Ultrafast Electron Microscopy Reveals Laser Control of Nanochannels

David L. Shenkenberg

Scientists are interested in optoelectronics applications, such as optical memory storage, and in molecular machines that mimic biological molecules, which usually exhibit precision superior to that of man-made devices. These diverse fields may benefit from the fact that a laser can open and close with precision tiny rod-shaped channels made of certain crystals, a discovery made by Nobel laureate Ahmed H. Zewail and members of his laboratory at California Institute of Technology in Pasadena.

Using ultrafast electron microscopy, researchers watched as short laser pulses with high fluence opened channels 10 to 100 nm across in these rod-shaped crystals. Using longer pulses with lower fluence, they closed the nanochannels. Reprinted with permission of Angewandte Chemie.

To observe these phenomena, the researchers used ultrafast electron microscopy, a technology that they developed. Instead of using a conventional electron gun, ultrafast electron microscopy uses the pulse train from the femtosecond laser as the source of electrons that probe the sample of interest. The result is a transmission electron microscope that not only has high spatial resolution but also high temporal resolution, owing to the femtosecond laser. The 776-nm laser pulses used to open and close the nanochannels coincided in space and time with the electron pulses.

The researchers used ultrafast electron microscopy to investigate nanochannels forming in semiconductor crystals of copper and TCNQ (7,7,8,8-tetracyanoquinodimethane, C₁₂H₄N₄). The crystals are shaped like long, thin rods or needles, seemingly with only length and no width or depth, or are quasi-one-dimensional.

When the scientists focused short laser pulses with high fluence on the crystals, they observed the formation of nanochannels — small openings that measured 10 to 100 nm across — whereas longer pulses with lower fluence caused the crystals to stretch until the channels closed. The researchers could open and close the nanochannels repeatedly and reversibly. At much higher fluences than those used to open the nanochannels, the copper separated from the TCNQ molecules and was deposited in isolated spots.

The nanochannels open because negatively charged TCNQ molecules, which normally resemble stacks of rings, become excited by the laser pulse, causing energy transfer that leaves the stacked molecules uncharged. When uncharged, stacking becomes unfavorable, and the crystals move outward, leaving a nanochannel. The size of the channel depends on the fluence of the laser pulse.

The researchers concluded that this work will enable numerous studies, including nucleation, charge and energy transport, and ultra-fast dynamics of dislocations of coherent nuclear motions, broadly spanning disciplines from materials science and nanotechnology to biology.

Angewandte Chemie, Dec. 10, 2007, pp. 9206-9210.