When needle-shaped microcrystals are irradiated with laser pulses, microscopic fissures open and close themselves as if by magic. A research team in the United States observed the phenomenon using ultrafast electron microscopy. In the future, switchable nanofissures of this kind could play a role in nanoelectronics and for nanoscopic "machines".

Ahmed H. Zewail and his team at the California Institute of Technology in Pasadena discovered that needle-shaped microcrystals made of copper and the organic compound TCNQ, a crystalline quasi-one-dimensional semiconductor, demonstrate special optomechanical phenomena that could prove interesting for nanoelectronic applications: When irradiated with laser pulses, the needles become longer under the microscope, but not wider. As a result, tiny fissures open in the crystals. When the irradiation is switched off, the needles contract and the gaps close again. The phenomenon was reported in the journal "Angewandte Chemie" (Applied Chemistry).

The team of researchers observed the effect with the help of its latest development, ultrafast electron microscopy. It combines a femtosecond optical system with a high-resolution electron microscope. The result is a new instrument with extremely high spatial and temporal resolutions. In 1999, Ahmed H. Zewail received the Nobel Prize for Chemistry for developing ultra-high-speed laser techniques that can be used to observe the movement of individual atoms in a molecule during a chemical reaction.

The change in the microcrystals is particularly visible when one of the needles is broken by the shock of a very strong laser pulse, creating a small crack some ten to one hundred nanometers at the break. When the crystal is irradiated and expands, the nanoscopic channel closes. When the crystal contracts, the channel is there again.

The composition of the crystal is responsible for this effect. The negatively charged TCNQ ions are positioned in it in such a manner that their central, flat, six-membered rings are piled up on top of one another in the long direction of the needle. The energy from the laser pulses excites electrons, and part of this energy is transferred back, creating uncharged TCNQ molecules whose stack arrangement is now unfavorable. That is why they require more space and cause the crystal to grow longer. The degree of stretching depends on the strength of the energy absorbed.