Vibrational energy in materials is sustained by vibration modes. These are characterised by a wavelength (λ) and an energy (E=hν, where ν is the vibrational frequency). When the wavelength of the mode approaches the size of an object, the whole object starts to oscillate. These oscillations can be used to study the subtle dynamic changes in an object's shape, all the way from planet-sized objects to nanoscale particles.
Most studies to date, however, have focused on highly symmetric and elastically isotropic objects like spheres, such as metallic clusters and nanobeads, through techniques like inelastic light scattering. Now, Bruno Stephanidis and colleagues at the University of Lyon have discovered that nanoparticle vibration modes are very sensitive to the internal elasticity and the detailed morphology of these particles, whether they are perfect isotropic spheres or anisotropic-faceted.
The researchers obtained their results through the inelastic scattering of laser light, or Raman scattering, detecting exceedingly small energy changes of the scattered photons from gold nanoparticles. They did this using a very-high-resolution spectrometer, called a tandem Fabry-Perot interferometer.
"Until now, low-frequency Raman scattering spectra from nanoparticles were essentially interpreted by assuming that the nanoparticles were elastically isotropic and spherical," Stephanidis told nanotechweb.org. "This simple approach was needed because most commercially available high-resolution spectrometers are unable to measure the fine structure of the relevant Raman lineshapes. By using the tandem Fabry-Perot with our samples containing anisotropic gold nanoparticles, we can now resolve substructures in the lineshapes."
These lineshapes correspond to a lift of degeneracy induced by lowering the symmetry of the nanoparticles as they go from being perfect spheres to anisotropic cubaoctahedra, explains Stephanidis. "Our results provide a very nice physics rationale about degeneracy lift upon symmetry lowering. This is important as applications are dealing more and more with non-spherical anisotropic nanoparticles."
The results could be used as the basis for studying elastically anisotropic non-spherical metallic nanoparticles with more complex shapes, such as nanoprisms and nanostars. "They may also be useful for the study of polyhedral biological nanoparticles, like viruses," said Stephanidis.
The team will now continue working on similar systems like gold/silver alloy nanoparticles and will also focus on the vibration modes of biological nanospheres.
The work was published in Physical Review B.