Metal nanostructures have unique optical properties that, if better understood, could lead to an entire new generation of artificial materials that improve solar energy harvesting, and chemical and biological sensing. Unfortunately, observing and imaging optical properties at such small scales is next to impossible due to the diffraction limit of visible light. So, researchers at MechSE used electrons--instead of visible light--to create high-resolution images of metal nanostructures and particles.
In a September 2009 issue of the American Chemical Society’s ACS Nano Journal, the researchers described how they bombarded triangular-shaped metal nanostructures with a tightly focused beam of electrons that caused the metal to emit visible light from an area only 10 to 20 nanometers in size. They then created a high-resolution map of each nanostructure’s local optical activity by scanning and measuring the wavelength and intensity of light emitted at each point.
Although materials scientists had previously used this method, known as cathodoluminescence spectroscopy, to examine quantum dots and other structures in semiconductors, they did not expect it could be used to image metals. In fact, it took almost two years for MechSE graduate student Pratik Chaturvedi and his adviser, Assistant Professor Nicholas Fang to convince their colleagues in materials sciences to take a look at what they were seeing.
“They believed that metal was an electron-absorbing material, so they did not expect it to emit visible light ,” Fang said.
But, the silver nanostructures did emit visible light, and when the researchers focused the electron beam on the corners of the triangles, the whole nanostructure lit up. Fang believes this near-field enhancement was caused by electron clouds inside the metal that scattered the boundary of the particle.
“Electron excitation is a local mode, but it’s different from light excitation, because all modes in a particle can be excited,” he said.
When Fang and his colleagues compared the images created via electron excitation with those made via traditional light excitation, they could see that there were optical modes in the nanoparticles that did not respond to light but that showed up when a focused electron beam was aimed at them.
“These are dark modes that are not excitable to free space light waves but can contribute to near-field enhancement,” Fang said.
Such near-field enhancement could be very useful in sensing, as scientists could use it to spot out and design regions on nanostructures that are resonant at specific wavelengths that can detect electron emissions from nearby molecules.