We now know how gold crystals begin to form on an atomic scale.
For the first time, scientists have observed and filmed! – The first milliseconds of gold crystal formation and found it to be much more complicated than previous research suggested. Instead of a single, irreversible transition, atoms come together and disintegrate several times before stabilizing in a crystal.
This discovery has implications for both materials science and manufacturing, as it reinforces our understanding of how materials come together from a stack of disordered atoms.
“While scientists are looking 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 the formation of crystals, in which atoms begin to assemble, the process is quite linear. Group a lot of atoms in the right conditions and they will turn into a crystal.
This process, however, is not easy to observe. It is a dynamic process that occurs on extremely small scales, both spatially and temporally, and often has a random element. But our technology has improved to the point that we can now observe processes on an atomic scale.
Just earlier this year, a team of Japanese scientists revealed that they had been able to observe the nucleation of salt crystals. Now, an American and Korean team led by engineer Sungho Jeon of Hanyang University in the Republic of Korea has done the same with gold.
In graphene support films, the team cultured tiny gold cyanide nanoribons, 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 milliseconds of nucleation in incredible detail.
The results were amazing. The gold atoms would come together in a crystal configuration, disintegrate and reunite in a different configuration, repeating the process several times, fluctuating between disordered and crystalline states before stabilizing.
It’s no different than what Japanese scientists actually observed with salt crystals; these atoms also fluctuated between uncharacteristic and semi-ordered states before meeting in a crystal. But this process was filmed at 25 fps; the gold atoms fluctuated much, much faster.
According to Ercius, only the 625 fps detector speed was hoping to catch her.
“A slower observation would miss this very fast reversible process and I would only see a blur instead of transitions,” he said.
What causes it, then? Heat. Nucleation and crystal growth are exothermic processes, which release energy in the form of heat into their environment. Think of a tiny little bomb. This repeatedly melts the crystal configurations, which attempt to reform.
But the reform process is not aided by recurring collisions of incoming atoms that disrupt the cluster of atoms dynamically. Eventually, however, the atoms come together in a way that can withstand the heat released by them.
And there! We have a stable gold crystal on which more atoms can be built without collapsing again in a disordered state.
“We have found that crystalline nucleation of gold clusters on graphene progresses through reversible structural fluctuations between 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.”
The next step is to develop an even faster detector in hopes of finding even more hidden nucleation processes.
The team’s research has been published in Science.