Inside a quantum dot

Until now, physicists who wanted to understand how electrons behaved at the nanoscale needed to choose between instruments which had good spatial resolution or fast time resolution, but not both. But researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a new machine able of tracking electrons at trillionths of a second.

Until now, physicists who wanted to understand how electrons behaved at the nanoscale needed to choose between instruments which had good spatial resolution (down to tens of nanometers or below) or fast time resolution (down to picoseconds), but not both. But researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a new machine able of tracking electrons at trillionths of a second. This system can work with any semiconductor and may lead to new discoveries in physics of nanoscale phenomena.

This system took four years of development to a team led by Benoît Deveaud-Plédran of EPFL's Laboratory of Quantum Optoelectronics (LOEQ). Here is what the researchers did.

The EPFL researchers replaced the standard electron gun filament on an off-the-shelf electron microscope with a 20 nanometer-thick gold photocathode. The gold is illuminated by an ultraviolet mode-locked laser, generating an electron beam that pulses 80 million times per second. Each pulse contains fewer than 10 electrons. The electrons excite the sample, causing it to emit light. The spectroscopic information is collected and analyzed to recreate the surface morphology and to trace the path the electrons follow through the sample.

Below is a diagram of the EPFL device showing its resolution in space and time (Credit for image and caption: EPFL, via Nature).

The EFL device for tracking electrons

a, Schematic implementation of our time-resolved cathodoluminescence system. b, Secondary electron images of gold nanoparticles deposited on an amorphous carbon film. The spatial resolution on such a specimen is estimated to be better than 50 nm. The spatial resolution in secondary electron mode is mainly determined by the size of the electron probe onto the specimen. c, Time-resolved spectra of a GaN specimen at room temperature. From the rise time of the signal, the temporal width of the electron pulse is estimated to be about 10 ps.

Deveaud-Pledran and his colleagues tested their new machine on pyramidal gallium-arsenide quantum dots and you can see below some of these nanopyramids (Credit: EPFL).

Nanopyramids of gallium arsenide

This research work has been published by 'Nature' under the title "Probing carrier dynamics in nanostructures by picosecond cathodoluminescence" (Volume 438, Number 7067, Pages 479-482, November 24, 2005). Here is a link to the first paragraph.

As I wrote above, this new machine might lead to a better understanding of the dynamics of materials at the nanoscale. But let's finish with some words from Deveaud-Pledran.

I can't tell you exactly what this machine will lead to because that depends on who uses it and what we find. But there's no question that it will help us make progress, and that the potential applications are exciting.

So, if even the research leader isn't so sure of what can be achieved with this machine, who can?

Sources: EPFL news release, via EurekAlert!, November 23, 2005; and various web sites

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