Tracking Electrons at Trillionths of a Second

Even the most sophisticated methods used to explore material properties and dynamics run into limits when applied at the nanoscale. Current techniques either have good spatial resolution or an ultrafast time resolution, but not both.

The machine developed by Professor Benoit Deveaud-Pledran and his EPFL colleagues is the first tool that can track the passage of an electron in a nanostructure – at a time scale of ten picoseconds and a spatial resolution of 50 nanometers.

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.

Deveaud-Pledran and his colleagues tested their new machine on pyramidal quantum dots. These 2-micron-high nano-objects contain several different nanostructures, making them ideal test objects. When the electron beam impacts the pyramid, the electrons diffuse towards the closest nanostructure. From there, the diffusion continues until the point of lowest energy is reached - the quantum dot at the tip of the pyramid. The time traces corresponding to each of these nanostructures reveal just how critical that 10- picosecond time resolution is; with even a 100-picosecond resolution, important information would be lost.

The machine will not only give us a glimpse into nanoscale dynamics, but because it will work on any semiconductor, it will also allow researchers to study previously intractable materials.; Source: Ecole Polytechnique Fédérale de Lausanne