Colloidal nanocrystals are composed of inorganic nanoparticle cores with organic ligands bound to the surface of each core. We have previously demonstrated that electrophoretic deposition, using DC electric fields, is a convenient way to deposit high quality, uniform films of nanoparticles from a solvent solution either over large areas or, selectively, in patterned regions. These films can be considered a new class of "interfacial solids" with the interfaces consisting of bonds between inorganic cores and organic ligands. The complex interplay between the nature and strength of these bonds, the ligand-ligand dynamics, and residual solvent trapped in the nano-pores, is expected to greatly affect the film properties, in particular the mechanical behavior. The dependence of both elastic and plastic mechanical properties on deposition conditions and interfacial chemistry needs to be understood to control and optimize the film mechanical behavior and stability. In nanoindentation, the force and indentation displacement are monitored as a sharp probe indents a materials, with controlled load. Nanoindentation shows that the mechanical response of these CdSe nanocrystal films exhibit viscoplasticity, which means that the response depends on how fast the load is applied and after deformation the material does not return to its initial size and shape. This viscoplasticity suggests polymeric features that are attributable to the organic ligands bound to the nanocrystal cores; with fewer ligands, features of deformation consistent with granular media emerge. The findings concerning the relative degree of polymeric and granular behavior should apply to films of other core/ligand nanocrystals, other methods of nanocrystal film formation, and to ordered nanocrystal arrays. Numerical simulations further elucidate the role the ligands play in the overall mechanical behavior of the films and point to the deformation modes during plastic flow.

(For more, see Phys. Rev. Lett. 98, 026103 (2007).)

Posted on: April 23, 2007