Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage.

For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of these particles are increased, favoring deposition. In situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insight into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass-transport-limited to reaction-rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering, from stable, well-dispersed colloidal starting materials.