Researchers at the University of Tokyo have combined novel techniques to record the first-ever atomic resolution video of salt crystals as they form in real-time. The team used the novel technique of atomic-resolution real-time video recording and the similarly novel technique of conical carbon nanotube confinement to achieve this impressive feat.

Before diving in, it’s important to give context. Salt crystals, like other crystals such as snowflakes and diamonds, comprise repetitive arrangements of molecules. While crystals can take many different forms, something they all have in common is a highly ordered microscopic structure. The process of disorganized molecules arranging into an ordered crystalline structure is called nucleation. The time it takes for the first crystal to appear in this process is called primary nucleation time. Secondary nucleation is a new crystal structure being produced by a preexisting crystal.

Nucleation has been the subject of scientific study for centuries. Scientists have been able to observe molecules at an atomic level and see what a crystal looks like. Still, until now, nobody has observed the dynamic process of crystal formation. Observing the nucleation process should shed light on the process and help expand our understanding of molecular structure and crystallization.

Credit: American Chemical Society / University of Tokyo

‘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.’

SMART-EM is a single-molecule atomic-resolution real-time electronic microscopy technique developed by students at the Department of Chemistry at the University of Tokyo. The technique captures images at 25 frames per second.

Nakamuro and his team looked at Sakakibara’s videos and, per, ‘were the first people ever to see tiny cuboid crystals made of tens of molecules of NaCl emerging from the chaotic mixture of separate sodium and chloride ions.’ They observed a statistical pattern in the frequency of crystal emergence that followed a normal distribution. A normal distribution of crystal emergence had long been a held theory but had yet to be verified through experiment.

Credit: American Chemical Society / University of Tokyo

University Professor Eiichi Nakamura added, ‘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.’

The team hopes to better understand polymorphism, which is an important process to produce various pharmaceutical and electronic components. To read more about the ongoing study, refer to ‘Capturing the Moment of Emergence of Crystal Nucleus from Disorder,’ by Takayuki Nakamuro, Masaya Sakakibara, Hiroki Nada, Koji Harano and Eiichii Nakamura.

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