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, the atoms come together and disintegrate several times before stabilizing into a crystal.
This discovery has implications for materials science as well as manufacturing, as it strengthens our understanding of how materials come together from a messy stack of atoms.
“As scientists try to control matter at smaller length scales to produce new materials and devices, this study helps us understand exactly how some crystals are formed,” explains physicist Peter Ercius of Lawrence Berkeley National Laboratory.
According to the classical understanding of nucleation – the very first part of crystal formation, in which atoms begin to assemble themselves – the process is quite linear. You put a number of atoms together under the right conditions, and they will gradually build themselves into a crystal.
However, this process is not easy to observe. It is a dynamic process that takes place on an extremely small scale, both spatially and temporally, and often involves an element of randomness. But our technology has improved so much that we can now observe processes on an atomic scale.
Just earlier this year, a team of Japanese scientists revealed they could have observed 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 gold.
On graphene-supporting films, the team grew tiny nanoribbons of gold cyanide, 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 surprising. Gold atoms would converge in a crystal configuration, disintegrate and reassemble in another configuration, repeating the process several times, fluctuating between disordered and crystalline states before stabilizing.
It is actually no different from what the Japanese scientists observed with the salt crystals; those atoms also fluctuated between characterless and semi-ordered states before converging in a crystal. But that process was filmed at 25 fps; the gold atoms fluctuated much, much faster.
Only the detector speed of 625 fps had hope of catching it, Ercius said.
“Slower observations would miss this very fast, reversible process and just see a blur instead of the transitions,” he said.
So what does it cause? Warmth. Nuclear formation and crystal growth are exothermic processes in which energy is released to their environment in the form of heat. Think of a very tiny bomb. This repeatedly melts the crystal configurations trying to reform.
But the reform process is not aided by the recurring collisions of incoming atoms that dynamically disrupt the cluster of atoms. Ultimately, however, the atoms come together in a way that can withstand the heat they release.
And there you go! We have a stable gold crystal on which more atoms can build without falling back into the disordered state.
“We found that crystal nucleation of gold clusters on graphene proceeds through reversible structural fluctuations between disordered and crystalline states,” the researchers wrote in their paper.
“Our findings clarify the fundamental mechanisms underlying the nucleation phase of material growth, including thin film deposition, interfacial-induced precipitation, and nanoparticle formation.”
Their next step is to develop an even faster detector in the hope of finding even more hidden nucleation processes.
The team’s research is published in Science