Using Raman Scattering to Understand Stress and Strain in Films of Nanoparticles

In films composed of nanocrystals, the particles are bound by interactions between the molecular ligands on the surfaces of the cores. They constitute a new type of materials with unexplored mechanical properties. Films deposited from solutions of nanocrystals can fracture after deposition because of strain that formed after solvent evaporation.
Professor Herman, in collaboration with Professor Kysar and others, has studied this for CdSe nanocrystals of 3 nm diameter deposited in an electric field (electrophoretic deposition). Very thick films fracture (optical micrograph on the right). He measured the strain in the CdSe nanocrystal cores by a method known as Raman scattering, which determines the vibration frequencies of the atoms in the particles from the shift in wavelength of the light that scatters from the film. This change in wavelength (or frequency) is also very sensitive to strain, and our measurements (left) indicate that the strain can exceed 1%. From this information, he was also able to deduce properties of the films, such as the biaxial elastic modulus, which relates the stress and strain in the film.


Raman scattering measurements (left) of the fractured CdSe nanocrystal film (right).

Posted on: May 12, 2006
(For more, see Nano Letters 6, 175-180 (2006).)