Center for Functional Nanomaterials Seminar

The charge on nanocrystals is not only used to stabilize the colloidal systems but also to assemble these materials into novel films and superlattices. Here, we propose a model for the charging of colloidal CdSe nanocrystals in non-aqueous solvents involving the dissociation of ligand molecules from specific surface sites. We also develop a mechanistic model to explain the electrophoretic deposition of nanocrystal films based on electrophoretic mobility measurements, photoluminescence from nanocrystal solutions and films, and observations from deposition experiments. Even though equally thick nanocrystal films are obtained on both negative and positive electrodes, the numbers of positive and negative nanocrystals are not equal in solution. After appropriate reprecipitation cycles, the surface ligands on colloidal CdSe nanocrystals are partially removed, giving rise to unpassivated surface sites that may enable the nanocrystals to better "stick" to electrodes during electrophoretic deposition. Thus, the surface charge of the nanocrystals is very significantly influenced by the presence of coordinating ligands. The factor limiting the maximum thickness achievable by electrophoretic deposition is thought to be the concentration of the minority charged nanocrystals (negatively charged nanocrystals in our experiments). The mechanical properties of these elctrophoretically deposited films have been measured by nanoindentation. The elastic modulus and hardness of 3.2 nm diameter CdSe nanocrystal films are around 10 GPa and 450 MPa, respectively. Furthermore, after particle cross-linking and partial ligand removal, the films exhibit compaction of the cores. This mechanical response suggests these nanocrystal films have polymeric features that can be attributed to the organic ligands and granular characteristics due to the inorganic cores. Both characteristics have also been confirmed by investigating larger nanocrystals and by removing the capping ligands.