Opening the Van der Waals’ sandwich

Eighty years after the theoretical prediction of the force required to overcome the van der Waals’ bonding between layers in a crystal, engineering researchers at Tohoku University have measured it directly. They report their results this week in the Journal of Applied Physics, from AIP Publishing.

In its proof-of-concept, the team also created more durable gallium selenide crystals. The accomplishment could advance the development of terahertz and spintronics technologies, used in a range of applications from medical imaging to quantum computers.

“This is the first time anyone has directly measured the van der Waals bonding force in the layers of a crystal,” Tadao Tanabe, one of the authors, said. “Even high school students know of this force, but in crystals it was very difficult to measure directly.”

Though considered promising for many technologies, the use of gallium selenide crystals has been hampered by the fact that they’re notoriously fragile. To make them stronger, Tanabe’s team, including Department of Materials Science colleague Yutaka Oyama, imagined growing crystals with small amounts of the selenium replaced with the rare element tellurium.

The researchers surmised that tellurium’s larger electron cloud would produce greater van der Waals’ forces between the crystal layers, strengthening the overall structure. Van der Waals’ are weak electric forces that attract atoms to one another through subtle shifts in the atom’s electron configurations.

The team grew and compared three different types of crystals: one pure gallium selenide, one with 0.6 percent tellurium and one with 10.6 percent tellurium. To test the effect on the tellurium on interlayer bonding, the team invented the equivalent of a crystal sandwich opener. Their system is able to measure with exquisite detail the tensile strength, the force required to pull the crystal until it breaks.

“The tensile testing system is very simple in some ways,” Tanabe said. “But it was very difficult to develop a way to identify the exact point at which the crystal broke.”

The crystals tested were about 3 millimeters in width, and only 1/5 of a millimeter thick, about half the thickness of a piece of standard printer paper. Each crystal is comprised of hundreds of individual layers.

The team used special double-sided tape on either side of a crystal to hold it between an anchored stage and a moveable one that could be pulled away slowly, at a rate of 50 millionths of a meter per second. “This enabled us to very precisely measure the interlayer force at which the crystal broke,” Tanabe said.

The researchers found that the interlayer van der Waals bonding in the tellurium-doped crystals was seven times stronger than in pure gallium selenide ones.

With the addition of tellurium, the soft and cleavable gallium selenide crystal becomes rigid by enhancement of the van der Waals’ bonding force, the authors report, paving the way for using this system to improve crystal-based technologies.

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