We now know how gold crystals begin to form on an atomic scale.
For the first time, scientists have observed and filmed. – the first few milliseconds of gold crystal formation and found it to be much more complicated than previous research suggested. Instead of a single irreversible transition, the atoms come together and fall apart several times before settling into a crystal.
This discovery has implications for both materials science and manufacturing, as it strengthens our understanding of how materials come together from a messy pile of atoms.
“As scientists seek to control matter at shorter scales to produce new materials and devices, this study helps us understand exactly how some crystals form,” explained physicist Peter Ercius of the Lawrence Berkeley National Laboratory.
According to the classical understanding of nucleation, the first part of crystal formation, in which atoms begin to self-assemble, the process is quite linear. If you put a bunch of atoms together in the right conditions, they will gradually turn into a crystal.
However, this process is not easy to observe. It is a dynamic process that occurs on extremely small scales, both spatially and temporally, and often has an element of randomness involved. But our technology has improved to the point that we can now observe processes at the atomic scale.
Earlier this year, a team of Japanese scientists revealed that they had been able to observe the nucleation of salt crystals. Now, a Korean and American team led by engineer Sungho Jeon of Hanyang University in the Republic of Korea has done the same with the gold.
On graphene support films, the team grew tiny gold cyanide nanoribbons, using one of the world’s most powerful electron microscopes to observe it, Berkeley Lab’s TEAM I. At speeds of up to 625 frames per second (fps), extremely fast for electron microscopy. – TEAM I captured the first few milliseconds of nucleation in incredible detail.
The results were amazing. The gold atoms would come together in a crystalline configuration, disintegrate and come back together in a different configuration, repeating the process several times, fluctuating between disordered and crystalline states before stabilizing.
It is not very different from what the Japanese scientists observed with the salt crystals, actually; Those atoms also fluctuated between featureless and semi-ordered states before merging into a crystal. But that process was shot at 25 fps; the gold atoms fluctuated much, much faster.
According to Ercius, only the detector’s speed of 625 fps had any hope of catching him.
“The slower observations would lose this reversible process very quickly and would only see a blur instead of the transitions,” he said.
So what causes it? Hot. Crystal nucleation and growth are exothermic processes, releasing energy in the form of heat into their surroundings. Think of a really tiny bomb. This repeatedly melts the crystal configurations, which are trying to reshape.
But the reform process is not favored by recurring collisions of incoming atoms that dynamically disrupt the group of atoms. However, eventually, the atoms bond together in a way that can withstand the heat released by doing so.
Et voila! We have a stable gold crystal in which more atoms can be built without collapsing back to the disordered state.
“We found that the crystalline nucleation of gold clusters in graphene progresses through reversible structural fluctuations between the disordered and crystalline states,” the researchers wrote in their paper.
“Our findings clarify the fundamental mechanisms underlying the nucleation stage of material growth, including thin-film deposition, interface-induced precipitation, and nanoparticle formation.”
His next step is to develop an even faster detector in the hopes of finding even more hidden nucleation processes.
The team’s research has been published in Science.