Electron Microscopy Images Working Nanodevices

Daniel S. Burgess

To enable a better understanding of the behavior of electronic and mechanical systems constructed of nanoscale parts, a research team at the University of California, Berkeley, and at Lawrence Berkeley National Laboratory, also in California, has developed a sample-handling approach for transmission electron microscopy that makes possible the live imaging of nanotube-based devices in operation. The method may affect photonics in the development of semiconductor fluorescent probes for biology applications and of LEDs and photodetectors based on nanostructures.

In a demonstration, the scientists fabricated an electron-transparent membrane by growing a 500- to 800-nm-thick layer of SiO on a silicon wafer. They then deposited a 10- to 20-nm-thick layer of Si₃N₄ atop the oxide and used chemical etches on the back side to selectively take away the silicon and to remove the exposed SiO. Nanostructure-based devices may then be placed on the Si₃N₄ layer that is left behind, against which they appear in relief under an electron microscope.

Although Si₃N₄ membranes have been used previously for imaging nanostructures, the investigators note that they have developed a means to incorporate them into functioning devices.

To illustrate the potential of the approach, they imaged 9.5-nm-diameter, 12-wall carbon nanotubes as they were thinned with exposure to increasing voltage. As the voltage increased from 200 mV to 2.56 V, individual walls of the tube would fail, causing the current passing through the tubes to decrease in regular steps of approximately 13.5 μA with each structural failure.

Conduction model

The observations suggest that current in a multiwall carbon nanotube is carried neither solely in the outermost wall nor equally in all the walls. Rather, the researchers propose, conduction in the nanostructures in the high-bias limit is better modeled as involving a hollow tube of bulk material.

Applied Physics Letters, Aug. 22, 2005, 083103.