Now we know how gold crystals begin to form on an atomic scale.
For the first time, scientists noticed – and filmed! – the first milliseconds of the formation of gold crystals and found that it is much more complicated than previous research has suggested. Rather than 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 production, as it strengthens our understanding of how materials come together in a disordered pile of atoms.
“As scientists try to control matter at shorter scales to produce new materials and devices, this study helps us understand exactly how some crystals form,” said physicist Peter Ercius of Lawrence Berkeley National Laboratory. .
According to the classical understanding of nucleation – the first part of the formation of crystals, in which atoms begin to self-assemble – the process is quite linear. Put a bunch of atoms together in the right conditions and they will gradually build 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 a random element involved. But our technology has improved to the point where we can now observe atomic-scale processes.
Earlier this year, a team of Japanese scientists revealed that they could observe the nucleation of salt crystals. Now, a Korean and American team led by engineer Sungho Jeon from Hanyang University in the Republic of Korea has done the same with gold.
On graphene backing films, the team developed small gold cyanide nanoribones, using one of the most powerful electron microscopes in the world to observe it, TEAM I. Berkeley Lab 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 surprising. Gold atoms come together in a crystalline configuration, disintegrate and reunite in a different configuration, repeating the process several times, fluctuating between disordered and crystalline states before stabilization.
It’s no different from what Japanese scientists have observed about salt crystals, in fact; these atoms also fluctuated between uncharacteristic and semi-ordered states before uniting in a crystal. But this process was filmed at 25 fps; gold atoms fluctuated much, much faster.
Only the speed of the 625 fps detector was hoping to catch it, according to Ercius.
“Slower observations would lose this reversible process very quickly and would only see a blurring instead of transitions,” he said.
So what causes it? The heat. Nucleation and crystal growth are exothermic processes that release energy in the form of heat in their surroundings. Think of a tiny bomb. This repeatedly melts the crystal configurations, which are trying to reform.
But the reforming process is not helped by the recurring collisions of the input atoms that dynamically disrupt the group of atoms. Eventually, however, the atoms come together in a way that can withstand the heat released by them in doing so.
And here you are! We have a stable gold crystal on which several atoms can be built without collapsing back into a disordered state.
“We found that the crystalline nucleation of gold groups 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.
Their next step is to develop an even faster detector in hopes of finding even more hidden nucleation processes.
The team ‘s research was published in Science.