A sodium chloride crystal growing in a vibrating carbon nanohorn. Image: © 2021 American Chemical Society.
A sodium chloride crystal growing in a vibrating carbon nanohorn. Image: © 2021 American Chemical Society.

Two novel techniques – atomic-resolution real-time video and conical carbon nanotube confinement – are allowing researchers to view never-before-seen details about crystal formation. These observations, reported in a paper in the Journal of the American Chemical Society, confirm theoretical predictions about how salt crystals form and could inform general theories about the way in which crystal formation produces different ordered structures from an otherwise disordered chemical mixture. 

Crystals include many familiar things, such as snowflakes, salt grains and even diamonds. They are regular and repeating arrangements of constituent molecules that grow from a chaotic sea of those molecules. The process of growth from this disordered state to an ordered one is known as nucleation, and although it has been studied for centuries, the exact goings-on at the atomic level have never been experimentally confirmed, until now.

With crystals, it's not just enough to be able to see molecules at the atomic level, because crystal growth is a dynamic process and observations of its development are as important as observations of crystal structure. Luckily, researchers in the Department of Chemistry at the University of Tokyo in Japan have now resolved this issue with their single-molecule atomic-resolution real-time electron microscopy technique, or SMART-EM, which can capture details of chemical processes at 25 images per second. 

"One of our master's students, Masaya Sakakibara, used SMART-EM to study the behavior of sodium chloride (NaCl) – salt," said project assistant professor Takayuki Nakamuro. "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."

Nakamuro and his team looked at the videos Sakakibara had captured and were the first ever to see tiny cuboid crystals made of tens of molecules of NaCl emerging from the chaotic mixture of separate sodium and chloride ions. Straight away, they noticed a statistical pattern in the frequency at which the crystals emerged; it followed what's known as a normal distribution, which has long been theorized but only now experimentally verified.

"Salt is just our first model substance to probe the fundamentals of nucleation events," said university professor Eiichi Nakamura. "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."

The team doesn't just have diamonds in mind: polymorphism in crystal growth is an essential process in the production of some pharmaceutical and electronic components too.

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.