Impact Dynamics of Aerosolized Colloidal Quantum-Dot Nanospheres

Lejun Qi, David J. Norris, Peter H. McMurry, STEVEN L. GIRSHICK

University of Minnesota

     Abstract Number: 700
     Last modified: May 14, 2010

Abstract
Quantum-dot nanospheres are agglomerates formed by aerosolizing semiconductor nanocrystals from the colloidal dispersion in which they are synthesized. Each agglomerate consists of multiple nanocrystals, with organic capping ligands and other organic residue from the colloid. Such structures are of potential interest for fabrication of optoelectronic devices by direct-write techniques.

We here report an experimental study of the impact dynamics of aerosolized colloidal quantum-dot nanospheres consisting of cadmium selenide nanocrystals (diameter 3-5 nm, and close to monodisperse for given synthesis conditions) that are capped by trioctylphosphine oxide and dispersed in hexane, and then aerosolized using a nebulizer. Measurements by tandem differential mobility analysis-aerosol particle mass analysis indicate that the aerosolized agglomerates are close to spherical. These measurements also show that the distance between neighboring nanocrystals in the agglomerate is reduced by increasing their concentration in the colloidal dispersion.

Experiments were conducted in which the agglomerates were impacted on carbon substrates by either electrostatic precipitation or aerodynamic focusing, produced either low-speed (1-2 cm/s) or high-speed (100-150 m/s) impact, respectively. The resulting deposits were examined by scanning transmission electron microscopy. The impact deformation mode was found to undergo a solid-like to liquid-like transition as the concentration of nanocrystals in the colloidal dispersion was increased. This is explained in terms of the cohesive interaction between neighboring nanocrystals in the agglomerate, which becomes stronger as the spacing between the nanocrystals is reduced. Impact behavior in the solid-like regime is similar to that observed in granular materials. The results also show the range of conditions under which deposits consisting of single monolayers (or other desired number of monolayers) of nanocrystals can be obtained, which is of potential interest for device applications.

This work was partially supported by the U.S. National Science Foundation under grant CTS-0506748.