Light and electron microscopy unite for new view of nanoparticles


Rebecca Pool

Thursday, November 8, 2018 - 11:45
TEM image of a palladium nanocube with a gold disc that promotes never-before-seen photochemical reactions [Michal Vadai]
Stanford University researchers have developed a custom specimen holder to couple light into and out of their environmental TEM, and study individual catalysts with nanometre resolution.
Despite heavy scepticism from the microscopy community, Professor Jennifer Dionne and colleagues are combining light microscopy and transmission electron microscopy, to study single nanoparticles undergoing a light-activated reaction.
They have already observed a photochemical reaction sweeping across a palladium nanoparticle.
As Dionne highlights: "It's kind of a tool that brings together the best of both optical and electron microscopy."
"We bring light of various 'colours' to the electron microscope," she adds. "Our measurements are direct; one can visibly see the photochemical reaction as it unfolds within the nanoparticle."
As part of their research, Dionne and colleagues wanted to look at a photocatalytic reaction, in which visible light initiates a chemical reaction in individual nanocubes of palladium, with a 30 nm side-length.
To study this, they joined forces with Gatan to design a TEM cathodoluminescence cryogenic holder that fitted within the 5 mm pole piece gap of their FEI Titan 80–300 environmental STEM.
As part of the holder set-up, the sample is surrounded with parabolic mirrors to focus light onto it, via optical fibres.
The mirrors also include a 500 μm wide aperture to allow the electron beam to pass while light is focused onto the sample.
To test the setup, Dionne and colleagues coupled a palladium nanocube to a gold nanodisc, which acted as an antenna to promote a chemical reaction with light.
The researchers then placed the palladium nanocube-gold nanodisc pair into the sample holder, pumped hydrogen into the specimen chamber, and observed the palladium nanocube change phase. 
They then excited the nanocube with light, and watched as a water-like wave flowed gracefully across the particle - the hydrogen leaving the palladium -  to sub-2 nm spatial resolution.
“I cannot stress how exciting it was to make it work the first time. It was a huge technological challenge,” says Michal Vadai from the Dionne Group. “The first time we got the beginning of an experimental result, we were shouting out loud. It was very, very exciting that we could see and control what was happening to this nanoparticle with light.”
Observations revealed that this photochemical reaction takes place ten times faster under light.
The researchers could also see how each step of the reaction was affected by different wavelengths of light, including the hydrogen leaving the nanocube and the lattice structure of the nanocube rearranging.
This video, captured by the Dionne lab using a transmission electron microscope, shows palladium nanocubes changing phase - losing hydrogen - after being illuminated. 
"With this prototype set-up, we've been able to show how illumination at different wavelengths can modify the rate and mechanism of a reaction... eventually we might be able to use light to control which products of a photochemical reaction are generated," says Dionne.
The researchers now aim to add spectroscopy capabilities to their set-up.
“If you’re talking about a single particle, you usually have to fight to see these weak signals,” says Vadai. “Looking forward, this will be a complete suite of tools that you can use to study interaction of light and matter in the nanoscale in real time, at very high resolution, at a single-particle level.”
Research is published in Nature Communications.
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