Torn crystal reveals interlayer van der Waals forces

Tearing apart a crystal made of atomically thin layers has allowed researchers to measure and tweak interlayer van der Waals forces for the first time.

Although intermolecular forces are weak compared with covalent bonds, they allow geckos to stick to windows and are responsible for water’s unusually high boiling point, compared with other group 16 hydrides – a fact that might be the basis for life on Earth. Only last year the van der Waals attraction between individual atoms was measured for the first time.

Now scientists in Japan have measured the van der Waals bonding energy between individual layers of gallium selenide, an atomically thin semiconductor, for the first time using macroscopic physical testing. Previous studies had relied on transmission electron microscopy measurements or had been purely theoretical.

The researchers simply stuck both sides of a crystal consisting of hundreds of gallium selenide layers to the two arms of a tensile testing machine using double-sided tape. The crystal was then very slowly pulled apart at a rate of 50 millionths of a metre per second until it broke.

To increase its tensile strength, the team added tellurium – an element with a larger electron cloud and therefore a better ability to participate in van der Waals bonding – to the material. Pure gallium selenide has an interlayer bonding energy of just 0.023x10⁶N/m². Adding 10% tellurium boosts the van der Waals force seven-fold, though the researchers note that the heavy element also adds covalent and ionic bonding components.