AsianScientist (Aug. 11, 2014) – Scientists have utilized silver nanoparticles to boost the signal of a highly sensitive electron microscopy technique nearly twenty-fold, enabling new methods of probing matter on the atomic scale. This research has been published in Nature Physics.
In the technique known as electron energy loss spectroscopy, electrons are fired at a sample and then collected after they have lost energy in inelastic collisions. The energy lost by each individual electron depends not only on the atoms hit, but the state they were in, allowing scientists to probe atomic details such as how atoms of different elements arrange themselves in various layers of a crystal and how they interact with their neighbors.
However, electron energy loss spectroscopy is hampered by how rarely these energy-losing inelastic collisions occur. In any given experiment the majority of electrons that hit a target will collide elastically, bouncing off at exactly the same speed and energy they previously had, while only two to five percent of electrons lose energy and undergo inelastic collisions.
Professor Chen Xiang Jun and his team at the University of Science and Technology of China have now succeeded in raising the proportion of inelastic collisions to as high as 40 percent, using their custom-built electron probe device.
In their device, energetic electrons are emitted from a charged tungsten tip held just micrometers above the surface of their sample. After the electrons strike the sample and recoil, they are collected by an analyser which sorts them by energy, allowing the team to determine what proportion of electrons underwent inelastic collision and how much energy was lost.
The secret to increasing the signal, however, lay in the team’s choice of target, as Prof. Chen explains: “When we used silver nanoparticles, we found that incoming electrons could readily excite the collective oscillation of electrons, known as ‘surface plasmon resonance’, thus losing some of their energy and undergoing an inelastic collision.
“The strength of the effect increased according to the square of the voltage used on the tungsten tip, meaning that a comparatively small increase in voltage yielded a large increase in the signal. This was confirmed when thicker layers of silver, which normally have smooth surfaces instead of nano-sized bumps and gaps, failed to show the same effect,” he said.
This novel technique, which they have titled “nonlinear electron scattering spectroscopy”, could see wide application in studying nature at the atomic scale. Its high sensitivity would allow researchers to measure the electronic structures of single atoms or molecules deposited on the metal surfaces. This would not only help in understanding adsorption better in general, but also aid in designing better industrial catalysts. The enhanced inelastic electrons could also be used in nano-lithography, or printing and fabricating at a very small scale.
The article can be found at: Xu et al. (2014) Nonlinear Inelastic Electron Scattering Revealed by Plasmon-enhanced Electron Energy-loss Spectroscopy.
Copyright: Asian Scientist Magazine; Photo: Chen Xiang Jun.
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