoring how nanopar-
ticles ripen in solution in real time — was
enabled by a custom-made, ultrathin “liq-
uid cell” that secures a tiny amount of liq-
uid between two carbon-film membranes
on a copper grid.
The researchers observed the liquid sam-
ple through ThemIS, a specialized elec-
tron microscope at the Molecular Foundry
that is capable of recording atomic-scale
changes in liquids at a speed of 40-400
frames per second. lbl.gov