Crystal formation is one of the most commonplace processes in the world. For example, ice is crystallized water. And salt crystals grow with just table salt and water. Many familiar things possess crystalline structures, such as salt grains, snowflakes, and diamonds.
Crystals are regular and repeating arrangements of molecules grown from a chaotic sea of those molecules. Yet, how crystals form on the atomic level is poorly understood – notably the first step in the crystallization process (known as nucleation).
How is something so typical still a mystery? Because nucleation is a dynamic process that can be somewhat random, it happens on such small scales. Those factors make it a difficult thing to study.
Crystals themselves have been studied for centuries. But the precise happenings of how the molecules arrange themselves from a messy soup of molecules hasn’t been experimentally confirmed until now. A research team from the University of Tokyo, led by chemist Takayuki Nakamuro, used a process they’ve been developing since 2005 to film the crystallization of salt on the atomic scale for the first time. The observations of the video confirmed theoretical predictions about how salt crystals form.
Their method involves two novel techniques:
- Conical carbon nanotube confinement, a unique sample preparation method;
- Single-molecule atomic-resolution real-time electron microscopy (SMART-EM), which captures details of chemical processes at 25 frames per second.
Project Assistant Professor Takayuki Nakamuro said:
One of our master’s students, Masaya Sakakibara, used SMART-EM to study the behavior of sodium chloride (NaCl) — salt. To hold samples in place, we use atom-thick carbon nanohorns, one of our previous inventions. With the stunning videos Sakakibara captured, we immediately noticed the opportunity to study the structural and statistical aspects of crystal nucleation in unprecedented detail.
The team recorded as the water evaporated from a sodium chloride solution and tens of salt molecules arranged themselves into cube-shaped crystals amid the chaos.
They re-did the same experiment nine times. Every time the molecules arranged themselves in the same way: from featureless to semi-ordered before suddenly forming into a crystal six atoms long by four atoms wide. There was also a statistical pattern in the frequency of crystal formation (something known as normal distribution), averaging 5.07 seconds from start to finish. The phenomenon was a theory that’s now been experimentally verified.
Next, the team plans to study more complex crystallization. They also want to investigate how to control the crystal size and shape.
 University Professor Eiichi Nakamura said:
Salt is just our first model substance to probe the fundamentals of nucleation events. Salt only crystallizes one way. But other molecules, such as carbon, can crystallize in multiple ways, leading to graphite or diamond. This is called polymorphism, and no one has seen the early stages of the nucleation that leads to it. I hope our study provides the first step in understanding the mechanism of polymorphism.
Polymorphism in crystal growth doesn’t only apply to diamonds. It’s an essential process in the production of some electronic components and pharmaceuticals. The better researchers can control the process, the more precisely they’ll create materials for medicine and next-generation technology.
